Information

16.1: Sensory Systems - Biology


We experience reality through our senses. Our exteroceptors detect stimulation from the outsides of our body: this includes smell, taste, and equilibrium. The interoceptors receive stimulation from the inside of our bodies: this includes blood pressure dropping, changes in the glucose, and pH levels. This has to do with how the brain interprets the stimuli that are received.

Chemoreception

The senses of gustation (taste) and olfaction (smell) fall under the category of chemoreception. Specialized cells act as receptors for certain chemical compounds. As these compounds react with the receptors, an impulse is sent to the brain and is registered as a certain taste or smell. Gustation and olfaction are chemical senses because the receptors they contain are sensitive to the molecules in the food we eat, along with the air we breathe.

Gustatory System

In humans, the sense of taste is transduced by taste buds and is conveyed via three of the twelve cranial nerves. Cranial nerve VII, the facial nerve, carries taste sensations from the anterior two thirds of the tongue (excluding the circumvallate papillae, see lingual papilla) and soft palate. Cranial nerve IX the glossopharyngeal nerve carries taste sensations from the posterior one third of the tongue (including the circumvallate papillae). Also a branch of the vagus nerve carries some taste sensations from the back of the oral cavity (i.e., pharynx and epiglottis). Information from these cranial nerves is processed by the gustatory system. Though there are small differences in sensation, which can be measured with highly specific instruments, all taste buds can respond to all types of taste. Sensitivity to all tastes is distributed across the whole tongue and indeed to other regions of the mouth where there are taste buds (epiglottis, soft palate).

Papilla

Papilla are specialized epithelial cells. There are four types of papillae: filiform (thread-shape), fungiform (mushroom-shape), foliate (leaf-shape), and circumvallate (ringed-circle). All papillae except the filiform have taste buds on their surface. Some act directly by ion channels, others act indirectly.

  • Fungiform papillae: as the name suggests, are slightly mushroom shaped if looked at in section. These are present mostly at the apex (tip) of the tongue.
  • Filiform papillae: these are thin, longer papillae that don’t contain taste buds but are the most numerous. These papillae are mechanical and not involved in gustation.
  • Foliate papillae: these are ridges and grooves towards the posterior part of the tongue.
  • Circumvallate papillae: there are only about 3–14 of these papillae on most people and they are present at the back of the oral part of the tongue. They are arranged in a circular-shaped row just in front of the sulcus terminalis of the tongue.

Olfactory System

Olfaction is the sense of smell. In humans the sense of smell is received in nasopharynx. Airborne molecules go into solution on moist epithelial surface of nasal passage. An olfactory receptors neuron sends an impulse via Cranial nerve I the olfactory nerve. Although 80–90 percent of what we think is “taste” actually is due to smell. This is why when we have a head cold or stuffed up nose we have a harder time tasting our foods.

Receptors

Humans have 347 functional odor receptor genes; the other genes have nonsense mutations. This number was determined by analyzing the genome in the Human Genome Project; the number may vary among ethnic groups, and does vary among individuals. For example, not all people can smell androstenone, a component of male sweat.

Each olfactory receptor neuron in the nose expresses only one functional odor receptor. Odor receptor nerve cells may function like a key-lock system: if the odor molecules can fit into the lock the nerve cell will respond. According to shape theory, each receptor detects a feature of the odor molecule. Weak-shape theory, known as odotope theory, suggests that different receptors detect only small pieces of molecules, and these minimal inputs are combined to create a larger olfactory perception (similar to the way visual perception is built up of smaller, information-poor sensations, combined and refined to create a detailed overall perception). An alternative theory, the vibration theory proposed by Luca Turin[1], posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunneling. However, the behavioral predictions of this theory have been found lacking[2].

An olfactory receptor neuron, also called an olfactory sensory neuron, is the primary transduction cell in the olfactory system. Humans have about 40 million olfactory receptor neurons. In vertebrates, olfactory receptor neurons reside on the olfactory epithelium in the nasal cavity. These cells are bipolar neurons with a dendrite facing the interior space of the nasal cavity and an axon that travels along the olfactory nerve to the olfactory bulb.

Many tiny hair-like cilia protrude from the olfactory receptor cell’s dendrite and into the mucus covering the surface of the olfactory epithelium. These cilia contain olfactory receptors, a type of G protein-coupled receptor. Each olfactory receptor cell contains only one type of olfactory receptor, but many separate olfactory receptor cells contain the same type of olfactory receptor. The axons of olfactory receptor cells of the same type converge to form glomeruli in the olfactory bulb.

Olfactory receptors can bind to a variety of odor molecules. The activated olfactory receptor in turn activates the intracellular G-protein GOLF, and adenylate cyclase and production of Cyclic AMP opens ion channels in the cell membrane, resulting in an influx of sodium and calcium ions into the cell. This influx of positive ions causes the neuron to depolarize, generating an action potential.

Individual olfactory receptor neurons are replaced approximately every 40 days by neural stem cells residing in the olfactory epithelium. The regeneration of olfactory receptor cells, as one of the only few instances of adult neurogenesis in the central nervous system, has raised considerable interest in dissecting the pathways for neural development and differentiation in adult organisms.

In the Brain

The axons from all the thousands of cells expressing the same odor receptor converge in the olfactory bulb (Figure 1). Mitral cells in the olfactory bulb send the information about the individual features to other parts of the olfactory system in the brain, which puts together the features into a representation of the odor. Since most odor molecules have many individual features, the combination of features gives the olfactory system a broad range of odors that it can detect.

Odor information is easily stored in long term memory and has strong connections to emotional memory. This is possibly due to the olfactory system’s close anatomical ties to the limbic system and hippocampus, areas of the brain that have long been known to be involved in emotion and place memory, respectively.

Pheromonal Olfaction

Some pheromones are detected by the olfactory system, although in many vertebrates pheromones are also detected by the vomeronasal organ, located in the vomer, between the nose and the mouth. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a face called flehmen to direct air to this organ. In humans, it is unknown whether or not pheromones exist.

Olfaction and Gustation

Olfaction, taste and trigeminal receptors together contribute to flavor. It should be emphasized that there are no more than 5 distinctive tastes: salty, sour, sweet, bitter, and umami. The 10,000 different scents which humans usually recognize as “tastes” are often lost or severely diminished with the loss of olfaction. This is the reason why food has little flavor when your nose is blocked, as from a cold.

