Muscle Flashcards
What is Muscle?
All cells can move but some are specialised to move parts of the body, cell contraction relies on interactions of actin and myosin (contractile proteins) and can act as isolated cells or cells organised into tissues.
A muscle cell is also known as a myofibre, the smaller components of which are known as myofibrils.
What are the cells and tissues associated with Muscle?
Tissues - skeletal muscle, cardiac muscle & smooth muscle.
Isolated Cells - myoepithelial cells, myofibroblasts & pericytes
Explain the concept of Skeletal Muscle.
Also known as voluntary, somatic or striated muscle; skeletal muscle attached to bones and allows movements. Giant cells (10-100 microns) in diameter with multinuclear peripheral nuclei form muscle fibres that can stretch over the length of a muscle.
Histology: There is more than one nucleus per cell and they are right around the edge of the cells as seen in a cross section. The longitudinal section may be able to show the striated nature of the muscle cells.
Explain the concept of Cardiac Muscle.
Only found in the heart, another example of striated muscle that works very similarly to skeletal muscle. It is made up of single cells that are connected into a network. it also has intrinsic properties that allow it to contract spontaneously.
Histology: Nuclei are centrally located and there is usually only one. Longitudinally it is easy to see that there are chains and have striations
Explain the concept of Smooth Muscle.
Spindle shaped (fusiform) cells that are between 200-300 microns by 5-6 microns that are often associated with hollow viscera, blood vessels and hair follicles. It is designed for long sustained contractions without fatigue.
Histology: small in diameter, central nucleus. Longitudinal section shows long and spindle like branches.
In skeletal Muscle, how does contraction work?
Contraction depends on the interaction between actin and myosin. It is trigger by an incoming action potential coming from a nerve which triggers release of neurotransmitter which then triggers an action potential in the nerve of the muscle.
The cytoskeleton is not an organelle but is part of the cell that is organised into myofibrils. Myofibrils are organised subparts of the cytoskeleton and consist on repeating subunits called sarcomeres. These myofibrils stretch the length of the cell.
In light microscopy, skeletal muscle is shown to have the myofibrils edge to edge, lining up the sarcomeres in each proceeding myofibrils.
Explain the ultrastructure of myofibrils?
the myofibrils are made up of repeating subunits called sarcomeres; these sarcomeres of neighboring sarcomeres line up across the cell and each sarcomere is made up of overlapping actin and myosin filaments.
What are Sarcomeres?
Each sarcomere is made of overlapping thick (myosin) and thin (actin) filaments. Z-discs anchor the thin filaments and form the boundary with the next sarcomere. The zone of overlap, at the cross bridges of the myosin with the bound actin that is crucial for contraction.
The sliding filament mechanism refers to the concept in which neither of the thick or thin filaments shorten themselves, but instead the sarcomere shortens. Because sarcomeres are in series, the myofibrils shorten overall, leading to muscle
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What is the sarcoplasmic reticulum?
a network of tubules. The SR membrane is very rich in calcium ATPases that pump calcium back into the lumen of the SR.
What are T-Tubules?
these tubules surround each myofibril, two per sarcomere. They are situated at the overlap of the thick and thin filaments and conduct the muscle action potential into the muscle cell SR.
Both the SR and T-tubules are visible via electron microscope with lines of mitochondria for ATP production as muscle cells require large amounts of energy.
How does Cardiac Muscle contract?
Contracts in the same manner as skeletal muscle. It is also a form of striated muscle that contains myofibrils and sarcomeres that work in the same general way. Single cells work as a part of a network.
Unlike he skeletal muscle which stretches from bone to bone, cardiac muscle is attached to other cardiac muscle cells which during contraction are trying to pull apart from each other and so the connections must be really strong.
On a light microscope, there are many capillaries that can be seen. The cells can be either mono- or binucleated. Also visible are the interconnections between cells called intercalated discs.
What are intercalated discs?
Individual cells joined by intercalated discs (array of specialised cell junctions); fascia adherentes join myofibrils across cell boundaries and desmosomes reinforce the join between myofibrils. (where the t tubules and mitochondria are found)
Gap junctions are need on the longitudinal section of the intercalated disc because the intercalated disc runs in zig zag patterns and the pressure on the muscle is only left/right and not up/down.
Cardiac muscle cells must coordinate contractions; Gap junctions electronically couple cells and coordinate action potentials in neighbouring cells.
How does smooth muscle contract?
Spindle shaped (fusiform), 200-300 micron by 5-6 micron cells with a central nucleus, perinuclear organelles, no myofibrils, sarcomeres or t-tubules. They are closely packed and generate their own connective tissue. When contracted they form a corkscrew shape. Under electron microscope, it can be seen that they contain thick (myosin) and thin (actin) filaments as well as some dense bodies known as Z discs.
During the contraction of smooth muscle, the dense bodies are drawn together which shortens the cell via the sliding filament mechanism.
What are some other contractile cells?
MYOEPITHELIAL CELLS
These cells surround some exocrine glands and contract via actin/myosin to squeeze out contents. They are found in organs such as the mammary glands.
MYOFIBROBLASTS
Fibroblasts make connective tissue; when connective tissue is injured, scar tissue contains activated fibroblasts that become myofibroblasts that pull the wound closed through contraction.
PERICYTES
These are contractile cells via actin/myosin interactions. They extend around capillaries and regulate capillary blood flow.
