THE NEURON

The nervous system is a huge ensemble consisting of two distinct classes of cells: nerve cells or neurons, and glial cells or glia.

The brain analyzes information coming from the peripheral receptors and creates perceptions some of which are stored in memory. On the basis of this information it coordinates complex patterns of muscle contractions and thus, behavior... Any cerebral process, ranging from the so called higher functions (learning, imaginery, language) to the simplest spinal reflex has the same cellular substrate: the neuron.

In its best estimate the human brain contains as many as 1011 neurons. Although there are many different types of nerve cells they share common features. The functional versatility of neurons rest mainly in a few principles that give rise to considerable complexity. Thus, it is possible to understand a great deal about the functioning of the brain by paying attention to the following general features.



GENERAL STRUCTURE

In general any typical neuron has four morphologically defined regions:

Dendrites which serve an input role. A typical neuron has several dendrites, normally arranged in an extremely branched fashion, in order to establish contacts with many other neurons.

The cell body also known as the soma which includes the nucleus and perikaryon. It is involved in the main metabolic functions of the cell.

The axon, representing the conduction component of the neuron. Every neuron has only a single axon, a thin cylindrical process that can ramify and extend up to 1m. The axon arises from a specialized region of the soma called the axon hillock where an all or none signal (the action potential) is initiated once a critical threshold is reached.

The presynaptic terminals. These are distinct structures located at the very end of an axon. Since an axon can ramify, it give rise to many presinaptic terminals. By means of its terminals one neuron transmits information about its own activity to the receiving structures (mainly dendrites) of other neurons.This transmission is achieved by releasing in the terminal a specific chemical signal: the neurotransmitter.


The point of contact between two neurons is called a synapse. The neurotransmitter is released into this synaptic space also known as the synaptic cleft. The neuron sending out information is called the presynaptic cell and, therefore, the cell receiving information is called the postsynaptic cell. Normally a presinaptic neuron terminates at the postsynaptic neuron´s dendrites but communication may take place at the cell body, or even , although less often, at the initial or terminal portions of the axon.



Neurons display an unambiguous polarity in the direction of information flow: Information flows from the receiving sites of the neuron (mainly dendrites and cell body) to the trigger zone at the axon hillock. From there the action potential is initiated and propagates unidirectionally along the axon to the presinaptic release sites; the synaptic terminals.





Morphologically, on the basis of the number of processes arising from the cell body neurons are classified into three groups: unipolar, bipolar and multipolar neurons.


Unipolar cells have one primary process that give rise to several branches. One of these is the axon and the rest serve as dendritic receiving structures. Unipolar cells have no dendrites arising directly from the cell's soma. These cells occur in certain ganglia of the autonomic nervous system of vertebrates.


Bipolar cells have two processes emerging from the cell soma: a peripheral process or dendrite which conveys information from the periphery and a central process, the axon, which carries information toward the brain. These cells have mainly sensory functions: retina, olfactory epithelium and sensory cells of the spinal ganglia ( the later cells are also classified as pseudo unipolar since they initially develop as bipolar, but the two processes latter fuse to form a single process that emerges from the cell soma and divides into two processes running in opposite directions: to the periphery and to the central nervous system).




Multipolar neurons have a single axon and one or more dendritic branches emerging from all parts of the cell body. Multipolar cells vary in the number and length of their dendrites and the length of their axons. The number and extent of dendritic processes depend on the number of synaptic contacts that other neurons make onto it. For instance a spinal motor cell, with a moderate dendritic tree, receives about 10,000 contacts. The much larger dendritic tree of a purkinje cell in the cerebellum, receives up to 150,000 contacts from other neurons.


See the general signaling mechanism.

Go to the list of topics