The key nutrition players in our taste is the olfactory function, 80–90 percent of what we consider taste is dependent on our senses of smell. With aging our olfactory function declines. In the elderly careful monitoring of appetite is necessary due to the alterations in the olfactory function.

The Sense of Vision

Vision needs to have the work of both the eyes and the brain to process any information. The majority of the stimuli is done in the eyes and then the information is sent to the brain by the way of nerve impulses. At least one-third of the information of what the eye sees is processed in the cerebral cortex of the brain.

Anatomy of the Eye

The human eye is a elongated ball about 1-inch (2.5 cm) in diameter and is protected by a bony socket in the skull. The eye has three layers or coats that make up the exterior wall of the eyeball, which are the sclera, choroid, and retina.

Sclera

The outer layer of the eye is the sclera, which is a tough white fibrous layer that maintains, protects and supports the shape of the eye. The front of the sclera is transparent and is called the cornea. The cornea refracts light rays and acts like the outer window of the eye.

Choroid

The middle thin layer of the eye is the choroid, also known as the choroidea or choroid coat, it is the vascular layer of the eye lying between the retina and the sclera. The choroid provides oxygen and nourishment to the outer layers of the retina. It also contains a nonreflective pigment that acts as a light shield and prevents light from scattering. Light enters the front of the eye through a hole in the choroid coat called the pupil. The iris contracts and dilates to compensate for the changes in light intensity. If the light is bright the iris then contracts making the pupil smaller, and if the light is dim, the iris dilates making the pupil bigger. Just posterior to the iris is the lens, which is composed mainly of proteins called crystallins. The lens is attached by the zonules to the ciliary body that contains the ciliary muscles that control the shape of the lens for accommodation. Along with the ciliary body and iris, the choroid forms the uveal tract. The uvea is the middle of the three concentric layers that make up an eye. The name is possibly a reference to its almost black color, wrinkled appearance and grape-like size and shape when stripped intact from a cadaveric eye.

Eye Movement

The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second, thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities. Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.

Try This Experiment

Hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about one hertz, the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand.

Depth Perception

Depth perception is the visual ability to perceive the world in three dimensions. It is a trait common to many higher animals. Depth perception allows the beholder to accurately gauge the distance to an object. Depth perception is often confused with binocular vision, also known as Stereopsis. Depth perception does rely on binocular vision, but it also uses many other monocular cues.

The Senses Of Hearing

The ear is the sense organ that collects and detects sound waves and plays a major role in the sense of balance and body position. The sensory receptors for both hearing and equilibrium are mechanoreceptors found in the inner ear; these receptors are hair cells that have stereocilia (long microvilli) that are extremely sensitive to mechanical stimulations.

Anatomy of the Ear

The ear has three divisions: the outer ear, the middle ear, and the inner ear (Figure 3).

Outer Ear: Auricle, Ear Canal, Surface of Ear Drum

The outer ear is the most external portion of the ear. The outer ear includes the pinna (also called auricle), the ear canal, and the very most superficial layer of the ear drum (also called the tympanic membrane). Although the word “ear” may properly refer to the pinna (the flesh covered cartilage appendage on either side of the head), this portion of the ear is not vital for hearing. The complicated design of the human outer ear does help capture sound, but the most important functional aspect of the human outer ear is the ear canal itself. This outer ear canal skin is applied to cartilage; the thinner skin of the deep canal lies on the bone of the skull. If the ear canal is not open, hearing will be dampened. Ear wax (medical name: cerumen) is produced by glands in the skin of the outer portion of the ear canal. Only the thicker cerumen-producing ear canal skin has hairs. The outer ear ends at the most superficial layer of the tympanic membrane. The tympanic membrane is commonly called the ear drum.

Middle Ear: Air Filled Cavity behind the Ear Drum, includes most of the Ear Drum, and Ear Bones

The middle ear includes most of the ear drum (tympanic membrane) and the 3 ear bones ossicles: malleus (or hammer), incus (or anvil), and stapes (or stirrup). The opening of the Eustachian tube is also within the middle ear. The malleus has a long process (the handle) that is attached to the mobile portion of the ear drum. The incus is the bridge between the malleus and stapes. The stapes is the smallest named bone in the human body. The stapes transfers the vibrations of the incus to the oval window, a portion of the inner ear to which it is connected. It is the final bone in the chain to transfer vibrations from the eardrum to the inner ear. The arrangement of these 3 bones is a sort of Rube Goldberg device: movement of the tympanic membrane causes movement of the first bone, which causes movement of the second, which causes movement of the third. When this third bone pushes down, it causes movement of fluid within the cochlea (a portion of the inner ear). This particular fluid only moves when the stapes footplate is depressed into the inner ear. Unlike the open ear canal, however, the air of the middle ear is not in direct contact with the atmosphere outside the body. The Eustachian tube connects from the chamber of the middle ear to the back of the pharynx. The middle ear in humans is very much like a specialized paranasal sinus, called the tympanic cavity, it, like the paranasal sinuses, is a hollow mucosa lined cavity in the skull that is ventilated through the nose. The mastoid portion of the temporal bone, which can be felt as a bump in the skull behind the pinna, also contains air, which ventilates through the middle ear.

