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Flashcards in MSK - Skeletal Muscle Deck (56):

Why are skeletal and cardiac muscle classed as striated?

Due to the ordered arrangement of myofibrillar apparatus producing a characteristic banding pattern when viewed under the microscope


Describe the key features of skeletal, cardiac and smooth muscle

Skeletal - striated, multi nucleated, fused cells, attached to skeleton, voluntary movement

Cardiac - striated, branched, uninucleated, only found in the heart, has intercalated discs and gap junctions, involuntary

Smooth - non striated, distinct cells, spindle shaped, walls of internal organs, involuntary


What are the four main functions of skeletal muscle?

Joint stability
Heat generation


What surrounds muscle fibres?

What surrounds fascicles?

What layer surrounds muscle?



What is fasciculation?

What happens in an excess?

Small, local, involuntary muscle contractions and relaxation

Excessive fasciculation is an early sign of motor neurone disease


What are the three arrangements of skeletal muscle?

What is the most common type?

Circular muscle
Parallel muscle
Pennate muscles

Parallel muscle


What do circular muscles act as?

How would you describe their arrangement?

Where do they attach?

Where are they found?

Sphincters to adjust opening

Concentric fibres

Skin, ligaments and fascia (rather than bone)

Around the eyes and mouth


Describe parallel muscles.

What are the three main categories and where are they found?

Fibres run parallel to the force generating axis

Strap - shaped like a strap, fibres in longitudinally to contraction direction. e.g. Sartorius

Fusiform - wider and cylindrically shaed in the centre, taper off at the ends. e.g. Biceps brachii

Fan shaped - fibres converge at one end and spread over the broad area at the other end e.g. Pectoralis major


Describe Pennate muscles

What are the three types and give an example of where each Is found.

One or more aponeuroses run through muscle body from tendon

Fascicles attach to aponeuroses at angle

Unipennate - all fascicles on same side as tendon e.g. Extensor digitorum longus.
Bipennate - fascicles on both sides of central tendon e.g. Rectus femoris
Multipennate - central tendon branches e.g. deltoid


What is meant by bone origin and bone insertion?

Origin - bone, typically proximal which has a greater mass and is more stable during contraction than the muscles insertion

Insertion - structure the muscle attaches to, tends to be distal and moved by contraction. May be bone, tendon or connective tissue. Greater motion than origin during contraction


What are limbs divided into?

What are they delineated by?


Fascia - connective tissue surrounding groups of muscles


What could trauma in one limb compartment cause?

What can it give rise to?

Internal bleeding which exerts pressure on blood vessels and nerves

Compartment syndrome


What is compartment syndrome?

How can it be treated?

Deep constant poorly localised pain
Aggravated by passive stretch of muscle group
Parathesia (pins and needles)
Compartment may feel tense and firm
Swollen shiny skin sometimes with obvious bruising
Prolonged capillary refill

Fasciotomy - cutting skin and fascia to relieve pressure. Subsequently covered by skin graft


What are the five muscles roles in movement and what does each do?

Give an example using flexion of the elbow joint.

Agonist - prime movers
Antagonists - oppose prime movers
Synergistic - assist prime movers
Neuralisers - prevent the unwanted actions that an agonist can perform
Fixators - act to hold a body part immobile while another body part is moving

Agonist - biceps brachii
Antagonist - triceps brachii
Synergists - pronator teres, Brachioradialis


What are the two contraction types? What happens in each?

Isotonic - constant tension, variable muscle length

Isotonic can be broken down into: Concentric (muscle shortens) Eccentric (muscle exerts a force while being extended, excess = DOMS)

Isometric - constant length, variable tension


What are the three biomechanical levers?

What happens in each?

Give an example of each.

Which is the most common?

First class lever (see-saw). Effort at one end, load at the other. Usually at mechanical disadvantage e.g. Extension/flexion of head

Second class lever (wheelbarrow) - effort at one end, fulcrum at the other e.g. Plantar flexion of foot.

Third class lever - (fishing rod) - effort is between load and fulcrum e.g. Biceps brachii. Most common.


