Reflex patters initiated by the secondary sensory fiber endings of muscle spindles: a proposal.

Muscle spindles have two kinds of sensory fibers, each with stretch-sensitive endings on the nuclear bag muscle fibers and the nuclear chain muscle fibers within the spindle. The stretch reflex responses initiated by the primary sensory fibers have been well defines. There are other stretch reflexes of poorly defined origin and distribution. Clinical studies on spastic patients indicate that these stretch reflexes show regional responses or regional synergies. Stretch of flexor muscles of proximal joints of an extremity causes slowly recruiting, prolonged, reflex flexion in that extremity, in the contralateral extremity and also in the ipsilateral extremity, producing the crossed extension-flexion reflex and long spinal reflex. Likewise, stretch of extensor muscles of the proximal joints initiates a similar extension reflex in the same distribution. Stretch of the distal flexors of an extremity reflexly spreads flexion through that extremity. Stretch of the distal extensors of an extremity spreads extension throughout that extremity. These reflexes can be abolished by intramuscular neurolysis with phenol. They have long internuncial pathways and involve both alpha and gamma motoneurons. The demonstration that these reflex patterns are initiated by stretch of the muscle and show characteristics similar to those shown in physiological studies by the secondary sensory spindle reflexes suggests that they are initiated by stretch of the endings of the secondary sensory fibers of muscle spindles.