Info Transmission

---Signals course through the nervous system in a relay where electrical impulses alternate with chemical messengers. The electrical impulses flow through those nerve-cell pathways the axons and dendrites. Chemical messengers leap the gaps(synapses) between each neuron's axon and the dendrites of the other neurons with which that axon makes contact. What happens is this. Before an impulse arrives, an inactive neuron has more potassium ions(electrically charged potassium atoms) than sodium ions. Outside the neuron the opposite holds good. So there are opposite electrical charges on the inside and the outside of the neuron's membrane.

---When an impulse reaches the nearby axon tip of another neuron, this yields neurotransmitters. These chemicals burst from tiny sacs in the buttons forming the ends of the axon. Crossing the narrow synaptic gap between their axon and the inactive neuron's cell body or dendrites, the neurotransmitters lock onto receptor sites in the inactive neuron's membranes. This may let potassium ions out and sodium ions in, so altering the charge inside the membrane.

---Neurotransmitters like acetylcholine(in parts of the brain) and noradrenaline(in the brainstem and hypothalamus) make an electric impulse flow through the receiving neuron. Such chemicals are accordingly known as excitory neurotransmitters.

---But there are inhibitory neurotransmitters, too. Gamma amino butyric acid, or GABA, (in the brain's outer grey matter) and glycine(in the spinal cord) both act to block electrical impulses.

---At any instant, one brain cell may receive thousands of contradictory signals from the other cells with which it is in contact. Whether or not it fires off an electrical impulse depends on how many signals of each kind it receives. So neurons are really minute information processors, and their selective work prevents mental chaos.

---Once a cell fires, the strength and speed of the impulse it carries depends on its dendrites and axon. The farther a signal flows through the dendrites, the weaker it grows. This is not true in axons, however long or short these may be. An axon's insulating myelin sheath may even boost the signal a little.

---But axon size does affect axon speed. In the smallest axons, impulses crawl at a slothful 1mph(0.5m/sec). They zoom through the largest axons at 270mph(120m/sec). Yet even this rapid transit is considerably slower than an electric current flowing through a wire.

---As electroencephalography shows, the brain is always firing with electrochemical activity. Yet, somehow, it sorts out for attention only those signals that matter. It can do this because a neuron's cell membrane can switch from negative to positive and back again in no more than one-thousandth of a second, and strongly stimulated neurons fire faster than others. Thus an excited neuron's firings per second stand out from the general background "noise" as a burst of machine-dun fire stands out from rifle shots.

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