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22.1: The Nervous System

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Describe the structure of a neuron, and identify types of neurons.
  • Explain how nerve impulses are transmitted.
  • Identify parts of the central nervous system and their functions.
  • Outline the divisions and subdivisions of the peripheral nervous system.
  • Explain how sensory stimuli are perceived and interpreted.
  • State how drugs affect the nervous system.
  • Identify several nervous system disorders.


  • action potential
  • autonomic nervous system (ANS)
  • axon
  • brain
  • brain stem
  • cell body
  • central nervous system (CNS)
  • cerebellum
  • dendrite
  • drug abuse
  • drug addiction
  • interneuron
  • motor neuron
  • myelin sheath
  • nerve
  • nerve impulse
  • nervous system
  • neuron
  • neurotransmitter
  • peripheral nervous system (PNS)
  • psychoactive drug
  • resting potential
  • sensory neuron
  • sensory receptor
  • somatic nervous system (SNS)
  • spinal cord
  • synapse


A small child darts in front of your bike as you race down the street. You see the child and immediately react. You put on the brakes, steer away from the child, and yell out a warning—all in just a split second. How do you respond so quickly? Such rapid responses are controlled by your nervous system. The nervous system is a complex network of nervous tissue that carries electrical messages throughout the body (see Figure below). To understand how nervous messages can travel so quickly, you need to know more about nerve cells.

The human nervous system includes the brain and spinal cord (central nervous system) and nerves that run throughout the body (peripheral nervous system).

Nerve Cells

Although the nervous system is very complex, nervous tissue consists of just two basic types of nerve cells: neurons and glial cells. Neurons are the structural and functional units of the nervous system. They transmit electrical signals, called nerve impulses. Glial cells provide support for neurons. For example, they provide neurons with nutrients and other materials.

Neuron Structure

As shown in Figure below, a neuron consists of three basic parts: the cell body, dendrites, and axon. You can watch an animation of the parts of a neuron at this link: http://www.garyfisk.com/anim/neuronparts.swf.

  • The cell body contains the nucleus and other cell organelles.
  • Dendrites extend from the cell body and receive nerve impulses from other neurons.
  • The axon is a long extension of the cell body that transmits nerve impulses to other cells. The axon branches at the end, forming axon terminals. These are the points where the neuron communicates with other cells.

The structure of a neuron allows it to rapidly transmit nerve impulses to other cells.

The neuron is discussed at http://www.youtube.com/watch?v=ob5U8zPbAX4 (6:13).

Myelin Sheath

The axon of many neurons has an outer layer called a myelin sheath (see Figure above). Myelin is a lipid produced by a type of a glial cell known as a Schwann cell. The myelin sheath acts like a layer of insulation, similar to the plastic that encases an electrical cord. Regularly spaced nodes, or gaps, in the myelin sheath allow nerve impulses to skip along the axon very rapidly.

Types of Neurons

Neurons are classified based on the direction in which they carry nerve impulses.

  • Sensory neurons carry nerve impulses from tissues and organs to the spinal cord and brain.
  • Motor neurons carry nerve impulses from the brain and spinal cord to muscles and glands (see Figure below).
  • Interneurons carry nerve impulses back and forth between sensory and motor neurons.

This axon is part of a motor neuron. It transmits nerve impulses to a skeletal muscle, causing the muscle to contract.

Nerve Impulses

Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.

Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane (see Figure below). It uses energy in ATP to pump positive sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. As a result, the inside of the neuron is negatively charged, compared to the extracellular fluid surrounding the neuron. This is due to many more positivly charged ions outside the cell compared to inside the cell This difference in electrical charge is called the resting potential.

The sodium-potassium pump maintains the resting potential of a neuron.

Action Potential

A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential. It begins when the neuron receives a chemical signal from another cell. The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell. As a result, the inside of the cell becomes positively charged compared to the outside of the cell. This reversal of charge ripples down the axon very rapidly as an electric current (see Figure below). You can watch a detailed animation of an action potential at this link: http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf.

An action potential speeds along an axon in milliseconds.

