An intrinsic reflex is characterized by the fact that the stimulus site and the responding organ are identical. Most intrinsic reflexes are muscle stretch reflexes that protect us, in which brief muscle stretch-whether caused by a reflex hammer or a sudden buckling of the knee joint, for example-leads to contraction and thus twitching of the affected muscle. Thus, when the knee suddenly buckles, the patellar tendon reflex (PSR) prevents us from falling down, as the thigh muscle reflexively causes the lower leg to shoot forward, bringing us back into balance.
The switch occurs in the spinal cord
Muscle reflexes can be used to check the function of many spinal cord nerves, as nerve excitation travels from the site of stimulation to the spinal cord and is returned directly. Since there is only one switch in the spinal cord, these reflexes are called monosynaptic, and are usually named after the muscle or tendon being tested: In addition to the PSR, important intrinsic muscle reflexes are the Achilles tendon and adductor reflexes on the leg, the biceps and triceps reflexes on the arm, and the masseter reflex on the jaw.
In addition, many others exist, and a detailed neurological examination will reveal whether there is damage to individual nerve pathways (individual reflexes absent) or whether there is general nerve dysfunction (no or increased reflexes present).
Reaction in a roundabout way by extraneous reflex
In the more complex polysynaptic foreign reflex, a nervous switch occurs in the spinal cord after the stimulus and nerve excitation is switched to other organs, which then respond to the stimulus.
In clinical practice, especially when brain damage is suspected, the pupillary reflex is checked, which leads to constriction of the pupil (pupillary fibers react) when light enters the eye (optic nerve is irritated). Because the pupillary response is coupled in the brain, both pupils normally constrict even if only one eye is exposed to light. In massive brain damage and brain death, this reflex is no longer present.
