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The Nervous System

Master controlling and
coordinating system of
body:



Sensory input (incoming)
 E.g.. seeing, hearing, etc.
Integration (processing)
 CNS (brain & spinal cord)
Motor output (outgoing)
 Responding to stimulus
Structural Classification

Central Nervous System (CNS)
 Integrating command center
 Brain
 Spinal cord
Structural Classification (cont’d)

Peripheral Nervous
System (PNS)
 Communication
lines
 12 pair of
cranial nerves
 31 pair of
spinal nerves
Functional Classification


Only PNS
structures
2 subdivisions
 Sensory or
afferent division
 Convey
impulses
to the CNS from
sensory receptors
Functional Classification
(cont’d)
 Motor
or efferent division
 Carries
impulses from CNS to
effector organs
 “effect” a motor response
Functional Classification
(cont’d)
 Motor
divided into
2 subdivisions
 Somatic
(voluntary)
nervous system
 Allows us to
control
skeletal
muscles
Functional Classification
(cont’d)
 Autonomic
nervous
system (ANS)
(involuntary)
 Sympathetic
 Parasympathetic
Nervous Tissue

2 types:
 Supporting cells
 Neurons
Neuroglia
“nerve glue”
 Supporting cells in CNS
 Do not conduct impulses
 Support, insulate, & protect
the neurons
 Also called glia
 Never lose ability to divide

Types of Neuroglia

Astrocytes



Star shaped
Abundant (half of
neural tissue)
Anchor the nerve
cells in place and
connect them to
the blood supply
Types of Neuroglia (cont’d)

Microglia
 Spiderlike,
very small
 Phagocytes
 Dispose of
dead brain
cells,
bacteria, etc.
Types of Neuroglia (cont’d)

Ependymal cells


Line the nervous system cavities (in
brain and inside spinal cord)
By ciliary action, they move CSF
Types of Neuroglia (cont’d)

Oligodendrocytes
(oligodendroglia)
 Produces a
fatty insulating
covering called
myelin sheath
 Wraps around
nerve fibers in
CNS
Supporting Cells in PNS


Schwann Cells
 Form the myelin sheath around
nerve fibers in PNS
Satellite Cells

Covering, protective, cushioning
cells
Neurons

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Nerve cells
Transmit nerve impulses from
one part of the body to another
Structure of Neuron

Cell body

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Contains nucleus
Dendrites

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Carry impulses
toward the cell
body
Frequently
multiple
Structure of Neuron
(cont’d)

Axons

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Carry impulse
away from cell
body
Usually singular
Structure of Neuron
(cont’d)

Axonal terminals
(knobs, boutons)

Releases a chemical
(neurotransmitter) into
a space (synaptic cleft
or synapse) between
neurons
Structure of Neuron (cont’d)

Myelin
Whitish, fatty material
surrounding long nerve fibers
 Protects and insulates the
fibers
 Increases transmission rate of
nerve impulses

Structure of Neuron (cont’d)

Myelin (cont’d)
Axons in PNS myelinated by
Schwann cells
 Myelin sheath
 Neurilemma – cytoplasm
 Axons in CNS myelinated by
oligodendrocytes

 Lack
neurilemma
Structure of Neuron
(cont’d)

Nodes of
Ranvier
 Gap (nonmyelinated)
from one
Schwann cell
to next
Structure of Neuron (cont’d)

Clusters of neurons cell bodies
and collections of nerve fibers
named differently when in CNS
or PNS:
 Cell bodies:
 CNS – nuclei
 PNS - ganglia
Structure of Neuron (cont’d)
 Bundles
 CNS
of nerve fibers:
– tracts
 PNS - nerves
White & Gray Matter
Refer to CNS
 White matter
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Dense collections of myelinated
fibers (tracts)
Gray Matter
 Mostly unmyelinated fibers
and cell bodies
Functional Classification

Which way information is
traveling in relation to CNS
Functional Classification
(cont’d)
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Sensory (afferent) neurons
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Carry info to CNS
Cell bodies located in ganglia in PNS
Dendrites are associated with receptors
(heat, pain, sight, smell, etc.)
Functional Classification
(cont’d)
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Motor (efferent) neurons
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Carry impulses from CNS to viscera,
muscles, and glands
Cell bodies located in CNS
Functional Classification
(cont’d)

Association (interneurons)
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In-between
Connect motor and sensory neurons
Cell bodies in CNS
Structural Classification

