Synaptic and Neuromuscular Transmission Flashcards
Synpase
. Specialized connection btw neuron and receptor cell
. Presynaptic and postsynaptic regions and a space (synaptic cleft)
Electrical synapses
. Cell membrane of presynaptic and postsynaptic cells physically close in association
. APs transmitted directly via gap junctions
. No synaptic cleft
. NOT COMMON IN MAMMALS
Chemical synpase
. Electrical signal transmitted indirectly across cleft via chemical neurotransmitter
. STANDARD IN PEOPLE
Chemical synapse mechanism
. AP in presynaptic cell causes fusion of vesicles w/ membrane and release of neurotransmitter (NT) into cleft
. NT diffuses across cleft and binds to receptors on postsynaptic membrane
. Results in opening or closing of ion channel
. Change in ion channel activity causes graded potentials in postsynaptic cell that depolarize or hyperpolarize cell
. Delay seen btw pre and post cells due to chemicals having to diffuse and bind
Things that could happen to neurotransmitter floating around synaptic cleft
. Reuptake: taken up by presynaptic cell for recycling
. Diffusion: floats out of cleft
. Degradation: enzymes present in and around synapse degrade it
. Makes it so signal is present for a very short time in absence of further stimulation
Summation of signals
EPSPs and IPSPs need to sum together to reach threshold for firing APs
Integration of signals
. Input from multiple sources is combined through process of summation of large number of excitatory and inhibitory signals
Convergence of signals
Effect of large number of presynaptic neurons impinging on single postsynaptic neurons
. Most neurons receive input from many other neurons
Divergence of signals
. Single presynaptic neuron impinging on more than 1 postsynaptic neuron
. Normally is only 1 axon, but it may have collaterals that output onto a number of postsynaptic neurons
Physiological modulation
. Normal regulation of synaptic transmission
. Postsynaptic potentials can be variable in magnitude due to the changes in presynaptic and sometimes postsynaptic factors
. Much more common is presynaptic modulation
Presynaptic facilitation or inhibition is most often due to ____
. Changes in presynaptic Ca concentration that inc. or dec. vesicle fusion
Presynaptic facilitation
. Excitatory neurons impinging on presynaptic terminal or axon
. Inc. amt neurotransmitter released from presynaptic terminal
Presynaptic inhibition
Inhibitory neuron impinging on presynaptic terminal or axon
. Dec. amt of NT released from presynaptic terminal
. Impinging neuron has no direct effect on firing on postsynaptic neuron that is interacting with the neuron it is impinging on
. Only indirect effect by inhibiting the neuron it is impinging so there is no transmission to the final postsynaptic neuron
Modulation of synaptic transmission by drugs
. Every step in normal process can be enhanced or inhibited by drugs
. Most important sites of action for neurologically active drugs is synapses
How tetanus bacillus toxin interferes w/ motor neurons
. Stops inhibitory motor neurons of skeletal muscle so they are in constant activation
. Releases inhibitory transmitters from interneurons inn spinal cord causing no presynaptic transmission
. Causes proteolysis of SNARE proteins involved in fusion of synaptic vesicles to presynaptic plasma membrane
What kind of neuron excites skeletal muscle?
Alpha-motor neuron
Neuromuscular junction
. Motor axon terminals embedded in grooves in skeletal muscle fiber’s surface
. ACh-containing vesicles concentrated in active zone in synaptic terminal
. Active zones located over junctional folds in motor end-plate
Neurotransmitter in neuromuscular junction
. ACh
. Synthesized from choline and acetyl CoA via choline acetyltransferase (ChAT)
. Stored in vesicles in presynaptic terminal
. Found in active zone directly opposite moor end-plate
Exocytosis of ACh vesicles process
. Depolarization causes inc. in intracellular Ca via voltage-gated channels
. Inc. in Ca culminates in ACh release
. More intracellular Ca, more vesicles that fuse so more ACh released
Motor end-plate
. Specialized region of sarcolemma
. High contraction of ACh receptors (nicotinic cholinergic receptors)
. ACh-induced excitation of muscle fiber only happens here
. Have junctional folds to inc. surface area
Nicotinic cholinergic receptors
. Ligand-gated
. Several subunit form a pore
. 2 molecules ACh bind to alpha-subunits exposed on membrane surface
. Opens pore in portion of receptor embedded in lipid bilayer
. Na and K ions flow through open channel down electrochemical gradients
End plate potential (EPP)
. Channels opening in response to normal ACh release allow current to flow and depolarize end plate
Characteristics of EPP
. Graded
. Generated by mixed cation channel that is art of same protein as nicotinic receptors
. Conducts NA better than K so the equilibrium potential is near 0 mV
Effect of EPP
. Causes postsynaptic membrane potential to move towards 0mV
. Exhibits excremental conduction
. Augmented in amplitude and duration when rate of ACh degradation is dec. from drugs (negostigmine)