Dr Gawad Physiology Course Online Exam - Excitable - Course Lecture 5

The correct answer is acetyl choline because it is the neurotransmitter responsible for transmitting signals from motor neurons to muscle fibers at the neuromuscular junction. It binds to receptors on the muscle fibers, causing depolarization and ultimately leading to muscle contraction. Adrenaline and noradrenaline are not neurotransmitters at the motor endplate, but rather hormones involved in the fight-or-flight response. Calcium ions play a crucial role in muscle contraction, but they are not the neurotransmitter at the motor endplate. Atropine is a medication that blocks the effects of acetyl choline, but it is not the neurotransmitter itself.

The action potential of skeletal muscles spreads inwards to all parts of the muscle fiber via the T-tubules. The T-tubules are invaginations of the muscle cell membrane that penetrate deep into the muscle fiber. They allow the action potential to rapidly reach the interior of the fiber, ensuring synchronous contraction of all sarcomeres within the muscle. This is crucial for efficient muscle contraction and coordination.

The correct answer is EPP is a localized state of depolarization at the motor end plate. This is because the end plate potential (EPP) refers to the depolarization of the motor end plate, which is the specialized region of the muscle fiber where the nerve terminal synapses. The EPP is a localized event that occurs specifically at the motor end plate, and it is responsible for triggering the contraction of the muscle fiber.

Neuromuscular transmission involves the transmission of signals from the nerve to the muscle. One important step in this process is the permeability change to Na+ at the receptor side of the neuro-muscular junction. This means that when the nerve signal reaches the muscle, the receptors on the muscle membrane become permeable to Na+ ions, allowing them to enter the muscle cell. This influx of Na+ ions triggers a series of events that ultimately leads to muscle contraction. This step is crucial for the successful transmission of the nerve signal to the muscle.

The motor end plate contains acetylcholine receptors that are similar to those in skeletal muscle, not smooth muscle. The other statements are all true: the nerve ending contains many vesicles and mitochondria, lack of Ca diminishes the release of acetylcholine, there is a high concentration of the cholinesterase enzyme, and there is a delay of neuromuscular transmission of 0.5 milliseconds.

During neuromuscular transmission, the impulse is transmitted in one direction. This is because the release of Ach (acetylcholine) from the synaptic cleft triggers a cascade of events that ultimately lead to muscle contraction. Once Ach binds to its receptors on the muscle cell, it initiates an electrical impulse that travels along the muscle fiber, resulting in contraction. However, the impulse cannot travel back in the opposite direction due to the presence of a refractory period, which prevents the muscle from immediately responding to another impulse. This ensures that the muscle contraction occurs in a coordinated and controlled manner.

The firing level is reached after about +40 mV of depolarization. This statement is not true because the firing level is actually reached when the membrane potential reaches a threshold level of around -55 mV to -50 mV. At this threshold, an action potential is triggered and the muscle cell contracts. Therefore, the statement is incorrect.