Sliding filament theory

Lusuma notes


Sliding filament theory

Lusuma notes


What is the type of motor neurone innervation get skeletal muscle?

Where are the cell bodies of the neurons found?

What is the connection between the individual muscle fibres and alpha motor neuron?

How many motor neurones innervate an individual muscle fibre?

How many muscles can a single motor neurone innervate?

Alpha motor neuron

Located in ventral horn of spinal cord - for muscles of limb and trunk
Located in the motor nuclei of brainstem - for muscles of head and face

Neuromuscular junction


Many muscle fibres


Are the muscle fibres that make up a motor unit of the same or different contractile types?

Same - so each motor unit is fast of slow contracting


What are the three main muscle fibre types?

What is the classification based on?

What technique can you use to distinguish the types?

Slow type I
Fast type IIA
Fast type IIX

Myosin heavy chain expression

Histochemical staining for myosin ATPase


Name some properties of Type 1, Type IIA and Type IIX muscle fibres

Type 1 - aerobic, high myoglobin levels, red colour, many mitochondria, rich capillary supply, fatigue resistant. First type to be recruited.

Type IIA - aerobic, high myoglobin levels, red to pink colour, many mitochondria, rich capillary supply, moderate fatigue resistance

Type IIX - anaerboic glycolysis, low myoglobin levels, white colour, few mitochondria, poor capillary supply rapidly fatigable. Last type to be recruited.


What axons are muscle spindles innervated by?

What is the role of the gamma neurone?

Two sensory and one motor axon

Keeps fibres taught - can alter the sensitivity of muscle spindle


What is the role of the Type Ia sensory neurones in muscle spindles?

What is the role of the type II sensory neurones?

Relay rate of change in muscle length back to CNS

Provide position sense


What muscle fibres facilitate proprioception?

What are patients with large fibre sensory neuropathy able to do?


Perform accurate movements while watching affected limb, but in the absence of vision small movements are grossly inaccurate


What is the number of muscle spindle fibres dependent on?

Dependent on fine control for that muscle


In what order are motor neurones are recruited?

Small motor neurones are recruited before large

Slow type I --> fast IIa --> fast IIx


What are the three types of muscle cells?



When a muscle is in tetany what does this mean?

There will be no further increase from any subsequent action potential


What effect do more action potentials have on force?

More action potentials = more force.

Subsequent action potential produce summation (up to a limit).

Limit = tetany


Are healthy muscles ever fully relaxed?

No - they retain an amount of tension and stiffness (muscle tone) apart from in REM sleep


What is the baseline tone in muscles due to?

Motor neurone activity

Muscle elasticity


How is muscle tone controlled?

Via motor centres in the brain


What is hypotonia?

*It is a symptom rather than a condition*

Who is is most common in?

Give three examples.

A lack of skeletal muscle tone.

Babies after birth - floppy baby syndrome

Muscular dystrophies
Spinal muscular atrophy
Charcot-Marie-Tooth disease


What can disrupt muscle tone?

Primary degeneration of the muscles

Lesions of lower motor neurons

Lesion of sensory afferents from muscle spindles

Cerebral or spinal neural shock

Lesions of the cerebellum


How is a muscle action potential generated?

Action potential opens voltage gated Ca2+ channels and triggers vesicle fusions and acetylcholine release

Nicotinic Ach channels open, Na+ flows into the muscle cell

Ach rapidly broken down in the synaptic cleft by acetylcholinesterase

Depolarisations open voltage gated Na+ channels in the muscle cell

Muscle action potential generated


Where are the voltage gated Ca2+ channels concentrated in a muscle cell?

What are the Ca2+ release channels closely associated to in the sarcoplasmic recticulum? What does this allow?

T tubules where they come into contact with the sarcoplasmic reticulum at triads

With DHP receptors - allows rapid signalling from action potential to Ca2+ release


What receptors are associated with malignant hyperthermia?

Ryanoide receptors


When calcium binds to troponin what does it reveal?

Tropmyosin reveals actin binding site for myosin heads


How is relaxation in a muscle facilitated?

What pumps are used?

