Neuron is the main cell of the Nervous System. It is an electrical excitable cell that is responsible for the transmision of the nerve impulse. With it, they process (in the brain, ganglia and spinal cord) and transmit information.
A human brain has 100.000 millions of neurons.
The other type of cell is glia, which protects the former.
Neurons have three main parts:
- Cell body or Soma: it is the main part of the cell, the one that makes all metabolism and functions. It is very similar to other body cells, except for the difference that neurons do not replicate.
- Axon: it is the longest and most important part of the neuron, it transmits the impulse. A neuron has one only axon.
- Axon terminals: these terminals contain the synaptic bulbs which secrete neurotransmitters to initiate the impulse on the next neuron.
- Dendrites: they detect the neurotransmitter and have receptors to receive the neurotransmitter and they start the impulse if it is strong enough.
Neurons do not reproduce like many others type of cells (they are amitotic, they do not have mitosis). Their DNA copying is blocked. Thus, they usually don't regenerate (except for aferent and eferent neurons) and have to be protected from damage by the glial cells. The regeneration is done via Schwann cells (glia).
Neurons communicate via chemical and electrical synapses, in a process known as synaptic transmission. The fundamental process that triggers synaptic transmission is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron. This is also known as a wave of depolarization.
Neurons form nerves. Nerves can be up to 1 meter long (the nerves controlling the legs, for instance).
Classification by function
- Afferent (sensitive): they pick information of the environment to the brain and spinal chord. Examples: Corti organ cells, Meissner corpuscules, ytfytffftfftffff
- Efferent (motor): they transmit messages from the CNS to the organs and motor nerves.
- Associative (found on the brain): they integrate the information received from sensitive neurons and they send the resulting data to motor neurons.
Associative neurons are the most complex type of cells that exist. They form complex neural networks on the brain and its adjacent structures. They are responsible for conscience and voluntary movement in animals. See brain.
Classification by polarity
- Unipolar: only one prolongation (no dendrites) that functions as an axon, the impulse does not cross the soma.
- Bipolar: a dendrite and an axon only. Example: retine cells.
- Multipolar: they have a great quantity of dendrites branching from the cell body (soma). Golgi I and Golgi II are the two types.