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Chapter 5: Synaptic Transmission I (Types of Synapses (Gray's type 1…
Chapter 5: Synaptic Transmission I
Electrical and chemical synapse
Is synaptic transmission electrical or chemical? ANSWER: BOTH CAN OCCUR
Reticular theory: electrical
neurites of different cells fused into a nerve net
current flow was continuous between cells
Neuron doctrine: chemical
nerve cells are separate units connected at SYNAPSES
neurotransmitter is released by the presynaptic cell and does something to postsynaptic cell
electrical synapse: ION CHANNELS connect cytoplasm of pre and postsynaptic cell
chemical synapse: neurons separated by SYNAPTIC CLEFT
current flow at electrical and chemical synapse
electrical: current flows through channels (GAP JUNCTION) between pre and post synaptic cells
if depolarization > threshold, AP in postsynaptic cell
chemical: no current crosses
AP leads to release of NEUROTRANSMITTER that diffuses across synaptic cleft
Transmitter interacts with RECEPTORS on postsynaptic cell (causes either depolarization or hyper polarization)
Functional significance of electrical synapses
fast
reliable
good for SYNCHRONIZING activity of groups of neurons
Chemical synapses
pre and postsynaptic cells are NOT connected structurally
synaptic cleft is LARGER than extracellular space
presynaptic active zone, postsynaptic densification
synaptic vesicles contain neurotransmitter
Types of Synapses
axodendritic
axosomatic
axoaxonic
Gray's type 1
asymmetric
round vesicles
excitatory
Gray's type II
symmetric
flattened vesciles
inhibitory
Neurotransmitters
amines/amino acids
synthesis occurs in axon terminal
stored in SYNAPTIC VESICLES
acetylcholine
peptide neurotransmitters: large molecules of amino acids strung together by ribosomes in the rough ER
Large peptides are formed in the ribosomes in the rough ER
In the Golgi apparatus, the large PRECURSOR PEPTIDE is CLEAVED
the ACTIVE PEPTIDE neurotransmitter is packaged into SECRETORY GRANULES for transport and release
Terms
neurotransmitter: endogenous agonist
agonist: MIMICS the neurotransmitter
antagonist: BLOCKS the neurotransmitter
Neuromuscular junction
muscle cells are large
motor unit: motor neuron and muscle cell it innervates
depolarization (end plate potential)
General Model of Chemical Transmission
transmitter is synthesized and stored in vesicles
AP in presynaptic terminal
depolarization of presynaptic terminal cause opening on V-gated Ca++ channels
Influx of Ca++ through channels
Ca++ causes vesicles to FUSE with presynaptic membrane
transmitter is released into SYNAPTIC CLEFT via exocytosis
transmitter binds to receptor molecules in postsynaptic membrane
opening or closing of postsynaptic channels
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Transmitter secretion: requires Ca++ for vesicle fusion
SNARE proteins bring 2 membranes close togethe: synaptobrevin in synaptic vesicle membrane, syntaxin and SNAP-25 in plasma membrane
calcium binding protein: synaptotagmin in vesicle membrane
Neurotransmitter recovery and degradation
reuptake: neurotransmitter RE-ENTERS presynaptic axon terminal
diffusion: away from synapse
enzymatic destruction: inside terminal cytosol or synaptic cleft
2 different gating mechanisms at chemical synapses: direct and indirect
direct gating (ligand-gated ion channels)
receptor = effector
inotropic receptors
GABA-A receptor: only conducts Cl-
AMPA receptors: permeable to BOTH Na+ and K+
indirect gating (GPCRs)
receptor protein that binds transmitter is DISTINCT from the ion channel
effects of transmitter binding receptor are coupled to an EFFECTOR protein by G-proteins
receptor DOES NOT EQUAL effector
metabotropic receptors
GPCR flow
transmitter binds
G-protein is activated
GDP is replaced by GTP
activated G-protein splits and interacts with ion channels/enzymes that activate second messengers
cascade of events
ion channel opens
Reversal potential
when ABOVE reversal potential --> OUTWARD current
when BELOW reversal potential --> INWARD current
2 signaling mechanisms: axonal conduction of action potential and synaptic transmission
Endocytosis following neurotransmitter release: Clathrin protein coats vesicles for recycling
All receptors have 2 major functions: RECOGNIZE and BIND transmitter, ACTIVATE EFFECTOR proteins