Recording-2025-03-13T12:03:29.466Z

Overview of Action Potentials

• EPSPs and IPSPs: Understanding begins with the role of neurotransmitters, but it extends beyond that, particularly at the axon initial segment where the action potential is initiated.

Action Potential Initiation

• Resting Membrane Potential: At rest, the neuron's membrane potential is negative.

• Threshold Reached: When threshold is reached at the axon initial segment, voltage-gated sodium channels open, causing a rapid increase in membrane potential to approximately +30 mV.

• Depolarization Phase: Inside becomes more positive as sodium ions diffuse inward, pulling negative charges away from the membrane.

  • This effect continues down the axon, causing subsequent segments to reach threshold and initiate their action potentials.

Propagation of Action Potentials

• Continuous Propagation: In unmyelinated neurons, action potentials propagate continuously along the axon. Each segment depolarizes before the next does.

• Saltatory Propagation: In myelinated neurons, action potentials propagate via saltatory conduction, jumping between nodes of Ranvier, increasing membrane resistance and reducing ion leakage.

Refractory Periods

• Purpose of Refractory Periods: Essential for ensuring clear and effective signal propagation without confusion of signals bouncing back and forth.

  • Absolute Refractory Period: During this period, it is impossible to generate another action potential; occurs while sodium channels are open or inactivated.

  • Relative Refractory Period: Follows the absolute refractory period; a stronger-than-normal stimulus is required to initiate another action potential due to the membrane's potential being more negative from potassium efflux.

Coding of Action Potentials

• Frequency Coding: Changes in action potential frequency communicate varying intensities of stimuli, such as blood pressure changes. Higher frequency indicates higher intensity.

• Temporal Coding: Change in frequency rather than absolute frequency provides information; for example, sensory receptors may signal only upon a change in stimulus (e.g., touch).

Group Neuron Effects

• Spatial Summation in Motor Neurons: Different groups of motor neurons activate based on exertion levels; more neurons are recruited for heavier loads.

  • Recruitment Example:

    • Low effort (e.g., lifting an eraser): minimal neuron recruitment.

    • Medium effort (e.g., lifting 5 lbs): moderate recruitment of additional neurons.

    • High effort (e.g., lifting 20 lbs): maximal recruitment of motor neurons.

Membrane Potentials and Ion Flow

• Resting Membrane Potentials: Compare skeletal muscle (-90 mV) and liver cells (-58 mV); potassium permeability plays a vital role in establishing resting potentials.

  • Skeletal muscle maintains high permeability to potassium, influencing its more negative potential.

  • Liver cell has different permeability characteristics affecting its less negative potential.

Specific Effects and Their Mechanisms

• Tonic Current: Understanding that tonic current does not propagate in one direction underscores the importance of refractory periods.

  • Tonic current contributes to depolarization and relates to the fiber diameter (length constant).

• Impact of Neurotoxins: E.g., dendrotoxin from mamba snake blocking potassium channels leads to prolonged depolarization due to inability to repolarize.