PSY2061 Chapter Notes - Chapter 4: Cannabinoid, Antidromic, Reuptake
5 Jun 2018
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PSY2061 – Readings – Week 4 – Neural Conduction and Synaptic Transmission
• resting membrane potential
•
o the membrane potential is the difference in electrical charge between the inside
and the outside of a cell
o recording the membrane potential
o
▪ to record a neuron’s membrane potential, it is necessary to position the
tip of one electrode inside the neuron and the tip of another electrode
outside the neutron in the extracellular fluid
▪ intracellular electrodes are called micro electrodes
▪ the steady membrane potential of about -70mV is called the neuron’s
resting potential
o ionic basis of the resting potential
o
▪ the salts in neural tissue separate into positively and negatively charged
particles called ions
▪ focus on sodium ions Na+ and potassium ions K+
▪ in resting neurons there are more Na+ ions outside the cell than inside
and more K+ ions inside than outside
▪
▪ these unequal distributions are maintained even though there
are specialised pores called ion channels in neural membranes
through which ions can pass
▪ there is substantial pressure on Na+ ions to enter the resting neurons -
two types of pressure
▪
▪ electrostatic pressure - from the resting potential
membrane
▪
▪ because opposite charges attract -
the -70mV charge attracts the
positively charged Na+ ions into
resting neurons
▪ random motion
▪
▪ pressure from random motion for Na+
ions to move down their concentration
gradient
▪ why the resting potential remains
▪
▪ The sodium ion channels in resting neurons
are closed, thus greatly reducing the flow of
Na+ ions into the neuron. In contrast, the
potassium channels are open in resting
neurons, but only a few K+ ions exit
because they are largely held inside by the
negative resting membrane potential.
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▪ At the same rate that Na+ ions leaked into
resting neurons, other Na+ ions were
actively transported out; and at the same
rate that K+ ions leaked out of resting
neurons, other K+ ions were actively
transported in. Such ion transport is
performed by mechanisms in the cell
membrane that continually exchange three
Na+ ions inside the neuron for two K+ ions
outside. These transporters are commonly
referred to as sodium–potassium pumps.
• generation, conduction and integration of postsynaptic potentials
•
o generation and condition of postsynaptic potentials
o
▪ when neurons fire they release from their terminal buttons chemicals
called neurotransmitters which diffuse across the synaptic clefts and
interact with specialised receptor molecules
▪ when neurotransmitter molecules bind to postsynaptic receptors -
they have one of two effects - depending on the neurotransmitter and
postsynaptic neuron
▪
▪ depolarise - the receptive membrane - decrease the
resting potential
▪ hyperpolarise - increase the resting membrane potential
▪ postsynaptic depolarisations are called excitatory postsynaptic
potentials EPSPs
▪
▪ they increase the likelihood that the neuron will fire
▪ postsynaptic hyperpolarisations are called inhibitory postsynaptic
potentials IPSPs
▪
▪ decrease the likelihood that the neuron will fire
▪
▪ both
▪
▪ graded responses
▪
▪ the amplitudes are proportional to
the intensity of the signals that elicit
them
▪ travel passively
▪ transmission is
▪
▪ rapid
▪ decremental
▪
▪ decreases in amplitude as
they travel through
the neuron
o integration of postsynaptic potentials and generation of action potentials
o
▪ action potentials are generated in the adjacent section of the axon
called the axon initial segment
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▪ integration - adding or combing a number of individual signals into
over overall signal
▪ spatial summation
▪
▪ three possible combinations
▪ how local EPSPs that are produced simultaneously on different
parts of the receptive membrane sum to form a greater EPSP
▪ how simultaneous IPSPs sum to form a greater IPSP
▪ how simultaneous EPSPs and IPSPs sum to cancel each other
out
▪ temporal summation
▪
▪ how postsynaptic potentials produced in rapid succession at the
same synapse sum to form a greater signal
▪ firing of a neuron is an all or none event
• conduction of action potentials
•
o action potentials are produced and conducted along the axon through the action
of voltage-activated ion channels - ion channels that open or close in response to
changes in the levels of the membrane potential
o ionic basis of action potentials
o
▪ membrane potential of a neuron at rest is relatively constant - despite
high pressure acting to drive na+ ions into the cell - because the resting
membrane is relatively impermeable to na+ ions and because those few
that do pass in are pumped out
▪ there is a change when the membrane potential of an axon is depolarised
to the threshold of excitation by an EPSP
▪
▪ the voltage activated sodium channels in the axon membrane
open wide and na+ ions rise in suddenly the membrane potential
from -70 to +50 mV
▪ the rapid change in membrane potential associated with the
influx of Na+ ions then triggers the opening of voltage activated
potassium channels
▪ K+ ions are driven out of the cell through these channels marks
the end of the rising phase of the action potential and the
beginning of repolorisation - once achieved the potassium
channels gradually close
▪ because they close gradually - too many K+ ions flow out of the
neuron and it is left hyper polarised for a brief period of time
o refractory periods
o
▪ there is a brief period after the initiation of an action potential where it is
impossible to elicit a second
▪
▪ called the absolute refractory period
▪ followed by the relative refractory period
▪
▪ the period during which it is possible to fire the neuron again
but only be applying higher than normal levels of stimulation
▪ responsible for two important characteristics of neural activity
▪
find more resources at oneclass.com
find more resources at oneclass.com