Multipolar

Several processes extend from cell
body
 One axon and many dendrites
 Ex. Motor neurons,
interneurons
Structural Classification (cont’d)

Bipolar

One axon and one dendrite off the
cell body
 Rare
 Located in nose, retina of eye
 Sensory receptor cells
Structural Classification (cont’d)

Unipolar

Single extension from cell body that
divides into 2
 Axon carries info to and from cell
body
 Ex. Sensory neurons in PNS
Regeneration

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Not very much
Ability to replace neurons not
possible
In PNS – a little bit of
regeneration – neurilemma plays
a role in fiber regeneration
In CNS - none
Nerve Impulses

Movement of ions (Na+, K+)
across a cell membrane of
neuron
Resting Membrane

Resting, inactive neuron –
plasma membrane is polarized.
 Na+ more concentrated outside
& K+ more inside
 Inside more negative than
outside
 Resting membrane potential
 -70mv
Resting Membrane (cont’d)

As long as inside remains more
negative than outside - neuron
will stay inactive
Depolarization & Action
Potential
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Neurons respond to changes in
environment – excitability
If stimulus is above minimum
level, the membrane
permeability changes for Na+
ions.
-50mv to –55mv threshold – now
action potential will happen
Depolarization & Action Potential
(cont’d)
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Opens Na+ gates on cell – Na+
ions move inward
Makes inside positive and
outside negative
+30 mv
Depolarization & Action Potential
(cont’d)
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Depolarizes membrane down
entire length of neuron
Action potential are “all or none”
Repolarization
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K+ gates open
K+ moves out
Inside – negative
Outside –
positive
More K+ outside
and Na+ inside
Ions in wrong
place
Repolarization(cont’d)



Na+-K+ pump puts Na+ outside
and K+ inside
Return to –70 mv
Refractory period

No nerve impulses conducted until
ions are in proper place
Conduction of a Nerve Impulse
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Action potential spreads along
the length of the neuron
Causes a wave of negative
charges
Conduction of a Nerve Impulse
(cont’d)

Impulses increase in
myelinated fibers
 Saltatory
conduction
 Jumps from node of
Ranvier to node of
Ranvier
 Much faster than on
unmyelinated fibers
 130m/sec vs
1m/sec
Synaptic Transmission

Prior to a stimulus, the axonal knob
contains ACh
 Other neurotransmitters
 Serotonin
 Dopamine
 Norepinephrine
 GABA
 Glutamate
 Endorphin
Synaptic Transmission (cont’d)

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Neuron receives a threshold
stimulus
Impulse moves down the fiber to
the axonal knob
Knob (vesicle) releases ACh into
the synaptic cleft
Synaptic Transmission (cont’d)
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When second neuron is stimulated,
AChE released into synaptic cleft digests ACh (in 1/500 sec.)
Synaptic stimulus stops
Second neuron stops firing
2 molecules – Acetyl and Choline are
reabsorbed into vesicle of axonal
knob. Restructured into ACh for use
again.
Synaptic Transmission (cont’d)

Neurons can exhibit synaptic
fatigue – may stop firing –
because of lack of energy
Subthreshold Impulse

Will do nothing to illicit a
response unless there is
summation (additive effect) of
several subthreshold stimuli
Reflexes

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Rapid, predictable, involuntary
responses to stimuli
Autonomic reflexes
 Regulate the activity of smooth
muscles, the heart and glands
 Digestion, elimination, blood
pressure, sweating
 Salivary reflex and pupillary
reflex
Reflexes (cont’d)

Somatic reflexes
 Stimulate the skeletal muscles
Reflexes (cont’d)

Reflexes occur over neural
pathways called reflex arcs.
Reflexes (cont’d)

All reflex arcs have a minimum of 5
elements:
 Sensory receptor (reacts to stimulant)
 Effector organ (the muscle or gland
eventually stimulated)
 Afferent and efferent neurons to connect
the two
 Synapse between afferent and efferent
neurons – the CNS integration center
Reflexes (cont’d)

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Two-neuron reflex arc
 Simplest type in humans
 No interneuron
Three-neuron reflex arc
 Has an interneuron
 Most reflexes are bisynaptic
The more synapses in a reflex
pathway, the longer the reflex takes
to happen
Reflexes (cont’d)

