The Role of the Nervous System

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The Role of the Nervous System

• Applied Kinesiology
• 420151

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Agenda

• Introduction to the nervous system
• Structural considerations
• Motor efferents and gradations of force
• Sensory afferents
• Reflex movement

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Introduction to the NS

• Functions
• Sensory input ? afferent neurons
• Integration
• Motor output ? efferent neurons
• Properties
• Irritability
• Conductivity

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Introduction to NS

• Levels of Control
• Cerebral cortex
• Consciousness
• Basal ganglia
• Homeostasis ? posture and equilibrium
• Cerebellum
• Timing and intensity ? smooth and precise motion
• Brain stem
• Arousal and cardiorespiratory function
• Spinal cord
• Link b/w CNS and PNS ? interneurons and synapses

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Cerebral cortex
Basal ganglia
Cerebellum
Brain stem
Spinal cord
Overide?
Figure 4.14, Hamilton
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Introduction to the NS

• Basic divisions of the nervous system

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Figure 14.1, Marieb Mallett (2003)
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Agenda

• Introduction to the nervous system
• Structural considerations
• Motor efferents and gradations of force
• Sensory afferents
• Reflex movement

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Structural Considerations

• The neuron
• The nerve
• The synapse
• The motor unit

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The Neuron

• Functional unit of nervous tissue
• Three main types of neurons
• Sensory/afferent neurons
• Motor/efferent neurons
• Interneurons
• Common structures

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Figure 12.11, Marieb Mallett (2003)
Dendrites
Cell Body
Axon
Differences Peripheral body, location of
dendrites/synaptic knobs, direction of
transmission
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The Neuron

• Other considerations
• Cell body
• Nucleus
• Almost all cell bodies are in spinal cord
  (ganglia?)
• Dendrites
• Afferents ? cell body via peripheral body
• Efferents ? cell body via axon
• Axon
• Myelin sheath
• Axon collaterals
• Extensive terminal branching (10,000)
• Synaptic knobs

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Figure 12.4, Marieb Mallett (2003)
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Structural Considerations

• The neuron
• The nerve
• The synapse
• The motor unit

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The Nerve

• Nerve bundle of neurons
• Not unlike skeletal muscle architecture

Figure 12.17, Marieb Mallett (2003)
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The Nerve

• Nerves can contain both afferent and efferent
  neurons.
• Spinal/peripheral nerves connect to the spinal
  cord via
• Anterior root (motor efferent neurons)
• Posterior root (sensory afferent neurons)

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Posterior root
Anterior root
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The Nerve

• Thirty one pairs of spinal/peripheral nerves
• Cervical ? 8
• Thoracic ? 12
• Lumbar ? 5
• Sacral ? 5
• Coccygeal ?1

Figure 13.29, Marieb Mallett (2003)
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Structural Considerations

• The neuron
• The nerve
• The synapse
• The motor unit

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The Synapse
Figure 12.7, Marieb Mallett (2003)

• Synapse Area between the synaptic knob of one
  neuron and the membrane of another neuron

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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)
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Excitatory and Inhibitory Postsynaptic
Potentials EPSP, IPSP
EPSP - IPSP Stimulus
Stimulus gt Threshold Excitation of
impulse Stimulus lt Threshold Inhibition of
impulse
Impulse itself can be excitatory or inhibitory in
nature
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Structural Considerations

• The neuron
• The nerve
• The synapse
• The motor unit

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The Motor Unit

• Functional unit of neuromuscular system
• Consists of
• Neuron all muscle fibers
• Eye muscles vs. gastrocnemius (10-2000)
• Fewer fibers/neuron precision
• More fibers/neuron force

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Figure 14.6, Marieb Mallett (2003)
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Agenda

• Introduction to the nervous system
• Structural considerations
• Motor efferents and gradations of force
• Sensory afferents
• Reflex movement

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Efferents Gradations of Force

• Motor efferent Sends signal away from the CNS
  (skeletal muscle)
• Dendrites in spinal cord
• Synaptic knobs ? muscle
• Excitatory or inhibitory
• Gradation of force
• Concept Muscles are able to activate with
  varying degrees of force

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Efferents Gradations of Force

• Two factors influence the gradation of force
• Number coding The number of motor units
  participating
• Rate coding The frequency of stimulation

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Number Coding

• All-or-none principle of single motor units ?
  threshold
• Gradation of force
• 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

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Resting muscle tonus achieved via alternating
activation of some muscle fibers
Figure 19.13, Plowman Smith (2003)
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Rate Coding

• Effects of different stimulus frequencies on
  motor units
• 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

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Maximum number coding maximum rate coding
maximum force
As frequency increases, force/tension increases
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Agenda

• Introduction to the nervous system
• Structural considerations
• Motor efferents and gradations of force
• Sensory afferents
• Reflex movement

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Sensory Afferents

• Sensory afferents Sends signal towards the CNS
• Dendrites are all over body (not in CNS)
• Synaptic knobs are in spinal cord
• Classifications of afferents
• Exteroceptors
• Interoceptors (visceroceptors)
• Proprioceptors

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Figure 14.1, Marieb Mallett (2003)
Proprioceptors are main concern
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Proprioceptors

• Location Tendons, skeletal muscle, ligaments,
  joint capsules and inner ear
• Functions
• Transmit movement information ? CNS
• CNS integrates and initiates appropriate response
  (consciously/subconsciously)
• Provide sense of body awareness
• Provide stimulus for reflexes

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Proprioceptor Classification

