Membrane potential
2. Resting membrane potentials
3. Action potential
4. Refractory periods
5. Action potential propagation
6. Synaptic transmission
1. Membrane potentials
Ionic gradients are responsible for the generation of a membrane potential of cells.
Membrane potential is the charge difference between the outer and inner membrane.
At the state of rest the inside of the cell is electrically negative with respect to the outside. It is the resting membrane potential.
2. Resting membrane potentials
Membrane potentials arise because cell membranes contain ion channels that provide selective ion permeability and because there are stable ion diffusion gradients across the membrane.
The following steps are involved in the development of a membrane potential :
3. A voltage difference develops as the K+ ions leave the upper compartment, leaving a net negative charge behind.
4. The negative potential in the upper compartment attracts K+ ions and opposes the K+ concentration gradient. An equilibrium potential is established when the voltage difference and the concentration gradient are equal but opposite driving forces. At the equilibrium potential, there is no net movement of K+.
The equilibrium potential (resting membrane potential, RMP) is a function of the size of the ion concentration gradient and is calculated using the Nernst equation. In most cells, RMP is primarily a function of ECF [K+] because K+ conductance predominates in most cells at rest.
- Depolarization is a change to a less negative membrane potential (membrane potential difference is decreased).
- Hyperpolarization occurs when the membrane potential becomes more negative (membrane potential difference is increased).
- Repolarization is the return of the membrane potential toward RMP following either depolarization or hyperpolarization
3. Action potential
Excitable tissues (i.e., neurons and muscle) are characterized by their ability to respond to a stimulus by rapidly generating and propagating electrical signals. The signal assumes the form of an action potential, which is a constant electrical signal that can be propagated over long distances without decay.
An action potential is an all-or-none response that occurs when an excitable cell membrane is depolarized beyond a threshold voltage.
“ALL-OR-NONE RESPONSE” of the action potential
Only 2 results from many signal types:
Once the threshold has been exceeded, there is a phase of rapid depolarization, which ends abruptly at a peak voltage greater than 0 mV.
The overshoot is the amount that the peak voltage exceeds 0 mV. A slower repolarizing phase returns membrane potential toward RMP.
An afterhyperpolarization (undershoot) is observed in nerves (but not in muscle), in which the membrane potential is transiently more negative than the resting membrane potential.
The characteristic phases of an action potential are explained by specific changes in membrane Na+ and K+ conductance with time:
The characteristic phases of an action potential are explained by specific changes in membrane Na+ and K+ conductance with time:
1. Rapid depolarization after threshold voltage is exceeded is due to the opening of voltage-gated Na+ channels. Na+-ions go in due to electrical and chemical gradients.
2. The peak voltage where rapid depolarization abruptly ends and the membrane enters the repolarizing phase has two components: 1) Inactivation of Na+ channels; 2) Opening of voltage-gated K+ channels (there are late because there are very slow) and K+-ions flux due to electrical and chemical gradients.
3. Repolarization of the membrane potential progresses due to the decreasing Na+ conductance and the increasing K+ conductance.
4. Afterhyperpolarization occurs because K+ conductance exceeds that at rest, because K+-channels are very slow .
4. Refractory periods
In the nervous system, it is necessary to encode the intensity of a stimulus rather than merely indicating whether or not a stimulus is present (e.g., how loud is a sound?). Because individual action potentials all have the same amplitude and action potentials never summate, the stimulus intensity must be encoded by action potential frequency.
The maximum action potential frequency is limited because a finite period of time must elapse after one action potential before a second one can be triggered.
- The absolute refractory period (ARP) is the time from the beginning of one action potential when it is impossible to stimulate another action potential (stimulation is possible, but response is not possible). The absolute refractory period results from closure of inactivation gates in Na+ channels.
- The relative refractory period (RRP) is the time after the absolute refractory period when another impulse can be evoked, but only if a stronger stimulus is applied. A stronger stimulus is needed because a small number of Na+ channels have now recovered from inactivation and the membrane is less excitable due to high K+ conductance.
5. Action potential propagation
The impulse in one area causes local current flow, which depolarizes the adjacent area to threshold, generating a new action potential downstream. This process is repeated to propagate the action potential signal, conduction is unidirectional because the upstream region is in its refractory period.
The speed of action potential conduction is faster in larger diameter fibers because they have lower electrical resistance than small diameter fibers.
Conduction speed is also increased by the myelination of nerve axons. Myelin consists of glial cell plasma membrane, concentrically wrapped around the nerve. In the peripheral nerves, the myelin sheath is interrupted at regular intervals by uncovered nodes of Ranvier.
Action potentials are propagated from node-to-node rather than conducting along the whole nerve membrane because voltage-gated Na+ channels are only expressed at nodes of Ranvier. This process call saltatory conduction
Action potentials are propagated from node-to-node rather than conducting along the whole nerve membrane because voltage-gated Na+ channels are only expressed at nodes of Ranvier. This process call saltatory conduction
6. Synaptic transmission