Inner Ear: Cochlea, Vestibule, and Semi-Circular Canals

The inner ear includes both the organ of hearing (the cochlea, Figure 4) and a sense organ (the labyrinth or vestibular apparatus) that is attuned to the effects of both gravity and motion. The balance portion of the inner ear consists of three semi-circular canals and the vestibule. The inner ear is encased in the hardest bone of the body. Within this ivory hard bone, there are fluid-filled hollows. Within the cochlea are three fluid filled spaces: the tympanic canal, the vestibular canal, and the middle canal. The eighth cranial nerve comes from the brain stem to enter the inner ear. When sound strikes the ear drum, the movement is transferred to the footplate of the stapes, which attaches to the oval window and presses into one of the fluid-filled ducts of the cochlea. The hair cells in the organ of Corti are stimulated by particular frequencies of sound, based on their location within the cochlea. High pitch sounds are at a higher frequency and, due to the shorter wavelength they “hit” the membrane “faster” (ie. close to the oval window). In contrast, low frequency sounds have large wavelengths, and will travel further through the scala vestibuli before “hitting” the tectorial membrane near the apex of the cochlea. The fluid inside the cochlea is moved, flowing against the receptor (hair) cells of the organ of Corti, which fire in a graded response based on the volume of the sound. The hair cells then stimulate the nerve cells in the Spiral Ganglion, which sends information through the auditory portion of the eighth cranial nerve to the brain. Humans are able to hear sounds between about 20 Hz and 20,000 Hz. Mammals that can hear lower frequency sounds, such as whales and elephants, have a longer cochlea. Humans tend to lose high-frequency hearing first, which has led some teenagers to using high-frequency ring tones (above 17,000 Hz) that may go undetected by their middle-aged teachers.

Hair Cell

Hair cells are columnar cells, each with a bundle of 100–200 specialized cilia at the top, for which they are named. These cilia are the mechanosensors for hearing. Lightly resting atop the longest cilia is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the cilia and allowing electric current into the hair cell. Hair cells, like the photoreceptors of the eye, show a graded response, instead of the spikes typical of other neurons.

Immediately over the hair cells of the organ of Corti is an overhanging “tectorial membrane.” When the Bones of the Middle Ear vibrate the oval window, these vibrations are transmitted to the fluid within the cochlea and eventually cause the round window on the cochlea to bulge outward. These vibrations deflect the membrane on which the Organ of Corti is located, causing the three rows of outer hair cells to “rub” against the overhanging tectorial membrane. By their muscle-like activity they ampify the weakest vibrations for the inner hair cells. The louder sounds are not amplified. The disturbed inner hair cells will then activate the cochlear nerve fibers.

The current model is that cilia are attached to one another by “tip links,” structures which link the tips of one cilium to another. Stretching and compressing the tip links may open an ion channel and produce the receptor potential in the hair cell. These graded potentials are not bound by the “all or none” properties of an action potential. There are far fewer hair cells than afferent (leading to the brain) nerve fibers in the cochlea. The nerve that innervates the cochlea is the cochlear nerve, and forms cranial nerve number VIII with the vestibular nerve from the balance organ. Neuronal dendrites innervate cochlear hair cells. The neurotransmitter itself is thought to be glutamate. At the presynaptic juncture, there is a distinct “presynaptic dense body” or ribbon. This dense body is surrounded by synaptic vesicles and is thought to aid in the fast release of neurotransmitter. Efferent projections from the brain to the cochlea also play a role in the perception of sound. Efferent synapses occur on outer hair cells and on afferent dendrites under inner hair cells.

Process of Hearing

Detection of sound motion is associated with the right posterior superior temporal gyrus. The superior temporal gyrus contains several important structures of the brain, including: marking the location of the primary auditory cortex, the cortical region responsible for the sensation of sound. Sections 41 and 42 are called the primary auditory area of the cerebrum, and processes the basic characteristics of sound such as pitch and rhythm. The auditory association area is located within the temporal lobe of the brain, in an area called the Wernicke’s area, or area 22. This area, near the lateral cerebral sulcus, is an important region for the processing of acoustic energy so that it can be distinguished as speech, music, or noise. It also interprets words that are heard into an associated thought pattern of understanding. The gnostic area of the cerebrum, (areas 5, 7, 39 and 40) helps to integrate all incoming sense patterns so that a common thought can be formed (correlated) using all arriving sensory information.

Hearing Under Water

Hearing threshold and the ability to localize sound sources are reduced underwater. in which the speed of sound is faster than in air. Underwater, hearing is by bone conduction and localization of sound appears to depend on differences in amplitude detected by bone conduction.

Localization of Sound by Humans

Humans are normally able to hear a variety of sound frequencies, from about 20Hz to 20kHz. Our ability to estimate just where the sound is coming from, sound localization, is dependent on both hearing ability of each of the two ears, and the exact quality of the sound. Since each ear lies on an opposite side of the head, a sound will reach the closest ear first, and its amplitide will be loudest in that ear. Much of the brain’s ability to localize sound depends on interaural (between ears) intensity differences and interaural temporal or phase differences.

Two mechanisms are known to be used. Bushy neurons can resolve time differences as small as the time it takes sound to pass one ear and reach the other (10 milliseconds). For high frequencies, frequencies with a wavelength shorter than the listener’s head, more sound reaches the nearer ear. Human echolocation is a technique involving echolocation used by some blind humans to navigate within their environment.

Process of Equilibrium

Equilibrioception or sense of balance is one of the physiological senses. It allows humans and animals to walk without falling. Some animals are better in this than humans, for example allowing a cat (as a quadruped using its inner ear and tail) to walk on a thin fence. All forms of equilibrioception can be described as the detection of acceleration.

It is determined by the level of fluid properly called endolymph in the labyrinth: a complex set of tubing in the inner ear.

When the sense of balance is interrupted it causes dizziness, disorientation and nausea.

You can temporarily disturb your sense of balance by closing your eyes and turning rapidly in circles five or six times. This starts the fluid swirling in circles inside your ear canal. When you stop turning it takes a few seconds for the fluid to lose momentum, and until then the sense from your inner ear conflicts with the information coming from your vision, causing dizziness and disorientation. Most astronauts find that their sense of balance is impaired when in orbit, because there is not enough gravity to keep the ear’s fluid in balance. This causes a form of motion sickness called space sickness.

Touch

Touch is the first sense developed in the womb and the last sense used before death. With 50 touch receptors for every square centimeter and about 5 million sensory cells overall, the skin is very sensitive and is the largest and one of the most complex organs in our bodies. These touch receptors are grouped by type and include mechanoreceptors (sensitive to pressure, vibration and slip), thermoreceptors (sensitive to changes in temperature), and nocioreceptors (responsible for pain).

Pacinian Corpuscles

Pacinian corpuscles detect gross pressure changes and vibrations. They are the largest of the receptors. Any deformation in the corpuscle causes action potentials to be generated, by opening pressure-sensitive sodium ion channels in the axon membrane. This allows sodium ions to influx in, creating a receptor potential. Pacinian corpuscles cause action potentials when the skin is rapidly indented but not when the pressure is steady, due to the layers of connective tissue that cover the nerve ending [3]. It is thought that they respond to high velocity changes in joint position.

