Question

With respect to the sliding filament mechanism, explain the function of: (pp. 147-149) a. Acetylcholine b. Calcium ions c. Myosin and actin d. Troponin and tropomyosin e. Cholinesterase

Short answer

Question: Explain the function of various components involved in the sliding filament mechanism of muscle contraction. Answer: Components involved in the sliding filament mechanism include acetylcholine, calcium ions, myosin and actin, troponin and tropomyosin, and cholinesterase. Acetylcholine transmits signals from motor neurons to muscle fibers, triggering an action potential. Calcium ions, released from the sarcoplasmic reticulum, bind to troponin, causing tropomyosin to expose myosin-binding sites on actin. Myosin heads then bind to actin, creating cross-bridges and sliding the filaments, resulting in muscle contraction. Troponin and tropomyosin regulate this process by controlling myosin and actin interactions. Cholinesterase degrades acetylcholine in the synaptic cleft, allowing muscle relaxation and preventing continuous stimulation.

Step-by-step solution

a. Acetylcholine

Acetylcholine is a neurotransmitter responsible for transmitting signals from motor neurons to muscle fibers. During muscle contraction, acetylcholine is released into the synaptic cleft, where it binds to receptors on the muscle fiber's membrane (sarcolemma). This binding triggers an action potential in the muscle cell, which then initiates the process of muscle contraction through the sliding filament mechanism.

b. Calcium ions

Calcium ions play a crucial role in the initiation of muscle contraction. The action potential generated in the muscle cell causes the release of calcium ions from the sarcoplasmic reticulum (a specialized endoplasmic reticulum in muscle cells). Calcium ions then bind to troponin, causing a conformational change that moves tropomyosin from its blocking position on actin filaments. This exposes the myosin-binding sites on actin, allowing myosin heads to bind and initiate the sliding filament mechanism.

c. Myosin and actin

Myosin and actin are the primary protein components of muscle fibers. Myosin forms the thick filaments, while actin forms the thin filaments. During muscle contraction, myosin heads bind to the exposed myosin-binding sites on actin, creating cross-bridges. These myosin heads then utilize ATP to "pull" the actin filaments towards the center of the sarcomere by performing a power stroke. This sliding of actin filaments past myosin filaments creates muscle contraction.

d. Troponin and tropomyosin

Troponin and tropomyosin are regulatory proteins found on the actin filaments. Tropomyosin covers the myosin-binding sites on actin at rest, preventing interaction between actin and myosin. Troponin is a complex protein with three subunits - one that binds to actin, one that binds to tropomyosin, and one that binds to calcium ions. When calcium ions bind to troponin, it causes a conformational change in tropomyosin, exposing the myosin-binding sites on actin and allowing the sliding filament mechanism to proceed.

e. Cholinesterase

Cholinesterase is an enzyme responsible for breaking down acetylcholine in the synaptic cleft. After acetylcholine has transmitted the signal to the muscle fiber, cholinesterase quickly degrades the acetylcholine molecules, preventing continuous stimulation of the muscle. This degradation allows the muscle to relax after contraction and ensures that the muscle only contracts when it receives a new signal from a motor neuron.