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Lecture 6

Lecture 6 - Synaptic Communication.docx

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Psychology 1000

Psychology 1000 Synaptic Communication September 25, 2012 Next Time: The Brain Scan: pg. 80-90  How do neurons produce an electrical signal?  How does a neuron code intensity?  How does one neuron communicate with another?  Caffeine mimics adenosine  Caffeine locks into the adenosine receptors, and instead of feeling sleepy, you feel wide awake  Not just caffeine -> all drugs work like this  They resemble neurotransmitters that are supposed to do one thing, but the drugs do another and change the entire neural system  The destruction of the myelin sheath will most likely result in slower neural conduction  Most neurons in the brain are multipolar  How do neurons work?  Cortex is packed with nerve cells  2/3 of neurons work here  Each neuron has a job to communicate with other neurons  Networks are formed amongst these cells  Neurons use communication lines to communicate with each other with electric and chemical signals  Neurons look like they fuse together, but they don’t actually touch one another  A tiny gap between neurons called a synapse  Neurons have receptor sites that bond with the molecules, and when they do, special gates open which allow the flow of K+ and Na+ through them  100 billion neurons in the brain  How does a cell actually produce an electrical signal?  Neuron is a semi-porous bag of fluid  Cell membrane is semipermeable – lets some stuff flow in and out of the cell  Inside the cell, there are ions (particles that have gained/lost electrons; +/- charged)  Na+, K+, Cl-, A- inside the cell  Na+, K+ and Cl- in the extracellular fluid; a lot of positively charged sodium outside  Due to the ionic concentration, there is a charge on the cell membrane  Positive charge on the outside, negative charge on the inside, kind of like a battery  The Action Potential depolarization  Depolarization is the result of sodium coming into the neuron (Na+ Inflow)  As the neuron is stimulated, the Na+ continues to flow Psychology 1000 Synaptic Communication September 25, 2012  If it reaches -55 mV (threshold), all hell breaks loose  If threshold is not reached, nothing happen and it goes back to -70mV  Once the threshold is reached, the charge shoots up past 0 up until +40 mV  Neuron must go back down to resting potential: repolarization  Repolarization is caused by potassium outflow (K+ ions leaving the neuron)  The act of kicking K+ out generates the inflow of Na+ to the areas adjacent to it, creating a new action potential down the axon  More and more potassium leaves until it drops below resting potential (under -70mV)  Kicked out too much potassium, it is now under -70mV  But eventually, it will drift back up and reach its resting potential  Whenever the neuron is below its resting potential, it is known as hyperpolarization  Entire process takes about 5-7 milliseconds; very quick, but not instantaneous  During repolarization: top of when it hit +40mV to when it reached the bottom of the trough: this is called the absolute refractory period  During the absolute refractory period, the neuron will not respond at all to ANY stimulation  Relative refractory period is when the neuron is below -70mV. It can still be stimulated, but it will take a stimulus stronger than normal to get the neuron to do anything  Neural Communication  Action potentials only occurs in axons: not in any other cells  Generated in the axon hillock  Any change in the electrical activity in cell body and dendrites is called the graded potential  Coding Intensity  Neuron fires on all-or-none fashion; either an action potential is generated at the hillock or it isn’t  Height of “spike” is always fixed regardless of the stimulus  So how is intensity coded??  Neurons have different thresholds  Number of neurons that fire -> stronger stimulus = more neurons  The frequency between action potentials; the frequency of firing = number 1 method of coding intensity  The intensity of a stimulus is directly proportional to the frequency of the firing of a neuron  A neuron fires more rapidly with a more intense stimulus  If stimulus intensity is increased, there will be more neurons being fired  There is a point wher
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