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15.4A: Functions of the Nervous System - Biology


The primary function of the nervous system is to coordinate and control the various body functions.

Learning Objectives

  • Describe the functions of the nervous system

Key Points

  • The nervous system is a highly integrated system. The nervous system has three overlapping functions based on sensory input, integration, and motor output.
  • At a more integrative level, the primary function of the nervous system is to control and communicate information throughout the body.

Key Terms

  • hormone: A molecule released by a cell or a gland in one part of the body that sends out messages affecting cells in other parts of the organism.
  • nervous system: The organ system that coordinates the activities of muscles, monitors organs, constructs and processes data received from the senses, and initiates actions.

The nervous system has three overlapping functions based on the sensory input, integration, and motor output. The nervous system is a highly integrated system.

Sensory Input

Sensory input comes from the many sensory receptors that monitor changes occurring both inside and outside the body. The total sum of the information gathered by these receptors is called sensory input. The nervous system processes and interprets sensory input and decides what actions should be taken. The nervous system activates effector organs such as muscles and glands to cause a response called motor output.

Integration

At a more integrative level, the primary function of the nervous system is to control and communicate information throughout the body. It does this by extracting information from the environment using sensory receptors. This sensory input is sent to the central nervous system, which determines an appropriate response.

Motor Response

Once the response is activated, the nervous system sends signals via motor output to muscles or glands to initiate the response.

In humans, the sophistication of the nervous system allows for language, abstract representation of concepts, transmission of culture, and many other features of society that would not otherwise exist.


Nervous System Function

The basic nervous system function includes—receiving sensory information (input), processing of the input and production of motor responses. The nervous system is formed of two parts that are integrally linked with each other. The brain and the nervous system have multiple functions that are extremely important for normal functioning of the body. The sensory information originating from stimulation of receptors by changes in the external and internal environment are carried to CNS through afferent nerves fibers. In the CNS, this information is processed through complex neuronal (synaptic) pathways.


As a result of above, commands are discharged through efferent nerve fibers to effectors organs (that is, muscles and glands) for production of motor responses or change in activity of the effectors organs. The central processing also causes stimulation of feelings (sensations) and storage of information as memory. The stored memory as the nervous system function in turn helps in dispensation of future input for other mental functions like emotion, power of thinking and judgment, intelligence, personality etc.
From our above discussion, it is apparent that the following nervous system functions are performing:—

Conscious sensation as a nervous system function

Nervous system is responsible for stimulation of conscious sensations by which we can feel the changes occurring within the body or around us. A conscious sensation such as, sight, hearing, smell, taste and feeling of touch, temperature, pressure, pain etc. are aroused when the afferent nerve impulses reach the highest centre of central nervous system or cerebral cortex of brain.

Mental functions as the nervous system function

Nervous system is responsible for higher mental functions like memory, intelligence, power of thinking, judgment, emotion, personality etc. These functions are specially developed in higher animals as well as humans having a highly developed brain.

Control of voluntary movements is the nervous system function

When a skeletal muscle is moved as contraction or relaxation according to our desire or will power, it is called voluntary movement. Such movements are governed by conscious sensation and initiated by cerebral cortex, and they help in locomotion, speech etc. is another nervous system function.

Control of reflex actions is a nervous system function

When a sensory input elicits a motor response without or prior to stimulation of conscious sensation, it is called reflex action. Nervous system regulates the activity of all the three types of muscles, those are skeletal, smooth and cardiac muscles and glands through various reflexes. Reflex control of skeletal muscles helps to uphold body posture, breathing etc., whereas reflex responses of other muscles and glands maintain co-ordination between the activities of different systems so that the whole body may function as an incorporated unit.


It is often a challenge to add a human anatomy and physiology activity to the classroom that is easy to implement and is also inexpensive. I have used this experiment with students to test the sensitivity of nerves located in different parts of the body.

Time required: As little as 20 minutes if the concept has already been introduced and the data table is supplied.

Classroom set up: Supply one paper clip per student and at least one ruler per table. (Students can share rulers.) Students should record their own data.

  • This is an easy experiment for students to perform individually, but most students prefer help from a partner. If done in partners, warn students to poke gently. The first time I did this experiment, one of my students ended up with a bloody nose from an ill placed paperclip poke (and I teach high school students). Ever since then, I have warned students. They scoff and roll their eyes, but there haven’t been any more injuries.
  • Certain students may insist they feel two points even when only one point is used. For these students, working with a partner may work best so they can concentrate on what they feel rather than what they are doing. Sharing data as a class will also help these students relate what they feel to what everyone else experienced.
  • More advanced students should be able to produce the data table on their own without the pdf printout or instructions.
  • Have students write a formal hypothesis explaining their reasoning and conclusion explaining their data and why it either supported or did not support their initial hypothesis.
  • This experiment could lead to follow up questions or experiments giving the students the opportunity to design and implement their own experiment. Examples include testing which finger is most sensitive, comparisons to other parts of the body including feet or toes, or reactions to hot and cold.

