chapter 8 Nervous System

Chapter 8
The 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

ØConsist 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

ØNeurons 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)

ØSupport cells, bringing the cells of nervous tissue together structurally and functionally

ØThree 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

ØSchwann cells form myelin sheaths on axons of the PNS (Figure 8-2)

NERVES AND TRACTS
(Figure 8-4)

˜Nerve—bundle of peripheral axons

ØTract—bundle of central axons

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

ØGray matter—tissue composed primarily of cell bodies and unmyelinated fibers

˜Nerve coverings—fibrous connective tissue

ØEndoneurium—surrounds individual fibers within a nerve

ØPerineurium—surrounds a group (fascicle) of nerve fibers

ØEpineurium—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

ØAt rest, the neuron's membrane is slightly positive on the outside—polarized—from a slight excess of Na+ on the outside

ØA stimulus triggers the opening of Na+ channels in the plasma membrane of the neuron

ØInward 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)

ØThe 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)

ØBrainstem

•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.)

ØCerebellum

•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

 

CENTRAL NERVOUS SYSTEM (cont.)

 

˜Divisions of the brain (cont.)

ØDiencephalon

•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

 

 

CENTRAL NERVOUS SYSTEM (cont.)

˜Divisions of the brain (cont.)

ØDiencephalon (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)

CENTRAL NERVOUS SYSTEM (cont.)

˜Divisions of the brain (cont.)

ØCerebrum

•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

˜Spinal cord (Figure 8-11)

ØColumns 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

ØInterior composed of gray matter made up mainly of neuron dendrites and cell bodies

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

ØSpinal 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

˜Coverings and fluid spaces of the brain and spinal cord

ØCoverings (Figure 8-13)

•Cranial bones and vertebrae

•Cerebral and spinal meninges—the dura mater, the pia mater, and the arachnoid mater

ØFluid spaces

•Subarachnoid spaces of meninges

•Central canal inside cord

•Ventricles in brain

PERIPHERAL NERVOUS SYSTEM

˜Cranial nerves (Figure 8-16 and Table 8-2)

ØTwelve pairs—attached to undersurface of the brain

ØConnect brain with the neck and structures in the thorax and abdomen

˜Spinal nerves

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

ØConduct impulses necessary for sensations and voluntary movements

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

 

AUTONOMIC NERVOUS SYSTEM

 

˜Functional anatomy

ØAutonomic 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)

ØAutonomic 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

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

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

 

˜Autonomic conduction paths

ØConsist of two-neuron relays (that is, preganglionic neurons from the central nervous system to autonomic ganglia, synapses, postganglionic neurons from ganglia to visceral effectors)

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

˜Sympathetic nervous system

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

ØAxons 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

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

ØFunctions 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

˜Parasympathetic nervous system

ØStructure

•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

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

˜Autonomic neurotransmitters

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

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

˜Autonomic nervous system as a whole

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

ØMany 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)