Anglický jazyk
6.3 Medical English - The nervous systém
The basic unit of the nervous system is the neuron. It consists of the cell body and its processes. Each neuron has two types of processes: several short fibres called dendrites and a single process called the axon which can grow branches along its course. The axon is the main conducting fibre. Neurons which provide links between other axons are called internuncial neurons. The dendrites relay impulses to the cell body while the axon relays impulses away from the cell body. The axon varies in length in different kinds of neurons. In a motor neuron it can be very long, for example from the cell body in the spinal cord to a muscle in the foot. Axons in the internuncial neurons are usually short and difficult to distinguish from the dendrites.
An unactivated nerve fibre maintains a state of chemical stability with a certain concentration of potassium inside and outside the lining membrane of a neuron. Its ratio inside:outside is 30:1. Thus the nerve fibre at rest is electrically charged. A nerve impulse is a wave of depolarization created by a chemical imbalance. Sodium passes through the cell membrane and releases potassium. The depolarization of a part of the nerve cell causes the next segment to be depolarized too. This process goes on until the end of the fibre. The end is not firmly connected to the next cell but there is a small gap that the chemicals skip. This functional junction is called a synapse. The most important transmitters are acetylcholin and noradrenalin, and there are many others, some of them unknown. Once the synapse was made these chemicals are quickly destroyed by enzymes. The nerve fibre recharges within milliseconds.
The central nervous system (CNS) consists of the brain and the spinal cord. The peripheral nerves are those that arise from the CNS and supply all parts of the body. These nerves are conducting fibres which can be sensory or motor. Sensory nerves innervate our senses such as the touch, the sight, the taste etc. Motor nerves innervate the muscles, so they are important for movement.
There are 12 pairs of nerves that arise from the brain. These are called cranial nerves. Five of them are both sensory and motor nerves. Of these, the most important is the vagus which supplies the heart, most of the digestive tract, the pharynx and the larynx. Then there are four pairs of motor nerves and three pairs of sensory nerves.
In contrast, all spinal nerves contain both sensory and motor nerves. There are 8 pairs of cervical nerves, twelve thoracic, five lumbar, five sacral and one coccygeal. The spinal nerves divide in two branches. The posterior branch serves the muscles and the skin on the back parts. The anterior branches of the thoracic nerves circle the thorax and supply the intercostal muscles and skin. All the other anterior branches form plexuses or networks of nerve fibres. These plexuses supply the cervical and pelvic regions and the upper and lower limbs. Thus each limb nerve contains fibres from several spinal nerves. The sciatic nerve, which emerges from the sacral plexus to serve the back of the thigh and the leg, contains fibres from the fourth and fifth lumbar nerves and the first, second and third sacral nerves.
The brain is a human computer. It contains inner white matter and outer grey matter which is called the cerebral cortex. The brain can be divided in the left and right hemispheres and four lobes: the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. Another diferentiation divides the brain in the forebrain, midbrain and hindbrain, and some older structures such as the cerebellum, medulla oblongata, hypothalamus etc.
Lobes have various functions. For example, the frontal lobe governs memory and intellect, controls the movement of muscles, determines attitudes and vocal cords. The parietal lobe analyses pain. The occipital lobe receives visual images. And finally the temporal lobe receives hearing sensations and controls balance.
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process
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výběžek
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branch
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větev, větvit se
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internuncial
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interneuron
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imbalance, inequilibrity
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nerovnováha
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release
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uvolnit
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gap
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mezera
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functional junction
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funkční spojení
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recharge
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znovu nabít
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innervate
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inervovat
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intercostal
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mezižeberní
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plexus
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pleteň
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sciatic nerve
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ischiatický nerv
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matter
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hmota
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lobe
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lalok
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forebrain
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přední mozek
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midbrain
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mezimozek
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hindbrain
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střední mozek
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cerebellum
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mozeček
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medulla oblongata
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prodloužená mícha
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