6.5 neurons and synapses Flashcards
what are neurons?
specialised cells that transmit electrical impulses within the nervous system
what are the three components of neurons?
- dendrites
- axons
- soma
what are dendrites?
branched protoplasmic extensions that can convert chemical signals to electrical signals to be conducted to the cell body
what part of the neurons convert chemical signals to electrical signals?
dendrites
which part of the neuron conducts electrical impulse away from the cell body?
axons
what are the functions of axons
they transmit electrical signals to terminal regions for communication with other neurons or effectors
what is the axon surrounded by in some neurons?
the myelin sheath
the myelin sheath ________ the conduction speed of electrical impulses
improves
what ions are used to generate a resting potential?
Na+ and K+
what is the effect of the pumping of Na+ and K+?
electrical signals are generated and conducted in neurons
where are K+ and Na+ pumped?
across the neuron membrane
at rest the neuron cell is not ________
neutral
when the neuron is at rest, the ________(where?) of the cell is slightly more ________ charged than the _______(where?) of the cell
inside, negatively, outside
what is the resting potential of a nerve cell?
-70mV
why is there a potential difference between the inside and outside of the neuron?
the inside of the neuron is more negative than the outside
what maintains the resting potential?
Na+ K+ pumps
is ATP used to maintain the resting potential?
yes
the Na+-K+ pump is a _____________ protein
transmembrane
what type of transport protein is the Na+/K+ pump?
antiport
for every __ K+ ions admitted, __ Na+ ions are expelled
2, 3
how is an electrochemical gradient created?
- for every 2 K+ ions admitted, 3 Na+ ions are expelled
- there are more +vely charged ions outside of the cell and more -vely charged ions in the cell
how is a resting potential generated? (4)
- 3 Na+ ions are pumped out of the cell by active transport with the expenditure of ATP
- 2 K+ ions are pumped into the cell
- greater electrochemical gradient where cell interior is relatively negative compared to extracellular environment
- thus resting potential created
action potentials are the ______ changes across the membrane that occur when ____________________
rapid, a neuron is firing
what are the three stages of action potentials
- depolarisation
- repolarisation
- refractory period
depolarisation involves ___ and repolarisation involves ____ (what ions)
Na+, K+
which part of the neuron is depolarised?
the membrane
what is depolarisation?
it refers to a sudden change in membrane potential, usually from a relative negative to positive change
which part of the neuron initiates the signal?
the dendrite
what is achieved through a “wave of depolarisation”?
the transmission of a nerve impulse
during depolarisation, Na+ ions [enter/exit] the inside of the neuron
enter
what happens during depolarisation? (5)
- a signal initiated at the dendrite
- in response, gated Na channels open within the membrane of the axon
- Na ions move in -> change in membrane potential causes mass opening of voltage gated Na channels
- the opening of Na channels causes a passive influx of Na+
- this causes the membrane potential to become more +ve
what are the two channels involved in depolarisation and repolarisation?
Na+ channels and K+ channels respectively
where are Na+ and K+ channels located?
on the membrane of the axon
does the opening of K channels causes K to [enter/exit] the inside of the neuron
exit
what is potential difference?
the diff in membrane potential btw the inside and outside of the neuron
what happens during repolarisation? (5)
- Na channels close at peak of action potential -> at the same time, gated K channels open at e membrane of e axon
- opening K channels causes a passive efflux of K
- this causes the membrane potential to return to a more -ve membrane potential
- K channels close slowly -> brief undershoot in membrane potential
- Na/K pumps readjust membrane potential to back to resting potential
what happens in an action potential? (long, 10)
- depolarisation
- a signal initiated at the dendrite
- in response, gated Na channels open within the membrane of the axon
- Na ions move in -> change in membrane potential causes mass opening of voltage gated Na channels
- the opening of Na channels causes a passive influx of Na+
- this causes the membrane potential to become more +ve - repolarisation
- Na channels close at peak of action potential -> at the same time gated K channels open at e membrane of e axon
- opening K channels causes a passive efflux of K
- this causes the membrane potential to return to a more -ve diff in internal differentiation
- K channels close slowly -> brief undershoot
- Na/K pumps readjust membrane potential to back to resting potential
what is the refractory period? (short)
the period of time following a nerve impulse before a neuron is able to fire again
why must the resting potential be restored after the depolarisation and repolarisation? (2)
- resting potential must be restored for neuron to fire again
- the ionic distribution of more Na+ outside the neuron and and more K+ inside the neuron is reversed
how is the resting potential restored?
via the antiport action of the Na+/K+ pump
what happens during the refractory period?
