Neural Network Operations

Abstract

Neuronal networks in the brain produce very complex, multi-component pathways, and only the simplest of these are understood in any detail. Two examples of well-studied networks are the monosynaptic spinal stretch reflex and the tri-synaptic circuit of the hippocampus. These two circuits are sketched in figure 16.1. It is very misleading to define a neuronal network by simply counting synapses. The circuit of a spinal stretch reflex (figure 16.1A) includes afferent fibers from muscle spindle proprioceptors and motor neurons that innervate the muscle fibers of the same muscle. The basic reflex is modulated by inputs from spindle proprioceptors of antagonistic muscles and from tendon proprioceptors of the same muscle. To add to the complexity of the simple circuit shown in this figure, a muscle such as the quadriceps may contain several hundred spindle proprioceptors, each of which makes contact with 100 to 150 motor neurons. The afferent axons from each proprioceptor branch and make only some of the 10,000 individual synapses found on a typical motor neuron. Each of these synapses is capable of producing only a small response in the motor neuron, and 50 to 100 afferent fibers must fire reasonably simultaneously in order to produce an action potential in a motor neuron.