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10/4/18
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Copyright © 2016 by Elsevier Inc. All rights reserved.
Nervous System
Chapter 23
1 Copyright © 2016 by Elsevier Inc. All rights reserved.
Introduction
• Nervous system uses transmitted electrical signals called impulses to relay messages and to stimulate change
• Body’s master communication and regulating system
• Every thought, action, and sensation reflects nerve activity
2
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Anatomy
• Brain• Spinal cord• Cranial and spinal nerves• Sense organs• Neurotransmitters
3 Copyright © 2016 by Elsevier Inc. All rights reserved.
Physiology
• Sensory input• Interpretive functions• Motor output• Higher mental functioning and
emotional responsiveness
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Central Nervous System
• Interprets sensory information⎼ Issues instructions in the form of motor
responses• Governs thoughts and emotional
responses• Surrounded by bones of skull and spinal
column
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Major Components of the Central Nervous System
• Brain • Spinal cord• Meninges• Cerebrospinal fluid (CSF)
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Peripheral Nervous System
• Cranial nerves: Exit brain• Spinal nerves: Exit spinal cord• Subdivisions of PNS are:
⎼ SNS: Voluntary (responses consciously controlled)
⎼ ANS: Involuntary • ANS contains sympathetic and
parasympathetic divisions
7 Copyright © 2016 by Elsevier Inc. All rights reserved.
Cells of the Nervous System
• Nervous system made up of billions of cells
• Two types: ⎼ Neurons⎼ Neuroglia
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Neuroglia
• Supports, protects, and insulates neurons
• Smaller and more numerous than neurons⎼ Makes up more than 50% of CNS
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Neuroglia
• Glial cells in CNS are:⎼ Astrocytes⎼ Microglia⎼ Oligodendrocytes
• Glial cells in PNS are:⎼ Schwann cells⎼ Satellite cells
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Neurons
• Impulse-conducting cells • Properties include:
⎼ Excitability: Converts stimulus into impulse⎼ Conductibility: Transmits impulses to
neurons, muscles, and glands⎼ Secretion: Secretes neurotransmitters
11 Copyright © 2016 by Elsevier Inc. All rights reserved.
Parts of a Neuron
• Cell body⎼ Contains nucleus and other organelles
• Dendrite⎼ Transmits impulses to cell body
• Axon⎼ Transmits impulses away from cell body
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Myelin
• Insulation• Speed conduction
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Functional Classification of Neurons
• Afferent (sensory): Carry impulses to CNS
• Efferent (motor): Carry impulses from CNS to muscles or glands
• Interneurons (association): Carry impulses between sensory and motor neurons and perform integrative functions
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Connective Tissue Layers of Nerves
• Epineurium⎼ Outer layer around entire nerve
• Perineurium⎼ Surrounds fascicles
• Endoneurium ⎼ Surrounds individual neurons and their
extensions
15 Copyright © 2016 by Elsevier Inc. All rights reserved.
Nerve Impulses
• Electrical signals that convey information along neurons
• Action potential• Change in the electrical charge of a cell
membrane ⎼ Needed to conduct an impulse⎼ Caused by movement of charged particles
(ions)
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Sodium-Potassium Pump
• Produces and maintains polarization• Pumps ions in opposite directions at an
unequal rate• Ions are:
⎼ Sodium (Na+)⎼ Potassium (K+)
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All-or-None Response
• Impulse is conducted along entire neuron at maximum capacity⎼ No fluctuations or decrease in magnitude
• Impulse continues until it reaches end of neuron
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Refractory Period
• Inability of a neuron to conduct another impulse ⎼ Essentially, a neuron's unresponsive
period
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Synapse
• Junction between:⎼ Neuron and neuron⎼ Neuron and muscle⎼ Neuron and gland
• Impulses are transmitted across synapse by with help of neurotransmitters
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Synaptic Structures
• Synaptic bulbs ⎼ Located at end of axon⎼ Contain neurotransmitters
• Synaptic gap (cleft)⎼ Space between synaptic bulb and plasma
membrane
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Neurotransmitters
• Chemical messengers involved in synaptic transmission
• Stored in vesicles of presynaptic neuron• Can be excitatory or inhibitory
22
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Examples of Neurotransmitters
• Acetylcholine is most common• Epinephrine and norepinephrine• Serotonin• Dopamine• Endorphins
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Central Nervous System
• Brain and spinal cord• Protected by
⎼ Skull ⎼ Vertebral column⎼ Meninges⎼ Cerebrospinal fluid (CSF)
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Brain
• It interprets sensory information and governs intellectual activity, consciousness, memories, and emotions
• Uses glucose as an energy source⎼ Cerebrum⎼ Diencephalon⎼ Cerebellum ⎼ Brainstem
