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The primary function of the nerve cells of the body is communication. This is accomplished by passing electrical and/or chemical messages from neuron to neuron, or from neuron to other target cells.
There are many types of neurons (figure 1), but in general neurons have certain typical structures (figure 2). The cell body (also called the soma or perikaryon) of the neuron contains the nucleus which often has a prominent nucleolus (figure 3). Spreading out from the cell body are long projections: the dendrites and axons. Dendrites carry electrical impulses towards the cell body, while axons carry electrical messages away from the cell body. Where the axon joins the cell body there is a region call the axon hillock which is important in producing action potentials. The cell body usually contains abundant rough endoplasmic reticulum, which in the neuron is called Nissl substance.
The axon of a neuron may be very long, and it is the axons which form the large nerve fibres which we can see easily on dissection, or even palpate through the skin. Neurons in the peripheral nervous system (outside of the brain and spinal cord) are protected by Schwann cells which wrap around the axons to form a neurilemma which aids in regeneration of injured cells (figure 4). There is no neurilemma in the central nervous system, and this may be one reason why injuries to the brain and spinal cord do not heal very well.
In some peripheral axons, the Schwann cells along the length of the axon may wrap around the axon many times. Collectively, this wrapping of Schwann cells is called a myelin sheath, and the points where successive Schwann cells touch are called nodes of Ranvier (figure 5). Axons with a myelin sheath conduct electricity relatively quickly compared to axons which only have a normal neurilemma. Many pain fibres are unmyelinated, and so pain information travels rather slowly towards the brain and spinal cord. Postural information, on the other hand, travels quickly and the axons which carry this information have a thick myelin sheath.
At rest, a neuron is polarized, with a weak positive charge outside of the cell membrane and a weak negative charge inside. When a dendrite is stimulated, the cell membrane become depolarized. An electrical wave of depolarization, also called an action potential, travels from the dendrite to the cell body and then along the axon. At the end of the axon are vesicles containing chemical neurotransmitters. Examples of neurotransmitters are acetylcholine and norepinephrine. When the depolarization reaches the end of the axon, the vesicles fuse with the cell membrane and release the neurotransmitters into the synapse. The neurotransmitters then stimulate adjacent neurons or other target cells such as muscles (figure 6).
Neurons which send impulses into the central nervous system are called sensory neurons or afferent neurons. Neurons which send impulses out of the central nervous system are called motor neurons or efferent neurons.
acetylcholine: アセチルコリン (asechirukorin); action potential: 活動電位(katsukoudeni); afferent neuron: 求心性ニューロン (kyuushinseinyuuron); axon: 軸索 (jikusaku); axon hillock: 軸索起始部 (jikusakukishibu), 軸索小丘 (jikusakushoukyuu); cell body: 細胞体 (saiboutai); central nervous system: 中枢神経系 (chuusuushinkeikei); dendrite: 樹状突起 (shushoutokki); efferent neuron: 遠心性神経 (enshinseinyuuron); electrical wave: 電波 (denpa); norepinephrine: ノルエピネフリン (noruepinefurin); motor neuron: 運動ニューロン (undounyuuron); myelin: ミエリン (mierin); neurilemma: 神経鞘 (shinkeishou), 神経線維鞘 (shinkeisenishou) neurotransmitter: 神経伝達物質 (shinkeidentatsubusshitsu); Nissl substance: ニッスル小体 (nissurushoutai); node of Ranvier: ランビエの絞輪 (ranbienokourin); nucleolus: 核小体 (kakushoutai); peripheral nervous system: 末梢神経系 (masshoushinkeikei), Schwann cell: シュワン細胞 (shuwansaibou); sensory neuron: 感覚ニューロン (kankakunyuuron); synapse: シナプス (shinapusu)
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