Nerve Issues

image_spineLets look first at the central nervous system.
Brain. Many invertebrate and vertebrate animals have a nerve cord running from anterior to posterior. Almost invariably this is larger at the anterior end, where sense organs are situated. In vertebrates three sets of paired sense organs, covering smell, sight and hearing/balance are located anteriorly and dorsally. Corresponding to these are three outpushings of the roof of the brain containing groups of nerve cells. The primitive smell brain, sight brain and hearing brain have undergone many changes but are still recognisable in man. The smell brain has become the cerebral hemispheres, the sight brain the tectum (less important in mammals than birds) and the hearing brain the cerebellum. Interestingly in the ventral midline a downpushing produces the infundibular stalk - the link to the endocrine system.
Spinal cord The spinal cord is made up of two distinct regions, grey matter and white matter which look like this. The white matter is around the outside and the grey forms an H in the middle. The grey matter is made up of accumulations of the bodies of conducting cells, neurones. These cells have long processes which may pass up and down the cord or out into the peripheral nervous system. Through both white and grey matter run different types of non-conducting glial cells, which provide nutrients and wrap nerve fibres in the cord with myelin. The white matter is made up of nerve cell processes, axons, wrapped in myelin which appears white in fresh tissue: confusingly most fat stains turn the white matter black.

Because there are afferent and efferent cell processes we need a way in and out of the cord. The way in is dorsal, the dorsal root which is a continuation of the top limb of the H in each spinal segment. The way out, the ventral root is not seen on this diagram because it is at a different level and tends to be a series of rootlets. If we want to see them both at once we have to make the section thicker. It is convenient in many cases to have the wiring to and from a particular part of the body running together, so the two roots join to form a mixed spinal nerve which runs round beneath each rib in the thorax. We will come to the bump in the dorsal root in a moment. The spinal nerve covers a whole body segment, sending off motor sensory or mixed branches as it does so.
We can now put in the simplest possible wiring diagram. This consists of three neurons.

  1. Sensory neuron. The cell body of this is in the dorsal root ganglion, the dorsal swelling I mentioned earlier, made up of thousands of similar cell bodies. A long process comes from a sense organ in the skin and a shorter one runs to the spinal cord. Once in the spinal cord the possibilities are endless: it could go up to the brain or down to another spinal level, but lets keep it simple and say it synapses with
  2. Connector neuron whose body is in the dorsal horn and synapses in turn with
  3. Motor neuron whose body is in the ventral horn, and whose axon passes out along the ventral root to a muscle, which contracts.

So far so good, but we have only dealt with connection to skeletal muscle. Smooth muscle is wired rather differently via the autonomic nervous system. This is in two parts, sympathetic and parasympathetic. Lets deal with the sympathetic and look at a sense organ in a tendon. This uses all the familiar bits of the circuit plus another part the sympathetic chain. The sympathetic chains, or sympathetic trunks are made up of (originally ) segmental ganglia and run antero-medially to the spinal cord. Connections are nowadays made only in some parts of the body - our example is thoracic, between T1 and L2.
The same three neurons are used, but in slightly different places. The sensory neuron is exactly the same except that it synapses not in the dorsal horn, but laterally. The thousands of synapses make up a lateral horn in the thorax. The connector neuron is longer, passing out of the cord and into the sympathetic ganglion via the white ramus communicans.
There are three possible variants from here.

  1. The connector neuron synapses with the motor cell body, and the axon leaves via the grey ramus communicans to the smooth muscle.
  2. The connector neurone may travel along the cord and synapse in another ganglion: it then leaves, not by a spinal nerve but by a special pathway to travel with blood vessels to, say, the heart.
  3. The connector neurone does not synapse but runs out of the sympathetic chain as part of a splanchnic nerve whose preganglionic fibres synapse in a ganglion near the aorta before supplying abdominal viscera. So only the proportions of the three neurones and the location of their cell bodies have changed. Outside the area served by direct connections to the spinal cord the sympathetic fibres have to run up or down the sympathetic chain first. The other part of the autonomic nervous system, the parasympathetic, is more specialised. Whereas all the body receives voluntary and sympathetic fibres parasympathetic innervation is restricted to some viscera (excepting the adrenal and gonad). Their connector neurones are restricted to certain levels in the brain and to S 2,3,4.

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