Chapter 9 The Nervous System - Linn-Benton Community ...cf.linnbenton.edu/mathsci/bio/klockj/upload/Chapter_009.pdf · Chapter 9 The Nervous System. ... •Peripheral nervous system

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Chapter 9

The Nervous System


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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) – a

subdivision of the peripheral nervous

system


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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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• 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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• 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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• 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 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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• 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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– 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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• 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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– 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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– 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 movements


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• 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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• 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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• 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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• 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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• 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 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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– 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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– 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 or visceral effectors—tissues

to which autonomic neurons conduct

impulses (i.e., cardiac and smooth

muscle and glandular epithelial tissue)


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• 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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• 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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– 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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• 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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• Parasympathetic nervous system

– Function—dominates control of many

visceral effectors under normal, everyday

conditions


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• 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 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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• 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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• Disorders of the autonomic nervous

system

– Neuroblastoma—highly malignant tumor of

the sympathetic nervous system, primarily

affecting young children

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Chapter 9 The Nervous System - Linn-Benton Community ...cf.linnbenton.edu/mathsci/bio/klockj/upload/Chapter_009.pdf · Chapter 9 The Nervous System. ... •Peripheral nervous system