Muscle cell component terminology
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
Muscle tissue
STRIATED MUSCLE - skeletal - cardiac NON-STRIATED MUSCLE - smooth
Skeletal muscle development
- 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
Skeletal muscle fibre types
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
Skeletal muscle structure (incredibly well vasculated)
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
Arrangement of skeletal muscle
- 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
Clinical importance
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)
Actin filaments
- 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)
Calcium and contraction
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
Rigor (death) conformation
Myosin heads are tightly bound to actin molecule
Lack of ATP in death perpetuates this binding —> rigor mortis
Neuromuscular junctions, T tubules and sarcoplasmic reticulum
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
Contraction of skeletal muscle
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
Cardiac muscle
- 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)
Purkinje Fibres
- 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
Smooth muscle
- 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
Smooth muscle locations
- 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.
Smooth muscle cells
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
Repair of mature muscle
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)
Definitions
Myalgia - muscle pain Myasthenia - muscle weakness Myocardium - muscular component of the heart Myopathy - any disease of the muscle Myoclonus - a sudden muscle spasm
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Light Microscopy22
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Epithelial Tissues14
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Glandular Tissues And How Cells Secrete21
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Internal Surfaces Of The Body10
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Cell Ultrastructure9
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Bugs In The System14
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Viruses19
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The Pre-embryonic Period13
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Connective Tissues12
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Skin14
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Cartilage And Bone13
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Gastrulation16
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Ossification And Bone Disease15
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Muscle19
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Alteration In Skeletal Muscle Function12
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The Nervous System8
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The Autonomic Nervous System10
