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Neuromuscular Junction Notes

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This is an extract of our Neuromuscular Junction document, which we sell as part of our Physiology and Pharmacology Notes collection written by the top tier of Oxford students.

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Neuromuscular junction
-if asked to talk about NMJ in essay- always start by describing the organisation of the synapse
-site of information transfer from a motor neuron to a muscle cell
-chemical synapse- SYNAPSE- specialised structure where information transfer takes place without physical interactions between 2 participating cells. CHEMICAL SYNAPSE- an electrical signal, an action potential is turned into a chemical signal, neurotransmitters that diffuses from the presynaptic neuron into the post synaptic neuron where it binds to receptors and converts the chemical signal back into an electric signal of an action potential
-evidence that is a chemical synapse- delay between the action potential in the muscle cell and the action potential in the presynaptic axon terminal which has a calcium dependent neurotransmitter release, quantal release of NT
-cholinergic transmission: motor nerves supplying skeletal muscle morphology and function of NMJ
-motor unit: whole assembly of muscle fibres that are innervated by the axon of one motor neuron
-the axon subdivides into many branches known as the axon terminalsunmyelinated nerve processes, bulbous shaped ending- boutons- and each of these make a single point of contact with a skeletal muscle fibre, midway along the length of the muscle fibre. The post synaptic membrane of the muscle fibre is directly under the axon terminal and has extensive invaginations- post junctional folds. The gap between the axon nerminal and the post synaptic membrane is known as the synaptic cleft- 50nm wide and is filled with a meshwork of proteins and proteoglycans Steps involved in a neuromuscular transmission 1) Synthesis of neurotransmitter occurs in the presynaptic membrane Cholinergic transmission: the neurotransmitter, acetylcholine is synthesised in the presynaptic membrane outside of the vesicle using choline and acetyl CoA and the enzyme choline acetyl transferase

-nerves can't make enough choline, choline is taken up from blood, comes from diet and liver
-the Ach is then transported into vesicles, formed from the cell bodies of the motor neurones that are then transported to the axon terminal by microtubules, by ACH-H exchange protein. The carrier protein couples the influx of Ach, with the efflux of H and is an example of secondary active transport. The protons move down the electrochemical gradient which is form by the proton pump powered by the ATP
-vesamicol- drug that inhibits this carrier protein and prevents the uptake of ACH into vesicles Evidence that the neurotransmitter is stored in vesicles
-electron microscope shows clear vesicles - Key evidence: must mention this in essay in conjuction with MEPP
-the evidence that NT are stored in vesicle is shown by the quantal release
-when a microelectrode is placed in the end plate of a frog muscle fibre to measure the resting membrane potential- found that there were depolarisation of O.4 mv at random intervals. These depolarisations known as miniature end plate potentials- size of these miniature end plate potentials showed that potential were discrete multiples of unitary amplitude. 1 quantum- corresponding to the release of ACH from one synaptic vesicle
-the miniature end plate potentials are purely spontaneous release (basal release), keeps th receptors positioned in the right place beneath the presynaptic terminal
-when the end plate potential is measured, the potential is of integer multiples of the miniature end plates- quantal nature 2) action potential in the presynaptic membrane When an action potential arrives in the presynaptic membrane it depolarises and stimulates the opening of voltage gated calcium ions.
-the voltage gated calcium channels are held near the active zone where the vesicles are docked and primed by the t snare protein syntaxin- this ensures that there is a high calcium concentration locally, this increases the speed and there is a minimal delay in calcium binding to the vesicles

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