Meissner’s Corpuscle

Meissner’s corpuscles are distributed throughout the skin, but concentrated in areas especially sensitive to light touch, such as the fingertips, palms, soles, lips, tongue, face, nipples and the external skin of the male and female genitals. They are primarily located just beneath the epidermis within the dermal papillae. Any physical deformation in the Meissner’s corpuscle will cause an action potential in the nerve. Since they are rapidly adapting or phasic, the action potentials generated quickly decrease and eventually cease. If the stimulus is removed, the corpuscle regains its shape and while doing so (i.e., while physically reforming) causes another volley of action potentials to be generated. This is the reason one stops “feeling” one’s clothes. This process is called sensory adaption. Because of their superficial location in the dermis, these corpuscles are particularly sensitive to touch and vibrations, but for the same reasons, they are limited in their detection because they can only signal that something is touching the skin. Meissner’s corpuscles do not detect pain; this is signaled exclusively by free nerve endings.

Ruffini Corpuscles

Ruffini corpuscles are thermoreceptors, aiding in the detection of temperature changes. Named after Angelo Ruffini, the Ruffini ending is a class of slowly adapting mechanoreceptor thought to exist only in the glabrous dermis and subcutaneous tissue of humans. This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and movement.



16.1: Sensory Systems - Biology

Since publication of the first edition, huge developments have taken place in sensory biology research and new insights have been provided in particular by molecular biology. These show the similarities in the molecular architecture and in the physiology of sensory cells across species and across sensory modality and often indicate a common ancestry dating back over half a billion years.

Biology of Sensory Systems has thus been completely revised and takes a molecular, evolutionary and comparative approach, providing an overview of sensory systems in vertebrates, invertebrates and prokaryotes, with a strong focus on human senses.

Written by a renowned author with extensive teaching experience, the book covers, in six parts, the general features of sensory systems, the mechanosenses, the chemosenses and the senses which detect electromagnetic radiation. Other sensory systems are also dealt with, including pain, thermosensitivity and minority senses. Finally the book provides an outline and discussion of philosophical implications.

Greater emphasis on molecular biology and intracellular mechanisms

New chapter on genomics and sensory systems

Sections on TRP channels, synaptic transmission, evolution of nervous systems, arachnid sensory systems, electroreception in the Monotremata, language and the FOXP2 gene, mirror neurons and the molecular biology of pain

Updated passages on animal and human olfaction and gestation

Over four hundred illustrations, boxes containing supplementary material and self–assessment questions and a full bibliography at the end of each part make Biology of Sensory Systems essential reading for undergraduate students of biology, zoology, animal physiology, neuroscience, anatomy and physiological psychology. The book is also suitable for postgraduate students in more specialised courses such as vision sciences, optometry, neurophysiology, neuropathology, developmental biology.

Since publication of the first edition, huge developments have taken place in sensory biology research and new insights have been provided in particular by molecular biology. These show the similarities in the molecular architecture and in the physiology of sensory cells across species and across sensory modality and often indicate a common ancestry dating back over half a billion years.

Biology of Sensory Systems has thus been completely revised and takes a molecular, evolutionary and comparative approach, providing an overview of sensory systems in vertebrates, invertebrates and prokaryotes, with a strong focus on human senses.

Written by a renowned author with extensive teaching experience, the book covers, in six parts, the general features of sensory systems, the mechanosenses, the chemosenses and the senses which detect electromagnetic radiation. Other sensory systems are also dealt with, including pain, thermosensitivity and minority senses. Finally the book provides an outline and discussion of philosophical implications.

Greater emphasis on molecular biology and intracellular mechanisms

New chapter on genomics and sensory systems

Sections on TRP channels, synaptic transmission, evolution of nervous systems, arachnid sensory systems, electroreception in the Monotremata, language and the FOXP2 gene, mirror neurons and the molecular biology of pain

Updated passages on animal and human olfaction and gestation

Over four hundred illustrations, boxes containing supplementary material and self–assessment questions and a full bibliography at the end of each part make Biology of Sensory Systems essential reading for undergraduate students of biology, zoology, animal physiology, neuroscience, anatomy and physiological psychology. The book is also suitable for postgraduate students in more specialised courses such as vision sciences, optometry, neurophysiology, neuropathology, developmental biology.

Preface to Second Edition.

1.3 Membrane Signalling Systems.

Chapter 2 Membranes, Action Potentials, Synapses.

2.1 The Measurement of Resting Potentials.

2.2 The Ionic Bases of Resting Potentials.

2.3 Electrotonic Potentials and Cable Conduction.

2.4 Receptor and Generator Potentials.

2.7 Synapses and Synaptic Transmission.

Chapter 3 General Features of Sensory Systems.

3.1 Classification of the Senses.

3.6 Maps of Sensory Surfaces.

3.7 Hierarchical and Parallel Design.

3.8 Feature Extraction and Trigger Stimuli.

Chapter 4 Classification and Phylogeny.

4.2 Classification into Six Kingdoms.

4.5 Protostomes and Deuterostomes.

4.6 Classification of the Metazoa.

4.7 Evolution of Nervous Systems.

Chapter 5 Genes, Genomics and Neurosensory Systems.

5.3 Genomes and Neurosensory Systems.

Part I: Notes, References and Bibliography.

PART II: MECHANOSENSITIVITY.

Chapter 6 Mechanosensitivity of Cell Membranes.

6.1 Mechanosensitive Channels in E. coli.

6.2 Detection of Osmotic Swelling by Hypothalamic Cells in Mammals.

7.1 Kinaesthetic Mechanisms in Arthropods.

7.1.1 Stretch Receptors in Crustacean Muscle.

7.2 Kinaesthetic Mechanisms in Mammals.

8.1 Mechanoreception in Caenorhabditis Elegans.

8.4 Tactile Receptors in Mammalian Skin.

8.5 Cerebral Analysis of Touch.

8.6 Plasticity of the Somaesthetic Cortex.

Chapter 9 Equilibrium and Hearing: The Uses of Hair Cells.

