Muscle is derived from mesoderm. Actin and myosin are abundant in muscle. The three categories of muscle cells are smooth, skeletal, and cardiac, the last two of which are striated. Skeletal muscle cells develop as a consequence of several cells fusing end to end, hence the increased number of nuclei. Intercalated discs in cardiac muscle tissue represent areas of gap junctions between cells, which permits the myocardium to contract as a unit.
Homeostasis is the maintenance of constant extracellular conditions within the body.
A negative feedback loop prevents the departure of a controlled variable such as pH or temperature from its normal, or ideal, value. Changes in the controlled variable trigger responses that oppose the change and restore the variable to its ideal level. An example of negative feedback is the regulation of body temperature. Positive feedback refers to a condition in which a change in a controlled variable actually causes the body to drive the controlled variable even further away from the initial value. An example of positive feedback is the contractions of the uterus during childbirth.