Synaptic Transmission Flashcards
1
Q
Neuronal morphology
A
- neurons are specialized to rapidly send and receive info
- dendrites-tapered processes arising from the cell body that greatly increase the receptive surface; membranes have receptors for transmitters and voltage gated ion channels that can amplify the graded synaptic signal
- cell soma- surrounds nucleus and contains the ER, Golgi, performs house keeping functions-membrane contain receptors that bind chemical transmitters released by afferent neurons
- axon
- presynaptic terminals
2
Q
Axon
A
- single thing processes arising from the cell body at the axon hillock
- transmits all or none action potentials to the terminals after integrating transmitter mediated bioelectrical changes received in the dendrites and cell body
- some azons can be a meter or longer in length containing 1000 times as much cytoplasm called axoplasm as cell body
- many axons are surrounded by a glial-derived myelin sheath which greatly increases the speed of impulse propagation via the process of saltatory conduction
3
Q
Presynaptic terminals
A
- specialized structures that convert electrical signals propagated down the axon (action potentials) into chemical signals (neurotransmitter), released from presynaptic vesicles and transmitted to the target cell at the synapse (the point of contact between the pre- and post- synaptic neuron/target cell)
- postsynaptic potentials differ from action potentials; they are small graded changes
4
Q
Axoplasmic transport
A
- axoplasm contains parallel arrays of microtubules and neurofilaments that provide structural stability and a means to transport materials back and forth between the cell body and presynaptic terminals
- fast axoplasmic transport occurs between thecell soma and presynaptic terminals in both anterograde and retrograde directions
- cancer patients -> microtubule disrupting agents -> develop peripheral neuropathies due to gradual vesicle depletion especially in axons innervating distal muscles
- microtuble motor kinesin mediates fast anterograde transport of mitochondria and vesicles from soma to terminals, wherease dynein mediates fast retrograde transport of degraded vesicular membranes and absorbed toxins/viruses/growth factors from terminal to soma
5
Q
Electrical synapses provide speed and synchony
A
- are extremely fast and most are bidirectional- they allow direct passive flow of electrotonic current between cells via specialized elements called gap junctions
- gap junctions are comprised of intramembrane channel proteins called connexins- to hemi channels termed connexons combine to form a gap junction between two cells
- current flow changes the postsynaptic membrane potential, which can lead to generation of an action potential
- pores are larger than voltaged gated or ligand gated channels- so there is unselective diffusion of ions and substances such as ATP and metabolites
6
Q
Chemical neurotransmitters
A
- present in the presynaptic terminal
- released in a voltage and calcium dependent manner (calcium is high outside and low inside cells)
- specific receptors present in postsynaptic target cell
- means to inactivate the transmitter (eg enzymatic breakdown/AChE or re-uptake via pumps/glutamate/GABA
7
Q
Chemical synapse in synaptic transmission
A
- transmitter molecules are synthesized and packaged in vesicles
- an action potential arrives at the persynaptic terminal
- depolarization of terminal opens voltage gated calcium channels
- increased calcium in terminal triggers vesicle fusion
- transmitter diffuses across cleft and binds to postsynaptic receptors
- a postsynaptic response occurs
- transmitter molecules are cleared/inactivated by enzymatic degradation, uptake, or diffusion
8
Q
Synapses
A
- presynaptic terminals have mitochondria and numerous vesicles
- some vesicles are docked in active zones
- these vesicles were preferentially released in response by action potential, activation of voltage sensitive calcium channels and influx of calcium
- the postsynaptic density was an area of electron dense areas with many receptos
- glucose is major source of energy in brain and neurons, the presynaptic terminals consume most of the energy- fMRI
9
Q
Exocytosis and vesicle fusion
A
- neurotransmitter release from presynaptic terminal exocytosis
- calcium dependent process involves tight interaction of proteins on the vesicle membrane (V-SNARES- synaptobrevin and Synaptotagmin) with proteins on presynaptic target membrane (T-SNARES- SNAP-25 and syntaxin)
- vesicle membrane that has fused to the presynaptic membrane is recycled through endocytosis
10
Q
Botulinum toxin
A
-causes muscle weakness by cleaving SNAP-25 or synaptobrevin proteins in the presynaptic terminal of alpha motor neurons
11
Q
Chemical transmitters
A
- numerous chemical transmitters: glutamate, acetylcholine, norepinephrine, dopamine, GABA, and glycine
- others are large peptides such as enkephalins and vasoactive intestinal polypetide
- depending on chemical structures and receptor interactions: rapid (ligand gated NMDA, AMPA, nicrotinic cholinergic receptors); moderately (metabotrophic glutamate and muscarinic cholinergic receptors); or slowly (receptors for various peptides and proteins) with time courses ranging from msec to days
12
Q
Ionotrophic receptor
A
- contain an ion channel as part of their structure and transmitter binding triggers a rapid response
- nicotinic ACh receptor channel activation-> membrane depolarization-> action potential excitation-> muscle contraction
13
Q
Metabotropic Receptor
A
- commonly linked to G proteins that transduce a slower biochemical signal
- muscarinic ACh receptor activatiom
- release of alpha-GTP and beta-gamma from the heterotrimeric G proteins -> activation of inward rectifier K+ channel by beta gamma -> membrane hyperpolarization -> decrease in heart rate
14
Q
Postsynaptic potentials-
A
- produced through conductance changes due to ion channel openings (and sometimes closings) lead to ionic current flow through the channels that in turn lead to changes in the membrane potential
- excitatory PSPs (EPSPs) increase the probability that an action potential will be triggered whereas inhibitor PSPs (IPSPs) decrease the likelihood
- resting neuron: -65mV
- Excited neuron- less negative intraneuronal potential caused by sodium influx -45mV
- Inhibited Neuron- inhibited state, more negative intraneuronal membrane potential (-70mV) cased by K efflux, Cl- influx or both
15
Q
Glutamate
A
- major excitatory transmitter in the brain
- binding of glutamate to specific ligand-gated ion channels in the target cell produces both fast and slow depolarizing responses (EPSP or currents EPSCs) because distinct receptors
- AMPA- selectively activated- mediate a fast EPSP via the flow of Na+ and K+ ions
- NMDA receptor channels selectively activated by NMDA and mediate a slower component of EPSP via the flow of Na+, K+, Ca2+ (do not open unless the membrane is sufficiently depolarized b/c they are blocked in a voltage dependent manner by Mg2+
- metabotropic glutamate receptors are even slower- because G-protein linked receptors
