INTRODUCTION Skeletal muscle Voluntary muscle Contracts only if the motor nerve supplying it is stimulated What happens in Stimulation – biological, CARDIAC &mechanical, electrical, chemical SMOOTH MUSCLES
• 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
CharacteristicsExcitability: 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 bodys 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 extensionTetanizabilityFatiguabilityRecruitment
Nerve and Blood Supply1. 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 nerves 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
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 Depolarization of the – Less movement of K+ outward membrane• Result is an END PLATE POTENTIAL
• 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
Acetylcholine2 ACh BIND Receptors Na+ Initiation of EPP AP
Acetylcholine Few msec After release subsequently destroyed by acetylcholinesterase Enzyme located on the Motor End Plate membraneMuscle cell response terminated EPP terminatedEnd products = Acetyl CoA + Choline recycled
What is EPP? EPSP--A small depolarization of post synaptic membrane• Similar to an Excitatory in response to neurotransmitter binding Post Synaptic Potential Brings the membrane closer To threshold• Ionic basis: A small trigger can now generate Action Potential Large movement of Na+ inward Less movement of K+ outward• 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
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 CHROMOSOME• A protein called #1 SECRETES rapsyn is then GENE- AGRIN recruited to the primary MuSK scaffold, to induce the additional clustering of acetylcholine receptors (AChR). This is thought of as the secondary scaffold. PHOSPHORYLATION OF MUSK RECEPTOR• A protein called Dok-7 has shown to be additionally required for the formation of the secondary scaffold; it is apparently recruited after RECRUITS CASEIN KINASE 2 MuSK phosphorylation REQD FOR CLUSTERING and before acetylcholine receptors are clustered.
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 Blocks ACh-R sites Prevents Acetylcholine inactivation of ACh Snake venom OrganoBlack Widow bungarotoxin Myasthenia PhosphatesSpider venom Pesticides/ gravis Nerve Gas Botulinum Curare toxin Neostigmine/ Autoimmune antibodies Physostigmine ACh – R- inactivated Blocks release of acetylcholine Irreversibly inhibits ACh-esterase Reversibly bindsExplosive release of ACh Temporarily inhibits With ACh - R 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- ethylmaleimidesensitive 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.