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The brain contains between 1 billion and 100 billion neurons, depending on the species. The neuron consists of a

cell body, dendrites, and an axon. The cell body contains the nucleus and cytoplasm. The electrically excitable axon extends from the cell body and often gives rise to many smaller branches before ending at nerve terminals. Dendrites extend from the neuron cell body and receive messages from other neurons. Synapses are the contact points where one neuron communicates with another. The dendrites and cell body are covered with synapses formed by the ends of axons from other neurons. Neurons signal by transmitting electrical impulses along their axons, which can range in length from a tiny fraction of an inch to three feet or more. Many axons are covered with a layered myelin sheath, which speeds the transmission of electrical signals along the axon. This sheath is made of specialized cells called oligodendrocytes in the brain and Schwann cells in the peripheral nervous system. Nerve impulses involve the opening and closing of ion channels, which are selectively permeable, water-filled molecular tunnels that pass through the cell membrane and allow ions — electrically charged atoms — or small molecules to enter or leave the cell. The flow of these ions creates an electrical current that produces tiny voltage changes across the neuron’s cell membrane.

Author:   magludi1  Version:  1  Language: English  Category:   Views: 135  Grades: 9-12

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