Nerves & Neurons
---The billions of tiny active units making up the nervous system are neurons or nerve cells - cells designed to communicate electrochemically with one another. Several kinds of noncommuicating glial("glue") cells support, nourish, insulate and far outnumber the neurons.
---Each neuron receives and transmits signals through thousands of tiny "wires" linking it with other neurons in the nervous system. In fact these wires belong to the neurons themselves.
---Each neuron has three main parts: cell body, axon and dendrites.
---The cell body is a minute blob made up of a central nucleus surrounded by cytoplasm - a rather sticky fluid containing special microscopic structures. Nutrients and waste products filtering in and out through the parmeable cell wall keep the cell body alive.
---Thus far, neurons seem like the other cells that build our bodies. It is their projecting axons and dendrites that make them so remarkable. An axon("axis") is a long, slim "tree-trunk" transmitting signals from the cell body to other cells via junctions known as synapses. Axons linking nearby regions of nerve tissue may be no more than a few millimeters long, while others sending signals from remote parts of the body to the brain or vice versa can measure more than a yard(about 1m).
---Dendrites("like trees") are networks of short fibres that branch out from an axon and synapse with the ends of axons from other neurons. Dendrites are receivers, bringing signals to their neuron's own cell body.
---Neurons come in many varieties, but all may be grouped in one of several ways, according to their purpose, size or shape. For instance, afferents(or sensory neurons) bring signals to the central nervous system from elsewhere in the body. Efferents(or motor neurons) send signals out. Interneurons - 97 percent of all neurons, but found only in the brain and spinal cord - communicate between the other groups.
---The grey matter of the brain and spinal cord consists of cell bodies. White matter comprises the nerve fibres, or axons, sheathed in a white fatty substance(myelin) which wraps around them in a roll. Bundles of such insulated axons form nerve fibres. Most central nervous system nerves comprise cell bodies with short axon tracts or bundles. The longest axons are in the peripheral nervous system. Outside the central nervous system, cell bodies clumped together form ganglia.
[Nerves] [Info Transmission] [Spinal Cord Tracts] [Cranial & Spinal Nerves]