Ever wonder what your doctor is looking for when she pokes and prods during a physical exam, then blinds you with that little flashlight? Most of this abuse is intended to test neural reflexes that yield clues about the state of your nervous system—a good way to diagnose all sorts of health problems because nerve cells and fibers permeate almost every nook and cranny of your body.

Nerve cells are exquisitely sensitive to disturbances from tumors, trauma, circulatory problems, metabolic disorders, and a host of other diseases whose locations can be pinpointed by determining which reflexes show abnormalities.

Here are some simple tests you can try at home:

1. Sit with your right leg crossed over your left and briskly tap the large tendon beneath your kneecap with the handle of a kitchen knife. This tap will pull on your thigh muscle, elongating stretch receptors embedded there, which in turn will excite a motor neuron in your spinal cord that will stimulate your thigh muscle to quickly contract, extending your leg below the knee in the famous knee-jerk reflex. Its magnitude tells the doctor a lot about the state of your peripheral and central nervous systems. Sluggish or absent responses suggest a problem in the spinal cord, peripheral nerves, or muscles, while exaggerated responses indicate problems above the spinal cord.

2. In a dimly lit room, stand in front of a mirror with a flashlight held away from your face. Look at your right eye while you shine the light directly into your left pupil. Your right pupil should constrict in a consensual reflex. The impulse starts with excitation of the left retina, then travels down the optic nerve to cells in the midbrain and brain stem, which excite neurons near both eyes that cause the pupils to constrict. If only your left pupil had constricted, your doctor would know that the left retina, optic nerve, and brain stem were intact but that there was a problem somewhere between your midbrain and brain stem and right eye.

3. Firmly press the end of a tablespoon into the bare sole of your foot near the heel, then quickly run the spoon handle toward your toes. Your toes should flex inward. In contrast, the toes of infants, whose neural circuits have not fully matured, and those of certain types of neurology patients, will typically extend out in a response called the Babinski reflex.