Chapter 13 - The Peripheral Nervous System and Reflex Activity - Review Questions - Page 522: 11

The answer is "C", the tendon reflex. In this polysynaptic reflex, the creation of increased tension in the tendon organ ( through passive stretching or by muscle contraction) will cause the tendon organ to fire-- when threshold is reached; The afferent impulses from the tendon organ go to the brain and the spinal cord. The afferent impulse that goes to the cerebellum is used for adjusting or smoothening muscle action, The afferent neuron that goes to the spinal cord branches, one branch synapses with a Renshaw cell which synapses with and inhibits the motor neuron to the agonist muscle. The result is that the alpha motor neuron to the initially contracting muscle inhibits its contraction and causes it to relax. But a branch of the afferent neuron also synapses with the an excitatory motor to the antagonist muscle. The antagonist muscle is therefore stimulated or activated while simultaneously the agonist is inhibited or relaxed. This is called reciprocal activation.

An example of reciprocal inhibition is the tendon organ reflex (Golgi tendon reflex). This is a polysynaptic reflex that incorporates a reciprocal activation of an antagonist muscle. When muscle contraction (or passive stretch) raises the tension in the tendon organ (Golgi tendon ) to threshold , it sends afferent impulses to the spinal cord and to the cerebellum. The brain uses the its received impulses to monitor and adjust muscle contraction by way of gamma motor neurons (finer fibers than alpha motor neuron). The afferent fiber to the spinal cord branches; one branch synapses with an inhibitory neuron (a Renshaw cell) which in turn synapses with a large alpha neuron going back to the initially contracting muscle (the agonist). This arrangement results in inhibition and relaxation of the agonist muscle. Another branch of the sensory neuron synapses with an excitatory interneuron in the spinal gray matter. This excitatory interneuron in turn synapses with an alpha motor neuron which in this case carries an excitatory impulse to the antagonist. This process is called reciprocal activation: the agonist muscle on one side a joint relaxes to accommodate the activation of an antagonist muscle on the other side of the joint The classical example of a tendon reflex is the patellar tendon reflex in which the quadriceps is the agonist and the hamstring muscle the antagonist. The flexing of the biceps is also an example of this reflex, in which actions inolve the principles of the tendon reflex.