Efferent neurons carry information from the central nervous system (CNS) to the peripheral nervous system (PNS). Afferent neurons carry information from the peripheral nervous system to the central nervous system.

In general, afferet neurons will relay sensory information about the environment (both internal and external), carrying these signals from the periphery to the central nervous system for integration and interpretation in the brain. Efferent neurons, in contrast, will carry motor signals or other internal stimuli to respond to the sensory information provided.

The amygdala is responsible for emotional memory and is part of the limbic system, which is located in the temporal lobe.

The nervous system is divided into the central nervous system (brain and spinal cord) and peripheral nervous system (all other nerves). The peripheral nervous system is then divided into the sensory and motor divisions. Then, the motor division is further divided into the somatic and autonomic divisions. The autonomic division is then divided into the parasympathetic ("rest and digest") and sympathetic ("fight or flight") divisions.

Myosin and actin are the two major proteins in muscle cells that allow for contraction. Actin is the thin filament; myosin is the thick filament. During muscle contractions the overlap between these two proteins results in a shorter muscle fiber, and a shorter muscle, that pulls on the tendon. The result is movement. The other answers contain other structural elements of muscles but are not the direct cause of muscle contraction.

The force-tension curve is used to measure the optimal muscle length for maximum muscle contraction. This length corresponds to the optimal overlap of actin and myosin filaments to generate force. The length of actin and myosin filaments determines the minimum and maximum possible overlap. Titin is the protein responsible for the elasticity of the sarcomere after it is stretched past maximum actin-myosin overlap. Titin allows force production to exist at a maximum tension slightly beyond only actin and myosin, thus affecting the force-tension curve.

In addition to the subunits of myosin that link it to actin, myosin is also an ATP hydrolase, or ATPase. Myosin must hydrolyze ATP to ADP to allow for the power stroke that propels myosin forward on the actin polymers.

Upon contraction, actin filaments will be pulled by myosin heads resulting in the shortening of the sarcomeres. The I band is composed of only actin filaments, and will begin to overlap with the myosin filaments, shortening the band. The A band, however, is the section composed of myosin filaments. Since this section is not altered by contraction, it stays the same length. Unlike the I band, the A band can contain regions of overlap without changing length.

The H zone, in contrast, refers to the region of myosin that is not overlapped by action. As the region of overlap increases, the H zone decreases. The distance between Z discs represents the total length of the sarcomere and must shorten in order for the muscle to contract.

Z-discs are the dividing points between sarcomeres. Actin filaments extend from this region and are joined together by several complex protein structures.

The M-line is the middle of the sarcomere, marking the central point of the myosin filaments. The I-band consists of actin filaments that are not overlapped by myosin; this region contains the Z-disc. The A-band marks the length of an entire thick filament (myosin), including the overlap region with actin.

Titin is a massive protein that spans the length of half of a sarcomere (from the Z-disc to the M-line) and allows for the passive elasticity of muscle. It is not directly involved in the process of contraction; that function is performed by actin and myosin.

Collagen proteins play an important role in providing tensile strength and building connective tissue throughout the body, but play only a minor role in the properties of muscle tissue in the extracellular matrix. Collagen is not found in the sarcomere.

Recall that during sarcomere contraction, the myosin filaments attach to actin filaments and slide along the actin filaments. By this mechanism, the region of overlap between the fibers is increased and the total sarcomere length shortens. Neither actin, nor myosin actually change length; they simply move in relation to one another.

The H zone refers to the region of myosin at the center of the sarcomere that is not overlapped by actin. When the sarcomere shortens, the region of overlap increases and the H zone decreases.