Download Chapter_4_Neuromuscular

Chapter 4:
The Neuromuscular
Basis of Human Motion
Lecture Notes
Objectives
1. Name and describe the function of the basic
structure of the nervous system
2. Explain how gradations in strength of muscle
contraction and precision of movements occur
3. Name and define the receptors important in
musculoskeletal movement
4. Explain how the various function, and describe
the effect each has on musculoskeletal
movement
Objectives
5. Describe reflex action, and enumerate and
differentiate among the reflexes that affect
musculoskeletal action
6. Demonstrate a basic understanding of volitional
movement by describing the nature of the
participation of the anatomical structures and
mechanisms
7. Perform an analysis of the neuromuscular
factors influencing the performance of a variety
of motor skills
THE NERVOUS SYSTEM AND BASIC
NERVE STRUCTURES
I. Central nervous system (CNS)
A. Brain
B. Spinal cord
II. Peripheral nervous system (PNS)
A. Cranial nerves (12 pairs)
B. Spinal nerves (31 pairs)
III. Autonomic nervous system
A. Sympathetic
B. Parasympthetic
Neurons

Is a single
nerve cell
consisting
of a cell
body and
one or
more
projections
Motor Neurons
Situated in anterior horns of spinal cord
 Dendrite that synapse with sensory
neurons
 Axon emerges from spinal cord, travels by
way of a peripheral nerve to muscle
 Each terminal branch ends at the motor
end plate of a single muscle fiber

Sensory
Neurons
Situated in a dorsal
root ganglion just
outside the spinal
cord
 Neuron may
terminate in spinal
cord or brain
 A long peripheral
fiber comes from a
receptor

Connector Neurons
Exist completely within the CNS
 Serve as connecting links
 May be a single neuron, connecting
sensory to motor neurons
 To an intricate system of neurons,
whereby a sensory impulse may be related
to many motor neurons

Nerves
A bundle of fibers, enclosed within a
connective tissue sheath, for transmission
of impulses
 A typical spinal nerve consist of

– Motor outgoing fibers
– Sensory incoming fibers
Nerves
Each spinal nerve
is attached to
spinal cord by an
anterior (motor)
root and a
posterior (sensory)
root
 Posterior root
bears a ganglion –
a collection of cell
bodies

Spinal Nerves

31 pairs – exit both sides of the vertebral
column
– 8 Cervical
– 12 Thoracic
– 5 Lumbar
– 5 Sacral
– 1 Coccyx

Table 4.1 outlines spinal
The Synapse





Connection between neurons
May be thousands between any two neurons
Proximity of the membrane of one axon to
another dendrites
The more often a synapse is used the faster a
signal will pass through it
The greater the number of synapses for receptor
to effector, the longer the time form stimulus to
response
The Synapse
Transmission across
depends on a
chemical transmitter
 Substance diffuses
synapse and produces
an action potential in
postsynaptic neuron

Action Potentials





Threshold level is the minimum level of stimulus
(chemical transmitter) necessary to initiate or
propagate a signal
Facilitation – an excitatory stimulus
Inhibition – an inhibitory stimulus
Stimulus may be from more than one neuron
The sum total of excitatory and inhibitory
determine if the postsynaptic neuron will
produce an action potential
THE MOTOR UNIT (MU)
Consist of a single MU
and all the muscle
fibers its axon
supplies
 All muscle fibers in a
MU are of the same
muscle fiber type

Size of Motor Units
Vary widely in the number of muscle fibers
 Gastrocnemius: 2,000 or more muscle
fibers
 Eye muscles: may have fewer than 10
fibers
 Small ratio of muscle fibers to MU is
capable of more precise movements
 Size of MU has direct bearing on the
precision of movement

Gradations in the Strength of
Muscular Contractions

Experience tells us that the same muscles
contract with various gradations of
strength
How do they adjust to such extremes?
1. Number of motor units that are activated
2. Frequency of stimulation
All-or-None Principle
Recruitment of Motor Units

