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Chapter 4

PSYC 2410 DE S12 Textbook Notes Chapter 4

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PSYC 2410
Elena Choleris

Chapter 4 Neural Conduction and Synaptic TransmissionThe Lizard A Case of Parkinsons DiseaseLike many patients with Parkinsons Disease Roberto looked much older than he actually wasUsed to be an active thriving business man but now is like granite and shuffles around when he movesThe first symptom had been tremors at resttremors that become worst when the muscle is relaxedOther symptoms include rigid muscles a marked poverty of spontaneous movements difficulty starting to move and slowness in executing voluntary movements once they have been initiatedTerm reptililan stare is often used to describe the characteristic face of Parkinsons diseaselack of blinking widely open eyes gazing out motionless faceCaused by a small group of neurons called the substantia nigra unaccountably dyingoThese neurons produce dopamine which helps control voluntary movementoWithout dopamine the brain begins to degrade in certain functions Although low dopamine levels are the cause of Parkinsons Disease administering dopamine directly is not a solution because dopamine does not readily cross the bloodbrain barrierA knowledge of dopaminergetic transmitters has led to the development of an effective treatmentoLdopa is a precursor to dopamine which does readily cross the blood brain barrieroLdopa is converted to dopamine once within the brain41 Resting Membrane PotentialMembrane Potential The difference in electrical charge between the inside and the outside of the cellRecording the Membrane PotentialoIn order to record the membrane potential of a cell the tip of one electrode must be positioned in the space outside the cell and the tip of another electrode must be placed inside a celloThe electrode which penetrates the cell membrane must be fine enough to pierce it without causing severe damageoMicroelectrodes Finetipped electrodes whose tips are less than onethousandth of a millimeter in diameter Microelectrodes are too small to be seen by the naked eyeResting Membrane PotentialoThe difference in potential in the extracellular fluid is 0oThe difference in potential between the extracellular fluid and the internal environment of a neuron is 70 mV The resting neuron has about 70mV less electric potential energy than the extracellular fluid outside the celloResting Potential The steady membrane potential of a neuron at rest usually about 70mVoWhen a neuron has a charge built up across its membrane in this manner it is said to be polarizedIonic Basis of the Resting PotentialoIons Positively or negatively charged particles An ion is formed when a particle gains or loses electrons and hence alters their electric charge A positive ion has lost electrons while a negative ion has gained electronsoSalts in solution naturally separate into positively and negatively charged ionsoHow charge is distributed evenly across the membrane can be explained by 2 reasonsRandom Motion Ions in neural tissue are in constant motion and particles in random motion tend to become evenly distributed because they tend to move from areas of dense concentration to areas of low concentrationElectrostatic Pressure Any accumulation of charges in one area tends to be dispersed by the repulsion among like charges in the vicinity and the attraction of opposite charges concentrated elsewhereoDespite homogenization no single class of ions is distributed equally on the two sides of the membraneoFour kinds of ions contribute significantly to the resting potential of the neuronSodium Ions NaConcentration is greater outside a resting neuron than insideSymbol developed from latin name for Sodium NatriumNa ions tend to be driven into the neurons by both the high concentration of Na ions outside the neuron and the negative internal resting potential of 70 mVThe membrane of the neuron is resistant to the passive diffusion of NaSodiumpotassium pumps are able to maintain a high external concentration of Na by pumping them out at the same rate they flow inPotassium Ions KConcentration is greater inside a resting neuron than outsideSymbol developed from latin name for Potassium KaliumK ions tend to move out of the neuron because of their high internal concentrationoThis tendency is partially offset since the inside of the neuron has a negative potential which attracts positively charged potassium ionsThe membrane of the neuron offers little passive resistance to the flow of K ions from the inside to the outside so they flow out of the cell body at a fairly substantial rateTo maintain the high concentration of K ions the sodiumpotassium pumps in the cell membrane pump K ions into the neuron at the same rate as they move outChlorine Ions ClConcentration is greater outside a resting neuron than insidePassage of Cl is not inhibitted by the neural membraneCl ions are actively forced out of neuron because of its negative charge of 70mVAs Cl ions accumulate in the area directly outside the cell they begin to move down their concetration gradient back into the neuronThe neuron is in equilibrium when the electrostatic pressure for Cl ions to move out of the neuron is equal to the tendency for them to move down the gradient back into the celloThe equilibrium point occurs at 70mVVarious negatively charged protein ionsMost negatively charged protein ions are synthesized in the neuron and remain thereoTwo properties are responsible for the uneven distribution of ions across the membraneOne property is passive and does not require energy the other process is active and does require energyPassive Property In resting neurons K and Cl ions pass readily through the neural membrane whereas Na ions have trouble passing through it and negatively charged proteins do not cross the resting membrane at all
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