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1H06 Anatomy & Physiology 1H06: Neurotransmission.docx

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Health Sciences
Alexander Ball

Neurotransmission Continuous conduction  Depolariztion and repolarization of each adjacent segment of plasma membrane  Unmyelinated axons and muscle fibers exhibit continuous conduction  Action potentials travel faster along myelinated axons than unmyelinated axons Saltatory conduction  Uneven distribution of voltage-gated channels in membrane.  Action potentail at first node generates ionic currents in cytosol and extracellular fluid that depolarize membrane to threshold, opening voltage- gated Na+ channels at second node  Action potential leaps from node to node as each nodal area depolarizes to threshold  More energy-efficient mode of conduction Factors that Affect Speed of Propagation 1. Amount of myelination 2. Axon diameter a. Larger diameter axons propagate action potentials faster than smaller ones because of increased surface area 3. Temperature a. Action potentials travel faster when heated. Classification of Nerve Fibers 1. A Fibers a. Largest-diameter b. Myelinated c. Brief absolute refractory period and conduct from 12 to 130m/sec d. A Fibers’ axons propagate impulses associated with touch, pressure position of joints, and some thermal and pain sensations 2. B fibers a. Myelinated, conduct up to 15m/sec b. Longer absolute refractory period than A fibers c. Conduct sensory nerve impulses from viscera to brain and spinal cord 3. C fibers a. Smallest-diameter axons b. All unmyelinated c. Conducts from 0.5 to 2m/sec d. Longest absolute refractory period e. Conduct some sensory impulses for pain, touch, pressure, and heat Presynaptic neuron – nerve cell that carries nerve impulse towards synapse Postsynaptic cell – cell that receives nerve impulse signal Postsynaptic neuron – carries nerve impulse away from synapse or an effector cell that responds to impulse at synapse Most synapses are axodendritic (from axon to dendrite), axosomatic (from axon to cell boy), or axoaxonic (from axon to axon). They may also be chemical or electrical and differ both structurally and functionally. At electrical synapse, action potentials conduct directly between plasma membranes of adjacent neurons through gap junctions. Electrical synapses have two main advantages: 1. Faster communication a. Action potentials conduct directly through gap junctions b. At electrical synapse, action potential passes directly from presynaptic cell to post-synaptic cell 2. Synchronization a. Large number of neurons or muscle fibers can produce action potentials in unison if connected by gap junctions Excitatory postsynaptic potential: depolarizing postsynaptic potential Inhibitory postsynaptic potential: hyperpolarizing postsynaptic potential Two receptors: http://www.youtube.com/watch?v=8jPH2pKzIDY Metabotropic receptor – neurotransmitter receptor containing neurotransmitter binding site but lacks an ion channel as part of its structure  Coupled to separate ion channel by G protein, which tiehr directly opens or closes the ion channel or may act indirectly by activating another molecule (second messenger) in cytosol, which opens or closes the ion channel Neurotransmitters removed by: 1. Diffusion a. Some of the released neurotransmitter molecules diffuse away from the synaptic cleft 2. Enzymatic degradation a. Certain neurotransmitters are inactivated through enzymatic degradation 3. Uptake by cells a. Many neurotransmitters are actively transported back into the neuron that r
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