Foundations of Body Functions: Action Potentials

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Questions and Answers

What is the latent period in nerve fiber recording?

The time between stimulation and the start of the action potential (correct)

The time it takes for the nerve to return to resting potential

The time from resting potential to action potential

The time taken by sodium ions to enter the nerve fibers

Which factors affect the duration of the latent period?

The type of stimulating electrode used

The amount of stimulus applied

The resting membrane potential before stimulation

The distance between the stimulating and recording electrodes (correct)

What does the ascending limb of the spike potential represent?

Gradual sodium ion influx leading to depolarization (correct)

The time taken for nerve repolarization

Rapid potassium efflux during action potential

Return of the nerve to resting potential

What is the maximum voltage spike potential can reach during depolarization?

$+35 mV$ Signup and view all the answers

What mainly causes rapid repolarization during the spike potential?

Increased potassium efflux Signup and view all the answers

What is the primary change in membrane potential during depolarization?

Increase in sodium ion permeability Signup and view all the answers

What occurs during the repolarization phase of an action potential?

Sodium channels close and potassium channels open Signup and view all the answers

What is the threshold membrane potential necessary to trigger an action potential?

-55 mv Signup and view all the answers

Which method would be used to record a biphasic action potential in neurons?

Using two electrodes placed on the surface of the membrane Signup and view all the answers

What ion primarily causes depolarization during the action potential?

Sodium (Na+) Signup and view all the answers

What characterizes the propagation of action potentials in myelinated fibers?

Saltatory conduction between nodes of Ranvier Signup and view all the answers

Which statement correctly explains the refractory period?

It is a period where action potentials can occur but require a stronger stimulus. Signup and view all the answers

What does the term 'polarization' refer to in the context of action potentials?

The maintenance of a resting membrane potential Signup and view all the answers

What is the initial effect of the stimulus on the membrane potential?

It causes a passive depolarization to -63 mV. Signup and view all the answers

At which membrane potential do voltage-gated Na+ channels begin to open?

-63 mV Signup and view all the answers

What occurs at the threshold potential of -55 mV?

Na+ channels fully open leading to rapid Na+ influx. Signup and view all the answers

What is the main reason for repolarization after depolarization?

Closure of voltage-activated Na+ channels. Signup and view all the answers

What is hyperpolarization in the context of membrane potential?

A temporary over-exit of K+ ions beyond -70 mV. Signup and view all the answers

What role does the Na-K pump play after membrane depolarization and repolarization?

It stabilizes ionic composition by transporting Na+ out and K+ into the cell. Signup and view all the answers

What immediate effect does the influx of Na+ ions have on the membrane potential?

It increases the membrane potential towards zero. Signup and view all the answers

What is the role of voltage-activated K+ channels during repolarization?

They open to allow K+ ions to diffuse out of the cell. Signup and view all the answers

What effect does increasing the thickness of the myelin sheath have on action potential conduction?

It increases the membrane resistance to current. Signup and view all the answers

Which phase of action potential occurs when voltage activated Na+ channels open?

Depolarization Signup and view all the answers

What is a characteristic of the conduction of action potentials in myelinated nerve fibers?

It occurs via saltatory conduction. Signup and view all the answers

During the repolarization phase, which ion channels are typically closed?

Voltage gated Na+ channels Signup and view all the answers

What distinguishes monophasic recording of action potentials from biphasic recording?

It consists of a single spike potential. Signup and view all the answers

In multiple sclerosis, what is the primary consequence of losing myelin sheaths?

Decrease in conduction velocity. Signup and view all the answers

What best characterizes continuous conduction in nerve fibers?

It is typically slower than 0.5 to 2.0 meters/second. Signup and view all the answers

What is the role of K+ ions during the repolarization phase of action potential?

K+ ions diffuse out of the cell to restore resting potential. Signup and view all the answers

What characterizes the Negative After-Potential?

Due to slow K+ efflux Signup and view all the answers

Which conduction method is faster?

Salutatory Conduction Signup and view all the answers

In which type of nerve fibers does Continuous Conduction occur?

Unmyelinated nerve fibers Signup and view all the answers

What is the primary energy consumption difference between Continuous and Salutatory Conduction?

Continuous Conduction uses more energy Signup and view all the answers

How does the potential difference facilitate the generation of an action potential at the resting node?

By creating a local current flowing between nodes Signup and view all the answers

Which of the following is true about the speed of Continuous Conduction?

It ranges from 0.5 to 2.0 m/sec Signup and view all the answers

What happens to the membrane during a Positive After-Potential?