- after depolarisation and repolarisation -> ionic distribution is largely reversed
- resting potential must be restored before a neuron can fire again
- thus restored via antiport action of the Na+/K+ pump
what are nerve impulses?
action potentials that move along the length of an axon as a wave of depolarisation
what characteristic of the Na/K channels enable action potential to work as a ‘kallang wave’?
- it is voltage-gated, meaning the channels open in response to changes in membrane potential
- depolarisation at one point of e axon triggers opening of ion channels in next segment of axon
what is the threshold potential?
the minimum electrical stimulus for voltage-gated ion channels to open
what is the value of the threshold potential?
-55mV
the propagation of nerve impulses is possible as the cytoplasm is a ____________ _____________
continous medium
what is the propagation of nerve impulses? (5)
- majority of Na+ channels are voltage gated (VG) Na channels
- open when membrane potential is depolarised to arnd -55 mV
- during depolarisation of one part of the neuron -> mass movement of Na+ into cell
- some +vely charged Na will move into more -vely charged adjacent region of the neuron -> cytoplasm is a continuous medium
- this movement causes that part to be more +vely charged
- triggers the opening of more VG Na channels -> initiates an action potential
- thus this causes the propagation of nerve impulses
label the oscilloscope
-
what feature of the nerve fibres enable saltatory conduction?
myelination of nerve fibres
what is the role of the myelin sheath?
to increase the speed of electrical transmissions via saltatory conduction
the part of the axon membrane that is not myelinated are the ______________
nodes of Ranvier
the nodes of Ranvier facilitate _________ __________
saltatory conduction
what is a disadvantage of mylineation?
it takes up significant space within an enclosed environment
in myelinated nerves, depolarisation occurs only at…
the nodes of Ranvier
synapses are junctions found btw ______ and btw ______ and _______ or ______________
neurons, neurons, receptor, effector cells
what does a synapse consist of (3, short)
- synaptic cleft
- pre-synaptic end of one neuron
- post-synaptic end of next neuron
how is an impulse transmitted from one neuron to another? (short)
it is chemically transmitted
neurotransmitters are…
chemical messengers
what is triggered when an action potential reaches the presynaptic ending? (short)
this triggers the opening of voltage gated Ca channels
what is the end of a neuron called?
axon terminal
what does the influx of Ca ions into the cell causes the _________ of ____________ containing neurotransmitters with the ________________ (cell part)
fusion, vesicles, cell membrane
neurotransmitters are released by _______ during synapse transfer? (
exocytosis
what causes special channels (in the post-synaptic neuron) to open (in the context of synapse transfer)?
the binding of neurotransmitters to specific receptors in the post-synaptic membrane
when neurotransmitters bind to receptors in the post-synaptic membrane, ____________ ion channels open
ligand-gated
how is an electrical impulse generated in the post-synaptic neuron? (2, short)
- neurotransmitters bind to specific receptors on post-synap membrane
- this opens ligand-gated ion channels
- opening of ion channels generates an electrical impulse
what happens to neurotransmitters after they synapse transfer? (short)
- recycled by reuptake pumps
- or degraded by enzymatic activity
how are impulses transferred from neuron to neuron? (6)
- when an action potential reaches e axon terminal -> it triggers the opening of voltage gated Ca channels
- Ca ions diffuse into cell -> causes the fusion of vesicles containing neurotransmitters with cell membrane
- neurotransmitters released by exocytosis -> diffuse across synaptic cleft
- neurotransmitters then bind to specific receptors in post-synaptic neuron membrane -> this opens ligand-gated ion channels
- opening of ion channels allows Na+ to enter cell in post-synaptic neuron
- influx of sodium ions causes depolarisation -> generates action potential on post-synap neuron
a neurotransmitter used by both the CNS and PNS is…
acetylcholine!
why must acetylcholine be continually removed from the synapse?
- otherwise overstimulation occurs
- this may cause convulsions + paralysis
how is acetylcholine removed? (3)
- enzyme acetylcholinesterase is released into synaptic cleft by the presynaptic neuron
- rapidly breaks down acetylcholine back into choline and an acetyl group
- choline is reabsorbed back into the presynaptic neuron -> where it is converted back into acetylcholine in the vesicles
acetylcholine triggers… (2)
- muscular contraction
- parasympathetic responses
what pesticides block synaptic transmission?
neonicotinoid pesticides
how does neonicotinoid pesticides kill insects?
- it contains compounds that can bind irreversibly to acetylcholine receptors in postsynaptic cells
- acetylcholinesterase cannot break them down -> continue to cause depolarisation of cell
- leads to paralysis and death
why are neonicotoid pesticides suitable pesticides?
they bind more strongly to acetylcholine receptors in insects than mammals
neurons only fire when the potential difference reaches the __________ ________ which is at -55mV
threshold potential
