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2.3 Neurons and Their Characteristics

Neurons are the building blocks of the signaling unit in the nervous system. Nerve cells come in different shapes, sizes, connections, and excitabilities. Therefore, the impression of uniformity of character which is often given for the cells is a vast oversimplification in almost all cases. However, certain properties such as excitability, development of an action potential, and synaptic linkage are considered as general characteristics of all nerve cells, and mathematical models of neurons are constructed based on these general features.

Each of the nerve cells has a nucleus and presumably a DNA. They do not normally divide in adult life, but they do die; and an old person may perhaps have only a third of the number of neurons at the time of birth.

Almost all outputs of the brain through neuronal transmission culminate in muscular activity. Thus, motoneurons — the neurons that signal the muscle fibers to contract — are deployed most frequently in the neuronal activities.

A sketch of a motoneuron is shown in Figure 2.2. It consists of three parts: The center is known as the cell-body or otherwise known as the soma (about 70 μm across in dimension). The cell body manufactures complex molecules to sustain the neuron and regulates many other activities within the cell such as the management of the energy and metabolism. This is the central-processing element of the neural complex.

Referring to Figure 2.2, the hair-like branched processes at the top of the cell emanating from them are called dendrites (about 1 mm or longer). Most input signals from other neurons enter the cell by way of these dendrites; and that leading from the body of the neuron is called the axon which eventually arborizes into strands and substrands as nerve fibers. There is usually only one axon per cell, and it may be very short or very long. For nerve cells (other than motoneurons where most branches go to muscle fibers), the axons terminate on other nerve cells. That is, the output signal goes down the axon to its terminal branches traveling approximately 1-100 meters/sec. The axon is the output element and it leads to a neighboring neuron. It may or may not be myelinated, that is, covered with a sheath of myelin. An axon in simple terms is a cylindrical semipermeable membrane containing axoplasm and surrounded by extracellular fluid.


Figure 2.2  The biological neuron
1. Nucleus; 2. Nucleolus; 3. Soma; 4. Nissl body; 5. Ribosome; 6. Cell membrane; 7. Synaptic region; 8. Incoming axon; 9. Outgoing axon; 10. Axon hill; 11. Dendrite; 12. Axon sheath

The connection of a neuron’s axonic nerve fiber to the soma or dendrite of another neuron is called a synapse. That is, the axon splits into a fine arborization, each branch of which finally terminates in a little end-bulb almost touching the dendrites of a neuron. Such a place of near-contact is a synapse. The synapse is a highly specialized surface that forms the common boundary between the presynaptic and the postsynaptic membrane. It covers as little as 30 nanometers, and it is this distance that a neurotransmitter must cross in the standard synaptic interaction. There are usually between 1,000 to 10,000 synapses on each neuron.

As discussed in the next section, the axon is the neuron’s output channel and conveys the action potential of the neural cell (along nerve fibers) to synaptic connections with other neurons. The dendrites have synaptic connections on them which receive signals from other neurons. That is, the dendrites act as a neuron’s input receptors for signals coming from other neurons and channel the postsynaptic or input potentials to the neuron’s soma, which acts as an accumulator/amplifier.

The agglomeration of neurons in the human nervous system, especially in the brain, is a complex entity with a diverse nature of constituent units and mutual interconnections. The neurons exist in different types distinguished by size and degree of arborization, length of axons, and other physioanatomical details — except for the fact that the functional attributes, or principle of operation, of all neurons remain the same. The cerebellar cortex, for example, has different types of neuron multiplexed through interconnections constituting a layered cortical structure. The cooperative functioning of these neurons is essentially responsible for the complex cognitive neural tasks.

The neural interconnections either diverge or converge. That is, neurons of the cerebral cortex receive a converging input from an average of 1000 synapses and are delivered through the branching outlets to hundreds of other neurons. There are specific cells known as Purkinje cells in the cerebellar cortex which receive in excess of 75,000 synaptic inputs; and there also exists a single granule cell that connects to 50 or more Purkinje cells.


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