The membrane is partially depolarized Signup and view all the answers

What is the role of the node of Ranvier in action potential propagation?

It allows for jumping of impulses from node to node Signup and view all the answers

Study Notes

Foundations of Body Functions and Biophysics

Course title: Foundations of Body Functions and Biophysics
Institution: New Mansoura University

Initiation and Propagation of Action Potential in Neurons

Lecturer: Dr. Abdelaziz M. Hussein
Department: Medical Physiology

Lecture Objectives

Define action potential and its phases
Explain the ionic basis of action potentials in neurons
Identify methods and record of monophasic action potentials in neurons
Explain how action potentials propagate along myelinated and unmyelinated nerve fibers
Identify the basis of biphasic recording of action potentials in neurons

Definition, phases, and ionic basis of action potentials in neurons
Recording of action potentials (biphasic and monophasic recording)
Propagation of action potentials in myelinated and unmyelinated nerve fibers

Action Potential

Definition: Electrical changes in resting membrane potential (RMP) due to stimulation
Phases:
- Depolarization: Membrane potential becomes less negative
- Repolarization: Membrane potential returns to its resting state

Ionic Basis of Action Potential in Neurons

Depolarization: Influx of sodium ions (Na+)
Repolarization: Influx of potassium ions (K+)

Depolarization

Definition: Loss of normal polarized state of membrane
Mechanism: Stimulus increases cell membrane permeability to Na+ ions, causing Na+ diffusion and membrane potential reversal from -70 mV to +35mV
Steps:
- Electrotonic potentials and firing levels: Stimulus (cathode) generates negative charges and reduces potential difference between membrane's surfaces, resulting in passive depolarization
- Voltage gated Na+ channels open: At -63mV further depolarization triggers more voltage gated sodium channels to open , increasing Na+ influx, and membrane potential rapidly rises causing active depolarization and reversing polarity to +35 mV

Repolarization

Definition: Restoration of the resting potential (-70 mV)
Mechanism:
- Closure of voltage-gated Na+ channels and slower opening of voltage-gated K+ channels
- K+ ions rapidly diffuse out of the cell, restoring the negative membrane potential.

Voltage-Gated Sodium Channels

Activation Gate: Opens rapidly at the threshold potential
Inactivation Gate: Closes slowly after the peak potential, preventing further Na+ influx
Resting State: Closed, but capable of opening

Voltage-Gated Potassium Channels

Delayed opening: Opens more slowly than Na+ channels, contributing to repolarization

After-Potentials

Brief changes in membrane potential following the spike potential
Characteristics:
- Short duration (4 msec): Negative after potential, partially depolarized membrane, due to slow K+ efflux
- Long duration (40 msec): Positive after potential, hyperpolarized membrane, continuous K+ efflux due to slow closure of K+ channels

Propagation of Action Potentials

Initiation: Starts at the initial (axon hillock) segment
Propagation: Signal travels along the axon to the terminal ending
Types:
- Continuous Conduction: Action potentials propagate along the entire axon membrane, step by step
- Saltatory Conduction: Action potentials jump between Nodes of Ranvier, relying on local current flow between nodes

Myelin Sheath and Propagation of Action Potentials

Myelination: Increases membrane resistance to current flow, leading to faster conduction velocity
Multiple Sclerosis: Demyelinating disease, decreasing conduction velocity

Recording of Action Potentials

Apparatus: Cathode Ray Oscilloscope (CRO)
Method: Microelectrode inserted into nerve fiber (recording) and another electrode outside stimulated area
- Measures potential difference across the nerve membrane
Parts:
- Latent period: Time from stimulation to start of action potential
- Spike potential: Rapid depolarization and repolarization
- After-potentials: Short-lived changes after spike potential, representing K+ efflux processes and ion redistribution
Importance: Allows for velocity of nerve impulse conduction calculations.

Redistribution of Ions

After depolarization and repolarization, the ionic composition is slightly disturbed
Na+/K+ pump actively works to restore the normal resting ionic concentrations

Role of Na+ and K+ Channels in Action Potentials

Critical for creating the potential difference across the cell membrane
Their opening and closing are tightly coordinated to determine the shape and progression of the electric signal

Questions

Listed specific questions related to depolarization and repolarization processes in nerve fibers, types of conduction and their mechanism of propagation, duration of latent period, etc.

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Description

This quiz covers the initiation and propagation of action potentials in neurons within the Foundations of Body Functions and Biophysics course. It explores the definition, phases, and ionic basis of action potentials, as well as recording techniques and propagation in nerve fibers. Test your knowledge on these essential topics in medical physiology.