Answer and Explanation:
The function of neurons is to transmit nerve impulses. The transmission depends on the unequal distribution of ions in the interior and the exterior of the cell.
The cell interior has many negative ions –anions- and k+ ions that cannot get out of the cell, while the extracellular space has many Na+ and Cl- ions.
The charge difference between the interior and the cell exterior is named resting potential difference, representing a non-exited state.
When the neuron is stimulated, many changes in channel proteins occur that let the ions move through the membrane altering the resting-potential difference.
The central nervous system generates an action potential that travels to the target activating the calcium channels. Calcium triggers vesicles fusion to the presynaptic membrane releasing a neurotransmitter into the synaptic space. Once there, the molecule binds to its receptors on the postsynaptic membrane, causing ion channels to open. The activation of different receptors will cause a change in the postsynaptic membrane modifying the cell activity.
Two types of receptors can be found the in te postsynaptic membrane:
- Ion channel receptors -or ionotropic- that produce a fast change in the membrane potential when opening. But when the neurotransmitter disappears, the response does it too.
- Metabotropic receptors that trigger a slow change in the membrane potential. They produce a slower response, but it lasts longer.
Depending on the ion flow through the membrane channel receptors, the postsynaptic potential may be depolarizing - excitatory postsynaptic potential, EPP-, or hyperpolarizing - inhibitory postsynaptic potential, IPP-. For instance, the entrance of Na+ ion depolarizes de cell. But if K+ gets out of the cell, it produces hyperpolarization.
Depolarizations or hyperpolarizations can be taken to other regions of the cell. The depolarizing or hyperpolarizing effect will gradually fade as it increases the distance from the potential change origin.
Under physiological conditions, when the axon is stimulated occurs a depolarization. If the stimulus exceeds a threshold level, many sodium gates open simultaneously, and the action potential occurs, responsible for the nerve impulse transmission. Once Na+ channels get closed, K+ channels open. Ions traffic consists of the pass of sodium to the interior of the cell and potassium to the exterior. K+ follows a concentration gradient, while Na+ follows an electrochemical gradient.
When the electrical stimulus is applied to the membrane of the axon it produces depolarization by activating the sodium voltage-dependent channels all along the axon. Depolarization propagates to inactivated regions adjacent to the membrane opening more sodium channels.
There is a directly proportional relationship between the fiber diameter and the conduction velocity. The greater the diameter is, the faster the conduction is. Most axons contain a myelin sheath formed by the envelopment of the axon by glial cells. Myelin is fatty and isolates the axon, so in myelinic regions, there is no ion exchange. So to conduct the impulse, there must be regions without myelin. These regions are the Ranvier nodes, which have channels for ion interchange.
The conduction speed increases in myelinated fibers because of the high concentrations of sodium channels in the Ranvier nodes. Mielinized fibers are thick, and their conduction velocities are very fast.