9.1 Anatomy and Physiology of Hair Cells.

9.3 Evolution of the Vertebrate Ear.

Box 9.1 Biophysics of Outer Hair Cells.

Box 9.2 Genetics and Deafness.

Chapter 10 Cerebral Analysis.

10.1 The Mammalian Vestibular Pathway and Reflexes.

10.2 The Mammalian Auditory Pathway.

10.3 The Avian Auditory Pathway and the Mapping of Auditory Space by the Barn Owl.

10.4 The Mammalian Auditory Cortex.

10.5 The Bat Auditory System and Echolocation.

10.6 The Human Auditory Cortex and Language.

10.7 Lateralization and the Neuroanatomy of Language.

10.8 Language and the FOXP2 Gene.

10.9 Callosectomy and After.

Box 10.1 Broca and Wernicke.

Part II: Notes, References and Bibliography.

PART III: CHEMOSENSITIVITY.

Chapter 11 Chemosensitivity in Prokaryocytes.

11.1 Chemosentivity in E. coli.

Chapter 12 Mammalian Chemo– Enteroreceptors.

12.1 Location of Mammalian Chemoreceptors for PaO<sub>2</sub> and PaCO<sub>2</sub>.


PHYSICS 141: The Physics of Sensory Systems in Biology

Shopping Period

Prof. Samuel will hold several "drop-in" zoom meetings during Shopping Period to meet with students and discuss the course and its contents (see Zoom). Short videos describing the academic goals and overall organization of the class will be made available (see Panopto).

Shopping Period Meetings

  • Monday, August 17th at 9 am (link)
  • Monday, August 17th at 12 pm (link)
  • Monday, August 17th at 4 pm (link)
  • Tuesday, August 18th at 9 am (link)
  • Tuesday, August 18th at 12 pm (link)

There is no required textbook for the course. Readings will be posted in Weekly Modules on this website (and also in the #Readings channel of our Slack website where we can engage in discussion threads about them). Much of the biology will be drawn from Sensory Transduction by Gordon Fain. Much of the physics will be drawn from Biophysics: Searching for Principles by Bill Bialek and from Random Walks in Biology by Howard Berg.

Pre-recorded lectures

All of the background physics, mathematics, and biology will be delivered in sets of pre-recorded lectures (See Panopto). Students are expected to view these pre-recorded lectures and complete the associated reading assignment before each of the scheduled class meetings.

Class meetings with Prof. Samuel

The scheduled class meetings (Tu, Th 9-10:15 AM EST) will be discussions of the material presented in the pre-recorded lectures and reading assignments. If any students are unable to attend at 9 AM (e.g., because of time zone), Prof. Samuel will hold a second class meeting later on Tuesdays and Thursdays (which will be regularly scheduled based on a student poll at the start of class).

Attendance at either the 9 AM or later meeting is mandatory. Students are free to join both. All zoom meetings will be recorded and made available on the website. Class participation is expected either live during these class meetings or through online discussions on Slack.

Section meetings with David Zimmerman, Teaching Fellow

We will schedule a weekly 1.5 h section meeting with David Zimmerman, a biophysics graduate student who is the teaching fellow for this course. The section meeting will focus on problem-solving relevant to completing the homework and programming assignments.

Student problem-solving workshops

Students are encouraged to work together to complete weekly homework and coding assignments. Slack will provide a continuous forum for exchanging ideas and discussions among students and conversing with Prof. Samuel and David Zimmerman. Students will be encouraged to schedule their own zoom meetings with one another.

We will also schedule a regular zoom weekly workshop 7-10 PM EST on Thursday evenings so that students can drop in and work together synchronously on problem sets. Prof. Samuel or David will be able to drop-in at these problem-solving workshops.

Video and written completion of assignments

Students will individually upload their written assignments to the course website. Each problem set will also include a "concept question" that will be answered by a short self-made video (5-10 minutes).

Midterm project

Students will choose two problems from a list, and have a week to prepare written solutions and a self-made video presenting their own solutions. Midterm projects are expected to be solved individually.

Final student presentations

Throughout the course, most of our class meetings will center on discussions of classic papers in the field of sensory systems. At the end of the semester, each student will present a recently published paper of their choosing. Students will 1) write their own review-style paper 2) prepare a video seminar that describes the importance of their chosen paper and communicates any essential physical and mathematical principles needed as background and 3) construct their own coding project that involves either data analysis or simulation that helps to illuminate the chosen paper.

50% Video and written completion of weekly problem sets and coding assignments

20% Final student presentation

The syllabus page shows a table-oriented view of the course schedule, and the basics of course grading. You can add any other comments, notes, or thoughts you have about the course structure, course policies or anything else.


36.1 Sensory Processes

By the end of this section, you will be able to do the following:

  • Identify the general and special senses in humans
  • Describe three important steps in sensory perception
  • Explain the concept of just-noticeable difference in sensory perception

Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Vestibular sensation , which is an organism’s sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (such as light, or sound, or the position of the body) into an electrical signal in the nervous system. This process is called sensory transduction .

There are two broad types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor , a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment: this neuron constitutes the sensory receptor. Free nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. For example, pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or pressure.

Reception

The first step in sensation is reception , which is the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor’s receptive field . Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with the body. For the sense of hearing, a stimulus can be a moderate distance away (some baleen whale sounds can propagate for many kilometers). For vision, a stimulus can be very far away for example, the visual system perceives light from stars at enormous distances.

Transduction

The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential . How is sensory input, such as pressure on the skin, changed to a receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 36.2) possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. Recall that in the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.

Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.

Encoding and Transmission of Sensory Information

Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus, and this segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus—a sound.

The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the CNS, and the brain will further process incoming signals.

Perception

Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord.

All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex (Figure 36.3) dedicated to processing that particular sense.

How are neural signals interpreted? Interpretation of sensory signals between individuals of the same species is largely similar, owing to the inherited similarity of their nervous systems however, there are some individual differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ.

Scientific Method Connection

Just-Noticeable Difference

It is easy to differentiate between a one-pound bag of rice and a two-pound bag of rice. There is a one-pound difference, and one bag is twice as heavy as the other. However, would it be as easy to differentiate between a 20- and a 21-pound bag?

