Please enable JavaScript.
Coggle requires JavaScript to display documents.
Spinal Control of Movement - Coggle Diagram
Spinal Control of Movement
How does the spinal cord’s command and control of coordinated muscle contraction take place? What neurons are involved? ,,,
The Lower Motor Neurons
somatic motor neurons in the ventral horn of the spinal cord
called lower motor neurons to distinguish them from the higher order upper motor neurons of the brain that supply
input
to the spinal cord.
only the lower motor neurons directly command muscle contraction.
innervate the somatic musculature
Segmental Organization
axons of lower motor neurons bundle together to form ventral roots
motor neurons that provide fibers to one spinal nerve are said to belong to a spinal segment
named for the vertebra where the nerve originates
The segments are cervical (C) 1–8, thoracic (T) 1–12, lumbar (L) 1–5, and sacral (S) 1–5
Skeletal muscles are not distributed evenly throughout the body, nor are lower motor neurons distributed evenly within the spinal cord.
e.g.
innervation of the more than 50 muscles of the arm originates entirely from spinal segments C3–T1
dorsal and ventral horns in this region appear swollen
spinal segments L1–S3
two categories of lower motor neurons
alpha motor neurons
directly trigger the generation of force by muscles
organization
One alpha motor neuron and
all
the muscle fibers it innervates collectively make up the elementary component of motor control:
the motor unit
The collection of alpha motor neurons that innervates a single muscle is called a
motor neuron pool
grading of control
need amount of force to be
just right
:girl:
NS uses several mechanisms to control the force of muscle contraction in a finely
graded
fashion.
recruiting additional synergistic motor units
provides extra tension
1 more item...
varying the firing rate of motor neurons :lightning:
continual barrage of action potentials
1 more item...
regulation
controlled by synaptic inputs in the ventral horn
only
three
major sources of input to an alpha motor neuron
dorsal root ganglion cells
axons innervate a specialized sensory apparatus embedded within the muscle known as a
muscle spindle.
:straight_ruler:
1 more item...
upper motor neurons
from the motor cortex and brain stem
1 more item...
interneurons in the spinal cord
largest input to an alpha motor neuron
may be excitatory or inhibitory input
gamma motor neurons
Excitation-Contraction Coupling
Muscle contraction is initiated by the release of acetylcholine (ACh) from the axon terminals of alpha motor neurons
ACh produces a large EPSP in the postsynaptic membrane
excitation
membrane of the muscle cell
(sarcolemma)
contains voltage-gated sodium channels that open → the EPSP is sufficient to evoke an action potential
AP sweeps down the sarcolemma and into the T tubules.
Depolarization of the T tubules triggers the release of Ca2+ from the SR
contraction
of the fiber
Ca2+ binds to troponin.
1 more item...
Spinal Control of Motor Units
deep within most skeletal muscles are specialized structures called
muscle spindles
also called a
stretch receptor
consists of several types of specialized skeletal muscle fibers contained in a fibrous capsule
what happens in the swollen middle (equatorial) region?
group Ia sensory axons wrap around the muscle fibers of the spindle
spindles + their associated Ia axons =
proprioceptors
Other sensors
Golgi tendon organs
3 more items...
mechanosensitive axons in the connective tissues of joints
specialized for the detection of changes in muscle length (stretch)
within Group I axons, Ia axons are the largest and fastest
conduct action potentials very rapidly.
enter the spinal cord via the dorsal roots, branch repeatedly, and form excitatory synapses upon:
both interneurons and alpha motor neurons of the ventral horns
very powerful inputs
reciprocal inhibition
:muscle:
inside spindle:
intrafusal
fibers
receive their motor innervation by
gamma motor neuron
circuit
(gamma loop)
:
gamma motor neuron
→ intrafusal muscle fiber
1 more item...
stretch (myotatic) reflex
When a muscle is pulled on, it tends to pull back
motor neurons must receive a continual synaptic input from the muscles
monosynaptic stretch reflex arc
The stretching of the equatorial region of the spindle leads to depolarization of the Ia axon endings
due to the opening of mechanosensitive ion channels
1 more item...
“monosynaptic” because only one synapse separates the primary sensory input from the motor neuron output.
provides additional
control
of alpha motor neurons and muscle contraction.
discharge of Ia sensory axons is closely related to the length of the muscle
As the muscle is stretched, the discharge rate goes up.