Monosynaptic and polysynaptic reflexes

Motor responses to discrete stimuli that require only two or more neurons to complete the circuit or arc.  When the arc consists of one sensory and motor neuron, it is referred to as a monosynaptic reflex (with ‘monosynaptic’ referring to one synapse).  Monosynaptic reflexes include, for example, the biceps and triceps reflexes, and patellar stretch reflex or knee jerk (see figure below).  In contrast, polysynaptic reflexes are mediated through at least two synapses and one interneuron.  One example is the automatic withdrawal of the leg when the foot steps on a sharp object.  Most reflexes are polysynaptic, and unlike their monosynaptic counterparts they can be inhibited by conscious control. 

Patellar reflex: a sensory neuron in the quadriceps (or thigh muscle) detects the stimulation created by a hammer tap to the knee cap. This neuron transmits an impulse to the synapse of a motor neuron in the gray matter of the spinal cord.  An impulse from here is conducted along a motor axon to the effector or agonist muscle (viz., the quadriceps) resulting in an extension movement of the lower leg. An inhibitory interneuron, not part of the arc, serves to relax the antagonist muscle (viz., hamstrings), thus preventing co-contraction between it and the agonist muscle.            

See Agonist muscle, Antagonist muscle, Golgi tendon neuron, Gray matter, Interneurons, Muscle spindle, Neuron, Reflex, Reflex arc