The Nervous System

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25
The Nervous System
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Learning Outcomes

• 25.1 Explain the difference between the central
  nervous system and the peripheral nervous system.
• 25.2 Describe the functions of the nervous
  system.
• 25.3 Describe the structure of a neuron.
• 25.4 Describe the function of a nerve impulse and
  how a nerve impulse is created.

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Learning Outcomes (cont.)

• 25.5 Describe the structure and function of a
  synapse.
• 25.6 Describe the function of the blood-brain
  barrier.
• 25.7 Describe the structure and functions of
  meninges.
• 25.8 Describe the structure and functions of the
  spinal cord.

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Learning Outcomes (cont.)

• 25.9 Describe the location and function of
  cerebrospinal fluid.
• 25.10 Define reflex and list the parts of a
  reflex arc.
• 25.11 List the major divisions of the brain and
  give the general functions of each.
• 25.12 Explain the functions of the cranial and
  spinal nerves.

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Learning Outcomes (cont.)

• 25.13 Describe the differences between the
  somatic nervous system and autonomic nervous
  system.
• 25.14 Explain the two divisions of the autonomic
  nervous system.
• 25.15 Describe the causes, signs and symptoms,
  and treatments of various diseases and disorders
  of the nervous system.

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Introduction

• Highly complex system of two parts
• Central nervous system (CNS)
• Peripheral nervous system (PNS)

• Controls all other organ systems and is important
  for maintaining balance within those systems

Disorders are numerous and often difficult to
diagnose and treat
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General Functions of the NS

• CNS
• Brain
• Spinal cord

• PNS
• Peripheral nerves
• Two sections
• Somatic nervous system (SNS)
• Skeletal or voluntary muscles
• Autonomic nervous system (ANS)
• Automatic functions

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General Functions (cont.)

• Three types of neurons
• Afferent or sensory nerves
• Sensory information from environment or inside
  body to CNS for interpretation
• Efferent or motor nerves
• Impulses from CNS to PNS to allow for movement or
  action
• Interneurons
• Interpretive neurons between afferent and
  efferent nerves in the CNS

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Apply Your Knowledge
ANSWER
Match the following ___ Somatic nervous
system A. Motor nerves ___ Autonomic nervous
system B. Governs skeletal or voluntary
muscles ___ Afferent nerves C. Governs
respiratory and GI systems ___ Efferent
nerves D. Go-betweens or interpreters ___
Interneurons E. Sensory nerves
B
C
A
E
D
Correct!
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Neuron Structure

• Functional cells of NS
• Transmit electrochemical messages called nerve
  impulses to
• Other neurons
• Effectors (muscles or glands)

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Neuron Structure (cont.)

• Neurons lose their ability to divide
• If destroyed, not replaced
• Neuralgia
• Support cells for neurons that can divide
• Astrocytes anchor blood vessels to nerves
• Microglia act as phagocytes
• Oligodendrocytes assist with production of
  myelin sheath

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Neuron Structure (cont.)

• Dendrites short
• Receive nerve impulses for the neuron
• Axons long
• Send nerve impulses away from the cell body

• Neurons have a cell body and processes called
  nerve fibers that extend from the cell body.

13
Neuron Structure (cont.)

• White matter axons with myelin sheath
• Schwann cells
• Wrap around some axons
• Cell membranes contain myelin
• Myelin insulates axons and enables axons to send
  nerve impulses more quickly
• Gray matter axons without myelin sheath

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Apply Your Knowledge
True or False ___ Effectors are neurons. ___
Neurons can reproduce. ___ Astrocytes anchor
blood vessels to nerve cells. ___ Microglia act
as phagocytes. ___ Oligodendrocytes are
reproductive cells. ___ Repolarization is the
return to the resting state.
ANSWER
They are the muscles or glands.
F
F
Neurons cannot reproduce.
T
T
They take part in myelin production.
F
T
GOOD JOB!
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Nerve Impulse

• Membrane potential
• Neuron cell membrane at rest is in a polarized
  state
• Inside of cell membrane is negative
• Outside of cell membrane is positive due to more
  Na and K
• As Na and K move into the cell, the membrane
  becomes depolarized
• Inside becomes more positive
• Action potential (nerve impulse) is created
• Repolarization occurs when K and later Na move
  to the outside of the cell membrane
• Return of the cell to polarized (resting) state

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Nerve Impulse (cont.)