All-or-None Principle: If the stimulus is of
threshold value, all muscles of MU will
contract
– Applies to muscle fibers not whole muscle

MU recruitment: has an orderly sequence to
– Smaller slow twitch fibers are recruited first
 They have lower thresholds
– Larger fast twitch fibers are recruited later
 They have higher thresholds
Frequency of Stimulation
At low frequency, muscle fibers relax between
impulses
 At high frequency, fibers do not have time to
relax and result in summation or maximal
contraction


A combination of maximum number of fibers
stimulated and high frequency results in a
maximal strength of contraction
SENSORY
RECEPTORS

Respond to different
stimuli
– Exteroceptors: near
body surface stimuli
come from outside the
body
– Interoceptors: sense
heat, cold, pain and
pressure
Proprioceptors
Respond to
degree,
direction, & rate
of change of
body movements
 Transmit
information to
CNS
 Muscle receptors
 Joint & skin
receptors

Muscle Proprioceptors
Muscle Spindles
Located in muscle belly, parallel with
fibers
 When stretched, sensory nerve sends
impulses to CNS, which activates the
motor neurons causing contraction of the
muscle
 More spindles are located in muscle
controlling precise movements

Muscle Proprioceptors
Muscle Spindles
Extrafusal fibers “regular” muscle fibers
 Intrafusal fibers muscle fibers inside spindles

– Noncontractile central portion
Muscle Proprioceptors
Muscle Spindles
Spindles contains two type of nerve
endings
 Primary or annulospiral endings: coiled
around noncontractile midsection

– Sensitive to velocity of change (phasic)
– Sharp decline in impulses to static changes

Flower-spray endings: at end of
noncontractile midsection
– Respond to static muscle length
– Impulses directly proportional to length
Muscle Proprioceptors
Muscle Spindles

Gamma motor neurons: stimulate the
intrafusal fibers to contract, shortening the
muscle spindle
Muscle Proprioceptors
Golgi Tendon Organ (GTO)
Embedded “in series”
in the tendon
 As tension in tendon
increases GTO is
activated
 Signals CNS to relax
muscle
 Protective mechanism

Joint and Skin Proprioceptors
Pacinian Corpuscles
In regions around joint capsules, ligament,
and tendons sheaths
 End-organ has concentric layers of capsule
 Activated by joint angle changes & pressure
 Transmits impulses for only a very brief time
 Predict where body part will be at any time
 Appropriate adjustment in position can be
anticipated and effected

Joint and Skin Proprioceptors
Ruffini Endings
In deep layers of skin and joint capsule
 Activated by mechanical deformation
 Signal continuous states of pressure
 Adapt slowly, then transmit a steady signal
 Stimulated strongly by sudden joint movement
 Sense joint position and changes in joint angle
 The CNS knowing which receptors is stimulated
can tell the joint angle

Joint and Skin Proprioceptors
Cutaneous Receptors
Meissner corpuscles: touch
 Pacinian corpuscles: pressure
 Free nerve endings: pain
 Serve as proprioceptors when they show
sensitivity to texture, hardness, softness
and shape, and participate in reflexes

Labyrinthine and Neck
Proprioceptors
Cochlea: is concerned with hearing
 Semicircular canals: sense balance

– Labyrinth filled with fluid and is lined with hair
cells, senses motion of fluid as head moves
Joint receptors of the neck: sensitive to
angle between the body and the head
– Prevent labyrinthine proprioceptors from
producing feeling of imbalance
REFLEX
MOVEMENT
A specific pattern of
response without
volition form the
cerebrum
 Receptor organ,
Sensory neuron,
Motor neuron, Muscle
 Connector neurons

Exteroceptive Reflexes
Extensor Thrust Reflex

Pressure on sole of stimulates reflex contraction of
extensor muscles
Exteroceptive Reflexes

Flexor Reflex:
– Most frequent in response to pain
– Quick withdraw from source of pain

