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8 The Nervous System Learning Outcomes These Learning Outcomes tell you what you should be able to do after completing the chapter. They correspond by number to this chapter’s sections. 8-1 Describe the anatomical and funct...

8 The Nervous System Learning Outcomes These Learning Outcomes tell you what you should be able to do after completing the chapter. They correspond by number to this chapter’s sections. 8-1 Describe the anatomical and functional divisions of the nervous system. 8-2 Distinguish between neurons and neuroglia on the basis of structure and function. 8-3 Describe the events involved in the generation and propagation of an action potential. 8-4 Describe the structure of a synapse, and explain the process of nerve impulse transmission at a synapse. 8-5 Describe the three meningeal layers that surround the central nervous system. 8-6 Discuss the roles of gray matter and white matter in the spinal cord. 8-7 Name the major regions of the brain, and describe the locations and functions of each. 8-8 Name the cranial nerves, relate each pair of cranial nerves to its principal functions, and relate the distribution pattern of spinal nerves to the regions they innervate. 8-9 Describe the steps in a reflex arc. 8-10 Identify the principal sensory and motor pathways, and explain how it is possible to distinguish among sensations that originate in different areas of the body. 8-11 Describe the structures and functions of the sympathetic and parasympathetic divisions of the autonomic nervous system. 8-12 Summarize the effects of aging on the nervous system. 8-13 Give examples of interactions between the nervous system and other body systems. Clinical Notes Aphasia and Dyslexia, p. 302 Spotlights Demyelination Disorders, p. 279 Seizures, p. 303 The Generation of an Action Potential, Epidural and Subdural Hemorrhages, Cerebral Palsy, p. 317 pp. 282–283 p. 290 Alzheimer’s Disease, p. 324 Propagation of an Action Potential, Spinal Cord Injuries, p. 292 pp. 284–285 271 M08_MART6934_07_GE_C08.indd 271 4/18/16 4:03 PM  272 NERVOUS SYSTEM An Introduction to the Nervous System Two organ systems coordinate organ system activities to main- and thinking about them. At the subconscious level (outside tain homeostasis in response to changing environmental your awareness), it is doing much more. Your nervous system conditions. They are the nervous system and the endocrine is also monitoring the external environment and your inter- system. The nervous system responds relatively swiftly but nal systems and issuing commands as needed to maintain briefly to stimuli. In contrast, responses by the endocrine system homeostasis. Yet in a few hours—at mealtime or while you are develop more slowly but last much longer. For example, the sleeping—your pattern of nervous system activity will be very nervous system adjusts your body position and moves your different. The change from one pattern of activity to another eyes across this page. At the same time, the endocrine system can take place in almost an instant because neural function is adjusting your body’s daily rate of energy use and directing relies on electrical events that proceed at great speed. such long-term processes as growth and maturation. (We will In this chapter we examine the structure and function of consider the endocrine system in Chapter 10.) the nervous system. We will consider its cells through their The nervous system is your most complex organ system. For organization into two major divisions: the central nervous example, right now you are consciously reading these words system and the peripheral nervous system. 8 Build Your Knowledge Recall that the nervous system directs immediate responses coordinating the activities of other organ systems. It also to stimuli (as you saw in Chapter 1: An Introduction provides and interprets sensory information about internal to Anatomy and Physiology). It usually does so by and external conditions. p. 35 receptors in body tissues and organs. Receptors are sensory 8-1 The nervous system has structures that either detect changes in the environment anatomical and functional divisions (internal or external) or respond to specific stimuli. The efferent division (effero, to bring out) of the PNS carries mo- Learning Outcome Describe the anatomical and functional divisions of the nervous system. tor commands from the CNS to muscles and glands. These target organs and tissues respond by doing something and The nervous system carries out three main functions. It are called effectors. (1) monitors the body’s internal and external environments, The efferent division of the PNS has two parts. The somatic (2) integrates sensory information, and (3) coordinates volun- nervous system (SNS) controls skeletal muscle contractions. tary and involuntary responses of many other organ systems. Voluntary contractions are under conscious control, such as The nervous system has two major anatomical divisions. when you lift a glass of water to your lips. Involuntary con- The central nervous system (CNS) consists of the brain and tractions are simple or complex movements controlled at the the spinal cord. It integrates and coordinates the processing of subconscious level. For example, if you accidentally place your sensory data and the transmission of motor commands. The hand on a hot stove, you will pull it back immediately, even CNS is also the seat of higher functions, such as intelligence, before you notice any pain. This type of automatic response memory, and emotion. All