6.2 Nervous conditions Flashcards
what is the militated sheath
insulating cover over axon formed by the Schwann cells to increase speed of nerve impulse
what is the nodes of ranvier
gaps between the Schwann cells
why does a militated axon conduct impulses faster than a non-myelinated one
action potential is only at the nodes which jumps from node to node so doesn’t travel along whole sheath
what is the voltage of the resting state
-70mV
how is the resting potential maintained in a neurone
the sodium potassium pump actively transports 2 potassium ions into the axon for every 3 sodium ions out of the axon.
This results in a chemical gradient where more Na+ are outside and more K+ inside. the membrane is more permeable to the potassium than the Na+ diffuse in maintaining the -70ve
5 steps of an action potential
- Depolarisation - the Na+ gates are open and Na+ rushes into the cell so inside the cell becomes less negative
- Threshold is met as more action potentials occur and increasingly more Na+ gates open
- Repolarisation - K+ voltage gates open but are slower to respond, K+ moves out and the Na+ gates are triggered to close once certain level of depolarisation has been reached
- Hyperpolarisation - outflow of K+ leads to a slight overshoot, this leads to the closing of the K+ voltage gate
- Resting state - the Na+ and K+ gates are then closed
what is the refectory period
where the axon can’t be depolarised to initiate a new action potential
what is the synapse
gap between neurones
what are the two functions of a synapse
- a single impulse to be transmitted to a number of different neurones creating simultaneous responses
- A number of impulses to be combined at a synapse allowing stimuli from different receptors to interact for a single response
what does unidirectionally mean for the synapse
synapses can only travel in one direction which is from the presynaptic neurone to the postsynaptic neurone
what does spatial summation mean for the synapse
low frequency action potentials often produce insufficient amounts of neurotransmitter to trigger a new action potential in the post synaptic neurone so together a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the post synaptic neurone. Together they therefore trigger a new action potential
what does temporal summation mean for the synapse
A single presynaptic neurone releases neurotransmitter many times over a short period. If the total amount exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered
6 steps of transmission across synapse
- calcium channels open – the calcium ions flood into the synaptic knob from the incoming action potential causing depolarisation opening the calcium channels
- neurotransmitter released – the influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane, this releases neurotransmitter called acetylcholine into the cleft
- sodium channels – acetylcholine binds to the receptor site on the sodium ion channels causing them to open and sodium ions diffuse in causing the postsynaptic neurone to depolarise.
- new action potential – if this depolarisation is above a certain threshold a new action potential is sent along the axon of the postsynaptic neurone
- acetylcholinesterase – a hydrolytic enzyme breaks up the acetylcholine into acetyl and choline, these diffuse back across the cleft into the presynaptic neurone allowing the neurotransmitter to be recycled
- remaking acetylcholine – ATP from mitochondria is used to recombine the acetyle and choline. This is stored in synaptic vessels for future use, more acetylcholine can be made at the SER, sodium ion channels close in the absence of acetylcholine at their receptor sites and the synapse can now be used again
