SPINAL CONTROL OF MOTOR UNITS
Proprioception from Muscle Spindles
Deep within most skeletal muscles are specialized structures called muscle spindles (see Fig.
13.15). It consists of several types of specialized skeletal muscle fibers contained in a fibrous
capsule. Group Ia sensory axons wrap around the muscle fibers of the spindle the middle third.
The spindles and their axons are specialized for the detection of changes in muscle length and
are examples of proprioceptors.
These receptors inform us about how our body is positioned and moving in space
Group I axons are the thickest myelinated axons in the body and conduct action potentials very
rapidly. Within this group, Ia axons are the largest and fastest. Ia axons enter the spinal cord via
the dorsal roots and for excitatory synapses upon both interneurons and alpha motor neurons of
the ventral horns.
A single Ia axon synapses on virtually every alpha motor neuron in the pool innervating
the same muscle that contains the spindle.
The Myotatic Reflex
When a muscle is pulled on, it tends to pull back (contract). This is the myotatic reflex (or
stretch reflex) and it involves sensory feedback from the muscle.
See Fig. 13.16. When a weight is placed on a muscle and the muscle starts to lengthen, the
muscle spindles are stretched. When the equatorial region of the spindle is stretched, the Ia
axon endings are depolarized (due to opening of mechanosensitive ion channels) and discharge
of action potentials. This will depolarize the alpha motor neurons and cause them to discharge
action potentials and causes the muscle to contract.
Example: the knee-jerk reflex.
Gamma Motor Neurons
The muscle spindle contains modified skeletal muscle fibers within its fibrous capsule called
intrafusal fibers. These are different from the extrafusal fibers mainly because the extrafusal
fibers are innervated by alpha motor neurons whilst the intrafusal fibers are innervated by
gamma motor neurons.
When the alpha motor neurons respond, the extrafusal fibers contract, and the muscle shorten
(see Fig. 13.19). Gamma motor neurons then innervate the intrafusal muscle fiber at the two
ends of the muscle spindle which causes contraction of the two poles. This pulls on the
noncontractile equatorial region and keeps the Ia axons active.
The activation of alpha and gamma motor neurons has opposite effects on Ia output. Alpha
activation decreases Ia activity while gamma activation increases Ia activity.
Changing the activity of the gamma motor neurons changes the set point of the myotatic
feedback loop. This circuit: Gamma motor neuron intrafusal muscle fiber Ia afferent axon alpha motor
neuron extrafusal fibers is called the gamma loop.
Alpha and gamma motor neurons are simultaneously activated by descending commands from
the brain. By regulating the set point of the myotatic feedback loop, the gamma loop provides
additional control of alpha motor neurons and muscle contraction.
Proprioception from Golgi Tendon Organs
The Golgi tendon organ monitors muscle tension (the force of contraction). These sensors are
located at the junction of the muscle and the tendon and are innervated by group Ib sensory
axons (slightly smaller than Ia) (see Fig. 13.20). Golgi tendon organs are situated in a series to
muscle fibers unlike spindles which are situated in parallel with muscle fibers (see Fig. 13.21).
Note: Ia activity from the spindle encodes muscle length information while Ib activity from
the Golgi tendon organ encodes muscle tension information.
Ib axons enter the spinal cord and synapse on interneuron in the ventral horn. Some of these
interneurons form inhibitory connections with the alpha motor neurons innervating the same
muscle. This is the basis for the reverse myotatic reflex (see Fig. 13.22).
It protects the muscle from being overloaded in extreme cases. However, normally, it
regulates muscle tension within an optimal range
o As muscle tension increases, the inhibition of the alpha motor neuron slows
muscle contraction. As muscle tension falls, the inhibition of the alpha motor
neuron is reduced and muscle contraction increases.
Proprioception from the Joints
Besides muscle spindles and Golgi tendon organs, a variety of proprioceptive axons is present
in connective tissues of joints (esp. in fibrous tissue surrounding joints and ligaments). They
respond to changes in the angle, direction, and velocity of movement in a joint. Most are rapidly
adapting. This means that sensory information about a moving joint is plentiful but nerve
encodings the rest