Contactins

I Introduction

The nervous system functions by virtue of neural networks that interconnect large varieties of nerve cells in a highly organized and controlled manner. These networks are assembled during development and are under constant adaptation during the whole lifespan. Cell adhesion molecules allow nerve cells, neurons, as well as glial cells to interact. Their key role is evident in neurodevelopmental processes such as migration, axon guidance, axon fasciculation, and synaptogenesis and in plastic processes of the mature brain such as synaptic rearrangements, dendritic dynamics, and regeneration. The repertoire of neural cell adhesion molecules is dominated by several large protein families, one of which is the immunoglobulin (Ig) superfamily of cell adhesion molecules, IgCAMs. These proteins are type I transmembrane proteins, share an architecture built on Ig domains, and are subdivided by the presence of additional conserved protein domains. The best known members of neural IgCAMs are the NCAM and L1-CAM families.

A peculiar family of neural IgCAMs is constituted by a six-member group of IgCAMs that are linked to the cell surface by a glycophosphatidylinositol (GPI)-anchor, the contactins ( Shimoda and Watanabe, 2009 Fig. 1 ). Prototypic for the contactins are contactin-1 (Cntn1, aka F3/contactin) and contactin-2 (Cntn2, aka TAG-1). These two proteins as well as their biological functions in neuron–glia interactions and formation of the nodes of Ranvier have been scrutinized in pivotal studies for over two decades ( Salzer et al., 2008 ). These studies revealed principles of structure and function that directed research into the other members of this family. However, contactin-3 (Cntn3 aka BIG-1), contactin-4 (Cntn4, aka BIG-2), contactin-5 (Cntn5, aka NB-2), and contactin-6 (Cntn6, aka NB-3) have remained underexposed despite multiple in-depth studies by the groups of Watanabe and Yoshihara. Recently, the genetics of neuropsychiatric neurodevelopmental disorders have encountered several of these members and raised the question how they participate in the pathogenesis of disorders such as autism. To understand their role in developmental disorders of the brain, it will be essential to determine the biological and molecular pathways in which these contactins participate. In this chapter, we provide an overview on biological and structural properties that are required to answer these questions.

Fig. 1 . Primary structure of contactin family members. (A) Phylogenetic analysis of human CNTN proteins. Amino acid sequences were aligned using CLUSTALW as implemented in MEGA5, and the tree was generated using MEGA5 ( Tamura et al., 2007 ). (B) Cartoon representing the domain architecture of CNTN family members along with amino acid identity between individual domains of human CNTN2, -3, -4, -5, and -6 with CNTN1. (C) Same as (B) but showing the amino acid identity of individual domains of CNTN3, -5, and -6 with CNTN4.


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Your Infringement Notice may be forwarded to the party that made the content available or to third parties such as ChillingEffects.org.

Please be advised that you will be liable for damages (including costs and attorneys’ fees) if you materially misrepresent that a product or activity is infringing your copyrights. Thus, if you are not sure content located on or linked-to by the Website infringes your copyright, you should consider first contacting an attorney.

Please follow these steps to file a notice:

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Nervous System

As you read this, your nervous system is performing several functions simultaneously. The visual system is processing what is seen on the page the motor system controls your eye movements and the turn of the pages (or click of the mouse) the prefrontal cortex maintains attention. Even fundamental functions, like breathing and regulation of body temperature, are controlled by the nervous system. The nervous system is one of two systems that exert control over all the organ systems of the body the other is the endocrine system. The nervous system’s control is much more specific and rapid than the hormonal system. It communicates signals through cells and the tiny gaps between them rather than through the circulatory system as in the endocrine system. It uses a combination of chemical and electrochemical signals, rather than purely chemical signals used by the endocrine system to cover long distances quickly. The nervous system acquires information from sensory organs, processes it and then may initiate a response either through motor function, leading to movement, or in a change in the organism’s physiological state.

Nervous systems throughout the animal kingdom vary in structure and complexity. Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Flatworms have both a central nervous system (CNS), made up of a ganglion (clusters of connected neurons) and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia. These ganglia can control movements and behaviors without input from the brain.

Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally (toward the stomach) whereas the vertebrate spinal cords are located dorsally (toward the back). There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.

The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons. There is great diversity in the types of neurons and glia that are present in different parts of the nervous system.

Neurons and Glial Cells

The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors—from basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors.

Most neurons share the same cellular components. But neurons are also highly specialized—different types of neurons have different sizes and shapes that relate to their functional roles.

Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures for receiving and sending the electrical signals that make communication between neurons possible ([link]). Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, most have one or many dendrites.

The bilayer lipid membrane that surrounds a neuron is impermeable to ions. To enter or exit the neuron, ions must pass through ion channels that span the membrane. Some ion channels need to be activated to open and allow ions to pass into or out of the cell. These ion channels are sensitive to the environment and can change their shape accordingly. Ion channels that change their structure in response to voltage changes are called voltage-gated ion channels. The difference in total charge between the inside and outside of the cell is called the membrane potential.

