CHAPTER 28 The Nervous System Summary of Key Concepts Concept 28.1 The nervous system links sensation to response. (pp. 610 611) The nervous system has two subsystems. The central nervous system (CNS), which includes the brain and spinal cord, processes information. The peripheral nervous system (PNS) includes all the rest of nervous system tissue. The PNS carries information to and from the CNS through nerves. A nerve is a bundle of neurons surrounded by connective tissue. The nervous system has three main functions: sensory input, integration, and motor output. Sensory input occurs when the PNS receives a stimulus (plural, stimuli). Most stimuli are detected by specialized cells called sensory receptors. Some stimuli, such as pain, are detected directly by sensory neurons. Sensory neurons also carry information about the stimulus to the CNS. Integration occurs when neurons in the CNS, called interneurons, interpret the sensory information. Motor output occurs when the CNS orders a response and the response is carried away from the CNS by motor neurons.a response that is rapid and automatic is known as a reflex. A well-known reflex is jerking the lower leg when the knee is tapped. 1. Contrast the CNS and PNS. 2. State the three main functions of the nervous system. Concept 28.2 Neurons conduct nerve impulses. (pp. 612 617) A typical neuron has a cell body that contains the nucleus. Two types of fibers stick out from the cell body. Shorter fibers, called dendrites, receive signals and carry them toward the cell body. A longer fiber, called the axon, carries the signals away from the cell body and toward other cells. Many axons are covered by a thick coat of material called a myelin sheath. Uncovered spaces, called nodes, lie between sections of myelin sheath. Signals cannot travel along the axon where it is covered by the myelin sheath, so they jump from node to node. This jumping allows signals to travel very quickly along the neuron. A neuron that is not transmitting a signal is called a resting neuron. A resting neuron stores electrical energy. The inside of a nerve cell is negative in electrical charge compared to the outside. This difference in charge is called the resting potential. It is caused by a greater concentration of sodium ions (Na + ) outside the cell than inside the cell. When the neuron is stimulated, Na + flows into the cell. The inside of the cell becomes positive compared to the outside. This change in electrical charge is called depolarization. If the neuron is stimulated to a certain level, called the threshold, additional Na + rushes into the cell, causing greater depolarization. This stronger depolarization, called the action potential, is the start of a nerve signal. The action potential travels along the rest of the neuron. Guided Reading and Study Workbook/Chapter 28 115
When the action potential reaches the end of an axon, it may move on to another cell. The place where one neuron meets another is called a synapse. In an electrical synapse, the action potential at the end of an axon directly causes an electrical change in the next neuron. In a chemical synapse, the action potential is changed to a chemical signal that travels across a tiny space between the neurons, called the synaptic cleft. The chemical signal consists of molecules called neurotransmitters. Neurotransmitter molecules are released by the end of the axon and cross the synaptic cleft to the next neuron. Some neurotransmitters trigger action potentials in the next neuron. Other neurotransmitters block action potentials. Problems with neurotransmitters may cause depression and some other mental illnesses. 3. Describe the structure of a neuron. 4. Explain how a neuron transmits a signal. Concept 28.3 The PNS carries information to and from the CNS. (pp. 618 619) The PNS includes a sensory division and a motor division. The sensory division includes two sets of sensory neurons that carry information to the CNS. One set carries information about the outside environment from the sense organs. The other set carries information about the internal environment, such as temperature and heart rate. The motor division of the PNS carries response messages from the CNS to muscles and glands. The motor division can be divided into the somatic and autonomic nervous systems. The somatic nervous system carries signals from the CNS to skeletal muscles. It controls mostly voluntary actions, such as walking. The autonomic nervous system carries signals from the CNS to organs and glands. It controls mostly involuntary responses, such as heart rate. The autonomic nervous system is further divided into sympathetic and parasympathetic divisions. The sympathetic division generally increases the level of activity in the body. For example, when you are under stress, the sympathetic division increases your heart rate. The parasympathetic division calms the body and returns it to regular functions. 5. State the functions of the sensory and motor divisions of the PNS. 6. How do the sympathetic and parasympathetic divisions differ? Concept 28.4 The CNS integrates nervous information. (pp. 620 622) The CNS consists of the spinal cord and brain. The spinal cord is contained within the backbone and is surrounded by fluid. The spinal cord carries signals to and from the brain. It also responds directly to some types of information. 116 Biology: Exploring Life
