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Principles of Manual Medicine

Monosynaptic Reflex Arc

Spinal reflexes regulate activities of the body. The simplest example of a spinal reflex is the monosynapic reflex arc, having four components:
  1. A receptor (in this case, the muscle spindle).
  2. An afferent component (sensory input).
  3. A central component (spinal processing).
  4. An efferent component (motor output).
Consider the patellar reflex. You should relate the numbered items below with the number appearing in the lower right corner of the image):
  1. Afferent activity, originating from muscle spindles in the intrafusal fibers (which are in parallel with the extrafusal fibers) in response to a brief, sudden stretch of the muscle, is transmitted along sensory neurons to the spinal cord.
  2. Sensory neurons synapse directly on motorneurons. Efferent activity is transmitted along these motoneurons back to the extrafusal fibers of the muscle that was stretched.
  3. The muscle contracts, resisting the force that initially caused it to be stretched.

While the monosynaptic reflex demonstrates the basic function of a spinal reflex, it should be understood that general spinal reflexes have the central component receiving sensory (afferent) input from:
  • The brain.
  • The viscera via sympathetic or parasympathetic pathways.
  • Somatic afferents.
Sensory inputs can be locally processed at the level of the cord, or they can be passed along to the brain for further processing. The central component can also send efferent activity back to the visceral and somatic structures and may include one or more interneurons.