Final answer:
The process of muscle contraction starts when the neurotransmitter acetylcholine (ACh) is released by a motor neuron at the neuromuscular junction, binds to receptors on the muscle fiber, and causes an action potential that leads to calcium release and the sliding filament model of contraction. This neurotransmitter-induced activity is critical for initiating and sustaining muscle contraction.
Step-by-step explanation:
Role of Neurotransmitters in Muscle Contraction
The process of stimulating skeletal muscle contraction begins at the neuromuscular junction, where the terminal end of a motor neuron releases the neurotransmitter achetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the muscle fiber's membrane, leading to the depolarization of the sarcolemma. This depolarization triggers an action potential that propagates along the sarcolemma and tunnels deep into the muscle fiber through the transverse tubules. The action potential activates the sarcoplasmic reticulum to release calcium ions (Ca2+).
When Ca2+ is released into the sarcoplasm, it binds to the troponin-tropomyosin complex on the actin filaments within the muscle fiber. This binding causes a conformational change, moving the tropomyosin away from the active sites on actin, allowing the myosin heads to bind to actin and form cross-bridges. The subsequent 'power stroke' is when the myosin heads pull on the actin filaments, powered by ATP, causing the muscle to contract. This process is part of the sliding filament model of muscle contraction.
As the action potential ceases, ACh is broken down by acetylcholinesterase, Ca2+ is pumped back into the sarcoplasmic reticulum, and the muscle fiber relaxes. The level of muscle tension produced depends on the cross-sectional area of the muscle fiber, the frequency of neural stimulation, and the number of motor units recruited. Thus, the neurotransmitter ACh is crucial for the initiation of the contraction cycle in skeletal muscles, with its binding setting off a cascade of events leading to muscle shortening and movement.