Human Physiology, 14th edition (2016)

Muscle 389

a tendon from excessive muscle contraction, or from muscle

contraction that could add to the tension on a tendon during

passive stretching of the muscle.

Reciprocal Innervation and the Crossed-

Extensor Reflex

In the knee-jerk and other stretch reflexes, the sensory neuron

that stimulates the motor neuron of a muscle also stimulates

interneurons within the spinal cord via collateral branches.

These interneurons inhibit the motor neurons of antagonist

muscles via inhibitory postsynaptic potentials (IPSPs). This

dual stimulatory and inhibitory activity is called reciprocal

innervation ( fig. 12.30 ).

Golgi Tendon Organs

The Golgi tendon organs continuously monitor tension in the
tendons produced by muscle contraction or passive stretch-
ing of a muscle. Sensory neurons from these receptors syn-
apse with interneurons in the spinal cord; these interneurons,
in turn, have inhibitory synapses (via IPSPs and postsynap-
tic inhibition—chapter 7) with motor neurons that innervate
the muscle ( fig. 12.29 ). The inhibitory Golgi tendon reflex is
a disynaptic reflex because two synapses are crossed in the
CNS. One is an excitatory synapse between a sensory neuron
and a spinal interneuron, and the other is an inhibitory syn-
apse between the spinal interneuron and the alpha motoneuron.
This inhibitory reflex may help prevent dangerous tension on

Figure 12.29 The

action of the Golgi tendon

organ. An increase in muscle

tension stimulates the activity

of sensory nerve endings

in the Golgi tendon organ.

This sensory input stimulates

an interneuron, which in

turn inhibits the activity of a

motor neuron innervating that

muscle. This is therefore a

disynaptic reflex.

Golgi

tendon

organ

Spinal cord Sensory neuron

Alpha motoneuron

inhibited

+

2. Sensory neuron
   stimulates
   interneuron


3. Interneuron
   inhibits
   motoneuron


4. Tension on tendon
   activates sensory
   neuron


5. Tension on
   tendon is reduced

Figure 12.30 A
diagram of reciprocal
innervation. Afferent
impulses from muscle
spindles stimulates
alpha motoneurons to
the agonist muscle (the
extensor) directly, but (via an
inhibitory interneuron) they
inhibit activity in the alpha
motoneuron to the antagonist
muscle.

Motor neuron

(inhibited)

Flexor muscles

Motor neuron

Ta p

Extensor

muscles

Sensory neuron

Dorsal root

ganglion

+

1. Muscle stretch
   activates spindle
   apparatus


2. Agonist muscle
   contracts in
   stretch reflex


3. Antagonist
   muscle relaxes