Introduction to Physiology Lecture 3

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Questions and Answers

What does a threshold potential of -55mV indicate?

The nerve's membrane is at rest

Stimulus is too weak to open voltage-gated channels

The nerve is fully depolarized

A full action potential will occur (correct)

Which statement correctly describes the all-or-nothing principle?

Different nerves can have different action potential sizes

All action potentials for a given nerve are the same size (correct)

Action potentials only occur in response to subthreshold stimuli

Action potentials vary in size depending on the strength of the stimulus

In neuronal activity, what occurs during the refractory period?

The neuron cannot fire another action potential regardless of stimulus strength

The neuron can easily fire another action potential

The neuron is hyperpolarized and can only respond to strong stimuli (correct)

The neuron is fully depolarized

How do local potentials differ from action potentials?

Local potentials reflect the shape and size of the stimulus, while action potentials do not Signup and view all the answers

What is the result when a stimulus exceeds the threshold potential?

An action potential occurs Signup and view all the answers

If two stimuli both exceed the threshold, what happens according to the all-or-nothing law?

Only one action potential will occur regardless of stimulus strength Signup and view all the answers

What occurs if a stimulus is below the threshold?

The neuron will exhibit graded depolarization Signup and view all the answers

What happens when sodium ions (Na+) enter a neuron?

It triggers the generation of an action potential Signup and view all the answers

What is the main event that occurs during depolarization of an action potential?

Na+ entry into the neuron Signup and view all the answers

Which phase of an action potential is characterized by the return of the membrane potential to resting levels?

Repolarization Signup and view all the answers

What is the threshold for initiating an action potential as indicated in the content?

-55 mV Signup and view all the answers

Which ion channel is primarily responsible for repolarization in an action potential?

Voltage-gated K+ channels Signup and view all the answers

What describes the phenomenon when an action potential overshoots the resting membrane potential?

Hyperpolarization Signup and view all the answers

What phenomenon explains why changes in membrane potential decrease as you measure further from the stimulation electrode?

Passive properties Signup and view all the answers

What mechanism indicates that local potentials can trigger action potentials?

A substantial depolarizing stimulus Signup and view all the answers

During which phase of an action potential do voltage-gated Na+ channels primarily operate?

Depolarization Signup and view all the answers

What is the process called that describes the change from a negative to positive membrane potential?

Depolarization Signup and view all the answers

Which area of the neuron is primarily responsible for integrating signals and generating action potentials?

Trigger Zone Signup and view all the answers

Study Notes

Course Information

Course: MD137
Course title: Introduction to Physiology
Lecturer: Leo Quinlan
Lecturer email: [email protected]
Academic year: 2024-2025

Lecture 3: Action Potentials

Topic: Action potentials (AP)
Subtopic: Neural Signaling
Learning Outcomes:
- Define action potential
- Detail the main ion movements during AP formation
- Explain the role of ion channels and the concept of threshold
- Describe the kinetics of an AP
- Students are encouraged to think positively about action potentials

### Polarization States of the Membrane

Action potentials are represented by changes in membrane potential difference (Vm).
Depolarization occurs when the membrane potential becomes less negative.
Repolarization occurs when the membrane potential returns to its resting state.
Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential.
The voltage, measured in milivolts (mV), changes during these phases.

Passive Properties

Henry and Huxley discovered that the membrane potential responds to stimulation, but its response decreases as the electrode is moved further from the stimulation site.
These properties characterize passive signal transmission in a neuron.

Active Properties

A small depolarizing stimulus (e.g., -65mV) produces a local potential change.
Stimulus recordings are measured at different locations (e.g.,REC 1, REC 2, REC 3).
Increasing the depolarizing stimulus (e.g., -60mV, -55mV, -40mV) causes action potentials to emerge at different recording locations.
As the stimulus is increased, propagation of the stimulus leads to an increasing frequency of action potentials.
Local potentials are graded potentials.

Generating Action Potentials

The threshold potential is -55 mV
Stimulus initiating the action potential at the trigger zone.
Action potentials are characterized by depolarization, repolarization, and recovery (resting state with refractory period).

Phases of Action Potentials (AP)

Depolarization: Voltage-gated Na+ channels open, Na+ ions enter the cell.
Repolarization: Voltage-gated Na+ channels close, and K+ channels open, K+ ions leave the cell.
Hyperpolarization: K+ channels stay open for a brief period, leading to membrane potential becoming more negative than resting potential.

Ion Channel Inaction During APs

Voltage-gated Na+ and K+ channels are crucial for action potential generation.
Na+ channels open rapidly, leading to depolarization.
Na+ channels inactivate, stopping further Na+ influx.
K+ channels open, leading to repolarization and hyperpolarization.

All-or-Nothing Principle

The amplitude of action potentials is consistent for a given neuron.
Action potentials are "all-or-none," meaning that either an action potential is initiated or it isn't, with no in-between values.

Regeneration of Action Potentials

Action potentials regenerate along the length of the axon.
The signal does not diminish as it propagates down the axon.

Membrane Potential Reversal During Action Potentials

During depolarization, the inside of the axon becomes positive relative to the outside.
Reversal in the membrane potential occurs due to ion movement across the membrane.

Refractory Period

Absolute refractory period: The neuron cannot generate another action potential because sodium channels are inactivated.
Relative refractory period: The neuron can generate another action potential, but a larger-than-normal stimulus is needed.

Different Action Potential Times

The duration of an action potential varies depending on the type of neuron (e.g., cardiac muscle, skeletal, motoneuron).

Local/Graded Potentials Vs Action Potentials

Local/Graded potentials:
- Vary in size and shape according to the stimulus.
- Decrease as they move away from the stimulus site.
Action potentials:
- Uniform in size, shape.
- Regenerated along the axon, without decreasing in size or magnitude.

Learning Outcomes (Recap)

Students should understand action potential characteristics.
- Definition
- Ion movements
- Role of ion channels
- Kinetics

Studying That Suits You

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Description

This quiz covers Lecture 3 on Action Potentials from the Introduction to Physiology course. It focuses on the definitions, mechanisms, and the electrical changes involved in neural signaling and membrane polarization. Students will explore the ion movements during action potential formation and the significance of threshold levels.