PSYC 2410 Chapter Notes - Chapter 4: Resting Potential, Axon Hillock, Axon Terminal

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19 Apr 2016
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Sunday, March 13, 2016
Chapter 4
Biopsych
Introduction:
The Lizard, a case of Parkinson's Disease:
-Roberto Garcia d’Orta a tall thin man in his 60’s suffering from Parkinson’s
-disease makes him appear much older
-had a symptom called “tremor-at-rest” which caused his hands to shake worse when
they weren't doing anything at all
-other symptoms of parkinson’s disease are not quite so benign, they can change a
vigorous man into a lizard
-Other symptoms of Parkinson’s include:
rigid muscles
lack of spontaneous movements
difficulty in starting to move
slowness in executing voluntary movements once they have been initiated
lack of blinking and widely opened eyes (“reptilian stare”)
-What happened to d’Orta’s brain?
a small group of nerve cells called the substantia nigra (black substance) were
unaccountably dying
substantia nigra neurons make the chemical dopamine, which they deliver to
another part of the brain called the striatum
as the cells of the substantia nigra die, the amount of dopamine they can deliver
diminishes
striatum = helps to control movement, but to do this normally it needs dopamine
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Sunday, March 13, 2016
-dopamine is not an effective treatment for Parkinson’s patients because it is unable to
penetrate the blood brain barrier
-knowledge of dopaminergic transmission has led to the development of an effective
treatment: L-Dopa
L-Dopa is the chemical precursor of dopamine, which readily penetrates the blood
brain barrier and is converted into dopamine once inside the brain
4.1 Resting Membrane Potential
Recording the Membrane potential:
-membrane potential = the difference in electrical charge between the inside and
outside of a cell
-to record a neurons membrane potential it is important to position the tip of one
electrode inside the neuron and the tip of another electrode outside of the neuron in
the extracellular fluid
-micro-electrodes = the intracellular electrodes; their tips are less than one-thousandth
of a mm in diameter
-when both tips are in the extracellular fluid, the voltage difference between them is
zero
-when the tip os the intracellular electrode is inserted into a neuron a steady potential
of about —70 millivolts(mV) is recorded
this indicates that the potential inside the resting neuron is about 70 mV less than
that outside the neuron
-neuron resting potential = the steady membrane potential of about —70 mV
also known as the state of being “polarized”
Ionic Basis of the Resting Potential:
-ions = positively and negatively charged particles
-salts in neural tissues separate into ions
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Sunday, March 13, 2016
-the plus signs indicate that each Na+ and K+ ion carries a single positive charge
-in resting neurons (—70 mV) there are more Na+ ions outside the cell than inside,
and more K+ ions inside than outside
-ion channels = specialized pores in neural membranes through which ions can pass
each type of ion channel is specialized for the passage of particular ions
-There are two types of substantial pressure on Na+ ions to enter the resting neurons:
electrostatic pressure: pressure from the resting membrane potential — because
opposite charges attract, the —70 mV charge attracts the positively charged Na+
ions into testing neurones
pressure from random motion: pressure for Na+ ions to move down their
concentration gradient — the ions in neural tissue are in constant random motion,
and particles in random motion tend to become evenly distributed because they are
more likely to move down their concentration gradient than up them; that is, they
are more likely to move from areas of high concentration to areas of low
concentration
-Alan Hodgkin and Andrew Huxley:
interested in the stability of the resting membrane
potential
some Na+ ions manage to enter resting neurons despite
closed sodium channels and some K+ ions exit
Why does the resting membrane potential stay fixed if
some Na+ enter and some K+ exit?
-at the same rate that Na+ ions are leaked into resting
neurons, other Na+ ions are actively transported out
-at the same rate that K+ ions are leaked out of resting
neurons, other K+ ions are actively transported in
-sodium-potassium pump = ion transport mechanism in
the cell membrane that continually exchanges 3 Na+
ions inside the neuron for 2 K+ ions outside
-SODIUM POTASSIUM PUMP KEEPS RESTING
POTENTIAL CONSTANT (—70 mV)
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