2. 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
3. • 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
5. 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
6. 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.
7. 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)
8. 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
11. 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
12. • 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.
13. – 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.
14. • 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
15. • 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
17. Acetylcholine Few
msec
After release
subsequently destroyed by
acetylcholinesterase
Enzyme located on the
Motor End Plate membrane
Muscle cell response terminated
EPP terminated
End products = Acetyl CoA + Choline recycled
19. 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
21. 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).
22.
23. 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.
24. 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.
25. • 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.
26. 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.
27. 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.
28. 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
29. 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
30. 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 Organo
Black Widow bungarotoxin Myasthenia Phosphates
Spider venom Pesticides/
gravis
Nerve Gas
Botulinum Curare
toxin
Neostigmine/
Autoimmune antibodies Physostigmine
ACh – R- inactivated
Blocks release of
acetylcholine Irreversibly inhibits
ACh-esterase
Reversibly binds
Explosive release of ACh Temporarily inhibits
With ACh - R
ACh-esterase
31.
32. • 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
33. • 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.
