Muscle Flashcards

1
Q

Muscle cell component terminology

A

Sarcolemma - the outer membrane of a muscle cell
Sarcoplasm - the cytoplasm of a muscle cell
Sarcoplasmic reticulum - the smooth endoplasmic reticulum of a muscle cell

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2
Q

Muscle tissue

A
STRIATED MUSCLE 
- skeletal 
- cardiac
NON-STRIATED MUSCLE 
- smooth
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3
Q

Skeletal muscle development

A
  • mesodermally-derived (multipotent myogenic stem cells give rise to myoblasts)
  • fusion of myoblasts forms a primary myotube
  • nuclei displaced to the periphery by actin and myosin myofilaments
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4
Q

Skeletal muscle fibre types

A

RED
- smaller
- well vasculated, rich in myoglobin and numerous mitochondria
- contraction is slow, repetitive and relatively weak
- fatigues slowly
- rich in oxidative enzymes, poor in ATP-ase
- fewer neuromuscular junctions
- located in the limb muscles of animals, postural muscles of back and breast muscle of migrating birds
WHITE
- larger
- not well vasculated, not much myoglobin and few mitochondria
- faster and stronger contractions
- fatigues rapidly
- poor in oxidative enzymes, rich in ATP-ase
- more neuromuscular junctions
- located in extraocular muscles (eye), muscles controlling fingers, breast muscle of a domestic hen

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5
Q

Skeletal muscle structure (incredibly well vasculated)

A

Outer elimysium
Perimysium ‘wraps’ the fascicles
Endomysium between muscle fibres (the cells)

Interdigitates with tendon collagen bundles at myotendinous junctions - sarcolemma always lies between the collagen bundles and the muscle fibres myofilaments

(M in H band within) Dark A band, (Z band within) light I band
A band stays the same size regardless if it is stretched or not

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6
Q

Arrangement of skeletal muscle

A
  • Convergent - e.g. pectoralis major (pecs)
  • Circular - e.g. orbicularis oris (lips)
  • Multipennate - e.g. deltoid
  • Fusiform - e.g. biceps brachii
  • Bipennate - e.g. rectus femoris (quad)
  • Unipennate - e.g. extensor digitorum longus
  • Parallel - e.g. satorius
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7
Q

Clinical importance

A

Actin, tropomyosin and troponin molecules complex form the thin filaments of skeletal and cardiac muscles
Useful clinically
- diagnostic tool for heart attacks (within 20hrs) - released from cardiac muscle during cardiac ischaemia (quality is not always proportional to the degree of damage)

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8
Q

Actin filaments

A
  • actin filament forms a helix
  • tropomyosin molecules coil around the actin helix reinforcing it
  • troponin complex is attached to each tropomyosin molecule
  • in the centre of the sarcomere the thick filaments (bend causing contraction) are devoid of myosin heads (which extend towards the actin filaments in regions of potential overlap)
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9
Q

Calcium and contraction

A

Increased amounts of ionic calcium binding to TnC of troponin - leads to a conformational change - that moves tropomyosin away from actin’s binding sites
The displacement allows myosin heads to bind to actin and contraction begins

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10
Q

Rigor (death) conformation

A

Myosin heads are tightly bound to actin molecule

Lack of ATP in death perpetuates this binding —> rigor mortis

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11
Q

Neuromuscular junctions, T tubules and sarcoplasmic reticulum

A

T tubules
- where I and A bands meet
Neuromuscular junction
- small terminal swelling of axon contain vesicles of acetylcholine
- a nerve impulse causes the release of acetylcholine which binds receptors on the sarcolemma to initiate an action potential propagated along the muscle

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12
Q

Contraction of skeletal muscle

A

1) initiation
- nerve impulse along motor neuron axon arrives at neuromuscular junction
- impulse causes release of acetylcholine into synaptic cleft - local depolarisation of sarcolemma
- voltage gated Na+ channels open causing an influx of Na+ into cell
- depolarisation spreads over sarcolemma and into T tubules
- voltage sensor proteins of T tubule membrane change their conformation
- gated Ca2+ release channels of adjacent terminal cisternae are activated
- Ca2+ is rapidly released from terminal cisternae into the sarcoplasm
- Ca2+ binds to TnC subunit of troponin
- the contraction cycle is initiated and Ca2+ is returned to the terminal cisternae of the sarcoplasmic reticulum

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13
Q

Cardiac muscle

A
  • striated
  • centrally positioned nuclei (1 or 2 per cell)
  • intercalated disks (for central or mechanical)
  • coupling with adjacent cells
  • BRANCHING
  • rich supply of capillaries in endomysium
  • distinct myofibrils are absent - instead myofilaments of actin and myosin form continuous masses in the cytoplasm
  • gap junctions for electrical coupling
  • adherens-type junctions to anchor cells and provide anchorage for actin filaments
  • the T tubules lie in register with the Z bands (not with the A-I junctions like in skeletal muscle)
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14
Q

Purkinje Fibres

A
  • all cardiac muscle exhibit a spontaneous rhythmic contraction
  • action potentials in the heart are generated in the sinoatrial node - to the atrioventricular node and then to the ventricles
  • impulses are carried by specialised myocardial cells –> Purkinje fibres

Are large cells with

  • abundant glycogen
  • sparse myofilaments
  • extensive gap junction sites
  • action potentials (3-4m/s) cardiac muscle fibres only 0.5m/s
  • rapid conduction allows ventricles to contract in a synchronous manner
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15
Q

Smooth muscle

A
  • spindle shaped (Fusiform)
  • central nuclei
  • not striated
  • no sarcomeres, no T tubules (caveoli instead)
  • contraction relies on actin-myosin interactions (calcium is required)
  • contract in a twisting way
  • slower contraction, sustained for longer (can remain contracted for hours or days) and requires less ATP
  • capable of being stretched
  • stimuli in the form of nerve signals, hormones, drugs, local concentrations of blood gases
  • forms sheets, bundles or layers containing thousands of cells
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16
Q

Smooth muscle locations

A
  • form contractile walls of passageways or cavities of vascular structures, the gut, respiratory and genitourinary systems
  • is of clinical significance in disorders such as high blood pressure, asthma, atherosclerosis etc.
17
Q

Smooth muscle cells

A

Myoepithelial cells
- radiating cells form a basket work around the secretory units of some exocrine glands (e.g. sweat, salivary and mammary)
- contraction assists secretion
- in the iris they contract to dilate the pupil
Myofibroblasts
- abundant in actin and myosin
- at all sites of wound healing - produce a collagenous matrix but also contract
- prominent in wound contraction and tooth eruption

Are innervated by autonomic nervous system fibres that release their neurotransmitters from varicosities (swellings) into a wide synaptic cleft
- swellings can release acetyl-choline which stimulates the adjacent muscle cells - the gaps between are much wider

18
Q

Repair of mature muscle

A

SKELETAL
- cannot divide but tissue can regenerate by mitotic activity of satellite cells - hyperplasia follows muscle injury
- satellite cells can fuse with existing muscle to increase mass (hypertrophy)
CARDIAC
- is incapable of regeneration
- after damage fibroblasts invade, divide and lay down scar tissue
SMOOTH
- retain their mitotic activity and can form new smooth muscle cells
- evident in the pregnant uterus where muscle wall becomes thicker by (hypertrophy - swelling) and (hyperplasia - mitosis)

19
Q

Definitions

A
Myalgia - muscle pain 
Myasthenia - muscle weakness 
Myocardium - muscular component of the heart 
Myopathy - any disease of the muscle 
Myoclonus - a sudden muscle spasm