• Impulse travels down axon to synaptic knob
• Vesicles or small sacs in synaptic knob
• Produce chemicals called neurotransmitters
• Neurotransmitters are released by synaptic knob
• Allow impulse transmission to postsynaptic
  structures
• Dendrites
• Cell bodies
• Axons of other neurons

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Nerve Impulse (cont.)

• Functions of neurotransmitters
• Cause muscles to contract or relax
• Cause glands to secrete products
• Activate or inhibit neurons

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Apply Your Knowledge
What is the function of neurotransmitters?
ANSWER Neurotransmitters cause muscles to
contract or relax, cause glands to secret
products, activate neurons to send nerve
impulses, or inhibit neurons from sending them.
Right!
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Central Nervous System

• Includes the spinal cord and brain
• Blood-brain barrier
• Protects layers of the membranes of the CNS
• Formed by tight capillaries
• Prevents unwanted substances from entering the
  CNS tissues
• Inflammation can make more permeable

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Central Nervous System (cont.)

• Meninges protect brain and spinal cord
• Dura mater
• Tough outer layer
• Arachnoid mater
• Middle layer (web-like)
• Pia mater
• Innermost and most delicate
• Directly on top of brain and spinal cord
• Holds blood vessels on the surface of these
  structures

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Central Nervous System (cont.)

• Epidural space
• Above dura mater
• Subdural space
• Below dura mater
• Subarachnoid space
• Between arachnoid mater and pia mater
• Contains cerebrospinal fluid (CSF)
• Cushions CNS

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Spinal Cord

• Slender structure continuous with the brain
• Descends into the vertebral canal and ends around
  the level of the first or second lumbar vertebra
• 31 spinal segments
• 8 cervical segments
• 12 thoracic segments
• 5 lumbar segments
• 5 sacral segments
• 1 coccygeal segment  

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Spinal Cord (cont.)

• Cervical enlargement
• Motor neurons that controlmuscles of the arms
• Lumbar enlargement
• Motor neurons that control muscles of the legs

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Spinal Cord (cont.)

• Gray matter
• Inner tissue with darker color
• Contains neuron cell bodies and their dendrites
• Divisions are called horns
• Central canal runs down the entire length of the
  spinal cord through the center of the gray
  matter

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Spinal Cord (cont.)

• White matter
• Outer tissue
• Contains myelinated axons
• Divisions are called columns (funiculi)
• Columns contain groups of axons called nerve
  tracts

Spinal Cord/Nerve
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Spinal Cord (cont.)
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Spinal Cord (cont.)

• One function of the spinal cord is to carry
  sensory information to and from the brain
• Ascending tracts
• Carry sensory information up to the brain
• Descending tracts
• Carry motor information down from the brain to
  muscles and glands

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Spinal Cord (cont.)

• Reflexes
• Another function of the spinal cord is to
  participate in reflexes
• Reflex a predictable, automatic response to
  stimuli

Receptor
Sensory Neurons
Effectors
Motor Neurons
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Brain

• Four sections
• Cerebrum
• Diencephalon
• Brain stem
• Cerebellum

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Cerebrum

• Largest section
• Two cerebral hemispheres
• Connected by a thick bundle of nerve fibers
  called the corpus callosum
• Longitudinal fissure between hemispheres
• Sulci grooves on surface
• Gyri or convolutions bumps of brain matter
  between sulci

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Cerebrum (cont.)

• Lobes
• Frontal
• Motor areas for voluntary body movements
• Parietal
• Somatosensory interprets sensations
• Temporal
• Auditory interprets sounds
• Occipital
• Interprets what a person sees

Parietal
Frontal
Temporal
Occipital
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Cerebrum (cont.)

• Cortex
• Outer layer gray matter
• Contains about 75 of all neurons
• Inner layer white matter
• Functions
• Interpret sensory information
• Initiate body movements
• Stores memories and creates emotions

• Ventricles
• Interconnected cavities within the brain
• Filled with CSF

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Diencephalon

• Between the cerebral hemispheres superior to the
  brain stem
• Thalamus
• Relay station for sensory information going to
  the cerebral cortex for interpretation
• Hypothalamus
• Maintains homeostasis by regulating vital
  activities

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Brain Stem

• Connects the cerebrum to the spinal cord
• Midbrain
• Just beneath diencephalon
• Controls both visual and auditory reflexes
• Pons
• Rounded bulge on underside of brain stem
• Between midbrain and medulla oblongata
• Regulates respiration

• Medulla oblongata
• Inferior portion of brain stem
• Directly connected to spinal cord
• Controls many vital activities, such as heart
  rate, blood pressure, and breathing