A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (–70 mV). This voltage is called the resting membrane potential it is caused by differences in the concentrations of ions inside and outside the cell and the selective permeability created by ion channels. Sodium-potassium pumps in the membrane produce the different ion concentrations inside and outside of the cell by bringing in two K + ions and removing three Na + ions. The actions of this pump are costly: one molecule of ATP is used up for each turn. Up to 50 percent of a neuron’s ATP is used in maintaining its membrane resting potential. Potassium ions (K + ), which are higher inside the cell, move fairly freely out of the neuron through potassium channels this loss of positive charge produces a net negative charge inside the cell. Sodium ions (Na + ), which are low inside, have a driving force to enter but move less freely. Their channels are voltage dependent and will open when a slight change in the membrane potential triggers them.

A neuron can receive input from other neurons and, if this input is strong enough, send the signal to downstream neurons. Transmission of a signal between neurons is generally carried by a chemical, called a neurotransmitter, which diffuses from the axon of one neuron to the dendrite of a second neuron. When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, the neurotransmitter opens ion channels in the dendrite’s plasma membrane. This opening allows sodium ions to enter the neuron and results in depolarization of the membrane—a decrease in the voltage across the neuron membrane. Once a signal is received by the dendrite, it then travels passively to the cell body. A large enough signal from neurotransmitters will reach the axon. If it is strong enough (that is, if the threshold of excitation, a depolarization to around –60mV is reached), then depolarization creates a positive feedback loop: as more Na + ions enter the cell, the axon becomes further depolarized, opening even more sodium channels at further distances from the cell body. This will cause voltage dependent Na + channels further down the axon to open and more positive ions to enter the cell. In the axon, this “signal” will become a self-propagating brief reversal of the resting membrane potential called an action potential.

An action potential is an all-or-nothing event it either happens or it does not. The threshold of excitation must be reached for the neuron to “fire” an action potential. As sodium ions rush into the cell, depolarization actually reverses the charge across the membrane form -70mv to +30mV. This change in the membrane potential causes voltage-gated K + channels to open, and K + begins to leave the cell, repolarizing it. At the same time, Na + channels inactivate so no more Na + enters the cell. K + ions continue to leave the cell and the membrane potential returns to the resting potential. At the resting potential, the K + channels close and Na + channels reset. The depolarization of the membrane proceeds in a wave down the length of the axon. It travels in only one direction because the sodium channels have been inactivated and unavailable until the membrane potential is near the resting potential again at this point they are reset to closed and can be opened again.

An axon is a tube-like structure that propagates the signal from the cell body to specialized endings called axon terminals. These terminals in turn then synapse with other neurons, muscle, or target organs. When the action potential reaches the axon terminal, this causes the release of neurotransmitter onto the dendrite of another neuron. Neurotransmitters released at axon terminals allow signals to be communicated to these other cells, and the process begins again. Neurons usually have one or two axons, but some neurons do not contain any axons.

Some axons are covered with a special structure called a myelin sheath, which acts as an insulator to keep the electrical signal from dissipating as it travels down the axon. This insulation is important, as the axon from a human motor neuron can be as long as a meter (3.2 ft)—from the base of the spine to the toes. The myelin sheath is produced by glial cells. Along the axon there are periodic gaps in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is “recharged” as it travels along the axon.

It is important to note that a single neuron does not act alone—neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.

Neurogenesis At one time, scientists believed that people were born with all the neurons they would ever have. Research performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues into adulthood. Neurogenesis was first discovered in songbirds that produce new neurons while learning songs. For mammals, new neurons also play an important role in learning: about 1,000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) each day. While most of the new neurons will die, researchers found that an increase in the number of surviving new neurons in the hippocampus correlated with how well rats learned a new task. Interestingly, both exercise and some antidepressant medications also promote neurogenesis in the hippocampus. Stress has the opposite effect. While neurogenesis is quite limited compared to regeneration in other tissues, research in this area may lead to new treatments for disorders such as Alzheimer’s, stroke, and epilepsy.

How do scientists identify new neurons? A researcher can inject a compound called bromodeoxyuridine (BrdU) into the brain of an animal. While all cells will be exposed to BrdU, BrdU will only be incorporated into the DNA of newly generated cells that are in S phase. A technique called immunohistochemistry can be used to attach a fluorescent label to the incorporated BrdU, and a researcher can use fluorescent microscopy to visualize the presence of BrdU, and thus new neurons, in brain tissue ([link]).

Visit this link interactive lab to see more information about neurogenesis, including an interactive laboratory simulation and a video that explains how BrdU labels new cells.

While glial cells are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of 10. Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. When glia do not function properly, the result can be disastrous—most brain tumors are caused by mutations in glia.

