Neurons, or nerve cells, can be categorized both by their function and by their anatomical features. In terms of function there are sensory neurons, motor neurons, local interneurons (found in dense nervous tissues such as the brain and retina), projection interneurons (carrying information to and from the CNS), and neuroendocrine cells (which secrete hormones and signaling substances directly into the microcirculation of the CNS, or into the general circulation upon neural stimulation). Exocrine glands are stimulated to secrete by the synaptic action of acetylcholine (ACh) secreted by postganglionic autonomic neurons. The adrenal medullae which secrete epinephrine and norepinephrine into the blood are innervated by sympathetic preganglionic neurons. All neurons are found in association with various types of glial cells.
In terms of anatomical features, all neurons have a cell body (also called a soma or perykaryon) with a nucleus. The output element of a neuron is its axon. The axon generally carries information in the form of propagated action potentials (spikes) from an originating site (the spike generator locus) to the output end (motor end plates in the case of a motoneuron, synaptic boutons for just about every interneuron with chemical synapses). To complicate things, there are neurons that do not generate spikes; they conduct signals electrotonically on their axons. Such nonspiking neurons can be found in the vertebrate retina or the optic lobes of arthropod compound eyes.
Depending on the number of axonal processes that originate on the soma, neurons are called unipolar, pseudo-unipolar, bipolar, or multipolar (Kandel et al., 1991). See Figure 1.1-1 for a general description of neuron anatomical features.
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