Biology Of The Archetype
Biology Of The Archetype
Nerve Cells and the Nervous System( 1 )
The Nerve Cell. As the basic functional structure of the nervous system, the nerve cell , or neuron as it is commonly called, has many functions, foremost of which is the processing of information. While this statement holds true for the neurons of all animal species, the observations made of the neuron's anatomy and physiology in this chapter refer specifically to the nerve cells of vertebrate species, which have many structural and functional similarities despite the many differences between the various vertebrate species.
Like any other vertebrate cell, the nerve cell contains intracellular organelles, such as the cell nucleus, endoplasmic reticulum, and mitochondria, which are integral to the cell's metabolic processes. The major intracellular organelles surround the cell nucleus, and this portion of the cell, including the major organelles, is referred to as the cell body . Emerging from the cell body are two varieties of structural projection: one is longer and is known as the axon , which is principally involved in information or neural signal output - where signals are emitted; and the others are shorter and referred to as dendrites , which are generally involved in signal input - where signals are received. The axon of the nerve cell facilitates long-distance signalling; and the dendrites, which often have a branching character, provide for variability in the number of signal input sites. Surrounding a nerve cell is a plasma membrane termed the cell membrane .
Neural signals are carried not only within neurons but between neurons. Existing between nerve cells, usually the axon of one nerve cell and a dendrite of another, is a junction known as a synapse ; research has determined that hundreds or even thousands of synapses can join one neuron with other neurons. The most widely occurring type of synapse is the chemical synapse, and the synaptic gap separating two nerve cells has been measured at between 20 and 30 nanometers (1 nanometer is one millionth of a millimeter). The presynaptic , usually the axonal, side of the synaptic gap is concerned with signal transmittance emission; and the postsynaptic , usually the dendritic, side is involved in signal reception. From the presynaptic axon the signal or transmitter , also referred to as a neurotransmitter , is secreted as a specific chemical. The neurotransmitter spreads across the synaptic gap to the postsynaptic dendrite of another neuron, and given sufficient neurotransmittance signal input during reception, the postsynaptic cell will respond in turn by transmitting the signal to a succeeding neuron. Signals are thus neurotransmitted in a specific direction, both within an individual neuron and between neurons. Neurons connected in this way form specific neural pathways, building neural circuits and other neural structures.
As with any biological structure, nerve cells can be classified and can be classified in various ways. Viewed functionally, neurons can be placed within three basic classes: sensory neurons respond to external stimuli, motor neurons have axons terminating in muscle tissue, and interneurons are those neurons which do not fulfil the criteria of the other two classes of neuron. The vast majority of neurons in the nervous system are interneurons. Nerve cells can be classified in other ways, such as in reference to cell shape or location. Pyramidal cells , for example, are so-called because their cell bodies have a pyramidic shape; and ganglion cells are located so as to receive signal inputs from large aggregations of interneurons. Whatever neurons are involved, those nerve cells occurring later in the neural pathway are known as higher-order cells: in the vertebrate retina, for instance, sensory neurons are first or