Crossed Extensor Reflex:
– As flexor reflex respond to pain, extensor
muscle muscle of opposite ling contract to
support additional weight
Proprioceptive Reflexes
Stretch Reflex
A reflex contraction of stretched muscle and
synergists and relaxation of antagonists
 Phasic Type: Knee jerk reflex

– Weight placed in hand elbow at 900
Proprioceptive Reflexes
Stretch Reflex

Tonic Type: muscle is gradually stretched and will
result in a more tonic response
Proprioceptive Reflexes
Stretch Reflex
Phasic
preparatory
phase can
take
advantage of
the stretch
reflex
 Result in a
stronger
contraction

Proprioceptive Reflexes
Stretch Reflex
Slow preparatory
phase should be used
when the desired
outcome is accuracy
 Result in a stronger
contraction

Tendon Reflex
The reflex Inhibit impulses form motor
neuron to the muscle and synergists,
causing muscle to relax, antagonists is
facilitated
 Protective mechanism to prevent muscles
from being torn or ruptured
 Feedback mechanism to control tension
 May effect skills of beginners until GTO
threshold develops

VOLITIONAL MOVEMENT
CNS: Levels of Control
1. Cerebral cortex: where consciousness
occurs, initiation of voluntary movement
2. Basal ganglia: responsible for
homeostasis, coordination & some learned
acts of posture
3. Cerebellum “little brain”: key role in
sensory integration, regulates timing &
intensity of muscle contraction
VOLITIONAL MOVEMENT
CNS: Levels of Control
4. Brain stem: arousal and monitoring of
physiological parameters, key facilitory
and inhibitory centers
5. Spinal cord: contains cell bodies of lower
motor neurons, common pathway
between CNS & PNS, final point for
integration and control
Functions of the 5 levels overlap depending
on classification scheme used
VOLITIONAL MOVEMENT
Pyramidal & Extrapyramidal Tracts
The two tracts originate in cerebral cortex
and end at the spinal cord
 Pyramidal Tracts: predominately axon of
motor neurons, controls muscles for
precision
 Extrapyramidal Tracts: synapse with all
levels of CNS, functions in stabilization
and control of posture

Kinesthesis
The conscious awareness of position of
body parts and the amount and rate or
joint movement
 Without rapid transmission & processing,
accurately controlled movements could
not proceed
 Kinesthetic perception and memory are
the basis for voluntary movement and
motor learning

Reciprocal Inhibition
When motor neurons are transmitting
impulses to an agonist, antagonistic are
simultaneously & reciprocally inhibited
 Antagonist remain relaxed & agonists
contract without opposition
 Automatic in reflexes & familiar
movements
 More complicated movements depends on
the degree of skill developed by performer

Coactivation or
Reciprocal Activation
Most frequently appears in movement
when there is uncertainty about
movement task
 Practice increases familiarity, and
coactivation decreases in favor of
reciprocal inhibition
 Efficiency of movement increase
 Coactivation also occurs to maintain joint
stiffness

NEUROMUSCULAR ANALYSIS
Muscle-response patterns of well-learned
motor skills involve the integrated action
of many reflexes and the inhibition of
others
 After repeated viewing, students should
be able to name and discuss the reflexes
that could be acting at various points in
each phase

Summary
1. Know the function of the basic structure of the
nervous system
2. Know how gradations in strength of muscle
contraction and precision of movements occur
3. Name and define the receptors important in
musculoskeletal movement
4. Know various neurological function, and
describe the effect each has on musculoskeletal
movement
Summary
5. Know reflex action, and enumerate and
differentiate among the reflexes that affect
musculoskeletal action
6. Have a basic understanding of volitional
movement by describing the nature of the
participation of the anatomical structures and
mechanisms
7. Be able to perform an analysis of the
neuromuscular factors influencing the
performance of a variety of motor skills
Summary
KNOW THE NERVOUS SYSTEM AND BASIC
NERVE STRUCTURES (CNS, PNS, ANS)
 KNOW THE PROPRIOSEPTORS,
REFLEXES, CARPUSLES
