Lecture 28 Action Potentials Part 2

Action Potential Overview

• The action potential (AP) is a brief fluctuation in membrane potential.

• It lasts a few milliseconds in axons and slightly longer in cell bodies.

• It is caused by the transient opening of voltage-gated ion channels (mainly Na^+ and K^+).

• AP spreads like a wave along the neuron and can also occur in muscle fibers.

Stages of the Action Potential

• There are three key stages of the action potential:

  1. Fast depolarization to approximately +30mV (reversal of polarization or overshoot) when the membrane potential reaches threshold.

  2. Repolarization.

  3. Hyperpolarization.

Threshold

• The stages are always preceded by a period where the cell membrane is depolarized to a threshold (typically -55 mV) by a physical or chemical stimulus.

• The resting membrane potential may vary (for example, -70 mV instead of the standard -65 mV).

Learning Objectives

• Describe the three stages of the action potential and associated changes in membrane permeability to Na^+ and K^+.

• Explain molecular events leading to Na^+ channel activation and inactivation.

• Explain how action potentials are generated by an electrical stimulus.

• Understand the concept of passive spread of potential along the cell membrane.

• Explain the mechanism of action potential transmission in unmyelinated and myelinated axons.

• Explain how action potentials are generated in sensory neurons (receptor potential).

Ionic Mechanisms

• The stages are associated with opening and closing of voltage-gated ion channels, affecting membrane permeability to Na^+ and K^+.

Activation and Inactivation of Voltage-Gated Channels

• Local hyperpolarization and depolarization influence the state of voltage-gated Na^+ channels.

• The process includes activation, inactivation, and return to the resting state upon membrane repolarization.

Generation of Action Potentials by Electrical Stimulation

• Local depolarization.

• Passive current spread (inside and outside the axon).

• Depolarization at adjacent parts of the membrane.

Passive Spread of Current

• Current spreads passively over a short distance.

• The length constant (\lambda) indicates the distance at which the potential drops to 37% (\frac{1}{e}) of the initial value.

Action Potential Transmission in Unmyelinated Axons

• Action potential is generated.

• Passive current spread occurs.

• Adjacent parts of the membrane are depolarized to the threshold.

• Voltage-gated Na^+ channels are activated.

• New action potentials are generated on both sides of the initial AP.

Action Potential Transmission in Myelinated Axons

• Myelination increases the action potential (AP) conduction velocity.

• It increases AP speed by enhancing the efficiency of passive spread.

• Action potentials are generated only at the nodes of Ranvier, with current flowing passively between them (saltatory conduction).

• The typical distance between nodes is approximately 1 mm.

Generation of Action Potentials in Sensory Neurons

• A stimulus acting on receptors in sensory neurons evokes a graded depolarization called the receptor potential.

• The receptor potential spreads passively to the trigger zone, where APs are generated.

• APs spread along the axon towards the CNS.

• Information about the strength of the stimulus is coded in the amplitude of the receptor potential and the frequency of APs.

Axons

• Two types of axons:

  1. Unmyelinated axons: small diameter (~1 um); transmission of APs is slow and continuous.

  2. Myelinated axons: larger diameter (5-10 um); transmission of APs is fast and saltatory.

Two Stages of Action Potential Transmission

• Passive spread

• Generation of Action potentials

Passive Spread of Current

• If subthreshold, depolarization occurs at one region of the membrane.

• Passive current flows inside and outside the axon.

• Adjacent parts of the membrane are depolarized.

Limitations of Passive Spread

• Current can spread passively only over a short distance, typically less than 1mm.

Action Potential Transmission in Unmyelinated Axons

• A small amount of current inside will trigger a change. Voltage-gated Na+ channels in adjacent parts of the membrane open.

• New full-sized APs are generated in adjacent parts of the membrane

Benefit

• The benefit of action potential is that there are full-sized action potentials regenerated at each point of the axon.

• Regeneration occurs along the entire axon via passive current spread.

Speed of AP Transmission

• Speed of AP transmission in unmyelinated axons is ~ 1 m/sec.

• Passive current flow between two adjacent points is fast, but AP must be regenerated at every point on the membrane, which takes time and slows conduction velocity.

• AP propagates much faster in myelinated axons at ~ 20 to 100 m/sec.

Structure of Neurons with Myelinated Axons

• Myelin sheath is formed by oligodendrocytes in the CNS and Schwann cells in the PNS.

• Myelination is discontinuous, interrupted at nodes of Ranvier.

Impact of Myelination

• Increases passive spread of current due to the insulating properties of myelin, resulting in less current dissipation as it flows along the axon.

• Passive conduction occurs in both directions.

Advantages of Myelination

• Myelination increases the speed of AP conduction by increasing the efficiency of passive spread.

• APs do not need to be regenerated at every part of the cell membrane but are generated only at nodes of Ranvier.

• This process is called “saltatory conduction”.

Comparison of Unmyelinated and Myelinated Axons

• Myelinated Axons: Less time required for generation of action potentials compared to Unmyelinated axons

Direction of AP Conduction

• AP arrives at a node going towards the right, but 'passive' conduction goes both ways.

Peripheral Nervous System (PNS)

PNS contains:

• Axons and cell bodies of sensory neurons

• Axons of motor neurons

• Neurons forming the 'autonomic nervous system'

Generation of APs in Sensory Neurons

• When a stimulus acts on receptors in sensory neurons, it evokes a graded depolarization known as 'the receptor potential’.

• The receptor potential spreads passively to the trigger zone, where APs are generated.

• APs then spread along the axon towards the CNS.

Receptor Potential Details

• Dependent on stimulus

• Not voltage-gated (ligand/mechanical)

Coding Stimulus Strength

Information about the strength of the stimulus is coded in the amplitude of the receptor potential and the frequency of APs.