Feeling for Life


How and why do I feel pain and other sensations? 

Wouldn't it be great to not have pain?  Then you could do whatever you want, right?  Well, not exactly!  You could pick up, or step on, something very hot or sharp and although you wouldn't feel pain, you could severely injure your hand or foot.  After all, your body is mostly made up of flesh with physical limitations that if exceeded can cause serious damage.  Pain and other sensations provide your body with information about what’s going on inside and outside of it so it can avoid injury and do what you want it to do.  This sensory information is provided by the nerve cells (neurons) in your skin and other places within your body and is an important part of your nervous system.      

Neurons are said to be excitable because when properly stimulated, like flipping a switch, they send out electrical signals.  It’s like when you trigger a motion detector that sends out a signal to open the doors as you approach a store.  So too, your skin has sensory receptors that detect light touch, pressure, stretch, vibration, and heat and cold.  It also has pain receptors that react to chemicals related to cell injury and extremes of pressure, stretch and temperature.  Your body also has pain and stretch sensitive neurons in and around its joints, ligaments and deep soft tissues in addition to other ones within your major organs that provide information about its internal environment.  Then there are the special sensory organs like the eye, triggered by light, to give us sight, the ear, by sound waves, to give us hearing, the nose and tongue, by chemicals, to give us smell and taste, and the vestibular apparatus, by motion, to give us our sense of balance.  When these sensory organs are triggered they send out electrical signals to connecting neurons which are bundled together into nerves.  From here the signals are passed on, like a relay race, to the central nervous system which consists of the spinal cord and the brain.  It’s here where all of the sensory information is integrated and decisions are made about what to do.

We usually have an awareness of what our sensory nerves tell us about what’s going on inside and outside of our body.  Most of us know what it’s like to feel light touch, pressure, stretch, vibration, hot and cold and pain. That also goes for being able to see, hear, smell and taste things and stay balanced.  And we’re usually aware of what’s happening in our joints, ligaments, deep soft tissues and sometimes even our major organs.   But there is one set of sensory neurons that works totally on the unconscious level and without them life would be impossible.  These are the stretch receptors within your muscles and tendons that tell your body where all of its parts are located in space and what they’re doing.  They send messages about the length of each muscle and the angle of each joint along with how much tension is being applied.  After all, how can your body control its actions if it doesn’t know what its muscles and bones are doing and where they’re located in space?

Finally, we must look at one other important aspect of nerve function, conduction velocity.  In other words, how long does it take for the information from these sensory devices to reach the central nervous system?  If the motion detector signaling the automatic doors in a store took a minute to react, rather than a split second, not only could that cause serious injury but there’d be a lot of angry customers which wouldn’t be good for business.  Think about it!  When it comes to life surviving within the laws of nature, real numbers have real consequences.  Your sensory nerves must send their information to your central nervous system fast enough to let it know where all the parts of your body sit in space, how fast they’re moving and in what direction or else you’re going to have problems staying balanced and being able to do what you want to do.   

For emergencies, where time is of the essence, your body is equipped with automatic reflexes that immediately turn on to try to prevent injury.  They work by sensory information going to the spinal cord or the brainstem that automatically triggers a specific motor response.  Examples include the blink reflex to protect your eyes from injury and the cough reflex to protect your airway from foreign matter obstructing air flow to your lungs.  Another important but more complex motor pattern used by the body to protect itself from injury is the withdrawal (flexor) and crossed extensor reflexes.  Step on something sharp and the pain messages quickly go to the spinal cord where it turns on a series of muscles to contract so you immediately flex your leg and withdraw your foot.  But when you lift your leg off the ground to avoid further injury you’re at risk of losing your balance and falling to the ground.  So a split second later the crossed extensor reflex straightens out your opposite leg and your body weight shifts over to maintain its balance.  This all takes place automatically without us having to think about it because by that time we’d be on the ground.  Isn’t that incredible?  Now, let’s look at how conduction velocity affects this important reflex.  

Step on something sharp in a parking lot and you may begin to fall backwards and be at risk of landing hard on your buttocks.  When you’re standing up your buttocks is about one meter off the ground.  Since we know that gravity makes all things accelerate downward at 10 m/sec2 this means that, if you couldn’t correct the situation in time, it would take about a half a second for you to hit the pavement.  So, to not fall to the ground your reflexes would have to do their jobs fast enough which would be dependent on them receiving the sensory information fast enough.

Your body has different types of nerves with different conduction velocities.  The impulse velocity of a nerve is faster if it’s larger in diameter and is coated with a fatty substance called myelin and it’s slower when it’s smaller in diameter and isn’t surrounded by myelin.  Normally, the sensory nerves servicing the reflexes mentioned above and the ones sending messages back to the muscles are large and myelinated with a conduction velocity of 100 m/sec (200 mph).  Being that it’s about one meter from the foot to the spinal cord it would take about 0.02 seconds for the sensory nerve impulse to travel to the spinal cord (0.01 sec) and back along the motor nerve (0.01 sec) to keep you balanced and on your feet.   Since it would take a half a second (0.5 sec) for you to hit the ground this would give the spinal cord plenty of time to process the sensory information and send out the signals to the muscles.
    
In contrast, the impulse velocity of the smaller unmyelinated nerves that inform your body of deep pain is only about 1 m/sec.  Since it’s about one meter from the foot to the spinal cord, if the conduction velocity for the sensory and motor nerves for this protective reflex were like the ones for deep pain it would take about two seconds for the impulses to go from the leg to the spinal cord (1 sec) and back along the motor nerve to the muscles (1 sec).  In other words, since you’d be on the ground in half a second (0.5 sec), by the time you hit the pavement the sensory messages from your feet would only have gone about half way up your leg.  Clearly, this wouldn’t have allowed our earliest ancestors to stay standing and be able to survive within the laws of nature.  It also explains why when you suffer a severe injury, you can reflexively move quickly to avoid further damage but you don’t feel the severe pain until a few seconds later.

So, it’s your sensory receptors that inform your body of what’s going on inside and outside of it that allows it to make decisions about protecting it from injury and doing what you want it to do.  And that’s why you feel pain and other sensations. 

Three Questions for Mr. Darwin

  1. How did my body anticipate the need to have so many different sensory modes, where did the information come from to produce each of them, how do they know what to do, and how did the intermediate organisms leading up to me survive without any one of them? 

  2. Where did the information come from that tells my central nervous system what to do with all the sensory information it receives to allow my body to do what I want it to do?

  3. How did my body figure out that the nerves supplying the sensory information for its reflexes needed to have a fast enough conduction velocity to beat the effects of gravity?   

Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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