The Spinal Cord
The spinal cord is the part of the central nervous system that connects the brain to the body. Connecting to the brain as the brainstem in the upper neck, every sensory or muscle control nerve we have, save for twelve pairs which control the head, run through at least part of the spinal cord.
The spinal nerves form a tight bundle at the core of the brainstem, whilst nine of the twelve pairs of cranial nerves - nerves which do not use the spinal cord - are wrapped round the outside, splitting from the brainstem at varying heights.
The brainstem is the part of the brain that descends just in front of the cerebellum. It drops down from the brain to meet and meld with the spinal cord rising from the body.
For all the complexity of the spinal cord, it is tiny. A roughly oval shaped, twin bundle of nerves, which when combined together is about the same width as your index finger - end on.
Below the brainstem, the spinal cord begins in earnest. This long tube flows inside the spinal vertibre, to protect its soft tissue. Like the brain it is a mix of grey and white nerve tissue, and contains a vaccule, where spinal fluid flows, as with the brain.
Unlike the brain, it is not composed of millions of small dendrites. Instead, each nerve is a single length, from the point it enters the spinal cord, right up to the base of the brain. This is why severing the spinal cord is such a large problem. There are millions of such nerves in the spinal cord, and if a group of them are severed, they must be reconnected correctly. If the wrong lower section isa joined to the wrong upper section, it completely changes how the brain interacts with the body - movement and sensation both have to be relearnt.
Not all nerves travel right along the spinal cord. Instead, they are broken into distinct groupings.
Firstly, there are ascending and descending nerves.
Obviously if you joined an ascending nerve and descending nerve together by mistake, neither would actually work, at best. At worst, you could create an interference signal as both ends continually fire.
Secondly, you have the spinal nerves. These are nerves which leave the spinal cord at each vertibre, to transmit information to the rest of the body, or carry it back.
These are split into four groupings:
In total there are 31 pairs of spinal nerves which branch off from the spinal cord. This does not sound like millions, until you realise that each of these nerves, is actually a nerve bundle, and each branches agaain and again and again, as individual nerve fibres travel in scores, to every extremity of the body.