The soma, or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. The word "soma" is from a Greek word meaning "body" and the soma of a neuron is often called the "cell body". There are many different specialized types of neurons and the size of the soma can range from about 5 micrometres (5 millionths of a metre) to over 1 millimetre for some of the largest neurons of invertebrates. The cell nucleus is a key feature of the soma. The nucleus is the source of most of the RNA that is produced in neurons and most proteins are produced from mRNAs that do not travel far from the nucleus. This creates a challenge for supplying new proteins to axon endings that can be several feet away from the soma. Axons contain microtubule-associated motor proteins that allow for the transport of protein-containing vesicles from the soma to the distant ends of axons. Such transport of molecules away from the soma allows the nucleus to help maintain critical cell functions in all parts of the neuron. The survival of some sensory neurons depends on axon endings making contact with sources of survival factors that prevent apoptosis. The survival factors are molecules such as nerve growth factor (NGF). NGF interacts with receptors on axon endings, and this produces a signal that must be transported up the length of the axon to the nucleus. A current theory of how such survival signals are sent from axon endings to the soma includes the idea that NGF receptors are endocytosed from the surface of axon tips and that such endocytotic vesicles are transported up the axon.