Question: What is the smallest detectible weight difference between a one-pound bag of rice and a larger bag? What is the smallest detectible difference between a 20-pound bag and a larger bag? In both cases, at what weights are the differences detected? This smallest detectible difference in stimuli is known as the just-noticeable difference (JND).

Background: Research background literature on JND and on Weber’s Law, a description of a proposed mathematical relationship between the overall magnitude of the stimulus and the JND. You will be testing JND of different weights of rice in bags. Choose a convenient increment that is to be stepped through while testing. For example, you could choose 10 percent increments between one and two pounds (1.1, 1.2, 1.3, 1.4, and so on) or 20 percent increments (1.2, 1.4, 1.6, and 1.8).

Hypothesis: Develop a hypothesis about JND in terms of percentage of the whole weight being tested (such as “the JND between the two small bags and between the two large bags is proportionally the same,” or “. . . is not proportionally the same.”) So, for the first hypothesis, if the JND between the one-pound bag and a larger bag is 0.2 pounds (that is, 20 percent 1.0 pound feels the same as 1.1 pounds, but 1.0 pound feels less than 1.2 pounds), then the JND between the 20-pound bag and a larger bag will also be 20 percent. (So, 20 pounds feels the same as 22 pounds or 23 pounds, but 20 pounds feels less than 24 pounds.)

Test the hypothesis: Enlist 24 participants, and split them into two groups of 12. To set up the demonstration, assuming a 10 percent increment was selected, have the first group be the one-pound group. As a counter-balancing measure against a systematic error, however, six of the first group will compare one pound to two pounds, and step down in weight (1.0 to 2.0, 1.0 to 1.9, and so on), while the other six will step up (1.0 to 1.1, 1.0 to 1.2, and so on). Apply the same principle to the 20-pound group (20 to 40, 20 to 38, and so on, and 20 to 22, 20 to 24, and so on). Given the large difference between 20 and 40 pounds, you may wish to use 30 pounds as your larger weight. In any case, use two weights that are easily detectable as different.

Record the observations: Record the data in a table similar to the table below. For the one-pound and 20-pound groups (base weights) record a plus sign (+) for each participant that detects a difference between the base weight and the step weight. Record a minus sign (-) for each participant that finds no difference. If one-tenth steps were not used, then replace the steps in the “Step Weight” columns with the step you are using.

Step Weight One pound 20 pounds Step Weight
1.1 22
1.2 24
1.3 26
1.4 28
1.5 30
1.6 32
1.7 34
1.8 36
1.9 38
2.0 40

Analyze the data/report the results: What step weight did all participants find to be equal with one-pound base weight? What about the 20-pound group?

Draw a conclusion: Did the data support the hypothesis? Are the final weights proportionally the same? If not, why not? Do the findings adhere to Weber’s Law? Weber’s Law states that the concept that a just-noticeable difference in a stimulus is proportional to the magnitude of the original stimulus.


