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Interneurons are located exclusively in the central nervous system, meaning that they are located in the brain or spinal cord, and consist of all nerve cells that are not specifically sensory or motor neurons. Local interneurons have short axons and form circuits with nearby neurons and analyze small pieces of information. Relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions. There are an estimated 100 billion interneurons, making them the most abundant type of neuron in the CNS.

Generally, sensory (afferent) neurons have cell bodies that lie in the peripheral dorsal root ganglion and axons that terminate in sensory endings in skeletal muscles. These afferent neurons detect a stimulus in the external or internal environment. Motor (efferent) neurons stimulate the contraction of a muscle or the secretion of a gland. One group of motor neurons that has origins in the ventral horn of the spinal cord projects to the flexor muscles on a muscle, while the other connects to the extensor muscles. Relay interneurons are responsible for serving as the connection point for motor and sensory neurons, and have the ability to modulate the input-output linkage.

The knee jerk reaction is a special type of reflex called a myotatic spinal reflex. When a sensory neuron detects painful stimuli such as a hot surface or pressure, a message is sent down the afferent axon to that neurons terminal buttons, which are located in the spinal cord. The terminal buttons of the sensory neuron release an excitatory neurotransmitter which stimulates the interneuron, causing the interneuron to fire. The terminal buttons of the interneuron then release a neurotransmitter that excites the motor neuron, which then sends messages down it’s axon. The motor neuron axon joins a nerve and travels to the muscle and releases a neurotransmitter, causing the muscle to contract or pull away from the painful stimuli.