Stretch myotatic reflexknee jerk Figure 29 is monosynaptic

a. Muscle is stretched, and the stretching stimulates group la afferent fibers.

Table 2-7. Summary of Muscle Reflexes

Number of

Afferent

Reflex

Synapses

Stimulus

Fibers

Response

Stretch reflex (knee jerk)

Monosynaptic

Muscle is

la

Contraction of the

stretched

muscle

Golgi tendon reflex (clasp-

Disynaptic

Muscle

lb

Relaxation of the

knife)

contracts

muscle

Flexor-withdrawal reflex

Polysynaptic

Pain

II, III,

Ipsilateral flexion;

(after touching a hot

and IV

contralateral

stove)

extension

Muscle Afferents Group Iii
Figure 2-9. The stretch reflex.

b. Group la afferents synapse directly on a-motoneurons in the spinal cord. The pool of a-motoneurons that is activated innervates the homonymous muscle.

c. Stimulation of a-motoneurons causes contraction in the muscle that was stretched. As the muscle contracts, it shortens, decreasing the stretch on the muscle spindle and returning it to its original length.

d. At the same time, synergistic muscles are activated and antagonistic muscles are inhibited.

e. Example of the knee-jerk reflex. Tapping on the patellar tendon causes the quadriceps to stretch. Stretch of the quadriceps stimulates group la afferent fibers, which activate a-motoneurons that make the quadriceps contract. Contraction of the quadriceps forces the lower leg to extend.

-Increases in y-motoneuron activity increase the sensitivity of the muscle spindle and therefore exaggerate the knee-jerk reflex.

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