Impulses and Sensations

[stevo247]

Are impulses and sensations created by nerves or only transmitted by them?

[Glider]

Transmitted. Nerves conduct action potentials.

 

Sensory 'organs' (specialised cells or nerve endings) transduce specific stimuli; chemical, mechanical, heat and light, into action potentials. All action potentials are the same, regardless of what nerve is conducting them.

 

'Sensation' is the result of interpretation within the brain and the factor determining what these incoming action potentials are interpreted as, is simply the target area, i.e. where they end up.

 

Signals ending up in the visual cortex are interpreted as patterns of light, because that's what the cells in those areas have evolved to do. The same applies for sound (auditory areas) and touch (primary somatosensory areas) and so-on.

[stevo247]

Thanks for your response. That was helpful.

 

Is the speed of transmission of an action potential the same regardless of what nerve is transmitting it?

 

Also, a unicellular organism such as an amoeba, appears to have a sensitivity to external stimulations. They react to touch, pressure, temperature, light, and chemical irritations. But they don't have an organized nervous system. Is there any research on how that is accomplished?

[Glider]

Thanks for your response. That was helpful.

 

Is the speed of transmission of an action potential the same regardless of what nerve is transmitting it?

No. Action potential velocity will always be the same in the same fibre (axon), but there are other factors that influence AP velocity, namely fibre diameter and myelination.

 

The larger the diameter of the fibre, the faster the conduction velocity. The presence of myelin (provided by schwann cells) also increases conduction velocity.

 

So it basically breaks down like this: Large myelinated efferent (motor) fibres have conduction velocities of over 300 metres per second. Large, myelinated afferent (sensory) fibres (e.g. A alpha and large C) have conduction velocities of 80 to 120 metres per second, but they're smaller in diameter than large motor fibres at around 13 to 20 micrometres (um).

 

Smaller afferent fibres (A-beta fibres at around 6 to 12 um dia.) have conduction velocities of around 35 to 75 metres per second. The smallest of the myelinated afferent fibres (A-Delta at around 1 to 5 um dia.) are only thinly myelinated and conduct at around 3 to 30 metres per second.

 

The smallest of our sensory fibres (also the oldest in evolutionary terms) is the C fibre. This is non-myelinated and has a diameter of only 0.2 to 1.5 um. Its condiction velocity is only 0.5 to 2.0 metres per second.

 

This is why, when you stub your toe, you get the initial sensation; the immediate sharp shock, followed by the slower dull, agonising aching. The latter is mediated by C fibres.

 

Also, a unicellular organism such as an amoeba, appears to have a sensitivity to external stimulations. They react to touch, pressure, temperature, light, and chemical irritations. But they don't have an organized nervous system. Is there any research on how that is accomplished?

There's bound to be, but it's not my area I'm afraid. I do know that whilst complex organisms tend to have complex systems for detecting external stimuli (e.g. light), for example we have eyes with highly complicated retinae; cone and rood cells etc. At the basic level, our light sensitivity is ultimately based on the sensitivity of single molecules of rhodopsin (opsin and retinal) that respond to light by altering their conformation.

 

Presumably then, single celled organisms having, as the name would suggest, only a single cell, would rely simply on these molecules for their light sensitivity without all the other clutter of rods, cones, ganglion cells and so-on.

 

I would imagine it's much the same for touch. Humans have a complex system for detection/transduction, transmission (to the CNS), integration and interpretation of physical sensation, but the complex bits are only to allow us to 'sense' in a conscious way so we can integrate the sensation into our model of the world.

 

At the most basic level however, our sense of touch depends upon relatively simple molecules in the sensory cell membrane that respond to mechanical deformation. I would imagine amoebae rely upon these molecules (or something like them) without all the other junk necessary for higher processing.

 

I imagine you could find information on amoebae in most basic biology books, or Google.