Human Body System

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LEVELS OF ORGANIZATION OF NERVOURS SYSTEM:

NEURONS:
* STRUCTURE AND FUNCTION: Specialized cell transmitting nerve impulses; a nerve cell. Neurons have specialized projections called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body. Neurons communicate with each other through an electrochemical process. Information is transmitted to the receiving cell at junctions via chemical messengers called neurotransmitters. At this juncture, the neuron transmitting the information/signal is referred to as the presynaptic cell, while the cell receiving the signal is the postsynaptic cell. Glial cells, or glia, are support cells that nourish the neurons and insulate the axons, while regulating the extracellular fluid around the neurons. At the end, the axon is divided into many branches, from where the signal passing through the axon is transmitted to the next cell via a junction known as the synapse. Hence, the branch of the axon involved in this "specialized" conjugation is called a synaptic terminal.

TYPES OF NEURONS:
* Neurons are the basic unit of the nervous system. All cells of the nervous system are comprised of neurons. While there are many different kinds of neurons, there are three broad categories based on function:

* Sensory neurons are sensitive to various non-neural stimuli.  There are sensory neurons in the skin, muscles, joints, and organs that indicate pressure, temperature, and pain.  There are more specialized neurons in the nose and tongue that are sensitive to the molecular shapes we perceive as tastes and smells.  Neurons in the inner ear are sensitive to vibration, and provide us with information about sound.  And the rods and cones of the retina are sensitive to light, and allow us to see. * Motor neurons are able to stimulate muscle cells throughout the body, including the muscles of the heart, diaphragm, intestines, bladder, and glands. * Interneurons are the neurons that provide connections between sensory and motor neurons, as well as between themselves.  The neurons of the central nervous system, including the brain, are all interneurons.

Most neurons are collected into "packages" of one sort or another, sometimes visible to the naked eye.  A clump of neuron cell bodies, for example, is called a ganglion (plural: ganglia) or a nucleus (plural: nuclei).  A fiber made up of many axons is called a nerve.  In the brain and spinal cord, areas that are mostly axons are called white matter, and it is possible to differentiate pathways or tracts of these axons.  Areas that include large number of cell bodies are called gray matter. THE SYANAPSE:

* Each neuron has only one axon, but the axon usually has extensive branching at its end, enabling it to communicate with many other neurons. Each branch ends in a synaptic terminal that is filled with a chemical, or neurotransmitter. * Neurotransmitters are chemicals made by the neuron (usually from amino acids) that transmit signals from the neuron to a target cell across a synapse (in tiny bubbles of chemicals called vesicles). When the axon is stimulated, the neurotransmitter is released into the opening between neurons, called the synapse. A synapse is a tiny space or gap that serves as a junction between neurons. At a synapse, the axon membrane closely adjoins the membrane of the target cell's dendrites. When released, the neurotransmitter diffuses across the synapse and binds to receptors in the membrane of the target cell (on the postsynaptic side)( The neurotransmitter acts like a little key, and the receptor site like a little lock.  When they meet, they open a passage way for ions, which then change the balance of ions on the outside and the inside of the next neuron. And the whole process starts all over again). This transmits the signal, or nerve impulse, across the synapse to the next neuron, which is either the next neuron or an effector cell.

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