The brain has several regions. The cerebrum is the largest and most complex part. It is divided into two halves, called hemispheres, which are connected by a thick band of nerve fibers called the corpus callosum. The outside part of the cerebrum is the cerebral cortex. It is highly folded and makes up most of the brain s total mass. The cerebral cortex is divided into several areas called lobes that have different functions. The cerebellum coordinates body movements. The brainstem filters all the sensory and motor information traveling to and from the brain. It also controls sleep and breathing. The thalamus filters information traveling to and from the cerebral cortex. It blocks some signals and increases others. The hypothalamus helps to regulate body temperature, blood pressure, hunger, thirst, and emotions. The hypothalamus also is the master control center of the endocrine system. Memory is the ability to store and recall information from past experiences. Memories last longer when they are associated with emotions such as fear or senses such as sight. The limbic system is the region of the brain where connections between memory, emotions, and senses are made. 7. List three parts of the brain and one function of each part. 8. What is memory? Concept 28.5 Sensory receptors link the environment to the nervous system. (pp. 623 628) Sensory receptors detect stimuli from the internal and external environments. The receptors are found in sense organs. When sensory information reaches your brain, you may experience an awareness of that information called a sensation. Your brain also may form a perception, which is a meaningful interpretation of sensory information. Five types of sensory receptors detect stimuli from inside and outside the body. Pain receptors detect pain. Thermoreceptors detect heat and cold. Photoreceptors are found in the eye and detect light. Light enters the eye through a clear area on the surface, called the cornea. Next, light passes through the pupil. The pupil is the dark opening in the iris, which is the colored part of the eye. After passing through the pupil, light passes through the disk-shaped lens. The lens focuses the light on the retina, the inner surface at the back of the eye. The retina contains millions of photoreceptor cells of two types: cones and rods. Cones distinguish colors. Rods are sensitive to dim light. Mechanoreceptors are found in the ear and detect pressure and motion. The outer ear collects sound waves and channels them through the tunnellike auditory canal to the eardrum. The eardrum is a sheet of tissue that vibrates when sound waves strike it. The eardrum passes the vibrations on to three small bones in the middle ear. These bones pass the vibrations to the cochlea in the inner ear. The cochlea is a coiled tube that contains fluid and is lined with tiny hairs. The vibrations make waves in the fluid, causing the hairs to move. Movement of the hairs, in turn, causes action potentials that are sent to the brain. The auditory tube lets air move between the inner ear and throat. This movement of air keeps the air pressure equal on both sides of the eardrum. The inner ear also has five fluid-filled structures lined with hairs that help you keep your balance. Guided Reading and Study Workbook/Chapter 28 117
Chemoreceptors in your nose start action potentials when they are stimulated by chemicals in the air. When the brain integrates the signals, you perceive an odor. Chemoreceptors in the taste buds on your tongue start action potentials when they are stimulated by dissolved chemicals in your mouth. When the brain integrates the signals, you perceive a taste. 9. What is the difference between sensation and perception? 