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Cerebellum

• Location
• Inferior to the occipital lobes of the cerebrum
• Posterior to the pons and medulla oblongata
• Coordinates
• Complex skeletal muscle contractions that are
  needed for body movements
• Fine movements

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Preventing Brain and Spinal Cord Injuries

• Wear appropriate protective gear for sports
• Avoid diving into unknown waters
• Always wear seat belts
• Follow safety rules on playgrounds

• Children should be in car seats appropriate for
  age and weight
• Know how to get help quickly in emergencies
• Follow traffic rules and signs while walking,
  biking, or driving

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Apply Your Knowledge
SUPER!
Answer
Match the following ___ Meninges A. Carry motor
information from brain ___ Ascending tracts B.
Stores memories and creates emotions ___
Descending tracts C. Grooves on the surface of
the cerebrum ___ Cerebral cortex D. Carry sensory
information to the brain ___ Hypothalamus E.
Predictable, automatic response to stimuli ___
Sulci F. Maintains homeostasis ___
Cerebellum G. Coordinates skeletal muscle
contractions ___ Reflexes H. Protects the brain
and spinal cord
H
D
A
B
F
C
G
E
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Peripheral Nervous System

• Nerves that branch off the CNS
• Peripheral nerves
• Two types
• Cranial nerves
• Spinal nerves

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Cranial Nerves

• I.   Olfactory nerves
• Carry smell information to the brain for
  interpretation
• II.  Optic nerves
• Carry visual information to the brain for
  interpretation
• III. Oculomotor nerves
• Found within the muscles that move the eyeball,
  eyelid, and iris
• IV. Trochlear nerves
• Act in the muscles that move the eyeball.

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Cranial Nerves (cont.)

• V.  Trigeminal nerves
• Carry sensory information from the surface of the
  eye, the scalp, facial skin, the lining of the
  gums, and the palate to the brain for
  interpretation
• Also found within the muscles needed for chewing
• VI. Abducens nerves
• Act in the muscles that move the eyeball

42
Cranial Nerves (cont.)

• VII. Facial nerves
• Found in the muscles of facial expression as well
  as in the salivary and tear glands
• Also carry sensory information from the tongue
• VIII. Vestibulocochlear nerves
• Carry hearing and equilibrium information from
  the inner ear to the brain for interpretation

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Cranial Nerves (cont.)

• IX. Glossopharyngeal nerves
• Carry sensory information from the throat and
  tongue to the brain for interpretation
• Also act in the muscles of the throat
• X. Vagus nerves
• Carry sensory information from the thoracic and
  abdominal organs to the brain for interpretation
• Also found within the muscles in the throat,
  stomach, intestines, and heart

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Cranial Nerves (cont.)

• XI. Accessory nerves
• Found within the muscles of the throat, neck,
  back, and voice box
• XII. Hypoglossal nerves
• Found within the muscles of the tongue

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Spinal Nerves

• Peripheral nerves originating from the spinal
  cord
• 31 pairs of spinal nerves
• 8 pairs of cervical nerves (C1 through C8)
• 12 pairs of thoracic nerves (T1 through T12)
• 5 pairs of lumbar nerves (L1 through L5)
• 5 pairs of sacral nerves (S1 through S5)
• 1 pair of coccygeal nerves (C0)

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Spinal Nerves (cont.)

• Dermatome
• Skin segment innervated by spinal nerve
• C1 is not associated with a dermatome
• Ventral root
• Axons of motor neurons only
• Dorsal root
• Axons of sensory neurons only
• Dorsal root ganglion
• Contains cell bodies of sensory neurons

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Spinal Nerves (cont.)

• Main portions of spinal nerves fuse to form nerve
  plexuses
• Cervical supply skin and muscles of neck
• Phrenic nerve originates from this plexus and
  controls the diaphragm
• Brachial control muscles in the arms
• Lumbosacral lower abdomen, external genitalia,
  buttocks, thighs, legs, and feet
• Sciatic nerve originates from this plexus and
  controls muscles of legs

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Somatic Nervous System

• Nerves that connect the CNS to skin and skeletal
  muscle
• Voluntary nervous system
• Controls skeletal muscles, which are under
  voluntary control

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Autonomic Nervous System

• Nerves that connect the CNS to organs and other
  structures
• Involuntary nervous system
• Motor neurons located in ganglia
• Neuron cell bodies outside the CNS

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Autonomic Nervous System (cont.)

