7 B-The Nervous system, Neurons and synapses

7 B-The Nervous system, Neurons and synapses - The Nervous...

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The Nervous System Neurons and Synapses-B Lecturer: Dr. R. Ahangari University of Central Florida, Orlando Human Physiology by S.I. Fox and human anatomy by Marieb & Mallat
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Synapses: Nervous system consists of neurons that are linked together to form functional conducting pathways. Synapses are the sites where two neurons come into close proximity. The term also implies to the nerve-muscle contact as well. Various forms of synapses: Axodendritic, axosomatic and axoaxonic. The first two are the most common forms. Axons can have a terminal expansion or a series of expansions called bouton de passage which make several contacts as they pass through a dendritic tree. Types of synapses: 1- Chemical (most common) 2- Electrical
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Chemical synapses involve the neurotransmitters released from a pre-synaptic neuron that becomes attached to a protein (receptor) at post-synaptic membrane. Chemical synapses are unidirectional. Ultrastructure of chemical synapses: The opposed surfaces of terminal axonal expansion and the neuron are termed the presynaptic and postsynaptic (with sub synaptic web) membranes, respectively, and are separated by a synaptic cleft (20- 30nm). Membranes are thicker here. Presynaptic cytoplasm contains vesicles , mitochondria and lysosomes among others. On postsynaptic side, the cytoplasm contains parallel cysternae. Synaptic cleft contains polysaccharides.
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Neurotransmitters at chemical synapses: The presynaptic vesicles contain the neurotransmitter substance and the mitochondria provide ATP for neurotransmitter synthesis. Neurotransmitters: Acetylcholine (Ach), norepinephrine, epinephrine, dopamine, glycine, serotonin, gamma-aminobutyric acid (GABA), enkephalines, substance P and glutamic acid. The majority of neurons produce and release only one principal neurotransmitter. Distribution of neurotransmitters: Ach: is found at neuromuscular junction, in autonomic ganglia, parasympathetic nerve, Norepinephrine: found at sympathetic nerve endings, in CNS: in hypothalamus. Dopamine: found in high concentrations in basal ganglia and hypothalamus. Glycine: is found principally in synapses in the spinal cord. Glutamate: is an excitatory amino acid neurotransmitter in many central nervous neurons.
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Action of neurotransmitters: Neurotransmitters are released from the nerve endings after the nerve being stimulated (arrival of action potential). This results in an influx of Ca ++ ions into the presynaptic part which causes the synaptic vesicles to fuse with the presynaptic membrane. The neurotransmitters are then released
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This note was uploaded on 06/20/2008 for the course PCB 3303 taught by Professor Anghari during the Fall '07 term at University of Central Florida.

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7 B-The Nervous system, Neurons and synapses - The Nervous...

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