Re: Are sensory nerves physically different from 'regular' nerves?

>Is there any physical difference between sensory nerves/neurons and other 
nerves/neurons?  

*Yes there are many differences. For one, there are groupings of sensory 
neurons that conduct electrical signals at different rates or conduction 
velocities. The reasons for this differ somewhat from motor neurons in the 
fact that some sensory neurons do not have as many wrappings of the myelin 
sheath around them. Thus, they have less "membrane resistance" and lose 
current by standard electrical cable properties as compared to other 
sensory neurons with many wrappings. You may have heard of ‘C' or pain 
fibers (meaning C type of sensory axons or slow pain conducting neurons). 
These are the "unmyelinated" sensory axons. They still have some wrappings 
but not as much as other axons. Thus, they conduct slower than other 
neurons (From Neuron to Brain, Sinauer Publishers, by Nicholls, Martin, 
Wallace, Fuchs, ISBN 0-87893-439-1, 4th edition year 2001)
This description above is mostly for mammals, but if one broadens the topic 
to invertebrates there are many different examples of motor and sensory 
neurons without multiple wrappings, but there are also examples of sensory 
and motor neurons with  myelin-like wrappings in invertebrates. Some recent 
articles on this are by Dr. Dan Hartline (Journal of Comparative 
Physiology, vol 186, pp 337-345, 2000).

Also the transmitter that sensory and motor neurons use to communicate to 
other neurons or a muscle cell are different. There are many variations  
when one considers all the animals (invertebrates as well as vertebrates). 
For example, in the crayfish - a standard preparation for years to 
investigate synaptic transmission, the neurotransmitter released from a 
motor neuron to communicate with the muscle is glutamate for the excitatory 
motor neurons. However, the crayfish also have inhibitory motor neurons 
that release GABA to inhibit the muscle fiber. For the most part these 
animals use Ach (acetylcholine) as the neurotransmitter for its sensory 
neurons. This is just the opposite way around in most of the cases for 
mammals- Ach for motor neurons and glutamate for sensory neurons. So there 
are differences in the biochemical properties within motor and sensory 
neurons.


>Or do they differ only in function? 

* As mentioned above biochemical differences also occur as well as 
function.

>Are they formed by the same proteins?  

* Since there are differences in biochemical properties, such as in 
enzymatic process, then different proteins will be found in sensory and 
motor neurons. Also they do share many similar processes so a number of 
proteins would likely be the same or very close (isoforms of proteins).

>Do all senses (vision, hearing, etc.) use sensory nerves?  

* As far as I am aware yes. But recall when one starts to consider the 
invertebrates (which consists of the majority of animals in the world), 
some sensory neurons can be within the ventral nerve cord. For example- in 
crayfish a sensory neuron that can detect light is in the 6th abdominal 
ganglion within the ventral nerve cord. This neuron serves as a sensory 
neuron and possibly as an interneuron for communication of other sensory 
information. But, I believe by definition if a neuron is the primary cell 
that detects the sensory input then it is a sensory neuron, and as 
mentioned this neuron may also serve other functions as well.


>What else is required to send sensory information to the central nervous 
system?  Are these things specific to sensory input only?  Or do they also 
perform other functions?

*Taking these questions together- To send sensory input means conducting 
electrical events. This of course means movement of ions. The basic 
processes are common to all neurons but many neurons have differences in 
the ratios of the various ions that move across the membrane and timing of 
channel opening and closing. So different types of channels may exsit. One 
major difference you might consider is the inherent difference of sensory 
and other neurons which is sensory neurons vary depending of what they are 
to sense. Like the rods and cones within the eye to detect light as well as 
of different colors of light. Motor neurons and other sensory neurons do 
not have the pigment molecules to absorb light in the same way or to 
transfer the light into an electrical event within the neuron. So yes, 
there are differences between sensory neurons. Other such examples are in 
the transduction from the stimulus to the electrical event. For movement 
detecting neurons- hairs on the skin or on the surface of a fly, when the 
hair deflects, the neuron endings sense this by their association with the 
hair. In some cases, the stretch of the neuron's membrane will open 
‘stretch-activated channels' to allow ions to flow across the membrane and 
depolarize the cell. Motor neurons would not have such channels in their 
dendrites, but now that you mention it I am not really sure if any one has 
looked for such channels in the dendrites of motor neurons.

I hope I have answered some of your questions. If you have more please ask.
RLCOOP1@pop.uky.edu