In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.

The action potential is discussed at http://www.youtube.com/watch?v=gkQtRec2464 (18:53).

The Synapse

The place where an axon terminal meets another cell is called a synapse. The axon terminal and other cell are separated by a narrow space known as a synaptic cleft (see Figure below). When an action potential reaches the axon terminal, the axon terminal releases molecules of a chemical called a neurotransmitter. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron, this starts an action potential in the other cell. You can view animations of neurotransmission at a synapse at the following links:

At a synapse, neurotransmitters are released by the axon terminal. They bind with receptors on the other cell.

The synapse is further discussed at http://www.youtube.com/watch?v=Tbq-KZaXiL4.

Central Nervous System

The nervous system has two main divisions: the central nervous system and the peripheral nervous system (see Figure below). The central nervous system (CNS) includes the brain and spinal cord (see Figure below). You can see an overview of the central nervous system at this link: http://vimeo.com/2024719.

The two main divisions of the human nervous system are the central nervous system and the peripheral nervous system. The peripheral nervous system has additional divisions.

This diagram shows the components of the central nervous system.

The Brain

The brain is the most complex organ of the human body and the control center of the nervous system. It contains an astonishing 100 billion neurons! The brain controls such mental processes as reasoning, imagination, memory, and language. It also interprets information from the senses. In addition, it controls basic physical processes such as breathing and heartbeat.

The brain has three major parts: the cerebrum, cerebellum, and brain stem. These parts are shown in Figure below and described in this section. For a video of the parts of the brain and their functions, go to this link: http://www.teachers.tv/video/13838.

You can also take interactive animated tours of the brain at these links:

In this drawing, assume you are looking at the left side of the head. This is how the brain would appear if you could look underneath the skull.

  • The cerebrum is the largest part of the brain. It controls conscious functions such as reasoning, language, sight, touch, and hearing. It is divided into two hemispheres, or halves. The hemispheres are very similar but not identical to one another. They are connected by a thick bundle of axons deep within the brain. Each hemisphere is further divided into the four lobes shown in Figure below.
  • The cerebellum is just below the cerebrum. It coordinates body movements. Many nerve pathways link the cerebellum with motor neurons throughout the body.
  • The brain stem is the lowest part of the brain. It connects the rest of the brain with the spinal cord and passes nerve impulses between the brain and spinal cord. It also controls unconscious functions such as heart rate and breathing.

Each hemisphere of the cerebrum consists of four parts, called lobes. Each lobe is associated with particular brain functions. Just one function of each lobe is listed here.

Spinal Cord

The spinal cord is a thin, tubular bundle of nervous tissue that extends from the brainstem and continues down the center of the back to the pelvis. It is protected by the vertebrae, which encase it. The spinal cord serves as an information superhighway, passing messages from the body to the brain and from the brain to the body.

Peripheral Nervous System

The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside the central nervous system. It is shown in yellow in Figure below. It is connected to the central nervous system by nerves. A nerve is a cable-like bundle of axons. Some nerves are very long. The longest human nerve is the sciatic nerve. It runs from the spinal cord in the lower back down the left leg all the way to the toes of the left foot. Like the nervous system as a whole, the peripheral nervous system also has two divisions: the sensory division and the motor division.

  • The sensory division of the PNS carries sensory information from the body to the central nervous system. How sensations are detected and perceived is described in a later section of this lesson.
  • The motor division of the PNS carries nerve impulses from the central nervous system to muscles and glands throughout the body. The nerve impulses stimulate muscles to contract and glands to secrete hormones. The motor division of the peripheral nervous system is further divided into the somatic and autonomic nervous systems.

The Nervous System

The nerves of the peripheral nervous system are shown in blue in this image. Can you identify the sciatic nerve?

Somatic Nervous System

The somatic nervous system (SNS) controls mainly voluntary activities that are under conscious control. It is made up of nerves that are connected to skeletal muscles. Whenever you perform a conscious movement—from signing your name to riding your bike—your somatic nervous system is responsible.

