A baseball pitcher planning a
pitch…
The mental body
image seems to be
generated by
somatosensory,
proprioceptive, and
visial inputs to the
posterior parietal
cortex (area 5, area
7)
The motor control hierarchy have
three levels
• The highest level---strategy
• Represented by the association areas of
neocortex and basal ganglia of the forebrain
• Is concerned with strategy:
• The goal of the movement and the
movement strategy that best achieves the
goal.
The motor control hierarchy have
three levels
• The middle level---tactics
• Represented by the motor cortex and
cerebellum
• Is concerned with tactics
• The sequences of muscle contractions
• Arranged in space and time
• Required to smoothly and accurately
achieve the strategic goal.
The motor control hierarchy have
three levels
• The lowest level—execution
• Represented by the brain stem and spinal
cord
• Is concerned with execution
• Action of the motor neuron and interneuron
pools that generate the goal-directed
movement and make any necessary
adjustments of posture.
organization of motor subsystems
Part 1
Spinal Control of
Movement
Introduction
• The rhythmic movements could be elicited
in the hind legs of cats and dogs long after
their spinal cords had been severed from the
rest of the central nervous system
---Charles Sherington Graham Brown(English)
The importance of circuitry within the spinal cord
for the coordinated control of movements
Spinal motor neurons
Alpha motor neurons
Innervate skeletal muscle (extrafusal muscle)
Lower
Function: directly command muscle contract
motor
Gamma motor neurons
neuro
ns
Innervate muscle spindle (intrafusal muscle)
Function: regulating the muscle spindle
Interneurons
Allows coordinated motor programs to be generated
Final common pathway
Directly command muscle contract
Compare: Upper motor neurons
αmotor neuron
Size
γmotor neuron
30-150μ
10-30μ
Low
High, spontaneous
Controlled
High center
Peripheral
High center
Effector
Largeα：fast muscle
Smallα：slow muscle
Intrafusal muscle fiber
Effect
Muscle contraction
Muscle spindle
Excitability
Spinal interneurons
• Most of the input to the alpha motor neurons
comes from interneurons of the spinal cord.
• Interneurons receive synaptic input from primary
sensory axons, descending axons from the brain,
and collaterals of lower motor neuron axons.
• The interneurons are themselves networked
together in a way that allows coordinated motor
programs to be generated in response to their
many inputs.
The Lower Motor Neurons
• Alpha motor neurons are directly responsible for
the generation of force by muscle
• Motor unit: one alpha motor neuron and all of the
muscle fibers it innervates collectively make up
motor unit, the elementary component of motor
control
• Motor neuron pool: the collection of alpha motor
neurons that innervates a single muslce.
I. Final Common Pathway and Motor Unit
Anterior motor neurons of the spinal cord
Anterior horns of the
cord gray matter
α，β，γ
Muscle innervation by lower motor
neurons
The ventral horn of the spinal
cord contains motor neurons
that innervate skeletal muscle
fibers
30 mixed spinal
nerves
Cervical 1-8
Thoracic 1-12
Lumbar 1-5
Sacral 1-5
Ventral roots+Dorsal root=Spinal nerve
Inputs to alpha motor neurons
This input may be excitatory or inhibitory
and is part of the circuitry that generates This input provides feedback about
muscle length
the spinal motor programs
This input is important for the initiation
and control of voluntary movement
Alpha-motoneuron – final common pathway
organization of motor systems
Motor Cortex
Brain Stem
Spinal Cord
-motor
neuron
Final common
pathway
Skeletal muscle
motor neuron:
final common
pathway
Spinal control of Motor Function
Motor neurons in the spinal cord
–α motor neurons
Causes contraction of the innervated muscle.
Final common pathway.
– γ motor neurons
innervates intrafusal muscle to adjust the
sensitivity of muscle spindle to stretch.
•Every striated
muscle has
encapsulated
muscle fibers
scattered
throughout the
muscle called
muscle spindles.
•Extrafusal and
intrafusal muscle
fibers
The extrafusal
muscle fibers are
innervated by
Alpha motor
neuron
skeletal muscle contraction
The intrafusal
muscle fibers are
innervated by
Gamma motor
neurons
regulate sensitivity of muscle spindle to stretch stimulation
Motor unit
※
 A motor unit consists of a single α-motor
neuron and the group of muscle fibers
which it innervates. The elementary component of motor
control.
