MSS11 Neuromuscular Junction And Motor Unit Flashcards
Peripheral synapse vs Central synapse (Neuromuscular junction (NMJ) vs Hippocampal synapses)
NMJ has been widely studied because
Advantages of NMJ:
- Size
- Accessibility
- Availability of α-bungarotoxin
(neurotoxic protein that is known to bind competitively and in a relatively irreversible manner to the nicotinic ACh receptor found at the NMJ, blocking the action of ACh
–> causing paralysis, respiratory failure, and death
–> numerous applications in neuroscience)
Tripartite synapse
NMJ a type of Tripartite synapse:
Functional integration and physical proximity of the **presynaptic differentiation, **postsynaptic differentiation, and their intimate association with surrounding ***glia (Perisynaptic Schwann cell)
–> combined contributions of 3 synaptic components to the activity at the chemical synapse
Synthesis and degradation of ACh
Choline + Acetyl CoA (in mitochondrion: CoA –> Acetyl CoA)
- -> by ***Choline acetyltransferase inside axon terminal
- -> ACh
- -> rapidly broken down by AChE on postsynaptic membrane in synaptic cleft
- -> Acetate + Choline
- -> ***Choline transported back to axon terminal
Activation of AChR by ACh
Voltage-gated Ca channel (pre-synaptic membrane) open
- -> trigger release of ACh
- -> 2 ACh bind to 1 AChR
- -> conformational changes
- -> opening of ion channel
- -> influx of Na + small efflux of K
- -> membrane depolarisation
- -> muscle contraction
AChR clustering
- By **contact with Neurites (projection from the cell body of a neuron)
- -> Nerve innervation of muscle fibres
- -> induce AChR clustering via **Agrin-Lrp4-MuSK signaling pathway (rather than have to be activated in order to cluster)
- -> Agrin activates receptor complex (binding protein Lrp4 + muscle-specific kinase MuSK)
- -> MuSK activation (mins)
- -> initiate AChR phosphorylation + clustering (hrs) - By Extracellular matrix factors
- -> New myofibers formed beneath basal lamina of muscle cell after damage of muscle cells, and AChR clusters were observed to reform at the original synaptic site (even if motor nerve were absent/cut)
- -> Clustering signals found to be secreted ***proteoglycan which incorporated into the extracellular matrix - Agrin-MuSK signaling required
NMJ disease: Myasthenia Gravis
Progressive disease:
- ***Ocular symptoms
- eyelid drooping
- double / blurred vision
- weakness of eyeball muscle - ***Oral symptoms
- Generalised MG (head, arms, legs)
Pathogenesis: Autoimmune: AutoAb destroy postsynaptic AChR (85% seropositive) --> endocytosis + degradation of AChR --> ↓ postsynaptic AChR density --> muscle weakness
Pathophysiology:
- Activation of **complement cascade –> MAC formation –> **Disruption of NMJ structure
- AutoAb –> Induction of AChR ***endocytosis (↓ AChR density)
- AutoAb –> Inhibition of AChR ***functions (i.e. channel properties)
Serological diagnosis: ELISA
Detect and quantify circulating ***IgG AutoAb in serum sample:
Recombinant proteins coated surface (with AChR) + Serum sample (with IgG AutoAb)
–> on-top incubate with enzyme-conjugated (horseradish peroxidase) secondary anti-human Ab (stick to IgG AutoAb)
–> incubate colorimetric substance
–> color detection
Limitation:
False +ve caused by hydrophobic binding of immunoglobulin components in sample to solid surfaces
Serological diagnosis: RIPA / RIA (Radioimmunoprecipitation assay / Radioimmunoassay)
Radioactive α-bungarotoxin to indicate **presence of pathogenic AutoAb against AChR:
Mixture of **radioactive α-bungarotoxin (or other antigens) + detergent extracts of human muscle (contain AChR) + patient’s serum sample (contain AutoAb)
—> form complex
—> incubate and pull down with secondary anti-human Ab by centrifugation (precipitate AutoAb with Anti-Ig)
—> radioactive signals in the pellet
Limitation:
Safety precautions required when using ***radioactive isotopes
Serological diagnosis: CBA (cell-based assay)
Transfection of DNA plasmid encoding protein of interest (AChR / MuSK) into heterologous cells
- -> exogenously express proteins on cell surface
- -> incubate with patient’s serum (with AutoAb)
- -> incubate with fluorescent secondary anti-human Ab (stick to AutoAb)
- -> fluorescence signals
Limitation:
Time-consuming, labour intensive
Current treatment for MG
- Anti-AChE therapy
- ***Pyridostigmine (AChE inhibitor) (要多d ACh)
–> reduce degradation of ACh
–> strengthen neuromuscular transmission
Limitation: effect wear off after a few hours - Immunosuppressant therapy
- ***Corticosteroids
–> induce steroid hormones to suppress immune system
Limitation: not good for chronic disease, harder to fight infection - Intravenous immunoglobulin (IVIG)
Motor unit in skeletal muscle
- contain hundreds of muscle fibres
- contract at the same time
- controlled by single motor neurone
- Muscles fibres from different motor units are intermingled
- -> forces applied to tendon remain roughly balanced regardless of which motor units stimulated
Motor unit recruitment
- Successive activation of the same / additional motor units
- Increasing strength of voluntary muscle contraction
By:
- ↑ no. of active motor units (***Spatial recruitment)
- ↑ firing frequency (***Temporal recruitment –> could lead to tetanus)
- Both mechanisms concurrently
Electromyography (EMG)
- Diagnostic procedure to assess health of muscles and motor neurone
- Translate electrical signals (MUAP motor unit action potential) between motor neurones and muscles –> graphs, sounds, values
- Surface / Intramuscular
Surface EMG vs IM EMG
Surface:
- non-invasive
- limited assessment of muscle activity
- surface electrode recordings restricted to ***superficial muscle
IM:
- insertion of needle electrodes through skin into muscle tissue
- common for neuromuscular disorders
- involves voluntary contraction of muscle –> less informative in patients unwilling / unable to cooperate
Motor unit recruitment by EMG recording
minimal effort of muscle contraction:
single motor unit firing at 6Hz
↑ muscle strength:
recruitment of second motor unit (second waveform with ↑ frequency)
–> further ↑ in muscle strength
–> third motor unit recruited (third waveform with ↑ frequency)
