Question

A drug that inactivates acetylcholinesterase a. inhibits the release of \(\mathrm{ACh}\) from presynaptic endings. b. inhibits the attachment of ACh to its receptor protein. c. increases the ability of ACh to stimulate muscle contraction. d. does all of these.

Short answer

The correct answer is \(c\): A drug that inactivates acetylcholinesterase increases the ability of ACh to stimulate muscle contraction.

Step-by-step solution

Understanding Acetylcholinesterase

Acetylcholinesterase is an enzyme that breaks down the neurotransmitter acetylcholine (ACh) at synaptic clefts. This breakdown allows for the termination of the nerve impulse transmission at the synapses, preventing continuous muscle contraction.

Inactivating Acetylcholinesterase

Inactivating acetylcholinesterase will prevent the breakdown of ACh in the synaptic clefts. As a result, ACh remains attached to its receptors, prolonging the nerve impulse transmission and continuous muscle contractions.

Evaluating Options a, b, and c

a. Inactivating acetylcholinesterase does not inhibit the release of ACh from presynaptic endings: it only prevents the breakdown of ACh in the synaptic clefts. Therefore, option a is incorrect. b. Inactivating acetylcholinesterase does not inhibit the attachment of ACh to its receptor protein: it just lets ACh stay attached to its receptors longer. Option b is also incorrect. c. Inactivating acetylcholinesterase increases the ability of ACh to stimulate muscle contraction because it prevents the breakdown of ACh in the synaptic clefts. This allows ACh to stay attached to its receptors longer, stimulating continuous muscle contractions. Option c is correct.

Conclusion

The correct answer is the option c: A drug that inactivates acetylcholinesterase increases the ability of ACh to stimulate muscle contraction. Options a and b are incorrect, and since not all of these options are correct, option d is also incorrect.

Key concepts

Acetylcholine function

Acetylcholine (ACh) is a key neurotransmitter in both the central and peripheral nervous systems. It plays an essential role in transmitting signals across synapses. When a nerve impulse reaches the end of a neuron at the synapse, ACh is released into the synaptic cleft, the small gap between the neurons.
Once released, ACh binds to receptor proteins on the postsynaptic cell, such as muscle cells or another neuron. This binding initiates a cascade of events that result in the opening of ion channels, allowing for changes in membrane potential. This is crucial for passing the signal to the next neuron or inducing a response, such as muscle contraction.
Acetylcholine is also involved in a range of functions including:

• Stimulating muscle movements, including those necessary for breathing.
• Regulating memory and alertness in the brain.
• Controlling the release of other neurotransmitters.

Without proper management of ACh levels, the signaling in the nervous system could be either too weak or overly persistent, leading to inadequate or excessive responses.

Synaptic transmission

Synaptic transmission is the process by which neurons communicate with each other or with muscles. It begins when an action potential, or electrical signal, travels down a neuron to the synapse at its end.
Once the electrical signal reaches the synapse, the following steps occur:

• Neurotransmitters, such as acetylcholine, are released from presynaptic vesicles into the synaptic cleft.
• These neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic cell's membrane.
• This binding results in either excitatory or inhibitory effects, depending on the type of neurotransmitter and receptor involved.

In the case of acetylcholine, the usual effect is excitatory, often leading to muscle contraction. After transmitting the signal, acetylcholine must be broken down quickly by acetylcholinesterase to prevent prolonged stimulation. Otherwise, neurotransmitters would keep binding to the receptors, leading to continuous postsynaptic activity, which is why acetylcholinesterase plays an important role in resetting the synapse for the next set of signals.

Muscle contraction

Muscle contraction is a complex process that begins at the neuromuscular junction, where nerve cells connect with muscle fibers. This process is mediated by acetylcholine released during synaptic transmission.
Here's a simplified sequence of events leading to muscle contraction:

• The arrival of an action potential at the synapse causes acetylcholine to be released into the neuromuscular junction.
• Acetylcholine binds to receptors on the muscle cell membrane, triggering an electrical change (action potential) that travels along the muscle fiber.
• This electrical signal prompts the release of calcium ions within the muscle cell, which interact with muscle proteins to facilitate contraction.

Inactivating acetylcholinesterase keeps acetylcholine in the synaptic cleft longer, thus prolonging muscle contraction. If left unchecked, this could lead to continuous and unwanted muscle contractions, underscoring the importance of acetylcholinesterase in regulating muscle movement.