ET : M - Smooth Muscle Flashcards
What is the structure of smooth muscles?
Very long and thin (typically 100-400 μm long and 5-10 μm in diameter), with a single central nucleus and is tapered at the ends
What is the cell shape?
Spindle
Examples of where smooth muscles are found
- Airways
- Bladder and reproductive organs
- Blood vessels
- Iris and ciliary muscles in the eye
What is the structure of single-unit (unitary/visceral) smooth muscle tissues?
Sheets of cells that are electrically connected by ‘dense bodies’ or gap junctions (action potential can spread from cell to cell) and act in unison (i.e. as one unit), often spontaneously active
Where are single-unit smooth muscle tissues mainly found?
In most blood vessels and hollow organs (respiratory, digestive, urinary and reproductive tracts)
What is the structure of multi-unit muscle tissues?
Tissue made of discrete bundles of independent cells which are densely innervated and contract only in response to its innervation (each cell is electrically isolated)
Generally, what are smooth muscles richly innervated by?
Automatic nervous system whose nerves form varicosities, releasing various neurotransmitters on many target cells
How can motor units arise?
From either a single nerve innervating several cells or from the spread of excitation from one cell to another
What is poorly organised?
The contractile machinery, with actin and myosin filaments spread throughout the cell with no obvious pattern
What’s an advantage of having a poorly organised contractile machinery?
Greater shortening can occur without filaments colliding with each other
Since smooth muscles don’t have sarcomeres, what does it allow?
A cell can contract from a relaxed length of ~150μm to ~30μm, which allows smooth muscle cells to change the diameter of hollow organs over a wide range (more contraction)
What is poorly developed?
SR and is spread througout the cell
What do smooth muscles not have?
Troponin and T-tubules (have caveolae instead which acts to increase surface area and helps to get a little bit more action potential into the centre)
What do smooth muscles have relatively of?
Few mitochondria spread throughout the cell
What is the intermediate filament?
Cytoskeleton element
What do dense bodies act like?
Z discs to “anchor” actin to sarcolemma (it’s where all the filaments join)
Do smooth muscles contain striations?
No (no “banded” appearance)
How is contraction initiated?
Involuntary (myogenic - neural, hormonal, spontaneous)
What are Ca2+ sources in smooth muscles?
Extracellular and SR (opening of channels regulated by voltage, hormones, neurontransmitters and specific ions)
How can Ca2+ enter the cell in smooth muscles?
Ca2+ enters the cell via voltage-gated Ca2+ channels or voltage-independent Ca2+ channels or from the SR
What is contraction initiated by?
An increase in intracellular Ca2+, which is supplied by Ca2+ channels in the surface membrane and/or Ca2+ release from the SR
How are the Ca2+ channels in the surface membrane activated and regulated?
Activated by depolarisation, tends to be regulated by second messengers (some may be regulated directly by certain neurotransmitters). A lot of Ca2+ within the cell is released due to hormones which, trigger receptors through second messenger pathway, usually involving IP3, causing Ca2+ to be released from the SR
What does Ca2+ bind to and activate?
Calmodulin
What does the activated calmodulin activate?
Calmodulin-dependent protein kinase (Myosin Light Chain Kinase (MLCK) - an enzyme)