Saturday, August 21, 2010

Bipolar Neuron

A bipolar neuron is a nerve cell which has two processes; a dendrite and an axon. Bipolar neurons are sensory nerve cells specialized for the transmission of special nerve impulses. As such, they are part of the sensory pathways for smell, sight, taste, hearing and vestibular functions. The most common examples are the bipolar neuron of the retina, and the ganglia of the vestibulocochlear nerve. When used without further detail, the term usually refers to the retinal cells.

The bipolar neuron of the retina exists between photoreceptors (rod cells and cone cells) and ganglion cells. Their function is the transmission signals from the photoreceptors to the ganglion cells. Bipolar cells receive synaptic input from either rods or cones, but not both, and they are designated rod bipolar or cone bipolar cells respectively. There are roughly 10 distinct forms of cone bipolar cells, however, only one rod bipolar cell, due to the rod receptor arriving later in the evolutionary history than the cone receptor.

Bipolar neurons are also found in the spinal ganglia, when the cells are in an embryonic condition. They are best demonstrated in the spinal ganglia of fish. Sometimes the processes, come off from opposite poles of the cell, and the cell then assumes a spindle shape; in other cells both processes emerge at the same point. In some cases where two fibers are apparently connected with a cell, one of the fibers is really derived from an adjoining nerve cell and is passing to end in a ramification around the ganglion cell, or, again, it may be coiled spirally around the nerve process which is issuing from the cell.