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A reflex arc is a neural pathway that controls a reflex action. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex is carried out. Analysis of the signal takes place after action has been taken.
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There are two types of reflex arc: autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). However, autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord itself.
During a somatic reflex, nerve signals travel along the following pathway:
- Somatic receptors in the skin, muscles, and tendons
- Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem
- An integrating center is the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse
- Efferent nerve fibers carry motor nerve signals to muscles
- Effector muscle innervated by the efferent nerve fiber that carries out the response.
It is the pathway followed by sensory nerve in carrying the sensation from receptor organ to spinal cord and then the pathway followed by motor nerve in carrying the order from spinal cord to effector organ during a reflex action.
Monosynaptic vs. polysynaptic
When a reflex arc consists of only two neurons in an animal (one sensory neuron, and one motor neuron), it is defined as monosynaptic. Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in contraction of the agonist or effector muscle. By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the brain.
The patellar reflex (knee jerk)
When the patellar tendon is tapped just below the knee, the patellar reflex is initiated and the lower leg kicks forward (via contraction of the quadriceps). The tap initiates an action potential in a specialized structure known as a muscle spindle located within the quadriceps. This action potential travels to the L3 and L4 nerve roots of the spinal cord, via a sensory axon which chemically communicates by releasing glutamate (see synapse) onto a motor nerve. The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee. Ultimately, an improper patellar reflex may indicate cerebellar injury.
The sensory input from the quadriceps also activates local interneurons that release the inhibitory neurotransmitter glycine onto motor neurons of antagonist muscles, blocking the innervation of these antagonistic (hamstring) muscles. The relaxation of the opposing muscle facilitates (by not opposing) the extension of the lower leg.
In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.