Chapter 8 The Nervous System

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Chapter 8The Nervous System
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Nervous System Three Branches - Central
Nervous System (CNS) - Peripheral Nervous
System (PNS) - Autonomic Nervous System
(ANS) Nervous System Physiology
Anatomy CNS PNS ANS
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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)
  Parasympathetic/Sympathetic NS

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CELLS OF THE NERVOUS SYSTEM

• Neurons
• Consist of three main partsdendrites cell body
  of neuron and axon (Figure 8-2)
• Dendrites conduct impulses to cell body of neuron
• Axons conduct 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

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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 glial cells of the CNS
  (Figure 8-3)
• Astrocytesstar-shaped cells that anchor small
  blood vessels to neurons
• Microgliasmall cells that move in inflamed brain
  tissue carrying on phagocytosis
• Oligodendrocytesform myelin sheaths on axons in
  the CNS
• Schwann cells form myelin sheaths on axons of the
  PNS (Figure 8-2)

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NERVES AND TRACTS(Figure 8-4)

• Nervebundle of peripheral axons
• Tractbundle of central axons
• White mattertissue composed primarily of
  myelinated axons (nerves or tracts)
• Gray mattertissue composed primarily of cell
  bodies and unmyelinated fibers
• Nerve coveringsfibrous connective tissue
• Endoneuriumsurrounds individual fibers within a
  nerve
• Perineuriumsurrounds a group (fascicle) of nerve
  fibers
• Epineuriumsurrounds the entire nervea

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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
  (that is, contraction by a muscle or secretion by
  a gland)
• The simplest reflex arcs are two-neuron
  arcsconsisting 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)

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NERVE IMPULSES

• Definitionself-propagating wave of electrical
  disturbance that travels along the surface of a
  neuron membrane
• Mechanism
• A stimulus triggers the opening of Na channels
  in the plasma membrane of the neuron
• 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

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THE SYNAPSE

• Definitionchemical compounds released from axon
  terminals (of a presynaptic neuron) into a
  synaptic cleft
• 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 neurotransmittersacetylcholine,
  catecholamines (norepinephrine, dopamine, and
  serotonin), and other compounds

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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
• Structurewhite matter with bits of gray matter
  scattered through it
• Functiongray matter in the brainstem functions
  as reflex centers (e.g., for 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

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CENTRAL NERVOUS SYSTEM

• Divisions of the brain (cont.)
• Diencephalon
• Structure and function of the 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
• Structure and function of the thalamus
• Dumbbell-shaped mass of gray matter in each
  cerebral hemisphere
• Relays sensory impulses to cerebral cortex
  sensory areas
• In some way produces the emotions of pleasantness
  or unpleasantness associated with sensations

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CENTRAL NERVOUS SYSTEM

• Divisions of the brain (cont.)
• Cerebellum
• Second largest part of the human brain
• Helps control muscle contractions to produce
  coordinated movements so that we can maintain
  balance, move smoothly, and sustain normal
  postures
• Cerebrum
• Largest part of the human brain
• Outer layer of gray matter is 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 (that is, nerve fibers arranged in bundles
  called tracts)
• Functions of the cerebrummental processes of all
  types, including sensations, consciousness,
  memory, and voluntary control of movements

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CENTRAL NERVOUS SYSTEM

• Spinal cord (Figure 8-11)
• 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
• Functions as the center for all spinal cord
  reflexes sensory tracts conduct impulses to the
  brain, and motor tracts conduct impulses from the
  brain

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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 meningesthe dura mater, the
  pia mater, and the arachnoid mater (Figure 8-13)
• Fluid spacessubarachnoid spaces of meninges,
  central canal inside cord, and ventricles in
  brain (Figure 8-14)

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PERIPHERAL NERVOUS SYSTEM

• Cranial nerves (Figure 8-16 and Table 8-2)
• Twelve pairsattached to undersurface of the
  brain
• Connect brain with the neck and structures in the
  thorax and abdomen
• Spinal nerves
• Structurecontain dendrites of sensory neurons
  and axons of motor neurons
• Functionsconduct impulses necessary for
  sensations and voluntary movements

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AUTONOMIC NERVOUS SYSTEM

• Autonomic nervous systemmotor neurons that
  conduct impulses from the central nervous system
  to cardiac muscle, smooth muscle, and glandular
  epithelial tissue regulates the bodys automatic
  or involuntary functions (Figure 8-18)
• Autonomic neuronspreganglionic 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 epithelial tissue

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