Nerve Impulse Excitability and Conduction

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Questions and Answers

What is the effect of electrical stimulation under a cathode?

It has no effect on excitability.

It increases excitability.

It reduces excitability. (correct)

It inverts the membrane potential.

Which factor is most significant in affecting the conduction of nervous impulses?

Extracellular ion concentration.

Type of neurotransmitter used.

Length of the nerve fiber.

Nerve fiber diameter. (correct)

What is the primary focus of local circuit theories in nervous impulse propagation?

Synchronous firing of multiple neurons.

The generation of action potentials.

Movement of ions across the membrane. (correct)

The distribution of neurotransmitters.

In the context of demyelination, which clinical application can result from nerve fiber damage?

Decreased conduction velocity. Signup and view all the answers

How does the presence of myelin affect nervous conduction?

It insulates the nerve fiber and increases conduction speed. Signup and view all the answers

Which property of an axon contributes to a low cytoplasmic resistance?

Large axon diameter Signup and view all the answers

What effect does a high membrane resistance have on conduction velocity in an axon?

Increases conduction speed Signup and view all the answers

How does a low membrane capacitance affect an axon's conduction properties?

Allows faster changes in membrane potential Signup and view all the answers

Which of the following combinations of properties would lead to the highest conduction velocity in an axon?

Large diameter, low capacitance Signup and view all the answers

In the context of axonal properties, capacitance is defined as the ability to:

Store charge Signup and view all the answers

What initiates an action potential in a segment of the axon?

Depolarization of that segment to threshold Signup and view all the answers

How does local current spread affect conduction velocity in an axon?

It increases conduction velocity by facilitating faster depolarization Signup and view all the answers

What primarily determines how far local currents can spread along an axon?

The diameter of the axon Signup and view all the answers

What role does depolarization play in the conduction of signals within an axon?

It enables local current spread to adjacent areas of the axon Signup and view all the answers

Which of the following statements is true regarding local currents in axons?

Local current spread is essential for initiating action potentials Signup and view all the answers

What effect does a lower membrane resistance have on ion channels?

It increases the loss of local current across the membrane. Signup and view all the answers

What happens when more ion channels are open in a membrane?

There is a decrease in the spread of the local current effect. Signup and view all the answers

Why does a decrease in membrane resistance limit the local current effect?

Because there is a loss of local current across the membrane. Signup and view all the answers

Which statement accurately describes the relationship between ion channels and membrane resistance?

More open ion channels decrease membrane resistance. Signup and view all the answers

What characteristic distinguishes large diameter axons like motoneurons from smaller axons such as C-fibers?

Large diameter axons are myelinated. Signup and view all the answers

What is one consequence of increased loss of local current across the membrane?

Isolation of local current activities. Signup and view all the answers

What effect does myelination have on the properties of an axon?

Decreases capacitance and increases resistance. Signup and view all the answers

Which of the following statements is true regarding C-fibers?

C-fibers are not myelinated. Signup and view all the answers

Why is myelin important for axons in the nervous system?

It reduces the capacitance of the axon. Signup and view all the answers

How do capacitance and resistance relate to signal transmission in myelinated axons?

Decreased capacitance and increased resistance enhance signal speed. Signup and view all the answers

What is the primary function of Schwann cells?

To myelinate peripheral axons Signup and view all the answers

Which cells are responsible for myelination in the central nervous system (CNS)?

Oligodendrocytes Signup and view all the answers

What does demyelination refer to?

The loss of the myelin sheath from axons Signup and view all the answers

Which of the following correctly identifies where Schwann cells are found?

In the peripheral nervous system only Signup and view all the answers

What can result from demyelination in axons?

Impaired nerve signal conduction Signup and view all the answers

Study Notes

Electrical Excitability of Nerve Impulse

The lecture aims to outline how the resting nerve fiber membrane reaches threshold, describe results of extracellular recordings, demonstrate local circuit theories of nerve impulse propagation, and discuss the effects of nerve fiber diameter and myelin on conduction, and distinguish clinical applications such as demyelination.

Electrical Stimulation

Excitability is reduced under an anode (positive charge), and increased under a cathode (negative charge).
This principle can be used to stimulate axons to threshold, triggering action potentials.

Extracellular Recording

Extracellular recordings, diphasic and monophasic, provide information about conduction velocity.
Diphasic recording shows a biphasic waveform.
Monophasic recording shows a single wave from a damaged axon.

Conduction Velocity

Calculated by the distance between stimulating and recording electrodes, and the time gap between the stimulus and the action potential registration.
Conduction Velocity = Distance / Time

Action Potential Conduction

A change in membrane potential in one part of an axon spreads to adjacent areas.
Local current spread is the mechanism for this.
Conduction velocity depends on how far along the axon local currents can spread.
Local current spread triggers depolarization to threshold, initiating an action potential at that point.

Length Constant

λ (length constant) is the distance a potential falls to 37% of its original value.
Influenced by membrane resistance and capacitance. Higher membrane resistance and lower capacitance lead to a larger length constant.

Factors Affecting Conduction Velocity

Conduction velocity increases with the distance local currents can travel.
Factors contributing to fast conduction include high membrane resistance, low membrane capacitance, and large axon diameter. (Lower cytoplasmic resistance is also implicated for large diameter axons.)

Capacitance

Capacitance is the ability to store charge due to the lipid bilayer of the axon membrane.
Higher capacitance slows the spread of local current.

Action Potential Propagation

Local currents cause the action potential to propagate down the axon.
Forward propagation only occurs; the refractory period prevents backward propagation.

The Myelin Sheath

Myelination considerably increases conduction velocity in axons.
Larger diameter axons (e.g., motoneurons) are myelinated.
Smaller diameter axons (e.g., sensory fibers) are often unmyelinated.
Myelin reduces capacitance and increases axon resistance.
- Schwann cells myelinate peripheral axons, while oligodendrocytes myelinate axons in the CNS.

Fiber Diameter and Conduction Velocity

Conduction velocity in myelinated axons is roughly proportional to the fiber diameter, while in unmyelinated it is proportional to the square root of diameter.
Myelinated mammalian axons have high maximal conduction velocities (120 m/s) relevant to motoneuron function, while unmyelinated axons have a lower maximal velocity (20 m/s).

Demyelination

Demyelination, as in multiple sclerosis, impairs the ability of myelinated axons to conduct action potentials.
It can lead to decreased conduction velocity, complete blockades, or partial transmission of action potentials.

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Description

This quiz explores the principles of electrical excitability in nerve impulses, including the effects of nerve fiber diameter and myelin. It covers aspects such as extracellular recordings and the correlation between nerve stimulation and action potentials. Ideal for students studying neurophysiology and related fields.