why do pediatricians check for reflexes with those hammer knee things? What is it actually for?
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Testing a reflex involves striking a tendon and observing a sudden contraction of the muscle attached to the tendon. It is a component of a medical physical examination intended to test the nervous system.
A “reflex arc” involved in this test includes the tendon, a sensory nerve innervating that tendon, a connection in the spinal cord (a synapse), a motor neuron that provides signal to the muscle attached to the tendon, and the muscle itself. As mentioned elsewhere in this thread, a reflex arc is capable of moving a muscle without input from the brain. However the behavior of the reflex is dependent on the health of the entire nervous system, and indeed the entire body.
If there is impairment of any of the local components of the arc mentioned (e.g. a problem with the motor nerve such as a traumatic injury), then the contraction of the muscle will be diminished. Other things that can diminish reflexes include damage to muscle, damage to sensory nerves, or damage to the synapse, such as with compression of the spinal cord. Many other culprits are possible.
However, if there is a problem in the nervous system “upstream” of the reflex, such as in the brain or spine above the level of the reflex arc, the muscle contraction will actually be hyper responsive. This is because there are normally constant signals from the brain that suppress reflex arcs. So if you have damage to the thoracic spinal cord, the normal inhibitory signal gets interrupted and the patellar (knee) reflex test yields an exaggerated contraction of the quadriceps muscle.
In theory every muscle in the body will exhibit a reflex arc if its tendon can be struck the right way. In practice, certain tendons (Achilles, patellar, triceps, biceps) are anatomically more convenient than others to strike with a hammer. Each of these reflex arcs has been mapped anatomically to a segment of the spinal cord. If you test all the reflexes, and pay attention to which are normal, which are hyporeflexive, and which are hyperreflexive, you can try to identify how far up the spinal cord an injury must be. This must be done in concert with the rest of the physical examination which includes testing sensitivity of skin to touch, as well as strength of specific muscle groups. Performing and interpreting a comprehensive, informative neurological exam takes years to master.
The next step, once the neurological “level” of injury has been identified (the “where”), the next step is to figure out the pathological cause of the injury (the “why”). This is yet another level of complex reasoning that takes years to master, and often requires tests that including bloodwork and imaging (CAT scans, MRIs, etc) or even more invasive testing such as electromyography. Finally, once you’ve got the “where” and the “why,” you come up with a plan to treat the issue.
To really understand exhaustively the complete answer to your question, one should take on a career in neurology. Obviously that’s beyond the scope of this thread. Hopefully my post at least clarified something for you.
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