CHAPTER 9The Nervous System

Objectives

• List the organs and divisions of the nervous system and describe the generalized functions of the system as a whole

• Identify the major types of cells in the nervous system and discuss the function of each

• Identify the anatomical and functional components of a three-neuron reflex arc. Compare and contrast the propagation of a nerve impulse along a nerve fiber and across a synaptic cleft.

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Objectives• Identify the major anatomical components of the brain and spinal cord and briefly comment on the function of each

• Identify and discuss the coverings and fluid spaces of the brain and spinal cord

• Compare and contrast spinal and cranial nerves• Discuss the structure and function of the two divisions of the autonomic nervous system

• Describe major nervous system disorders

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Organs and Divisions of the Nervous System

•Central nervous system (CNS)—brain and spinal cord

•Peripheral nervous system (PNS)—all nerves

•Autonomic nervous system (ANS)

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Cells of the Nervous System

•Neurons•Consist of three parts

• Cell body of neuron—main part• Dendrites—branching projections that conduct impulses to cell body of neuron

• Axon—elongated projection that conducts impulses away from cell body of neuron

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Cells of the Nervous System•Neurons

• Neurons classified according to function or direction of impulse• Sensory neurons: conduct impulses to the spinal cord and brain; also called afferent neurons

• Motor neurons: conduct impulses away from brain and spinal cord to muscles and glands; also called efferent neurons

• Interneurons: conduct impulses from sensory neurons to motor neurons; also called central or connecting neurons

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Cells of the Nervous System•Glia (neuroglia)

• Support cells, bringing the cells of nervous tissue together structurally and functionally

• Three main types of connective tissue cells of the CNS • Astrocytes—star-shaped cells that anchor small blood vessels to neurons

• Microglia—small cells that move in inflamed brain tissue carrying on phagocytosis

• Oligodendrocytes—form myelin sheaths on axons in the CNS (Schwann cells form myelin sheaths in PNS only)

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Cells of the Nervous System•Disorders of nervous tissue

• Multiple sclerosis—characterized by myelin loss in central nerve fibers and resulting conduction impairments

• Tumors• General name for nervous system tumors is neuroma

• Most neuromas are gliomas, glial tumors• Multiple neurofibromatosis—characterized by numerous benign tumors

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Nerves •Nerve—bundle of peripheral axons

• Tract—bundle of central axons• White matter—brain or cord tissue composed primarily of myelinated axons (tracts)

• Gray matter—brain or cord tissue composed primarily of cell bodies and unmyelinated fibers

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Nerves

•Nerve coverings—fibrous connective tissue• Endoneurium—surrounds individual fibers within a nerve

• Perineurium—surrounds a group (fascicle) of nerve fibers

• Epineurium—surrounds the entire nerve

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Reflex Arcs• Nerve impulses are conducted from receptors to effectors over neuron pathways or reflex arcs; conduction by a reflex arc results in a reflex (i.e., contraction by a muscle or secretion by a gland)

• The simplest reflex arcs are two-neuron arcs—consisting of sensory neurons synapsing in the spinal cord with motor neurons; three-neuron arcs consist of sensory neurons synapsing in the spinal cord with interneurons that synapse with motor neurons

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Nerve Impulses

•Definition—self-propagating wave of electrical disturbance that travels along the surface of a neuron membrane; sometimes called action potentials

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Nerve Impulses

•Mechanism• At rest, the neuron’s membrane is slightly positive on the outside—polarized—from a slight excess of Na+ on the outside

• A stimulus is the change in the internal or external environment

• triggers the opening of Na+ channels in the plasma membrane of the neuron

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Nerve Impulses

•Mechanism• Inward movement of Na+ depolarizes the membrane by making the inside more positive than the outside at the stimulated point; this depolarization is a nerve impulse (action potential)

• The stimulated section of membrane immediately repolarizes, but by that time the depolarization has already triggered the next section of membrane to depolarize, thus propagating a wave of electrical disturbances (depolarizations) all the way down the membrane

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The Synapse

• Definition—the place where impulses are transmitted from one neuron to another (the postsynaptic neuron)

• Synapse made of three structures—synaptic knob, synaptic cleft, and plasma membrane

• Neurotransmitters bind to specific receptor molecules in the membrane of a postsynaptic neuron, opening ion channels and thereby stimulating impulse conduction by the membrane