communication between the CNS is called a reflex. and the rest of the body takes place through the peripheral The autonomic nervous system (ANS), or visceral motor nervous system (PNS), which includes all the neural tissue system, automatically regulates smooth muscle, cardiac mus- outside the CNS. cle, and glandular secretions at the subconscious level. The Figure 8-1 presents an overview of the functional relation- ANS includes a sympathetic division and a parasympathetic di- ships of the CNS and PNS. Notice that the PNS itself consists vision, which commonly have opposite effects. For example, of two divisions. The afferent division (afferens, to bring activity of the sympathetic division speeds up your heart to) of the PNS brings sensory information to the CNS from rate, but the parasympathetic division slows your heart rate. M08_MART6934_07_GE_C08.indd 272 4/18/16 4:03 PM  Chapter 8 The Nervous System 273 Figure 8-1 A Functional Overview of the Nervous System. Central Nervous System The central nervous system Information processing (CNS) consists of the brain integrates, processes, and and spinal cord. coordinates sensory input and motor commands. Peripheral The afferent division The efferent division carries motor commands Nervous System brings information to the from the CNS to peripheral tissues and systems. The peripheral CNS from the receptors. nervous system (PNS) includes all includes the neural tissue outside the CNS. Somatic Autonomic nervous system nervous system Parasympathetic Sympathetic division division 8 Receptors Effectors Smooth Somatic sensory Visceral sensory muscle receptors (monitor receptors (monitor the outside world internal conditions Cardiac and our position in it) and the status of muscle Skeletal other organ systems) Glands muscle Receptors are sensory structures that detect Effectors are target organs whose activities changes in the internal or external environment. change in response to neural commands. CHECKPOINT two kinds of cells, neurons and neuroglia. p. 141 Neurons 1. Identify the two anatomical divisions of the nervous (neuro-, nerve) are the basic functional units of the nervous system. system. All neural functions involve the communication of neurons with one another and with other cells. The neuro- 2. Identify the two functional divisions of the peripheral glia (noo-ROG-le-uh; glia, glue) regulate the environment. nervous system, and describe their primary functions. around neurons, provide a supporting framework for neu- 3. What would be the effect of damage to the afferent ral tissue, and act as phagocytes. Although they are much division of the PNS? smaller cells, neuroglia (also called glial cells) far outnumber See the blue Answers tab at the back of the book. neurons. Unlike most neurons, most glial cells retain the ability to divide. 8-2 Neurons are specialized for intercellular communication and are Neurons supported by cells called neuroglia The General Structure of Neurons Learning Outcome Distinguish between neurons and neuroglia on the basis of structure and function. Neurons can have a variety of shapes. Figure 8-2 shows a multipolar neuron, the most common type of neuron in the The nervous system includes all the neural tissue in the CNS. A multipolar neuron has (1) a cell body; (2) several body. Neural tissue (introduced in Chapter 4) consists of branching, sensitive dendrites, which receive incoming  274 NERVOUS SYSTEM Figure 8-2 The Anatomy of a Representative Neuron. The relationships of the four parts of a neuron (dendrites, cell body, axon, and axon terminals) are shown in the multipolar neuron depicted here. Mitochondrion Golgi apparatus Axon (may be myelinated) Axon Axon hillock Collateral terminals Nissl bodies Nucleus Nucleolus Nerve cell body Nucleolus Nucleus Axon hillock Cell body Nissl bodies Dendrites 8 Nerve cell body LM × 1500 signals; and (3) a single long axon, which carries outgoing Most neurons lack centrioles, organelles involved in the signals toward (4) one or more axon terminals. movement of chromosomes during mitosis. p. 98 As a The cell body contains a large, round nucleus with a result, typical CNS neurons cannot divide, so they cannot prominent nucleolus. The cytoplasm of the cell body con- be replaced if lost to injury or disease. Neural stem cells are tains organelles that provide energy and synthesize organic present in the adult nervous system, but they are typically compounds. The numerous mitochondria, free and fixed ribo- inactive. Exceptions occur in the nose, where the regenera- somes, and membranes of the rough endoplasmic reticulum tion of olfactory (smell) receptors maintains our sense of (RER) give the cytoplasm a coarse, grainy appearance. Clusters smell, and in the hippocampus, a portion of the brain in- of RER and free ribosomes are known as Nissl bodies. They volved in storing memories. Researchers are studying the give a gray color to areas containing neuron cell bodies and processes that trigger neural stem cell activity, with the goal account for the color of gray matter seen in brain and spinal of preventing or reversing neuron loss due to trauma, dis- cord dissections. ease, or aging. Projecting from the cell body are a variable number of den- drites and a single large axon. The plasma membrane of the dendrites and cell body is sensitive to chemical, mechanical, Structural Classification of Neurons or electrical stimulation. In a process described later, such The billions of neurons in the nervous system are variable in stimulation often leads to the generation of an electrical im- form. Based on the relationship of the dendrites to the cell body pulse, or action potential, that travels along the axon. Action and axon, neurons are classified into three types (Figure 8-3): potentials begin at a thickened region of the cell body called the axon hillock. The axon may branch along its length, pro- 1. A multipolar neuron has two or more dendrites and a ducing branches called collaterals. Axon terminals (also called single axon (Figure 8-3a). These are the most common synaptic terminals and synaptic knobs) are found at the tips neurons in the CNS. All the motor neurons that control of each branch. An axon terminal is part of a synapse, a site skeletal muscles are multipolar. Their axons may be a where a neuron communicates with another cell. meter or more in length.  