10. Identify the structures of the eye. 11. Explain how the nose collects and transmits smell stimuli to the brain. Concept 28.6 Certain drugs alter brain function. (pp. 629 631) A drug is a substance that is used to produce an effect on the body. Drugs affect the nervous system in various ways. Some drugs increase the rate at which neurons release neurotransmitters. Other drugs decrease this rate. Any drug, even a legal drug, can be abused. Abuse occurs when a drug is overused or taken in a way that it was not intended to be used. Abuse may lead to tolerance of a drug, which means that more and more of the drug is needed to produce the same effect. Use of some drugs may result in addiction, an uncontrollable dependence on a drug. A person who is addicted will experience symptoms of withdrawal when the drug is not taken. Drugs can be classified according to their effects on the body. Drugs called stimulants generally increase CNS activity and include caffeine, nicotine, and cocaine. Most stimulants are highly addictive. Drugs called depressants generally decrease CNS activity. They may interfere with coordination and judgment. Examples of depressants are alcohol, tranquilizers, and marijuana, which is made from the hemp plant. Drugs called opiates are made from the opium poppy and act like natural brain chemicals. Opiates include morphine and heroin and may produce feelings of joy and block pain signals. They also are highly addictive. Drugs called hallucinogens are substances such as LSD that cause the user to see, hear, and perceive things that do not exist. Ecstasy is a drug that acts like a stimulant and hallucinogen. Using Ecstasy may harm the brain. Inhalants such as aerosols produce vapors that have mindaltering effects. Inhalants can damage major organs and even be fatal. 12. Define tolerance and addiction. 13. List three types of drugs and give an example of each type. Reading Skills Practice Taking notes Take notes on the material on the brain on pages 620 622. Rewrite the information in your own words, and include each of the Key Terms in your notes. 118 Biology: Exploring Life
Vocabulary Review and Reinforcement In 1 8, fill in the blanks with the appropriate terms from the chapter. Nerve cells are called neurons. A bundle of neurons is called a(n) 1.. There are three types of neurons. The type of neuron that carries information about stimuli to the CNS is a(n) 2.. The type of neuron that integrates information in the CNS is a(n) 3.. The type of neuron that carries signals away from the CNS is a(n) 4.. A neuron has fibers called 5. that carry signals toward the cell body. A neuron also has a fiber called the 6. that carries signals away from the cell body. Some fibers may be covered by a thick coat of material called a(n) 7.. Uncovered spaces on the fibers are called 8.. In 9 17, write the letter of the correct definition on the line next to each term. 9. sensory receptors 10. resting potential 11. action potential 12. synaptic cleft 13. corpus callosum 14. cerebral cortex 15. auditory canal 16. auditory tube 17. somatic nervous system a. thick band of nerve fibers connecting the hemispheres of the cerebrum b. structure that allows air to move between the middle ear and throat c. highly specialized cells that receive most sensory stimuli d. outside part of the cerebrum e. difference in charge between the inside and outside of a resting neuron f. stronger depolarization that is the start of a nerve signal g. system of the motor division that carries signals to skeletal muscles h. tiny space between neurons in a chemical synapse i. structure that channels sound waves to the eardrum In 18 26, write true if the statement is true. If the statement is false, replace the underlined term with a term that makes the statement true. 18. An environmental change that stimulates sensory neurons is called a(n) sensation. 19. A rapid, automatic response is known as a(n) synapse. 20. A change in charge inside a neuron, from negative to positive, is referred to as depolarization. 21. The level of stimulation that results in an action potential is called the stimulus. 22. A(n) interneuron is a chemical signal that crosses the synaptic cleft. 23. The hypothalamus helps to regulate body temperature, blood pressure, hunger, thirst, and emotions. 24. Connections between memory, emotions, and senses are made in the limbic system. Guided Reading and Study Workbook/Chapter 28 119
GRSW C01-36 4/16/03 12:37 M Page 120 Name Class Date 25. Light enters the eye through a transparent area called the iris. 26. Opiates cause the user to see, hear, and perceive things that do not exist. WordWise Answer the questions by writing the Key Terms in the blanks. Then put the numbered letters in order to find the hidden Key Term. Write a definition for the hidden Key Term. 1. What is the synthetic drug that acts as both a stimulant and a hallucinogen? 1 2. What is the name for the inner surface of the eye where photoreceptors are found? 2 3. What part of the eye gives your eyes their color? 3 4. What do you call an awareness of a sensory stimulus? 4 5. What section of the brain includes the medulla oblongata and the pons? 5 6. What type of drug is made from opium poppies? 6 7. What area of the brain filters information going to and coming from the cerebral cortex? 7 8. What is the term for the dark opening in the center of the iris? 9. What type of drug is nicotine? 8 9 10. What part of the brain is divided into two hemispheres? 10 Hidden Key Term: 1 2 3 4 5 6 7 8 9 10 Definition: 120 Biology: Exploring Life