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Cerebrum
• Largest area of brain• Language centers interpret
written/spoken words as well as speech• Cerebral cortex covers outer region of
cerebrum⎼ Sulci and gyri separate cerebrum into
lobes
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Cerebral Hemispheres
• Cerebrum contains right and left hemispheres ⎼ Research indicates they possess
specialized functions• Longitudinal fissure
⎼ Separates hemispheres• Corpus callosum
⎼ Transverse fibers connecting hemispheres
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Hemispheric Specialization
• Left hemisphere
⎼ Language: Receptive and expressive
⎼ Governs many mathematical abilities, as
well as reasoning and analytical skills
• Right hemisphere specializes in sounds
⎼ Melodies
⎼ Art
⎼ Emotional expression
⎼ Spatial relationships
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Cerebral Lobes
• Frontal lobe⎼ Motor function⎼ Personality/mood⎼ Intellect ⎼ Speech
• Parietal lobe⎼ Sensation ⎼ Speech⎼ Understanding
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Cerebral Lobes
• Temporal lobe⎼ Auditory ⎼ Naming⎼ Memory
• Occipital lobe⎼ Vision ⎼ Whole object integration
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Brain Waves and States of Consciousness
• Consciousness: Degree of mental alertness and responsiveness
• Levels of consciousness recorded as brain wave patterns⎼ Beta⎼ Alpha⎼ Theta⎼ Delta
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Brain Wave Patterns
• Beta⎼ Wakeful consciousness and mental activity⎼ High-intensity waves are associated with
extreme stress• Alpha
⎼ Awake and relaxed
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Brain Wave Patterns
• Theta⎼ Drowsiness and dreamlike awareness,
subconscious, out-of-body experiences• Delta
⎼ Deep sleep from which the subject is not easily aroused
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Diencephalon
• Located in center of brain• Contains two primary structures:
⎼ Thalamus ⎼ Hypothalamus
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Thalamus and Hypothalamus
• Thalamus
⎼ Relays sensory information (except
olfaction) to appropriate parts of cerebrum
• Hypothalamus
⎼ Regulates ANS
⎼ Controls hunger and thirst, anger and
aggression, hormones, sexual behavior,
and sleep patterns
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Cerebellum
• Located posterior and inferior to cerebrum⎼ Concerned with muscle tone⎼ Coordinates complex muscular movements⎼ Regulates posture and balance
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Brainstem
• Vegatative function• Continuous with spinal cord • Three divisions:
⎼ Midbrain⎼ Pons⎼ Medulla oblongata
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Midbrain and Pons
• Midbrain⎼ Conducts nerve impulses from cerebrum to
pons ⎼ Conducts sensory impulses from spinal
cord to thalamus• Pons
⎼ Connects cerebellum and cerebrum with spinal cord
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Medulla Oblongata
• Transmits sensory and motor impulses between brain and spinal cord
• Contains: ⎼ Respiratory center⎼ Cardiovascular center⎼ Vasomotor center
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Spinal Cord
• Exits skull via foramen magnum • Integrating center and information
highway• Cauda equina
⎼ Lower portion of cord fans out like a horse tail
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Spinal Cord
• Cross section reveals:⎼ White matter: Located on periphery⎼ Gray matter: Located in deeper regions
• H-shaped
• Central canal⎼ Center of spinal cord; contains circulating
CSF
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Spinal Cord: Tracts
• Two types: ⎼ Ascending
• Sensory (afferent) impulses travel up cord⎼ Descending
• Motor (efferent) impulses travel down cord
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Meningeal Layers
• Pia mater⎼ Innermost delicate layer⎼ Attaches to surface of CNS
• Arachnoid⎼ Middle layer; forms loose covering around
CNS• Dura mater
⎼ Outermost dense layer; lies against bones of CNS
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Cerebrospinal Fluid
• Fluid circulating around brain and spinal cord
• Functions include:⎼ Supplies oxygen and nutrients to tissues of
brain and spinal cord⎼ Removes metabolic waste during sleep⎼ Acts like a shock absorber
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Peripheral Nervous System
• Cranial nerves⎼ Arise from inferior surface of brain⎼ 12 pairs
• Spinal nerves⎼ Arise from spinal cord ⎼ 31 pairs
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Cranial Nerves
• CN I: Olfactory - smell• CN II: Optic - sight• CN III: Oculomotor - eye movement• CN IV: Trochlear - eye movement• CN V: Trigeminal - face sensation and
chewing • CN VI: Abducens - eye movement • CN VII: Facial - facial expression
46
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Cranial Nerves
• CN VIII: Vestibulocochlear - hearing and equilibrium
• CN IX: Glossopharyngeal - oral sensation, taste and salivation
• CN X: Vagus - 90% of parasympathetic nervous system
• CN XI: Accessory (spinal accessory) -trapezius and SCM
• CN XII: Hypoglossal - tongue movement47 Copyright © 2016 by Elsevier Inc. All rights reserved.
