October 11, 2026

The knee-jerk reflex is controlled by a simple circuit of sensory and motor neurons.

Tapping the kneecap with a reflex hammer pulls on the tendon of the quadriceps femoris, a muscle that extends the lower leg. When the muscle stretches in response to the pull of the tendon, information regarding this change in the muscle is conveyed to the central nervous system by sensory neurons. In the spinal cord, the sensory neurons form excitatory synapses with extensor motor neurons that contract the quadriceps, the muscle that was stretched. The sensory neurons act indirectly, through interneurons, to inhibit flexor motor neurons that would otherwise contract the opposing hamstring muscles. These actions combine to produce the reflex behavior. In the drawing, each extensor and flexor motor neuron represents a population of many cells.

Thus, the electrical signals that produce the stretch reflex carry four kinds of information:

1. Sensory information is conveyed to the central nervous system (the spinal cord) from muscle.
2. Motor commands from the central nervous system are issued to the muscles that carry out the knee jerk.
3. Inhibitory commands are issued to motor neurons that innervate opposing muscles.
4. Information about local neuronal activity related to the knee jerk is sent to higher centers of the central nervous system, permitting the brain to coordinate different behaviors simultaneously or in series.

Pattern of connection, in which one neuron activates many target cells, is called divergence. It is especially common in the input stages of the nervous system.

A single motor cell in the knee-jerk circuit receives 200 to 450 input contacts from approximately 130 sensory cells. This pattern of connection is called convergence. It is common at the output stages of the nervous system

Each sensory neuron input produces relatively weak excitation, so convergence also ensures that a motor neuron is activated only when a sufficient number of sensory neurons are activated together.

Diverging and converging neuronal connections are a key organizational feature of the brain.

A. In the sensory systems, each receptor neuron usually contacts several neurons that represent the second stage of processing. At subsequent processing stages, the incoming connections diverge even more. This allows sensory information from a single site to be distributed more widely in the spinal cord and brain. B. By contrast, motor neurons are the targets of progressively converging connections. With this arrangement, input from many presynaptic cells is required to activate the motor neuron.

Inhibitory interneurons can produce either feed-forward or feedback inhibition.

A. Feedforward inhibition enhances the effect of the active pathway by suppressing the activity of pathways mediating opposing actions. Feedforward inhibition is common in mono-synaptic reflex systems. For example, in the knee-jerk reflex circuit afferent neurons from extensor muscles excite not only the extensor motor neurons but also inhibitory interneurons that prevent the firing of the motor cells innervating the opposing flexor muscles. B. Feedback inhibition is a self-regulating mechanism. Here extensor motor neurons act on inhibitory interneurons that in turn act on the extensor motor neurons themselves and thus reduce their probability of firing. The effect is to dampen activity within the stimulated pathway and prevent it from exceeding a certain critical level.