[Sorry for anyone that might get this blog twice... I had to edit and repost to test an email feature...]
We finish the "Parts of the Brain" portion of The Lab Rats' Guide to the Brain by examining the one brain part that does not occupy the same space as the "brain." In other words, the spinal cord is more properly considered an extension of the brain, than simply a conduit for nerves to reach the parts of the brain.
On the other hand, that is precisely what the spinal cord does - provide a tract for ascending (sensory) and descending (motor) neurons to send their axons to the respective targets (brain and muscles). The correspondence with level of the spinal cord and the region it connects ("innervates" is the term used by neuroscientists and physicians) is fairly obvious for the major limbs - i.e. arms are innervated by axons that enter/exit the spinal cord at the neck and shoulder level; legs are innervated by axons that enter/exit at the lumbar, or lower back level. However, what may not be as obvious is the correspondence of the nerves serving internal organs. The chart at right shows the origin of the axons entering and leaving the spinal cord (in particular the ganglia that run parallel to the spinal cord) with different levels of the "spinal column" (spinal cord, vertebrae plus all associated nerves entering, exiting and running parallel to the spine). The internal organs usually cannot be sensed by themselves, which is why internal pain is often associated with the nearest "peripheral" nerves, i.e. heart with arms, stomach with chest, kidneys with lower back.
The statement above about ganglia, however is very important - axons do not travel all the way between brain and muscle or organ. Sensory nerves send projections to the spinal cord, where the axons form synapses on neuron cell bodies *in* the spinal cord, which then send *their* axons to the brain (typically to the thalamus). Motor neuron axons descending from the brain likewise form synapses with neurons with cell bodies either in the spinal cord or in the spinal ganglia (parallel to the spinal cord). Those neurons then project their axons to the various muscles and organs.
The diagram above shows a cross-section of the spinal cord. The central "circle" is the spinal cord. It is divided in half, just like the brain, to serve the symmetrical halves of the body. At 4 o'clock and 8 o'clock positions are the projections from brain to body - called the anterior or "ventral roots". Axons are from neurons in the spinal cord that project to the muscles and organs. The posterior or "dorsal roots" at the 10 and 2 o'clock positions are neurons entering the spinal cord. These "roots" contain a swelliing or "ganglia" that contain the cell bodies for neurons connecting the sensory receptors to the spinal cord. The dorsal and ventral roots comprise the spinal nerves, and project out between the vertebrae, or bones of the spine. [A "pinched nerve" or "slipped disk" results in unusual pressure on the spinal nerves, damaging the neurons, producing pain and possibly altering muscle movement.] The dark shaded "butterfly" in the center of the spinal cord is Gray Matter, consisting of the cell bodies of the neurons which serve each level of the spinal column. The lighter shading is White Matter and consists of the axons ascending and descending in well defined columns serving each part of the body.
At the right is a series of sections through the spinal cord at different levels - C.VIII or C8 is in the lower cervical (neck) area. TH.VI or T6 is in the thoracic, or mid-back area. L.I - Lumbar or L1 - is in the lower back, while S.I - Sacral or S1 - is just above the tailbone or sacrum. The size of the section shows the relative diameter of the spinal cord at each level. C8 and L1 have a greater diameter than T6 and S1 because the major nerve connections to the arms are C6-8 and for the legs at L1-4. Note also that there is very little white matter in S.1, due to the fact that most of the axons enter and exit the spinal cord *above* the S1 level. Likewise, while T6 has a smaller diameter than L1, it has more white matter, since it includes the axons for all body regions served below T6 - including those from L1.
Thus the spinal cord acts as a mediary between muscle/sensory systems and brain. This is in fact a very important function, in that many of the *reflex* actions are controlled solely by the spinal cord. The sensation of extreme cold or heat on the hand, followed by a reflex withdrawal of the hand from the heat/cold source is a reflex. Some reflex still exists when the spinal cord is severed and does not require the brain to function. Every muscle used to *extend* a limb, whether arm, leg, finger, etc. is counterbalanced by a *flexor* which produces opposite, bending motion. If *both* muscles were active at the same time, our limbs would be rigid, unable to move. Neurons in each muscle and in the tendons where muscles attach to bone sense the stretch and contraction of muscles. Neurons in the spinal cord alter the signals to the *opposing* muscle group to tell the muscles to stop resiting the motion. There *is* conscious control of the reflex. Once a target muscle position is achieved, other signals from brain (cerebellum and basal ganglia) turn the opposing muscles back on so enable a smooth stop with no overshoot, oscillation or tremor. Again, the connections are made in the spinal cord and, and result in the enlarged Gray Matter regions in the spinal cord at the major input/output points for those muscle systems.
In this manner, the spinal cord is in essence an extension of the brain - or at least the brainstem, thalamus and basal ganglia - down into the remainder of the body. It is well armored by the vertebrae of the spine, and cushioned by cerebrospinal fluid (in the subarachnoid space), just like the brain. Sampling and analyzing the cerebrospinal fluid at the level of the spinal cord (i.e. a spinal tap) can tell doctors many things about the functioning of the nervous system. Delivery of drugs directly to the spinal cord can effectively anesthetize *only* the parts of the body connected to that specific region of the spinal cord (epidural or spinal anesthesia).
At the same time, the spinal cord is easily damaged, and we will cover aspects of spinal cord damage, paralysis, paresis and chronic pain when The Lab Rats' Guide to the Brain moves on to the section on diseases and disorders of the brain.
On that note, this is the final blog post on parts of the brain. There will be a brief recess - one or two blogs - before starting the next section. This would be a good time for mailbag questions, some miscellaneous musings on science and fiction, and perhaps time to exercise the LabRats.
So please, send in questions, fill out the poll in the upper right, recommend this blog to your friends ... and take care of your spinal cord - it has a way of telling you when you don't!