• Sympathetic division
• Fight or flight Prepares body for stressful
  or emergency situations
• Neurons release neurotransmitter norepinephrine
• Increases heart and breathing rates
• Slows down muscles of the stomach and intestines
• Dilates pupils
• Constricts blood vessels increases blood
  pressure

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Autonomic Nervous System (cont.)

• Parasympathetic division
• Prepares body for rest and digesting
• Controls most of the bodys organs
• Releases acetylcholine
• Slows heart and breathing rates
• Constricts pupils
• Activates muscles of stomach and intestine
• No communication with blood vessels
• Sympathetic and parasympathetic divisions are
  antagonistic

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Apply Your Knowledge
What is the difference between the sympathetic
and parasympathetic nervous systems?
ANSWER The sympathetic nervous system prepares
the body for stress. It releases norepinephrine,
causing an increase in heart and respiratory
rate, slows down the GI system, and dilates
pupils. The sympathetic system also controls
constriction of blood vessels. The
parasympathetic system prepares the body for
resting and digesting. It releases acetylcholine,
which slows heart and respiratory rates,
constricts pupils, and stimulates the GI system.
It has no effect on most blood vessels.
Impressive!
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Neurologic Testing

• Typical neurologic examination
• State of consciousness
• Reflex activity
• Speech patterns
• Motor patterns

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Diagnostic Procedures

• Lumbar puncture
• Magnetic resonance imaging (MRI)
• Positron emission tomography (PET) scan
• Cerebral angiography

• Computerized tomography (CT) scan
• Electroencephalogram (EEG)
• X-ray

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Cranial Nerve Tests

• Olfactory nerves (Cranial nerve I)
• Have patient smell various substances
• Cranial nerves III, IV, and VI
• Have patient track movement of finger
• Cranial nerve V
• Have patient clench teeth, feel jaw muscles

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Cranial Nerve Tests (cont.)

• Cranial nerve VII
• Check patients facial expression
• Cranial nerve XII
• Have patient extend his tongue and move it from
  side to side

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Reflex Testing

• Areflexia
• Absence of a reflex
• Hyporeflexia
• Decreased reflex
• Hyperreflexia
• Stronger-than-normal reflex

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Reflex Testing (cont.)

• Biceps reflex
• Absence indicates spinal cord damage in the
  cervical region
• Knee reflex
• Absence may indicate damage to lumbar or femoral
  nerves
• Abdominal reflexes
• Used to evaluate damage to thoracic spinal nerves

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Apply Your Knowledge
Excellent!
Match the following ___ State of
consciousness A. Determines the health of
peripheral nerves ___ Reflex activity B. Loss
of balance, abnormal posture ___ Speech
patterns C. Stupor, delirium, vegetative ___
Motor patterns D. Loss of ability to form words
ANSWER
C
A
D
B
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Common Diseases and Disorders
Disease/Disorder Description
Alzheimers disease Progressive, degenerative disease of the brain
Amyotrophic lateral sclerosis (ALS) Lou Gehrigs disease Degeneration of neurons in the spinal cord and brain
Bells palsy Weak or paralyzed facial muscles
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Common Diseases and Disorders (cont.)
Disease/Disorder Description
Brain tumors and cancers Abnormal growths Can be primary or secondary tumors Most common gliomas
Epilepsy and seizures Occurs as a result of bursts of electrical signals that disrupt normal brain functioning
Guillain-Barré Syndrome Bodys immune system attacks the PNS
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Common Diseases and Disorders (cont.)
Disease/Disorder Description
Headaches Tension Migraines Cluster Episodic or chronic With aura/without aura Form of migraines occurs in groups
Meningitis Inflammation of meninges
Multiple sclerosis (MS) Chronic disease of CNS Myelin is destroyed
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Common Diseases and Disorders (cont.)
Disease/Disorder Description
Neuralgias Disorders causing nerve pain
Parkinsons disease Progressive and degenerative motor system disorder
Sciatica Damage to sciatic nerve
Stroke Brain cells die because of an inadequate blood flow brain attack
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Apply Your Knowledge
Bravo!
ANSWER
True or false ___ Alzheimers disease is a
progressive degenerative disease of the
brain. ___ Epilepsy is due to degenerative
neurons in the spinal cord and brain. ___
Neuralgias are motor system disorders. ___
Stroke occurs when brain cells die because of
inadequate blood flow.
T
F
Due to a burst of electrical signals that disrupt
brain function.
F
They are group of disorders referred to as nerve
pain.
T
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In Summary

• 25.1 The central nervous system is composed of
  the brain and spinal cord. The peripheral nervous
  system consists of the peripheral nerves located
  throughout the body. The peripheral nervous
  system is further divided into two distinct
  systems the somatic and autonomic nervous
  systems.
• 25.2 There are three separate types of neurons to
  carry out the functions of the nervous system.
  The afferent (sensory) nerves, the efferent
  (motor) nerves, and the interpretive interneurons
  that act as interpreters between the afferent
  and efferent nerves.