Contents

Preface to Second Edition. Preface to First Edition. PART I: PRELIMINARIES. Chapter 1 Elements. 1.1 Allosteric Effectors. 1.2 Membranes. 1.3 Membrane Signalling Systems. 1.4 Channels and Gates. 1.5 Concluding Remarks. Chapter 2 Membranes, Action Potentials, Synapses. 2.1 The Measurement of Resting Potentials. 2.2 The Ionic Bases of Resting Potentials. 2.3 Electrotonic Potentials and Cable Conduction. 2.4 Receptor and Generator Potentials. 2.5 Sensory Adaptation. 2.6 Action Potentials. 2.7 Synapses and Synaptic Transmission. 2.8 Concluding Remarks. Chapter 3 General Features of Sensory Systems. 3.1 Classification of the Senses. 3.2 Modality. 3.3 Intensity. 3.4 Adaptation. 3.5 Receptive Fields. 3.6 Maps of Sensory Surfaces. 3.7 Hierarchical and Parallel Design. 3.8 Feature Extraction and Trigger Stimuli. 3.9 Concluding Remarks. Chapter 4 Classification and Phylogeny. 4.1 Systematics. 4.2 Classification into Six Kingdoms. 4.3 Unicellularity. 4.4 Multicellularity. 4.5 Protostomes and Deuterostomes. 4.6 Classification of the Metazoa. 4.7 Evolution of Nervous Systems. 4.8 Concluding Remarks. Chapter 5 Genes, Genomics and Neurosensory Systems. 5.1 Introduction. 5.2 Comparative Genomics. 5.3 Genomes and Neurosensory Systems. 5.4 Concluding Remarks. Part I: Self Assessment. Part I: Notes, References and Bibliography. PART II: MECHANOSENSITIVITY. Chapter 6 Mechanosensitivity of Cell Membranes. 6.1 Mechanosensitive Channels in E. coli. 6.2 Detection of Osmotic Swelling by Hypothalamic Cells in Mammals. 6.3 Concluding Remarks. Chapter 7 Kinaesthesia. 7.1 Kinaesthetic Mechanisms in Arthropods. 7.1.1 Stretch Receptors in Crustacean Muscle. 7.2 Kinaesthetic Mechanisms in Mammals. 7.3 Concluding Remarks. Chapter 8 Touch. 8.1 Mechanoreception in Caenorhabditis Elegans. 8.2 Spiders. 8.3 Insects. 8.4 Tactile Receptors in Mammalian Skin. 8.5 Cerebral Analysis of Touch. 8.6 Plasticity of the Somaesthetic Cortex. 8.7 Concluding Remarks. Chapter 9 Equilibrium and Hearing: The Uses of Hair Cells. 9.1 Anatomy and Physiology of Hair Cells. 9.2 Lateral Line Canals. 9.3 Evolution of the Vertebrate Ear. 9.4 Concluding Remarks. Box 9.1 Biophysics of Outer Hair Cells. Box 9.2 Genetics and Deafness. Chapter 10 Cerebral Analysis. 10.1 The Mammalian Vestibular Pathway and Reflexes. 10.2 The Mammalian Auditory Pathway. 10.3 The Avian Auditory Pathway and the Mapping of Auditory Space by the Barn Owl. 10.4 The Mammalian Auditory Cortex. 10.5 The Bat Auditory System and Echolocation. 10.6 The Human Auditory Cortex and Language. 10.7 Lateralization and the Neuroanatomy of Language. 10.8 Language and the FOXP2 Gene. 10.9 Callosectomy and After. 10.10 Concluding Remarks. Box 10.1 Broca and Wernicke. Part II: Self Assessment. Part II: Notes, References and Bibliography. PART III: CHEMOSENSITIVITY. Chapter 11 Chemosensitivity in Prokaryocytes. 11.1 Chemosentivity in E. coli. 11.2 Concluding Remarks. Chapter 12 Mammalian Chemo- Enteroreceptors. 12.1 Location of Mammalian Chemoreceptors for PaO<sub>2</sub> and PaCO<sub>2</sub>. 12.2 Structure. 12.3 Physiology. 12.4 Biochemistry. 12.5 Concluding Remarks. Chapter 13 Gustation. 13.1 Gustation in Insects. 13.2 Gustation inMammals. 13.3 Concluding Remarks. Chapter 14 Olfaction. 14.1 Insect Olfactory Systems. 14.2 Mammalian Olfactory Systems. 14.3 The Vertebrate Vomeronasal Organ (VNO) and Pheromones. 14.4 Concluding Remarks. Part III: Self Assessment. Part III: Notes, References and Bibliography. PART IV: PHOTOSENSITIVITY. Box I4.1 Bacteriorhodopsin. Chapter 15 Invertebrate Vision. 15.1 Designs of Invertebrate Eyes. 15.2 Examples of Invertebrate Eyes. 15.3 Concluding Remarks. Box 15.1 The Evolution of Opsins. Box 15.2 Early Genetics of Eyes. Chapter 16 The Human Eye. 16.1 Anatomy. 16.2 Embryology. 16.3 Detailed Anatomy and Physiology. 16.4 Movements of the Eyeball. 16.5 Concluding Remarks. Box 16.1 Genetics of Cataract. Chapter 17 The Retina. 17.1 Retinal Pigment Epithelium (RPE). 17.2 Retina. 17.3 Concluding Remarks. Chapter 18 Visual Pathways and Cortices. 18.1 Visual Pathways into the Brain. 18.2 Primary Visual Cortex. 18.3 Extrastriate Cortices. 18.4 Face Recognition. 18.5 Prosopagnosia. 18.6 Concluding Remarks. Box 18.1 The Reality of Cortical Columns. Box 18.2 Blindsight. Chapter 19 Other Vertebrate Visual Systems. 19.1 Visual Pigments. 19.2 Photoreceptors. 19.3 Tapeta. 19.4 Retinae. 19.5 Dioptric Apparatus. 19.6 Median Eyes. 19.7 Visual Pathways. 19.8 Visual Centres in the Brain. 19.9 Concluding Remarks. Part IV: Self Assessment. Part IV: Notes, References and Bibliography. PART V: OTHER SENSES. Chapter 20 Thermosensitivity. 20.1 Molecular Biology. 20.2 Poikilotherms. 20.3 Homeotherms. 20.4 Concluding Remarks. Chapter 21 Minority Senses. 21.1 Infrared Radiation. 21.2 Polarized Light. 21.3 Electric Fields. 21.4 Magnetic Fields. 21.5 Concluding Remarks. Chapter 22 Pain. 22.1 The Biological Significance of Pain. 22.2 Neurophysiology of Pain. 22.3 Neuropharmacology of Pain Pathways. 22.4 Referred Pain. 22.5 Gate Theory. 22.6 Concluding Remarks. Part V: Self Assessment. Part V: Notes, References and Bibliography. PART VI: CODA. Chapter 23 Summing Up. 23.1 Molecular Themes. 23.2 Cellular Themes. 23.3 Sense Organs. 23.4 Central Analysers. 23.5 Homeostasis. 23.6 Different Sensory Worlds. 23.7 From Abiotic to Biotic: Communication. 23.8 From Biotic to Social Communication: Mirror Neurons. 23.9 Concluding Remarks. Chapter 24 Philosophical Postscript. 24.1 Descartes. 24.2 Qualia. 24.3 Tabula Rasa? 24.4 Epigenetic Epistemology. 24.5 Evolutionary Epistemology. 24.6 Beyond Descartes. 24.7 Concluding Remarks. Part VI: Self Assessment. Part VI: Notes, References and Bibliography. Appendix: Some Techniques. Acronyms and Abbreviations. Glossary. Index.


Biology of Sensory Systems , Second Edition

Biology of Sensory Systems has thus been completely revised and takes a molecular, evolutionary and comparative approach, providing an overview of sensory systems in vertebrates, invertebrates and prokaryotes, with a strong focus on human senses.

Written by a renowned author with extensive teaching experience, the book covers, in six parts, the general features of sensory systems, the mechanosenses, the chemosenses, the senses which detect electromagnetic radiation, other sensory systems including pain, thermosensitivity and some of the minority senses and, finally, provides an outline and discussion of philosophical implications.

  • Greater emphasis on molecular biology and intracellular mechanisms
  • New chapter on genomics and sensory systems
  • Sections on TRP channels, synaptic transmission, evolution of nervous systems, arachnid mechanosensitive sensilla and photoreceptors, electroreception in the Monotremata, language and the FOXP2 gene, mirror neurons and the molecular biology of pain

Updated passages on human olfaction and gustation.

Over four hundred illustrations, boxes containing supplementary material and self-assessment questions and a full bibliography at the end of each part make Biology of Sensory Systems essential reading for undergraduate students of biology, zoology, animal physiology, neuroscience, anatomy and physiological psychology. The book is also suitable for postgraduate students in more specialised courses such as vision sciences, optometry, neurophysiology, neuropathology, developmental biology.

Praise from the reviews of the first edition:

"An excellent advanced undergraduate/postgraduate textbook." ASLIB BOOK GUIDE

"The emphasis on comparative biology and evolution is one of the distinguishing features of this self-contained book. . this is an informative and thought-provoking text. " TIMES HIGHER EDUCATIONAL SUPPLEMENT


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Sensory Systems

VIII TYPES OF SENSORY SYSTEMS

A U-Shape Curve of the System Reactivity

As for any neuronal network, overall activity in the sensory systems depends on the sensory input according to sigmoidal function ( Fig. 10.17 ) while reactions of the system to the same stimulus obey to the inverted U-law (see Introduction). In short, the law declares that the system reacts poorly to the stimulus when the overall activity of the system is too low (threshold effect) or too high (ceiling effect).

