Central Nervous System
---Disconnected from input and output, a computer's control unit would be useless. Similarly, your brain depends on nerves that wire it up to the body's sense organs and muscles. This chapter looks beyond the brain to the broader wiring of the nervous system, then investigates nerve cells and how these send their signals to and fro.
---In evolutionary terms, the
brain itself is just a complex outgrowth of the spinal cord.
Indeed, both make up the body's central nervous system - its key
roles being receiving sensory data, making decisions, and sending
out commands to muscles. Compared with the convoluted brain, the
spinal cord appears a simple structure, in keeping with its
relatively lowly functions: performing reflex actions and
transmitting nervous impulses to and from the brain. But cord and
brain share some important features, and though the cord may be
more primitive, physiologists have by no means yet unravelled all
its operations.
---A soft, curved cylinder, the spinal cord descends inside the backbone for about 18 inches(46cm) from a big hole at the base of the skull, where the cord's upper end joins the brainstem, to the level of the first lumbar("apron for the loins") vertebra, well above the bottom of the spine. The cord bulges at those levels from which nerves branch off to supply the arms and legs. But below the first lumbar vertebra it continues downward as no more than a fine thread, tethered to the back of the coccyx(loosely meaning "cuckoo's bill"), the tail end of the backbone.
---Arched bones of the vertebrae
enclose and shield the vulnerable cord. Inside this bony cave the
cord is safely-wrapped by three membranes - continuations of the
meninges that surround the brain. Similarly, a cerebrospinal
fluid cushion surrounds the cord, protecting it from jarring. The
spinal cord has a white outer layer of insulated nerve fibres
which carry signals - some up, some down the cord. Enclosed by
this white layer lies a butterfly-shaped grey section of the
nerve cells that forms a fluted column running up inside the
outer cord.
---The butterfly's lower "wings," facing chest or belly, hold nerve fibres controlling muscles. Upper "wings," which face the back, contain cells receiving sensory signals from outside the spinal cord. Different areas inside the spinal cord affects different regions of the body.
---At intervals along the cord, each wingtip sprouts a nerve root - four per "butterfly." From each wing, both roots join to form a mixed(part-sensory, part-motor) spinal nerve that exists from the spine through one of the two gaps in the bone between each pair of vertebrae.
---The 31 pairs of spinal nerves roughly tally in number with the gaps available. But because the spine grows longer than the spinal cord, the cord's lower nerve roots get out of step with their exit holes and must grow far downward through the spinal canal before they can escape. The resulting fan of nerve roots in the lower spine creates the cauda equina("horse's tail").
Nerves---> Next
[Nerve Pathways] [Info Transmission] [Spinal Cord Tracts] [Cranial & Spinal Nerves]
