Microstructure and function of Human Muscle Systems Flashcards
5 tissue types
Epithelium (sheets of cells) Connective tissue (support and strength) Blood Muscle tissues Neural tissues (CNS & nerves)
Muscle
a contractile tissue
classes of muscle
3 main?
2 less major?
Striated (striped)
Skeletal
Cardiac
Non-striated
Smooth muscle
Myoepithelium (in glands, iris of eye)
Myofibroblasts (in healing wounds)
Differential functions of muscles
skeletal?
cardiac?
smooth?
Skeletal - rapid contraction but subject to fatigue. Voluntary control.
Cardiac (heart only) - quite rapid, resists fatigue. Can contract regularly for 90+ years. Involuntary control (most people).
Smooth - slower contraction but very powerful, energy-efficient, and little fatigue. Usually involuntary control. E.g. uterus, gut.
Skeletal muscle fibres
length?
nuclei?
Very long (up to ~4 cm), giant thread-like cells with thousands of nuclei (= multinucleate or syncytial cells)
How can you get a multinucleate cell?
Skeletal muscle fibres are formed by thousands of precursor cells in the embryo (myoblasts) fusing together.
Thick and thin filaments
what does transverse section show?
The thick and thin filaments are made of many molecules of the proteins myosin and actin - the core of the contractile apparatus.
Transverse sections show the myosin is in a hexagonal array, with more actin filaments than myosin.
What gets narrower in sliding filament theory?
I gets narrower BUT not A
How does skeletal muscle know when to contract?
what carry signal to all parts of the cell?
Neural stimulation via motor end-plates hence one nerve can supply many muscle fibres
Special intracellular membrane systems carry the signal to all parts of this very large cell. (Diffusion not fast enough)
How does the muscle carry the signal to all parts of the cell?
Membrane systems: convey stimulus rapidly inside fibre, via T-tubules to SR, and initiate contraction.
Example of skeletal muscle pathology: Duchenne muscular dystrophy
Dystrophic. Smaller fibres, more connective tissue. Damage/ death and repair of fibres occurring.
Overview of cardiac muscle fibres (in heart) versus skeletal
similar?
;ength?
key difference?
nucleus?
Similar (striated) but
Much smaller fibres, joined end-to-end by specialized junctions, intercalated disks
Fibres have only one nucleus (or two)
Junction types in the intercalated disk
FA: Fascia adherens. Special sheet-like intermediate junction, strong adhesion. Actin attached
GJ: Gap junction. Ionic communication, contraction wave
D: Desmosome. Adhesion
Z: Z-line
Functional aspects - cardiac muscle
why branching fibres?
why smaller diameter fibres?
why numerous mitochondria?
why disks?
Branching fibres provide extra strength and resistance to splitting (by high-pressure blood).
Smaller diameter fibres (than skeletal muscle) allow rich blood supply and additional connective tissue (for strength).
Numerous mitochondria allow continuous energy supply - resistance to fatigue.
Intercalated disks give very strong attachments and ionic communication between fibres.
Initiation of contraction in cardiac muscle
starts where? how does it start?
spreads where? how?
what carry it to ventricles?
Myogenic stimulus – muscle-generated. Action potential starts from pacemaker region in R. atrium, and carries a wave of contraction across heart, assisted by ion diffusion through the gap junctions in the intercalated disks.
Purkinje fibres (larger, modified cardiac muscle fibres) carry stimulus rapidly to ventricles.
Sites & features of smooth muscle
where?
why smooth?
Generally internal organs other than heart. Digestive system, uterus, lungs, bladder, also glands, blood vessels, etc.
Also called “involuntary” muscle. Why?
“Smooth” because no striations.
Initiation of contraction is varied - neural, hormonal etc.
Smooth muscle - overview of fibres
shape of fibres?
nucleus?
Spindle-shaped fibres.
One sausage-shaped nucleus.
Long, spindly cells, one nucleus each. No stripes.
Trichrome stain shows collagen (blue-green) around each individual smooth muscle fibre.
The fibres are attached to this collagen, thus to each other
Contractile system in smooth muscle
features?
what is visbile/
what is caveolae?
Features:
no myofibrils
no striations
no complex membrane systems
But:
actin and myosin filaments visible
plasma membrane has caveolae, containing many kinds of receptors hence signals to contract
nearby ER may be a version of sarcoplasmic reticulum
Dense Bodies
what are they?
what do they contain?
anchors for contraction
Dense bodies in smooth muscle function like Z lines.
Contain α-actinin, and actin attaches to them.
Smooth muscle overview
transverse section shows? hexagonal shape? effect?
Transverse section shows actin and myosin filaments, but relatively few myosin (thick). No hexagonal array.
Structure allows force to be exerted at greater degree of stretch than for striated muscle.
Smooth muscle is the simplest and looks the most primitive, least organized type of muscle in mammals. But well adapted to function.
Contraction is slower than in skeletal, but tissue more elastic, energy-efficient, well adapted to functions in organs.
