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Chapter 44

bioa02 chapter 44

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Biological Sciences
Mary Olaveson

Chapter 44 44.1 What cells are unique to the nervous system? - nervous systems are composed of two unique categories of cells o nerve cells or neurons and glial cells or glia - neurons are excitable o can generate and propagate electrical signals, which are known as nerve impulses, or action potentials o have long extensions called axons that enable them to conduct action potentials over long distances - Glial cells do not conduct action potentials o They support neurons physically, immunologically, and metabolically - A nerve is a bundle of axons that come from many different neurons - Afferent neurons carry sensory information into the nervous system o That information comes from specialized sensory neurons that transducer (convert) various kinds of sensory input into action potentials - Efferent neurons carry commands to physiological and behavioral effectors such as muscles and glands - Interneurons integrate and store information and facilitate communication between sensors and effectors - Numbers of neurons organized into clusters called ganglia o Ganglia serving different functions may be distributed around the body - Animals that are bilaterally symmetrical, ganglia frequently come in pairs, one on each side of the body o One pair of ganglia is larger than the others, and is therefore given the designation of brain - In vertebrates, most cells f the nervous system are found in the brain and the spinal cord the sites of most information processing, storage and retrieval o The brain and the spinal cord are called central nervous system (CNS) - Information is transmitted from sensory cells to the CNS and from the CNS to effectors via neurons that extend or reside outside of the brain and the spinal cord o These neurons and their supporting cells are called peripheral nervous system (PNS) - The human nervous system contains an estimated 10^11 neurons - Information is passed from one neuron to another where they come into close proximity at structures called synapses o the cell that sends the message is the presynaptic neuron o the cell that receives it is the postsynaptic neuron - the human brain must contain up to 10^14 synapses, which can be highly plastic, strengthening with use and weakening with disuse - neurons behave similarly in animals as different as squids and humans www.notesolution.com - the transmission of action potentials can be rapid, up to 100 meters per second or more - most neurons have four regions o cell body contains the nucleus and most of the cells organelles many projections may sprout from the cells body most of these projections are shrub like dendrites o dendrites bring information from other neurons or sensory cells to the cell body o axons usually carry information away form the cell body length differs among different types of neurons are the telephone lines of the nervous system o at the tip of each of these tiny nerve endings is a swelling called axon terminal, that comes very close to the target cell to form a synapse - an action potential arriving at an axon terminal causes the release of chemical messenger molecules called neurotransmitters, from the axon terminal o they diffuse across the space and bind to receptors on the plasma membrane of the postsynaptic or target cell - There are many more glial cells than neurons in the human brain - Glial cells are not excitable, and do not transmit electrical signals o Some physically support and orient the neurons and help them make the right contract during embryonic development o Other supply neurons with nutrients, maintain the extracellular environment, consume foreign particles and cellular debris, or insulate axons - The in CNS, some glial cells are called oligodendrocytes, which wrap around the axons of neurons, covering them with concentric layers of insulating plasma membrane - In the PNS, glia called Schwann cells perform this function - Myelin is the covering produced by oligodendrocytes and Schwann cells, and it gives many parts of the nervous system a glistening white appearance - Not all axons are myelinated, but those that are can conduct action potential more rapidly than those axons that are not myelinated - Glia called astrocytes contribute to the blood-brain barrier, which protects the brain from toxic chemicals in the blood o Help form the blood-brain barrier by surrounding the smallest, most permeable blood vessels in the brain o Since the barrier consists of plasma membranes, it is permeable to fat- soluble substances such as anesthetic and alcohol, which explains why these substances have such rapid and marked effects on the nervous system 44.2 How do neurons generate and conduct signals? www.notesolution.com
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