Answer:
Birds can keep themselves warn without any external heat source which is not the case with reptiles and amphibians
Explanation:
Birds are able to generate heat means they are endothermic. They generate heat through various metabolic processes. They have air pockets beneath their feathers to insulate their bodies during cold temperatures. Feathers which are modified reptilian scales help birds to streamline their exterior body and structure feathers in a way that it insulates the body.
A sarcomere is the basic contractile unit of muscle fiber. Each sarcomere is composed of two main protein filaments—actin and myosin—which are the active structures responsible for muscular contraction. The most popular model that describes muscular contraction is called the sliding filament theory.
Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem. So i beleive it is 4 but not too sure. Hope this helped a bit (: