Areflex arc is aneural pathway that controls areflex. Invertebrates, mostsensory neuronssynapse in thespinal cord and the signal then travels through it into thebrain. This allows for faster reflex actions to occur by activating spinalmotor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.
There are two types:autonomic reflex arc (affecting inner organs) andsomatic reflex arc (affecting muscles). Autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord.[1]
During a somatic reflex, nerve signals travel along the following pathway:[1]
A reflex arc, then, is the pathway followed by nerves which (a.) carry sensory information from the receptor to the spinal cord, and then (b.) carry the response generated by the spinal cord to effector organs during a reflex action.The pathway taken by the nerve impulse to accomplish a reflex action is called the reflex arc.
When a reflex arc in an animal consists of only onesensory neuron and onemotor neuron, it is defined asmonosynaptic, referring to the presence of a singlechemical synapse. In the case of peripheral muscle reflexes (patellar reflex,achilles reflex), brief stimulation to themuscle spindle results in contraction of the agonist or effector muscle.By contrast, inpolysynaptic reflex pathways, one or more interneurons connectafferent (sensory) andefferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the brain.[clarification needed]
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When thepatellar tendon is tapped just below the knee, the tap initiates anaction potential in a specialized structure known as amuscle spindle located within thequadriceps. This action potential travels to the L3 and L4 nerve roots of the spinal cord,[2]via a sensoryaxon which chemically communicates by releasingglutamate onto amotor nerve. The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee (i.e. the lower leg kicks forward). Ultimately, an improper patellar reflex may indicate an injury of thecentral nervous system.[2]
The sensory input from the quadriceps also activates localinterneurons that release the inhibitoryneurotransmitterglycine onto motor neurons ofantagonist muscles, blocking their stimulation (in this case thehamstring muscles). The relaxation of the opposing muscle facilitates (by not opposing) the extension of the lower leg.
In invertebrates reflex interneurons do not necessarily reside in the spinal cord, for example as in thelateral giant neuron ofcrayfish.