Structure and Function of the Muscular System

The Skeletal Muscle Contraction Cycle

When a muscle is stimulated, the contraction cycle occurs in three stages: excitation−contraction coupling, contraction, and relaxation.

Muscle stimulation turns into muscle contraction through a process called excitation−contraction coupling. Basically, the muscle is stimulated (excitation) and it reacts (contraction). The contraction cycle occurs in three stages: excitation−contraction coupling, contraction, and relaxation. Muscle fibers are stimulated by motor neuron axons, which end at muscle fiber end plates or neuromuscular junctions. At an end plate, the axon branches and forms many synaptic boutons. Each synaptic bouton is closely located over a region of muscle fiber membrane, called the junctional fold. Junctional folds are specialized regions of muscle fiber membrane filled with acetylcholine receptors. The area between the presynaptic bouton and the postsynaptic junctional fold is called the synaptic cleft.

Action potentials create a signal from the brain. The message travels along motor neuron axons to reach the synaptic bouton. At that point, action potentials stimulate the release of a synaptic vesicle, which is a small membrane-lined sac filled with neurotransmitters. At the neuromuscular junction, joining the point of nerves and muscles, the synaptic vesicles fill with the neurotransmitter acetylcholine, a chemical compound in the nervous system that stimulates muscle activity. Following a stimulus, synaptic vesicles release their contents into the synaptic cleft. This stimulates the acetylcholine receptors in the junctional folds of the muscle fiber end plate.

The muscle fiber membrane becomes depolarized, meaning the electrical change inside the cell changes. Depolarization sets off voltage-gated sodium channels and begins a signal across the muscle fiber. This signal spreads down the transverse tubules of the muscle fiber. This releases the calcium stored in the cell, which begins a muscle contraction. For the muscle to relax, calcium is actively pumped back into the sarcoplasmic reticulum, where calcium within the cell is stored.

Skeletal Muscle Contraction and Relaxation

Muscle contractions require the input of ATP to move the actin and myosin filaments across each other.

The sliding filament model explains how the increase in calcium caused by a muscle fiber action potential causes muscles to contract. As discussed above, each sarcomere contains myosin filaments in the center, with actin filaments on either side. These thick myosin filaments contain projections called myosin heads. Myosin heads have a binding site that forms a cross-bridge with actin molecules; however, in a muscle at rest a protein called tropomyosin blocks these cross-bridge sites on the actin molecule.

Myosin heads also have a site that binds ATP (adenosine triphosphate)⎯the primary molecule used in cells throughout the body to transfer energy. Energy from ATP moves myosin heads into a cocked position so that, in muscles at rest, the myosin heads are ready to release energy. In this resting position myosin heads are still bound to ADP (adenosine diphosphate) at their ATP binding site. ADP is the molecule created when energy is released by the removal of one of the phosphates from ATP.

When a muscle is stimulated, it releases calcium from intracellular stores in the sarcoplasmic reticulum. This calcium binds to troponin, which causes tropomyosin to expose the active myosin-binding site on the actin molecule. Once this binding site is exposed, myosin heads bind to the actin filament, forming an actin−myosin cross-bridge. While in this cross-bridge position, the myosin head snaps back from its cocked position. This movement slides the filaments together and increases the area of overlap within the sarcomere, causing shortening of the I-bands and H-zone, thus shortening the muscle. This movement is called a power stroke.

During the power stroke the myosin head releases its bound ADP, freeing the ATP binding site for a new molecule. When a new molecule of ATP binds to the myosin head, it causes the head to let go of the actin filament. Energy is released as ATP is dephosphorylated to ADP, and that energy moves the myosin head back into its cocked position. As long as calcium and ATP are still present in the muscle fiber, this cycle continues and myosin heads continue to "walk" up the actin filaments, contracting the muscle.

In order to relax the muscle, the muscle fiber actively pumps calcium back into the sarcoplasmic reticulum. Once calcium is released from the troponin, and when calcium is no longer present, tropomyosin returns to its previous configuration, blocking the active site on the actin filament. No longer able to form a cross-bridge between actin and myosin filaments, the muscle relaxes.

