Stretch reflex  The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle. A spinal reflex is a fast response that involves an afferent signal into the spinal cord and an efferent signal out to the muscle. The stretch reflex can be a monosynaptic reflex which provides automatic regulation of skeletal muscle length, whereby the signal entering the spinal cord arises from a change in muscle length or velocity. When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length. Gamma motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle. There are several theories as to what may trigger gamma motoneurons to increase the reflex's sensitivity. For example, "alpha-gamma co-activation" might keep the spindles taut when a muscle is contracted, preserving stretch reflex sensitivity even as the muscle fibers become shorter. Otherwise the spindles would become slack and the reflex would cease to function. This reflex has the shortest latency of all spinal reflexes including the Golgi tendon reflex and reflexes mediated by pain and cutaneous receptors. The stretch reflex is accomplished through several different structures. In the muscle, there are muscle spindles, whose extrafusal muscle fibers lie parallel to the muscle and sense changes in length and velocity. The afferent sensory neuron is the structure that carries the signal from the muscle to the spinal cord. It carries this action potential to the dorsal root ganglion of the spinal cord. The efferent sensory neuron is the structure that carries the signal from the spinal cord back to the muscle. It carries the action potential from the ventral root of the spinal cord to the muscle down the alpha motor neuron. This synapsis on the first structure discussed, the extrafusal fibers of the muscle spindle. A person standing upright begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the opposite side will stretch. The muscle spindles in those muscles will detect this stretching, and the stretched muscles will contract to correct posture. Other examples (followed by involved spinal nerves) are responses to stretch created by a blow upon a muscle tendon:  Another example is the group of sensory fibers in the calf muscle, which synapse with motor neurons innervating muscle fibers in the same muscle.