A reflex is an automatic response to a stimulus. For example, the lower leg jerks when the tendon below the kneecap is gently tapped with a small rubber hammer. The pathway that a reflex follows (reflex arc) does not directly involve the brain. The pathway consists of the sensory nerve to the spinal cord, the nerve connections in the spinal cord, and the motor nerves back to the muscle. Doctors test reflexes to determine whether all parts of this pathway are functioning. (This explanation is taken from the Merk manual. I hope it is O.K. to give credit and name the book.)
There are some things besides just the reflex arc pathway itself that determine how well a reflex works. One that I particularly know about is the thyroid hormone concentration, ie. hypothyroidism causes the reflexes to be slow.
When I went to the neurosurgeon for an evaluation he checked my reflexes and I didn't have any reflex in my left elbow any time he checked it. He'd go from my left to my right and then back again. It ended up being a pinched nerve at C5-C6 area. I had ACDF surgery seven weeks ago and as soon as I woke from surgery my pain, tingling, numbness, and muscle spasms were all gone (aside from my throat.) When I went for my four week check-up, the reflex at the elbow was back!
Your question uses the word reflexes
. Would you be willing to be more specific? There are several who read/write on this board who can enlighten you even more.