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The Nervous System Introduction • The nervous system is the communication and control system of an animal’s body. • Function: • _____________ (senses changes within or outside the body and sends info to the spinal cord and brain) • _____________ (brain and spinal cord receive, analyze, store, and integrate the info to produce a response) • _____________ (an instruction of the body to do something) • Composed of two main divisions: • _______________ Nervous System (CNS) - brain and spinal cord • _______________ Nervous System (PNS) - nerves that arise from the CNS and innervate rest of body Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. • Neuron: basic functional units of nervous system • High requirement for ____________. • Cannot ___________ but can ________________ if the cell body remains intact • Neuroglia (AKA glial cells): provide structural/functional support and protection to neurons Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. • Soma • Central cell body • Cell processes • _____________ – receive stimuli/impulses and transmit the information ___________ soma. • Short and branched • _____________ - conducts nerve impulses ________ from soma towards another neuron or an effector cell (muscle, gland) • Long, single process Neuron Myelin • Axons are often covered in myelin • Nervous tissue containing myelinated axons is called ___________ matter. • Myelin: cell membrane of glial cells tightly wrapped around the axon • _____________________ are glial cells of the brain and spinal cord • ____________ ______ are glial cells of the peripheral nerves • Myelinated axons conduct impulses faster than unmyelinated ones. Nodes of Ranvier • Multiple Schwann cells or oligodendrocytes cover the entire length of the axon • Nodes of Ranvier are ______ in the myelin sheath between adjacent glial cells • Myelin sheath and nodes of Ranvier work together to enhance the ___________ of conduction of nerve impulses along the axon. Organization of Nervous System 1. Anatomical • ____________ nervous system (CNS) • Brain and spinal cord • ____________ nervous system (PNS) • Nerves extending outward from the central axis toward the periphery of the body • Cranial nerves originate directly from the brain • Spinal nerves emerge from the spinal cord 2. Direction of Impulses • _____________ nerves - conduct impulses TOWARD CNS • a.k.a. sensory nerves - conduct sensations from sensory receptors in skin and other locations to CNS • ______________ nerves conduct impulses AWAY from CNS • a.k.a. motor nerves – cause muscle contraction/movement or glandular secretion • Some nerve fibers are sensory (optic), motor (oculomotor), or Organization of Nervous System Organization of Nervous System 3. Function: Somatic vs. Autonomic • _________________ nervous system - actions under conscious, or voluntary control • Motor nerves lead to skeletal muscle and cause limb or body movement • Example: turning your head when your name is called • _________________ nervous system - controls and coordinates automatic functions • Motor nerves lead to smooth muscle, cardiac muscle, and glands • Example: increasing of the heart rate after a car crash, stomach releasing HCl when food is present • ______________________ Division (Stressed state of mind: fight/flight) • ______________________ Division (Relaxed state of mind: feed/breed rest/digest) Neuron Function: DEPOLARIZATION AND REPOLARIZATION • Resting __________ - when neuron is not being stimulated to transmit information • But, the cell membrane is not truly at rest (Na/K pump is very active) • Resting __________ ____________- difference in electrical charge across neuronal membrane • Due to differences in distribution of positive and negative charges from sodium, potassium, and other ions on either side of neuronal membrane • Resting membrane potential is a negative number (-70 mV), indicating the negative charge inside the cell Neuron Function • Na+/K+ pump: specialized molecules located in the neuron’s cell membrane that maintain the resting membrane potential. • Pumps ____ (Na+) out of neuron • Pumps ____(K+) into neuron Cell membrane becomes ________________ Depolarization STEPS: • Neuron receives external stimulus • ______ channel opens on cell membrane • Na+ flow into cell by passive _____________ • Down the concentration gradient • Electrical gradient (opposites attract) Depolarization STEPS: • During depolarization, inside of neuron goes from being ____________ to _____________ charged due to influx of Na+ ions • This change in electrical charge is also known as the ______________ POTENTIAL Neuron Function: Depolarization STEPS: • Na+ channels close • ______ channels open • K+ diffuses out of cell Repolarization • Electrical and concentration gradient, just like Na • Resting membrane potential restored • Cell is ___________________ • But, Na+ is inside, K+ outside Repolarization AFTER Repolarization • Na+/K+ pump moves ions back to their original sides • Resting state restored • Stimulus must be sufficient to exceed threshold and make the neuron respond. Will cause complete depolarization. __________ stimuli do not cause depolarization. • “_________________________ principle” - neuron depolarizes to its maximum strength or not at all • Strong stimuli cause numerous sodium channels open, which creates a WAVE OF DEPOLARIZATION • Conduction of the Action Potential —spreading wave of opening Na+ channels in sufficient numbers to allow sodium influx and depolarization • Wave of Depolarization = Conduction of Action Potential = Nerve Impulse Threshold Stimulus Neuron Function: Action Potential Propagation • Time period when sensitivity of neuron is reduced —Cell is still in depolarization/early repolarization • _____________ refractory period - during Na+ influx and K+ outflow • No amount of stimulus can cause depolarization • ____________ refractory period - during end of repolarization period • Strong stimulus can cause depolarization Refractory Period Saltatory Conduction • RAPID means of conducting an action potential (more rapid than in unmyelinated axons) • Depolarization in myelinated axons can only take place at _______________________________ • Myelin sheath prevents sodium ions from entering the cell. • The depolarization wave is able to skip from one NOR to the next, accelerating the rate at which the wave moves from the cell body to the end of the axon Local Anesthetics • Work by blocking the propagation of the action potential • Blocks _______ channels • Sodium cannot flow into the cell, so threshold is not achieved • Example: Lidocaine • ____________ – junction between two neurons or a neuron and target cell • ___________ ________– space between adjacent neurons/effector cell • Presynaptic neuron - neuron bringing the depolarization wave to the synapse • Releases _______________________ • Postsynaptic neuron - contains receptors for the neurotransmitter Synaptic Transmission Neurons: synapses Synaptic Transmission • The axon of the presynaptic neuron ends as a branched structure (_____________________). • Synaptic knob- slightly enlarged bulb at end of axon • Vesicles contain neurotransmitter (a chemical messenger) • When depolarization wave reaches axon terminal, calcium channels open and cause vesicles to fuse with cellular membrane and release neurotransmitter into the synapse Synaptic Transmission • Neurotransmitters diffuse across synaptic cleft toward postsynaptic membrane • Receptors on postsynaptic membrane bind neurotransmitter • Receptors are VERY specific for each neurotransmitter (similar to a lock and key) Types of Neurotransmitters • ______________ neurotransmitters (acetylcholine, norepinephrine, dopamine) • Usually cause an influx of Na+; postsynaptic membrane moves toward threshold (more positive) • _______________ neurotransmitters (acetylcholine, norepinephrine, dopamine, GABA, glycine) • Move the charge of postsynaptic cell farther away from threshold (more negative) • May open K+ channels/Cl- channels Recycling the Neurotransmitter • Acetylcholinesterase - found on postsynaptic membrane; breaks down acetylcholine • Monoamine oxidase (MAO) - breaks down reabsorbed epi/norepinephrine in the synaptic knob • Catechol-O-methyl transferase (COMT) breaks down epi/norepinephrine that is not reabsorbed