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.
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.
Textbook Reference: Concept 34.5 Neurons Are Organized into Nervous Systems