(also: neurone) Neurons are elementary units of the nervous system that are specialized for communication. For this purpose, neurons have excitable membranes and, in response to various signals such as NEUROTRANSMITTERS, generate action potentials and conduct them to their targets (see ACTION POTENTIAL). In contrast, GLIAL CELLS, which are the other main cell type in the nervous system, do not generate action potentials. Another unique feature of a neuron is that their final MITOSIS (see CELL DIVISION) occurs mostly before birth. The brain has only limited abilities to generate new neurons after birth. The notion that the nervous system is not a mesh of tissues but consists of individual nerve cells is called the NEURON DOCTRINE and established in the late nineteenth century. This discovery involved many scientists, and Ramon y Cajal (1852–1934), Golgi (1843–1926) and Sherrington (1857–1952) played a major role. The term neuron was coined by von Waldeyer (1836–1921) in 1891.
A neuron consists of the SOMA or cell body, dendrites (see DENDRITE) and the AXON. The soma is the site of the synthesis of PROTEINS, and contains the NUCLEUS and various subcellular organelles including MITOCHONDRIA, RIBOSOMES, ENDOPLASMIC RETICULUM, GOLGI APPARATUS and LYSOSOMES. The term PERIKARYA is often used interchangeably with the soma, but strictly speaking, it refers to the CYTOPLASM around the nucleus. From the soma emerge a number of dendrites, usually two to five, which branch into secondary and higher-order dendrites. The function of dendrites is to receive synaptic inputs. The axon emerges from either the soma or a main dendrite. Although a neuron usually has a single axon, axons may branch to innervate multiple targets. The function of the axon is to conduct action potentials, and release neurotransmitters at axon terminals. Axons can be myelinated (see MYELIN) or unmyelinated. Myelinated axons have the NODES OF RANVIER, which are gaps in myelin that allow faster conduction of action potentials. The action potential is generated at the AXON HILLOCK, the cone-shaped site where the axon originates.
Individual neurons display characteristic morphologies which have fascinated neuroanatomists for a century, and these morphological features have been used to classify neurons. Neurons with a single process (‘processes’ being axons and dendrites) are found in the peripheral ganglia (see GANGLION), and are called PSEUDOUNIPOLAR NEURONS because the process eventually divides into two. Neurons with two processes 180° apart are called BIPOLAR NEURONS, and in the CEREBRAL CORTEX they are INTERNEURONS. Neurons that have multiple, branching dendrites are called MULTIPOLAR NEURONS and usually have long axons that project to other regions of the CENTRAL NERVOUS SYSTEM. Certain neurons are known for their characteristic dendritic morphologies. PURKINJE CELLS in the CEREBELLUM have unmistakable patterns of dendritic trees which look like flattened orange trees. Large PYRAMIDAL NEURONS in the cerebral cortex have a triangle-shaped cell body, a long APICAL DENDRITE which ascends toward the surface of the cortex, and shorter, BASALDENDRITES. MEDIUM SPINY NEURONS in the STRIATUM are known for their extremely dense DENDRITIC SPINES.
Neurons have two principal functions: the generation and conduction of action potentials, and neurotransmission. Action potentials are generated in response to various inputs received by the soma and dendrites, mostly at synapses. The probability of generating action potentials is determined by the temporal and spatial summation of excitatory, inhibitory and modulatory inputs as well as the intrinsic membrane properties that are unique to individual neurons. The pattern of dendritic branching has a crucial role in determining the electrophysiological properties of neurons. For neurotransmission, neurons have elaborate systems to synthesize NEUROTRANSMITTERS and release them from their terminals. Neurons are also themselves a target of neurotransmitters, and transmitter receptors are present on dendrites, soma and axon terminals. It should noted that although neurotransmission is a unique property of neurons, the uniqueness lies in the direct apposition of target cells against the signalling cell—that is, the SYNAPSE. Secretion of chemical messengers per se is not unique to neurons but occurs in ENDOCRINE cells as well. The difference is that HORMONES secreted by endocrine cells are released into circulation (the blood or CEREBROSPINAL FLUID) which delivers them to distant target cells. Therefore, neurotransmission can be viewed as one end of a spectrum of chemical communication used by cells, with the endocrine action at the other end, and the PARACRINE action or VOLUME TRANSMISSION in the middle of the spectrum.
There are aspects of neuronal functions that add complexity to the concept of the neuron as a functional unit. First, there is evidence that components of dendrites in a single neuron can represent separate functional units in certain mammalian neurons including cerebellar Purkinje cells. Second, although DALE’S PRINCIPLE stipulates that all terminals of the same neuron release the same set of neurotransmitters, differential release of transmitters has been demonstrated in invertebrate neurons. Third, a variety of GAP JUNCTIONS is now known to exist between neurons, allowing passage of small molecules and electrical coupling of two and more neurons. Although these new find-ings do not refute the NEURON DOCTRINE, they provide further complexity to the function of neurons.
