Neuronal Function: Membrane Potentials, Stimuli, and Action Potentials

Sodium-Potassium Pump and Resting Membrane Potential

• The sodium-potassium pump is vital for maintaining the electrochemical gradient of neurons.
  • Pumps 3 sodium ions (6 3Na^+) out of the cell and 2 potassium ions (6 2K^+) into the cell.
  • ATP usage: The pump uses ATP, consuming about 70% of the energy in nerve cells.
  • Result: Higher concentration of sodium outside the cell and higher concentration of potassium inside the cell, creating a positive charge outside and a negative charge inside, establishing a resting membrane potential of -70 mV.

Unique Ion Channels

• Ion channels play a crucial role in cellular communication and membrane potentials.

  • Potassium Channels: Allow potassium ions (6 K^+) to flow down their concentration gradient.

  • More potassium is present inside the cell; therefore, there is a tendency for potassium to move out.

  • However, the negative charge inside (due to proteins) opposes this, causing a limited net movement of potassium out of the cell.

  • Sodium Channels: Facilitate the movement of sodium ions (6 Na^+) into the cell.

  • Sodium tends to move inside due to both diffusion and attraction to negative proteins inside the cell.

  • However, these channels are small, allowing only a limited influx of sodium ions.

Resting Membrane Potential

• The resting membrane potential of -70 mV arises from the combined effects of the sodium-potassium pump, the movement of ions through channels, and the permeability characteristics of the neuron's cell membrane.

Action Potentials and Local Potentials

• Stimulus Definition: A stimulus is any event that evokes a reaction in the neuron, such as stretch, touch, or a chemical signal.

  • This initiates the local potential at the dendrites.

• Local Potentials: Temporary, short-range changes in membrane potential that occur at the dendrites and soma when stimulated.

  • Local potentials can cause a slight depolarization as sodium ions enter, leading to a less negative voltage, for example, changing from -70 mV to -55 mV (depolarization).
  • The degree of local potential is graded; it can increase in magnitude with stronger stimuli.
  • Reversibility: If the stimulus stops, the membrane potential returns to resting state/controllers.)

• Action Potentials: Rapid, large changes in membrane voltage that travel along the axon after reaching a certain threshold at the axon hillock (trigger zone).

  • Characterized by a dramatic up-and-down change in membrane potential, fundamentally different from local potentials.

Mechanism of Local Potentials

• Local potentials involve the activation of receptors that open ion channels in response to stimuli, allowing ions (commonly sodium) to enter the neuron.
  • This causes depolarization, which spreads toward the soma. If strong enough at the axon hillock, it can initiate an action potential.
  • Local potentials can also be excitatory or inhibitory.
  • Excitatory Local Potentials: Make the inside of the cell less negative (depolarization), increasing the likelihood of a subsequent action potential.
  • Inhibitory Local Potentials: Make the inside of the cell more negative (hyperpolarization), decreasing the likelihood of an action potential.
  • Example of hyperpolarization: Influx of chloride ions, which are negatively charged, enhances negativity inside the cell.

Role of the Axon Hillock

• The axon hillock serves as the site of action potential initiation, gathering inputs from local potentials.
  • Contains numerous voltage-gated channels that open in response to changes in membrane voltage.
  • These channels include both sodium and potassium ion channels, crucial for generating action potentials once the threshold is reached.

Conclusion

• Understanding the roles of the sodium-potassium pump, channels, local potentials, and action potentials is essential in grasping neuronal function and communication processes.