The somatic nervous system also controls some unconscious movements called reflexes. A reflex is a very rapid motor response that is not directed by the brain. In a reflex, nerve impulses travel to and from the spinal cord in a reflex arc, like the one in Figure below. In this example, the person jerks his hand away from the flame without any conscious thought. It happens unconsciously because the nerve impulses bypass the brain.

A reflex arc like this one enables involuntary actions. How might reflex responses be beneficial to the organism?

Autonomic Nervous System

All other involuntary activities not under conscious control are the responsibility of the autonomic nervous system (ANS). Nerves of the ANS are connected to glands and internal organs. They control basic physical functions such as heart rate, breathing, digestion, and sweat production. The autonomic nervous system also has two subdivisions: the sympathetic division and the parasympathetic division. You can watch an animation comparing these two subdivisions at this link: http://www.garyfisk.com/anim/autonomicns.swf.

  • The sympathetic division deals with emergency situations. It prepares the body for “fight or flight.” Do you get clammy palms or a racing heart when you have to play a solo or give a speech? Nerves of the sympathetic division control these responses.
  • The parasympathetic division controls involuntary activities that are not emergencies. For example, it controls the organs of your digestive system so they can break down the food you eat.

The Senses

The sensory division of the PNS includes several sense organs—the eyes, ears, mouth, nose, and skin. Each sense organ has special cells, called sensory receptors, that respond to a particular type of stimulus. For example, the nose has sensory receptors that respond to chemicals, which we perceive as odors. Sensory receptors send nerve impulses to sensory nerves, which carry the nerve impulses to the central nervous system. The brain then interprets the nerve impulses to form a response.


Sight is the ability to sense light, and the eye is the organ that senses light. Light first passes through the cornea of the eye, which is a clear outer layer that protects the eye (see Figure below). Light enters the eye through an opening called the pupil. The light then passes through the lens, which focuses it on the retina at the back of the eye. The retina contains light receptor cells, like those in the photograph on the first page of this chapter. These cells send nerve impulses to the optic nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are seeing. To learn more about the eye and the sense of sight, you can go to the link below. Be sure to take the quick quiz at the end of the animation. http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP14304

The eye is the organ that senses light and allows us to see.


Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. Sound waves enter the auditory canal and travel to the eardrum (see Figure below). They strike the eardrum and make it vibrate. The vibrations then travel through several other structures inside the ear and reach the cochlea. The cochlea is a coiled tube filled with liquid. The liquid moves in response to the vibrations, causing tiny hair cells lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are hearing.

The ear is the organ that senses sound waves and allows us to hear. It also senses body position so we can keep our balance.


The ears are also responsible for the sense of balance. Balance is the ability to sense and maintain body position. The semicircular canals inside the ear (see Figure above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the peripheral nervous system. This system maintains the body’s balance by triggering contractions of skeletal muscles as needed.

Taste and Smell

Taste and smell are both abilities to sense chemicals. Like other sense receptors, both taste and odor receptors send nerve impulses to the brain, and the brain “tells” us what we are tasting or smelling.

Taste receptors are found in tiny bumps on the tongue called taste buds (see Figure below). There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty taste is called umami. You can learn more about taste receptors and the sense of taste by watching the animation at the following link: http://www.bbc.co.uk/science/humanbody/body/factfiles/taste/taste_ani_f5.swf.

Taste buds on the tongue contain taste receptor cells.

Odor receptors line the passages of the nose (see Figure below). They sense chemicals in the air. In fact, odor receptors can sense hundreds of different chemicals. Did you ever notice that food seems to have less taste when you have a stuffy nose? This occurs because the sense of smell contributes to the sense of taste, and a stuffy nose interferes with the ability to smell.

Odor receptors. Odor receptors and their associated nerves (in yellow) line the top of the nasal passages.


Touch is the ability to sense pressure. Pressure receptors are found mainly in the skin. They are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Some touch receptors sense differences in temperature or pain. How do pain receptors help maintain homeostasis? (Hint: What might happen if we couldn’t feel pain?) See http://www.youtube.com/watch?v=xRkPNwqm0mM (0.51) for a summary.

