Slide 1 Mosby items and derived items © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. Chapter 8 The Nervous System.

Slide 1Mosby items and derived items © 2012 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 8The Nervous System


ORGANS AND DIVISIONS OF THE NERVOUS SYSTEM (Figure 8-1)

Central nervous system (CNS)—brain and spinal cord

Peripheral nervous system (PNS)—all nerves Autonomic nervous system (ANS)



CELLS OF THE NERVOUS SYSTEM Neurons

Consist of three main parts—dendrites; cell body of neuron; axon (Figure 8-2) Dendrites—branching projections that conduct impulses to

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

from cell body of neuron Neurons classified according to function

Sensory (afferent) neurons—conduct impulses to the spinal cord and brain

Motor (efferent) neurons—conduct impulses away from brain and spinal cord to muscles and glands

Interneurons—conduct impulses from sensory neurons to motor neurons or among a network of interneurons; also known as central or connecting neurons



CELLS OF THE NERVOUS SYSTEM (cont.)

Glia (neuroglia) Support cells, bringing the cells of nervous

tissue together structurally and functionally Three main types of glial cells of the CNS

(Figure 8-3) 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 on axons of

the PNS (Figure 8-2)



NERVES AND TRACTS(Figure 8-4) Nerve—bundle of peripheral axons

Tract—bundle of central axons White matter—tissue composed primarily of

myelinated axons (nerves or tracts) Gray matter—tissue composed primarily of

cell bodies and unmyelinated fibers Nerve coverings—fibrous connective tissue

Endoneurium—surrounds individual fibers within a nerve

Perineurium—surrounds a group (fascicle) of nerve fibers

Epineurium—surrounds the entire nervea



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 (that is, 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 (Figure 8-5)



NERVE IMPULSES Definition—self-propagating wave of electrical disturbance

that travels along the surface of a neuron membrane (also called action potential)

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 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


THE SYNAPSE Definition—the place where impulses are

transmitted from one neuron to another (the postsynaptic neuron) (Figure 8-7)

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

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



CENTRAL NERVOUS SYSTEM Divisions of the brain (Figure 8-9 and Table 8-1)

Brainstem Consists of three parts of brain; named in ascending

order: the medulla oblongata, pons, and midbrain Structure—white matter with bits of gray matter

scattered through it Functions

All three parts of brainstem conduct impulses to the higher parts of the brain 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




CENTRAL NERVOUS SYSTEM (cont.) Divisions of the brain (cont.)

Cerebellum Structure

Second largest part of the human brain Gray matter outer layer is thin but highly folded, forming a

large surface area for processing information Arbor vitae—internal, treelike network of white matter tracts

Function Helps control muscle contractions to produce coordinated

movements for maintaining balance, moving smoothly, and sustaining normal postures

Variety of additional coordinating effects, assisting the cerebrum and other regions of the brain



Diencephalon Hypothalamus

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

Acts as the major center for controlling the ANS; therefore, it 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 body temperature, appetite, wakefulness, and pleasure



Diencephalon (cont.) Thalamus

Dumbbell-shaped mass of gray matter extending toward each cerebral hemisphere

Relays sensory impulses to cerebral cortex sensory areas In some way produces the emotions of pleasantness or

unpleasantness associated with sensations Pineal gland (pineal body)

Small body resembling a pine nut behind the thalamus Adjusts output of “time-keeping hormone” melatonin in

response to changing levels of external light (sunlight and moonlight)



Cerebrum Largest part of the human brain Outer layers of gray matter are 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 Tracts—nerve fibers arranged in bundles Basal nuclei—islands of gray matter regulate automatic

movements and posture Functions of the cerebrum—mental processes of all

types including sensations, consciousness, memory, and voluntary control of movements


CENTRAL NERVOUS SYSTEM (cont.) Spinal cord (Figure 8-11)

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

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



CENTRAL NERVOUS SYSTEM (cont.) Coverings and fluid spaces of the brain and

spinal cord Coverings (Figure 8-13)

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

the pia mater, and the arachnoid mater Fluid spaces

Subarachnoid spaces of meninges Central canal inside cord Ventricles in brain



PERIPHERAL NERVOUS SYSTEM Cranial nerves (Figure 8-16 and Table 8-2)

Twelve pairs—attached to undersurface of the brain Connect brain with the neck and structures in the

thorax and abdomen Spinal nerves

Thirty-one pairs—contain dendrites of sensory neurons and axons of motor neurons

Conduct impulses necessary for sensations and voluntary movements

Dermatome—skin surface area supplied by a single cranial or spinal nerve




AUTONOMIC NERVOUS SYSTEM Functional anatomy

Autonomic nervous system—motor neurons that conduct impulses from the central nervous system to cardiac muscle, smooth muscle, and glandular epithelial tissue; regulates the body’s automatic or involuntary functions (Figure 8-18)

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

Autonomic or visceral effectors—tissues to which autonomic neurons conduct impulses (i.e., cardiac and smooth muscle and glandular epithelial tissue)

Composed of two divisions—the sympathetic system and the parasympathetic system



AUTONOMIC NERVOUS SYSTEM (cont.)

Autonomic conduction paths Consist of two-neuron relays (that is,

preganglionic neurons from the central nervous system 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


AUTONOMIC NERVOUS SYSTEM (cont.) Sympathetic nervous system

Dendrites and cell bodies of sympathetic preganglionic neurons are located in the gray matter of the thoracic and upper lumbar segments of the spinal cord

Axons leave the 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 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 triggered

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


AUTONOMIC NERVOUS SYSTEM (cont.) Parasympathetic nervous system

Structure Parasympathetic preganglionic neurons have

dendrites and cell bodies in the gray matter of the brainstem and the sacral segments of the 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

Function—dominates control of many visceral effectors under normal, everyday conditions, counterbalances sympathetic function


AUTONOMIC NERVOUS SYSTEM (cont.) 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)

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 (that

is, they receive fibers from parasympathetic and sympathetic divisions and are influenced in opposite ways by the two divisions)

Slide 1 Mosby items and derived items © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. Chapter 8 The Nervous System.

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