Consistent with other systems of the human body, the nervous system structure (anatomy) and function (physiology) are intricately linked. Thus, a thorough knowledge of nervous system anatomy and histology is essential for understanding neurophysiology.
Large proteins are interspersed between the phospholipid molecules of the neuron membrane (the number of different types of proteins in a cell membrane vary from approximately 100 to only a few). There are two primary classifications (see ):
Integral (transmembrane) proteins span the entire thickness of the cell membrane, enabling transport between extra- and intracellular environments. They are tightly embedded within and bound to the surrounding phospholipid molecules of the membrane.
Carrier proteins have ion-specific bonding sites which may transport ions passively (with gradient) or actively (against a gradient) across the membrane, depending on whether an energy source is available (i. e., sodium/potassium pump—ATP needed).
When a neuron is resting (not firing), its axon membrane is polarized at -70 mV. This polarization results from intracellular fluid being relatively negative in comparison to the extracellular fluid. The polarized state is achieved by integral channel and carrier proteins in the cell membrane distributing ions across the membrane (e. g., sodium [Na+] potassium [K+] pump carrier proteins and K+ channel proteins). Ion movement creates electrical signals.
Example: Na+–K+ pump carrier “transport” proteins move large numbers of Na+ out of the cell, creating a positive extracellular charge. Simultaneously, these proteins move some K+ into the cell’s cytoplasm. The cell becomes positive on the outside and negative on the inside because more Na+ ions are moved outside the cell than K+ ions are moved inside.
An action potential is the movement of ions across the neuron’s membrane resulting in (1) rapid depolarization (charge moves toward 0 mV; i. e., no difference in charge) followed by (2) repolarization, (3) brief hyperpolarization (overshoot, meaning greater than -70 mV), and return to (4) normal resting potential of -70 mV. Nerve impulses are transmitted via action potentials.
Threshold level stimulation activates opening of numerous voltage-gated ion channels. The amount of Na+ inside the cell increases and the cell membrane charge reverses (i. e., the inside of the cell becomes positively charged and the outside becomes negatively charged).
Potassium ions move outside the cell membrane and Na+ stays inside. This rapidly repolarizes the cell. However, the resulting polarization is now due to a greater amount of intracellular Na+ versus K+ (the Na+–K+ ion ratio is different compared to the initial “resting potential” polarization).
Step #3: Hyperpolarization (i. e., the membrane potential moves lower than normal resting potential of > -70 mV; i. e., it “overshoots”). This is due to a brief increase in intracellular Na+ ions relative to the normal resting potential ratio of Na+–K+ ions.
The Na+–K+ pump moves intracellular Na+ ions out to the extracellular environment in exchange for K+ ions. This returns the neuron membrane to its normal intracellular Na+–K+ ion ratio and polarized resting state.
The refractory period ensures that an action potential will only travel forward. As an action potential moves forward along an axon, a new action potential is incapable of occurring until the membrane resting potential is re-established behind it. This limits the number of signals/impulses a neuron can generate over a period of time.
During the absolute refractory period, an action potential cannot be produced regardless of the stimulus strength. Voltage-gated Na+ channels are either already open or inactivated, thus making them incapable of producing an action potential.
During the relative refractory period, voltage-gated Na + channels are recovering from inactivation. If the neuron receives a sufficiently strong stimulus (greater than normal), it may generate another action potential.
Myelinated axons enable the signal to continue along the axon through myelin insulated segments without the need for channel opening. Sodium channel opening only occurs periodically at uninsulated spots along the axon known as “nodes of Ranvier” ().