Explain spinal reflex
Answers
- Spinal reflexes are investigator-evoked artifacts arising from connections of stretch receptors in the muscle or nociceptors in the skin that activate a spinal motor neurons to evoke contractions/twitches in particular somatic muscles (e.g., the quadriceps muscle in a patellar tendon reflex).
The Spinal Cord Possesses Other Reflexes and Includes Locomotor Pattern Generators
The spinal reflexes discussed above are only part of the story. There are a variety of other reflexes such as the placing reflex, which helps maintain posture and support. The placing reflex refers to the reaction to tactile stimuli applied to the back of the paw of lower animals. The reaction is to flex the limb and then swing it forward and extend it. For example, a twig touching the dorsum of the foot during its swing phase results in the foot being lifted over the twig. In humans, this is the reaction upon stubbing one’s toe: the affected limb is flexed and then rapidly swung forward and extended to catch the falling body. This is a far more complicated reflex than what we have already discussed. It involves a complex sequence of motor neuron activation and inhibition.
The spinal cord also possesses central pattern generators that are the basis of gait. Lower animals, because of their quadripedal locomotion, show distinct differences in gaits from walking, trotting, pacing, and galloping. Humans also show differences in gait but, because of our bipedal locomotion, the differences center on speed of movement rather than differences in the sequence of flexion and extension of four legs. In the gallop, for example, the two hind legs are simultaneously or nearly simultaneously extended to push the animal forward. Out of phase with the extension of the hind legs is the extension and then flexion of the forelegs to pull the animal forward. In bipedal motion this would become a hop, which is extraordinarily inefficient locomotion. Humans pay for their bipedal locomotion by being one of the slowest animals around. The advantage is that it frees the hands to throw a sharp rock or a spear at much higher velocities than locomotion.
Experiments with lower animals show that part of locomotion control resides in pattern generators located in the spinal cord. When the thoracic spinal cord of a cat is cut, severing all connections to the lower motor neurons from higher centers in the brain, and the cat is suspended in a sling above a treadmill, the animal can still raise and place the hind legs, and this motion will keep pace as the treadmill is accelerated. Thus, the basic circuitry for movement of the hind legs resides in the spinal cord.