Mosby items and derived items © 2010, 2006, 2002, 1997, 1992 by Mosby, Inc., an affiliate of Elsevier Inc. Chapter 9 The Nervous System
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Chapter 9The Nervous System
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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)
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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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Cells of the Nervous System
• 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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Cells of the Nervous System
• 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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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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The Synapse
• Definition—the place where impulses are transmitted from one neuron to another (the postsynaptic neuron)
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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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Central Nervous System
• 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
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Central Nervous System
– 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
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Central Nervous System
– Cerebellum• Second largest part of the human brain• Equilibrium, balance and posture
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Central Nervous System
– 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
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Central Nervous System
• 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
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Central Nervous System
• 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
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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• glandular epithelial tissue
• regulates body’s automatic or involuntary functions
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Autonomic Nervous System
• Composed of two divisions: • sympathetic system (SNS)• parasympathetic system (PSNS)
• Visceral effectors-cardiac, smooth, glandular
• Somatic effectors-skeletal
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Autonomic Nervous System
– 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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Autonomic Nervous System
• Parasympathetic nervous system-
• Normal everyday conditions-• Slow heartbeat, increase peristalsis, increase
secretion of digestive juices and insulin
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Autonomic Nervous System
• Neurotransmitters-chemicals by which neurons communicate-examples:
• Endorphins, serotonin, dopamine, nitric oxide--Cholinergic fibers—acetylcholine– Adrenergic fibers—epinephrine/adrenalin
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Autonomic Nervous System
• 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)