Physiology 3120 Lecture Notes - Classical Conditioning, Sound Intensity, Protein Folding

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Neurophysiology Dr. Hore
Neuro I – Introduction
Anatomical Features
1011 neurons  1015 synaptic connections
CNS = brain & spinal cord
PNS = nerve
Types of brain cells
oNeurons
Information processing cells
Fire action potentials
Make synaptic connections
oGlial cells
Provide support – helper cells
Major function: contribute to the well-being of neurons
Remove neurotransmitters
Removing ammonia (by-product of metabolism)
Supply glucose to
neurons
Taking up excess
extracellular K+
Provide myelin sheaths
for axons
10 times more glial cells than
neurons
Neuron
Synapses
Chemical Synapse
oMost common
oHigh electrical resistance between
pre- & post-synaptic cells
oElectrical currents ahead of action
potentials do not cross synapse
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oSignal which propagates electrically down is conveyed across synapse by neurotransmitters
oNeurotransmitter attaches to receptor on postsynaptic neuron membrane
oSlower than electrical synapses
Electrical Synapse
oNeurons very close together
oCytoplasmic continuity via gap junctions
oLow resistance to electrical signal
oElectrical currents travel across synapse
oNo delay – very rapid
oBidirectional (action potential travelling from post
to pre)
Synaptic plasticity
oStrength of synapses can be altered
Equilibrium Potentials for Representative Neurons
Representative – not exact
Example neuron:
oRMP = - 70 mV
oNa+ = + 60 mV
oK+ = - 90 mV
oCl- = - 80 mV
oCa++ = + 120 mV
Neurons at rest
oStrongly permeable to K+
oRMP is not -90
oSome Na+ leaks in  bring in +ve charge  depolarize bringing it to -70 mV
What would happen if the
membrane became suddenly
permeable to:
oOnly Cl- – membrane
potential would go to -80
mV
oOnly K+ – membrane
potential would go to -90
mV
oOnly Cl- & K+ – membrane
potential would go to a
value between equilibrium
potentials of both ions i.e. approx. -85 mV
oOnly Na+ & K+ - somewhere between the two (experiments show value is 0 mV)
EPSP & IPSP
EPSP
oTransient (15 ms – doesn’t have enough time to reach 0 mV) increase in conductance to both
Na+ & K+ but Na+ dominates & cell depolarizes
Long change
oTakes membrane potential towards zero/threshold
If it was longer it would reach 0
Most of the time does not reach threshold – sub-threshold (if reaches threshold  AP)
oAt single synapse is < 1 mV in size
oTotal size depends on how many afferent axons were active
oOpens channel where both Na+/K+ travel – Na+ in & K+ out
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oNa+ rushes in due to chemical & electrical gradient  depolarization  +ve charge is brought in
IPSP
oTransient (15 ms) increase in conductance to Cl-, K+ or both
oProduce hyperpolarization  inhibits neuron
oIn some neurons ECl- may be close to -70 mV
oCl- mediated IPSP may be very near RMP
oEven if IPSP does not hyperpolarize it suppresses action potentials because it clamps the cell at
IPSP equilibrium potential (far from AP threshold)
Produced by different transmitters
Transmission at CNS Synapse vs. Transmission at NMJ
There are a variety of transmitters in CNS vs. Ach is only used at NMJ
Size of ESPS
oEPSP at single central synapse is < 1 mV in size
oNeed for 50-100 incoming Aps to produce EPSP that summate before membrane is depolarized
to threshold level & AP is triggered
oAt NMJ – one AP in α-MN produces an
end plate potential that always
reaches threshold
Central synapses can be excitatory or
inhibitory vs. NMJ which is only excitation
Main Signal Flow within Neuron & Action
Potential
Signal flow through neuron involves receiving
chemical signals at synapses by chemical
transmission
Excitatory synapses
oCauses Na+ to enter bringing in +ve
charge
oCauses current to flow to axon hillock because this region is -ve with respect to excited region
oAxon hillock is region of low electrical resistance compared to axon & has a high density of V.G.
Na+ channels
oIf membrane potential is depolarized sufficiently at axon hillock – large # of V.G. Na+ channels
will be open & action potential will be initiated at axon hillock & propagate down axon
Inhibitory synapses
oPositive current flow in the opposite direction – away from axon hillock
Electrical current is flow of positive charge (not electrons)
In excitable cells (neurons & muscle fibres)
current flow is produced by shuffling of +ve
positive ions in one directions & -ve ions in
opposite direction
Neuro II: Synaptic Transmission
Sequence of Synaptic Transmission
Presynaptic neurons synthesize
neurotransmitters which are stored in vesicles
Presynaptic depolarization by action potential
activates Ca2+ influx at V.G. Ca2+ channels
Ca2+ causes exocytosis & release of
neurotransmitter
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