Figure 10.17 . Two types of sensory systems. The sensory system output activity in relation to the input represents a sigmoidal function. If the sensory input is increased in intensity the output of the system is also increased but in different way depending on the initial state of the system. The system in which activity increases slower (faster) than the input is called the reducing (augmenting) system.

B Augmenting and Reducing Sensory Systems

Suppose that for a given system we study how the system reacts to stimuli with increasing intensities ( Fig. 10.17 ). As one judge from Fig. 10.17 the response of the system will increase. However, the way of this increasing will depend on the state of the system. If the system is characterized by the initial low input and low overall activity (the point at the bottom of the curve), relative changes in the response will be higher than relative changes in the sensory input. These systems can be labeled as augmenting sensory systems (see insertion of Fig. 10.17 ). If the system has much higher overall activity, then relative changes in response will be lower than the relative increase in stimulus intensity. These systems can be labeled as reducing sensory systems.

In animal research the serotoninergic neurons of the brain stem were found to innervate the auditory cortex. The serotoninergic innervation in its turn leads to a strong dependence of overall activity of the auditory cortex with the level of serotonin. Auditory N1/P2 component serve as a good indicator of functioning of the auditory system. So, if the level of serotonin and correspondingly the input activity is increased the loudness dependence of auditory evoked potential reduces (i.e., the system shifts from the left point on the curve of Fig. 10.17 to the right point). In the studies of Gallinat et al. (2000) this property of the auditory system and N1/P2 component was used as a predictor of the acute response to serotonin reuptake inhibitors in depression.

C Auditory P2 in Augmenters and Reducers

One practical way of assessing the type of the sensory system is presenting to a subject auditory stimuli with increasingly changing intensities. ERP over the primary cortical areas are recorded, the P200 amplitude is measured and is plotted against stimulus intensity. As predicted from Fig. 10.17 , on the basis of the slope of the P200 amplitude, normal subjects can be classified into two groups: augmenters and reducers.


Transduction and Perception

Transduction is the process that converts a sensory signal to an electrical signal to be processed in a specialized area in the brain.

Learning Objectives

Explain how stimuli are converted to signals that are carried to the central nervous system

Key Takeaways

Key Points

  • Sensory signals are converted to electrical signals via depolarization of sensory neuron membranes upon stimulus of the receptor, which causes opening of gated ion channels that cause the membrane potential to reach its threshold.
  • The receptor potentials are classified as graded potentials the magnitude of these potentials is dependent on the strength of the stimulus.
  • The sensory system shows receptor specificity although stimuli can be combined in processing regions of the brain, a specific receptor will only be activated by its specific stimulus.
  • The brain contains specific processing regions (such as the somatosensory, visual, and auditory regions) that are dedicated to processing the information which has previously passed through the thalamus, the ‘clearinghouse and relay station’ for both sensory and motor signals.
  • The four major components of encoding and transmitting sensory information include: the type of stimulus, the stimulus location within the receptive field, the duration, and the intensity of the stimulus.

Key Terms

  • membrane potential: the difference in electrical potential across the enclosing membrane of a cell
  • action potential: a short term change in the electrical potential that travels along a cell
  • transduction: the translation of a sensory signal in the sensory system to an electrical signal in the nervous system

Transduction

The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor. The change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? As an example, a type of receptor called a mechanoreceptor possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. In the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.

Mechanoreceptor activation: (a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the plasma membrane. A mechanosensitive channel is connected to the plasma membrane and the cytoskeleton by hair-like tethers. When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit the cell. (b) Stereocilia in the human ear are connected to mechanosensitive ion channels. When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve.

Sensory receptors for the various senses work differently from each other. They are specialized according to the type of stimulus they sense thus, they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.

Encoding and Transmission of Sensory Information

Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus. This segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system. The electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus: a sound.

The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials. Reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the central nervous system.

Perception

Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens, not at the level of the sensory receptor, but at the brain level. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus.

All sensory signals, except those from the olfactory system, are transmitted though the central nervous system: they are routed to the thalamus and to the appropriate region of the cortex. The thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex dedicated to processing that particular sense.

Sensation processing: The brain has dedicated areas to the processing of stimuli, including: (a) thalamus and (b) the auditory, visual and somatosensory processing regions.


Biology of Sensory Systems, 2nd Edition

Biology of Sensory Systems has thus been completely revised and takes a molecular, evolutionary and comparative approach, providing an overview of sensory systems in vertebrates, invertebrates and prokaryotes, with a strong focus on human senses.

Written by a renowned author with extensive teaching experience, the book covers, in six parts, the general features of sensory systems, the mechanosenses, the chemosenses, the senses which detect electromagnetic radiation, other sensory systems including pain, thermosensitivity and some of the minority senses and, finally, provides an outline and discussion of philosophical implications.

  • Greater emphasis on molecular biology and intracellular mechanisms
  • New chapter on genomics and sensory systems
  • Sections on TRP channels, synaptic transmission, evolution of nervous systems, arachnid mechanosensitive sensilla and photoreceptors, electroreception in the Monotremata, language and the FOXP2 gene, mirror neurons and the molecular biology of pain

Updated passages on human olfaction and gustation. 

Over four hundred illustrations, boxes containing supplementary material and self-assessment questions and a full bibliography at the end of each part make Biology of Sensory Systems essential reading for undergraduate students of biology, zoology, animal physiology, neuroscience, anatomy and physiological psychology. The book is also suitable for postgraduate students in more specialised courses such as vision sciences, optometry, neurophysiology, neuropathology, developmental biology.

Praise from the reviews of the first edition:

"An excellent advanced undergraduate/postgraduate textbook." ASLIB BOOK GUIDE

"The emphasis on comparative biology and evolution is one of the distinguishing features of this self-contained book. . this is an informative and thought-provoking text. " TIMES HIGHER EDUCATIONAL SUPPLEMENT


Watch the video: Chapter 50 Part 1: Sensory and Motor Mechanisms (January 2022).