Lecture 2 Brain: A closer look at neurons and the cortex Flashcards
Neuronal signalling
Neurons receive, transform and transmit information
Role of a single neuron
Receive input information from other neurons (convergence)- Integrates/associated this input information to produce an output
- Sends this integrated output information to many other neurons (divergence)
Information = signal = variation in:
- The electrical membrane potential/voltage
- Post-synaptic potential
- Action potential
- The quantity of certain chemical molecules (neurotransmitters) released at the synapse
Electrical neuronal signalling within a neuron: 3 transmembrane properties
- Na+/K+ ion pump: always actively pumping ions in and out of the cell
- Na+ and K+ voltage-gated ion channels
- Let either Na+ or K+ ions through when open
- Closed at rest
- Open/close depending on the value of the electrical membrane potential
Electrical neuronal signalling within a neuron: Resting membrane potential
Na+/ K+ ion pump creates different concentrations of ions on each side of the neuron’s cell membrane
More positive ions in the extracellular space
Resting membrane potential is -70 mV
Voltage-gated ion channels are closed below -55 mV
Changes in the value of the membrane potential will carry information
When does the membrane potential change from rest ?
When a post-synaptic neuron receives input from other pre-synaptic neurons at dendrites/synapses
What happens if the pre-synaptic input makes the post-synaptic cellular membrane Less negative/more positive input
Depolarisation
Excitatory (PSP)
What happens if the pre-synaptic input makes the post-synaptic cellular membrane more negative/less positive than -70mV
hyperpolarisation
Inhibitory (PSP)
What happens if there is more excitatory than inhibitory input on the neuron’s membrane
Membrane depolarises overall
When will a neuron fire
If depolarisation reaches -55 mV threshold
Action potential definition
Fast depolarisation, repolarisation and hyperpolarisation of the neuron’s membrane
What happens in the cell membrane when a -55mV depolarisation threshold is reached
Voltage gated ion channels open
Na+ channels open faster
K+ channels open more slowly
Na+ ions first enter the neuron
Depolarization (membrane positivity increases up to +40 mV
K+ ions then exit the neuron
Repolarisation
From +40 mV to 70 mV back to resting state
Hyperpolarisation
below -70mV
Evolution of the membrane potential during an action potential
1.Na+ ion channels open (depolarisation above -55 mV)
2.K+ ion channels open (depolarisation slows down and peaks at 40 mV)
3.Na+ ion channels close (repolarisation, hyperpolarisation)
4.K+ ion channels close (back to resting membrane potential, thanks to ion pumps)
Time for full action potential process
About 2ms
AP propagation along the axon
Strong depolarisation at location A on the membrane spreads to next location B
-55 mV threshold is reached at location B, triggering AP
so on at the other locations along the axon
AP does not go back to location AP reaches axon terminal and becomes the input for the next neuron
Myelination
Myelin sheaths are protrusions from oligodendrocytes, a type of glial cell
Insulates axons and speeds up the propagation of APs
Aps can jump from one nod to the next- Speeds up AP propagation by 10 or 100-fold
-Only long axon are usually myelinated
-Myelination concentration is high in white matter
Key properties of APs
All or nothing phenomenon
Self-propagation
Unidirectionally
Does not dissipate
Chemical communication between neurons at the synapse: Action potential arriving at the pre-synaptic axon terminal riggers
Depolarisation of the axon terminal
Release of neurotransmitter molecules into the synaptic cleft
Depolarization or hyperpolarization of the membrane on the postsynaptic dendrite (excitatory or inhibitory PSP)
Post-synaptic potential (PSP)
Neurotransmitter receptors on post-synaptic neuronal membrane are linked to ion channels.
Post-synaptic potential (PSP) Neurotransmitter binding on the receptor opens the ion channel, which either:
Hyperpolarizes the membrane potential - inhibitory post synaptic potential
Depolarises the membrane potential - excitatory post-synaptic potential
Properties of PSPs
They are near the post-synaptic dendrite
They dissipate
They are smaller in amplitude than APs
Many EPSPs must combine for the resting membrane potential to reach the AP threshold
From PSPs to action potentials
Summation of PSPs occur at the neuron’s hillock
If enough EPSPs sum together to reach threshold, an AP is generated
If too many inhibitory PSPs sum together, prevent AP in post-synaptic neuron