How Neurons Communicate

All functions performed by the nervous system—from a simple motor reflex to more advanced functions like making a memory or a decision—require neurons to communicate with one another. Neurons communicate between the axon of one neuron and the dendrites, and sometimes the cell body, of another neuron across the gap between them, known as the synaptic cleft. When an action potential reaches the end of an axon it stimulates the release of neurotransmitter molecules into the synaptic cleft between the synaptic knob of the axon and the post-synaptic membrane of the dendrite or soma of the next cell. The neurotransmitter is released through exocytosis of vesicles containing the neurotransmitter molecules. The neurotransmitter diffuses across the synaptic cleft and binds to receptors in the post-synaptic membrane. These receptor molecules are chemically regulated ion channels and will open, allowing sodium to enter the cell. If sufficient neurotransmitter has been released an action potential may be initiated in the next cell, but this is not guaranteed. If insufficient neurotransmitter is released the nerve signal will die at this point. There are a number of different neurotransmitters that are specific to neuron types that have specific functions.

The Central Nervous System

The central nervous system (CNS) is made up of the brain and spinal cord and is covered with three layers of protective coverings called meninges (“meninges” is derived from the Greek and means “membranes”) ([link]). The outermost layer is the dura mater, the middle layer is the web-like arachnoid mater, and the inner layer is the pia mater, which directly contacts and covers the brain and spinal cord. The space between the arachnoid and pia maters is filled with cerebrospinal fluid (CSF). The brain floats in CSF, which acts as a cushion and shock absorber.

The Brain

The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, cerebellum, brainstem, and retinas. The outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex. The cerebral cortex, limbic system, and basal ganglia make up the two cerebral hemispheres. A thick fiber bundle called the corpus callosum (corpus = “body” callosum = “tough”) connects the two hemispheres. Although there are some brain functions that are localized more to one hemisphere than the other, the functions of the two hemispheres are largely redundant. In fact, sometimes (very rarely) an entire hemisphere is removed to treat severe epilepsy. While patients do suffer some deficits following the surgery, they can have surprisingly few problems, especially when the surgery is performed on children who have very immature nervous systems.

In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere. This causes a condition called split-brain, which gives insights into unique functions of the two hemispheres. For example, when an object is presented to patients’ left visual field, they may be unable to verbally name the object (and may claim to not have seen an object at all). This is because the visual input from the left visual field crosses and enters the right hemisphere and cannot then signal to the speech center, which generally is found in the left side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group of objects with the left hand, the patient will be able to do so but will still be unable to verbally identify it.

Visit the following website to learn more about split-brain patients and to play a game where you can model split-brain experiments yourself.

Each hemisphere contains regions called lobes that are involved in different functions. Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital ([link]).

The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement. Areas within the motor cortex map to different muscle groups. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decision-making. Studies of humans who have damaged their frontal lobes show that parts of this area are involved in personality, socialization, and assessing risk. The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and also reading. Two of the parietal lobe’s main functions are processing somatosensation—touch sensations like pressure, pain, heat, cold—and processing proprioception—the sense of how parts of the body are oriented in space. The parietal lobe contains a somatosensory map of the body similar to the motor cortex. The occipital lobe is located at the back of the brain. It is primarily involved in vision—seeing, recognizing, and identifying the visual world. The temporal lobe is located at the base of the brain and is primarily involved in processing and interpreting sounds. It also contains the hippocampus (named from the Greek for “seahorse,” which it resembles in shape) a structure that processes memory formation. The role of the hippocampus in memory was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories (although he could remember some facts from before his surgery and could learn new motor tasks).

Interconnected brain areas called the basal ganglia play important roles in movement control and posture. The basal ganglia also regulate motivation.

The thalamus acts as a gateway to and from the cortex. It receives sensory and motor inputs from the body and also receives feedback from the cortex. This feedback mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states.

Below the thalamus is the hypothalamus. The hypothalamus controls the endocrine system by sending signals to the pituitary gland. Among other functions, the hypothalamus is the body’s thermostat—it makes sure the body temperature is kept at appropriate levels. Neurons within the hypothalamus also regulate circadian rhythms, sometimes called sleep cycles.

The limbic system is a connected set of structures that regulates emotion, as well as behaviors related to fear and motivation. It plays a role in memory formation and includes parts of the thalamus and hypothalamus as well as the hippocampus. One important structure within the limbic system is a temporal lobe structure called the amygdala. The two amygdala (one on each side) are important both for the sensation of fear and for recognizing fearful faces.

The cerebellum (cerebellum = “little brain”) sits at the base of the brain on top of the brainstem. The cerebellum controls balance and aids in coordinating movement and learning new motor tasks. The cerebellum of birds is large compared to other vertebrates because of the coordination required by flight.

The brainstem connects the rest of the brain with the spinal cord and regulates some of the most important and basic functions of the nervous system including breathing, swallowing, digestion, sleeping, walking, and sensory and motor information integration.

