Transcript Presentation 5: The Role of the Nervous System

The Role of the Nervous
System
Applied Kinesiology
420:151
Agenda
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Introduction to the nervous system
Structural considerations
Motor efferents and gradations of force
Sensory afferents
Reflex movement
Introduction to the NS
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Functions:
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Sensory input  afferent neurons
Integration
Motor output  efferent neurons
Properties:
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Irritability
Conductivity
Introduction to NS
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Levels of Control:
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Cerebral cortex
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Basal ganglia
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Timing and intensity  smooth and precise motion
Brain stem
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Homeostasis  posture and equilibrium
Cerebellum
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Consciousness
Arousal and cardiorespiratory function
Spinal cord
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Link b/w CNS and PNS  interneurons and synapses
Cerebral cortex
Basal ganglia
Cerebellum
Brain stem
Spinal cord
Overide?
Figure 4.14, Hamilton
Introduction to the NS
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Basic divisions of the nervous system:
Figure 14.1, Marieb & Mallett (2003)
Agenda
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Introduction to the nervous system
Structural considerations
Motor efferents and gradations of force
Sensory afferents
Reflex movement
Structural Considerations
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The
The
The
The
neuron
nerve
synapse
motor unit
The Neuron
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Functional unit of nervous tissue
Three main types of neurons
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Sensory/afferent neurons
Motor/efferent neurons
Interneurons
Common structures
Figure 12.11, Marieb & Mallett (2003)
Dendrites
Cell Body
Axon
Differences: Peripheral body, location of
dendrites/synaptic knobs, direction of transmission
The Neuron
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Other considerations
Cell body
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Dendrites
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Nucleus
Almost all cell bodies are in spinal cord (ganglia?)
Afferents  cell body via peripheral body
Efferents  cell body via axon
Axon
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Myelin sheath
Axon collaterals
Extensive terminal branching (10,000)
Synaptic knobs
Figure 12.4, Marieb & Mallett (2003)
Structural Considerations
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The
The
The
The
neuron
nerve
synapse
motor unit
The Nerve
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Nerve = bundle of
neurons
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Not unlike skeletal
muscle architecture
Figure 12.17, Marieb & Mallett (2003)
The Nerve
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Nerves can contain both afferent and
efferent neurons.
Spinal/peripheral nerves connect to the
spinal cord via:
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Anterior root (motor efferent neurons)
Posterior root (sensory afferent neurons)
Posterior
root
Anterior root
The Nerve
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Thirty one pairs of
spinal/peripheral nerves:
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Cervical  8
Thoracic  12
Lumbar  5
Sacral  5
Coccygeal 1
Figure 13.29, Marieb & Mallett (2003)
Structural Considerations
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The
The
The
The
neuron
nerve
synapse
motor unit
Figure 12.7, Marieb & Mallett (2003)
The Synapse
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Synapse: Area between the synaptic knob
of one neuron and the membrane of
another neuron
Neurons have thousands of synaptic knobs
Some neurons are excitatory, some inhibitory
Competition between excitation and inhibition
occurs  Threshold stimulus reached?
NMJ or motor
end plate
Neurotransmitter
Figure 12.8, Marieb & Mallett (2003)
Excitatory and Inhibitory
Postsynaptic Potentials: EPSP, IPSP
EPSP - IPSP = Stimulus
Stimulus > Threshold = Excitation
of impulse
Stimulus < Threshold = Inhibition
of impulse
Impulse itself can be excitatory or
inhibitory in nature
Structural Considerations
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The
The
The
The
neuron
nerve
synapse
motor unit
The Motor Unit
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Functional unit of neuromuscular
system
Consists of:
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Neuron + all muscle fibers
Eye muscles vs. gastrocnemius (10-2000)
Fewer fibers/neuron = precision
More fibers/neuron = force
Figure 14.6, Marieb & Mallett (2003)
Agenda
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Introduction to the nervous system
Structural considerations
Motor efferents and gradations of force
Sensory afferents
Reflex movement
Efferents: Gradations of Force
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Motor efferent: Sends signal away from
the CNS (skeletal muscle)
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Dendrites in spinal cord
Synaptic knobs  muscle
Excitatory or inhibitory
Gradation of force:
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Concept: Muscles are able to activate with
varying degrees of force
Efferents: Gradations of Force
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Two factors influence the gradation of
force:
Number coding: The number of motor
units participating
Rate coding: The frequency of
stimulation
Number Coding
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All-or-none principle of single motor
units  threshold
Gradation of force
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Small force = fewer motor units or motor
units with less fibers
Large force = more motor units or motor
units with more fibers
Orderly sequence  Size principle
Resting muscle
tonus achieved via
alternating
activation of some
muscle fibers
Figure 19.13, Plowman & Smith (2003)
Rate Coding
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Effects of different stimulus frequencies
on motor units:
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Single stimulus  twitch
Second stimulus added prior to full
relaxation  temporal summation
Multiple stimuli added so that any
relaxation is prohibited  irregular and
fused tetanus
Maximum number
coding + maximum
rate coding =
maximum force
• Temporal Summation
– increase in tension with
increased frequency of
stimuli
• Tetanus
– sustained tension
between stimulus
As frequency increases, force/tension increases
Agenda
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Introduction to the nervous system
Structural considerations
Motor efferents and gradations of force
Sensory afferents
Reflex movement
Sensory Afferents
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Sensory afferents: Sends signal towards
the CNS
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Dendrites are all over body (not in CNS)
Synaptic knobs are in spinal cord
Classifications of afferents:
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Exteroceptors
Interoceptors (visceroceptors)
Proprioceptors
Figure 14.1, Marieb & Mallett (2003)
Proprioceptors are main concern
Proprioceptors
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Location: Tendons, skeletal muscle,
ligaments, joint capsules and inner ear
Functions:
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Transmit movement information  CNS
