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.