Human Physiology, 14th edition (2016)

388 Chapter 12

neuron directly synapses with the motor neuron, without
involving spinal cord interneurons. The stretch reflex is thus
a monosynaptic reflex in terms of the individual reflex arcs
(although many sensory neurons are activated at the same
time, leading to the activation of many motor neurons).
Stretch reflexes maintain muscles at an optimal length, as
previously described under the heading “Length-Tension
Relationship” in section 12.3.
The stretch reflex is present in all muscles, but it is most
dramatic in the extensor muscles of the limbs. The knee-jerk
reflex (patellar tendon reflex) —the most commonly evoked
stretch reflex—is initiated by striking the patellar ligament
with a rubber mallet. This stretches the entire body of the
muscle, and thus passively stretches the spindles within the
muscle so that sensory nerves with primary (annulospiral)
endings in the spindles are activated. Axons of these sensory
neurons synapse within the ventral gray matter of the spinal
cord with alpha motoneurons. These large, fast-conducting
neurons stimulate the extrafusal fibers of the quadriceps femo-
ris muscles to produce concentric (shortening) contractions,
resulting in the reflex extension of the knee joint. This is an
example of negative feedback—stretching of the muscles (and
spindles) stimulates shortening of the muscles (and spindles).
These events are summarized in table 12.6 and illustrated in
figure 12.28.

Figure 12.28 The knee-
jerk reflex. This is an example
of a monosynaptic stretch reflex.

Patellar

ligament

Spindle Alpha

motoneuron

Patella

Sensory

neuron

Spinal cord

Extrafusal

muscle

fibers

Tendon

1. Striking patellar ligament
   stretches tendon and quadriceps
   femoris muscle.
   4. Alpha motoneuron
   stimulates extrafusal
   muscle fibers to
   contract.


2. Sensory neuron
   activates alpha
   motoneuron.


3. Spindle is stretched,
   activating sensory
   neuron.


4. Passive stretch of a muscle (produced by tapping its tendon)
   stretches the spindle (intrafusal) fibers.


5. Stretching of a spindle distorts its central (bag or chain)
   region, which stimulates dendritic endings of sensory
   neurons.


6. Action potentials are conducted by afferent (sensory) nerve
   fibers into the spinal cord on the dorsal roots of spinal
   nerves.


7. Axons of sensory neurons synapse with dendrites and cell
   bodies of somatic motor neurons located in the ventral horn
   gray matter of the spinal cord.


8. Efferent nerve impulses in the axons of alpha motoneurons in
   the ventral roots of spinal nerves are conducted to the ordinary
   (extrafusal) muscle fibers.


9. Release of acetylcholine from the endings of alpha
   motoneurons stimulates the contraction of the extrafusal fibers,
   and thus of the whole muscle.


10. Contraction of the muscle relieves the stretch of its spindles,
    thus decreasing activity in the spindle afferent nerve
    fibers.

Table 12.6 | Summary of Events in a

Monosynaptic Stretch Reflex