Muscle Contraction Essay

Smooth muscle contraction occurs when calcium is present in the smooth muscle cell and binds onto calmodulin to activate myosin light chain kinase (Wilson et al., 2002). Phosphorylation of myosin light chains result in myosin ATPase activity thus cross-bridge cycling occurs causing the muscle to contract (Horowitz et al., 1996). There are two known models of excitation and contraction in smooth muscle, electromechanical coupling (EMC) and pharmomechanical coupling

As result, the bonding causes the sodium and potassium gates to open. As result, end plate potential is formed and excites areas of muscle tissue. Action potential is created and the muscle moves or contracts.

When a muscle contracts, myosin heads in thick filaments bind to actin in thin filaments and pulls the thin filament, shortening the length of the muscle fiber. However, without Ca++ when troponin binds to actin, the tropomyosin moves into a position that

Muscle fibres, as shown in Diagram 1, consist of myofibrils, which contain the proteins, actin and myosin, in specific arrangements . The diagram illustrates how a muscle is made up of many fascicles, which in turn are made up of many endomysiums, and within them, many muscle fibres. Each muscle fibre is made up of many myofibrils that consist of sarcomeres bound end on end . Actin is a thin filament, about 7nm in diameter, and myosin is a thick filament, about 15nm in diameter , both of which reside in the sarcomere. They are held together by transverse bands known as Z lines . Diagram 2 shows actin and myosin filaments within a sarcomere, and the Z lines that connect them.

How is contraction ended? Ach is released and binds to receptors on the motor end plate, then an action potential is produced which releases Ca+. The Ca+ binds to troponin, then myosin binds to actin to form crossbridges. The myosin pulls the actin then releases from actin and ADP is bound to the myosin.

The power stroke is responsible for the contraction of the muscle and force generation. ATP binding to the myosin head detaches the myosin head from the actin filament and allows the cycle to repeat. The coordinated contraction and relaxation of many muscles in an antagonistic fashion is the basis for the kinematics of any movement.

2. What occurs in the muscle during this apparent lack of activity? Ca++ is being released from the sacroplasmic reticulum and filament movement is taking up slack.

Actin and Myosin proteins serve the primary role of producing muscle contraction. Myosin molecules will create pressure in the skeletal muscle, where ATP hydrolysis causes Myosin to bind to Actin. A conformational change of the molecule then result in Myosin being

1.a) Contractile protein molecules that are seen in skeletal muscle fibres are actin (thin filaments) and myosin (thick filaments). Together, they produce the force of muscle contractions by forming cross bridges, and moving via a power stroke. The regulatory proteins that are seen within a skeletal muscle are troponin and tropomyosin. These proteins play a role in starting or stopping muscle contractions. When a muscle fibre is relaxed, there are no contractions because actin is unable to bind with the cross bridge. This is because tropomyosin covers the myosin binding sites on the actin proteins. In addition, troponin is not bound to calcium when a muscle fibre is relaxed, thus keeping the tropomyosin in its blocking position. When calcium enters the muscle fibres, it binds with troponin. This binding causes the tropomyosin to move away

There is cross bridge form with exposed active sites and binding of heads of myosin. This allows contraction of the muscle. In relaxation the neurotransmitter acetylcholine is broken down and calcium ions are recaptured by the sarcoplasmic reticulum preventing the cross bridge formation hence relaxation of muscles occurs. The process involved allows movement. In the nervous system, when one wants to move, the motor cortex in the brain transmits electrical signal through the local nerves and spinal cord. The skeletal muscle is then able to contract and shortened allowing moving. In the process of movement, a feedback signal is send through the nerves to the cerebellum to enhance good coordination of movement. (Fredrick Martini et al,

1. Myosin is inactive and folded when smooth muscle is relaxed. The muscle is able to contract when the myosin becomes unfolded. Phosphorylating the myosin regulatory light chain activates the muscle contraction in vertebrates. This phosphorylation is dependent on calcium concentrations. A conformational change occurs when the calcium concentration rises and binds to CAM.This complex is then able to bind to and activate the myosin light chain kinase. This kinase is able to activate the light chain and cause the myosin to contract. Muscles are relaxed when calcium concentrations are low because the light chain is dephosphorylated.

If calcium ions levels become too low, insufficient ATP is available or if the action potentials are not passed into the muscle cells, the tropomyosin complex will return to its position covering the active site and the contraction of the muscle cell will no longer

Facilitation occurs when postsynaptic potentials evoked by a stimulus are increased when that stimulus closely follows a pervious stimulus. Five stimulus pulses were given at decreasing interpulse intervals. The data for this experimenet displays that when the interpulse interval changes from 10msec to 8 msec, the number of pulses needed to reach the maximum MAP increases rather than the expexted outcome. This is most likely due to movement of the muscle in the chamber, causing the recordings to be reading different areas of the muscle. Different neuromuscular junctions in the muscle require different amounts of Ach in order for muscle action potentials to reach threshold.

The action potential arrives at the axon terminal. Calcium channels open allowing calcium ions to enter the axon terminal. Acetylcholine is released into the synaptic cleft and calcium ions are pumped out of the axon terminal. Acetylcholine binds to receptor sites on the motor end plate. An action potential is generated which propagates and travels down t-tubulues. The action potential causes the release of calcium ions. Calcium ions trigger a contraction of the muscle cell.

A muscle fiber is a thin, elongated cylinder with rounded ends. Beneath its cell membrane, they cytoplasm contains many small, oval nuclei and mitochondria. This contains threadlike myofibrils, which are parallel to each other.