Physiology

1. The Cytoskeleton
• By definition, it is the cytoplasmic filamentous
network associated with cell shape and
movement.
• There are three classes of the cytoskeletal
filaments, starting from the thinnest:
1. microfilaments/actin filaments
2. Intermediate filaments
3. Microtubule

2. Structure, composition and function
of the cytoskeleton

3. Microfilaments
• They have a diameter of 7nm and are abundant in all eukaryotic cells as they were first
discovered in the skeletal muscle and are made up of the protein subunit g-actin(globular
actin)which assemble into a polymer of two twisting chains known as F-actin(filamentous)
• They compose of the contractile protein cutin, which makes up a major portion of the
cytoskeleton in all cells where they slide along filaments of another protein called myosin
to make the cells contract
• In none muscle cells actin filaments are less organised and much less prominent
• Like microtubules, actin filaments have a plus a minus ends with more ATP-powered
growth occuring at a filament’s plus end
• They can assemble and disassemble rapidly
• In many types of cells, networks of actin filaments are found beneath the cell cortex
,which is the meshwork of membrane-associated proteins that supports strengthens the
plasma membrane
• Such networks allow cells to hold- and- move specialised shapes ,such as the brush order
of microvilli
• They are also involved in cytokinesis and cell movement
• In short the main functions of the microfilaments are:
• In determining cell shape
• Ability of movement by the amoeboid like movements
• Cell division
• Muscle cell contraction

4. • Intermediate filaments
• They are composed of twisted strands of several
different proteins including keratin, desmin,and
lamin
• They have a diameter of 10nm
• Filaments also contribute to cell shape and help
anchor the nucleus
• They provide considerable strength to cells and
consequently are most extensively developed in
the region of cell subject to mechanical
stress(e.g in association with desmosomes)
• Once they are assembled they less readily
disassemble

5. • Microtubules
• these are hollow tubes of about 25nm in diameter
• They compose of the protein subunits tubulin
• Tubulin contains two polypeptide subunits string together to make long strands
called protofilaments,13 protofilaments then come together to form the hollow,
straw-shaped filaments of microtubules
• They are the most rigid of the cytoskeletal filaments and are present in the long
processes of neurons, where they provide the framework that maintains the
processes’ cylindrical shape
• Microtubules are ever changing ,with reactions constantly adding and subtracting
tubulin dimers at both ends of the filaments
• The rates of change at either end are not balanced, one end grows more rapidly
and is called the plus end and the other the minus end
• In cells, the minus ends of microtubules are anchored in structures called
microtubule organising centres(MTOCs)
• The primary MTOC in a cell is called the centrosome and it is usually located
adjacent to the nucleus
• Microtubules radiate from a region of the cell known as the centrosome which
sorrounds two cylindrical bodies called centrioles composed of nine sets of fused
microtubules
• Microtubules and actin filaments have also been implicated in the movement of
organelles within the cytoplasm