In a spinal reflex, sensory (afferent) signals enter the spinal cord and are converted to motor (efferent) signals without any participation from the brain. The simplest type of spinal reflex involves two neurons communicating through one synapse, and is called a monosynaptic reflex. Most spinal circuits are more complex and involve multiple synapses. For example, the withdrawal of a limb in response to a painful stimulus is controlled by several sets of neurons that control antagonistic sets of muscles.
The accompanying animation depicts a spinal reflex that involves multiple sets of neurons and antagonistic sets of muscles.
The cell bodies of sensory neurons are located in the dorsal root ganglia. From the ganglia, sensory axons enter the spinal cord through the dorsal roots, where they synapse upon interneurons. The interneurons, in turn, project to the motor neurons.
Motor neurons are located in the ventral horn of the spinal cord and send their axons out the ventral root to innervate muscles. Some motor neurons innervate flexor muscles while others innervate the extensor muscles.
Whether a particular motor neuron is excited or inhibited depends on the interneuron. In this example, an interneuron sends an inhibitory signal to the motor neuron controlling the extensor muscles. Another interneuron sends an excititory signal to the motor neuron controlling the flexor muscles. This permits the dog to withdraw the limb from the painful stimulus.
With withdrawal of one limb, the weight will shift to the limb on the other side of the body. Interneurons also connect to motor neurons on the opposite side of the spinal cord so that the muscles in the other limb can adjust to the shift in weight distribution. In this case, note that it is the extensor muscle that contracts and the flexor muscle that relaxes.
Sensory information is also relayed up the spinal cord to the brain, where it can be further processed. Ultimately, signals from the brain travel back down the spinal cord along motor pathways and can thus modify the actions of the spinal circuits.
The information from higher nervous centers permits fine-tuning of muscular control, and also allows the dog to avoid painful stimuli in future encounters.
As you have just seen, a great deal of information processing takes place in the spinal cord without any input from the brain. This function of the spinal cord has important implications for research and the development of treatments for spinal cord injury and repair. Patients with spinal cord injuries, where information from the brain can no longer reach target motor neurons, may benefit from an experimental exercise program. This program hopes to exploit the reflex feature of the spinal cord by retraining the spinal cord to use its internal circuitry to coordinate limb movement without the need for input from the brain.