Spinal cord

Connecting to the brainstem and extending down the body through the spinal column is the spinal cord. The spinal cord is a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the body. The spinal cord is contained within the meninges and the bones of the vertebral column but is able to communicate signals to and from the body through its connections with spinal nerves (part of the peripheral nervous system). A cross-section of the spinal cord looks like a white oval containing a gray butterfly-shape ([link]). Axons make up the “white matter” and neuron and glia cell bodies (and interneurons) make up the “gray matter.” Axons and cell bodies in the dorsa spinal cord convey mostly sensory information from the body to the brain. Axons and cell bodies in the spinal cord primarily transmit signals controlling movement from the brain to the body.

The spinal cord also controls motor reflexes. These reflexes are quick, unconscious movements—like automatically removing a hand from a hot object. Reflexes are so fast because they involve local synaptic connections. For example, the knee reflex that a doctor tests during a routine physical is controlled by a single synapse between a sensory neuron and a motor neuron. While a reflex may only require the involvement of one or two synapses, synapses with interneurons in the spinal column transmit information to the brain to convey what happened (the knee jerked, or the hand was hot).

The Peripheral Nervous System

The peripheral nervous system (PNS) is the connection between the central nervous system and the rest of the body. The PNS can be broken down into the autonomic nervous system, which controls bodily functions without conscious control, and the sensory-somatic nervous system, which transmits sensory information from the skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles.

The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control it can continuously monitor the conditions of these different systems and implement changes as needed. Signaling to the target tissue usually involves two synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ ([link]). There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.

The sympathetic nervous system is responsible for the immediate responses an animal makes when it encounters a dangerous situation. One way to remember this is to think of the “fight-or-flight” response a person feels when encountering a snake (“snake” and “sympathetic” both begin with “s”). Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend off a predator.

While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system allows an animal to “rest and digest.” One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord ([link]). The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated including slowing of heart rate, lowered blood pressure, and stimulation of digestion.

The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (what it sees, feels, hears, and so on) and could not control motor movements. Unlike the autonomic nervous system, which usually has two synapses between the CNS and the target organ, sensory and motor neurons usually have only one synapse—one ending of the neuron is at the organ and the other directly contacts a CNS neuron.

Section Summary

The nervous system is made up of neurons and glia. Neurons are specialized cells that are capable of sending electrical as well as chemical signals. Most neurons contain dendrites, which receive these signals, and axons that send signals to other neurons or tissues. Glia are non-neuronal cells in the nervous system that support neuronal development and signaling. There are several types of glia that serve different functions.

Neurons have a resting potential across their membranes and when they are stimulated by a strong enough signal from another neuron an action potential may carry an electrochemical signal along the neuron to a synapse with another neuron. Neurotransmitters carry signals across synapses to initiate a response in another neuron.

The vertebrate central nervous system contains the brain and the spinal cord, which are covered and protected by three meninges. The brain contains structurally and functionally defined regions. In mammals, these include the cortex (which can be broken down into four primary functional lobes: frontal, temporal, occipital, and parietal), basal ganglia, thalamus, hypothalamus, limbic system, cerebellum, and brainstem—although structures in some of these designations overlap. While functions may be primarily localized to one structure in the brain, most complex functions, like language and sleep, involve neurons in multiple brain regions. The spinal cord is the information superhighway that connects the brain with the rest of the body through its connections with peripheral nerves. It transmits sensory and motor input and also controls motor reflexes.

The peripheral nervous system contains both the autonomic and sensory-somatic nervous systems. The autonomic nervous system provides unconscious control over visceral functions and has two divisions: the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is activated in stressful situations to prepare the animal for a “fight-or-flight” response. The parasympathetic nervous system is active during restful periods. The sensory-somatic nervous system is made of cranial and spinal nerves that transmit sensory information from skin and muscle to the CNS and motor commands from the CNS to the muscles.


Nervous System

In the human beings as well as other higher animals the nervous system and the endocrine system have evolved for regulating the function of different organs of the body and for maintaining synchronization between their activities. The nervous system receives information from different sensory organs which are called receptors and integrates them to determine the response to be made by day organs of the body. Regulation of the activities of different organs according to the need of the body is done by it. So, the nervous system keeps the body aware of the changes occurring in its internal or external environment and helps the body to react properly so that it can deal with different situation. The nervous system acts very rapidly but its actions are relatively short lusting, on the other hand the endocrine system acts slowly for a prolonged period.

Organisation of nervous system: –

By two types of cells the nervous system is made up, those are neurons and neuroglia. The highly specialised cells neurons carry out the functions of nervous system by transmitting information in the form of nerve impulses from one part of the body to another through the complex path involving synapses. There are more than 10,000 million of neurons in the nervous system of the human beings which are formed during the foetal life. New neurons are never formed during their lifespan as after birth the neurons do not divide. However the neurons increase in length and new synapses are formed with the growth and development of the body after birth. The neuroglia cells support and nourish the neurons. They continue to multiply after birth and increase in number.

