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What Is a Spinal Reflex?

A knee jerk is a spinal reflex.
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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 21 March 2014
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A spinal reflex is a reflexive action mediated by cells in the spinal cord, bypassing the brain altogether. A classic example is the kneejerk or patellar reflex, where the leg jerks when the kneecap is struck with a brisk tap. These reflexes have ancient origins and have evolved to allow the body to respond quickly to threats and hazards without the time delay involved when the brain is consulted about how to respond to a stimulus.

In a spinal reflex, a sensation is felt at the site and relayed to neurons in the spinal cord via a sensory pathway. The spinal cord returns a signal along a motor pathway, signaling a movement in response to the sensation. This happens in fractions of a second, allowing people to jerk away before the brain is even aware of a problem. A signal is also sent to the brain, alerting it to the sensation and response. This happens so quickly that people may experience the sensation of pain almost simultaneously with the development of the reflex, even though the brain is actually the last to know in this situation.

The nerve pathways carry signals like heat from burning, sharp sensations, and other painful feelings. These reflexes are highly primitive and simple, and probably evolved in humans a very long time ago. Many other animals demonstrate similar reflexes in response to pain, illustrating the way the body's instinct for self-preservation is consistent across species large and small.

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It's possible to test a spinal reflex in a clinical setting. This may be done if a medical professional suspects that a patient has a neurological problem like damage to the spine or a nerve pathway. The doctor will use a tool to provide a stimulus and take note of how the patient responds. Failure to respond with a reflex movement indicates that there is a problem with how the signals responsible for the reflex are sent.

In a regular neurological exam, testing of reflexes is a common step to collect information about the patient's nervous system. General physical exams may also include a quick check with the goal of seeing whether the patient needs a neurological consult and further evaluation. Such tests are especially common in pediatrics, where checkups include a brief neurological assessment to confirm that a baby is reaching developmental milestones in a timely fashion.

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pastanaga
Post 2

@umbra21 - I don't know about fish but we share some reflexes with frogs. Like the scratch reflex. There have been a lot of studies done on frogs to investigate that one (which is just as it sounds, irritating the skin makes you want to scratch it).

I think that one is a spinal reflex too.

There's another interesting one, the diving reflex which can be found in mammals and birds. It makes you hold your breath when cold water splashes your face. I guess that one helps you not to drown if you fall in a river or something.

umbra21
Post 1

It's so interesting that we share this kind of reflex with different kinds of animals. I wonder how far back the evolutionary chain we share them. With all animals that have developed a spine?

I suppose that would include fish, so it would be difficult to tell, but I think scientists are sometimes able to work out which parts we all have in common from looking at embryos, which are surprisingly similar across species.

So, you can look at the cells in different embryos and see where they end up in a fish and where they end up in a person and figure out if one reflex, like jerking away from heat, is made by the same cells.

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