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George E. Carvell, PhD, PT   University of Pittsburgh
fig 13.17
p 454
Deep tendon reflexes are clinical tests of the most basic spinal reflex arc. Primary endings in the dynamic nuclear bag 1 element of the muscle spindle are exquisitely sensitive to brief, high velocity stretches. The spindle endings in the muscle(s) attached to the tendon that is struck by a reflex hammer will normally send a brief burst of impulses along the Group Ia afferents back to the spinal cord. Ia afferents have powerful monosynaptic connections on alpha motoneurons in the ventral horn of the spinal gray. The motoneurons that typically have sufficient depolarization to bring them to threshold are those that innervate motor units in the homonymous agonist muscle. Other motoneurons in adjacent agonist and synergist motoneuron pools may be depolarized at a subthreshold level. The normal response to this tendon tap is a twitch of a few slow-twitch (S) motor units in the homonymous muscle; the S motor units have the strongest monosynaptic excitatory drive from the Ia afferents. Hyperactive reflexes due to an upper motor neuron lesion (e.g., stroke) show large amplitude responses in the homonymous muscle & may radiate to include synergist muscles.
GMOMM  2001