Today I am going to talk about small nerve fibres. That network which forms the “peripheral nervous system”. As we now know that, there are two main groups of fibres: Motor & Sensory nerve fibres.
From the anterior horn cells, signals travel outside of the spinal cord through a very sophisticated network starts with Roots, then Trunks, then Divisions, then Cords, then Branches; to end at muscle fibbers in a connection called “myo-neural junction” where electrical stimulation is converted into mechanical action through an important chemical called “Acetyl Choline”.
The motor nerve fibres are those fibres which carry messages from the central nervous system(Brain & Spinal cord) to the peripheral nervous system. The motor signals are formed in the motor neurons of the brain which are located in the pre-central gyrus, then they travel down through the brain stem to the bottom of the medulla where they cross to the opposite side before they enter the frontal half of the spinal cord to end at the final (lower) motor neurons at the end of the spinal cord in the Anterior Horn Cells, where motor neuron disease starts.
Obviously that network outside of the spinal cord is formed of both motor and sensory nerve fibres. The motor fibres will mostly end up at muscles through myo-neural junctions, and the sensory fibres do start mostly at the skin, and end up in the sensory neurons in the post-central gyrus where messages are transferred to the motor fibres for the required action or actions.
Let me here make things easier for those none-medical readers: Let us imagine the United Kingdom as a country. People feel the prices are going up, services are getting worse, and they become angry. They express their anger through different ways including of course the social media, formal papers, to their councils, and to their parliamentary representatives. The message will at the end reach the government to respond, but if the government does nothing, then the Parliament has to take action. Members of the parliament will discuss why the people are angry with the government and they end up by issuing orders to the government to act or resign !!. Members of the parliament would make decisions then pass them to the Prime minister who will pass them to the appropriate minister to execute them. If the government does its job at required; people will be happy, frustrations will stop, and the government will survive or even becomes popular !!.
As the motor fibres have no jobs but to execute functions, and they only execute functions following orders from higher centres; they have to receive orders before they decide to do anything. The government ministers will only do extra work if people demand it; otherwise they only do the ordinary and the “routine”. That is exactly the case for the motor system as a whole, and the motor fibres as messengers to the executive systems to act.
The start of any action has to be a response to a demand “feedback”, the feedback will only come from far away (people) to the centre (The State). Hence, I probably see it better to start at the “people”… the fist(farest) receptors in the body; which are the sensory receivers commonly in the skin. I will then talk about sensory nerve fibres.
There are three types of nerve fibres(A, B, and C fibres). These divisions are based on nerve calibre(size), and nerve velocity(speed). The “A” fibres are further sub-divided into 4 sub-divisions(Alpha, Beta, Gamma, and Delta). All “A” fibres are myelinated, hence they carry the fastest electrical signals such as sharp pain and burning. “B” fibres are smaller in size than A fibres, and some of them are myelinated but others are not. “C” fibres are the smallest, and are all not myelinated. The “C” fibres are the target of my talking about the “Gate Theory”.
In the next and final episode, I will talk about “C” fibres, the “Gate Theory”, and our usage of it to soften our pains and make ourselves happier !!.