Download Skeletal Muscle Motor Control

PHYSIOLOGY 1
LECTURE 20
SKELETAL MUSCLE
MOTOR CONTROL
SKELETAL MUSCLE
MOTOR CONTROL
• Objectives: The student should learn:
• 1. The muscle control system (afferents
and efferents)
• 2. The feedback information required for
limb movement
• 3. Muscle receptors (muscle spindles,
Golgi tendon bodies, free nerve endings)
• 4. Afferent / efferent nerve axon types,
receptors, and sensitivity
SKELETAL MUSCLE
MOTOR CONTROL
• 5. Types of muscle spindle fusiform
fibers and function
• 6. Golgi tendon bodies sensitivity
• 7. Overall muscle control system
SKELETAL MUSCLE
MOTOR CONTROL
•
Introduction:
– Skeletal muscle control is not a simple
matter of command move but rather
a complex series of continual
feedback loops which supply the brain
with continual information on
SKELETAL MUSCLE
MOTOR CONTROL
•Joint position
•State of muscle contraction
–Agonists muscle – concentric
contraction
–Antagonists muscle – eccentric
contraction
•Degree of tension on tendons
•Strain and tension on joints
SKELETAL MUSCLE
MOTOR CONTROL
– The feedback information supplies the
brain with exact information on
• Limb position (all the above)
• Ground conditions (Visual)
• Surroundings (Visual, hearing,
touch)
• Continual estimates of weights to be
lifted or moved
SKELETAL MUSCLE
MOTOR CONTROL
Speed
of limb movement
•Momentum
•Pain
•Muscle temperature
•Chemical environment of the muscle
fibers
•Degree of muscle recruitment
SKELETAL MUSCLE
MOTOR CONTROL
• Allows the motor cortex, cerebellum and
spinal column to continually adjust
required estimates on force, speed,
position, and limb angles needed to
accomplish the task required or in the
case of repetitive movements to initialize
the reverberatory spinal reflexes to
accomplish the movement.
SKELETAL MUSCLE
MOTOR CONTROL
•
Receptors in muscle control
–
Sensory Receptors – (Afferents) The classic
studies of Ruffini, Golgi, and Sherrington
showed that skeletal muscle was richly
endowed with a large variety of sensory
receptors.
Receptors in muscle control
•Muscle spindles
•Golgi tendon organ
•Secondary spindle endings
•Nonspindle endings
•Free nerve endings
Receptors in muscle control
– Control Innervations –
• (Efferent) These nerve fibers provide
skeletal muscle with the necessary
commands to maintain continual
information flow from the brain,
midbrain, and spinal column
– Alpha motor neuron
– Beta fibers (skeletofusimotor)
– Gamma fibers (fusimotor)
Receptors in muscle control
Afferent axon
type:
Receptor:
Sensitive to:
Group 1a
Primary spindle
endings
Muscle length
and rate of
change of length
Group 1b
Golgi tendon
organ
Muscle tension
Receptors in muscle control
Group II
Secondary
spindle endings
Muscle length
(little rate
sensitivity)
Group II
Nonspindle
endings
Deep pressure
Groups III & IV
Free nerve
endings
Pain, chemical
stimuli, and
temperature
Receptors in muscle control
Efferent
axon type:
Innervation Function
Alpha motor
Extrafusal
neuron
muscle fibers
(Skeletomuscle)
Control tension
of muscles,
strength and
rate of pull on
skeleton
Receptors in muscle control
Beta fibers
Intrafusal muscle
(Skeletofusimotor fibers (collateral
)
from alpha motor
neuron)
Control sensitivity
of muscle
spindles – no
independent
control of muscle
spindles
Gamma motor
neurons
(fusimotor)
Control sensitivity of
spindles
independent of
alpha motor
neurons - unloading
does not impair
Intrafusal fibers
Receptors in muscle control
• Muscle Spindles – Encapsulated sensory
receptor whose complexity is as great as the
eye – each individual muscle contains several
muscle spindles – a general rule is that
muscles requiring fine delicate control have a
greater density of muscle spindles
(intravertebral muscles) than do gross muscles
(gluteus maximums). They are randomly
distributed deep within the belly of the muscle
Receptors in muscle control
• Capsule – Muscle spindles are
encapsulated receptors with fusiform
(spindle) shape and range from 2 to 10
mm in length and 0.5 mm to 1 mm in
diameter containing a gel perhaps to
lubricate the intrafusal fibers. Capsule is
connected to the extrafusal skeletal
muscle fibers and to intrafusal fibers.
