Introduction to optometry foundation year at city university of london Introductory biological sciences module Lesson 8 action potentials . Everything you need to know about the action potentials in the course. With pictures and diagrams.
Neurons action potentials
Ionic composition of a resting neuron is negative inside the membrane and positive outside.
Is -70mV. More sodium outside cell and more potassium inside.
Selectively permeable membrane, which means K+ is able to flow out
through leakage channels, but Na+ struggles to flow in.
At rest, both chemical and electrostatic forces are trying to push Na+ back
into the cell, resisted by selectively membrane.
Nervous system uses change in membrane resting potential as a signal.
(e.g., an indication that something has happened [efferent signal], or
something should happen [afferent signal])
Depolarisation- less negative than -70mV
Hyperpolarisation- more negative than -70mV
Depolarisation occurs when
- Let Na+ flow back inside membrane
Leakage channels are permanently opened
Gated channels- proteins change shape in order to open and close
Can be opened and closed in three ways:
1. Voltage-gate- in response to electrical stimulus (key in action
potentials)
2. Ligand-gated in response to specific chemical binding to the
gate protein
3. Mechanically gated- mechanical deformation
Voltage gated ion channels
Activated by changes in electrical membrane potential near the
channel
3D structure formed by multiple transmembrane helices (groups of proteins) enclosing the
central pore
Action potential often raises the voltage so much that
the voltage crosses right through positive values
If enough channels open the process
becomes self-reinforcing (a positive
feedback loop)
- the more Na+ that enter, the
more voltage-gated Na+ channels
open so more Na+ enter
APs are an all-or-nothing phenomenon: either enough channels open to
overwhelm the pumps and trigger the feedback loop or they don’t.
Threshold is -55mV
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