Chapter 8 The Nervous System 275 Figure 8-3 A Structural Classification of Neurons. The neurons Receptors may be categorized according to the information are not drawn to scale; bipolar neurons are many times smaller than typical they detect. Two types of somatic sensory receptors detect unipolar and multipolar neurons. The arrows show the normal direction of information about the outside world or our physical position an action potential. within it. 1. External receptors provide information about the external environment in the form of sensations of touch, pressure, pain, and temperature and the more complex senses of taste, smell, sight, equilibrium, and hearing. a Multipolar neuron 2. Proprioceptors (pro -pre-o -SEP-torz; proprius, one’s... own + capio, to take) monitor the position and move- ment of skeletal muscles and joints. Visceral receptors, or internal receptors, monitor the activities of the digestive, respiratory, cardiovascular, urinary, and repro- ductive systems. They provide sensations of distension, b Unipolar neuron deep pressure, and pain. MOTOR NEURONS. The half million motor neurons, or efferent 8 neurons, of the efferent division carry instructions from the c Bipolar neuron CNS to other tissues, organs, or organ systems. These periph- eral targets are called effectors. For example, a skeletal muscle is an effector that contracts when it receives neural stimula- 2. In a unipolar neuron, the dendrites and axon are contin- tion. Neurons in the two efferent divisions of the PNS target uous, and the cell body lies off to one side (Figure 8-3b). separate classes of effectors. The somatic motor neurons of In a unipolar neuron, the action potential begins at the the somatic nervous system innervate skeletal muscles, and base of the dendrites, and the rest of the process is con- the visceral motor neurons of the autonomic nervous system sidered an axon. Most sensory neurons of the peripheral innervate all other effectors, including cardiac muscle, smooth nervous system are unipolar. Their axons can be as long muscle, and glands. as those of multipolar neurons. 3. Bipolar neurons have only one dendrite and one axon, INTERNEURONS. The 20 billion interneurons, or association with the cell body between them (Figure 8-3c). Bipolar neurons, are located entirely within the brain and the spinal neurons are small and rare. They occur in special sense cord. Interneurons, as the name implies (inter-, between), in- organs, where they relay information about sight, smell, terconnect other neurons. They are responsible for distributing or hearing from receptor cells to other neurons. sensory information and coordinating motor activity. The more complex the response to a given stimulus, the greater the num- Functional Classification of Neurons ber of interneurons involved. Interneurons also play a role in all higher functions, such as memory, planning, and learning. In terms of function, neurons are sorted into three groups: (1) sensory neurons, (2) motor neurons, and (3) interneurons. Neuroglia SENSORY NEURONS. The approximately 10 million sensory Neuroglia are abundant and diverse. They make up about half neurons, or afferent neurons, in the human body form the of the volume of the nervous system. They are found in both afferent division of the PNS. Sensory neurons receive infor- the CNS and PNS, but the CNS has a greater variety of glial cells. mation from sensory receptors monitoring the external and The CNS contains four types of neuroglial cells (Figure 8-4): internal environments and then relay the information to other neurons in the CNS (spinal cord or brain). The recep- 1. Astrocytes (AS-tro -sīts; astro-, star + cyte, cell) are the. tor may be a dendrite of a sensory neuron or specialized largest and most numerous neuroglia. These star- cells of other tissues that communicate with the sensory shaped cells have varied functions. Astrocytes maintain neuron. the blood-brain barrier that isolates the CNS from the M08_MART6934_07_GE_C08.indd 275 4/18/16 4:04 PM  276 NERVOUS SYSTEM Figure 8-4 Neuroglia in the CNS. This diagrammatic view of neural tissue in the CNS depicts the relationships between neuroglia and neurons. White matter Central canal of spinal cord Gray matter Neuron Neuron Neuroglia in the CNS Ependymal cell Ependymal cells are simple cuboidal epithelial cells that line fluid-filled passageways within the brain and spinal cord. Microglial cell 8 Microglia are phagocytes that move through neural tissue removing Gray unwanted substances. matter Astrocyte Astrocytes are star-shaped White cells with projections that matter Myelinated anchor to capillaries. They form axons the blood-brain barrier, which isolates the CNS from the general circulation. Oligodendrocyte Oligodendrocytes are cells with sheet-like