Acetylcholine Activity and Termination

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Questions and Answers

What is the primary function of voltage-gated calcium channels in nerve terminals?

To inhibit the release of acetylcholine

To facilitate the diffusion of Ca2+ into the nerve terminal (correct)

To regulate the activity of acetylcholine receptors

To directly stimulate muscle contraction

What is the location of acetylcholine receptors in muscle fibers?

In the cytoplasm of the muscle fiber

In the synaptic space

On the surface of the nerve terminal

In the muscle fiber membrane, below the dense bar areas (correct)

What is the process by which acetylcholine is released from synaptic vesicles into the synaptic space?

Diffusion

Endocytosis

Exocytosis (correct)

Osmosis

What is the effect of Ca2+ on acetylcholine vesicles in the nerve terminal?

It exerts an attractive influence on the vesicles, drawing them to the neural membrane Signup and view all the answers

What is the approximate number of synaptic vesicles in the terminals of a single end plate?

300,000 Signup and view all the answers

What is the function of linear dense bars in the neural membrane?

To anchor protein particles that form voltage-gated calcium channels Signup and view all the answers

What is the result of the sudden influx of sodium ions into the muscle fiber when the acetylcholine channels open?

An increase in the membrane potential of the muscle fiber Signup and view all the answers

What is the minimum increase in membrane potential required to initiate an action potential in the muscle fiber?

20-30 mV Signup and view all the answers

What is the primary mechanism by which the effect of acetylcholine binding is terminated?

Breakdown of acetylcholine by acetylcholinesterase Signup and view all the answers

What is the result of stimulating the nerve fiber at rates greater than 100 times/second for several minutes?

Fatigue of the neuromuscular junction Signup and view all the answers

What is the primary function of acetylcholinesterase in the synaptic cleft?

To break down acetylcholine into acetyl and choline Signup and view all the answers

What is the name of the local potential created when the acetylcholine channels open?

End plate potential Signup and view all the answers

What is the primary reason for the larger influx of sodium ions compared to potassium ions through the acetylcholine channel?

The higher concentration of sodium ions in the extracellular fluid Signup and view all the answers

What is the primary effect of acetylcholine binding to its receptors on the muscle fiber membrane?

Allowing large numbers of sodium ions to pour into the muscle fiber Signup and view all the answers

What is the outcome of the local positive potential change inside the muscle fiber membrane, also known as the end plate potential?

It initiates an action potential that spreads along the muscle membrane Signup and view all the answers

Which ions are repelled by the negative charges in the mouth of the acetylcholine channel?

Chloride ions Signup and view all the answers

What is the requirement for the acetylcholine channel to open and allow ions to pass through?

Two acetylcholine molecules attaching to the alpha subunit protein Signup and view all the answers

What is the purpose of the negative potential on the inside of the muscle membrane?

It prevents the efflux of positively charged potassium ions Signup and view all the answers

Study Notes

Voltage-Gated Calcium Channels

Primary function: Facilitate the influx of Ca2+ ions into nerve terminals, triggering the release of neurotransmitters.

Acetylcholine Receptors

Location: Distributed on the postsynaptic membrane of muscle fibers, specifically within the motor end plate region.

Acetylcholine Release

Process: Acetylcholine is released from synaptic vesicles into the synaptic space via exocytosis, triggered by increased intracellular calcium levels.

Calcium Ions and Acetylcholine Vesicles

Effect of Ca2+: Influx of Ca2+ ions plays a crucial role in stimulating synaptic vesicles to fuse with the presynaptic membrane, facilitating neurotransmitter release.

Synaptic Vesicles in End Plate

Count: Approximately 100-300 synaptic vesicles are present in the terminals of a single end plate.

Linear Dense Bars

Function: Serve as anchoring sites for synaptic vesicles, stabilizing their position in the presynaptic membrane.

Sodium Ion Influx

Result: Sudden influx of sodium ions when acetylcholine channels open causes depolarization of the muscle fiber membrane, leading to muscle contraction.

Action Potential Threshold

Minimum potential increase: Around -55 mV is required to initiate an action potential in the muscle fiber.

Termination of Acetylcholine Effect

Mechanism: The primary means of terminating acetylcholine's action is through enzymatic breakdown by acetylcholinesterase.

High-Frequency Nerve Stimulation

Result: Stimulating the nerve fiber at rates greater than 100 times/second leads to sequential release of acetylcholine and potential exhaustion of synaptic vesicle pools.

Acetylcholinesterase

Primary function: Breaks down acetylcholine in the synaptic cleft, preventing prolonged activation of acetylcholine receptors and ensuring quick signal termination.

Local Potential

Name: End plate potential is the local depolarization generated when acetylcholine channels open.

Sodium vs. Potassium Ion Influx

Primary reason: The channels are more permeable to sodium ions due to the electrochemical gradient, resulting in a larger influx compared to potassium ions.

Effects of Acetylcholine Binding

Primary effect: Results in depolarization of the muscle fiber membrane, which contributes to muscle contraction.

End Plate Potential Outcome

Result: The local positive potential change leads to the initiation of an action potential that propagates along the muscle fiber.

Ion Repulsion

Repelled ions: Negative charges in the mouth of the acetylcholine channel repel Ca2+ ions while allowing Na+ ions to pass through more readily.

Acetylcholine Channel Opening Requirement

Requirement: The binding of acetylcholine to its receptor is essential for the channel to open and permit ion flux.

Muscle Membrane Negative Potential

Purpose: The negative potential inside the muscle membrane creates an electrochemical gradient, essential for action potential generation and restoration of resting membrane potential.

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Description

Learn about the brief duration of acetylcholine activity and its termination mechanisms, including the role of acetylcholinesterase and diffusion. Understand how ACh is broken down and becomes unavailable to activate the ACh receptor.