Spinal Control of Motor Units Quiz

Spinal Control of Motor Units

• Muscle spindles are specialized structures within skeletal muscles that are proprioceptors specialized in detecting changes in muscle length.
• Muscle spindles consist 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 and are specialized for the detection of changes in muscle length (stretch).
• Ia axons are the thickest myelinated axons in the body and are the largest and fastest within the group.
• Ia axons 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.
• The discharge of Ia sensory axons is closely related to the length of the muscle.
• The Ia axon and the alpha motor neurons on which it synapses constitute the monosynaptic stretch reflex arc.
• The stretch reflex involves sensory feedback from the muscle and was first shown by Sherrington, who noted that when a muscle is pulled on, it tends to pull back (contract).
• The stretch reflex arc serves as an antigravity feedback loop.
• The knee-jerk reflex is one example of the stretch reflex that tests the intactness of the nerves and muscles in this reflex arc.
• Peripheral sensory and motor nerves are vulnerable to various types of injury, and the cut axons of peripheral nerves can often regenerate and reestablish their connections with muscle.
• Regenerated axons and synapses have been studied carefully in the neural circuits of the stretch reflex.