T15 Flashcards
Functions of Schwann cells and their myelin sheath
Some neurones are surrounded by the membranes of cells called Schwann cells.
Schwann cells have several functions:
Their membranes form the myelin sheath.
They remove debris via phagocytosis.
They aid regeneration.
The myelin sheath surrounds parts of the axon, acting as an insulator that prevents the passage of ions into or out of the axon at the regions it covers.
Myelinated axons conduct impulses rapidly through a process known as ‘saltatory conduction’. This means that the electrical impulse ‘jumps’ between gaps in the myelin sheath called the nodes of Ranvier, increasing transmission speed.
Describe how a resting potential is established and maintained
sodium-potassium pump uses active transport and ATP to pump out 3 Na+ for every 2 K+ ions pumped into the axon
potassium ions diffuse back out (of the axon) as their channels are open
membrane is less permeable to sodium so fewer sodium ions diffuse back in (to the axon)
this maintains a resting potential of -70mV as the membrane is negatively charged in comparison to the extracellular space
Describe how an action potential is generated
Resting potential - the membrane is a rest and polarised at around -70mV
Stimulus - Voltage-gated Na+ channels open, so more Na+ flows into the axon, making the inside less negative
Depolarisation - If the threshold potential of around -55mV is reached, more Na+ channels open, causing an influx of Na+
Repolarisation - At around +30mV, Na+ channels close and K+ channels open, so K+ flows out of the axon and the membrane starts repolarising
Hyperpolarisation - An excess of K+ leaves the axon, dropping the potential below the -70mv resting level
Refractory period - Various ion pumps and channels work together to restore the membrane back to the resting potential
Describe the all or nothing principle
The threshold phenomenon - Once the threshold potential is reached, an action potential is always triggered, regardless of the stimulus’ strength.
No partial response - Without reaching the threshold potential, no action potential is initiated.
Action potentials are always the same size - A stronger stimulus doesn’t increase the size of the action potential, but it does increase the frequency of action potentials generated.
What is the importance of the refractory period?
- Ensures action potentials don’t overlap
- Limits the frequency at which impulses are transmitted
- Guaranteeing that impulses travel in only one direction
How does an action potential travel as a wave of depolarisation?
- The opening of Na+ channels results in local depolarisation, allowing positive ions to spread sideways.
- Adjacent voltage-gated Na+ channels open in response to this change.
- This action leads to the depolarisation of nearby membrane areas.
- As each patch of membrane activates the next, an advancing wave is formed.
- Areas of the membrane that have just experienced depolarisation are in the refractory period and remain unresponsive while they repolarise (K+ exits the axon and Na+ channels are closed).
- This ensures that the wave moves in one direction, preventing the backward flow of the nerve impulse.
Describe spatial and temporal summation at synapses
Spatial summation:
Multiple presynaptic neurones converge on a single postsynaptic neurone or effector cell.
The combined input of neurotransmitters can trigger postsynaptic firing.
Inhibitory inputs have the potential to prevent this firing.
Temporal summation:
Repeated firing by a presynaptic neurone leads to continuous neurotransmitter release.
An increased amount of neurotransmitter makes it more likely to trigger postsynaptic firing.
Describe the steps involved in inhibitory synaptic transmission
- Inhibitory neurotransmitters are released into the synaptic cleft
- Inhibitory neurotransmitters bind to chloride channels on the postsynaptic membrane
- The opening of these channels allows and influx of Cl- into the postsynaptic neurone via facilitated diffusion
- K+ channels also open, and K+ leaves the postsynaptic neurone
- This results in the hyperpolarisation of the postsynaptic membrane, preventing the generation of an action potential
Key steps in synaptic transmission
An action potential arrives at the presynaptic knob.
This causes voltage-gated calcium ion (Ca2+) channels to open and Ca2+ flows into the presynaptic knob.
This causes synaptic vesicles, which contain neurotransmitters, to move towards and fuse with the presynaptic membrane.
The vesicles release neurotransmitters into the synaptic cleft through exocytosis, and the neurotransmitters rapidly diffuse across the synaptic cleft.
On reaching the other side, the neurotransmitters bind to receptor proteins on the postsynaptic membrane, causing the receptors to change shape.
This opens sodium ion channels in the postsynaptic membrane, leading to the depolarisation of the postsynaptic membrane.
If this depolarisation reaches a threshold level, an action potential is triggered in the postsynaptic neurone.
The role of acetylcholine in cholinergic synapses
Cholinergic synapses are specific types of synapses that use acetylcholine (ACh) as their neurotransmitter.
After ACh binds to receptors and triggers a response:
ACh is broken down by the enzyme acetylcholinesterase into choline and ethanoic acid (acetate).
These breakdown products are then reabsorbed into the presynaptic knob via active transport.
They can then be recycled to synthesise more ACh.
Ach is transported into synaptic vesicles, ready for another action potential.
It is important to remove neurotransmitters like ACh from the synaptic cleft to prevent the stimulus from being maintained and to allow another stimulus to affect the synapse. This prevents continuous stimulation and allows for neurotransmitter recycling.