KQED: The Flavor of Food: Smell + Taste + Touch

Did you know that about 95 percent of what we think is taste is actually smell? Or that the way we perceive flavor comes from a complex relationship between our senses, emotions and memories? As scientists decode how our taste and olfactory receptors work, top chefs are taking that knowledge and creating science in the kitchen. See the Science of Taste at the video below for more information.

Drugs and the Nervous System

A drug is any chemical that affects the body’s structure or function. Many drugs, including both legal and illegal drugs, are psychoactive drugs. This means that they affect the central nervous system, generally by influencing the transmission of nerve impulses. For example, some psychoactive drugs mimic neurotransmitters.

Examples of Psychoactive Drugs

Caffeine is an example of a psychoactive drug. It is found in coffee and many other products (see Table below). Caffeine is a central nervous system stimulant. Like other stimulant drugs, it makes you feel more awake and alert. Other psychoactive drugs include alcohol, nicotine, and marijuana. Each has a different effect on the central nervous system. Alcohol, for example, is a depressant. It has the opposite effects of a stimulant like caffeine.

Product Caffeine Content (mg)
Coffee (8 oz) 130
Tea (8 oz) 55
Cola (8 oz) 25
Coffee ice cream (8 oz) 60
Hot cocoa (8 oz) 10
Dark chocolate candy (1.5 oz) 30

Drug Abuse and Addiction

Psychoactive drugs may bring about changes in mood that users find desirable, so the drugs may be abused. Drug abuse is use of a drug without the advice of a medical professional and for reasons not originally intended. Continued use of a psychoactive drug may lead to drug addiction, in which the drug user is unable to stop using the drug. Over time, a drug user may need more of the drug to get the desired effect. This can lead to drug overdose and death.

Disorders of the Nervous System

There are several different types of problems that can affect the nervous system.

  • Vascular disorders involve problems with blood flow. For example, a stroke occurs when a blood clot blocks blood flow to part of the brain. Brain cells die quickly if their oxygen supply is cut off. This may cause paralysis and loss of other normal functions, depending on the part of the brain that is damaged.
  • Nervous tissue may become infected by microorganisms. Meningitis, for example, is caused by a viral or bacterial infection of the tissues covering the brain. This may cause the brain to swell and lead to brain damage and death.
  • Brain or spinal cord injuries may cause paralysis and other disabilities. Injuries to peripheral nerves can cause localized pain or numbness.
  • Abnormal brain functions can occur for a variety of reasons. Examples include headaches, such as migraine headaches, and epilepsy, in which seizures occur.
  • Nervous tissue may degenerate, or break down. Alzheimer’s disease is an example of this type of disorder, as is Amyotrophic lateral sclerosis, or ALS. ALS is also known as Lou Gehrig's disease. It leads to a gradual loss of higher brain functions. For a detailed video discussion of Alzheimer's disease see the the visualMD.com at http://www.thevisualmd.com/health_centers/neurological_health/what_is_alzheimer_s/what_is_alzheimer_s_disease_video.

KQED: Autism: Searching for Causes

Autism is a developmental disorder that appears in the first three years of life, and affects the brain's normal development of social and communication skills. Autism is a broad term given to a spectrum of disorders known as the autism spectrum disorders (ASDs). There are three types of ASDs: autistic disorder (also called “classic” autism), Asperger syndrome, and pervasive developmental disorder – not otherwise specified (PDD-NOS; also called “atypical autism”).

Individuals with autistic disorder usually have significant language delays, social and communication challenges, and "unusual" behaviors and interests. Many people with autistic disorder also have intellectual disability. People with Asperger syndrome usually have some milder symptoms of autistic disorder. They might have social challenges and "unusual" behaviors and interests. However, they typically do not have problems with language or intellectual disability. People who meet some of the criteria for autistic disorder or Asperger syndrome, but not all, may be diagnosed with PDD-NOS. People with PDD-NOS usually have fewer and milder symptoms than those with autistic disorder. The symptoms might cause only social and communication challenges.

