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Lecture 9

BIO304H5 Lecture Notes - Lecture 9: Cardiac Muscle, Pacemaker Current, Depolarization

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Ian Orchard

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Lecture :
Depolarizing afterpotential
- No hyperpolarization because competing sodium current for
longer period of time
- Time delay
Non-spiking neurons
- Not all neurons produce AP
- Transmit signals just in the form of graded potentials
- Signals degrade over length
oShorter axon helps prevent loss
oTherefore non-spiking neurons are smaller: otherwise they
would loose current as it leaked out the membrane
Graded Potentials
- Produced in result to
oChemical, photo (light)
oReceptor protein ion channels are activated by change in
- Can be depolarizing or hyperpolarizing
- If it depolarizes neuron = excitatory event
- If it hyperpolarizes neuron = inhibitory event
Characteristics of Graded Potential
- Cant carry signal over long distance cause current is lost
- Its graded: magnitude and duration is proportional to strength of
oAction potentials are all or none
Stronger threshold stimulus produces stronger
frequency of APs
Subthreshold stimulus will not produce any APs
Electrotonic Spread
- Graded potential decreases in magnitude as you move away
from source of stimulus
- Graded potential has to be strong enough to exceed threshold
Axon Hillock
- Site of integration
- High density of VG ion channels
oCell body = 1 Na VG channel/ mm2
oAxon Hillock = 100-200 VG Na channel/ mm2
- If graded potential exceeds threshold then AP will proceed down
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