Receptors in muscle control
• Intrafusal fibers – specialized muscle
fibers contained within the muscle
spindle capsule and embedded at
random within the extrafusal skeletal
muscle fibers
– Range from 2 to 12 per capsule
– Intrafusal fibers are arranged in
parallel with themselves and with
the extrafusal fibers
Receptors in muscle control
– Intrafusal fiber types
• 1] Nuclear bag fibers – contain numerous nuclei
•
•
•
concentrated near the center of the fiber and lying
parallel with each other – 2 types – one of each in
each muscle spindle
a] Dynamic fiber – motion sensor
b] Static fiber – position sensor
2] Nuclear chain fibers- contain nuclei lined up in
series – varying numbers usually 5 in mammals –
degree of stretch and strain
Receptors in muscle control
• d. Central regions of each fiber are non
contractile and innervated by one or more
neurons wrapped around the intrafusal
fiber (annulospiral endings – primary
spindle endings and secondary endings)
• e. Polar regions of the intrafusal fibers are
contractile – pull on central region from
both ends
Receptors in muscle control
• Primary spindle endings (afferents) – Group 1a
fiber axon – dendritic endings wrap around all
Intrafusal fibers (dynamic and static nuclear
bag fibers, and all nuclear chain fibers) - may
be one or more but usually just one - and
provide feedback information on muscle length
and rate of change in muscle length (stretch) –
nerve fires continually increase firing rate
when stretched and decrease firing rate when
relaxed - information on developed muscle
tension (force), speed of contraction, and
momentum
Receptors in muscle control
• Secondary spindle endings (afferents) - Group
II fiber axon – dendritic endings wrap around
nuclear chain fiber and static nuclear bag fiber
intrafusal fibers – may be as many as 5
dendrites on each fiber and provide feedback
on muscle length – nerve fires continually
stretch increases firing rate and relaxation
decreases firing rate but differs from primary
firing rate– gives information on joint angles
and muscle length even when muscle is
relaxed
Receptors in muscle control
• Beta fiber axon endings (efferents) –
Synapse with intrafusal muscle fibers –
these are collateral axons from the alpha
motor neuron – adjust tension in the
intrafusal fibers to maintain sensitivity as
the fiber contracts
Receptors in muscle control
• Gamma motor neurons (efferents) -
Synapse with intrafusal muscle fibers
near polar regions – Control spindle
sensitivity in unloaded state (non
contractile)
Receptors in muscle control
• D. Golgi Tendon Organs - Encapsulated
sensory receptor – much simpler than the
muscle spindles –
• Capsule – Golgi tendon organs are thin
encapsulated receptors with an elongated
tube shape approximately 1 mm in length
and 0.1 mm in diameter
Receptors in muscle control
• Type 1b nerve fiber enters the capsule and
•
splits into hundreds of dendritic endings which
are associated with collagen fibers at the
muscle tendon junction – nerve fires continually
– increased firing rate with increased tension
and decreased rate with relaxation
Collagen fibers twist together at this point so
that the Golgi tendon body may associate with
as many as 15 different muscle motor units
Receptors in muscle control
• Sensitive to minute changes in muscle
tension - Stretch of the tendons or
passive changes in tension
Receptors in muscle control
• Free Nerve Endings – Skeletal muscle is
richly endowed with numerous free
nerve endings (type II, III, and IV fiber
types). However very little is known
about these other than type and
conjecture as to function.
