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)
What allows myosin to interact with actin?
MLCK phosphorylating myosin light chains, thus, hydrolysing ATP (to become energised) and activating myosin ATPases
How does the cross-bridge cycle occur?
MLCK acts on myosin heads as it comes along and interacts with the myosin neck region where all the bending/enzyme activity occurs. Activated myosin (phosphate from ATP added to light chain) forms cross-bridges with actin of thin filaments and contraction begins
Why is MLCK myosin regulated?
All the action is on myosin (thick filaments)
The max. cross bridge cycling rate is ____ and contraction is quite ____
The max. cross bridge cycling rate is low and contraction is quite slow
When does contraction end (relaxation)?
A Myosin Light Chain Phosphatase (MLCP) dephosphorylates the light chains (by getting rid of phosphate on myosin head)
What is the primary mechanism for reducing intracellular Ca2+?
Ca2+ ATPase in the cytoplasm
Since smooth muscle contraction is ___ regulated, it’s ____ but _____
Since smooth muscle contraction is enzyme regulated, it’s slow but efficient
What is the contractile state of the cell dependent on?
Ca2+ levels within the cell and the activity of MLCK and phosphatase. The contractile state is therefore, the balance of both MLCK and phosphatase activities and contraction can be graded by differential modulation of these processes
Increased MLCK activity (Ca2+ reguluated) favours?
Contraction
Increased MLCP activity favours?
Relaxation
How can smooth muscle contraction be modulated?
- Neurotransmitters
- Hormones
- Environment (pH, O2, etc.)
- Histamine
- Adenosine
- Prostacyclin
- Nitric Oxide
- Stretch
These all can affect either the cytosol Ca2+, MLCK or MLCP activity (and if any of one of them changes, it will change the tone of blood vessel or gut etc.)
What innervates most smooth muscle fibres>
Automatic nerve fibres
What do automatic nerve fibres do?
Branch and form “diffuse junctions” with underlying smooth muscle fibres
What do varicosities in the terminal axon do?
Contain and release neurotransmitters into a wide synaptic cleft (a diffuse junction), it leaks the hormones out and can affect the neighbouring cells (in response to action potentials).
Where is neurotransmitters secreted and diffused?
Secreted into the matrix coating and diffuses to the muscle cells
When you stretch a smooth muscle, it will generally?
Initially contract, effectively resisting the stretch - it constricts so there’s a lot of stress and tension (e.g. blood vessels trying to maintain blood flow constant). This mechanism appears to reside in the presence of ‘stretch activated’ ion channels, which alow Ca2+ to enter the cell
What happens to smooth muscles after stretch overtime?
Slowly relaxes, adapting to the change in length (e.g. gut), via Ca2+ dependent K+ channels which, when activated, hyperpolarises the membrane potential and shut down Ca2+ influx via voltage-dependent Ca2+ channels