Anatomically the nervous system is divided into two parts those are central nervous system (CNS) and peripheral nervous system (PNS). The CNS comprises of brain and spinal cord which lie in the axial part of the body. The part expended part of CNS is the brain lying within the cranial cavity. The weight of a adult human brain is about 1.36 kilogram in males and 1.25 kilograms in females. From above downward, the human brain is divisible into three primary regions those are (i) forebrain, (ii) midbrain, and (iii) hindbrain.

The largest part of human brain is forebrain and further divided into two parts namely the cerebrum and the diencephalon. The diencephalon comprises of thalamus and hypothalamus.
The midbrain remains undivided but the hindbrain is further divided into two parts: met-encephalon and myelencephalon. The metencephalon comprises of pons and cerebellum. The brainstem he’s formed by midbrain, pons and medulla together. The spinal cord is the long, tubular lower part of the CNS lying within the cavity of vertebral column.
To study about anatomy of brain click here ►► HUMAN BRAIN
The peripheral nervous system includes all the nerve pathways of the body outside the brain and spinal cord. Contact with the CNS is made by these pathways in various part of the body. By 43 pairs of nerves the PNS is comprises of. Among them, 12 pairs of peripheral nerves are connected with brain and are called cranial nerves. The remaining 31 pairs of peripheral nerves are connected to spinal cord, which are called spinal nerves.

The total central nervous system is histologically divisible into two distinct zones – grey matter and white matter. The grey matter is composed of cell bodies of neurons hence, synapses are formed in the region of the CNS. On the other hand the white matter consists of nerve fibres called Axons. Due to the presence of myelin sheath on the actions it appears as white. In cerebrum and cerebellum, the grey matter is present on the outer side which is also called cortex and the white matter on the inner side. The remaining parts of CNS are called nerve tracts whereas in the PNS, they are called peripheral nerves. There are two types of peripheral nerve fibres – sensory or afferent fibres and motor or efferenr fibres.

Sensory fibres carry the information received by the receptors, from different part of the body to the central nervous system to make CNS aware to the change happening within the body or in the external environment. Motor fibres carry the commands of the CNS to different organs for necessary modification in their activities. Peripheral nerves may be of three types depending on the types of nerve fibres, those are – sensory, motor and mixed.

The nervous system can also be divided functionally into two parts, such as, somatic nervous system and automatic nervous system. The somatic nervous system is concerned with conscious sensations arising from the sense organ, muscles, tendons and joints and movement control of skeletal muscles. On the other hand the automatic nervous system is concerned with the control of involuntary organs.


What Are the Four Functions of the Nervous System?

The nervous system is perhaps the most important part of the body. The roles it serves are crucial for perceiving and responding to the world around us. Our nervous systems allow our body to respond to stimuli and coordinate important bodily functions.

Parts of the Nervous System

The nervous system consists of the brain, organs used to provide sensory information, spinal cord and all the nerves that connect them. Each part of the nervous system is responsible for providing some level of control over the body. The nervous system is made of two groups: the central nervous system and peripheral nervous system

The central nervous system, also known as the CNS, includes both the spinal cord and the brain. They are the control center of the body. These are the parts of the body in which you make decisions and evaluate the world.

The peripheral nervous system, or PNS, consists of sensory nerves and organs in the body. This part of the nervous system monitors the world around you and then sends information to the brain.

Sensory Function

The sensory function of the nervous system is the part that gathers information about both the world around you and the inside of the body. The nervous system collects the data and then interprets it in a way that the brain can understand and respond. For example, the eyes are one of the most important sensory organs. The eyes take in light and convert it into electrical signals that travel to the brain and create an image. Other sensations the body perceives and interprets include taste, smell, touch, and hearing. The nervous system also senses the body’s internal environment, though you may not be consciously aware of it.

Communicative Function

Communication is another crucial feature of the nervous system. Without the communicative function of the nervous system, the brain and spinal cord would miss out on important information coming from the sensory organs. For example, the nerves in your hand may experience a reaction when you place your hand on a hot burner, but without communicating the pain to your brain, you might not know to pull your hand away and prevent further burning.

Integrative Function

Information is processed via the nervous system’s integrative functioning. Integration occurs when a stimulus is sent to the area in which the information is processed. Stimuli may be compared with other stimuli, perhaps those that occur at the same time or memories of those past. This means that an individual can respond to a stimulus based on experience.

Motor Function

The nervous system also serves a motor purpose that results after the nervous system responds to perceived stimuli. The motor function creates the response to the stimuli, often in the form of contracting muscles. Some forms of response are voluntary, and some are completely involuntary, like a reflex. For example, it's the motor function of the nervous system that causes you to pull your hand away from a hot stove or to jump out of the way of a moving vehicle.


Nervous System Physiology

Functions of the Nervous System

The nervous system has 3 main functions: sensory, integration, and motor.

