10.4 The Process of Skeletal Muscle Contraction and Relaxation

I. The Neuromuscular Junction (NMJ)
A. Neuromuscular Junction: The synapse formed between a motor neuron and a muscle fiber plays a critical role in muscular movement. This junction acts as the point where the nervous system communicates with the muscular system, facilitating voluntary muscle control.

  1. Function: The primary function of the NMJ is to transmit nerve impulses (or neuronal action potentials) from the motor neuron to the muscle fiber's sarcolemma. This transmission triggers an action potential in the muscle fiber, which ultimately leads to muscle contraction.

B. Neuromuscular Junction Components

  1. Axon terminal - This is the end part of the motor neuron, containing synaptic vesicles filled with the neurotransmitter acetylcholine (ACh), essential for muscle activation. When an action potential arrives at the axon terminal, it initiates the release of ACh into the synaptic cleft through a process called exocytosis.

  2. Synaptic cleft - The synaptic cleft is the small gap between the axon terminal and the muscle fiber. This space is filled with a gel-like substance that provides structural support, ensuring that the neuron remains anchored in place, allowing for efficient signaling.

  3. Motor end plate - This specialized area of the muscle fiber plasma membrane has a highly folded surface, maximizing the number of ligand-gated Na+ channels. The binding of ACh to these channels causes them to open, which allows Na+ ions to flow into the muscle cell, leading to depolarization and ultimately muscle contraction.

  • Takeaway: the electrical signal that stimulates the muscle cell is turned into chemical signal -acetylcholine- which is then converted back into an electrical signal through the action potential of the muscle

II. Skeletal Muscle Contraction
A. Excitation Phase

  1. When an action potential travels from the brain or spinal cord to the axon terminal of a motor neuron, it prompts the vesicles (via calcium ions entering into the cytosol of the axon terminal, due to voltage-gated calcium ion channels opening upon the action potential) within the axon terminal to release acetylcholine into the synaptic cleft. This release is crucial as it is the first step in muscle activation.

  2. Once released, ACh diffuses across the synaptic cleft and binds specifically to the ligand-gated channels in the motor end plate of the muscle fiber's sarcolemma.

  3. The binding of ACh to these ligand-gated channels results in their opening. This allows Na+ ions to enter the muscle fiber, generating an end-plate potential (EPP) that depolarizes the membrane in that localized area.

  4. The depolarization caused by sodium ion influx creates the end-plate potential necessary for muscle activation.

  5. Multiple of these end-plate potentials must be generated to produce a contraction, but ACh is degraded almost immediately following reactions catalyzed by acetylcholinesterase (AChE), so the neuron must fire multiple action potentials for repeated contractions

B. Excitation-Contraction Coupling

  1. The end-plate potential is critical as it leads to the opening of voltage-gated Na+ channels in the sarcolemma surrounding the motor end plate, resulting in the generation of an action potential that spreads across the muscle fiber.

  2. As the action potentials propagate along the sarcolemma, the depolarization of one area triggers the sequential opening of adjacent voltage-gated sodium ion channels, creating a rapid chain reaction that facilitates the transmission of the action potential down the entire muscle fiber to the triads. These action potentials are able to continue through the sarcolemma because of the T-Tubules

  3. The action potential then signals the terminal cisternae (directly linked to T-Tubules by voltage gated proteins) in the sarcoplasmic reticulum, opening voltage-gated Ca2+ channels and leading to a significant release of Ca2+ ions into the cytosol.

  4. Released calcium ions bind to troponin, a regulatory protein that plays a vital role in muscle contraction.

  5. As calcium binds, tropomyosin, which is another regulatory protein, shifts to reveal the active sites on actin filaments.

C. Contraction Phase

  1. The contraction phase begins instantly after the active sites on actin are exposed, allowing for the initiation of the crossbridge cycle

    1. Troponin (regulatory protein that binds to tropomyosin) has a calcium ion bound to it

    2. This causes troponin to shift its position, allowing tropomyosin to move away from the active sites, thus allow for the myosin heads to bind to the actin sites

  2. The myosin head becomes cocked when ATP binds to it (due to ATPase enzyme within the myosin). The energy gathered from ATP hydrolysis enables the myosin head to position itself in a high-energy state, primed for contraction.

  3. In this cocked position, the myosin head binds to the exposed active site on actin, initiating the power stroke.

  4. The power stroke occurs when ADP and inorganic phosphate (Pi) are released from the myosin head , causing it to pull the actin filament toward the M line (the center of the sarcomere), while the myosin head pivots into a relaxed, low-energy state.

  5. After the power stroke, myosin can bind to another ATP molecule, facilitating the detachment from the actin active site. This mechanism ensures that the thin filaments do not slide backward as some myosin heads remain attached to actin during the process.

  6. The crossbridge cycle will continue as long as there is adequate stimulation for contraction and sufficient ATP availability. I. Excitation Phase 1. Action potentials from the brain/spinal cord trigger acetylcholine (ACh) release from motor neuron axon terminals into the synaptic cleft. 2. ACh binds to ligand-gated channels on the muscle fiber's motor end plate, allowing Na+ influx that generates an end-plate potential (EPP) necessary for activation. 3. Multiple EPPs needed for contraction as ACh is quickly degraded by acetylcholinesterase (AChE). II. Excitation-Contraction Coupling 1. The EPP opens voltage-gated Na+ channels, generating an action potential that propagates along the muscle fiber via T-Tubules. 2. The action potential signals the sarcoplasmic reticulum to release Ca2+ ions into the cytosol. 3. Ca2+ binds to troponin, causing tropomyosin to shift and expose active sites on actin filaments. III. Contraction Phase 1. The contraction begins when myosin heads bind to exposed actin sites, initiating the crossbridge cycle. 2. ATP binding to myosin allows the head to cock, priming it for contraction. 3. The power stroke occurs when myosin releases ADP and Pi, pulling actin filaments toward the M line. 4. Myosin detaches from actin by binding another ATP, and the cycle continues with adequate stimulation and ATP availability.

III. Skeletal Muscle Relaxation
A. Acetylcholinesterase is an enzyme that decomposes any remaining ACh in the synaptic cleft. This degradation is essential for preventing continuous stimulation of the muscle fiber, allowing it to relax.
B. Once ACh is broken down, the ligand-gated sodium channels close, ending the end plate potential and initiating repolarization.
C. The sarcolemma returns to its resting membrane potential due to the efflux of K+ ions through voltage-gated K+ channels.
D. As the T-tubules repolarize, calcium ion channels in the sarcoplasmic reticulum (SR) also close.
E. Calcium ions are actively pumped back into the SR, returning the cytosolic concentration of calcium to resting levels.
F. In the absence of calcium, both troponin and tropomyosin block the active sites on actin, leading to muscle relaxation as the myofilaments glide back to their original positions, aided by structural proteins like titin.