The Role of the Nervous System Applied Kinesiology 420:151.

The Role of the Nervous System

Applied Kinesiology420:151

Agenda

Introduction to the nervous system Structural considerations Motor efferents and gradations of

force Sensory afferents Reflex movement

Introduction to the NS

Functions: Sensory input afferent neurons Integration Motor output efferent neurons

Properties: Irritability Conductivity

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

Figure 4.14, Hamilton

Cerebral cortex

Basal ganglia

Cerebellum

Brain stem

Spinal cord

Overide?

Introduction to the NS

Basic divisions of the nervous system:

Figure 14.1, Marieb & Mallett (2003)

Agenda



Structural Considerations

The neuron The nerve The synapse The motor unit

The Neuron

Functional unit of nervous tissue Three main types of neurons

Sensory/afferent neurons Motor/efferent neurons Interneurons

Common structures

Dendrites Cell Body Axon

Differences: Peripheral body, location of dendrites/synaptic knobs, direction of

transmission


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

The Nerve

Nerve = bundle of neurons Not unlike skeletal

muscle architecture


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)

Anterior root

Posterior root

The Nerve

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


The Synapse 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 inhibitoryCompetition between excitation and inhibition occurs Threshold stimulus reached?

Neurotransmitter

NMJ or motor end plate


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

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

Agenda



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

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

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

Figure 19.13, Plowman & Smith (2003)

Resting muscle tonus achieved via alternating activation of some muscle

fibers

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

• Temporal Summation– increase in tension with

increased frequency ofstimuli

• Tetanus– sustained tension

between stimulus

As frequency increases, force/tension increases

Maximum number coding + maximum

rate coding = maximum force

Agenda



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


Proprioceptors are main concern

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

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

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)

Stretch

Interneurons

Excitatory activation of

agonists

Activation of synergists

Reciprocal inhibition of antagonists

Figure 14.5, Knutzen & Hamill (2004)

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

1. High muscle tension

2. High tendon tension

3. GTO activation

4. Inhibition of agonist

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

Skin and Joint Proprioceptors:Pacinian 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

Ruffini endings

Pacinian corpuscle

Free nerve endings

Labyrinthine Proprioceptors

Location: Inner ear Structure: Several structures

within the ear Function:

Detect orientation and movements of the head

Neck Proprioceptors

Location: Ligaments of cervical vertebrae

Function: Head/neck movement transmit

opposite signals Prevents sense of imbalance

Agenda



Reflexes

Reflex: Specific pattern response that occurs without volition

The reflex arc consists of: Receptor organ Afferent neuron Interneuron (sometimes) Efferent neuron

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

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

Exteroceptive: Flexor Reflex

General mechanism: Typically in response to pain excitatory impulse to flexors

Examples: Pricking or burning hand

Exteroceptive: Crossed Extensor Reflex

General mechanism: Functions cooperatively with flexor reflex Contralateral limb is extended

Examples: Stepping on tack Pricking or burning hand

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

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 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

Proprioceptive: Righting Reflex

General mechanism: Body tilting thrusting of limbs to restore balance

Example: A gentle push with eyes shut

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.

Proprioceptive: Labyrinthine Reflex

General mechanism: Movements of the head activation of limbs to maintain balance

The Role of the Nervous System Applied Kinesiology 420:151.

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