The more highly specialized receptor organs are complex and admirably designed to receive a particular type of stimulus. The neuromuscular receptors, or muscle spindles, have their own sensory and motor innervation comprising a monosynaptic reflex system known as the myotatic or stretch reflex. When muscle spindles are elongated due to passive stretching of the muscle, reflex contraction occurs. This appears to function not only to oppose the forces of gravity but also during reflex and voluntary contraction of muscles, both flexor and extensor.