Is it possible for an individual to override certain autonomic functions within the human body?

The body is equipped with several mechanisms to prevent override. For instance if you hold your breath long enough you will eventually pass out and begin breathing. Can a person at will override the knee tap reaction or slows his/her heart down to a stop at will? Just wondering if it is possible?
Answer:
there is a lot of research into cognitive development models for re-training people after, say, a stroke. In these studies / exercises the therapist help stroke victim re-train parts of their brains to take over the roles of damaged parts. This kind of research ties in with the work of Bio-Rhythm researchers over the last 40+ years in training individuals to actually control their pulse rate and raise / lower their blood pressure. You should look into the research done by many western universities on YOGI's who can also control body temperature and blood flow to specific parts of their bodies. The overwhelming answer seems to be that it takes training and practice but yes.

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Some Indian gurus can reportedly stop their heart for a few beats.

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There have been plenty of stories shown on TV that show Yogis in India doing such unbelievable feats. You might be able to search these happening on the internet.

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Slowing down the heart rate is theoretically possible (and there are anecdotal reports of it), because the heart rate is regulated by the sympathetic and the parasympathetic nervous sytems, both of which have a lot to do with higher brain functions, such as emotional state, stress etc.On the other hand, the knee jerk reflex is a spinal reflex, it exist even in the bodies of brain dead persons. It is not possible to supress that by any way other than damaging the spinal cord or the peripheral nerves.

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