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Neurons, Neurons, Neurons! Mrs. Hartley Anatomy and Physiology Remember:Divisions of Nervous System Nervous System Peripheral Nervous System Autonomic Cardiac and smooth muscle And Glands Parasympathetic “rest and digest” Sympathetic “fight or flight” Central Nervous System Receives sensory input and sends motor output Brain and Spinal Cord Peripheral Nervous System Somatic Sensory Input from Face, ears, nose, eyes Motor output from CNS To skin and skeletal muscle Neurons • Primary structural and functional unit of nerve tissue • Responsive cells that conduct impulses at great speeds • Cell body, axons, dendrites, myelin sheaths, synaptic terminals • Direction of impulse: dendrites to cell body to axon to axon terminal to synaptic end bulb Cell Body: cytoplasm enveloped by plasma membrane • Prominent nucleus/nucleolus • Many functional organelles (mitochondria, Golgi, etc.) Axon • Conducts impulses away from the cell body • Terminal end communicates with the next cell Dendrites • Thin, branching extensions of cell body • Receive impulses from adjacent neurons • Uni-directional: only towards cell body Myelin Sheath (pink) • White, fatty insulating barrier • Aids in nerve impulse conduction What Goes Wrong in MS? When myelin is damaged, dense, scar-like tissue forms around nerve fibers throughout the brain and spinal cord. These scars, sometimes referred to as sclerosis, plaques, or lesions, can slow down or completely prevent the transmission of signals between nerve cells. Messages from the brain and spinal cord cannot reach other parts of the body. Damage, or scarring, occurs in many places throughout the central nervous system, hence the term "Multiple Sclerosis." Nodes of Ranvier • Gaps in myelin sheath • Aid in impulse conduction Information from one neuron flows to another neuron across a synapse. The synapse is a small gap separating neurons. The synapse consists of: 1. a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles, 2. a postsynaptic ending that contains receptor sites for neurotransmitters and, 3. a synaptic cleft or space between the presynaptic and postsynaptic endings. http://www.blackwellpublishing.com/matthews/nmj.html So…How Are Impulses Created? • Excitability: neurons have the ability to respond when a stimulus becomes great enough to alter the resting membrane potential of a particular region of a membrane – Uneven distribution of ions (electrically charged particles) of sodium and potassium causing an electrical gradient Depolarization and Repolarization • Rapid change in membrane’s permeability to sodium ions causing the electrical charge inside the cell to become more +positive • Soon after, resting membrane potential is restored • Potassium flows out of the cell and sodium stops flowing in http://www.blackwellpublishing.com/matthews/channel.html Action Potential • Depolarization + rapid repolarization results in an action potential or nerve impulse • Rapid at 1/1000s and in rapid succession Myelinated vs. Unmyelinated • Unmyelinated: impulse continues along the length of the neuron unimpeded • 10 m/s • Myelinated • Nodes of Ranvier • Myelin disrupts impulse…saltatory conduction….impuls e skips, quickening the impulse • 130 m/s http://www.brainviews.com/abFiles/AniSalt.htm http://www.blackwellpublishing.com/matthews/actionp.html So Let’s Play a Game! • “Saltatory Conduction” • “All or None Game” • All or None response: if stimulus is large enough, impulse will be conducted along the entire length of the neuron