Nervous transmission Flashcards
What is the cell surface membrane at rest?
In a maintained state of polarisation, this is a resting potential at around -70mv
What happens to NA+ and K+ ion channels to achieve resting potential?
NA+ ion channels = closed
K+ channels = open
What is the sodium potassium pump?
Active transporters that move 3NA+ out of the neuron for every 2K+ moved in
What causes membrane polarisation at rest?
- extracellular space outside the axon accumulates more positive ions
- this makes the axon cytoplasm negatively charged in comparison
- this causes the membrane to be polarised
What are the stages of generating an action potential?
1) resting potential = membrane is at rest at -70mv
2) stimulus = VG NA+ ion channels open so more NA+ flows into the axon making it less negative
3) depolarisation = if the threshold potential of -55mv is reached more NA+ channels open causing an NA+ influx and action potential to be generated
4) repolarisation = at around +30mv NA+ channels close and K+ channels open so K+ flows out of the axon and the membrane starts repolarising
5) hyperpolarisation = excess of K+ leaves the axon dropping the potential below the -70mv resting level
6) refractory period = various ion pumps and channels work together to return the membrane to the resting potential
What is the generation of an action potential an example of and why?
Positive feedback
because the initial NA+ influx depolarises the axon membrane which opens more NA+ channels meaning a greater influx NA+ further depolarising the membrane
What is the all or nothing principle of action potentials?
signal transmission between neurons is not dependent on the strength of the stimuli but meeting the threshold so neurons will either transmit an impulse over the synapse to the next neuron completely or not at all
What are three characteristics of the all or nothing principle?
- once the threshold his met an action potential is always triggered regardless of stimuli’s strength
- without reaching the threshold potential no action potential is initiated
- stronger stimuli doesn’t increase the size of the action potential but increases the frequency of generation
What is the refractory period?
The recovery period after a neuron fires an action potential where the neuron membrane can’t generate another action potential because NA+ ion channels remain closed during repolarisation
Why is the refractory period essential?
- ensure action potentials don’t overlap
- limiting the frequency at which impulses are transmitted
- guaranteeing impulses travel in only one direction
What are the stages of an action potential travelling in waves of depolarisation?
1) opening of NA+ channels results in local depolarisation allowing positive ions to spread sideways
2) adjacent VGNA+ channels open in response to this change
3) this action leads to the depolarisation of local membrane areas
4) as each patch of membrane activates the next an advancing wave is formed
5) areas of the membrane that have just experienced depolarisation are in the refractory period and remain unresponsive while repolarising
6) ensures the wave moves in one direction preventing backward flow of nerve impulses
What are the three factors affecting speed of transmission of an action potential?
- myelination
- axon diameter
- temperature
How does myelination affect the transmission speed of an action potential?
myelinated neurons are insulated by a myelin sheath enabling saltatory conduction
What is saltatory conduction and how does it increase speed of transmission?
- process involves action potentials ‘jumping’ between nodes of ranvier)
- myelinated neurons transmit impulses faster than unmyelinated neurons
How does axon diameter affect the transmission speed of an action potential?
- large axon diameter means there’s less resistance in ion flow so the wave of depolarisation travels faster along the axon
- broader axons transmit impulses faster
How does temperature affect the transmission speed of an action potential?
- higher temp accelerates the diffusion of ions leading to faster depolarisation and impulse transmission
- however temp over 40 can cause proteins to denature resulting in slower transmission due to membrane damage
What is a synapse and what do they do?
Synapse = junction where info is transferred from one neuron to another neuron or affecter cell, coordinating activity in the nervous system
What role do synapses play in neural communication?
- transmit info through the release of neurotransmitter chemicals
- single impulse from a presynaptic neuron can initiate multiple postsynaptic neurons or affecter cells
- impulses from several pre-synaptic neurons can be combined into a single postsynaptic response
What is a synaptic knob, synaptic vesicle, synaptic cleft and neurotransmitter receptor?
synaptic knob = section at end of presynaptic neuron containing organelles needed for neurotransmitter production
synaptic vesicle = sacs within synaptic knob storing neurotransmitters until release
synaptic cleft = gap between presynaptic and post-synaptic neuron membranes
neurotransmitter receptor = specific molecules on postsynaptic membrane that bind with the neurotransmitters
Does an excitatory neurotransmitter generate an action potential, what effect is had on the membrane and give an example?
action potential = may be triggered if threshold potential is met
effect = depolarisation of post synaptic membrane
example = acetylcholine is an excitatory neurotransmitter in the CNS and at neuromuscular junctions
Does an inhibitory neurotransmitter generate an action potential, what effect is had on the membrane and give an example?
action potential = is prevented
effect = hyperpolarisation of postsynaptic membrane
example = acetylcholine is an inhibitory neurotransmitter at cardiac synapses
What is summation?
summation = process combining imputs to make the generation of an action potential in a postsynaptic neuron or effector cell more likely
What is spatial summation?
- multiple presynaptic neurons converge on a single post synaptic neuron or effector cell
- the combined imput of neurotransmitters can trigger postsynaptic firing
- inhibitory imputs have the potential to prevent this firing
What is temporal summation?
- repeated firing by a presynaptic neuron leading to continuous neurotransmitter release
- increased amount of neurotransmitter makes it more likely to trigger postsynaptic firing
What is synaptic transmission and why is it important?
synaptic transmission = process by which a nerve impulse is transmitted from one neuron to another across a synapse, essential for communication with the nervous system
What are the key steps in synaptic transmission?
1) action potential arrives at the presynaptic knob
2) causes VGCA2+ channels to open and CA2+ to flow into the presynaptic knob
3) causes synaptic vesicles which contain neurotransmitters to move towards and fuse with the presynaptic membrane
4) the vesicles release the neurotransmitter into the synaptic cleft through exocytosis and the neurotransmitter rapidly diffuses across the synaptic cleft
5) the neurotransmitter binds to receptor proteins on the postsynaptic membrane causing receptors to change shape
6) this opens sodium ion channels in the postsynaptic membrane leading to it’s depolarisation
7) if this depolarisation reaches the threshold level an action potential is triggered by the postsynaptic neuron
What are cholinergic synapses?
cholinergic synapses = specific synapses using acethlycoline (ACh) as their neurotransmitter
What’s the response triggered after ACh binds to receptors?
1) ACh is broken down by the enzyme acetylcholinesterase into choline and ethanoic acid (acetate)
2) these breakdown products are the reabsorbed into the presynaptic knob via active transport
3) they can be recycled to synthesise ACh
4) ACh is transported into synaptic vesicles ready for another action potential
Why is it important to remove to neurotransmitters like ACh from the synaptic cleft?
To prevent the stimulus from being maintained and to allow other stimuli to affect the synapse
prevents continuous stimulation and allows for neurotransmitter recycling
