This section has provided an overview of the many types of neurons used by an animal to survive in its environment. Sensory neurons respond to certain physical parameters its environment, both internal and external. In most cases, transduction of a physical quantity leads to the generation of nerve impulses, the frequency of which is a nonlinear, increasing function of the stimulus intensity. Receptors also code for rate of change of the stimulus; the output frequency thus can be thought of as having proportional-plus-derivative components.
Motorneurons make muscles contract; the higher their frequency, the higher the tension developed or the quicker the contraction. To a crude approximation, the conversion of motoneuron spike frequency to muscle tension can be modeled by treating the muscle as an electrical low-pass filter, where output voltage is proportional to tension. Autonomic, effector neurons that innervate glands behave in much the same way as motoneurons, where the low-pass filter output voltage models the rate of release of a hormone.
The most plentiful type of neuron is clearly the interneuron. Interneurons serve as control and computational elements for output (effector and motor) neurons in the spinal cord. In the CNS they have too many roles to list here. The cerebellum, which regulates the fine details of the motoneurons outputs, was seen to have more interneurons than the cerebral cortex. Interneurons are either excitatory or inhibitory, but their function lies in the details of their anatomies, synaptic inputs and outputs, and properties of their membranes.
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