Involuntary Nerve reactions

[spooky]

We're learning about CNS (central Nerve System) and the PNS (peripheral Nerve system). The other day when I was in class and we were discussing the involuntary reactions, such as the knee bending when a large enough hit goes to your knee. Does anyone know why we have these reactions? Or is it some strange left over from evolution like goosebumps.

[Glider]

Reflexes that persist to adulthood exist to prevent or limit injury. Other (infantile) reflexes like the grasping reflex seen in the feet of infants (e.g. the Babinski reflex) are left over from our evolutionary history (we no longer grasp with our feet) and dissapear within two years. If present after 2 years, it's considered abnormal.

 

The knee-jerk (and related reflexes) are deep tendon reflexes. Essentially, they protect the joints. They are a way for the body to detect sudden passive extension/flexion of a joint and to engage oposing motor muscles quickly to compensate and prevent injury due to over extension/flexion.

 

The speed is due to the reflex arc which a functional unit on its own and exists at the spinal level so signals don't have to travel to the brain to elicit a reaction. This saves time and also means that reflexes are entirely automatic and require no conscious input.

[spooky]

Thanks for your help Glider, so what part of the brain actually controls these involuntary reactions?

[Glider]

None. The reflex arc exists at the spinal level (althoough the spinal cord is still part of the CNS).

 

Basically, the reflex arc consists of an interneuron which connects an afferent (sensory) nerve fibre to an efferent (motor) nerve fibre. When an incoming afferent signal of sufficient intensity passes into the dorsal horn of the spinal cord, it triggers the interneuron, which passes the signal to the efferent neurons in the ventral horn of the spinal cord and down to the appropriate muscle.

 

Although the afferent signal will continue up the spinal cord to the brain (so we sense the contact), the motor impulse will have caused the muscle to contract by the time we feel it, so as far as the reflex goes, the afferent impulse continuing up to the brain is irrrelvant.

 

In short, the reflex arc is an autonomous, functional unit existing at the spinal level that requires no input from the brain, conscious or otherwise.

[immortal]

How does the brain sense whether the external environment is stable or not. I know it uses different systems such as proprioceptors, the circular canals of ears, eyes etc then how this complex system work together. It is interesting to study these designs created by evolution.

[Revenged]

Thanks for your help Glider, so what part of the brain actually controls these involuntary reactions?

 

The brain does control reflex activity... Whoever thought that it the brain doesn't have a control on reflexes is mistaken... There is a vitally important Golgi tendon reflex that was not mentioned... This neural pathway runs to the brain and inhibits reflexes to ensure that the tensions in tendons does not become extremely high so that the tendons would snap... With an upper motoneurone lesion, you lose this Golgi tendon reflex and it causes dramatically increased reflexes... Also, one small point... the sensory->interneurone->motoneurone story is not quite true as it only applies for the antagnoistic muscles in a reflex... The main muscle in the reflex only have a sensory->motoneurone pathway...

 

The reason that doctors test reflexes is to see whether a patient had a lower or upper motoneurone lesion... Basically, a lower motoneurone lesion would affect the peripheral nervous system and would be shown by a weakness or loss of reflex functions in a certain part (e.g. loss of the knee jerk reflex on the left)... Whereas an upper motoneurone lesion is shown by a central nervous system damage... E.g. if you had cut the entire of your spinal cord, your brain has no control over your reflexes... If it were true that the brain had no control over reflexes you would expect no difference in reflexes... but the fact that you get dramatically increased reflexes and reversed reflexes (e.g. Babinski sign is a reversed reflexes) shows that the brain is very much in control of reflexes...

[Glider]

No, the brain modulates simple reflex action. Input from the brain is not necessary for the action to occur. I accept that there is a range of reflex types but the OP asked about the patellar reflex, which along with other reflexes such as the escape reflex is a simple (i.e. spinal) reflex.

 

Spinal reflexes are those in which "the sensory stimuli arise from receptors in muscles, joints and skin, and in which the neural circuitry responsible for the motor response in entirely contained within the the spinal cord.(bold added)" (Kandel, Schwarts & Jessell, 1991. p565).

 

I accept that descending influences from higher CNS areas modulate the reflex response (the basis of diagnostic reflex testing) and can also use these simple spinal circuits to generate more complex behaviours, but that is not what was asked. The OP asked why these reflexes exist and I felt that the best response to a relatively simple question was a relatively simple answer.

 

 

 

Reference:

Kandel, E. R., Schwarts, J. H. and Jessell, T. M. (1991) Principles of Neural Science (3rd Ed.). Prentice Hall International: UK.

[Revenged]

The OP asked why these reflexes exist and I felt that the best response to a relatively simple question was a relatively simple answer.

 

ok, no worries... i thought you were suggesting that the brain has no influence on reflex responce that's all... but i agree with what you are saying anyway...

[Glider]

No, I wasn't suggesting that at all, just trying to keep things simple (habit from teaching). You were quite right to point out that higher CNS areas modulate reflex.