The muscles that start shivering represent which

part of the control system?

Primary motor center is a system which is responsible for shivering of muscles.

Explanation:

Primary motor center is an area which is responsible for the movement of muscles by producing impulses. Shivering is also the movement of muscles which occurs when the body feels cold. This shivering occurs for the production of heat in the body and soon the body gets warm.

The thermostatic control center in the hypothalamus.

Explanation:

In the hypothalamus, a control area presents which controls the temperature of the body. Primary motor center for shivering present in the posterior hypothalamus which is continuously inhibited by heat signals which come from the heart center present in the anterior hypothalamus. But when the body felt cold than from the skin and spinal code signals generated and excite the shivering area. The body starts shivering which involves muscles of the body.