Physiology Exam question

[tyler90012]

Hi, everyone. The following question was on an exam that I took recently in an undergraduate physiology course:

 

How is it that if all sensory information is encoded in the form of action potentials, we can differentiate between sensory stimuli?

 

My answer, which was marked incorrect was...

 

"The number and frequency of action potentials is different for different stimuli."

 

The answer written on the exam key was...

 

"Anatomical connections." (nothing more... that's it)

 

 

After reviewing physiology-related websites (including those for other physiology courses), I think my answer is correct, and I'm not sure what my professor's answer even means. Anyone have any insight?

[Glider]

Yes. The number and frequency of action potentials (APs) signals the duration and intensity of whatever stimulus triggers them. APs conform to an 'all or none' law, which means if the stimulation reaches a threshold an action potential will be triggered. APs are all the same. It doesn't matter where they happen, an AP in an optic nerve fibre is the same as an AP in an auditory nerve fibre, or a chemoreceptor or any other fibre.

 

The more intense the stimulus, the higher the frequency of APs and the longer the duration of the stimulus, the greater the number of APs (until habituation).

 

What allows us to differentiate between e.g. sound and light is quite simply the anatomical connections. Light stimulates the optic nerve which terminates in the primary visual cortex in the occipital lobes. Sound stimulates the auditory nerve which terminates in the auditory cortex in the temporal lobes.

 

These areas of cortex are speciallised to interpret incoming information as images and sounds respectively. However, they can't differentiate between sources of AP volleys, so, if you were to connect the auditory nerve to the visual cortex, any sound you heard would be percieved as forms of light and movement. The same would happen in you connected the optic nerve to the auditory cortex. Any images triggering APs in the optic nerve would result in the perception of sounds.

 

A little experiment you can do to show this is to close one eye, and push (gently) in one corner. Mechanical deformation of the retinal cells will also trigger APs. Yet, these will be percieved as blotches of light because the visual cortex cannot differentiate between causes of the incoming APs; light or mechanical force or indeed APs from any other source terminating in the visual cortex will always be interpreted as light/movement.

 

In short, the specialisation of the areas of brain receiving APs determines the eventual percept, not differences in the qualities of the APs, because there are none. Therefore, the anatomical connection determines the experience, not qualities of the APs or stimulus.

[tyler90012]

Thanks for the reply, Glider. That makes sense. I was misinterpreting the question.

[Glider]

You're welcome.