Muscle stretch (tendon) reflex activity and muscle tone depend on the status of the large motor neurons of the anterior horn (the alpha motor neurons), the muscle spindles and their afferent fibers, and the small anterior horn cells (gamma neurons), whose axons terminate on the specialized intrafusal muscle fibers (nuclear chain fibers) within the spindles. Some gamma motor neurons are tonically active at rest, keeping the intrafusal muscle fibers taut and more sensitive to active and passive changes in muscle length. Each anterior horn cell has on its surface membrane approximately 10,000 receptive synaptic terminals. Some of these terminals are excitatory, others inhibitory; in combination, they determine the activity of the neuron. Beta motor neurons effect cocontraction of both spindle and nonspindle fibers, but the physiologic significance of this innervation is not fully understood.
A tap on a tendon stretches causes a vibratory wave of the spindle and activates its nuclear bag fibers. Afferent projections from these fibers synapse directly with alpha motor neurons in the same and adjacent spinal segments; these neurons, in turn, send impulses to the skeletal muscle fibers, resulting in the familiar monosynaptic muscle contraction or monophasic (myotatic) stretch reflex, commonly referred to as the tendon reflex or “tendon jerk” (Fig. 3-1), more correctly called the muscle stretch or proprioceptive reflex. All this occurs within 25 ms of the sudden stretch. The alpha neurons of antagonist muscles are simultaneously inhibited but through disynaptic rather than monosynaptic connections. This is accomplished in part by inhibitory interneurons (reciprocal inhibition), which also receive input from descending pathways. Renshaw cells also participate by providing negative feedback through inhibitory synapses of alpha motor neurons (recurrent inhibition).
Thus the tension in the spindle fibers and the state of excitability of the alpha and gamma neurons (influenced greatly by descending fiber systems) determines the level of activity of the tendon reflexes and muscle tone (the responsiveness of muscle to stretch). Other mechanisms, of an inhibitory nature, involve the Golgi tendon organs, which detect the tension produced by active contraction of muscle. These encapsulated receptors, which lie in the tendinous portions of muscle, activate afferent fibers that end on internuncial cells, which project to alpha motor neurons, thus forming a disynaptic reflex arc. Golgi tendon receptors are silent in relaxed muscle and during passive stretch; they serve, together with muscle spindles, to monitor or calibrate the length and force of muscle contraction under different conditions. They also play a role in naturally occurring limb movements, particularly in locomotion.
The alpha motor neurons are located in the anterior gray matter (anterior horn) of the spinal cord. The medial parts of the anterior horn supply trunk or axial muscles, and neurons of the lateral parts supply the appendicular muscles. The largest neurons innervate large muscles with large motor units. Smaller anterior horn cells innervate small muscles and control more delicate movements, particularly those in the fingers and hand. Both groups of alpha neurons receive projections from the propriospinal neurons via the large fasciculi proprii from adjacent spinal segments. All the facilitatory and inhibitory influences supplied by cutaneous and proprioceptive afferent and descending suprasegmental neurons are coordinated at segmental levels. For further details the reader may consult Burke and Lance and also Davidoff (1992).