Instructions: Read and watch the following animation below to evaluate and inter
ID: 3520242 • Letter: I
Question
Instructions:
Read and watch the following animation below to evaluate and interpret the process involved in the activation of the actin-myosin cross bridge (sliding filament mechanism) in skeletal muscle contraction.
Electrical Excitation of a Muscle Fiber Skeletal muscle fiber cells can be stimulated by a somatic motor neuron resulting in depolarization of the sarcolemma.
If this depolarization reaches threshold, an action potential is initiated resulting in muscle contraction. Describe a somatic motor neuron and how it communicates with a skeletal muscle cell? Describe the process of depolarization within the sarcolemma and what is required for a stimulus to reach threshold, resulting in skeletal muscle contraction? Excitation-Contraction Coupling An action potential is transmitted along the sarcolemma and down the T tubules (transverse tubules).
This action causes calcium ions to be released from the terminal cisternae of the sarcoplasmic reticulum. Calcium ions couple electrical excitation to muscle fiber contraction by binding to troponin. Two events happen when calcium binds to troponin.
Describe these two events in great detail. Muscle Fiber Contraction due to the Sliding Filament Mechanism Watch the animated interaction between actin and myosin in the sliding filament mechanism during skeletal muscle contraction.
https://www.youtube.com/watch?v=hgLSLJ09fZM
Evaluate and interpret the process in the actin-myosin cross bridge (sliding filament mechanism) during skeletal muscle contraction and answer the following questions below:
What happens to the position of the myosin head when ATP is hydrolyzed and ADP is attached to the myosin head group?
What event happens immediately after the myosin head binds to the active site on actin?
Describe this process in great detail. What happens to the position of the myosin head when ATP binds to the myosin head group? Why does the actin filament only move in one direction? Describe in great detail.
Explanation / Answer
Skeletal muscles have a striped appearance and hence are called striated muscles. They are made up of number of muscle bundles or fascicles which are held together by a collagenous connective tissue layer called fascia. They work under the voluntary control of Somatic Nervous system.
The somatic nervous system is the part of the peripheral nervous system (that is an entire nervous system outside brain and spinal cord) associated with the voluntary control of body movements via skeletal muscles. The somatic nervous system is made up of two types of neurons- sensory and motor neurons. Somatic motor neuron (motor neuron transmits impulses from the central nervous system to a muscle, gland, or other effector tissue) originate in the central nervous system and project their axons to skeletal muscles (such as the muscles of the limbs, abdominal, and intercostal muscles), which are involved in locomotion. They are define as efferent to direct the flow of information from the central nervous system (CNS) to the periphery.
The somatic neuromuscular junction is a specialized synapse that is a site of communication between motor neurons and skeletal muscle fibres. Upon adequate stimulation, the motor neuron releases a flood of acetylcholine (Ach) neurotransmitters from the axon terminals from synaptic vesicles bind with the plasma membrane. The acetylcholine molecules bind to postsynaptic receptors found within the motor end plate. Two acetylcholine receptors activate an opening of ion channel and sodium ions are allowed to flow into the cell. Opening chemically gated ion channels upon ACh binding to ACh receptors allows Na+ & K+ to pass the sarcolemma. More Na+ in than K+ out into the cell causes depolarization (interior slightly less negative than outside and triggers a muscle action potential. T tubules of skeletal muscle sarcolemma are then stimulated to release calcium ion from the sarcoplasmic reticulum. This chemical release that causes the target muscle fiber to contract. Thus, the action potential travels very rapidly along the sarcolemma.
After getting signaled by a motor neuron, a skeletal muscle fiber contracts as the thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres. This process is known as the sliding filament model of muscle contraction. The myosin head is attracted to actin filament and myosin binds actin at its actin-binding site, forming the cross-bridge. Cross-bridge formation occurs when the myosin head attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate (Pi) are still bound to myosin.
Pi is then released, causing myosin to form a stronger attachment to the actin, after which the myosin head moves toward the M-line, pulling the actin along with it. As actin is pulled, the filaments move approximately 10 nm toward the M-line. This movement is called the power stroke, as movement of the thin filament occurs. This results in the myosin head moving towards the center of the sarcomere, after which the attached ADP and phosphate group are released. A new molecule of ATP attaches to the myosin head, causing the cross-bridge to detach. The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position. The myosin head is now in position for further movement.
There are always few myosin heads remain attached to the actin myofilament when other myosin heads are detaching. The cross bridge always remains in place, preventing the actin myofilament from sliding.The actin myofilament can only move in one direction relative to the myosin filament.