INTRODUCTION
To understand and appreciate what happens in MS, we need to know something about the nervous system.
Everything we do is electrical. Every stimulus ( mechanical, electrical, thermal, atmospheric, etc..) we receive has to be converted into electrical impulses in order for it to be transmitted from one place to another.
Electricity is generated in every cell of our body by forcing sodium ions from outside the cell to inside it through the cell membrane to produce polarity ( + inside & – outside). That gradient produces recordable electricity when many cells are activated (depolarized) at the same time.
Once the stimulus is removed, then the positive charges (ions) are balanced by pushing potassium to move out of the cell membrane, but that is not enough to balance the amount of sodium ions already inside the cell membrane. Ionized calcium moves out of the cell to add to the potassium positive charges which results in a balanced environment between in and outside the cell membrane. Sodium then moves back to its natural habitat to inactivate the cell membrane (repolarization).
This exactly what happens in the nerve cell.
The Nervous System
The nervous system is a network of cells called neurons which transmit information in the form of electrical signals. Your brain has around 100 billion neurons, and each communicates with thousands of others – as many connections as in the world’s telephone system, the biggest machine on the planet. Neurons communicate with each other at special junctions where chemicals help to bridge the gap between one neuron and the next.
The principle nerve cell is called neuron. The neuron is formed of cell (electrical generator), and an axon(electrical cable). Every neuronal axon is covered by a sheath called “myeline”, and the axon then called “myelinated nerve fiber”.
The myeline sheath has 3 main functions:
- Protection to the nerve fiber (like the plastic on the electrical wires).
- Facilitates nutrition to the nerve fiber.
- Faster transmission of electrical impulses ( around 10-20 times faster than its speed through the nerve fiber). This is very important to remember when we talk about MS.
There are two types of neurons ( motor & sensory) one of them brings messages from outside to the brain, and the other takes messages away from the brain.
Each nerve cell communicates with many other nerve cells via appendages called “dendrites”. The inter-communications between dendrites, and between nerve fibers are called “synapses”.
When a nerve cell is stimulated, an electrical impulse is produced; then the electrical impulse will be transmitted to many other neurons at the same time via dendrites (inter-neuronal synapses) and that will produce a bigger electrical charge that can be recorded via electrodes such as EEG or evoked potentials.
When many neurons are firing at the same time, a convulsion might happen such as epileptic seizure.
Around 100 billions of nerve cells, with their trillions of synapses form together what is known as the “grey matter”.
All of those billions of cells are concentrated in an area of 4mm thick, but around the size of a standard bedroom carpet. This large thin area of grey matter is in fact making the most of our brain generator of electricity. Most of our brain functions are produced in this area, predominantly cognition, recognition, memory, orientation, skills, and speech.
The bedroom size thin carpet of grey matter has to be folded into many folds for it to carried inside our heads ( folding a large carpet to put it in your small car to take it to the laundry !). The folds are called “gyri”, and the clefts between them are called “sulci”.
The myelinated axons converge to make a bulky network called the “white matter”, which apart from few grey islands is formed of nerve fibers (electric wires). The predominant function of the white matter is to carry electricity (remember that MS is a white matter disease !).
The gray and white matter, in addition to blood vessels and supportive structures form together the mighty brain with its famous four lobes.
Away from the brain, comes the spinal cord with its white and grey matter, but its a bundle of nerve fibers carrying functions in two directions (sensory or motor). Nerve fibers will end up eventually in the muscles (motor) to electrify the muscle for it to contract and help you to do things. The sensory fibers obviously in its majority start just under the skin (nerve endings and corpuscles) to carry stimuli back to the spinal cord and
eventually to the brain.
From the spinal cord, a network of nerve fibers emerge in a complex way of arrangements ( like the London underground map !), to eventually supply the muscles.
A nerve fiber is eventually connected to a number of muscle fibers to form the “motor unit”. Each muscle is formed of thousands of motor units which are responsible for the strength of the muscle contractions.
A connection between nerve and muscle fiber is called “myoneural junction” where acetyle-choline is the only chemical transmitter. There, an electricity is produced by converting chemical stimuli to electrical impulses which stimulate the muscle fibers and made them contracted.
If we can add another kind of nervous system called “the autonomic nervous system” to the somatic nervous system, we end up with a very complex, sophisticated, and highly efficient integrated nervous system that what we – neurologists – deal with and try to control ( you can probably imagine the task !).
Eventually, to make the whole picture in one context for simplicity, we will end up with this:
….. and that is the nervous system folks in its simplest form and shape for a lay person to understand !!.