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The Synapse• Names of neurotransmitters—acetylcholine, catecholamines (norepinephrine, dopamine, and serotonin), endorphins, enkephalins, nitric oxide (NO), and other compounds

• Parkinson disease (PD)—characterized by abnormally low levels of dopamine in motor control areas of the brain; patients usually exhibit involuntary trembling and muscle rigidity

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Central Nervous System

•Divisions of the brain •Brainstem

•Consists of three parts, named in ascending order: medulla oblongata, pons, and midbrain

•Structure—white matter with bits of gray matter scattered through it

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Central Nervous System

•Divisions of the brain • Brainstem

• Functions• All three parts of brainstem are two-way conduction paths• Sensory tracts in the brainstem conduct impulses to the

higher parts of the brain• Motor tracts conduct from the higher parts of the brain to

the spinal cord• Many important reflex centers lie in the brainstem

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Central Nervous System

•Diencephalon• Structure and function of the hypothalamus

• Consists mainly of the posterior pituitary gland, pituitary stalk, and gray matter

• Acts as the major center for controlling the ANS; therefore, helps control the functioning of most internal organs

• Controls hormone secretion by anterior and posterior pituitary glands; therefore, it indirectly helps control hormone secretion by most other endocrine glands

• Contains centers for controlling appetite, wakefulness, pleasure, etc.

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Central Nervous System

•Structure and function of the thalamus•Dumbbell-shaped mass of gray matter extending into each cerebral hemisphere

•Relays sensory impulses to cerebral cortex sensory areas

•In some way produces the emotions of pleasantness or unpleasantness associated with sensations

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Central Nervous System

•Cerebellum• Second largest part of the human brain• Helps control muscle contractions to produce coordinated movements so that we can maintain balance, move smoothly, and sustain normal postures

• Recent evidence shows the coordinating effects of the cerebellum may be more extensive, also assisting the cerebrum and other regions of the brain

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Central Nervous System•Cerebrum

• Largest part of the human brain• Outer layer of gray matter is the cerebral cortex; made up of lobes; composed mainly of dendrites and cell bodies of neurons

• Interior of the cerebrum composed mainly of white matter (i.e., nerve fibers arranged in bundles called tracts)

• Functions of the cerebrum—mental processes of all types, including sensations, consciousness, memory, and voluntary control of thought

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Central Nervous System•Brain disorders

•Destruction of brain tissue• Cerebrovascular accident (CVA)—hemorrhage from or cessation of blood flow through cerebral blood vessels; a “stroke”

• Cerebral palsy (CP)—condition in which damage to motor control areas of the brain before, during, or shortly after birth causes paralysis (usually spastic) of one or more limbs

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Central Nervous System

•Brain disorders• Dementia—progressive loss of memory, shortened attention span, personality changes, reduced intellectual capacity, motor control deficit• Alzheimer disease (AD)—brain disorder of the middle and late adult years characterized by dementia

• Huntington disease (HD)—inherited disorder characterized by chorea (purposeless movement) progressing to severe dementia

• HIV (also causes AIDS) can infect neurons and thus cause dementia

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Central Nervous System

•Brain disorders•Seizure disorders

• Seizure—sudden burst of abnormal neuron activity that results in temporary changes in brain function

• Epilepsy—many forms, all characterized by recurring seizures

• Electroencephalogram—graphic representation of voltage changes in the brain used to evaluate brain activity

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Central Nervous System

• Spinal cord • Columns of white matter, composed of bundles of myelinated

nerve fibers, form the outer portion of the H-shaped core of the spinal cord; bundles of axons called tracts

• Interior composed of gray matter made up mainly of neuron dendrites and cell bodies (Figure 9-18)

• Spinal cord tracts provide two-way conduction paths—ascending and descending

• Spinal cord functions as the primary center for all spinal cord reflexes; sensory tracts conduct impulses to the brain, and motor tracts conduct impulses from the brain

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Central Nervous System

• Coverings and fluid spaces of the brain and spinal cord• Coverings

• Cranial bones and vertebrae• Cerebral and spinal meninges—the dura mater, arachnoid

mater, and the pia mater

• Fluid spaces• Subarachnoid spaces of meninges• Central canal inside cord• Ventricles in brain

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Peripheral Nervous System

• Cranial nerves• 12 pairs—attached to undersurface of the brain• Connect brain with the neck and structures in the

thorax and abdomen• Spinal nerves

• 31 pairs—contain dendrites of sensory neurons and axons of motor neurons

• Conduct impulses necessary for sensations and voluntary movements

• Skin surface area supplied by a single nerve is called a dermatome

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Peripheral Nervous System• Peripheral nerve disorders

