Nerve Pathways

---A brain cut open seems a meaningless mesh of neurons. Yet a signal speeding through the nervous system manages to find the exact region of the brain designed to handle it. Somehow, too, that region may send directions that reach specific muscles. all this is possible because the brain is built rather like a telephone exchange, with nerve fibres as its wiring and special neuron clusters as the switchboards controlling signals flowing in or out.

---Special centres like the cerebral cortex or the cerebellum are relatively huge. Others, including some hidden in the brainstem, are cell clusters so small that you need a microscope to see them. On close inspection, the maze of "wires" between these switchboards largely consists of ordered bundles of nerve fibres known as tracts. Each tract is made of axons projecting from cell bodies located in one "switchboard," and ending at one or more other "switchboards."

---Hundreds of millions of these fibres supply different regions of the cerebrum. There are three main kinds of connection. Association fibres join different parts of the same cerebral hemisphere. Projection fibres fan out from the brainstem to all parts of the cerebrum. A dense mass of commissural fibres builds up the corpus callosum - the bridge 4 inches(10cm) long that links both hemispheres.

---Many fibre tracts are named according to where they start and finish. For example, corticospinal tract fibres send signals from cerebral cortex to spinal cord. Within such tracts, scientists have discovered many pathways serving special purposes. Among others they have mapped the closely associated pain and temperature pathways, and the general sensory pathways.

---From such studies we now know that different spinal nerves serve specific regions of the body surface, known as dermatones. This helps explain why disorder of an internal organ that affects a spinal nerve may produce pain that seems located in the skin some way from the actual trouble. This phenomenon is called referred pain and may result, for example, in gallbladder pain being felt in the right shoulder.

---Even when a pathway has been severed, its central nervous system "switchboard" may report incoming calls. Thus certain stimuli produce "phantom limb" sensation in amputees. Some actually claim to sense a wristwatch on a missing arm.

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[Nerves] [Nerve Pathways] [Spinal Cord Tracts] [Cranial & Spinal Nerves]

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