The Nervous System

Chapter 8: The Nervous System

I. Organs and divisions of the nervous system

A. Central nervous system

1. Brain

2. Spinal cord

B. Peripheral nervous system

1. Cranial nerves

2. Spinal nerves

C. Autonomic nervous system

1. Sympathetic nervous system

2. Parasympathetic nervous system

II. Cells of the nervous system

A. Neurons

1. Consist of three main parts

a Dendrites – “Receivers” conduct received impulses towards the cell body

b Cell body – the main body of the neuron; contains a nucleus and other organelles

c Axon – “Transmitter” conducts impulses away from the cell body

2. Neurons classified according to function

a Sensory (afferent) neurons – conducts nerve impulses from sense organs towards the central nervous system

b Interneurons – connects sensory neurons to motor neurons. Part of central nervous system. “Bridge” between sensation and action.

c Motor (efferent) neurons – conducts nerve impulses from central nervous system to muscles and glands.

B. Glia – three main types of connective tissue of the central nervous system

1. Astrocytes – star-shaped cells that anchor small blood vessels to neurons

2. Microglia – small cells that move in inflamed brain tissue carrying out phagocytosis

3. Oligodendrocytes – form myelin sheaths on axons in the central nervous system

C. Disorders of nervous tissue

1. Multiple sclerosis – characterized by myelin loss in central nerve fibers and resulting conduction impairments.

2. Tumors

a General term for nervous system tumors is neuroma

b Most neuromas are gliomas; glial tumors

c Multiple neurofibromatosis – characterized by numerous benign tumors that can progress to disfiguring, crippling soft tissue tumors.

III. Nerves

A. Nerve – bundle of peripheral axons

1. Tract – bundle of central axons

2. White matter – tissue composed primarily of myelinated axons (nerves of tracts)

3. Gray matter – tissue composed primarily of cell bodies and unmyelinated fibers

B. Nerve coverings – fibrous connective tissue

1. Endoneurium – surrouns individual fibers within a nerve

2. Perineurium – surrounds a group (fascicle) of nerve fibers

3. Epineurium – surrounds the entire nerve

IV. Reflex arcs

A. 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.)

B. Simplest reflex arcs are two-neuron arcs – consist 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.

V. Nerve impulses

A. Definition – self-propagating wave of electrical disturbance that travels along the surface of a neuron membrane

B. Mechanism

1. Stimulus triggers the opening of Na+ channels in the plasma membrane of the neuron.

2. Inward movement of positive sodium ions leaves a slight excess of negative ions outside at a stimulated point; marks the beginning of a nerve impulse (depolarization)

3. Impulse travels along the surface of the cell membrane as a “domino effect” of successive depolarization

VI. The synapse

A. Definition – chemical compounds released from axon terminals (of a presynaptic neuron) into a synaptic cleft.

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

C. Names of neurotransmitters – acetylcholine, catacholamines (norepinephrine, dopamine, and serotonin), and other compounds.

D. Parkinson’s Disease – characterized by abnormally low levels of dopamine in motor control areas of the brain; patients usually exhibit involuntary trembling and muscle rigidity.

VII. Central nervous system

A. Divisions of the brain

1. Brainstem

a Consisting of three parts of brain named in ascending order:

(1) Medulla oblangota

(2) Pons

(3) Midbrain

b Structure – white matter with bits of gray matter scattered through it (Reticular formation)

c Function – gray matter in the brainstem functions as reflex centers (for example – heartbeat, respirations, and blood vessel diameter); 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

2. Diencephalon

a Structure and function of the hypothalamus

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

(2) Acts as the major center for controlling the autonomic nervous system; therefore helps control the functioning of most internal organs

(3) Controls hormone secretion by anterior and posterior pituitary glands; therefore indirectly helps control hormone secretions by most other endocrine glands

(4) Contains centers for controlling appetite, wakefulness, pleasure, etc.

b Structure and function of the thalamus

(1) Dumbbell-shaped mass of gray matter in each cerebral hemisphere

(2) Relays sensory impulses to cerebral cortex areas

(3) In some way produces the emotions of pleasantness or unpleasantness associated with sensations

3. Cerebellum

a Second largest part of the human brain

b Helps control muscle contractions to produce coordinated movements so that we can maintain balance, move smoothly, and sustain normal postures

