Gerald Pollack).Įxamples of the great variety of neuronal form are shown in Figure 1-Figure 6. Two neurons in the auditory pathway of crickets named AN2 (red) and ON1 (green). The interneuron appears in yellow due to colocalization of the dyes ( reproduced with permission). The electrically coupled cells appear in red. C Confocal image of a leech ganglion from an embryo in which mechanosensory P cells were injected with a combination of the fluorescent dyes Alexa 488 and neurobiotin (visualized with Steptavidin-C圓) and a local bending interneuron was injected with Alexa 488. Giant neuron in a cricket (?) ganglion ( reproduced with permission). A dye-filled antennal lobe projection neuron, one of the second-order neurons of the Drosophila olfactory system ( ). peripheral sensory neurons, the input occurs via axons.įigure 3: Invertebrate neurons. There are, however, exceptions to these general rules of neuronal organization. There are some dendrites that also serve as output systems (see dendro dendritic synapses by G. The axon conducts propagating electrochemical signals termed action potentials (usually initiated at the axon hillock, green in Figure 2B) away from the soma.

the output pole, called the axon (coined in 1896 by Rudolph Albert von Kolliker red in Figure 1), arises as a single structure from the soma (and occasionally from a dendrite).With invertebrate neurons the dendrites arise most commonly from the axon (see Figure 3). the receiving or input pole generally consists of extensively branching tree-like extensions of the soma membrane known as dendrites (coined in 1889 by William His from dendros (Greek) meaning tree) which arise in vertebrate neurons directly from the cell body (the body is also a receiving site in most neurons).The point being that neurons can have many types of branching or, in fact, no branches at all. These bald neurons may be found in present day forms as receptors cells in, for instance, the carotid glomus, in gustatory system in vertebrate tongue or as photoreceptors in the retina. Early in the evolution of CNS, and during development, such cells were devoid of any plasmalemmal extensions. In general terms neurons are viewed as having an input and an output pole. Polarization of a developing spinal motor neuron that accompanies the clustering of sodium channels near the axon hillock (red). Spinal motor neuron labeled with antibodies to sodium channels at the axon initial segment (green) and neurofilaments (red) ( reproduced with permission). Avian optic lobe neuron showing axon emerging from dendrite (Cajal). The membranous system bounds and defines different intracellular compartments and includes the outer cell membrane (the plasmalemma) that encompasses the global compartment defining cellularity itself.Īssociated with the plasmalemma (but also with other intracellular elements) are transmembrane macromolecules that control the functionality of nerve cells (voltage and ligand activated ionic channels, ionic pumps, non-gated ‘leakage’ channels) and the machinery that keeps the cells alive by continuously taking up and replacing the molecular modules that constitute the cell’s functional matrix.įigure 2: A. The soma houses the cell nucleus and most of the genomic expression and synthetic machinery that elaborates the proteins, lipids, and sugars that constitute the neuronal cytoplasm and membranes.

Neurons are generically characterized by a central cell body or soma that comes in different shapes.

3.6 Intrinsic electrical properties and subthreshold oscillations.

Their ability to regulate extracellular background synaptic transmitter levels makes them, however, essential for CNS function. Their electrical response dynamics, being slow compared with that of neurons place them mostly in a modulatory role. It is now known that these cell types are essential in the maintenance of the neuronal network, in neuronal migration during development and in the generation of myelin. Other cellular types are also present in the central nervous system (CNS), most particularly several types of glial elements, initially considered a glue type cell serving as a support matrix for the neuronal circuits. The anatomical variation of these neurons is large, but the general morphology and their electrical and ligand dependent responsiveness allows these cells to be classed as neurons (coined in 1891 by Wilhelm von Waldeyer). Neurons are the principal cellular elements that underlie the function of the nervous system including the brain, spinal cord, peripheral sensory systems and enteric (gut) nervous system. Cell sensitive to water movement in the medicinal leech, (Gascoigne & A McVean 1991). Figure 1: Drawings of neurons showing soma and dendrites in black, axon in red.

Neuron dendrite synapse axon