For the refined motion
For the strength
Innervation ratio
Fibers per motor neuron
Extraocular muscle 3:1
Gastrocnemius 2000:1
•The muscle cells
of a motor unit are
not grouped, but
are interspersed
among cells from
other motor units
•The coordinated
movement needs
the activation of
several motors
organization of motor systems
Voluntary movements
Motor Cortex
Nociceptive stimulus
Stretch muscles
Reflex
Brain Stem
Posture maintenance
Spinal Cord
Final common
pathway
Receptor
Skin Muscle Joint
-motor
neuron
Effector
Skeletal muscle
Regulating Functions of the Spinal Cord
Spinal shock
Spinal Reflex
spinal animal:
is prepared by
transecting the
spinal cord at the
fifth cervical
segment
※
Spinal shock
• Concept:
Complete transection of the spinal cord
results in the immediate paralysis and loss
of sensation in all body regions innervated
by spinal cord segments below the lesion. All
reflex in the isolated segments of the spinal
cord are at once totally depressed. This is
called spinal shock.
Symptoms of spinal shock
During spinal shock:
complete loss of all reflexes,
no tone, paralysis, complete anaesthesia,
no peristalsis, bladder and rectal reflexes absent
(no defecation and micturition )(Feces is detained
in rectum and urine in bladder )
no sweating (Sweating reflex is suppressed)
arterial blood Pressure decrease(40mmHg)
(BP drops down, peripheral blood vessels are
dilated)
Spinal shock
The reason: Is result of the sudden withdrawal
of descending control from suprasegmental
centers in the brain.
The recovery of spinal neurons excitability.
The time course of recovery from spinal shock
varies considerably with the species studied. It
is increased with ascent of the phylogenetic
scale.
Extensor reflex weaken
HighFacilitate
central ?
Flexor reflex strengthen
Inhibit
※ Postural reflex
Flexion reflex and crossed
extension reflex
• -is a protective reflex
and a polysynaptic
reflex.
• -results in flexion on
the ipsilateral and
extension on the
contralateral.
• -can be innervated by
noxious stimulates.
Flexion reflex and
crossed extension reflex
There is a rapid lifting of
the affected foot (ipsilateral
withdrawal reflex ),
while the contralateral
response activates the
extensor muscles of the
opposite leg (contralateral
extensor reflex)
support the weight shifted
to it.
Flexion reflex and crossed extension reflex
Flexor reflex
•A noxious stimulus to the spinal animal
induces flexion upward and away from the
stimulus.
• Reflex response consists of facilitation of
flexor muscles and inhibition of extensor
muscle in the stimulated limb.
• That can provide protection from the injury.
Crossed extensor reflex
If the stimulus sufficiently strong,
flexion of ipsilateral limb and extension of
the contra-lateral limb which serves to
support the body weight and maintain the
body balance.
※
• Stretch reflex includes:
•
•
Tendon reflex (Phasic stretch reflex ) ：
short lasting and relatively intense (Ⅰa), strong
reflex contraction.(single synaptic reflex).
•
•
Muscle tonus (Tonic stretch reflex ) :
less intense but lasts longer, maintain the body
posture.（polysynaptic reflex）
Tendon reflex—knee-jerk reflex;
 Caused by rapid stretch of
the muscle,
 Transmitted from the Ia
sensory ending of the M. S.
 Causes an instantaneous,
strong reflex contraction of the
same muscle;
 Opposing sudden changes in
length of the M.S;
 A monosynaptic pathway:
being over within 0.7 ms;
elbow jerk
Achilles
tendon
reflex
Muscle tonus
 Caused by a weaker and continues stretch of the muscle,
 Transmitted from the Ia and II sensory ending of the M. S.
 Multiple synaptic pathway, continues for a prolonged
period.