4. Cerebrum

a Largest part of the human brain

b Outer layer of gray matter is the cerebral cortex; made up of lobes; composed mainly of dendrites and cell bodies of neurons

c Interior of cerebrum composed mainly of white matter (that is nerve fibers arranged in bundles called tracts)

d Functions of the cerebrum – mental processes of all types, including sensations, consciousness, memory, and voluntary control of movements

5. Brain disorders

a Destruction of brain tissue

(1) Cerebrovascular accident (CVA) – hemorrhage from or cessation of blood flow through cerebral blood vessels; a “stroke”

(2) Cerebral palsy – 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

(3) Dementia – syndrome that includes progressive loss of memory, shortened attention span, personality changes, reduced intellectual capacity, and motor control deficit

(a) Alzheimer’s Disease – brain disorder of the middle and late adult years characterized by dementia

(b) Huntington’s Chorea – inherited disorder characterized by purposeless movements; progressing to severe dementia

b Seizure Discorder

(1) Seizure – sudden burst of abnormal neuron activity that results in temporary changes in brain function

(2) Epilepsy – many forms, all characterized by recurring seizures

(3) Electroencephalogram – graphic representation of voltage changes in the brain used to evaluate brain activity

B. Spinal Cord

1. Outer part is composed of white matter made up of many bundles of axons called tracts; interior composed of gray matter made up mainly of neuron dendrites and cell bodies

2. Functions as the center for all spinal cord reflexes; sensory tracts conduct impulses to the brain, and motor tracts conduct impulses from the brain.

C. Coverings and fluid spaces of the brain and spinal cord

1. Coverings

a Cranial bones and vertebrae

b Cerebral and spinal meninges – the dura mater, arachnoid mater, and pia mater

2. Fluid spaces – subarachnoid spaces of meninges, central canal inside cord, and ventricles in brain

VIII. Peripheral nervous system

A. Cranial nerves

1. 12 Pairs – attached to undersurface of the brain

2. Connect brain with the neck and structures in the thorax and abdomen

B. Spinal nerves

1. Structure – contain dendrites of sensory neurons and axons of motor neurons

2. Function – conduct impulses necessary for sensations and voluntary movements

C. Peripheral nerve disorders

1. Neuritis – general term referring to nerve inflammation

a Sciatica is inflammation of the sciatic nerve that innervates the legs

b Neuralgia – nerve pain, often accompanies neuritis

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

3. Bell’s Palsy – paralysis of facial features resulting from damage to the facial (seventh) cranial nerve

4. Herpes zoster (shingles)

a Viral infection caused by chickenpox (varicella) virus that has invaded the dorsal root ganglion and remained dormant until an episode of shingles

b Usually affects a single dermatome, producing characteristic painful plaques or vesicles

IX. Autonomic nervous system

A. Autonomic nervous system – motor neurons that conduct impulses from the central nervous system to cardiac muscle, smooth muscle, and glandular epithelial tissue; regulates the bodies autonomic or involuntary functions

B. 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 tissue.

C. Autonomic or visceral effectors – tissues to which autonomic neurons conduct impulses

D. Composed of two divisions – the sympathetic and parasympathetic system

E. Autonomic conduction paths

1. Consist of two-neuron relays (that is, pre-ganglionic neurons from the central nervous system to autonomic ganglia, synapses, postganglionic neurons from ganglia to visceral effectors)

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

F. Sympathetic nervous systems

1. Structure

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

b 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.

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

2. Functions

a Serves as the emergency or stress system, controlling visceral effectors during strenuous exercise and strong emotions (anger, fear, hate, or anxiety)

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

G. Parasympathetic nervous system

1. Structure

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

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

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

2. Function – dominates control of many visceral effectors under normal, everyday conditions

H. Autonomic neurotransmitters

1. Cholinergic fibers – preganglionic axons of parasympathetic and sympathetic systems are parasympathetic postganglionic axons release acetylcholine.

2. Andrenergic fibers – axons of sympathetic postganglionic neurons release norepinephrine

I. Autonomic nervous system as a while

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

2. 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)

J. Disorders of the autonomic nervous system

1. Stress induced disease

a Prolonged or excessive response to stress can disrupt normal functioning throughout the body

b Examples of stress induced conditions

(1) Heart disease

(2) Digestive problems

(3) Reduced resistance to disease

2. Neuroblastoma – highly malignant tumor of the sympathetic nervous system, primarily affecting young children.