processes that wrap around axons. Myelin (cut) n Axo Nodes Capillary body’s general circulation. Cytoplasmic extensions of many compounds, such as hormones and amino acids the astrocytes end in expanded “feet” that wrap around that could interfere with neuron function. Astrocytes capillaries. The astrocytes secrete chemicals that cause also create a structural framework for CNS neurons and the capillaries of the CNS to become impermeable to perform repairs in damaged neural tissues. M08_MART6934_07_GE_C08.indd 276 4/18/16 4:04 PM  Chapter 8 The Nervous System 277 2. Oligodendrocytes (ol-i-go-DEN-dro-sīts; oligo-, few) do in the CNS. The other glial cells in the PNS are Schwann.. have smaller cell bodies and fewer processes (cytoplasmic cells (Figure 8-5a). extensions) than astrocytes. The plasma membrane at Schwann cells cover every axon outside the CNS. Wher- the tip of each process forms a thin, expanded sheet that ever a Schwann cell covers an axon, the outer surface of the wraps around an axon. This membranous wrapping is Schwann cell is called the neurilemma (nu-ri-LEM-uh). Unlike. called myelin (MI-e-lin). It serves as electrical insulation an oligodendrocyte in the CNS, which may myelinate por-. that increases the speed at which an action potential tions of several axons, a Schwann cell can myelinate only one travels along the axon. Each oligodendrocyte myelinates segment of a single axon (Figure 8-5a). However, a Schwann short segments of several axons, so many oligodendro- cell can enclose portions of several different unmyelinated cytes are needed to coat an entire axon with myelin. Such axons (Figure 8-5b). an axon is said to be myelinated. The areas covered in myelin are called internodes. Figure 8-5 Schwann Cells and Peripheral Axons. The small gaps between adjacent cell processes are called nodes, or the nodes of Ranvier (rahn-ve-A)... Myelin is lipid-rich, and on dissection, areas Nodes of the CNS containing myelinated axons appear glossy white. Areas dominated by my- 8 elinated axons are known as the white mat- ter of the CNS. Not every axon in the CNS is myelinated. Axons without a myelin coating Schwann cell nucleus are said to be unmyelinated. Areas contain- ing neuron cell bodies, dendrites, and unmy- Myelin covering elinated axons make up the gray matter of internode the CNS. Neurilemma 3. Microglia (mī-KROG-le-uh) are the smallest.. Axons and least numerous of the neuroglia in the CNS. Microglia are phagocytic cells derived from white blood cells that migrated into the Schwann CNS as the nervous system formed. They per- cell nucleus form protective functions such as engulfing cellular waste and pathogens. 4. Ependymal (ep-EN-di-mul) cells are simple cuboidal epithelial cells that line cavities in the CNS filled with cerebrospinal fluid (CSF). These cavities include the central canal of the spinal cord and the chambers, or ventricles, of the brain. The lining of Myelinated axon TEM × 14,048 epithelial cells is called the ependyma (ep- a A myelinated axon in the PNS is EN-di-muh). Unlike other epithelia, it lacks covered by several Schwann cells, a basement membrane. In some regions each of which forms a myelin of the brain, the ependyma produces CSF. sheath around a portion of the axon. This arrangement differs from Unmyelinated TEM × 14,048 In other locations, the cilia on ependymal the way myelin forms in the CNS; axon cells help circulate this fluid within and compare with Figure 8-4. b A single Schwann cell can around the CNS. encircle several unmyelinated The PNS contains two types of neuroglia. Satellite axons. Every axon in the PNS is cells surround and support neuron cell bodies in completely enclosed by the peripheral nervous system, much as astrocytes Schwann cells. M08_MART6934_07_GE_C08.indd 277 4/18/16 4:04 PM  278 NERVOUS SYSTEM Organization of Neurons in the ⦁ A collection of neuron cell bodies (gray matter) with a common function is called a center. A center with a dis- Nervous System crete boundary is called a nucleus. Portions of the brain Neuron cell bodies and their axons are not randomly scattered surface are covered by a thick layer of gray matter called in the CNS and PNS. Instead, they are organized into masses neural cortex (cortex, rind). The term higher centers refers or bundles that have distinct anatomical boundaries and are to the most complex integration centers, nuclei, and ar- identified by specific terms (Figure 8-6). We will use these eas of cortex in the brain. terms again, so you may find a brief overview here helpful. ⦁ The white matter of the CNS contains bundles of axons In the PNS: that share common origins, destinations, and functions. These bundles are called tracts. Tracts in the spinal cord ⦁ Neuron cell bodies (gray matter) are located in ganglia form larger groups called columns. (singular, ganglion). The neuron cell bodies are surrounded ⦁ Pathways include both gray matter and white matter. by satellite cells. They link the centers of the brain with the rest of the ⦁ The white matter of the PNS contains axons bundled together body. For example, sensory (ascending) pathways dis- in nerves. Spinal nerves are connected to the spinal cord, and tribute information from sensory receptors to processing cranial nerves are connected to the brain. Both sensory and centers in the brain. Motor (descending) pathways begin motor axons may be present in the same nerve. at CNS centers for motor activity and end at the skeletal 8 In the CNS: muscles they control. Figure 8-6 Anatomical Organization of the Nervous System. CENTRAL NERVOUS SYSTEM GRAY