 Non-synchronized contraction,
 for at least many seconds or minutes, maintaining the
posture of the body.
tendon reflex and the tonic stretch
reflex
Type
stimulation
Tendon reflex
suddenly stretched
Afferent fiber Ia sensory ending
Synapse
Tonic stretch reflex
Weaker, continuous
stretch
Ia and Ⅱ sensory
ending
Monosynaptic reflex Polysynaptic reflex
Significance Determine the
lesion areas
maintenance of posture
Stretch reflex Muscle sensors
• Types of muscle fibers
• a. Extrafusal fibers
• -make up the bulk of
muscle
• -are innervated by
α-motoneurons.
• -provide the force for
muscle contraction.
Stretch reflex Muscle sensors
• b. Intrafusal fibers
• -are smaller than
extrafusal fibers
• -are innervated by γmotoneurons.
• -are encapsulated in
sheaths to form
muscle spindles
Stretch reflex Muscle sensors
• Muscle spindles:
• -are distributed
throughout muscle
• -intrafusal fibers is in
parallel with extrafusal
fibers.
• -Nuclear bag fibers and
Nuclear chain fibers.
Stretch reflex
How the muscle spindle works
• -When a muscle is
stretched → the muscle
spindle is also stretched
→ stimulates group Ⅰa
and Ⅱ afferent fibers.
• →α-motoneurons →
• Innervates the extrafusal
fibers → the muscle
contracts
Stretch reflex
How the muscle spindle works
• γ-motoneurons
• -innervate intrafusal
muscle fibers.
• -adjust the sensitivity
of the muscle spindle.
• -receive control of
high center .
WHAT IS THE -LOOP?
Descending influence
Muscle spindle
Ⅰa
γ
Activation of the γ-loop
results in increased
muscle tone
α
MUSCLE
The Gamma loop
• Changing the activity of the gamma motor
neurons changes the set point of the myotatic
feedback loop
• The Gamma loop
• Gamma motor neuron—intrafusal muscle fiber--Ⅰa afferent---alpha motor neuron—extrafusal
muscle fibers
• Alpha and gamma motor neurons are
simultaneously activated by descending
commands from the brain.
Tendon organ and
inverse stretch reflex
Tendon organ
Change of tension
Ⅰb afferent fiber
Tendon
organ
Action of Tendon Organ
Tendon organ embedded
in the tendon at the end
of the muscle measure
the tension of muscles
can be activated by
isometric contraction of
extrafusal fibers. Tendon
organ is innervated by Ib
afferent fibers, which
synapses with inhibitory
interneurons and inhibit
α motor neurons .
Proprioception from Golgi tendon
organs
• Acts like a strain gauge, it monitors muscle
tension, or the force of contraction.
• Located at the junction of the muscle and
the tendon and are innervated by group Ⅰb
sensory axons that are slightly smaller than
the Ⅰa axons innervating the muscle
spindles.
Tendon organ
Mechanism of stretch reflex
• A skeletal muscle is stretched→ spindle is stretched
→the impulses are conducted to the spinal cord by
Ⅰa/Ⅱfibers
to
activate
the
α-motor
neurons→which supply the same muscle → the
muscle contracts to oppose the stretch (the
antagonist muscle relaxes ).
• if stronger stretch applied on muscle, it changes the
muscle tone → stretch the tendon organ → in turn
to inhibit αmotoneurons by inhibitory interneurons
→ to inhibits stretch reflex to prevent the stretched
muscles from injury.
Summary
• Spinal shock
• Stretch reflex
Question
• Describe the neuronal circuit and function
of the skeletal muscle stretch reflexes in
detail, and difference in the different types
of the reflexes.
(1) The basic circuit:
• Sudden stretch of a muscle excites the
muscle spindle, and Ia proprioceptor nerve
fiber sends signals to the spinal cord,
synapses directly with anterior motor
neurons that send nerve fiber back to the
extrafusal muscle fibers of the same muscle,
causing reflex contraction of the muscle.
(2) tendon reflex and the tonic
stretch reflex
Type
Tendon reflex
Tonic stretch reflex
suddenly stretched
Weaker, continuous
stretch
Afferent
fiber
Ia sensory ending
Ia and Ⅱ sensory
ending
Synapse
Monosynaptic reflex
Polysynaptic reflex
stimulation
Significance
Determine the lesion
maintenance of posture
areas
(3)Functions:
• Tendon reflex causes an instantaneous,
strong reflex contraction of the same muscle;
Muscle tonus is good for maintaining the
posture of the body.