A huge number of neuronal cells form the nervous system that regulates all aspects of body functions. The human brain contains about 1012 neurons (nerve cells) with each neuron forming thousands of connections to other neurons so that a large network of electrical connections with massively parallel information processing characteristics results. The nervous system also contains glial cells that occupy the spaces between neurons and modulate their function. They surround the soma and axons of the neurons and are metabolically coupled to the neurons. The output of a nervous system is the result of its inputs and its circuit properties, that is, of the wiring or interconnections (synapses) between the single neurons, and of the strength of these interconnections. The synaptic connections between neurons can be reorganized, which is known as synaptic plasticity, and is believed to be the mechanism of learning in our brain. Basically, three different types of neurons can be distinguished by their physiology and function in the body:
• interneurons, neurons in contact with other neurons (most of the inter-neurons are located in the brain)
• motor neurons, neurons, which control the actions of the organism mainly via contact with muscle cells
• sensor neurons, neurons, which receive stimuli from the external environment, such as the retina.
The neuron exhibits four distinct regions with different functions: the cell body, the dendrites, the axon and the axon terminals (Fig. 8.2). The cell body with a diameter of 10 to 100 pm contains the nucleus and is the production site of most neuronal proteins. Almost every neuron has a single axon, whose diameter varies from a micrometer in the human brain to a millimeter in the giant squid. Axons are specialized for the conduction of electrical pulses, termed action potentials, away from the cell body towards the axon terminus.
Neurons communicate with one another through specialized contact zones, which are called synapses. Synapses can be either electrical or chemical. Synapses have two important functions in the transmission of impulses from one cell to the other. The first is signal amplification, which is common at nerve-muscle synapses. A single motor neuron can cause a contraction of multiple muscle cells because the release of relatively few signaling molecules is required at the synapse to stimulate contractions. The second advantage is signal computation, which is common at synapses involving interneurons. A single neuron can be affected simultaneously by signals received at multiple excitatory, inhibitory and also electrical synapses. The
neuron averages these signals continuously and determines whether or not to generate an action potential. The connection between two neurons is therefore weighted by the property of the synapse, which connects the two neurons. This is the basis of the ability of neural networks to learn and to perform computation.
Most neurons have multiple dendrites, which extend outward from the cell body and are specialized to receive chemical or electrical signals from the axon terminals of other neurons via the synapses. Dendrites convert these signals into small electric impulses and transmit them towards the cell body. Particularly in the central nervous system, neurons have extremely long dendrites exhibiting complex branching.
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