synapses and neurotransmitters Flashcards
what is the criteria for neurotransmitters?
- be present in presynaptic terminals
- be released in response to stimulation
- act on the postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what are the main classes of neurotransmitters?
- amino acids
- amines
- peptides (made up of many amino acids)
evidence for neurons
- golgi stain
- physiological evidence from study of reflexes
- final evidence from electron microscopy
why are synapses important?
they enable flexible processing and allow for specialization and integration
what are electrical synapses?
gap junctions that allow current to pass directly between neurons
they directly connect the cytoplasm of two neurons
both depolarization and hyperpolarization are transmitted
how to tell if neurons are connected by gap junctions?
physical technique - dyes can be inserted into neurons using electrodes
dyes diffuse from one neuron to the other demonstrating presence of gap junctions
electrical technique - hyperpolarizing and depolarizing stimuli are passed from one neuron to the other
what are electrical synapses good for?
- fast communication
- synchronizing neurons
what are the steps in chemical synaptic transmission?
- package neurotransmitters in vesicles and put them in pre-synaptic terminal
- action potential arrives - voltage-gated ca2+ channels open
- Ca2+ influx - vesicles fuse to membrane, neurotransmitters released
- neurotransmitters diffuse across the synaptic cleft, activate receptors on postsynaptic cell
- neurotransmitters removed from cleft
what are the two types of synaptic vesicles?
synaptic vesicles - clear, recycled by endocytosis, small molecule transmitters
dense-core secretory granules - dense, ‘one and done’, peptide neurotransmitters
how do vesicles fuse with the membrane?
v-SNAREs on the vesicle have partner t-SNAREs on the membrane
calcium binds to synaptotagmin on v-SNAREs, causing a conformational change that makes the SNAREs ‘zipper’ together
what are SNAREs commonly used for?
they are targets for toxins which cause paralyses but small amounts can be used to get rid of wrinkled (botox)
what are the two types of neurotransmitter receptors?
ligand-gated ion channels (ionotropic receptors)
G-protein coupled receptors (metabotropic receptors)
what are ionotropic receptors?
ligand-gated ion channels
binding of neurotransmitter triggers a conformational change on ion channel allowing ions to pass through
what are metabotropic receptors?
G-protein-coupled receptors (not ion channels)
binding of neurotransmitter triggers conformational change of G-protein which has more complex effects
how are neurotransmitters removed from synaptic cleft?
- they diffuse away
- actively taken up by transporters for recycling (into presynaptic neuron/glia)
- destroyed in synaptic cleft by enzymes
differences between electrical and chemical synapses
- electrical signals pass in both directions, chemical only one direction
- electrical signals passed directly, chemical signals can be transformed (inverted, amplified etc)
- electrical synapses are fast, chemicals are slow
similarities between electrical and chemical synapses
- both are plastic (can be modified), but chemical more so
- allow summing up inputs by post-synaptic neuron
features of neuromuscular junction
- motor neuron action potentials always cause muscle cell action potentials
- uses neurotransmitter acetlycholine
- fast and reliable neurotransmission
how does the neuromuscular junction achieve such efficient transmission?
- presynaptic membrane has large number of active zones
- postsynaptic membrane contains junctional folds densely filled with neurotransmitter receptors
active zones and junctional folds are precisely aligned
how do neurotransmitters allow for convergence and divergence?
- each transmitter can activate multiple different receptors (convergence)
- each receptor can activate different downstream effectors
- different transmitters or receptors can activate the same downstream effector
how does dendritic integration work?
EPSPs from different neurons and dendrites summate to reach threshold, so AP generated at axon hillock, which is actively regenerated along axon
why does it matter how excitatory and inhibitory synapses are spatially arranged?
- an inhibitory synapse can block propagation of EPSP towards soma
- GABAa receptors can decrease membrane resistance so current leaks out
- they act by shunting inhibition
how can inhibition act presynaptically?
GABA produced by 3rd party neuron can activate GABAb receptors which can close calcium channels so less neurotransmitter can be released
how does cocaine and amphetamines affect monoamines?
block reuptake of dopamine, norepinephrine (noradrenaline)
how do antipsychotics affect monoamines?
block dopamine receptors
what are the main monoamines?
dopamine, serotonin, norepinephrine (noradrenaline), epinephrine (adrenaline)
what are the modulatory neurotransmitters?
acetylcholine, monoamines
how do antidepressants affect monoamines?
increasing serotonin
selective serotonin reuptake inhibitors (SSRIs)
MAO-A inhibitors (stop breakdown of serotonin)