• Neuritis—general term referring to nerve inflammation• Sciatica is inflammation of the sciatic nerve that innervates the legs• Neuralgia, or muscle pain, often accompanies neuritis

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Peripheral Nervous System• Peripheral nerve disorders

• Trigeminal neuralgia—recurring episodes of stabbing pain along one or more branches of the trigeminal (fifth cranial) nerve in the head

• Bell palsy—paralysis of facial features resulting from damage to the facial (seventh cranial) nerve

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Peripheral Nervous System• Peripheral nerve disorders

• Herpes zoster or shingles • Viral infection caused by chickenpox virus that has invaded the dorsal

root ganglion and remained dormant until an episode of shingles• Usually affects a single dermatome, producing characteristic painful

plaques or vesicles

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Autonomic Nervous System

• Autonomic nervous system—motor neurons that conduct impulses from the central nervous system to cardiac muscle, smooth muscle, and glandular epithelial tissue; regulates body’s automatic or involuntary functions

• Autonomic neurons—preganglionic autonomic neurons conduct from spinal cord or brainstem to an autonomic ganglion; postganglionic neurons conduct from autonomic ganglia to cardiac muscle, smooth muscle, and glandular epithelial tissue

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Autonomic Nervous System• Autonomic or visceral effectors—tissues to which

autonomic neurons conduct impulses (i.e., cardiac and smooth muscle and glandular epithelial tissue)

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Autonomic Nervous System

• Composed of two divisions: the sympathetic system and the parasympathetic system

• Autonomic conduction paths• Consist of two-neuron relays (i.e., preganglionic

neurons from the CNS to autonomic ganglia, synapses, postganglionic neurons from ganglia to visceral effectors)

• In contrast, somatic motor neurons conduct all the way from the CNS to somatic effectors with no intervening synapses

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Autonomic Nervous System

• Sympathetic nervous system• Dendrites and cell bodies of sympathetic

preganglionic neurons located in gray matter of thoracic and upper lumbar segments of spinal cord

• Axons leave spinal cord in the anterior roots of spinal nerves, extend to sympathetic or collateral ganglia, and synapse with several postganglionic neurons whose axons extend to spinal or autonomic nerves to terminate in visceral effectors

• A chain of sympathetic ganglia is in front of and at each side of the spinal column

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Autonomic Nervous System

• Functions of the sympathetic nervous system • Serves as the emergency or stress system, controlling visceral

effectors during strenuous exercise and when strong emotions (anger, fear, hate, or anxiety) are elicited

• Group of changes induced by sympathetic control is called the fight-or-flight response

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Autonomic Nervous System

• Parasympathetic nervous system• Structure

• Parasympathetic preganglionic neurons have dendrites and cell bodies in the gray matter of the brainstem and the sacral segments of spinal cord

• Parasympathetic preganglionic neurons terminate in parasympathetic ganglia located in the head and the thoracic and abdominal cavities close to visceral effectors

• Each parasympathetic preganglionic neuron synapses with postganglionic neurons to only one effector

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Autonomic Nervous System

• Parasympathetic nervous system• Function—dominates control of many visceral effectors under

normal, everyday conditions

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Autonomic Nervous System• Autonomic neurotransmitters

• Cholinergic fibers—preganglionic axons of parasympathetic and sympathetic systems and parasympathetic postganglionic axons release acetylcholine

• Adrenergic fibers—axons of sympathetic postganglionic neurons release norepinephrine (noradrenaline)

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Autonomic Nervous System• Autonomic nervous system as a whole

• Regulates the body’s automatic functions in ways that maintain or quickly restore homeostasis

• Many visceral effectors are doubly innervated (i.e., they receive fibers from parasympathetic and sympathetic divisions and are influenced in opposite ways by the two divisions)

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Autonomic Nervous System• Disorders of the autonomic nervous system

• Stress-induced disease• Prolonged or excessive response to stress can disrupt normal

functioning throughout the body• Examples of stress-induced conditions include heart disease, digestive

problems, and reduced resistance to disease

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Autonomic Nervous System• Disorders of the autonomic nervous system

• Neuroblastoma—highly malignant tumor of the sympathetic nervous system, primarily affecting young children

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