Spinal reflex – impulse does not
have to go to the brain

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Ex. Knee-jerk reflex
Cranial reflex
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Ex. Pupillary reflex
Spinal Cord
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Elongated extension of CNS
Two-way conduction pathway to
and from the brain
Major reflex center
Spinal Cord (cont’d)

Protected by meninges

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Dura mater
Arachnoid
Pia mater
Spinal Cord (cont’d)

Cross section of
spinal cord
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Outer white matter
(myelinated)
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Inner gray matter

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Contains ascending
and descending
tracts
Association neurons
Central canal - CSF
Spinal Cord (cont’d)

Spinal nerves
exit between
vertebrae
Spinal Cord (cont’d)
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Spinal cord
terminates at L1 (1st
lumbar vertebra)
Swellings at
cervical and lumbar
regions for the
limbs
Below L1, region
still has nerves –
cauda equina
PNS

Nerves and scattered groups of
neuronal cell bodies (ganglia)
outside of CNS
Structure of a Nerve

Nerve - bundle of neuron fibers
found outside of CNS
Structure of a Nerve (cont’d)
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Each fiber surrounded by
endoneurium
Perineurium –wrapping
that forms fiber bundles
– fascicles
Fascicles bound together
by epineurium to form
nerve
Classification of nerves

Classified according to direction
they transmit impulses
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Mixed – carry both sensory and
motor fibers
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All spinal are mixed
Afferent (sensory) – towards CNS
Efferent (motor) – carry only motor
fibers away
Cranial Nerves
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12 pairs
Primarily serve head and neck
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Vagus extends to ventral cavity
Numbered in order
Named according to function
Most are mixed nerves
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3 (optic, olfactory,
vestibulocochlear) – purely sensory
Cranial Nerves (cont’d)
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Olfactory Nerve (N I)
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Optic Nerve (N II)
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Vision
Oculomotor Nerve (N III)

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Smell
Eye movement (diameter of pupil)
Trochlear Nerve (N IV)

Eye movement (looking up)
Cranial Nerves (cont’d)

Trigeminal Nerve (N V)

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3 divisons go to face
 Opthalmic
 Maxillary
 Mandibular
Abducens Nerve (N VI)

Eye movement (lateral movement)
Cranial Nerves (cont’d)

Facial Nerve (N VII)
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Vestibulocochlear Nerve (N VIII)
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Expressions
Balance, equilibrium, hearing
Glossopharyngeal Nerve (N IX)
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Swallowing and taste
Cranial Nerves (cont’d)
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Vagus Nerve (N X)
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Accessory Nerve (N XI)
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Widely distributed area
Connects brain to viscera
Sternocleidomastoid and trapezius
Hypoglossal Nerve(N XII)

Tongue movements
Spinal Nerves
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Originate from spinal cord
31 pairs
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Named for region of cord from
which they derive
Mixed in function
Roots of Spinal Nerves
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Connect the spinal
nerves with the
spinal cord
Lie within the
vertebral canal
Dorsal roots contain
sensory fibers
Ventral roots contain
motor fibers
Rami

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Branches emerging from the
vertebral column
Dorsal ramus supplies deep back
muscles
Ventral ramus supplies the trunk
and limbs
ANS
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Also called involuntary nervous
system
Regulate the activity of smooth &
cardiac muscles & glands
Purely motor – no sensory
nerves
Subdivisions of ANS
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Sympathetic & parasympathetic
Both serve same organs, but
cause opposite effects

Counterbalance each other
Sympathetic
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Mobilizes body during extreme
situations (fear, exercise, rage)
“Fight-or-flight”
Prepares body to cope with some
threat
Activation results in increased
heart rate and blood pressure
Parasympathetic
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
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“House keeper” system
In control most of the time
Maintains homeostasis by seeing
that normal digestion and
elimination occur and that
energy is conserved
Sympathetic
Parasymp.
Heartbeat
Increase
Decrease
Resp. rate
Increase
Decrease
Bronchio
dilation
Dilate
Constrict
Blood
pressure
Increase
Decrease
Glands –
salivary,
lacrimal
Inhibits: dry mouth
and dry eye
Stimulates: more
saliva and tears
Digestion
Sympathetic
Parasymp.
Slow down
Increases
peristalsis &
secretions
No effect
Sweat glands Produces
of skin
perspiration
Prepares for
Eyes
distant vision
Liver
Prepares for close
vision
Releases glucose No effect
to blood