  1. Sensory. The sensory function of the nervous system involves collecting information from sensory receptors that monitor the body’s internal and external conditions. These signals are then passed on to the central nervous system (CNS) for further processing by afferent neurons (and nerves).
  2. Integration. The process of integration is the processing of the many sensory signals that are passed into the CNS at any given time. These signals are evaluated, compared, used for decision making, discarded or committed to memory as deemed appropriate. Integration takes place in the gray matter of the brain and spinal cord and is performed by interneurons. Many interneurons work together to form complex networks that provide this processing power.
  3. Motor. Once the networks of interneurons in the CNS evaluate sensory information and decide on an action, they stimulate efferent neurons. Efferent neurons (also called motor neurons) carry signals from the gray matter of the CNS through the nerves of the peripheral nervous system to effector cells. The effector may be smooth, cardiac, or skeletal muscle tissue or glandular tissue. The effector then releases a hormone or moves a part of the body to respond to the stimulus.

Unfortunately of course, our nervous system doesn’t always function as it should. Sometimes this is the result of diseases like Alzheimer’s and Parkinson’s disease. Did you know that DNA testing can help you discover your genetic risk of acquiring certain health conditions that affect the organs of our nervous system? Late-onset Alzheimer’s, Parkinson’s disease, macular degeneration - visit our guide to DNA health testing to find out more.

Divisions of the Nervous System

Central Nervous System

The brain and spinal cord together form the central nervous system, or CNS. The CNS acts as the control center of the body by providing its processing, memory, and regulation systems. The CNS takes in all of the conscious and subconscious sensory information from the body’s sensory receptors to stay aware of the body’s internal and external conditions. Using this sensory information, it makes decisions about both conscious and subconscious actions to take to maintain the body’s homeostasis and ensure its survival. The CNS is also responsible for the higher functions of the nervous system such as language, creativity, expression, emotions, and personality. The brain is the seat of consciousness and determines who we are as individuals.

Peripheral Nervous System

The peripheral nervous system (PNS) includes all of the parts of the nervous system outside of the brain and spinal cord. These parts include all of the cranial and spinal nerves, ganglia, and sensory receptors.

Somatic Nervous System

The somatic nervous system (SNS) is a division of the PNS that includes all of the voluntary efferent neurons. The SNS is the only consciously controlled part of the PNS and is responsible for stimulating skeletal muscles in the body.

Autonomic Nervous System

The autonomic nervous system (ANS) is a division of the PNS that includes all of the involuntary efferent neurons. The ANS controls subconscious effectors such as visceral muscle tissue, cardiac muscle tissue, and glandular tissue.

There are 2 divisions of the autonomic nervous system in the body: the sympathetic and parasympathetic divisions.

  • Sympathetic. The sympathetic division forms the body’s “fight or flight” response to stress, danger, excitement, exercise, emotions, and embarrassment. The sympathetic division increases respiration and heart rate, releases adrenaline and other stress hormones, and decreases digestion to cope with these situations.
  • Parasympathetic. The parasympathetic division forms the body’s “rest and digest” response when the body is relaxed, resting, or feeding. The parasympathetic works to undo the work of the sympathetic division after a stressful situation. Among other functions, the parasympathetic division works to decrease respiration and heart rate, increase digestion, and permit the elimination of wastes.

Enteric Nervous System

The enteric nervous system (ENS) is the division of the ANS that is responsible for regulating digestion and the function of the digestive organs. The ENS receives signals from the central nervous system through both the sympathetic and parasympathetic divisions of the autonomic nervous system to help regulate its functions. However, the ENS mostly works independently of the CNS and continues to function without any outside input. For this reason, the ENS is often called the “brain of the gut” or the body’s “second brain.” The ENS is an immense system—almost as many neurons exist in the ENS as in the spinal cord.

Action Potentials

Neurons function through the generation and propagation of electrochemical signals known as action potentials (APs). An AP is created by the movement of sodium and potassium ions through the membrane of neurons. (See Water and Electrolytes.)

  • Resting Potential. At rest, neurons maintain a concentration of sodium ions outside of the cell and potassium ions inside of the cell. This concentration is maintained by the sodium-potassium pump of the cell membrane which pumps 3 sodium ions out of the cell for every 2 potassium ions that are pumped into the cell. The ion concentration results in a resting electrical potential of -70 millivolts (mV), which means that the inside of the cell has a negative charge compared to its surroundings.
  • Threshold Potential. If a stimulus permits enough positive ions to enter a region of the cell to cause it to reach -55 mV, that region of the cell will open its voltage-gated sodium channels and allow sodium ions to diffuse into the cell. -55 mV is the threshold potential for neurons as this is the “trigger” voltage that they must reach to cross the threshold into forming an action potential.
  • Depolarization. Sodium carries a positive charge that causes the cell to become depolarized (positively charged) compared to its normal negative charge. The voltage for depolarization of all neurons is +30 mV. The depolarization of the cell is the AP that is transmitted by the neuron as a nerve signal. The positive ions spread into neighboring regions of the cell, initiating a new AP in those regions as they reach -55 mV. The AP continues to spread down the cell membrane of the neuron until it reaches the end of an axon.
  • Repolarization. After the depolarization voltage of +30 mV is reached, voltage-gated potassium ion channels open, allowing positive potassium ions to diffuse out of the cell. The loss of potassium along with the pumping of sodium ions back out of the cell through the sodium-potassium pump restores the cell to the -55 mV resting potential. At this point the neuron is ready to start a new action potential.

