lec 1 stu
TRANSCRIPT
Block : Musculoskeletal
Lectures : 5
Tutorials : 2
Practicals : 2
Review : 1
Skeletal muscle
Voluntary muscle
Contracts only if
the motor nerve supplying it is stimulated
Stimulation – biological, mechanical, electrical, chemical
What happens in
CARDIAC &
SMOOTH
MUSCLES
INTRODUCTION
• Motion is an essential body function resulting from contracting and relaxing of muscles. – Muscles provide force.
– Bones act as levers.
• Muscle tissue constitutes 40 to 50% of body weight.
motion maintenance of posture
heat production
Types of muscles
Characteristics Excitability: Ability of muscles to receive and respond to stimuli. Stimulus is
a change in the internal or external environment. Stimulus must be strong enough to create an action potential - nerve impulse. Response is the body's reaction to a stimulus.
Contractility: Ability to shorten and thicken. Extensibility: Ability to be stretched (extended). Many skeletal
muscles are in opposing pairs: extensor/contractor. Elasticity: Ability to return to original shape after contraction or
extension Tetanizability
Fatiguability
Recruitment
Nerve and Blood Supply
1. Innervation and vascularization are directly
related to contraction.
2. An artery and one or two veins accompany each
nerve that penetrates a skeletal muscle.
3. Capillaries branch through the edomysium.
Each muscle fiber is in contact with one or two
capillaries.
4. Each skeletal muscle fiber makes contact with a nerve's synaptic end bulb.
Skeletal Muscle • Controlled by Somatic NS
• Skeletal muscle specific terms:
– Neuromuscular junction
– Motor endplate – skeletal muscle on the receiving end of nm junction
– End Plate Potential (EPP) – generator potential of skeletal muscle
• ACh release is quantal (miniature end plate potential = 0.4 mV)
Some definitions… • Motor Unit
– Composed of an alpha
motorneuron and all the
myofibers innervated by that
neuron
• Motor Endplate
– The region of the myofiber
directly under the terminal
axon branches
• Neuromuscular junction
– Where the axon terminal and
the motor endplate meet
Neuromuscular Transmission
TRANSMISSION
• An action potential in a motor neuron is propagated to the axon terminal
• Triggers the opening of voltage gated Ca2+ channels
• Calcium enters into the terminal button
• Calcium helps the vesicles [ containing neurotransmitters] to arrange themselves along the border of the terminal membrane
• Release of Acetylcholine [ACh] by exocytosis
• FOUR ways in which Acetyl choline can be released: – Constant leak or molecular sieve – Spontaneous quantal release leading to
small transient depolarisations of 0.5mV giving rise to miniature end plate potential (MEPP) at a frequency of about 2Hz. • This is too small to cause muscle contraction.
The function of mepp is not yet known.
– Additional type of release that is quantal but
unrelated to nerve impulse and occurs only
when normal ion dependant quantal release is
impaired eg botulinum toxin.
– Nerve impulse evokes huge quantal release
(=300 quanta) of acetyl choline and leads to
the depolarisation of the post junctional
membrane.
• This constitutes full size end plate potential (EPP) and triggers excitation-contraction coupling followed by muscular activity.
• Acetylcholine diffuses across the space separating the nerve and the muscle cells and binds with receptor sites specific for it on the motor end plate of the muscle cell membrane
• Binding of ACh to its receptors opens cation channels [ ligand gated channels] – Large movement of Na+ inward
– Less movement of K+ outward
• Result is an END PLATE POTENTIAL
Depolarization of the membrane
• Local current flow occurs between depolarized end plate and the adjacent membrane
• Voltage gated Na+ channels opened more Na+ entry more depolarization
• This initiates an ACTION POTENTIAL which propagates throughout the muscle fiber
Acetylcholine Receptors
Initiation of
EPP
AP
2 ACh BIND
Na+
Acetylcholine
subsequently destroyed by acetylcholinesterase
Enzyme located on the Motor End Plate membrane
Muscle cell response terminated EPP terminated
End products = Acetyl CoA + Choline recycled
Few
msec
After release
Nicotinic type
What is EPP? • Similar to an Excitatory
Post Synaptic Potential
• Ionic basis:
• Magnitude of EPP much larger – More neurotransmitter released – Larger surface area of motor end plate
• High density pf receptors
• More binding sites
– More ion channels: greater influx of positive ions: larger depolarization
• Graded Potential – – magnitude depends on amount and duration of Ach at the end
plate
EPSP--A small depolarization
of post synaptic membrane
in response to neurotransmitter binding
Brings the membrane closer
To threshold
A small trigger can now generate
Action Potential Large movement of Na+ inward Less movement of K+ outward
http://www.jneurosci.org/cgi/reprint/30/1/13.pdf
SNARE
• SNARE proteins (an acronym derived from
"SNAP (Soluble NSF [N-ethylmaleimide
sensitive fusion proteins Attachment Protein)
REceptors"
• The primary role of SNARE proteins is to mediate vesicle fusion, that is, the exocytosis of cellular transport vesicles with the cell membrane at the porosome or with a target compartment (such as a lysosome).
Agrin
• is a large proteoglycan – Role is in the development of theneuromuscular
junction during embryogenesis.
– named based on its involvement in the aggregation of acetylcholine receptors during synaptogenesis.
