Week 3-Brain Communication And Development Flashcards
What’s resting potential?
-The opening of ion channels makes the inside of
the neuron more negative than the outside ‘extracellular’ space.
-This is primarily due to the action of ion channels (leak and pump) which allow and maintain a graded difference in prevalence of charged ions from inside, to outside, the cell.
Typically:
* 3 Na+ ions out
* 2 K+ ions in
How can the typical resting membrane potential
inside the neuron (-70 mV) be influenced?
This can influenced by incoming signals from other cells which can either:
* Further polarise the cell (more negative) this inhibits the likelihood of an action potential occurring
* Else it can depolarise the cell (make it more
positive, i.e., towards neutral) which is excitatory
and increases the chance of the cell generating an
action potential.
* We call these events post-synaptic potentials
What are some features of postsynaptic
potentials?
- Post synaptic potentials travel across the
neuron almost instantaneously (rapid) - But as they travel they decrease in size (decremental)
How does depolarisation affect postsynaptic potentials?
- If the cell receives excitatory input it will
depolarise. - If the membrane potential at the Axon
Hillock (red square) achieves the threshold of excitation (commonly -50 to -55 mV) the cell will fire. - However, remember that the incoming signals can only travel short distances before they expire (decremental)
How does hyperpolarisation affect postsynaptic potentials?
- If the cell receives inhibitory input it will hyperpolarise to become even more negative.
-If the inside of the cell is more negative you’ll
need a bigger stimulus to reach the threshold.
-Therefore the cell is inhibited from firing by
hyperpolarisation.
How do signals integrate?
- The resting potential of post-synaptic cell is polarised (-70mV).
- Cells are usually contacted by many incoming PSPs.
- Each PSP could have excitatory or inhibitory influences (usually many of each), by depolarising or hyperpolarising the post-synaptic neurone (respectively).
- The effect of these PSP’s transmit across the neuron decrementally.
- The balance between excitatory and inhibitory input (the net effect) determines whether an action potential fires
- If the net effect transmitted to the axon hillock results in depolarisation to the threshold of excitation then an action potential will fire.
How are action potentials all or nothing?
-In some ways the firing of a neurone is like
the firing of a gun
-As the trigger is squeezed it gradually moves
back (depolarising potential)
-Until it causes the gun to fire (action potential)
Like a gunshot, an action potential is an all or none phenomena
-Squeezing the trigger harder won’t make the bullet travel faster or further
How are action potentials generated?
When the integration of inputs achieves the
threshold of excitation at the axon hillock, it
initiates the generation of an action potential
(AP).
The AP itself is described according to various
components or phases:
1. Depolarisation: Na+ channels open, influx of Na+ into cell.
2. K+ channels open, K+ begins to leave cell.
3. Peak : Na+ channels begin to close, K+ channels still open.
4. Repolarization: Na+ stops entering cell, K+ ions move out.
5. Hyperpolarization: K+ channels start to close but some K+ ions continue to move out of cell.
PSP communication already occurs, so why do we need action potentials?
-The axon of a motor neurone is up to 10,000 times as long as the cell body is wide.
-This means that you need a non-decremental way to send information long distances.
* AP are large swings to opposite polarity
* Non-decremental so able to carry the original signal for long distances
How are action potentials able to transverse large distances without losing the integrity of the signal? (the original large swing in polarity is maintained)
This is due to a cascading effect whereby the rapid depolarisation at the axon hillock leads to the achievement of the threshold of excitation in the next section of the axon and this continues down the length of the axon.
The speed of transmission is affected by what?
The anatomy of the axon
*Myelinated axons (up to 150 m/s)
*Non-myelinated axons (0.5-10 m/s)
How does the axon communicate with other parts of the neuron?
-The ways that two cells can synapse are many and varied. Pre-synaptic terminal buttons to post-
synaptic soma, axon or dendrites
Axo-axonic (axon)
Axo-somatic (soma)
Axo-dendritic (dendrites)
What are electrical synapses?
- Electrical synapses are the result of a narrow gap between the pre- and postsynaptic neurons known as a gap junction.
- The close proximity (e.g., 4 nm {DNA is about 2 nanometers across}) means the cytoplasm of the two cells are interconnected
- This permits electrical signals (and even small molecules) to pass directly from one cell to the next.
- This system is FAST
What are electrical synapses part 2?
-Electrical synapses in the cerebral cortex allow each network of inhibitory neurons to fire in a
highly coordinated way
-They may relate to rhythmic activity in the cortex.
-The high speed of electrical synapses transmission means they are important for reflexive processes.
-A downside of electrical transmission is that there is no opportunity for ‘gain’ i.e., a small signal cannot bring about a large response.
What are chemical synapses?
- This method of transmission depends on the release of chemicals from presynaptic cell, which are received and have an effect on post synaptic
cell. - In chemical synapses the pre- and post-synaptic membranes are divided by the synaptic cleft (20 nm wide).
- The post-synaptic membrane contains receptors that can receive the chemical transmitters that will be used to communicate from the pre-synaptic cell.
