Cholinergic Receptors and Neuromuscular Transmission

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Questions and Answers

What is the initial change in membrane potential during depolarization?

Decrease from -70 to -35 mV
Increase from -70 to 0 mV
Decrease from -70 to -55 mV (correct)
Increase from -70 to -55 mV

What causes the rapid increase in depolarization after 15 mV?

The flow of Na+ causing more Na+ channels to open (correct)
K+ efflux from the nerve fiber
Closure of voltage-gated Na+ channels
Stoppage of Na+ influx

What is responsible for repolarization?

Stoppage of Na+ influx and K+ efflux (correct)
Flow of Na+ into the cell
Closure of voltage-gated K+ channels
Opening of voltage-gated Na+ channels

What is the characteristic of an action potential?

It is an all or none response (B) Signup and view all the answers

What happens to the membrane potential after reaching 0 mV?

It overshoots to +35 mV (C) Signup and view all the answers

What type of potential is produced by a subthreshold stimulus?

Graded potential (A) Signup and view all the answers

What is the primary ion involved in the depolarization phase?

Na+ (B) Signup and view all the answers

What happens to the voltage-gated Na+ channels during repolarization?

They start to close (C) Signup and view all the answers

What is the primary mechanism by which calcium ions affect neuromuscular transmission?

By stimulating the rupture of acetylcholine vesicles (D) Signup and view all the answers

What is the characteristic of an action potential produced by a suprathreshold stimulus?

It is the same as the action potential produced by a threshold stimulus (C) Signup and view all the answers

What is the effect of magnesium ions on neuromuscular transmission?

Inhibition of acetylcholine release (D) Signup and view all the answers

What happens during the absolute refractory period?

The nerve is unexcitable (C) Signup and view all the answers

Which of the following drugs is an example of a reversible anticholinesterase?

Neostigmine (A) Signup and view all the answers

What is the mechanism of action of curare on neuromuscular transmission?

It competes with acetylcholine for its receptors (A) Signup and view all the answers

How does the action potential propagate in unmyelinated nerves?

Through local current flow from the active point to the inactive point (B) Signup and view all the answers

What is the advantage of saltatory conduction in myelinated nerves?

It is faster and more economic in terms of ATP usage (B) Signup and view all the answers

What is the effect of succinylcholine on muscle contraction?

It causes sustained depolarization of the motor end plate (C) Signup and view all the answers

What is the structure of the neuromuscular junction?

Branched motor nerve, motor end plate, synaptic gutter, synaptic cleft, presynaptic membrane, postsynaptic membrane, and subneural folds (C) Signup and view all the answers

What is the primary mechanism of action of botulinum toxin on neuromuscular transmission?

It inhibits the release of acetylcholine (D) Signup and view all the answers

What is the characteristic of the action potential propagation in myelinated nerves?

It is faster and more economic in terms of ATP usage (B) Signup and view all the answers

What is the delay between the nerve impulse reaching the neuromuscular junction and the action potential generated in muscle?

0.5 msec (B) Signup and view all the answers

What is a characteristic of neuromuscular transmission?

It is unidirectional, from nerve to muscle (A) Signup and view all the answers

What is the state of the nerve during the relative refractory period?

It is partially excitable (B) Signup and view all the answers

What is the characteristic of the action potential?

It is self-propagated along the rest of the fiber (D) Signup and view all the answers

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Study Notes

Neuromuscular Transmission

One-way transmission: from nerve to muscle, not the opposite
Delay of 0.5 msec between nerve impulse and muscle action potential
Can be fatigued due to depletion of acetylcholine vesicles
Calcium influx stimulates NMT, while magnesium inhibits it

Effect of Drugs on Neuromuscular Transmission

Acetylcholine-like drugs: nicotine, carbachol, and methacholine
Anticholinesterase drugs: reversible (neostigmine) and irreversible (parathion and DFP)
NMT-blocking drugs: curare, succinylcholine, and botulinum toxin

Excitable Tissues

Include: nerves, skeletal muscle, and smooth muscles
Exclude: exocrine gland

Membrane Potential

Resting membrane potential is mainly due to potassium efflux
Na+ influx and efflux, and K+ influx and efflux affect membrane potential

Action Potential

Absolute refractory period: nerve is unexcitable, no stimulus can excite it
Relative refractory period: partial recovery of excitability
Action potential is self-propagated and travels long distances without decreasing in voltage

Propagation of Action Potential

In unmyelinated nerves: self-propagation through local current flow
In myelinated nerves: saltatory conduction, which is faster and more economic

Neuromuscular Junction

Structure: branched motor nerve, motor end plate, synaptic gutter, synaptic cleft, presynaptic and postsynaptic membranes, and subneural folds
Depolarization: gradual decrease in membrane potential from -70 to -55 mV, followed by rapid depolarization and overshoot to +35 mV
Repolarization: stoppage of Na+ influx, K+ efflux, and closure of voltage-gated Na+ channels

Membrane Potential Changes

Depolarization: Na+ influx, followed by rapid depolarization and overshoot
Repolarization: K+ efflux, closure of voltage-gated Na+ channels, and opening of voltage-gated K+ channels