"A neuron is a special type of cell that is found in the bodies of most animals (all members of the group Eumetazoa,
to be precise—this excludes only sponges and a few other very simple
animals). The features that define a neuron are electrical excitability
and the presence of synapses, which are complex membrane junctions used to transmit signals to other cells. "
"Neurons are highly specialized for the processing and transmission of
cellular signals. Given the diversity of functions performed by neurons
in different parts of the nervous system, there is, as expected, a wide
variety in the shape, size, and electrochemical properties of neurons.
For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.
- The soma is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter.
- The dendrites
of a neuron are cellular extensions with many branches, and
metaphorically this overall shape and structure is referred to as a
dendritic tree. This is where the majority of input to the neuron
- The axon
is a finer, cable-like projection which can extend tens, hundreds, or
even tens of thousands of times the diameter of the soma in length. The
axon carries nerve signals
away from the soma (and also carries some types of information back to
it). Many neurons have only one axon, but this axon may—and usually
will—undergo extensive branching, enabling communication with many
target cells. The part of the axon where it emerges from the soma is
called the axon hillock. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels.
This makes it the most easily-excited part of the neuron and the spike
initiation zone for the axon: in electrophysiological terms it has the
most negative action potential threshold.
While the axon and axon hillock are generally involved in information
outflow, this region can also receive input from other neurons.
Evolution of the mechanisms of connection between neurons: electrical, mixed, and chemical synapses.
"Investigation of the mechanisms of transmission of stimuli in synapses
of isolated perfused spinal cord of cyclostomes, amphibia, reptiles, and
mammals demonstrated that the ratio between electrical and chemical
synapses decreased progressively in favor of the latter in the
transition from primitive toward more highly developed forms. Electrical
transmission was not detected in synapses of spinal cords of reptiles
and mammals. On the basis of the data, the result of analysis of
elementary synaptic processes in synapses uniting electrical and
chemical mechanisms of transmission and dendrodendrite electronic
interdependences of the motor neurons, a hypothesis is formulated that
the number of electrical connections characterizes the union of similar
neurons, while in a sequential union of nerve cells of various functions
and origins, there are mixed or chemical synapses. The possible
cellular mechanisms which are the basis of this phenomenon are