MATTER ORGANIZATION PERIPHERAL Neural Cortex Centers NERVOUS SYSTEM Gray matter on the Collections of GRAY MATTER surface of the brain neuron cell bodies in the CNS; each Ganglia center has specific Collections of processing functions Nuclei neuron cell bodies in the PNS Collections of neuron cell bodies Higher Centers in the interior of The most complex WHITE MATTER the CNS centers in the brain Nerves Bundles of axons WHITE MATTER ORGANIZATION in the PNS Tracts Columns Bundles of CNS Several tracts that axons that share form an anatomically a common origin, distinct mass destination, and RECEPTORS function PATHWAYS Centers and tracts that connect the brain with other organs and systems in the body EFFECTORS Ascending (sensory) pathway Descending (motor) pathway M08_MART6934_07_GE_C08.indd 278 4/18/16 4:04 PM  Chapter 8 The Nervous System 279 of negative charges inside the cell. When positive and nega- CLINICAL NOTE tive charges are held apart, a potential difference is said to exist between them. This potential difference is called a membrane Demyelination Disorders potential, or transmembrane potential, because the charges are Demyelination is the progressive destruction of myelin sheaths, separated by a plasma membrane. both in the CNS and PNS. The result is a gradual loss of sensa- The unit of measurement of potential difference is the volt tion and motor control that leaves affected body regions numb (V). Most cars, for example, have 12 V batteries. The mem- and paralyzed. One demyelination disorder is multiple sclerosis (skler-O-sis; sklerosis, hardness), or MS. It affects axons in. brane potential of cells is much smaller and is usually reported the optic nerve, brain, and/or spinal cord. Common signs and in millivolts (mV, thousandths of a volt). The membrane po- symptoms of MS include partial loss of vision and problems tential of an unstimulated, resting cell is known as its resting with speech, balance, and general motor coordination. Other membrane potential. The resting membrane potential of a important demyelination disorders include heavy metal poisoning, neuron is –70 mV. The minus sign indicates that the cytoplas- Charcot-Marie-Tooth disease, and Guillain-Barré syndrome. mic surface of the plasma membrane contains an excess of negative charges compared to the extracellular surface. CHECKPOINT Factors Responsible for the Membrane Potential 4. Name the structural components of a typical neuron. Many factors influence membrane potential. In addition 5. Examination of a tissue sample reveals unipolar to an imbalance of electrical charges, the intracellular and 8 neurons. Are these more likely to be sensory neurons extracellular fluids differ markedly in chemical and ionic or motor neurons? composition. For example, the extracellular fluid con- 6. Identify the neuroglia of the central nervous system. tains relatively high concentrations of sodium ions (Na+ ) 7. Which type of glial cell would increase in number in and chloride ions (Cl - ). The intracellular fluid contains the brain tissue of a person with a CNS infection? high concentrations of potassium ions (K + ) and negatively 8. In the PNS, neuron cell bodies are located in charged proteins (Pr - ). ______________ and surrounded by neuroglial cells The selective permeability of the plasma membrane main- called ______________ cells. tains these differences between the intracellular and extracel- See the blue Answers tab at the back of the book. lular fluids. The proteins within the cytoplasm are too large to cross the membrane. The ions can enter or leave the cell only with the aid of membrane channels and/or carrier pro- 8-3 In neurons, a change in the plasma teins. p. 89 There are many different types of membrane membrane’s electrical potential may result channels: in an action potential (nerve impulse) ⦁ Some, called leak channels, are always open. Learning Outcome Describe the events involved in the generation and ⦁ Others, called gated channels, open or close under specific propagation of an action potential. circumstances. For example, gated channels may open or close due to the presence of a specific chemical or a The sensory, integrative, and motor functions of the nervous change in membrane potential, or voltage. system are dynamic and ever changing. All communications between neurons and other cells take place through their Both passive and active processes act across the plasma membrane surfaces. These membrane changes are electrical membrane to determine the membrane potential at any events that proceed at great speed. moment. The passive forces are chemical and electrical. Chemical concentration gradients move potassium ions out of the cell and sodium ions into the cell. (These ions The Membrane Potential move through separate leak channels.) However, it is easier A characteristic feature of all living cells is a polarized plasma for potassium ions to diffuse through a potassium channel membrane. An undisturbed, or unstimulated, cell has a plasma than for sodium ions to diffuse through sodium channels. membrane that is polarized because the membrane separates As a result, potassium ions diffuse out of the cell faster than an excess of positive charges outside the cell from an excess sodium ions enter the cell. M08_MART6934_07_GE_C08.indd 279 4/18/16 4:04 PM  280 NERVOUS SYSTEM Electrical forces across the membrane also affect the passive Changes in the Membrane Potential movement of sodium and potassium ions. The overall positive charge on the outer surface of the plasma membrane repels Any stimulus that (1) alters