nervous coordination and receptors Flashcards
Describe the structure of a myelinated motor neurone
dendrtie sparky thing
cell body the circle
axon
myelin sheath duck fat
node of ranvier between the duck fats
axon terminal opp sparky thing
describe resting potential
Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside)
Explain how a resting potential is established across the axon membrane in
a neurone
na k pump AT
3 na out 2 k into axon
Creating an electrochemical gradient:
○ Higher K + conc. inside AND higher Na + conc. outside
Differential membrane permeability:
○ More permeable to K+ → move out by facilitated diffusion
○ Less permeable to Na+ (closed channels)
stimulus
na channels open
memeb permeability to na increases
na diffuse into axon down electrochemical gradient
depol
threshold reached AP gen
as voltage gated channels open more na diffuse in rapdily
repol
voltage gated na channels close
voltage gated k channels open k diffuse out axon
hyperpol
k channels slow to cloe
slight over shoot too many k diffuse out
resting potential
restored by na k pump
all or nothing pricniple
● For an action potential to be produced, depolarisation must exceed threshold potential
● Action potentials produced are always same magnitude / size / peak at same potential
○ Bigger stimuli instead increase frequency of action potentials
non myelinated axon
● Action potential passes as a wave of
depolarisation
● Influx of Na+
in one region increases
permeability of adjoining region to Na+ by
causing voltage-gated Na+ channels to open
so adjoining region depolarises
myelinated axon
● Myelination provides electrical insulation
● Depolarisation of axon at nodes of Ranvier only
● Resulting in saltatory conduction (local
currents circuits)
● So there is no need for depolarisation along
whole length of axon
describe how damage to myelin sheat can cause slow and jerky movement
● Less / no saltatory conduction; depolarisation occurs along whole length of axon
○ So nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction
● Ions / depolarisation may pass / leak to other neurones
○ Causing wrong muscle fibres to contract
refactory period
● Time taken to restore axon to resting potential when no further action potential can be generated
● As Na
+ channels are closed / inactive / will not open
importnce of refractory period
● Ensures discrete impulses are produced (action potentials don’t overlap)
● Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)
○ Higher intensity stimulus causes higher frequency of action potentials
○ But only up to certain intensity
● Also ensures action potentials travel in one direction – can’t be propagated in a refractory region
factors affecting speed of conductance
Myelination
● Depolarisation at Nodes of Ranvier only → saltatory conduction
● Impulse doesn’t travel / depolarise whole length of axon
Axon diameter
● Bigger diameter means less resistance to flow of ions in cytoplasm
Temperature
● Increases rate of diffusion of Na
+ and K+ as more kinetic energy
● But proteins / enzymes could denature at a certain temperature
What are cholinergic synapses?
Synapses that use the neurotransmitter acetylcholine (ACh)
Describe transmission across a cholinergic synapse
pre synaptic neurone
depol at pre synaptic neurone causes voltage gated causes ca channels to open
ca diffuse into pre synap knob
causing vesciles contining ach to fuse with pre synaptic memebrane
○ Releasing ACh into the synaptic cleft (by exocytosis)
Describe transmission across a cholinergic synapse
post synaptic neurone
ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane
causing na channels to open
na diffuses into post synaptic memebrane causing depol
threshold met AP gen
what happens to ach after synaptic transmission
● It is hydrolysed by acetylcholinesterase
● Products are reabsorbed by the presynaptic neurone
● To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation
Explain how synapses result in unidirectional nerve impulses
● Neurotransmitter only made in / released from pre-synaptic neurone
● Receptors only on post-synaptic membrane
Explain summation by synapses
● Addition of a number of impulses converging on a single post-synaptic neurone
● Causing rapid buildup of neurotransmitter (NT)
● So threshold more likely to be reached to generate an action potential
spatial summation
● Many pre-synaptic neurones share
one synaptic cleft / post-synaptic
neurone
● Collectively release sufficient
neurotransmitter to reach threshold
to trigger an action potential
temporal summation
● One pre-synaptic neurone releases
neurotransmitter many times over
a short time
● Sufficient neurotransmitter to reach
threshold to trigger an action
potential
inhibiton by inhibitory synapses
● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:
○ Cl channels open → Cl diffuse in
○ K+ channels open → K+ diffuse out
● This means inside of axon has a more negative charge relative to outside / below resting potential
● So more Na+
required to enter for depolarisation
● Reduces likelihood of threshold being met / action potential formation at post-synaptic membranes
nmj strucure
very similar to synapse except
Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more
● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter
Cholinergic synapse
neurone to neurone
nt excibatory or inhibitory
Action potential may be initiated in postsynaptic neurone
NMJ
(Motor) neurone to muscle
Always excitatory
Action potential propagates along sarcolemma down T tubules
