Question

Neurotransmitters are released upon stimulation from a nerve impulse by the: A) myofibrils B) motor unit C) thick filaments D) axon terminals of the motor neuron E) sarcolemma of the muscle cell

Answer 1

Neurotransmitters are released by the axon terminals of the motor neuron upon receiving a nerve impulse. Acetylcholine is released into the synaptic cleft and binds to the sarcolemma of the muscle fiber, triggering muscle contraction.

Step-by-step explanation:

Neurotransmitters are released upon stimulation from a nerve impulse by the axon terminals of the motor neuron. When a nerve impulse, or action potential, travels down the motor neuron's axon, it reaches the axon terminals at the neuromuscular junction—the point where the motor neuron communicates with a muscle fiber.

At the neuromuscular junction, neurotransmitters such as acetylcholine (ACh) are contained within synaptic vesicles in the axon terminals. The arrival of the action potential causes the vesicles to fuse with the presynaptic membrane and release their contents into the synaptic cleft. The released acetylcholine then binds to receptors on the sarcolemma, which is the muscle fiber's plasma membrane, leading to depolarization and initiating muscle contraction.

The acetylcholine is crucial because it bridges the gap between the motor neuron's signal and the actual contraction of the muscle fiber. This entire process ensures precise control over muscle movements.

Answer 2

Neurotransmitters (such as acetylcholine) are released upon stimulation from a nerve impulse by the axon terminals of the motor neuron (Option D). This process is critical for muscle contraction and coordinated movement.

The release of neurotransmitters upon stimulation from a nerve impulse occurs at the neuromuscular junction, which is the point of contact between a motor neuron and a muscle cell (muscle fiber). Let's break down the process step by step:

1. Motor Neuron: The process begins with a motor neuron, which is a type of nerve cell responsible for transmitting signals from the central nervous system (CNS) to muscle cells. Motor neurons innervate (connect to) muscle cells, and each motor neuron can innervate multiple muscle cells.

2. Axon Terminal: At the end of the motor neuron's axon (a long, slender projection from the cell body), there are specialized structures called axon terminals or synaptic end bulbs. These axon terminals contain synaptic vesicles filled with neurotransmitters, such as acetylcholine (ACh) in the case of muscle cells.

3. Action Potential: When a nerve impulse, or action potential, reaches the axon terminal of the motor neuron, it triggers the opening of voltage-gated calcium channels in the axon terminal membrane.

4. Calcium Entry: The influx of calcium ions (Ca2+) into the axon terminal is a key step in the process. Calcium ions enter the axon terminal from the extracellular space.

5. Neurotransmitter Release: The increase in calcium concentration inside the axon terminal promotes the fusion of synaptic vesicles containing neurotransmitters (e.g., acetylcholine) with the neuronal membrane. This fusion allows the neurotransmitters to be released into the synaptic cleft, which is the tiny gap between the motor neuron's axon terminal and the muscle cell's membrane (sarcolemma).

6. Binding to Receptors: Acetylcholine molecules released into the synaptic cleft bind to specific receptors on the sarcolemma (muscle cell membrane). These receptors are called nicotinic acetylcholine receptors.

7. Muscle Cell Activation: Binding of acetylcholine to its receptors on the sarcolemma leads to changes in ion permeability of the muscle cell membrane. This results in the initiation of an action potential in the sarcolemma, which propagates along the muscle fiber's surface.

8. Muscle Contraction: The action potential travels deep into the muscle fiber via a network of tubules called the T-tubules. This triggers the release of calcium ions from the sarcoplasmic reticulum (a specialized organelle within the muscle cell).

9. Sliding Filament Theory: The released calcium ions enable the muscle contraction process to occur by allowing the myosin and actin filaments within the muscle cells to interact, resulting in muscle contraction.