membrane permeability to sodium positively charged potassium ions. At the same time, the nega- or potassium ions or (2) alters the activity of the exchange tively charged inner membrane surface attracts the positively pump will disturb the resting membrane potential of a cell. charged sodium ions. Potassium ions continue to leave the Some stimuli that can affect membrane potential include cell, however, because its chemical concentration gradient is exposure to specific chemicals, mechanical pressure, changes stronger than the repelling electrical force. in temperature, or shifts in the extracellular ion concentrations. To maintain a potential difference across the plasma mem- Any change in the resting potential can have an immediate brane, active processes work both to overcome the combined effect on the cell. For example, permeability changes in the sarco- chemical and electrical forces driving sodium ions into the lemma of a skeletal muscle fiber trigger a contraction. p. 227 cell and to maintain the potassium concentration gradient. In most cases, a stimulus opens gated ion channels that are The resting potential remains stable over time because of the closed when the plasma membrane is at its resting membrane actions of a carrier protein, the sodium–potassium exchange potential. The opening of these channels speeds up ion move- pump. p. 94 This ion pump exchanges three intracellu- ment across the plasma membrane and changes the mem- lar sodium ions for two extracellular potassium ions. At the brane potential. For example, the opening of gated sodium normal resting membrane potential of –70 mV, sodium ions channels speeds up the entry of sodium ions (Na + ) into the are ejected as fast as they enter the cell. The cell, therefore, cell. As the number of positively charged ions on the inner 8 undergoes a net loss of positive charges. As a result, the inte- surface of the plasma membrane increases, the membrane rior surface of the plasma membrane maintains an excess of potential shifts toward 0 mV. A shift in this direction is called a negative charges, primarily from negatively charged proteins. depolarization of the membrane. Figure 8-7 shows the plasma membrane at the resting mem- On the other hand, a stimulus that opens gated potassium brane potential. ion channels shifts the membrane potential away from 0 mV, Figure 8-7 The Resting Membrane Potential. Cl – + – + – –30 – + –70 0 + EXTRACELLULAR +30 + + + + FLUID + + 3 Na+ mV + – – + + + + + + + + + + + + + + + + + Sodium– K+ leak potassium Na+ leak Plasma channel exchange channel membrane pump – – – – – – – – – – – – – + + – – + ATP ADP – Protein – 2K + + – + + + + + + + – KEY – – – CYTOSOL Protein + Sodium ion (Na+) + Protein – Protein – – + + + – + Potassium ion (K+) – + – – – Chloride ion (Cl– ) M08_MART6934_07_GE_C08.indd 280 4/18/16 4:04 PM  Chapter 8 The Nervous System 281 because additional potassium ions (K + ) will leave the cell. Such a change may take the membrane potential from –70 mV to Build Your Knowledge –80 mV. This kind of shift is called a hyperpolarization. Information transfer between neurons and other cells in- Recall that a single axon of a motor neuron may branch volves two types of change in membrane potential: graded to control more than one skeletal muscle fiber (as you potentials and action potentials. Graded potentials, or local saw in Chapter 7: The Muscular System). Each potentials, are changes in the membrane potential that can- branch ends in an axon terminal that is part of a neuro- not spread far from the site of stimulation. For example, if a muscular junction. Each skeletal muscle fiber has only chemical stimulus to the plasma membrane of a neuron opens one neuromuscular junction. A motor unit is a single gated sodium ion channels at a single site, the sodium ions en- motor neuron and all the muscle fibers it innervates. tering the cell will depolarize the membrane at that location. p. 233 Attracted to surrounding negative ions, the sodium ions move along the inner surface of the membrane in all directions. The degree of depolarization decreases with distance from the stim- that were applied to the trigger have no effect on the speed of ulation site. Why? This happens because the cytosol resists ion the bullet leaving the gun. In an axon, the graded potential is movement and because some sodium ions are lost as they move the pressure on the trigger, and the action potential is like the back out across the membrane through leak channels. firing of the gun. An action potential will not appear unless Graded potentials occur in the plasma membranes of all cells the membrane depolarizes to a level known as the threshold. 8 in response to environmental stimuli. They often trigger specific Every stimulus (whether minor or extreme) that brings cell functions. For example, a graded potential in the membrane the membrane to threshold will generate an identical action of a gland cell may trigger secretion. However, graded potentials potential. This concept is called the all-or-none principle affect too small an area to have an effect on the activities of such because a given stimulus either triggers a typical action po- large cells as skeletal muscle fibers or neurons. In these cells, tential or none at all. The all-or-none principle applies to all graded potentials can influence activities in distant portions of excitable membranes. the cell only if they lead to the production of an action potential, an electrical signal that affects the surface of the entire membrane. The Generation of an Action Potential An action