Histology of Skeletal Muscle Flashcards
Types of muscle tissue
There are three types of muscle: skeletal, cardiac and smooth. All have parallel, elongated cells called myofibres or muscle fibres.
How does muscle tissue function?
In all muscles energy from hydrolysis of ATP (chemical energy) is transferred into mechanical energy that results in movement (contraction). This occurs via myofilaments (actin & myosin) which fill most of the muscle cell. Muscle cells also need an exciteable cell membrane which responds to a stimulus to initiate cellular contraction. There are other types of contractile cells found in the body: 1. Myoepithelial cells 2. Myofibroblasts 3. Pericytes
What are the different microscopic appearances classifications of muscle?
Smooth, striated (skeletal and cardiac).
Innvervation
(1) Voluntary (skeletal) (2) involuntary (smooth and cardiac)
Skeletal muscle
Skeletal muscle fibres are long tubular cells. The average length of skeletal muscle cells in humans is about 3cm (range 1mm – 100cm). Diameters vary from 10um to 100um. Skeletal muscle fibres contain up to several hundred small peripherally located nuclei.
Development of skeletal muscle
Origin is from mesoderm –> mesenchyme -> myoblasts. A muscle fibre is formed by the fusion of multiple small muscle cells (myoblasts) making a large multinucleated cell (syncytium). Initially the nuclei are centrally located, but as the cells produce their actin and myosin filaments the nuclei are pushed peripherally.
Skeletal muscle organisation
A muscle is made up of many fascicles. A fascicle has many myofibres (myofibre = muscle cell). Myofibres are polygon shaped in cross-section and has many myofibrils. A myofibril is composed of bundles of myofilaments (thick and thin).
Epimysium
surrounds the whole muscle (dense connective tissue), contains larger vessels and nerves.
Perimysium
dense CT surrounds the fascicles, contains blood vessels and nerves.
Endomysium
loose CT = reticular fibres, surrounds individual muscle cells contain very small blood vessels and nerves.
How is a tendon form and attached to a bone?
The coverings of the muscle (epimysium, perimysium and endomysium) continue beyond the muscle tissue and fuse to become a tendon which then blends with the fibrous layer of the periosteum and the bone via Sharpey’s fibres. Muscle-tendon (musculotendinous) junction is a weak point.
What are myofibrils composed of and what are their action?
Myofibrils contain actin and myosin (myofilaments). Myofilaments (actin and myosin) are proteins, hence the highly acidophilic staining pattern of muscle. Muscles contract when actin and myosin filaments slide over each other (sliding filament mechanism). Actin and myosin are proteins that form the filamentous structures called myofilaments.
Can mitochondria be identified under EM in skeletal muscle?
At low power EM the mitochondria can be identified as small electron dense bodies between myofibrils.
Actin
Actin are thin filaments, with a diameter of 7nm and a length of 1 micron. Thin filaments also contain troponin, tropomyosin (both wrap around actin filaments) and nebulin (anchors from Z line to the end of the thin filament, thought to help anchor the thin filaments and regulate where they lie).
Myosin
Myosin are thick filaments. They are 15nm in diameter and 1.5 microns in length. Thick filaments also contain titin (helps regulate where the thick filaments thick in between the thin filaments).
What is the structure of a sarcomere?
• M line tethers myosin • H-band = myosin only (no overlapping actin) • A-band = all myosin (plus actin between A and H bands) • I-band = actin only (no overlapping myosin) • Z line = disc in centre of I band from which actin extends.
Innervation of skeletal muscle
Innervation is by the somatic nervous system – this is conscious control of muscle contraction. The junction between a nerve and a skeletal muscle cell is called a neuromuscular junction. One axon (nerve cell) may innervate many muscle cells – motor unit.
Motor unit
A motor unit is composed of a neuron and all of the muscle cells it innervates.
The terminal branches of the neuron end on individual muscle fibres. A single neuron may innervate a few to many muscle fibres (e.g. eye one neuron to three muscle fibres; erector spinae one neuron to several hundred fibres).
Neuromouscular junction
The neuromuscular junction is composed of a synapse (axon terminal) and infoldings of sarcolemma (junctional folds). These would contain receptors for acetylcholine.
If muscle contraction starts at the plasma membrane (sarcolemma), but the cell is tightly packed with myofibrils how does the action potential travel to inside the cell?
T-tubule system
The t-tubule system is an invagination of the sarcolemma of the muscle cell into and between the myofibrils of the muscle cell.
teh sarcoplasmic reticulum
The sarcoplasmic reticulum: the endoplasmic reticulum of these cells is greatly expanded to form a network of cisternae that surrounds the myofibrils.
What is a triad?
In the histology of skeletal muscle, a triad is the structure formed by a T tubule with a sarcoplasmic reticulum (SR) known as the terminal cisterna on either side.
At death
At death there is initially still some ATP in the muscle. Ca no longer actively pumped into sarcoplasmic reticulum – leaks into cytosol and causes muscles to contract.
However, need ATP to release Ca and relax muscle = rigor mortis (starts about 3 hours after death).
Eventually muscle starts to break down and the body ‘relaxes’ (12 – 72 hours).
Type I skeletal muscle
(red) slow twitch-fatigue resistant but generated less tension. Many mitochondria, wider Z bands. Depends on cellular respiration for ATP production. Rich in myoglobin = red colour. Activated by slow-conduction motor neurons which have a small diameter.
