Recording-2025-03-11T12:01:50.520Z

Electrotonic Current

• Electrotonic current represents the passive flow of charge along the dendrites and axons of neurons due to the movement of ions.

• It spreads in all directions but tends toward the axon terminal in the case of neurons.

• This current weakens as it moves further from the source due to leakage of positive charges through the membrane.

Ion Movement and Resistance

• Positive charges can leak through the membrane, often represented by potassium ions.

• Lower internal resistance (Ri) means a higher likelihood for ions to encounter others and transmit current. Larger axons or dendrites, having more cytoplasm, facilitate easier current flow due to increased ion presence.

• Dendrites initially have a resting membrane potential around -60 mV.

Action Potentials and Summation

• Action potentials start with an excitatory post-synaptic potential (EPSP) which may not always reach threshold alone; hence, summation is necessary.

• Temporal Summation: Repeated action potentials arrive quickly in succession, leading to a cumulative effect where they increase the likelihood of reaching threshold together.

• Spatial Summation: Synaptic input from multiple sources occurs simultaneously, aggregating EPSPs across different locations on the neuron.

• The membrane must be sufficiently depolarized at the axon initial segment to activate voltage-gated channels and propagate an action potential.

Integration Points

• The axon initial segment (AIS) serves as a crucial integration point where excitatory and inhibitory signals (IPSPs) are summed.

• Strong IPSPs can inhibit the transmission of sensory information if they outbalance EPSPs.

• Signals are integrated regardless of their source, leading to a potential action that dictates whether to proceed with the signal transmission or not.

Mechanisms of Action Potential

• Once threshold is reached, sodium channels open, causing rapid depolarization (voltage steps up to +30 mV).

• After reaching +30 mV, sodium channels inactivate, and potassium channels begin to open, triggering repolarization, which overshoots resting potential, creating a hyperpolarization phase.

• The sodium-potassium pump restores the resting potential by moving sodium out and potassium in, preparing for the next action potential.

Propagation of Action Potentials

• Continuous propagation in unmyelinated axons requires frequent regeneration of action potentials. As sodium ions rush in, they affect adjacent segments, triggering further depolarization.

• This process is akin to a domino effect where the current travels along the axon, and if it drops below threshold, the action potential will cease.

• In contrast, myelinated axons exhibit saltatory propagation, where action potentials jump between nodes of Ranvier, significantly increasing transmission speed and efficiency (up to 30 miles/hr).

Myelination and Conduction Speed

• Myelination decreases leakage of ions and increases the length constant, allowing action potentials to travel further.

• Only the nodes of Ranvier contain sodium channels which reduce the need to open channels along the entire length of the axon, thereby enhancing conduction speed.

• The body selectively myelinates certain motor and sensory neurons for fast response and efficiency in signaling.

Impact of Demyelination in Conditions like Multiple Sclerosis

• Loss of myelin disrupts normal signaling, causing erratic transmission as signals fail to reach the synapse effectively due to increased leakage and slower conduction.

• Demyelination results in symptoms related to motor control and sensation as the axon cannot conduct signals as quickly or efficiently as before.

Refractory Periods

• Following depolarization, the cell enters a refractory state where sodium channels remain inactive, preventing immediate reactivation.

• The refractory period ensures one action potential follows another without overlap.

• It is crucial for directional signal propagation as it prevents the backtracking of signals to the soma, funneling action potentials towards the synapse.