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In Summary (cont.)

• 25.3 All neurons are composed of a cell body, the
  shorter and more numerous dendrites that receive
  information for the cell body, and the longer
  axons that bring an impulse from the cell body to
  the dendrite of the next neuron.
• 25.4 The function of a nerve impulse is the
  sending of information either from the CNS to the
  PNS or vice versa.
• At rest, a neuron is said to be in a polarized
  state, and when it responds to stimuli,
  depolarization takes place.
• Repolarization occurs after the electrical
  current (impulse) has passed.

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In Summary (cont.)

• 25.5 A synapse is the space between the axon of
  one neuron and the dendrite of the next. At the
  end of each axon is the synaptic knob, which
  contains vesicles that produce neurotransmitters.
  These are released by the synaptic bulb to allow
  impulse transmission to continue to the next
  neuron.
• 25.6 The blood-brain barrier is a layer of
  tightly woven capillaries that protects the
  delicate tissues of the CNS. This barrier
  prevents certain substances, such as various
  waste products and drugs, from easily crossing
  this barrier and entering the brain tissues

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In Summary (cont.)

• 25.7 The meninges, a triple-layered membrane
  protecting the brain and spinal cord, are
  composed of the following layers
• The tough outer layer is the dura mater.
• The space between the cranium and the dura mater
  is called the epidural space.
• Below the dura mater is the subdural space.
• The middle layer is the arachnoid, and below it
  is the subarachnoid space, which contains
  cerebrospinal fluid (CSF).
• The innermost layer is the pia mater.

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In Summary (cont.)

• 25.8 The spinal cord is continuous with the brain
  and consists of 31 spinal segments
• 8 cervical 5 sacral
• 12 thoracic 1 coccygeal
• 5 lumbar
• The basic function of the spinal cord is to
  carry sensory information from the body to the
  brain and motor information from the brain to the
  muscles and glands of the body.

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In Summary (cont.)

• 25.9 Cerebrospinal fluid (CSF) is located within
  the subarachnoid space of the brain and within
  the central canal of the spinal cord. It cushions
  the brain and spinal cord.
• 25.10 A reflex is a predictable automatic
  response to a stimulus. A typical reflex flows as
  follows Sensory receptors send information
  (impulse) to the interneurons, which in turn send
  the information on to the effectors of the motor
  neurons, producing a response.

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In Summary (cont.)

• 25.11 The cerebrum is the largest part of the
  brain and is divided into two hemispheres
  subdivided into four lobes frontal, parietal,
  temporal and occipital.
• The diencephalon contains the thalamus and
  hypothalamus.
• The brain stem consists of three parts the
  midbrain, pons, and medulla oblongata.
• The cerebellum is the final area of the brain.

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In Summary (cont.)

• 25.12 Twelve cranial nerves carry sensory
  information
• Olfactory Optic Oculomotor
• Trochlear Trigeminal Abducens
• Facial Vagus Vestibulocochlear
• Glossopharyngeal Accessory Hypoglossal
• There are 31 pairs of spinal nerves, each except
  C1 innervating a dermatome. Refer to Figure 25.8
  for a map of the spinal nerves.

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In Summary (cont.)

• 25.13 The somatic nervous system connects the CNS
  to the skin and skeletal muscle (voluntary
  functions).
• The autonomic nervous system connects the CNS to
  the internal organs (involuntary functions).
• 25.14 The autonomic nervous system is divided
  into the sympathetic system, which prepares the
  body for fight or flight (stressful)
  situations, and the parasympathetic system, which
  is the bodys everyday resting system for
  normal situations.

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In Summary (cont.)

• 25.15 There are many and varied diseases and
  disorders related to the nervous system. The
  signs, symptoms, and treatments of these diseases
  are as varied as the diseases themselves. Many of
  the more common diseases are included in the
  Pathophysiology section at the end of this
  chapter.

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End of Chapter 25
Activity of the nervous system improves the
capacity for activity, just as exercising a
muscle makes it stronger. Dr. Ralph Gerard