Friday, April 14, 2017

Nerve fibres : A clinico-physiological approach.

Hello everyone. So I've not been active at all lately , cause Final Year ! Pretty depressing 🙄. Anyway. Here's a post about the nerves and what we need to know for clinical application!

Nerves

So Erlanger and Gasser classified the nerves into A, B and C based on Myelination and size.

So you have :
A
(Which has Alpha , Beta , Gamma , Delta fibres )
B
C

Out of these , the first 3 : 
A - Alpha , Beta , Gamma = Large fibres which are largely Myelinated.

And next 3 :
A - Delta , B , C = Small fibres which are not Myelinated as much.

How I remember these fibres is as per evolutionary significance.
The least Myelinated fibres , which are the smallest are the ones all living creatures need. As we progress from C to B to A , we continue to gather more and more well developed and specialised fibres.

C -
The smallest fibres.
Least Myelinated.
Most basic fibres and most primitive from an evolutionary stand point !
Control sensations of Dull Pain and Temperature (Heat)

B -
Small fibres.
Low Myelination.
Next most Basic Instinct - Urination.
Controls your Autonomic nervous system.
Remember- B = Bladder

A Delta -
Moderately Small fibres.
Lower Myelination than other A fibres.
Responsible for sensation of Sharp pain and Temperature ( Cold )

Thus to summarize the small fibres -

We have C , B and A Delta.
Out of these
C and A Delta control Pain and Temperature
( where C controls Dull pain and Heat ; A Delta controls Sharp pain and Cold )
And B controls Bladder /ANS.

(How to remember A Delta vs C.
C is more primitive. Hence controls Dull pain. Sharp pain is a little more specialized and hence is controlled by the relatively more modern fibre - the A Delta)

Coming to the large fibres.

We progress from A gamma to A beta to A alpha.

A Gamma :
Large but smaller then Alpha and beta.
Myelinated but not as much as alpha and beta.
Responsible for muscle tone.
Remember : A Gamma = Gamma motor neuron which is responsible for tone.

A Beta :
Very large.
Well Myelinated.
Responsible for modern sensations like Fine touch, Pressure and Vibration.

A Alpha :
Largest.
Most Myelinated.
Responsible for Muscle Contraction and Most modern sense - Proprioception.
It's the Bomb of the fibres hence responsible for muscle contraction.

Thus , demyelinating diseases like Guillian Barre syndrome and CIDP would affect the fibres that are used to being Myelinated.
So your presentation in these diseases would generally involve loss of:
A Alpha - Motor + Proprioception
A Beta -  Modern sensations of fine touch and vibration.
A Gamma - Tone

And Axonal Polyneuropathic Diseases like Metabolic or Post infectious ones would involve loss and abnormalities of -
A Delta - Sharp Pain and Cold
B - ANS
C - Dull pain and Heat.

Hope this was helpful !
Happy studying !
Stay awesome.

~ A.P. Burkholderia

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