Information

Myelin sheath and nerve stimulation conduction


The propagation velocity of nerve stimuli in the membrane of a neuron ranges from 10cm / s to 1m / s.

Such speeds, however, are insufficient to coordinate the actions of large animals. In a giraffe, for example, a pulse traveling at a speed of 1m / s would take between three and four seconds to travel the distance from the hind paw to the brain. If this was really the giraffe's nerve-driving speed, she would be a slow, uncoordinated animal, unable to cope with situations that required quick responses.

The rapid propagation of nerve impulses is ensured by the presence of myelin sheath that covers the nerve fibers. The myelin sheath consists of concentric layers of plasma membranes of glial cells, mainly Shwann cells. Among the glial cells surrounding the axon are small spaces, the Ranvier's nodules, where the neuron membrane is exposed.

In myelinated nerve fibers, the nerve impulse, instead of continuously propagating through the neuron membrane, jumps directly from one Ranvier node to another. In these myelinated neurons, the pulse propagation velocity can reach speeds of up to 200 m / s (720 km / h).

Synapses: Neurons in Communication

Communication of one neuron with the cell body or dendrites of the other, or even with the membrane of a muscle cell, occurs through a region known as synapse (from the Greek synapsis = joining action). In it, a tiny synaptic cleft of approximately 20 nm separates the two cells. The axon message is delivered in the form of chemical mediators, also known as neurotransmitters or neurons, chemicals that come into contact with receptors located on postsynaptic membranes and trigger a change in the behavior of the second neuron or muscle cell. The best known neurotransmitters in the vertebrate nervous system are the acetylcholine and the norepinephrine (or epinephrine).