Nervous coordination Flashcards
describe the structure of a myelinated motor neurone
-dendrite
-cell body (soma)
-myelin sheath
-schwann cells
-axon
-node of ranvier
-axon terminal
explain how a resting potential is established across the axon membrane in a neurone
- Na+/ K+ pump actively transports: Na+ out of axon and K+ into axon
-creating an electrochemical gradient: higher K+ conc. inside and higher Na+ conc. outside
-differential membrane permeability:
*more permeable to K+ -> move by facilitated diffusion
*less permeable to Na+ (closed channels)
explain how changes in membrane permeability lead to depolarisation and the generation of an action potential
- stimulus=
-Na+ channels open; membrane permeability to Na+ increase
-Na+ diffuse into axon down electrochemical gradient( causing depolarisation) - depolarisation=
- if threshold potential reached, an action potential is generated
-as more voltage gated Na+ channels open
-so more Na+ diffuse in rapidly - repolarisation=
-voltage gated Na+ channels close
-voltage gated K+ channels open; K+ diffuse out of axon - hyperpolarisation=
-K+ channels slow to close so there’s a slight overshoot- too many K+ diffuse out - resting potential restored=
-restored by Na+/K+ pump
describe the all or nothing principle
- 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)
explain how the passage of an action potential along non-myelinated result in nerve impulses
-action potential passes as a wave of depolarisation
-influx of Na+ in one region increases permeability of adjoining regions to Na+ by causing voltage-gated Na+ channels to open so adjoining region depolarises
explain how the passage of an action potential along myelinated axons results in nerve impulses
-myelination provides electrical insulation
-depolarisation of axon at nodes of ranvier only
-resulting in saltatory conduction
-so there is no need for depolarisation along whole length of axon
describe the nature of the refractory period
-time taken to restore axon to resting potential when no further action potential can be generated
-as Na+ channels are closed
Describe the factors that affect 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
Explain the importance of the 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
What are cholinergic synapses?
Synapses that use the neurotransmitter acetylcholine (ACh)
Describe transmission across a cholinergic synapse
- Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ channels
= Ca2+ diffuse into pre-synaptic neurone / knob - Causing vesicles containing ACh to move and fuse with pre-synaptic membrane
=Releasing ACh into the synaptic cleft (by exocytosis) - ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane
- Causing Na+ channels to open
=Na+ diffuse into post-synaptic knob causing depolarisation
=threshold is met, an action potential is initiated
Explain what happens to acetylcholine 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
Describe 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
Describe temporal summation
● One pre-synaptic neurone releases neurotransmitter many times over a short time
● Sufficient neurotransmitter to reach threshold to trigger an action potential
Describe inhibition by inhibitory synapses
● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:
= Cl- channels open → Cl- diffuse in
= K+ channels open → K+ diffuse out
● More Na+ required for depolarisation
● Reduces likelihood of threshold being met / action potential formation at post-synaptic membrane
Describe the structure of a neuromuscular junction
Very similar to a synapse except:
● Receptors are on muscle fibre instead of postsynaptic membrane and there are more
● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter
Compare transmission across cholinergic synapses and neuromuscular junctions
In both, transmission is unidirectional but…
*Cholinergic synapse
=Neurone to neurone (or effectors, glands)
=Neurotransmitters can be excitatory or inhibitory
=Action potential may be initiated in postsynaptic
neurone
*Neuromuscular junction
=(Motor) neurone to muscle
=Always excitatory
=Action potential propagates along sarcolemma down T tubules
how do some drugs stimulate the nervous system, leading to more action potentials
=Similar shape to neurotransmitter
=Stimulate release of more neurotransmitter
=Inhibit enzyme that breaks down neurotransmitter → Na+ continues to enter
how do some drugs inhibit the nervous system, leading to fewer action potentials
=Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels
=Block receptors by mimicking shape of neurotransmitter
