Nervous System Flashcards
Describe how a generator potential is established in an Pacinian Corpuscle
- Mechanical Stimulus e.g Pressure deforms lamellae and stretch-mediated Na+ channels
- Na+ channels in membrane open and Na+ diffuses into Sensory Neurone
- Greater pressure causes more Na+ channels to open and more Na+ to enter
- Causes depolarisation, leading to a generator potential
- If generator potential reaches a threshold, triggers an action potential
Explain what the Pacinian Corpuscle illustrates
- Receptors only respond to a specific stimuli
- Pacinian Corpuscle only responds to mechanical pressure
- Stimulation of receptor leads to a generator potential
- When threshold is reached, triggers action potential (all or nothing principle)
Describe a resting potential
- Inside of Axon has more of a negative charge relative to outside (as more positive ions outside than inside)
Explain how a resting potential is established across the Axon Membrane in a neurone
- Na+/K+ pump actively transports 3NA+ out and 2K+ into axon
- Creates electrochemical gradient (higher K+ conc inside, higher Na+ conc outside axon)
- Differential membrane permeability
- More permeable to K+ (move out by facilitated diffusion)
- Less permeable to Na+ (closed channels)
Explain how changes in membrane permeability lead to depolarisation and generation of an action potential
- Stimulus
- Na+ channels open; membrane permeability to Na+ increases,
- 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 (positive feedback effect)
- 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 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 potential
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
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 / inactive / will not open
Describe transmission across a cholinergic synapse
1) Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca
2+ channels
○ Ca
2+ diffuse into pre-synaptic neurone / knob
2. 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
○ If 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
- Importance: low frequency action potentials release insufficient neurotransmitter to exceed threshold