Ca2+ being pumped back into the sarcoplasmic recticulum via Ca2+ pumps

SERCA pumps require ATP


What channels do skeletal muscle have a high concentration of?

High concentration of Cl- leak channels
as well as
K+ leak channels


What is the resting membrane potential for skeletal muscle and why?

-90mV due to the high concentration of Cl- leak channels

Close to the Nerst potential for Cl-


What is the high Cl- permeability important for in skeletal muscle?

Repolarisation after an action potential


What channels are found in skeletal muscle?

High concentration of leak Cl- channels
K+ leak channels
Voltage gated Na+ channels
Voltage gated K+ channels
Voltage gated Ca2+ channels


What are the two types of voltage gated channelopathies affecting skeletal muscle?

What is the one we focus on? Describe.

Myotonia - inability to relax muscles at will

Periodic paralyses

Myotonia congenita - mutation in skeletal muscle Cl- channel


What is myotonia congenita caused by?

What are the symptoms?

What makes them worse/relieves them?

What muscle does it particularly occur in?

What is the treatment?

What type of inheritance pattern is it?

Mutations in the chloride channel CLCN1

Muscle stiffness and hypertrophy

Enhances symptoms = cold and inactivity
Relieves = exercise

Leg muscles

Most cases require no treatment. Symptoms can be relived with anticonvulsant drugs

Recessive (Becker type) or dominant (Thomsent type) deadening on type of mutation


In most cells, what ion permeabilities majorly determine excitability?

Why is skeletal muscle unique?

Do Cl- ions actively set the resting potential?

What happens in myotonia?

Na+ an K+

Above 70% of the conductance in resting fibres is due to Cl- flow. Resting membrane potential usually equates to Cl- Nerst equilibrium potential

No, but when membrane potential deviates from rest, large
Cl- currents flow which tend to return the membrane voltage to its prior resting value

Buffering capacity from Cl- is lost and myotonic discharges result


What are the sources of energy for muscle contraction?

Short term stores of ATP in muscle fibre used fir initial burst

Creatine phosphate


Oxidative phosphorylation


What does anaerobic glycolysis of blood glucose form? (Inefficient)

What enzyme is involved in the reaction?

Why may muscle pain be experienced?

Pyruvate --> Lactate

Lactate dehydrogenase

Lactate causes build up of acidosis which leads to muscle pain?


What protein is known to be important in the contraction of striated muscle?

What does it do?


Connects the Z line to the M line in the sarcomere.

Contributes to force transmission at the Z line and resting tension in the I band region

Limits the range of motion of the sarcomere in tension


What are the type of motor neurons innervating skeletal muscle fibres?

Where are the cell bodies found?

What is the connection between the individual muscle fibres and alpha motor neuron?

Alpha motor neurones

Located in ventral horn of spinal cord - for muscles of limbs and trunk

Located in the motor nuclei of brainstem - for muscles of head and face

Neuromuscular junction


How do skeletal muscle fibres differ?

Speed at which they contract

Amount of force they generate

Energy requirements/susceptibility to fatigue


Why are fast IIB fibres not found in humans?

Found in small rodents

The gene for fast IIB myosin heavy chain is not expressed in humans presumably because the isoform contracts too quickly for large muscles with many sarcomeres in series


What two factors does the contractile force produced by a muscle depend on?

Size principle - small neurones recruited before large ones

Rate code - refers to the frequency at which the muscle fibres as stimulated by their alpha motor neuron.


What does consecutive action potential result in?

Summation - giving a slightly larger force to each contraction


What is malignant hyperthermia?

What does it lead to?

What is the most common genetic cause?

What effect does this have?

What is the treatment?

Rare life threatening condition triggered by anaesthetic agents and succinylcholine (neuromuscular blocking agent).

Uncontrolled increase in oxidative metabolism and increase in body temperate which can be fatal.

Polymorphism in the ryanoide receptor.

Receptor activated by some anaesthetic agents leading to a massive increase in intracellular Ca2+ from intracellular stores. SERCA pump working at a dramatically increased rate ultimately leading to excessive heat production.

Dantrolene - a muscle relaxant which antagonises the ryanodine receptor