Synapses

A synapse is the junction between a neuron and another cell. Synapses may form between 2 neurons or between a neuron and an effector cell. There are two types of synapses found in the body: chemical synapses and electrical synapses.

  • Chemical synapses. At the end of a neuron’s axon is an enlarged region of the axon known as the axon terminal. The axon terminal is separated from the next cell by a small gap known as the synaptic cleft. When an AP reaches the axon terminal, it opens voltage-gated calcium ion channels. Calcium ions cause vesicles containing chemicals known as neurotransmitters (NT) to release their contents by exocytosis into the synaptic cleft. The NT molecules cross the synaptic cleft and bind to receptor molecules on the cell, forming a synapse with the neuron. These receptor molecules open ion channels that may either stimulate the receptor cell to form a new action potential or may inhibit the cell from forming an action potential when stimulated by another neuron.
  • Electrical synapses. Electrical synapses are formed when 2 neurons are connected by small holes called gap junctions. The gap junctions allow electric current to pass from one neuron to the other, so that an AP in one cell is passed directly on to the other cell through the synapse.

Myelination

The axons of many neurons are covered by a coating of insulation known as myelin to increase the speed of nerve conduction throughout the body. Myelin is formed by 2 types of glial cells: Schwann cells in the PNS and oligodendrocytes in the CNS. In both cases, the glial cells wrap their plasma membrane around the axon many times to form a thick covering of lipids. The development of these myelin sheaths is known as myelination.

Myelination speeds up the movement of APs in the axon by reducing the number of APs that must form for a signal to reach the end of an axon. The myelination process begins speeding up nerve conduction in fetal development and continues into early adulthood. Myelinated axons appear white due to the presence of lipids and form the white matter of the inner brain and outer spinal cord. White matter is specialized for carrying information quickly through the brain and spinal cord. The gray matter of the brain and spinal cord are the unmyelinated integration centers where information is processed.

Reflexes

Reflexes are fast, involuntary responses to stimuli. The most well known reflex is the patellar reflex, which is checked when a physicians taps on a patient’s knee during a physical examination. Reflexes are integrated in the gray matter of the spinal cord or in the brain stem. Reflexes allow the body to respond to stimuli very quickly by sending responses to effectors before the nerve signals reach the conscious parts of the brain. This explains why people will often pull their hands away from a hot object before they realize they are in pain.

Functions of the Cranial Nerves

Each of the 12 cranial nerves has a specific function within the nervous system.

  • The olfactory nerve (I) carries scent information to the brain from the olfactory epithelium in the roof of the nasal cavity.
  • The optic nerve (II) carries visual information from the eyes to the brain.
  • Oculomotor, trochlear, and abducens nerves (III, IV, and VI) all work together to allow the brain to control the movement and focus of the eyes. The trigeminal nerve (V) carries sensations from the face and innervates the muscles of mastication.
  • The facial nerve (VII) innervates the muscles of the face to make facial expressions and carries taste information from the anterior 2/3 of the tongue.
  • The vestibulocochlear nerve (VIII) conducts auditory and balance information from the ears to the brain.
  • The glossopharyngeal nerve (IX) carries taste information from the posterior 1/3 of the tongue and assists in swallowing.
  • The vagus nerve (X), sometimes called the wandering nerve due to the fact that it innervates many different areas, “wanders” through the head, neck, and torso. It carries information about the condition of the vital organs to the brain, delivers motor signals to control speech and delivers parasympathetic signals to many organs.
  • The accessory nerve (XI) controls the movements of the shoulders and neck.
  • The hypoglossal nerve (XII) moves the tongue for speech and swallowing.

Sensory Physiology

All sensory receptors can be classified by their structure and by the type of stimulus that they detect. Structurally, there are 3 classes of sensory receptors: free nerve endings, encapsulated nerve endings, and specialized cells. Free nerve endings are simply free dendrites at the end of a neuron that extend into a tissue. Pain, heat, and cold are all sensed through free nerve endings. An encapsulated nerve ending is a free nerve ending wrapped in a round capsule of connective tissue. When the capsule is deformed by touch or pressure, the neuron is stimulated to send signals to the CNS. Specialized cells detect stimuli from the 5 special senses: vision, hearing, balance, smell, and taste. Each of the special senses has its own unique sensory cells—such as rods and cones in the retina to detect light for the sense of vision.

Functionally, there are 6 major classes of receptors: mechanoreceptors, nociceptors, photoreceptors, chemoreceptors, osmoreceptors, and thermoreceptors.