Depending upon the number of synapses in the reflex arc, the reflex may be monosynaptic, bisynaptic or polysynaptic. The α-motoneurons are involved in three types of reflexes.
Of these, one is monosynaptic, the other is bisynaptic and the third is polysynaptic.
(1) The stretch reflex. This is a monosynaptic reflex, which is triggered by stretching of the muscle. Sensory receptors in the muscle - the muscle spindeles - detect changes in muscle length and reflexly bring about α -motoneuron discharge.
(2) The inverse stretch reflex. This is a protective reflex, which is triggered by excessive rise in muscle tension. The tension-detecting receptors are located in the tendons and are called Golgi tendon organ. Stimulating of these receptors brings about a reflex inhibition of the α -motoneurons. The inhibition is possible by the interposition of an inhibitory Golgi bottle neuron in the reflex arc, which is bisynaptic.
(3) The withdrawal (flexor) reflex. This also is a protective reflex, which is triggered by painful stimuli. It results in flexion of the affected limb. The reflex arc is polysynaptic. Withdrawal reflex is associated with a crossed-extensor reflex, the physiological significance of which is discussed in the context of regulation of posture (see below).
Clinicians classify reflexes as superficial and deep. Superficial reflexes are actually polysynaptic withdrawal reflexes while the deep reflexes are monosynaptic stretch reflexes.