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CNS integrates and initiates appropriate
response (consciously/subconsciously)
Provide sense of body awareness
Provide stimulus for reflexes
Proprioceptor Classification
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Muscle proprioceptors:
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Joint and skin proprioceptors
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Muscle spindles
Golgi tendon organs
Ruffini endings
Pacinian corpuscles
Labyrinthine and neck proprioceptors
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Labyrinthine proprioceptors
Neck proprioceptors
Muscle Proprioceptors: Muscle
Spindles
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Location: Lay between and parallel to muscle
fibers
Structure:
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Tiny capsules (1 mm)
Filled with fluid and intrafusal muscle fibers
Nucleated and supplied with afferent neuron
Function:
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Sensitive to stretch and tension of skeletal muscle
tissue
Transmit to CNS
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Excitatory impulse  agonist and synergists
Inhibitory impulse  antagonists (reciprocal inhibition)
Figure 14.5, Knutzen & Hamill (2004)
Stretch
Interneurons
Excitatory activation of
agonists
Activation of
synergists
Reciprocal inhibition of
antagonists
Muscle Proprioceptors: GTOs
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Location: Musculotendon junction of skeletal
muscle
Structure:
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Mass of terminal endings in connective tissue
capsule
Connections both with tendon and fibers
Function:
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Sensitive to tension in tendon due to both stretch
and shortening of muscle
Transmit to CNS:
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Inhibitory impulse  agonists and synergists
Excitatory impulse  antagonists
1. High muscle tension
3. GTO
activation
4. Inhibition of agonist
2. High tendon tension
Joint and Skin Proprioceptors:
Ruffini Endings
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Location: Beneath skin, joint capsules
Structure: Spray of dendrites in
flattened connective tissue capsule
Functions: Sensitive to 
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Rapid changes in joint angle
Constant pressure resulting in deformation
of capsule
Skin and Joint Proprioceptors:
Pacinian Corpuscles
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Location: Beneath skin, joint capsules,
ligaments and tendons
Structure:
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Relatively large (naked eye)
Tip of single dendrite in connective tissue capsule
Function: Sensitive to 
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Rapid changes in joint angle
Rapid, short-term changes in pressure resulting in
deformation of capsule
Pacinian corpuscle
Ruffini endings
Free nerve endings
Labyrinthine Proprioceptors
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Location: Inner ear
Structure: Several structures within the
ear
Function:
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Detect orientation and movements of the
head
Neck Proprioceptors
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Location: Ligaments of cervical
vertebrae
Function:
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Head/neck movement  transmit opposite
signals
Prevents sense of imbalance
Agenda
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Introduction to the nervous system
Structural considerations
Motor efferents and gradations of force
Sensory afferents
Reflex movement
Reflexes
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Reflex: Specific pattern response that
occurs without volition
The reflex arc consists of:
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Receptor organ
Afferent neuron
Interneuron (sometimes)
Efferent neuron
Figure 12.18, Marieb & Mallett (2003)
Classification of Reflexes
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Exteroceptive reflexes: Respond to external
stimuli
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Extensor thrust reflex
Flexor reflex
Crossed extensor reflex
Proprioceptive reflexes: Response to internal
stimuli
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Stretch (myotatic) reflex
Tendon reflex
Righting reflex
Tonic neck reflex
Labyrinthine reflex
Exteroceptive: Extensor Thrust
Reflex
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General mechanism: Pressure
stimulates pacinian corpuscles 
excitatory impulse to extensors
Examples:
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Standing
Shifting weight  preparation for motion
Hands  cartwheel or back handspring
Exteroceptive: Flexor Reflex
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General mechanism: Typically in
response to pain  excitatory impulse
to flexors
Examples:
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Pricking or burning hand
Exteroceptive: Crossed
Extensor Reflex
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General mechanism: Functions
cooperatively with flexor reflex
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Contralateral limb is extended
Examples:
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Stepping on tack
Pricking or burning hand
Proprioceptive: Stretch Reflex
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General mechanism: Stretched muscle results
in stretched muscle spindle 
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Excitatory impulse to agonist for protection
Inhibitory impulse to antagonists
Two types:
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Phasic: Rapid stretching  rapid powerful
contraction that ends rapidly
Tonic: Slow stretching  smooth, less powerful
contraction that lasts as long as the stretch
Elbow flexed 90 degrees
while holding a bucket
1. Object dropped into
bucket
2. Object dropped in from
lesser height
3. Object placed into
bucket
Figure 14.12, Hamilton
Explosive movements: Long and rapid
prep phases (phasic stretch reflex)
Precise movements: Short and slow prep
phase (tonic stretch reflex)
Proprioceptive: Tendon Reflex
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General mechanism: Sensitive to tension in
tendon due to:
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Muscle lengthening
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Muscle shortening
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Tendon reflex vs. stretch reflex
Very sensitive (low threshold)
Threshold stimulus  inhibitory impulse to
agonists
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Extreme cases  total relaxation
Training or extreme stress can increase the
threshold
Proprioceptive: Righting Reflex
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General mechanism: Body tilting 
thrusting of limbs to restore balance
Example:
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A gentle push with eyes shut
Proprioceptive: Tonic Neck
Reflex
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General mechanism: Head movement results
in flexion or extension of limbs
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Obvious in infants
Surpressed in adults  evident under stress
Examples: Symmetric vs. asymmetric
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Neck flexion: Upper extremities flex
Neck extension: Upper extremities extend
Neck rotation:
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Extension/Abduction of contralateral arm
Flexion/abduction of ipsilateral arm.
Proprioceptive: Labyrinthine
Reflex
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General mechanism: Movements of the
head  activation of limbs to maintain
balance