• Muscle proprioceptors
• Muscle spindles
• Golgi tendon organs
• Joint and skin proprioceptors
• Ruffini endings
• Pacinian corpuscles
• Labyrinthine and neck proprioceptors
• Labyrinthine proprioceptors
• Neck proprioceptors

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Muscle Proprioceptors Muscle Spindles

• Location Lay between and parallel to muscle
  fibers
• Structure
• Tiny capsules (1 mm)
• Filled with fluid and intrafusal muscle fibers
• Nucleated and supplied with afferent neuron
• Function
• Sensitive to stretch and tension of skeletal
  muscle tissue
• Transmit to CNS
• Excitatory impulse ? agonist and synergists
• Inhibitory impulse ? antagonists (reciprocal
  inhibition)

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Figure 14.5, Knutzen Hamill (2004)
Stretch
Excitatory activation of agonists
Interneurons
Activation of synergists
Reciprocal inhibition of antagonists
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Muscle Proprioceptors GTOs

• Location Musculotendon junction of skeletal
  muscle
• Structure
• Mass of terminal endings in connective tissue
  capsule
• Connections both with tendon and fibers
• Function
• Sensitive to tension in tendon due to both
  stretch and shortening of muscle
• Transmit to CNS
• Inhibitory impulse ? agonists and synergists
• Excitatory impulse ? antagonists

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1. High muscle tension
3. GTO activation
4. Inhibition of agonist
2. High tendon tension
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Joint and Skin Proprioceptors Ruffini Endings

• Location Beneath skin, joint capsules
• Structure Spray of dendrites in flattened
  connective tissue capsule
• Functions Sensitive to ?
• Rapid changes in joint angle
• Constant pressure resulting in deformation of
  capsule

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Skin and Joint ProprioceptorsPacinian Corpuscles

• Location Beneath skin, joint capsules, ligaments
  and tendons
• Structure
• Relatively large (naked eye)
• Tip of single dendrite in connective tissue
  capsule
• Function Sensitive to ?
• Rapid changes in joint angle
• Rapid, short-term changes in pressure resulting
  in deformation of capsule

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Pacinian corpuscle
Ruffini endings
Free nerve endings
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Labyrinthine Proprioceptors

• Location Inner ear
• Structure Several structures within the ear
• Function
• Detect orientation and movements of the head

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Neck Proprioceptors

• Location Ligaments of cervical vertebrae
• Function
• Head/neck movement ? transmit opposite signals
• Prevents sense of imbalance

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Agenda

• Introduction to the nervous system
• Structural considerations
• Motor efferents and gradations of force
• Sensory afferents
• Reflex movement

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Reflexes

• Reflex Specific pattern response that occurs
  without volition
• The reflex arc consists of
• Receptor organ
• Afferent neuron
• Interneuron (sometimes)
• Efferent neuron

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Figure 12.18, Marieb Mallett (2003)
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Classification of Reflexes

• Exteroceptive reflexes Respond to external
  stimuli
• Extensor thrust reflex
• Flexor reflex
• Crossed extensor reflex
• Proprioceptive reflexes Response to internal
  stimuli
• Stretch (myotatic) reflex
• Tendon reflex
• Righting reflex
• Tonic neck reflex
• Labyrinthine reflex

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Exteroceptive Extensor Thrust Reflex

• General mechanism Pressure stimulates pacinian
  corpuscles ? excitatory impulse to extensors
• Examples
• Standing
• Shifting weight ? preparation for motion
• Hands ? cartwheel or back handspring

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Exteroceptive Flexor Reflex

• General mechanism Typically in response to pain
  ? excitatory impulse to flexors
• Examples
• Pricking or burning hand

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Exteroceptive Crossed Extensor Reflex

• General mechanism Functions cooperatively with
  flexor reflex
• Contralateral limb is extended
• Examples
• Stepping on tack
• Pricking or burning hand

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Proprioceptive Stretch Reflex

• General mechanism Stretched muscle results in
  stretched muscle spindle ?
• Excitatory impulse to agonist for protection
• Inhibitory impulse to antagonists
• Two types
• Phasic Rapid stretching ? rapid powerful
  contraction that ends rapidly
• Tonic Slow stretching ? smooth, less powerful
  contraction that lasts as long as the stretch

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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
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Explosive movements Long and rapid prep phases
(phasic stretch reflex)
Precise movements Short and slow prep phase
(tonic stretch reflex)
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Proprioceptive Tendon Reflex

• General mechanism Sensitive to tension in tendon
  due to
• Muscle lengthening
• Tendon reflex vs. stretch reflex
• Muscle shortening
• Very sensitive (low threshold)
• Threshold stimulus ? inhibitory impulse to
  agonists
• Extreme cases ? total relaxation
• Training or extreme stress can increase the
  threshold

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Proprioceptive Righting Reflex

• General mechanism Body tilting ? thrusting of
  limbs to restore balance
• Example
• A gentle push with eyes shut

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Proprioceptive Tonic Neck Reflex

• General mechanism Head movement results in
  flexion or extension of limbs
• Obvious in infants
• Surpressed in adults ? evident under stress
• Examples Symmetric vs. asymmetric
• Neck flexion Upper extremities flex
• Neck extension Upper extremities extend
• Neck rotation
• Extension/Abduction of contralateral arm
• Flexion/abduction of ipsilateral arm.

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Proprioceptive Labyrinthine Reflex

• General mechanism Movements of the head ?
  activation of limbs to maintain balance