– In humans, this protein is encoded by the AGRIN gene on chromosome 1
• It may also have functions in other tissues and during other stages of development.
• It is a majorproteoglycan component in the glomerular basement membrane and may play a role in the renal filtration and cell-matrix interactions, indicated in glomerulopathy.
Mechanism of action
• During development, the growing end of motor neuron axons secrete a protein called agrin.
• This protein binds to several receptors on the surface of skeletal muscle.
• The receptor which seems to be required for formation of the neuromuscular junction (NMJ) is called the MuSK receptor (Muscle specific kinase).
• MuSK is a receptor tyrosine kinase - meaning that it induces cellular signaling by causing the addition of phosphate molecules to particular tyrosines on itself and on proteins which bind the cytoplasmic domain of the receptor.
• In addition to MuSK, agrin binds several other proteins on the surface of muscle, including dystroglycan and laminin. Apparently these additional binding steps are required to stabilize the NMJ.
• The requirement for Agrin & MuSK in the formation of the NMJ was primarily demonstrated by "knockout" mouse studies. In mice which are deficient for either protein- Agrin or MuSK, the neuromuscular junction does not form.
• Many other proteins also comprise the NMJ, and are required to maintain its integrity. For example, MuSK also binds a protein called "dishevelled" (Dvl), which is in the Wnt signalling pathway. Dvl is additionally required for MuSK-mediated clustering of AChRs, since inhibition of Dvl blocks clustering.
CollagenQ (ColQ) • plays an important structural role at NMJs
– by anchoring and accumulating acetylcholinesterase (AChE) in the extracellular matrix (ECM).
– interacts with perlecan/dystroglycan and the muscle specific receptor tyrosine kinase (MuSK), key molecules in the NMJ formation.
• important regulatory functions at the synapse by controlling AChR clustering and synaptic gene expression through its interaction with MuSK, post synaptic differentiation/maintenance
• MuSK promotes acetylcholine receptor (AChR) clustering in a process mediated by rapsyn, – a cytoplasmic protein that stimulates AChR packing in clusters
and regulates synaptic gene transcription of ColQ both in vitro and in vivo.
Signaling • A protein called
rapsyn is then recruited to the primary MuSK scaffold, to induce the additional clustering of acetylcholine receptors (AChR). This is thought of as the secondary scaffold.
• A protein called Dok-7 has shown to be additionally required for the formation of the secondary scaffold; it is apparently recruited after MuSK phosphorylation and before acetylcholine receptors are clustered.
PHOSPHORYLATION OF
MUSK RECEPTOR
SECRETES
RECRUITS CASEIN KINASE 2 REQD FOR CLUSTERING
CHROMOSOME #1
GENE- AGRIN
Disease of Presynaptic NMJ
Mechanism of Disease • Autoimmune neuromyotonia (Isaacs' disease)
– Antibodies directed towards delayed rectifier potassium channels in terminal nerve fibres. Results in inefficient repolarization after action potential
• LEMS
– IgG antibodies directed towards presynaptic voltage-gated syndrome calcium channels. Results in decreased mobilization and, therefore, decreased release of ACh vesicles
• Botulism – Botulinum toxins inhibit Synaptic Vesicle [SV] exocytosis by proteolysis of
components of the SNARE complex
• Congenital MG – May be a result of deficiency of choline acetyltransferase leading to a
defect of presynaptic ACh resynthesis, or a result of paucity of SVs
Disease of Post synaptic NMJ Mechanism of Disease
• Acquired MG – Autoimmune disease in which IgG autoantibodies (Abs) are
directed towards the postsynaptic nAChR (in 85% of patients). Others have Abs directed towards the MuSK receptor
• Neonatal MG – Seen in babies born to mother with MG. A result of placental
transfer of anti-AChR AbsDrug-induced MG Most commonly seen after treatment with penicillamine. Reverses after withdrawal of drug
• Congenital MG – Most commonly due to genetic defects in the AChR subunit
morphology. Rare and complex group of diseases. • Slow channel syndromes are because of a defect producing
prolonged opening of AChR channels. Results in varying degrees of myopathy.
• Fast channel syndromes result in a decreased affinity of the AChR for ACh
Chemical agents & diseases can affect the NMJ
and thus affect the transmission & muscular
activity
Altered release of
Acetylcholine Blocks ACh-R sites Prevents
inactivation of ACh
Black Widow
Spider venom
Snake venom
bungarotoxin
Botulinum
toxin
Explosive release of ACh
Blocks release of
acetylcholine
Curare
Myasthenia
gravis
Reversibly binds
With ACh - R
Autoimmune antibodies
ACh – R- inactivated
Organo
Phosphates
Pesticides/
Nerve Gas
Neostigmine/
Physostigmine
Irreversibly inhibits
ACh-esterase
Temporarily inhibits
ACh-esterase
• Botulinum toxin is an enzyme produced by Clostridium botulinum, a spore-forming bacillus. C botulinum is an obligate anaerobic, motile, gram-positive (in young cultures), straight to slightly curved rod with oval, subterminal spores
• One gram of crystalline botulinum toxin is enough to kill 1 million people
• In normal muscle tissue (left), there are normal soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins with normal release of acetylcholine at the neuromuscular junction. At right, flaccid paralysis occurs when SNARE proteins are cleaved by botulinum toxin, resulting in inhibition of synaptic vesicle fusion and absence of acetylcholine release.