mosby items and derived items © 2010, 2006, 2002, 1997, 1992 by mosby, inc., an affiliate of...

Mosby items and derived items © 2010, 2006, 2002, 1997, 1992 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 9The Nervous System

Mosby items and derived items © 2010, 2006, 2002, 1997, 1992 by Mosby, Inc., an affiliate of Elsevier Inc.2

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)


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



• 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



• 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)


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


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


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


Nerve Impulses

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


The Synapse

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


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



• Divisions of the brain – Brainstem

• Functions– 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



– Diencephalon• Hypothalamus Controls 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

Thalamus-sensations, emotions, awareness sensation



– Cerebellum• Second largest part of the human brain• Equilibrium, balance and posture



– Cerebrum• Largest part of the human brain

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



• Spinal cord-• Occipital bone to the bottom of the first lumbar vertebra

– Spinal cord tracts provide two-way conduction paths—ascending -to brain and descending-from brain

– 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

– Spinal nerves named for what part of the vetebrae



• 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


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


Autonomic Nervous System

• Autonomic nervous system—motor neurons that conduct impulses from the central nervous system to

• cardiac muscle• smooth muscle• glandular epithelial tissue

• regulates body’s automatic or involuntary functions



• Composed of two divisions: • sympathetic system (SNS)• parasympathetic system (PSNS)

• Visceral effectors-cardiac, smooth, glandular

• Somatic effectors-skeletal



– 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



• Parasympathetic nervous system-

• Normal everyday conditions-• Slow heartbeat, increase peristalsis, increase

secretion of digestive juices and insulin



• Neurotransmitters-chemicals by which neurons communicate-examples:

• Endorphins, serotonin, dopamine, nitric oxide--Cholinergic fibers—acetylcholine– Adrenergic fibers—epinephrine/adrenalin



• 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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