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University of California - Irvine
Biological Sciences
Peter A.Bowler

e109 10/17/12 (Wedn before midterm) Myelin sheath-fast conduction Fig 8.16: what happens in some axons that have a myelin sheath -don’t need to memorize all the glial cells and what they do but know what they do -know the glial cells highlighted in Friday online lecture -myelin sheath is not continuous over the entire axon; areas without myelin are called nodes of Ranvier -axons with myelin sheath have a much faster axon potential -1. voltage gated sodium channels open, Na+ floods in (know all the steps of theAP) +this is where the action potential takes place -myelin sheath is electrically insulating so that there is no ion movement across the membrane at that spot (passive spread or a graded potential) AP -graded potential action potential again (occurs in the Nodes of Ranvier) -Without myelin sheaths, there are leaks of the ions, which slows conduction and does not lead to normal/any conduction of the action potential; electrical disturbance is not propagated as well -Sufficient depolarization doesn’t occur in the absence of insulation (myelin sheaths) so action potential is not generated after degenerated myelin sheath area. -action potentials ONLY occur at the Nodes of Ranvier, NOT at the myelin sheaths Synapse—region where neuron meets target cell (neuron, muscle cell, or glial cell) Two types of synapses: 1. Electrical (gap junction): particular type of gap junction a. if two cells have cytoplasmic continuity, any kind of electrical disturbance in one cell will be felt in the other cell b. communication is in either direction 2. Chemical synapse a. one way transmission of neurotransmitters across the synapse b. most common type of synapse c. presynaptic cells send the message and postsynaptic cells receive the message Chemical synapse 1. An action potential arrives from the axon to the axon terminal depolarizes the axon terminal 2. The depolarization opens voltage gated Ca2+ channels and Ca2+ enters the cell (intracellular Ca2+ is usually very low, so Ca2+ rushes in and is a very common secondary messenger inside the cell) 3. Calcium entry causes exocytosis of synaptic vesicle contents (neurotransmitter a. (can’t package stereoids: side note. So this is not a lipid) b. contents dumped in synaptic cleft 4. Neurotransmitter diffuses across synaptic cleft and binds with the receptors of the postsynaptic cell 5. Binding of neurotransmitters initiates a response in the postsynaptic cell 6. Neurotransmitters are taken up by cells, broken down, or diffuse away a. this step is very important because you are getting rid of old information b. an enzyme degrades the neurotransmitters Know these two major Neurocrine chemicals from pg. 267: (know all parts of these two included antagonist, agonists) 1. Ach (Acetylcholine): a. made from choline and acetyl Coa (neurons that secreteAch and receptors that bindAch are “cholinergic”)—stored in synaptic vesicle and are secreted by exocytosis to the postsynaptic cell b. cholinergic describes Ach’s receptors c. Acetylcholinesterase (AChE) breaks down theAch after it binds to the Cholinergic receptor of the postsynaptic cell d. Type: ICR: (Na+,K+) and GPCR i. ICR = ion channel receptors (neurotransmitter binds to the ICR to open it, but the neurotransmitter does not come in) 2. NE (Norepinephrine: a. Adrenergic receptor Agonist: molecule that combines with receptor and mimic responses (example: nicotine is an agonist forAch) Antagonist: molecule that opposes action of another by binding to receptor (example: curare) ICR=ion channel receptors In the post-synaptic cell: *Na+ channel::: would cause excitatory depolarization=EPSP(excitory post-synaptic potential) *More K+ out or Cl- in:::: would cause inhibitory hyperpolarization = IPSP *Can cause a cascaded potential (don’t worry about) Neural circuits can diverge or converge In a divergent pathway, one presynaptic neuron branches to affect a larger number of postsynaptic neurons In a convergent pathway, many presynaptic neurons converge to influence a smaller number of postsynaptic neurons How does a post-synaptic cell integrate multiple inputs? (EPSPs and IPSPs) 1. Three excitatory neurons fire. Their graded potentials separately are all below threshold. 2. Graded potentials arrive
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