Neuromuscular Junction Overview

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Questions and Answers

What triggers the exocytosis of vesicles containing acetylcholine in the presynaptic terminal?

Increased Ca 2+ levels in the cytosol (correct)

Binding of acetylcholine to postsynaptic receptors

Release of sodium ions from vesicles

Decrease in Ca 2+ levels

How many molecules of acetylcholine are contained in each vesicle?

20,000 molecules

5,000 molecules

10,000 molecules (correct)

50,000 molecules

What is the effect of acetylcholine binding to nicotinic–ACh receptors?

It causes the receptor to close the ion channels.

It prevents sodium ions from entering the muscle fiber.

It triggers a decrease in membrane potential.

It leads to a conformational change opening ion channels. (correct)

What occurs when sodium ions enter the muscle fiber during the end plate potential?

Depolarization and a localized positive potential change Signup and view all the answers

Which statement accurately describes the end plate potential?

It decays exponentially and is localized. Signup and view all the answers

What is the critical membrane potential that must be reached to trigger an action potential?

−60 mV Signup and view all the answers

What distinguishes the end plate potential (EPP) from an action potential?

EPP does not have a threshold and is non-propagated. Signup and view all the answers

What occurs after sufficient depolarization at the motor end plate?

An action potential is initiated in the muscle fiber. Signup and view all the answers

What is the function of the terminal buttons in neuromuscular transmission?

Release of acetylcholine into the synaptic cleft Signup and view all the answers

Which structure contains nicotinic acetylcholine receptors?

Postsynaptic membrane Signup and view all the answers

What role does cholinesterase play at the neuromuscular junction?

Degrades acetylcholine after signaling Signup and view all the answers

What causes the end plate potential at the postsynaptic membrane?

Entry of Na+ ions into the postsynaptic membrane Signup and view all the answers

What initiates the release of acetylcholine from the terminal buttons?

Opening of voltage-gated Ca2+ channels Signup and view all the answers

How is the synaptic cleft described in terms of its size?

It is 50-100 nanometers wide Signup and view all the answers

What is the primary source of energy for the synthesis of acetylcholine in terminal buttons?

Adenosine triphosphate (ATP) Signup and view all the answers

What describes the postsynaptic membrane's modifications at the neuromuscular junction?

Thickened and depressed to form a synaptic trough Signup and view all the answers

Study Notes

The Neuromuscular Junction

The axon of a neuron supplying a skeletal muscle loses its myelin sheath and divides into fine branches ending in small swellings called terminal buttons or end feet.
These terminal buttons form a neuromuscular junction at the center of a muscle fiber.
The nerve terminal contains vesicles filled with acetylcholine and mitochondria that provide energy for acetylcholine synthesis.

Components of the Neuromuscular Junction

Presynaptic membrane: The axonal membrane lining the terminal buttons.
Synaptic cleft: A 50-100 nm space between the presynaptic and postsynaptic membranes filled with extracellular fluid and reticular fibers.
Postsynaptic membrane: The muscle fiber membrane (sarcolemma) in the region of the neuromuscular junction, thickened and depressed to form the synaptic trough. Also known as the motor end plate.

Characteristics of the Postsynaptic Membrane

Only one nerve fiber ends on each end plate.
The postsynaptic membrane is folded into subneural clefts or pallisades, increasing its surface area.
It contains nicotinic acetylcholine (ACh) receptors, which are binding sites for acetylcholine.
The matrix of the subneural cleft contains the enzyme cholinesterase, which degrades acetylcholine.

Neuromuscular Transmission

The skeletal muscle is stimulated solely through its nerve.
The neuromuscular junction transmits impulses from the nerve to the muscle.

Events in Neuromuscular Transmission

Release of acetylcholine: When a nerve impulse reaches the terminal buttons, voltage-gated Ca²⁺ channels open, increasing the membrane permeability to Ca²⁺ ions.
Ca²⁺ entry: Ca²⁺ ions from the extracellular fluid enter the terminal buttons.
Vesicle fusion and exocytosis: The increased Ca²⁺ levels in the cytosol trigger the fusion of acetylcholine-containing vesicles with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.
Acetylcholine binding: Acetylcholine diffuses across the cleft and binds to nicotinic-ACh receptors on the motor end plate.
Channel opening: Binding of acetylcholine to its receptors opens acetylcholine-gated ion channels in the postsynaptic membrane.
Sodium influx: The opening of channels allows Na⁺ ions to enter the muscle fiber, causing depolarization.
End plate potential (EPP): The local positive potential change inside the muscle fiber membrane due to sodium influx.
EPP characteristics:
- Not an action potential, localized, non-propagative, decays exponentially away from the end plate.
Action potential generation: When the EPP reaches a critical level of -60 mV, it triggers action potential generation in the muscle fiber.
Muscle contraction: Action potentials propagate along the muscle fiber, leading to muscle contraction.
Miniature end plate potential (MEPP): Even at rest, small amounts of acetylcholine are randomly released, producing small depolarizations called MEPPs.
Acetylcholine removal: The enzyme cholinesterase in the synaptic cleft rapidly degrades acetylcholine, terminating its action.

Key Points

Acetylcholine is the neurotransmitter at the neuromuscular junction.
Calcium ions are crucial for acetylcholine release.
Nicotinic-ACh receptors on the postsynaptic membrane are responsible for acetylcholine binding.
End plate potential is a local depolarization that triggers action potential generation in muscle fibers.
Cholinesterase rapidly breaks down acetylcholine, preventing prolonged stimulation of the muscle fiber.

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Description

Explore the structure and function of the neuromuscular junction, a critical interface between nerve and muscle fibers. This quiz covers components like the presynaptic and postsynaptic membranes, the synaptic cleft, and the role of acetylcholine. Test your understanding of how these elements work together to facilitate muscle contraction.