Motor System Spinal Cord2
Motor System Spinal Cord2
Motor System Spinal Cord2
Spinal cord
Components of spinal motor control
system
• Spinal neurons
• Motor unit
• Muscle spindles
• Golgi tendon organs
Upper motor neuron
of extrapyramidal tract Upper motor
Dorsal root neuron of
ganglion cell corticospinal tract
α-motor neuron in
the spinal cord
Neuro muscular
junction
α-mn is directly
Muscle responsible for
spindle generation of
force by muscle muscle
Golgi Tendon organ
50 muscles of the
Figure 5.28
arm innervated
from spinal
segments C3-T1
Page 173
Cervical Cervical
cord nerves Vertebrae
Muscles of the leg
innervated from
spinal segments
L1-S3
Thoracic
Thoracic nerves
cord
Lumbar
Lumbar nerves Cauda
cord equina
Sacral
Sacral nerves
cord
Coccygeal
nerve
Cell body of White matter Gray matter
efferent neuron
Interneuron
Cell body of
afferent neuron Dorsal root
Dorsal root
Efferent fiber ganglion
From receptors
To effectors
Ventral root
Spinal nerve
Figure 5.29
Page 174
Figure 5.31 Page 176
Spinal cord
= Motor unit 1
A motor unit is one motor
= Motor unit 2
neuron and the muscle
= Motor unit 3 fibers it innervates
The size principle: the orderly
recruitment of motor units
• The first motor units to be activated are those
with smallest motor axons;
– these motor units generate the smallest contractile
forces
– and allow the initial contraction to be finely graded.
• As more motor units are recruited,
– the alpha motor neurons with progressively larger
axons become involved
– and generate progressively larger amounts of tension
Motor unit and motor neuron pool
Dorsal root
Dorsal root
ganglion
Ventral root
Figure 5.29
Page 174
Whole muscle tension depends on
• the size of the muscle,
• the extent of motor unit recruitment,
• the size of each motor unit.
• The number of muscle fibers varies among
different motor units.
– Muscles performing refined, delicate movements
have few muscle fibers per motor unit.
– Muscles performing coarse, controlled movements
have a large number of fibers per motor unit.
– The asynchronous recruitment of motor units
delays or prevents muscle fatigue.
• One group of motor neuron pools is
located in the medial part of the ventral
horn, and the other much larger group lies
more laterally.
Somatotopic organization of spinal cord motor neuron
trunk extremities
extensor
The ventral root s
Functional rule
• The motor neurons located medially project to
axial muscles (muscles of the neck and back):
those located more laterally project to limb
muscles (arms and legs).
• Within the lateral group the most medial motor
neuron pools tend to innervate the muscles of
the shoulder and pelvic girdles, while motor
neurons located more laterally project to distal
muscles of the extremities and digits.
• In addition the motor neurons innervating the
extensor muscles tend to lie ventral to those
innervating flexors.
Descending tracts
Dorsal surface
Lateral corticospinal
Ventral corticospinal
Vestibulospinal
Ventral surface
Alpha motor
neuron axon Intrafusal (spindle)
muscle fibers
Gamma motor
neuron axon Contractile end portions
of intrafusal fiber
Noncontractile
Secondary (flower-spray) central portion
endings of afferent of intrafusal
fibers fiber
Primary (annulospiral)
Extrafusal (“ordinary”) endings of afferent fibers
muscle fibers
Muscle spindles
• Can be stimulated by 2 ways
• Stretching the entire muscle
• Causing contraction of intrafusal fibers
while extrafusal fibers remain at the same
length.
Muscle spindles
• Group Ia afferent fibers form primary
endings on nuclear bag and chain fibers,
• Group II fibers form secondary endings on
nuclear chain fibers.
• Dynamic motor axons end on nuclear bag
fibers and static motor axons on nuclear
chain fibers.
Muscle spindles
• Primary endings demonstrate both static
and dynamic responses, which signal
muscle length and rate of change in
muscle length.
• Secondary endings demonstrate only
static responses and signal only muscle
length.
• Motor neurons cause muscle spindles to
shorten, which prevents the unloading
effect of muscle contraction.
Golgi tendon organs
• Located in the tendons of muscles and are
arranged in series.
• They are supplied by group Ib afferent
fibers and are excited both by stretch and
by contraction of the muscle (very
sensitive to changes in muscle tension)
Extrafusal
skeletal
muscle fiber
Spinal
cord
Intrafusal
muscle
spindle fiber
Patellar
tendon
Alpha motor
neuron
Figure 8.27
Page 288
Inverse stretch reflex
• Disynaptic (inhibitory interneuron+ α-mn )
• Inhibition of α-mn of same muscle
• Receptor: Golgi tendon organ (in series with
muscle fibers)
• Stimulus: increase in muscle tension by
– excessive stretch
– excessive active muscle contraction
• Result: relaxation (sudden stop in contraction)
• Safety:
– regulates muscle tension
– protects the tendon from tearing
Withdrawal reflex
• Polysynaptic
• Protective
• Painful stimulation of skin, subcutaneous
tissue or muscle
• Stimulation of flexorscontraction
• Reciprocal innervation
• Simultaneous inhibition of antagonists
relaxation
= Inhibitory interneuron Components of a
Receptor
= Inhibits
Page 178 = Stimulates Afferent pathway
Integrating center
Efferent pathway
Effector organs
Thermal
pain receptor
in finger
Ascending pathway
to brain
Afferent
Pathway
Stimulus
Afferent
pathway Efferent
pathway
Efferent
pathway Integrating center
Flexor (spinal cord)
Extensor Extensor
muscle Flexor
muscle muscle
contracts muscle
relaxes contracts
relaxes
Pain
Injured extremity
receptor
(effector organ)
in heel
Response
Response
Stimulus
Opposite extremity
(effector organ)
Upper motor
Dorsal root neuron of
ganglion cell Interneuron in corticospinal tract
the spinal cord
S Y α-motor neuron in
the spinal cord
Effector
W
U
X V
Receptor
Z
T