Spinal Nerves
• 31 pairs⎼ 8 cervical nerves ⎼ 12 thoracic nerves⎼ 5 lumbar nerves⎼ 5 sacral nerves⎼ 1 coccygeal nerve
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Structure of Spinal Nerves
• Part of spinal nerve that connects to spinal cord
• Two types:⎼ Ventral (anterior)
• Contains motor neurons⎼ Dorsal (posterior)
• Contains sensory neurons
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Ganglion
• Cluster of nerve cell bodies • Ventral and dorsal roots merge to form
a single nerve
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Nerve Plexuses
• Network of intersecting nerves in PNS• Major plexuses:
⎼ Cervical⎼ Brachial⎼ Lumbosacral
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Dermatomes
• Area of skin supplied by specific sensory nerve root
• Dermatomes are named by their corresponding nerve
• Distribution called a dermatome map⎼ Each map has its own variations
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Myotomes
• Group of skeletal muscles supplied by specific motor spinal nerve root
• Distribution is called a myotome map• Sometimes used in muscle tests
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Reflexes
• Involuntary and predictable• Cranial reflex means the brain mediated
the reflex⎼ Primitive reflexes may reappear in people
with dementia• Spinal reflex means the spinal cord
mediated the reflex
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Autonomic Nervous System
• Innervates cardiac and smooth muscles/glands, thus regulating:⎼ Heart and respiration rates⎼ Blood circulation⎼ Body temperature⎼ Gastrointestinal activity and metabolism
• Two divisions: ⎼ Sympathetic and parasympathetic
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Dual Innervation
• Innervated by both sympathetic and parasympathetic divisions
• Some have only sympathetic innervation ⎼ Example: Adrenal glands
• Some have only parasympathetic innervation ⎼ Example: Lacrimal apparatus
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Parasympathetic Division
• Supports functions that conserve and restore energy
• Regulates digestion⎼ "Housekeeping" division
• Nerves emerge from the brainstem and sacrum
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Sympathetic Division
• Dominates during physical exertion or emotional stress⎼ Effects include raises in heart rate, blood
pressure, and respiration• Suppresses activity of the digestive and
urinary systems• Also called the thoracolumbar division
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Touch
• Many different types of many receptors, such as: ⎼ Pressure⎼ Movement⎼ Temperature⎼ Nociception and pain perception
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Taste
• Chemoreceptors activated when a molecule from a particular size and shape fits into a particular receptor site
• Five primary tastes:⎼ Salty⎼ Sweet⎼ Bitter⎼ Sour⎼ Savory
• Strongly influenced by smell60
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Smell
• Olfaction: Sense of smell• Scent receptors located in superior
nasal cavity⎼ Molecules fit into receptor sites;
information travels into olfactory bulb and the brain
• Plays important role in sexual behavior
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Vision
• Photoreceptors transmit visual information and send to retina
• Rods are active in dim light, responsible for night vision
• Cones are active in bright light, specialized for detecting color
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Hearing
• Mediated by mechanoreceptors ⎼ Detect sound waves and vibrations
• Sound waves vibrate tympanic membrane, causing it to vibrate⎼ Transmitted through three small bones in
middle ear• Oval window covers opening to cochlea
⎼ Sound waves travel until they reach brain
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Balance and Equilibrium
• Fluid-filled canals contain mechanoreceptors that resemble hair⎼ These cells move when we move⎼ Information is sent along nerves, notifying
brain of body movements
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Receptors
• Respond to stimuli by creating a nerve impulse⎼ Our bodies avert harm and seek nourishment
• Specific receptors generally detect specific stimuli
• Perception: Organization, identification, and interpretation of sensory information
• Adaptation: Decrease in sensitivity to prolonged stimulus
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Proprioceptors
• Found in muscles, joints, fascia, and ears
• Detect body movements, body position, and muscle stretch
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Receptors Classified by Type of Stimuli Detected
• Chemoreceptors detect chemical stimuli or changes in chemical concentrations of fluids
• Nociceptors detect noxious stimuli, such as excessive heat and cold or tissue damage
• Mechanoreceptors detect mechanical stimuli
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Muscle Spindles
• Stretch receptors wrapped around intrafusal fibers
• Monitor changes in muscle length and rate of change
• Cause reflexive contraction when muscles are overstretched
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Golgi Tendon Organs
• Activated by muscle tension • Located at musculotendinous junctions• If tension is too great, motor efferent
neurons are inhibited ⎼ Causes inverse stretch reflex
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