Mechanism of Muscle Contraction

When a muscle is ready to contract, calcium in the muscle fibers exposes myosin-binding sites on actin filaments. Actin−myosin cross-bridges form. By hydrolyzing ATP, myosin heads push up against actin fibrils, contracting the sarcomere. Calcium and ATP presence enables cross-bridge cycling.

Skeletal Muscle Tension

Muscle tension is the force output of a muscle.
The amount of tension, or force, that a muscle can generate during a contraction depends on the frequency of the stimulus. A single input from a motor neuron causes a single muscle twitch or contraction cycle. The amount of calcium released from a single muscular action potential is fairly small and will not bind to all of the actin-binding sites, so relatively few cross-bridges can form in this single twitch. In addition, after the contraction, calcium is pumped back into the sarcoplasmic reticulum. When stimuli are provided in rapid succession, however, increasing amounts of calcium are released, allowing more cross-bridges to form, and increasing muscle tension. When enough calcium is released to continuously activate all of the cross-bridge binding sites, the muscle is said to have reached tetanus—that is, the muscle is contracting with as much force as it is able to. At tetanus, the muscle is generating its maximum tension. Tetanus is the point at which a muscle is in the state of sustained contraction. So, if the frequency of stimulation is fast enough, tetany occurs (sustained contraction).

Muscle Tension Depends on Stimulus Frequency

A single stimulus creates a single muscle twitch, while multiple low frequency stimuli produce short periods of contraction and relaxation, called unfused tetanus. High-frequency stimuli cause constant contractions called fused tetanus, when the muscle is contracting with as much tension as it can generate.
The tension that a muscle can generate also depends upon its length. There is an optimal length for muscle contraction. Skeletal muscles are stretched at rest, and this resting length maximizes the muscle's ability to contract when stimulated. This optimal length is a product of the sarcomere structure. If there is too much overlap between the thick and thin myofilaments when the muscle is stimulated, then the muscle is not able to contract much farther. If the muscle is stretched too far, then there is not enough overlap between the thick and thin myofilaments to allow cross-bridge formation.

Muscle Tension Depends on Muscle Length

Muscles have an optimal length for maximum tension with a muscle contraction. If muscle length decreases, sarcomeres cannot contract much farther. If muscle length increases, the lack of overlap produces muscle contraction.

Types of Skeletal Muscle Contractions

Isotonic contractions occur when a muscle contracts while changing length, and isometric contractions occur when a muscle contracts but does not change length.

All muscle contractions are caused by the cross-bridge cycling of actin and myosin. When these muscles work together in concert, however, the forces they exert on each other, on the skeleton, and on the environment produce contractions that can be described in different ways.

An isotonic contraction occurs when a muscle contracts and changes in length. Isotonic contractions can be concentric or eccentric. During a concentric muscle contraction, the muscle shortens in length, as described in the sliding filament model, and tension on the muscle remains constant. This generates force where the muscle is attached to the tendons, often causing the angle of a joint to change. For example, a concentric contraction of the biceps muscle causes the biceps to shorten and the arm to bend at the elbow.

During an eccentric contraction, a force is imposed on the muscle that is greater than the force generated by the muscle contraction. In this case, rather than shortening, the muscle lengthens, even as it contracts, increasing the tension on the muscle. The eccentric contraction serves to slow the lengthening movement or add control. For example, when the arm is straightened, the biceps muscle lengthens.

An isometric contraction occurs when a muscle contracts and generates force exactly equal to the force being exerted upon it, and therefore the tension increases while the actual muscle length does not change. For example, if the biceps muscle is activated to hold a weight in a steady position but does not actively curl the arm upwards toward the shoulder, the muscle is undergoing an isometric contraction.
Isotonic contractions occur when a muscle contracts and shortens (concentric contractions) or lengthens (eccentric contraction). Isometric contractions occur when a muscle contracts and generates force but muscle length does not change.