5. [Total: 8 pts] Muscle stretch reflex

The figure below shows a simplified model of muscle stretch reflex in an experimental anesthetized

animal. The model describes how the muscle stretch reflex regulates the muscle length. All variables

shown have been linearized and represent fluctuations around its mean value under normal resting

condition. Z represents external neural frequency stimulating the spinal cord. fa represents the

total change in afferent neural frequency. The spinal cord, represented by the gain Gc, converts the

incoming neural input to the neural output fe- The changes in the efferent neural frequency, fe, are

relayed through motor neurons to the muscle. The muscle converts the efferent neural frequency

to changes in muscle length L. The gain of this conversion is-G. Note that the gain is negative

because increases in efferent neural frequency lead to decreases in muscle length. The change in

the muscle length is sensed by the muscle spindle. The muscle spindle relays the neural output, fs,

to the spinal cord through sensory neurons. This process is represented by the gain Gg.

fa

fe

Gc

Z

-GM

Gs

fs

L

a) [4 pts] Suppose that animal A is injected with a drug that disrupts neurotransmission, blocking

the sensory neurons. Under such condition, is the system operating as a negative feedback

system? Explain your answer. Please also state which gain(s) is affected by the sensory nerve

blockade.

b) [4 pts] Under which condition between normal and sensory nerve blockade would result in

stronger muscle contraction for the same level of the external stimulation Z? Explain your

answer.