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Nervous system cells QuickTime™ and a decompressor are needed to see this picture. Ch. 12 Introduction to the nervous system video QuickTime™ and a decompressor are needed to see this picture. Nervous system Introduction Functioncommunication Components Brain, spinal cord, nerves QuickTime™ and a decompressor are needed to see this picture. Organization of the nervous system Subdivisions Central Nervous system (CNS) Peripheral Nervous system (PNS) Nerves Structural/functional center Brain,spinal cord Integrate sensory information Evaluate sensory information Initiate outgoing response Cranial-from brain Spinal-from spinal cord Afferent and efferent divisions Afferent - information from environment Efferent-from brain to muscles/glands Innervation Somatic - carries info to muscles Autonomic-carries info to smooth and cardiac muscle Efferent division Sympathetic - (fight or flight) Parasympathetic-normal resting activities (rest/repair) Visceral sensory division-communicating between viscera and brain - continual Cells of the nervous system Glia - supportive function Types Astrocytes-star shaped Microglia-small If brain is inflamed, phagocytosis Ependymal-thin sheets that line cavities of cns Most numerous Connect to neurons and capillaries Transfer nutrients from blood to neurons Make up blood brain barrier Produce and circulate fluid Oligodendrocytes-hold nerve fibers together and produce myelin sheath Schwann-in pns Form myelin sheath Gaps-nodes of ranvier Essential for nerve regrowth Microglia QuickTime™ and a decompressor are needed to see this picture. Cells of the nervous system Neurons-excitable cells that conduct impulses Structure Cell body-ribosomes make neurotransmitters which are packaged into vessicles Dendrites-one or more per neuron Conduct nerve signals to cell body Distal ends of sensory neurons are receptors Axon-single process extending from axon hillock Sometimes covered with myelin - fatty layer Conducts nerve impulses away from cell body Synaptic knob at end Cytoskeleton- neurofibrils - allow rapid transport Functional regions Input - dendrites/cell body Summation-axon hillock Conduction-axon Output -synaptic knobs Classification of neurons Structural classification Multipolar-1axon, several dendrites Bipolar-1axon,1dendrite Unipolar-1axon which divides into 2 Functional classification Afferent-sensory neuron(conduct impulses to spinal cord or brain) Efferent-motor neuron(conduct impulses from brain to muscle or gland) Interneurons (bridge gap between sensory and motor ) Reflex arc Signal conduction route - from receptor to and from CNS 3 neuron arc-most common - afferent neuron, interneuron and efferent neuron Two neuron arcsimplest form afferent and efferent neuron Synapse-Where nerve signals are transmitted from one neuron to another Types Electricalelectrical current jumps gap Chemical typical in adults Located at junction of synaptic knob of one neuron and dendrite/cell body of another Nerves-bundles of fibers held together with connective tissue Connective tissue Endoneurium-surround each fiber Perineurium-hold together bundles of nerves (fascicles) Epineurium-surround many fascicles/blood Tracts-bundles of nerve fibers in CNS White matter - myelinated nerves Gray matter-cell bodies, dendrites Nerves can be afferent, efferent or Mixed Repair of nerve fibers If the cell dies, the damage is permanent. If the axon is damaged, the axon can bypass the damage and re-grow. Stages of repair Axon degeneration and Removal of debris by phagocytosis. Growth bypasses the damaged axon with new axon formation Schwann cells cover the new growth Nerve Impulses Living cells maintain a difference in the concentration across membranes Membrane potential - excess of positively charged ions outside membrane, negatively charged inside Polarized membrane-exhibits this difference Magnitude measured in Volts or millivolts (mv). Resting membrane potential is normally -70mv. Sodium potassium pump (active transport)-produces slight excess of pos. ions outside. Transports sodium/potassium inside/outside. Local potential-slight shift away from resting potential Excitation occurs when -dditional sodium channels opened (sodium is positively charged)allows potential to move towards 0 (depolarization) Action potential occurs when a certain number of sodium ions diffuse inward. To resotre the neuron to resting potential, Inhibition occurs potassium channels open increasing potential ( QuickTime™ and a decompressor are needed to see this picture. Action potential mechanism Stimulus triggers sodium gates to open Sodium moves into cell Threshold potential point at which impulse is triggered All or none Gates stay open for a short time then close Movement to resting potential when potassium channels open (repolarization) Hyperpolarization precedes achieving resting potential again Refractory period-membrane resists repolarization (brief) QuickTime™ and a decompressor are needed to see this picture. Action potential conduction Reverse of polarity at peak of action potential Reversal causes electrical current to flow between membrane regions and triggers sodium channels to open in next segment. This repeats Action potential never moves backward because of refractory period In myelenated axons, action potentials only occur at nodes of ranvier, jumping to next node - called Saltatory conduction Speed of conduction-depends on diameter of fiber and presence or absence of myelin. Types of synapses Electrical-action potential continues to postsynaptic membrane Chemical-presynaptic cells release chemical messengers (neurotransmitters) across gap to postsynaptic cell, inducing action potential. Mechanism of synaptic transmission Action potential releases calcium Neurotransmitter release Neurotransmitter diffusion Post synaptic potential Neurotransmitter action termination by re-uptake or taken up by glia Neurotransmitters Description-chemical communicators Function - can be excitatory or inhibibitory Classes of neurotransmitters Acetylcholine Amines affect learning, emotions, motor control Amino acids - most common. Location PNS QuickTime™ and a decompressor are needed to see this picture. Neurotransmitter stimulation of post-synaptic membrane Direct stimulation second messenger Anaesthetics Reduce pain sensation by the following Blocks initiation or conduction of nerve impulse Inhibit sodium channel opening Antidepressants Depression - deficit of norepinephrine, dopamine, serotonin Possible causes: Neurotransmitters are not present in the synaptic cleft in enough quantity Neurotransmitters are receycled too quickly Neurotransmitters are not produced by the ribosomesin enough quantitiy Drugs inhibit the enzymes that inactivate neurotransmitters Inhibit reuptake of neurotransmitters Cycle of life Nerve tissue development Begins in fetal ectoderm Learning - formation of new synapses Aging - degeneration