Bio Tutoring

Neurons and Membrane Potential

  • Main Focus

    • Understanding membrane potential is crucial when studying neurons.

    • The movement of sodium ( ext{Na}^+) and potassium ( ext{K}^+) across the neuronal membrane is fundamental to neuronal function.

Key Concepts

  • Membrane Potential

    • Refers to the potential difference across a neuron’s membrane.

    • Changes in membrane potential are associated with neuronal signaling processes such as action potentials.

  • Ionic Movement

    • Ions Involved:

    • Potassium ( ext{K}^+): Naturally more concentrated inside the cell at resting potential.

    • Sodium ( ext{Na}^+): More concentrated outside the cell at resting potential.

    • Movement of ions can be due to various mechanisms:

    • Passive Transport (Diffusion): Movement occurs down the concentration gradient.

    • Active Transport: Requires energy to move ions against their concentration gradient, primarily through ion pumps.

Types of Ion Channels

  • Channel Proteins

    • Resting ext{K}^+ Channel: Always open, allowing ext{K}^+ ions to move out of the cell since they flow down their concentration gradient.

    • Voltage-Gated Na+ Channel: Opens transiently in response to a change in membrane potential, allowing rapid influx of ext{Na}^+ during action potentials.

    • Ligand-Gated Channel: Opens in response to specific neurotransmitters.

  • Ion Channel Dynamics

    • Depolarization: Occurs when ext{Na}^+ ions enter the cell, reducing the negative charge inside the cell relative to the outside (making the interior less negative).

    • Repolarization: Following depolarization, ext{K}^+ ions exit the cell, restoring the negative internal charge.

    • Hyperpolarization: Occurs when the potential becomes more negative than the resting potential due to excess ext{K}^+ exiting the cell.

Detailed Membrane Potential Changes

  • Phases of Action Potential

    • Resting Potential:

    • Typically around -70 mV.

    • Depolarization Phase:

    • The influx of ext{Na}^+ ions raises the membrane potential toward a positive value (threshold may be around -55 mV).

    • Repolarization Phase:

    • Voltage-gated ext{K}^+ channels open and ext{K}^+ leaves the cell, driving the membrane potential back down.

    • Return to Resting Potential:

    • The membrane is stabilized back to resting potential by the sodium-potassium pump, which actively transports ext{Na}^+ out and ext{K}^+ into the cell.

Active and Passive Transport

  • Active Transport

    • Sodium-Potassium Pump: Moves ext{Na}^+ out of the cell and ext{K}^+ into the cell against their concentration gradients, requires ATP.

  • Passive Transport

    • Facilitated diffusion of ions through specific channels, does not require energy.

    • For example, sodium can enter the cell via facilitated diffusion when channels open during an action potential.

Ion Concentration Gradients

  • Equilibrium Potential

    • ext{K}^+ equilibrium potential is approximately -90 mV.

    • ext{Na}^+ equilibrium potential is approximately +35 mV.

  • Graded Potentials

    • Caused by the influx of ext{Na}^+ via the ligand-gated channels leading to membrane depolarization which may trigger action potentials if the threshold is reached.

Synaptic Transmission

  • Neurotransmitter Release

    • Triggered by calcium ions ( ext{Ca}^{2+}) entering the axon terminal in response to an action potential.

    • Participates in exocytosis of neurotransmitters into the synaptic cleft to propagate a signal to the postsynaptic neuron.

  • Neurotransmitter Reuptake

    • Mechanism where neurotransmitters are taken back into the presynaptic neurons to terminate the signal and recycle components.

Effects of Toxins on Neuronal Activity

  • Increasing Action Potential Frequency:

    • Toxins can inhibit voltage-gated sodium channels from closing, leading to sustained depolarization.

  • Decreasing Action Potential Frequency:

    • Damage to the golgi apparatus prevents neurotransmitter synthesis, reducing synaptic transmission.

    • Enhancing enzymes that break neurotransmitters down would decrease their availability.

Summary of Ion Channel Functions

  • Voltage-Gated Sodium Channels: Open during depolarization, close during repolarization.

  • Ligand-Gated Sodium Channels: Open in response to neurotransmitters, leading to graded potentials that can escalate to action potentials.

  • Calcium Channels: Facilitate the influx of calcium for neurotransmitter release during synaptic transmission.

Important Terminology

  • Resting Membrane Potential: The electrical potential across the membrane of a neuron at rest (about -70 mV).

  • Action Potential: A rapid increase in membrane potential followed by a rapid decrease, forming an electrical signal that propagates along the neuron.

  • Hyperpolarization: Membrane potential becomes more negative than the resting potential.

  • Depolarization: Membrane potential becomes less negative, moving toward zero and possibly becoming positive.

  • Graded Potential: Changes in membrane potential that vary in size and can lead to action potentials.