MSK - Skeletal Muscle Flashcards
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?
Movement
Posture
Joint stability
Heat generation
What surrounds muscle fibres?
What surrounds fascicles?
What layer surrounds muscle?
Endomysium
Perimysium
Epimysium
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?
Compartments
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
One
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?
Intrafusal
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?
Skeletal
Cardiac
Smooth
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
Glycolysis
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?
Titin
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
