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SYNAPTIC & NEUROMUSCULAR
TRANSMISSION
Ass. Prof. Dr. Emre Hamurtekin
EMU Faculty of Pharmacy
INTRODUCTION
• Axodendritic synapses
• Axosomatic synapses
• Axo-axonal synapses
INTRODUCTION
• Each presynaptic terminal of a chemical synapse is
seperated from the postsynaptic structure by a synaptic
cleft.
• Postsynaptic density is a thickening located in the
postsynaptic structure and a complex of specific receptors,
binding proteins and enzymes.
• Synaptic vesicles: Membrane-enclosed vesicles inside the
presynaptic terminal which contain neurotransmitters.
 Small, clear synaptic vesicles (Ach, Gly, GABA, glutamate)
 Small vesicles with a dense core (catecholamines)
 Large vesicles with a dense core (neuropeptides)
• Ca enters the presynaptic neuron and triggers exocytosis of
neurotransmitters.
INTRODUCTION
EXCITATORY & INHIBITORY POSTSYNAPTIC
POTENTIALS
• A single stimulus produces an initial depolarizing response and
after reaching its peak, declines exponentially. During this
potential, the excitability of the neuron to other stimuli is
increased. This potential is called excitatory postsynaptic
potential (EPSP).
• The excitatory transmitter opens Na or Ca channels in the
postsynaptic membrane.
• Stimulation of some inputs produces hyperpolarizing responses
and excitability of the neuron to other stimuli decreases. This
potential is called inhibitory postsynaptic potential (IPSP).
• An IPSP can be produced by a localized increase in Cl transport;
negative charge is transferred into the cell.
• IPSP can also be produced by opening of K channels or closure of
Na and Ca channels.
EXCITATORY & INHIBITORY POSTSYNAPTIC
POTENTIALS
TEMPORAL & SPATIAL SUMMATION
• Temporal summation: If a second EPSP from a single
neuron is elicited before the first EPSP decays, the two
potentials summate and their additive effects are sufficient
to induce an action potential in the postsynaptic
membrane.
• Time constant of the postsynaptic neuron affects the
amplitude of the depolarization caused by consecutive
EPSPs produced by a single presynaptic neuron.
• Spatial summation: EPSPs from different presynaptic
neurons summate and their additive effects become
sufficient to induce an action potential in the postsynaptic
membrane.
• Length constant of the postsynaptic neuron affects the
amplitude of the depolarization caused by consecutive
EPSPs produced by diffrent presynaptic neurons.
INHIBITION AT SYNAPSES
• Inhibition in the CNS can be;
postsynaptic
presynaptic
• Postsynaptic inhibition occurs when an
inhibitory transmitter (i.e. glycine, GABA) is
released from a presynaptic nerve terminal
onto the postsynaptic neuron.
POSTSYNAPTIC INHIBITION
INHIBITION AT SYNAPSES
• Presynaptic inhibition is a process mediated by
neurons whose terminals are on excitatory endings,
forming axoaxonal synapses.
• There are 3 mechanisms for presynaptic inhibition:
Increase in Cl conductance and reduces Ca entry and
reduction in the amount of excitatory transmitter release.
Opening of voltage-gated K channels results with K efflux
and thus Ca entry decreases.
Direct inhibition of transmitter release independent of Ca
influx.
GABA is the first transmitter shown to produce presynaptic inhibition.
 Increase in Cl conductance (GABA-A receptors)
 Increase in K conductance (GABA-B receptors)
PRESYNAPTIC FACILITATION
• The action potential is prolonged and this
increases the duration that the Ca channels
stay open.
NEUROMUSCULAR JUNCTION
NEUROMUSCULAR TRANSMISSION
1. The impulse arriving in the end of the motor neuron increases the
permeability of its endings to Ca.
2. Ca enters the nerve ending.
3. Ca triggers the exocytosis of acetylcholine-containing synaptic
vesicles.
4. Acetylcholine in the synapse binds to nicotinic receptors located in
the motor end plate.
5. Binding of Ach to these receptors increases the Na and K
conductance.
6. Influx of Na produces a depolarizing potential (end-plate potential)
7. Local potential depolarizes the adjacent muscle plasma membrane
and action potential occurs in the muscle membrane.
8. The muscle membrane action potential initiates muscle
contraction.
THE END