potential is a propagated change in the membrane How is an action potential generated? An action potential be- potential of excitable cells. Excitable cells are the only cells that gins when the first portion of the excitable axon membrane, have an electrically excitable membrane that can be stimulated called the initial segment, depolarizes to threshold. The steps to propagate action potentials. Excitable membranes contain involved in generating an action potential begin with a graded voltage-gated channels that open or close in response to changes depolarization to threshold (from –70 mV to –60 mV) and end in the membrane potential. Skeletal muscle fibers, cardiac mus- with a return to the resting potential (–70 mV). The steps are cle cells, and the axons of neurons have excitable membranes. illustrated in Spotlight Figure 8-8. In a skeletal muscle fiber, the action potential begins at the The membrane cannot respond normally to further stimu- neuromuscular junction and travels along the entire mem- lation during most of these steps. This period is known as the brane surface, including the T tubules. p. 232 The resulting refractory period of the membrane. It lasts from the moment ion movements trigger a contraction. the voltage-gated sodium channels open at threshold until the In an axon, an action potential usually begins near the axon return to the resting potential, or repolarization, is complete. hillock and travels along the length of the axon toward the axon The refractory period limits the rate at which action potentials terminals, where its arrival activates the synapses. An action can be generated in an excitable membrane. (The maximum potential in a neuron is also known as a nerve impulse. (We rate of action potential generation is 500–1000 per second.) discuss action potentials in cardiac muscle cells in Chapter 12.) Action potentials are generated by the opening and closing Propagation of an Action Potential of voltage-gated sodium channels and voltage-gated potassium channels in response to a graded potential. This local depo- An action potential initially involves a relatively small portion larization acts like pressure on the trigger of a gun. A gun fires of the total membrane surface of the axon. But unlike graded only after a certain minimum pressure has been applied to the potentials, which diminish rapidly with distance, action po- trigger. It does not matter whether the pressure builds gradu- tentials affect the entire membrane surface. The basic processes ally or is exerted suddenly—when the pressure reaches a criti- of action potential propagation along unmyelinated and cal point, the gun will fire. Whenever the gun fires, the forces myelinated axons are shown in Spotlight Figure 8-9 (p. 284). M08_MART6934_07_GE_C08.indd 281 4/18/16 4:04 PM  SPOTLIGHT Figure 8-8 THE GENERATION OF AN ACTION POTENTIAL A neuron receives information on its dendrites and cell body, and communicates that information to another cell at its axon terminal. Because the two ends of the neuron may be a meter apart, such long-range communication relies on action potentials. Action potentials are propagated changes in the membrane potential that, once started, affect the entire excitable membrane of the axon. Action potentials depend on the presence of voltage-gated channels. Axon hillock Initial segment (first portion of excitable axon membrane to reach threshold) Steps in the generation of an action potential in an axon. The first step is a graded depolarization caused by the opening of chemically gated sodium ion channels, usually at the axon hillock. The axon membrane colors in steps 1–4 match the colors of the line graph showing changes in the membrane potential. Resting Potential 1 Depolarization to Threshold 2 Activation of Sodium Channels and Rapid Sodium ion Potassium ion Depolarization + + + + –60 mV +10 mV –70 mV + + + + + + + + + + + + + + – – – – – Local – – + – + + – – + + + current + + + + + + + + + + + + + + + + + + + The axon membrane contains both The stimulus that begins an action potential When the voltage-gated sodium channels voltage-gated sodium channels and is a graded depolarization large enough to open, sodium ions rush into the cytosol, voltage-gated potassium channels that open voltage-gated sodium channels. The and rapid depolarization occurs. The inner are closed when the membrane is at the opening of the channels occurs at a membrane surface now contains more resting potential. membrane potential known as the threshold. positive ions than negative ones, and the membrane potential has changed from –60 mV to a positive value. 282 M08_MART6934_07_GE_C08.indd 282 4/18/16 4:04 PM  Sodium channels close, voltage-gated potassium channels open, and potassium ions move out of the cell. Repolarization begins. Changes in the +30 3 membrane potential at one location D E P O L A R I Z AT I O N R E P O L A R I Z AT I O N during the generation of an 0 action potential. The circled numbers in 2 the graph correspond Resting to the steps Membrane potential (mV) potential Voltage-gated sodium Potassium channels illustrated below. –40 channels open and close, and both sodium sodium ions move into and potassium channels Threshold the cell. The membrane return to their –60 potential rises to +30 mV. normal states. –70 1 4 A graded depolarization brings an area of excitable membrane to threshold (–60 mV). REFRACTORY PERIOD During the refractory period, the membrane cannot respond to further stimulation. 