resting potential & synapses

Electrical Activity of Neurons

• Resting Membrane Potential (Transmembrane Potential)

  • Definition: The electrical potential of a cell's interior compared to its surroundings; occurs across the membrane.
  • Significance: Indicates the cell's state when not communicating (i.e., 'resting').
  • Typical value: Approximately -70 mV for an undisturbed cell.

• Charge Separation and Ion Distribution

  • Cells have a separation of positive and negative charges due to various ions:
  • Proteins: Large molecules with negative charges are trapped inside the cell, contributing to the overall negative charge.
  • Ions:
    • Sodium ions (Na⁺): Primarily located outside the cell.
    • Potassium ions (K⁺): Primarily located inside the cell.
    • Chloride ions (Cl⁻): Tend to accumulate outside the cell.

• Membrane Structure

  • The neuron membrane consists of a lipid bilayer with integral proteins and ion channels that are selectively permeable.
  • Ion Channels: Key players in maintaining resting potential, allowing ions to move across the membrane based on their electrochemical gradients.
  • Sodium-Potassium Pump: A vital active transport mechanism that moves Na⁺ out of the cell and K⁺ into the cell, maintaining the concentration gradient essential for action potentials.

Action Potentials

• Definition: A propagated change in transmembrane potential, initiated at the axon hillock and travels along the axon to the synapse.

• All-or-None Principle: Action potentials occur at full strength or not at all, analogous to flushing a toilet:

  • You must exert enough force (threshold) to initiate a flush (action potential) which always has the same magnitude and speed.

• Steps in Action Potential Generation:

  1. Resting State: Cell is at -70 mV; Na⁺ outside, K⁺ inside.
  2. Threshold Reached: Depolarization begins when the membrane potential reaches approximately -60 mV.
  3. Opening of Voltage-Gated Sodium Channels:
     • Sodium ions rush into the cell, causing a rapid depolarization (membrane potential moves towards +30 mV).
  4. Inactivation of Sodium Channels: At +30 mV, sodium channels close, and voltage-gated potassium channels open.
  5. Potassium Efflux: K⁺ rushes out of the cell, repolarizing the membrane and causing it to overshoot to about -90 mV (hyperpolarization).
  6. Return to Resting Potential: Voltage-gated K⁺ channels close, and the resting membrane is stabilized back to -70 mV.

• Refractory Periods:

  • Absolute Refractory Period: No new action potential can be initiated regardless of the stimulus strength.
  • Relative Refractory Period: A stronger-than-normal stimulus can initiate an action potential as the membrane is partially repolarized.

Propagation of Action Potentials

• The action potential moves forward due to local currents depolarizing adjacent membrane segments; previous regions cannot fire again due to the refractory period.

Synaptic Transmission

• Chemical Synapse: Communication between neurons occurs via neurotransmitters:

  • Process:
  1. Action potential reaches the presynaptic terminal, triggering release of neurotransmitters from synaptic vesicles through exocytosis.
  2. Neurotransmitters cross synaptic cleft and bind to receptors on the postsynaptic membrane, resulting in opening ligand-gated ion channels.
  3. This binding influences the postsynaptic transmembrane potential, altering its electrical state.

• Ligand-Gated Channels: These channels open in response to neurotransmitter binding, allowing ions to flow, potentially generating an impulse in the postsynaptic neuron.

• Neurotransmitter Reuptake: Quickly removes neurotransmitter from synaptic cleft to terminate signal transmission.

• Electrical Synapses:

  • Very fast communication by electrical impulses via gap junctions (direct contact).
  • Commonly found in cardiac muscle cells to enable synchronized contractions.
  • Less common than chemical synapses but important for rapid response mechanisms like in the heart.