The Meaning Of A Spinal Reflex
To really understand the meaning of a spinal reflex, one should first study an overview of the entire nervous system, which contains among other things, the brain and the spinal cord. Such an overview, even in summary form, makes one feel like they're reviewing the content of a post graduate class in computing, where the latest technical innovations are being discussed.
What the brain and nervous system does is truly amazing. The spinal reflex is only a small part of what goes on, but a very important part. If it were not for the spinal reflex we would probably suffer a great deal more pain than we normally do, and would not escape from dangerous situations nearly as often as we normally do.
One can think of the nervous system as a data processing system, getting information and requests from all parts of the body, processing that information, and sending out appropriate return messages or responses to muscles, glands, internal organs or wherever appropriate.
These messages are quite similar to data packets in a computer in that electricity is involved, and messages are transmitted throughout the nervous system by electrical impulses or synapses. Even in the fastest computers it takes a finite amount of time for an electrical signal to travel from one place to the next. The brain, as fast as it is, is no different. Furthermore, some actions require thought on the brains part, while others are done without conscious thought.
Reflex Action - There are some activities which occur very quickly, such as removing a finger that has touched a hot surface. We don't speculate on how warm or hot the surface is. In fact we don't think about it at all, until after the fact. The nervous system simply jerks the finger away the moment excessive heat is experienced. This is called a reflex action and is something we don't mull over in our brain, but something the nervous system takes control of on its own as a protective function.
Spinal Reflex Action - The fact is, the brain is not in control of everything that goes on within the nervous system. Some actions and responses are controlled by the spinal cord, which is an extension of the brain stem. The spinal cord has grey matter just as the brain does, but does not control conscious thought as the brain does. Thus, when we touch a hot surface with a finger, our biceps contract and the antagonist muscles, the triceps, simultaneously relax, both in an instant, and our hand jerks away. When the doctor taps us under the knee cap, our quadriceps muscle contracts immediately, while the hamstring, the antagonist muscle, simultaneously relaxes, the result being the lower leg kicking forward, the familiar knee jerk reaction. These are both example of the spinal reflex, where the brain is essentially uninvolved, and everything that happens is being controlled by the spinal cord.
The Brain Arrives Late - While the brain is uninvolved in the reflex reaction, it still receives the signal from the nerves that precipitated the reaction or reflex. The spinal cord however took immediate action (the reflex) without waiting for the brain to analyze the situation. If we touch a hot stove the brain will tell us that we've touched a hot stove, but long after (in relative terms) the spinal cord has initiated the reflex action or jerk. Medical dictionaries define a spinal reflex as a reflex action that is mediated from the center of the spinal cord.
The next time your doctor taps you under the knee with his little hammer, it's your spinal cord that's telling your leg to jerk, and it’s your brain telling you that your leg has jerked.