NEURONAL STRUCTURE AND SYNAPSES
A typical neuron of the central nervous system consists of three parts: dendritic tree, cell body (soma), and axon. The highly branched dendritic tree has a much greater surface area than the remainder of the neuron and is the receptive part of the cell. Incoming synaptic terminals make contact directly with the dendritic surface or with the small spines (gemmules) that protrude from it. The membrane potential induced in the dendrites spreads passively onto the cell soma, which allows all inputs acting on the neuron to summate in controlling the rate of neuronal discharge through the axon.
The soma contains the various organelles that control and maintain neuronal structure: nucleus, Golgi apparatus, lysosomes, ribosomes, mitochondria, and smooth and rough endoplasmic reticula. The rough endoplasmic reticulum, studded with ribosomes, is called the Nissl substance because of its characteristic blue staining with Nissl stain. The ribosomes are the site of synthesis of neuronal proteins; as in other cells, the ribonucleic acid (RNA) templates that control protein structure are transcribed from patterns in the nuclear deoxyribonucleic acid (DNA). The soma membrane is also covered with synaptic endings separated by glial processes. Because of their proximity to the origin of the axon, these synaptic endings have an especially potent effect on the rate of discharge of the neuron.
In humans, the axon can extend for several feet. Such lengths pose supply problems because the neuron must transport proteins and other synthesized substances as far as the axon terminals. Certain key substances are transported, at a rate as high as 400 mm/day, by rapid axonal transport, a process probably associated with the microtubules that originate in the soma and run the length of the axon. Other soluble and particulate substances move by slow axonal transport at a rate of 1 to 4 mm/day, aided partly by the peristalsis-like motion of the axon.
The axon originates from a conical projection (axon hillock) on the soma (as shown in Plate 2-14) or on one of the proximal dendrites. The axon membrane is specialized for the transmission of action potential. Because of its shape and high excitability, the initial segment of the axon is usually the site of action potential generation. The action potential then spreads down the axon and back to the soma and proximal dendrites. Because of the low excitability of the dendrites, the impulse usually does not spread very far into the dendritic tree. At its distal end, the axon divides into numerous branches, which end in synapses.
The most common central nervous system (CNS) synapses are those between axon terminals and dendrites (axodendritic) or between axon terminals and somata (axosomatic). Axodendritic synapses take several forms. Spine synapses are of particular interest, because they may be the site of morphologic changes accompanying learning. Axosomatic synapses are of the simple type shown in example A. Synaptic interconnections between a number of neurons occur within structures of a complex organization, such as the cerebellar glomerulus, although all synapses within the glomerulus are axodendritic.
Axons also form axoaxonic synapses with other axon terminals, and these are responsible for the phenomenon of presynaptic inhibition. Axoaxonic synapses are also seen in the efferent vestibular system and in connection with motor neuron dendrites and other terminals ending on those dendrites.
The CNS also contains several less common types of synapses. Dendrodendritic synapses are found in the olfactory bulb. In the internal plexiform layer of the retina, synaptic interactions involve synaptic t of bipolar, amacrine, and ganglion cell processes.
Other synapses are those formed between the peripheral axonal processes of sensory neurons and sensory receptor cells, as in the inner ear. Here, the axon terminal forms the postsynaptic element that is depolarized by the presynaptic sensory cell.
There are also specialized axosomatic synapses formed by efferent motor axons on muscle (motor end lates) and by autonomic axons on secretory cells.