P: Synapses Flashcards
Define stimulus
Stimulus: an environmental event that excites sensory receptors to provide information about the event to the CNS.
What happens if you block potassium channel or open a sodium channel
Blocking a K+ channel and opening a Na channel has same effect: builds up positive charge, so cell depolarises.
Acetylcholine (ACh)
Used by spinal cord neurons to control muscles. Used by neurons in brain to control memory. Usually excitatory
Dopamine (DA)
Produces feeling of pleasure. Usually inhibitory
Gamma-aminobutyric acid (GABA)
Major inhibitory neurotransmitter in brain
Glutamate
Major excitatory neurotransmitter in brain
Glycine
Mainly used by spinal cord neurons. Usually inhibitory
Norepinephrine/ Noradrenaline
Acts as neurotransmitter + hormone.
In PNS used for fight or flight.
In CNS regulates normal brain processes
Usually excitatory, but can be inhibitory in some brain areas.
Serotonin
Involved in mood, appetite, sensory perception.
Inhibitory action in pain pathways of spinal cord.
How can neurotransmitter be removed from synapse?
- Degradation by membrane bound enzymes
- Reuptake into presynaptic cell
- Diffusion out of synapse
How do antidepressants work?
- Block enzymes that degrade neurotransmitters so that neurotransmitter concentration remains higher for longer
- Block neurotransmitter uptake into presynaptic membrane, so neurotransmitter concentration remains higher for long and more neurotransmission will occur.
Excitatory postsynaptic potentials:
depolarisations bring membrane towards threshold
Inhibitory postsynaptic potentials
hyperpolarisations that bring membrane further from threshold.
EPSP characteristics:
- Summation
- No threshold
- No refractory period
- Occur in dendrites + cell bodies
Spatial summation
Excitatory potentials from many neurons trigger threshold point
Temporal summation
Many excitatory potentials from one neuron triggers threshold point.
SNARE proteins
soluble N-ethylmaleimide sensitive factor attachment protein receptor.
What are the types of SNARE proteins
- Synaptobrevin (v-SNARE)
- Synaptotagmin (v-SNARE)
- Syntaxin (t-SNARE)
- SNAP 25 (t-SNARE)
Describe the mechanism of SNARE proteins
- Action potential arrives at nerve terminal
- Syntaxin and SNAP 25 (t-SNARE) form complex.
- Synaptobrevin (v-SNARE) begins to join with Syntaxin and SNAP 25 complex and 3 SNARE proteins wind around each other to pull vesicle membrane and nerve terminal closer together
- Entry of calcium into presynaptic neuron which binds to synaptotagmin, causing fusion of vesicle and release of Ach into synaptic cleft
- ATP required to disassemble SNARE proteins
How does ACh release lead to a muscle AP
Binding of ACh to nicotinic receptors allows simultaneous flow of Na+ in and K+ out through the ligand gated ion channels
The nicotinic receptor leads to fast synaptic transmission The movement of ions leads to generation of end plate potentials
Tetrodotoxin
Blocks voltage gated Na+ channels
Novocaine, lidocaine –> block action potential rather than synapse.
Calciseptine
Blocks Ca2+ channels
Iberiotoxin
Blocks K+ channels
Saxitoxin
Blocks voltage gated Na+ channels
Compare absolute and relative refractory periods
- During absolute refractory period, another stimulus (AP) will not excite neuron
- During relative refractory period, another stronger stimulus can excite neuron
Primary hyperkalemic paralysis
- Painful spontaneous muscle contractions followed by periods of paralysis of affected muscles
- Plasma + extracellular fluid have elevated K+
- Due to mutated Na+ channels causing reduction in rate of voltage inactivation and longer lasting action potentials –> increased K+ secretion
- Na+ channels become refractory so muscles cannot contract in response to further stimulatory action potentials.
