Your answer MUST answer all the aspects of the question - write your answer clearly & use spell-check. a. What is a motor unit? b. For a skeletal muscle fiber to contract, it must receive a stimulus from its motor neuron. This occurs within the neuromuscular junction (NMJ). Discuss the ENTIRE mechanism of skeletal muscle fiber contraction at the NMJ: beginning with the mechanism for release of neurotransmitter from the pre-synaptic motor neuron and continuing through the entire mechanism leading to the contraction of the skeletal muscle fiber.

There are three major types of muscle present in our body - cardiac muscle, smooth muscle, and skeletal muscle. Skeletal muscle is a form of striated muscle tissue directly under the voluntary control of the nervous system. Its capacity to induce movement by contracting and relaxing its fibers at its own will is a unique feature of skeletal muscles.

1. Motor neurons are specialized nerve cells that connect the central nervous system with muscles and other target tissues. Their primary function is to transfer signals or instructions from the muscles or other target tissue to the central nervous system and vice versa.
2. A neuromuscular junction is a synapse formed between a motoneuron and a muscle fiber. The transmission of signals from the nervous system to the motoneuron, followed by its depolarization, results in the opening of voltage-gated calcium (Ca²⁺) channels of the presynaptic membrane. The inward flow of Ca²⁺ causes the release of acetylcholine (a chemical, neurotransmitter molecule) at the neuromuscular junction, which moves to the postsynaptic membrane at the muscle fiber. The postsynaptic membrane of the muscle fiber is also known as the motor endplate. Acetylcholine binds to the nicotinic receptors located at the motor endplate. This binding results in its depolarization, which initiates the action potentials in skeletal muscle fiber's sarcolemma. The sarcolemma's action potentials are responsible for propagating the action potential from the surface to the muscle fiber's interior. T-tubules contain dihydropyridine receptors adjacent to the terminal cisternae of the sarcoplasmic reticulum of the muscle fiber. When T-tubules become depolarized, their dihydropyridine receptors undergo a conformational change that mechanically interacts with the sarcoplasmic reticulum's ryanodine receptors. This interaction opens the ryanodine receptors causing Ca₂⁺ to release from the sarcoplasmic reticulum. The resulting increased intracellular Ca₂⁺ attaches to troponin C of the troponin complex on the thin filaments. The interaction between a Ca₂⁺ d troponin C exhibits cooperativity, which means that each Ca₂⁺ that binds troponin C increases the affinity of troponin C for the next Ca₂⁺ molecule, a total of four Ca₂⁺ ions can bind to one troponin C. As a result of Ca₂⁺ binding, the troponin complex undergoes a conformational change causing displacement of tropomyosin from the myosin-binding sites on F-actin. This leads to the cross-bridge cycle, an event in which the thick and thin filaments slide past one another and initiate skeletal muscles' contraction.