Neuro 5 Flashcards
Describe what is in an neuromuscular junction
Pre synaptic terminal
Synaptic cleft
Post synaptic endplate region of muscle fibre
What is a neuromuscular junction
Specialised structure involving distal axon terminal and muscle membrane, allowing for unidirectional chemical communication between nerves and muscle
Points of failure for NMJ
Demyelination - Leads to reduced conduction velocity and muscle weakness or uncoordinated movement
How many neurones supply a muscle fibre, and what happens otherwise
Usually 1 nerve supplies the bundles of synapses for 1 muscle fibre. If nerve is damaged reinnervation can cause a muscle fibre to have branched form multiple neurones or be innervated by branches from previously unconnected nerves
Describe what happens in synapse/NMJ
- AP opens VGCCs causing an influx of Ca2+
- Ca2+ influx triggers vesicle exocytosis so that ACh diffuses into the cleft and binds to receptor-cation channels, opening these on the post-synaptic neurone
- Local currents flow from depolarised region to adjacent region, triggering an action potential
- ACh broken down by acetylcholine esterase and muscle fibre response ceases
What happens when NMJ is at rest
Individual vesicle release Act at low rate causing miniature end plate propagation (MEPP)
Describe structure of myofibril
Covered by sarcolemma plasma membrane. Has T tubules which tunnel into centre. Sarcoplasm contains myoglobin and mitochondria. Sarcoplasmic Reticulum = fluid filled tubules contain Ca2+
What happens to structure of myofilaments during contraction
During contraction I band became shorter
A-band remained same length
H-zone narrowed or disappeared
Describe activation process when an AP arrives
Action potential propagates along surface membrane and into T-tubules
DHP (dihydropyridine) receptor in T-tubule membrane: senses V & changes shape of the protein link to Ryanodine receptor, opens the Ryanodine receptor Ca2+ channel in the SR membrane; Ca2+ released from SR into space around the filaments
Ca2+ binds to Troponin & Tropomyosin moves allowing
Crossbridges to attach to actin
Describe relaxation process in muscle after AP
Ca2+ is actively transported into the SR continuously while action potentials continue by ATP- driven pump
Ca2+ dissociates from TN when free Ca2+ declines; TM block prevents new crossbridge attachment; Active force declines due to net crossbridge detachment
What can disorders of NMJ cause
Muscle weakness
Describe botulism
Botulinum toxin disrupts production of stimulation induced ACH by presynaptic nerve terminal
Describe Lambert Eaton myastenic syndrome
Autoimmune diease. Antibodies target voltage gated calcium ion channels. Associated with lung cancer
Describe Myasthenia Gravis
Autoimmune disease. Antibodies target Act receptors.
Causes fatigable weakness
Usually will have personal or family history of other autoimmune diseases
Affects ocular, respiratory, limb, bulbar muscle.
Detected by EMG examination as antipodes are present in blood
If really ba, blood can be removed via plasma exchange
