Dendrites are one of three parts that make up most nerve cells, or neurons. Almost all neurons have a similar structure that contains one axon; one cell body, also called the soma; and many dendrites. While every neuron has an axon and cell body, it may or may not have dendrites. When they are present, they can be found in one of two forms, which are apical or basilar.
No matter the type of dendrite, the function is the same. The surface is where nerve impulses are received from the terminal ends of the axons of other neurons. Nerve impulses are transmitted or inhibited at a synapse by means of chemical stimulation through the release of neurotransmitters. Usually, this impulse is then transmitted directly to the cell body or along the axon to a centrally located cell body, based on the type of nerve cell.
The two types of dendrites differ in a few fundamental ways. A basilar dendrite does not have a stalk like that seen on an apical dendrite. In most cases, basilar dendrites are found in greater numbers than apical ones. Another difference is that apical dendrites are mostly found within the cerebral cortex of the brain.
Apical dendrites can be further subdivided into two more groups, distal and proximal. Those in the distal group are longer and project from the cell body at the opposite end of the axon. Due to their length, they are said to form non-local synapses, which are not close to the nerve cell. An apical dendrite that belongs to the proximal group is much shorter and receives impulses from other neurons that are much closer, including interneurons.
Another method of classifying dendrites is by whether they are spiny or smooth. These categories are based on the presence or absence of dendritic spines. Smooth ones do not, and the nerve impulse is received directly on the shaft. Spiny ones do have dendritic spines, so the nerve impulse is received along the shaft as well as along the spines that protrude from it.
Smooth and spiny dendrites often carry different types of messages to the nerve cell. In most cases, the synapses found at a smooth dendrite are inhibitory, which means that the neurotransmitter that is released will inhibit, or reduce, the chance of the impulse being transmitted along the neuron. The synapses found at spiny dendrites are usually excitatory. This means that the opposite action occurs here, so the neurotransmitter will stimulate the impulse to continue along the neuron.