Receptors in muscle control
• Type II fibers – respond mostly to pressure –
•
•
•
may be giving information on blood flow and
tissue pressures
Type III fibers – respond to pain, temperature,
and chemical stimuli
Type IV fibers - temperature and chemical
stimuli
There is some evidence that these fibers may
also act on the vasomotor and pulmonary
centers in the medulla, to signal for increased
blood flow and increased breathing rate during
exercise.
Receptors in muscle control
Receptors in muscle control
Receptors in muscle control
REFLEX CONTROL
• Reflex control – When a skeletal muscle is
passively stretched it responds with a
momentary contraction (Knee jerk reaction to
a hammer strike just below the patella)
– Classic experiments of Sherrington and
Lindel (1924 – 30) on decerebrate cats
showed that the stretch reflex has two
components and showed reciprocal and
synergist innervation
REFLEX CONTROL
Brisk phasic component – initiatial stretch
of the muscle results in a brief but fairly
powerful contraction of the skeletal
muscle
Weaker but longer lasting component
triggered by the new static stretch of
the lengthen skeletal muscle fibers
Reciprocal innervation – stretch of the
agonist muscle fibers produced
relaxation in the antagonists muscle
REFLEX CONTROL
Synergist innervation – stretch of the
agonists muscle fibers also produced
contractions in all synergistic muscles
REFLEX CONTROL
– Lloyd and Eccles (1946 – 48) showed that
group 1a axons from the muscle spindles
pass into the dorsal root of the spinal cord
and split into thousands of collaterals
• Group 1a collaterals form an excitatory
synapse with every alpha motor neuron
acting on the original muscle
REFLEX CONTROL
• Group 1a collaterals form an excitatory
•
synapse with all synergistic muscle alpha
motor neurons (Synergist innervation)
Group 1a collaterals form an excitatory
synapse with inhibitory interneurons
which synapse with all antagonist muscle
alpha motor neurons (Reciprocal
innervation)
REFLEX CONTROL
– Stretch reflex tends to resists passive
lengthing of the muscles from their
original resting position
– Chiropractically this reflex can be used
to test for vertebral and nerve damage
– compression or extension of the
dorsal root would tend to slow or block
the reflex
REFLEX CONTROL
– Flexion reflex afferent pathways – Group
II fibers from the secondary muscle
spindle nerves and some free nerve
endings (group III and IV fibers) seem
to stimulate homogenous alpha motor
neurons and inhibit antagonist muscle
alpha motor neurons
REFLEX CONTROL
• Motor control mechanism
– Central motor command – Alpha motor
neuron
• Single action potential produces a sub
max twitch
• Collaterals synapse on agonists muscle
alpha motor neurons and on antagonists
alpha motor neurons – agonists muscle
begins contraction and antagonist muscle
begins contraction at same time
REFLEX CONTROL
REFLEX CONTROL
• Feedback from muscle spindles, Golgi
•
tendon organ, and free nerve endings
constantly modify central motor command
to properly position limb in space and
time
Feedback from muscle spindles (Group 1a
fibers) excites synergistic muscles to
contract in concert
REFLEX CONTROL
• Feedback from muscle spindles (Group
1a fibers)
• cause inhibition of antagonists muscle
•
alpha motor neurons – begins to relax
antagonists muscle fibers to allow
eccentric contraction
Secondary command continues movement
while making necessary adjustments
based on feedback information to properly
align speed and force of the movement
SKELETAL MUSCLE
MOTOR CONTROL
• Summary
• 1. What is the difference between intrafusal
and extrafusal fibers?
• 2. How does fiber type tie into function?
(chart)
• 3. What is a muscle spindle body and how
does it operate?
• 4. What is a Golgi tendon body and how does
it operate?
SKELETAL MUSCLE
MOTOR CONTROL
• Summary cont.
• 5. What is reflex control, how does it operate?
• 6. How does skeletal muscle control operate,
affect on synergistic muscle, agonist muscle
and antagonists muscle?