Basic Neurophysiology

Neurophysiology is the study of the function of the nervous system.

Membrane Ion Channels

All animal cells are enclosed in a plasma membrane, which separates its cytoplasm with the extracellular environment. Cell membrane has the structure of a lipid bilayer, with large molecules embedded in it. Concentration of ions are different across the cell membrane, with more sodium ions (Na+) and chloride ions (Cl-) outside the cell, and more potassium ions (K+) and negative charged protein molecules (A-) inside the cell. Selective ion channels at rest allow potassium ions (K+) cross the membrane easily, creating a voltage more negative inside the cell than outside, which is called resting membrane potential. The resting membrane potential of a neuron is about -70 mV.

Action Potential

An action potential is the change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell in response to a stimulus, which occurs as the electrical potential briefly (about 1ms) rises and falls (Hodgkin and Huxley). Specifically, an action potential occurs when the threshold is reached (-65 mV) which activates the voltage-gated ion channels to open. When the threshold is reached: Na+ channels open and Na+ rushes into the cell while K+ channels open slowly and K+ leaves the cell, the cell then becomes hyperpolarized (Pinel, 2017). There are three phases of an action potential: the rising phase (Na+ and K+ channels open), repolarization (Na+ channels close), and hyperpolarization (K+ channels start to close).

Propagated Neural Activity

An action potential is generated near the axon hillock when the threshold is reached (-65 mV), the signal is then propagated down (travels) the axon. Active axonal conduction is due to the diffusion of Na+ into the cell which creates waves of depolarization whereas passive axonal conduction occurs as the signal travels down a myelinated axon along each segment, an action potential is generated at each node of Ranvier between the segments (Pinel, 2017) propagated from one node to the next (saltatory conduction).