Today, many tens of thousands of children receive treatment for the most severe form of autism. This is a tremendous increase from 15 years ago, prompting officials to call the situation a public health crisis. Autism researchers are analyzing everything from saliva samples to carpet dust in hopes of cracking the autism mystery. See the video below for additional information.

KQED: Alzheimer's Disease: Is the Cure in the Genes?

By 2050, as the population ages, 15 million Americans will suffer from Alzheimer's disease-- triple today's number. But genetic studies may provide information leading to a cure. See the video below for more information.

In April 2011, an international analysis of genes of more than 50,000 people led to the discovery of five new genes that make Alzheimer's Disease more likely in the elderly and provide clues about what might start the Alzheimer's disease process and fuel its progress in a person’s brain. See http://www.nytimes.com/2011/04/04/health/04alzheimer.html for additional information.

Why I do Science

If you kids don't like broccoli, it may not be their fault, it may just be their genes talking. Dr. Danielle Reed is a geneticist working to understand the genetics of taste. Can all people detect the same tastes? No. Why not? It has to do with a person's genes. People may actually taste foods differently.

Lesson Summary

  • Neurons are the structural and functional units of the nervous system. They consist of a cell body, dendrites, and axon. Neurons transmit nerve impulses to other cells. Types of neurons include sensory neurons, motor neurons, and interneurons.
  • A nerve impulse begins when a neuron receives a chemical stimulus. The impulse travels down the axon membrane as an electrical action potential to the axon terminal. The axon terminal releases neurotransmitters that carry the nerve impulse to the next cell.
  • The central nervous includes the brain and spinal cord. The brain is the control center of the nervous system. It controls virtually all mental and physical processes. The spinal cord is a long, thin bundle of nervous tissue that passes messages from the body to the brain and from the brain to the body.
  • The peripheral nervous system consists of all the nervous tissue that lies outside the central nervous system. It is connected to the central nervous system by nerves. It has several divisions and subdivisions that transmit nerve impulses between the central nervous system and the rest of the body.
  • Human senses include sight, hearing, balance, taste, smell, and touch. Sensory organs such as the eyes contain cells called sensory receptors that respond to particular sensory stimuli. Sensory nerves carry nerve impulses from sensory receptors to the central nervous system. The brain interprets the nerve impulses to form a response.
  • Drugs are chemicals that affect the body’s structure or function. Psychoactive drugs, such as caffeine and alcohol, affect the central nervous system by influencing the transmission of nerve impulses in the brain. Psychoactive drugs may be abused and lead to drug addiction.
  • Disorders of the nervous system include blood flow problems such as stroke, infections such as meningitis, brain injuries, and degeneration of nervous tissue, as in Alzheimer’s disease.

Lesson Review Questions


1. List and describe the parts of a neuron.

2. What do motor neurons do?

3. Define resting potential.

4. Name the organs of the central nervous system.

5. Which part of the brain controls conscious functions such as reasoning?

6. What are the roles of the brain stem?

7. Identify the two major divisions of the peripheral nervous system.

8. List five human senses.

9. What is a psychoactive drug? Give two examples.

10. Define drug abuse. When does drug addiction occur?

11. Identify three nervous system disorders.

Apply Concepts

12. Tony’s dad was in a car accident in which his neck was broken. He survived the injury but is now paralyzed from the neck down. Explain why.

13. Multiple sclerosis is a disease in which the myelin sheaths of neurons in the central nervous system break down. What symptoms might this cause? Why?

Think Critically

14. Explain how resting potential is maintained and how an action potential occurs.

15. Compare and contrast the somatic and autonomic nervous systems.

Points to Consider

In this lesson, you learned that the nervous system enables electrical messages to be sent through the body very rapidly.

  • Often, it’s not necessary for the body to respond so rapidly. Can you think of another way the body could send messages that would travel more slowly? What about a way that makes use of the network of blood vessels throughout the body?
  • Instead of electrical nerve impulses, what other way might messages be transmitted in the body? Do you think chemical molecules could be used to carry messages? How might this work?

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