0 Time (msec) 1 2 3 Inactivation of Sodium 4 Closing of Potassium Channels Return to Resting Potential Channels and Activation of Potassium Channels + + +30 mV + + + –90 mV + –70 mV + + + + + + + + + + + + + + + – – – – – – + + + + + +– – – – – + + + + + + + + + + + + + + + + As the membrane potential approaches The voltage-gated sodium channels remain As the voltage-gated potassium +30 mV, voltage-gated sodium channels inactivated until the membrane has channels close, the membrane close. This step coincides with the opening repolarized to near threshold levels. The potential returns to normal resting of voltage-gated potassium channels. voltage-gated potassium channels begin levels. The action potential is now Positively charged potassium ions move out closing as the membrane reaches the normal over, and the membrane is once of the cytosol, shifting the membrane resting potential (about –70 mV). Until all have again at the resting potential. potential back toward resting levels. closed, potassium ions continue to leave the Repolarization now begins. cell. This produces a brief hyperpolarization. 283 M08_MART6934_07_GE_C08.indd 283 4/18/16 4:04 PM  SPOTLIGHT Figure 8-9 PROPAGATION OF AN ACTION POTENTIAL Continuous Propagation Axon hillock along an Unmyelinated Axon Initial segment In an unmyelinated axon, an action potential moves 1 2 3 along by continuous propagation. The action potential spreads by depolarizing the adjacent region of the axon membrane. This process continues to spread as a chain reaction down the axon. 1 Extracellular Fluid KEY As an action potential develops Action Resting potential at the initial segment 1 , the +30 mV + + –70 mV + –70 mV potential membrane potential at this site Na+ + + + Graded depolarization depolarizes to +30 mV. + + + + + + 1 2 3 Rapid + – – – – – – – – – depolarization + + + ++ + Cell membrane Cytosol Repolarization 2 As the sodium ions entering at Graded depolarization 1 spread away from the open –60 mV + + –70 mV + + + + + voltage-gated channels, a graded + + + + + + + depolarization quickly brings the 1 2 3 membrane in segment 2 to – + + + + – + – – – – – – – threshold. + + + Loc al c urrent 3 An action potential now occurs in Repolarization segment 2 while segment 1 (refractory) +30 mV + + –70 mV begins repolarization. + Na+ + + + + + + + + 1 2 3 – – – + + + + – – – – – – + + + + + + 4 As the sodium ions entering at –60 mV segment 2 spread laterally, a + graded depolarization quickly + + + + + + + + + + + brings the membrane in segment 1 2 3 3 to threshold, and the cycle is – – – – – – – +L o c+ – t+ repeated. + + + a l c u r re n + + + + 284 M08_MART6934_07_GE_C08.indd 284 4/18/16 4:04 PM  Saltatory Propagation along a Myelinated Axon 1 2 3 Because myelin limits the movement of ions across the axon membrane, the action potential must “jump” from node to node during propagation. This results in much faster propagation along the axon. 1 + + + Extracellular Fluid + An action potential +30 mV + + + –70 mV + –70 mV + + + + + + develops at the + + + + + + + + + + + initial segment 1. + + + + + + + Na+ + + Myelinated + Myelinated + Myelinated 1 internode 2 internode 3 internode + + + – – – – – – – – – – – – – – – – – – + + + + Plasma membrane Cytosol 2 + + + A local current + + + + + + –60 mV + –70 mV + + + produces a + + + + + + + graded + + + + + + + + + + + + + + depolarization + + + that brings the 1 2 3 axon membrane + + + + Local – + – – – – – – – – – – – – – – – – – + + current + + at the next node + + to threshold. 3 Repolarization + + + + + An action potential (refractory) + + + + +30 mV –70 mV + + + + develops at + + + + + + + + + + + node 2. + + + + Na+ + + + + 1 2 3 – – – – – + + – – – – – – – – – – – – + + + + + + + ++ + 4 + + + + + A local current + + + + + –60 mV + + produces a graded + + + + + + + + + + depolarization that + + + + + + + + + + + + brings the axon + + + membrane at node 1 2 3 3 to threshold. – – – – – – – –+ + + + Local + – – – – – – – – + + current + + + + 285 M08_MART6934_07_GE_C08.indd 285 4/18/16 4:04 PM  286 NERVOUS SYSTEM For simplicity, think of the plasma membrane as a series of 12. Two axons are tested for propagation velocities adjacent segments. (speeds). One carries action potentials at 50 meters per The action potential begins at the axon’s initial segment. second, the other at 1 meter per second. Which axon For a brief moment at the peak of the action potential, the is myelinated? membrane potential becomes positive rather than negative 1 See the blue Answers tab at the back of the book. ( 1 ). A local current then develops as the sodium ions begin moving in the cytosol and the extracellular 1fluid ( 2 ). The 2 current of moving sodium ions spreads in all directions, local 8-4 At synapses, communication takes 1 depolarizing adjacent portions of the membrane. 3 2 3 axon (The place among neurons or between neurons hillock cannot respond with an action 2 potential 3 because 4 it and other cells does 4 not have voltage-gated sodium channels.) The process Learning Outcome Describe the structure of a synapse, and explain the then continues in a chain reaction ( 3 and 4 ). 5 5 process of nerve impulse transmission at a synapse. Each time a local current develops, the action potential 4 5 6 moves 6 forward along the axon. It does not move backward, be- Recall that a synapse is a site where a neuron communicates cause the previous segment of the axon 5 is still in 6 the refractory 7 with another cell. In the nervous system, information moves period. 7 As a result, an action potential always proceeds away from one location to another in the form of action potentials 6 from its generation site and cannot reve