M7 L4: Smooth muscle cells Flashcards
What is the structure smooth muscle : two tissue types
Long spindle shape with central nucleus.
Tissue can be single unit: sheets of electrically coupled cells that act in unison and spontaneously active.
Multiunit tissue made of discrete bundles of independent cells which are densely innervated and contract only in response to innervation: eg. vas deferens, iris, piloerectors
Where is smooth muscle found
airways, blood vessels, bladder and reproductive organs, iris and cillary muscles in eye
What is the arrangement of smooth muscles in the walls of hollow organs
Outside there is a longitudinal layer of smooth muscle moves along food and a inside is a circular layer of smooth muscle that mashes it up
What is the basic cellular structure of smooth muscle: has and doesn’t have. connecting, cytoskeleton
No T-tubules but have caveolae instead to increase surface area. Dense bodies that are like z lines to anchor actin to sarcolemma. Gap junctions connect smooth muscle cells in unitary muscle. Intermediate filament is cytoskeleton diamond. It has poorly developed SR
How well is smooth muscle as a contractile protein
Actin and myosin filaments are less organised so allows for greater shortening of the cell without filaments colliding with each other - 60-75% shortening possible
How is contraction initiated in smooth muscle cell
Trigger for contraction is increase in intracellular calcium. The CA2+ entering through channels in the cell membrane is important source of calcium compared i to SR stores. Electrical behaviour is very complex but primarily due to voltage gated Ca2+ channels (not many Na+ channels).
What are the three ways smooth muscle contractions can be initiated and what type of contraction is this
neural, hormonal or spontaneous (myogenic) to initiate an involuntary contraction
Ca2+ regulation in smooth muscle is done by what
voltage, hormones, neurotransmitters and specific ions. There are lot of different sources of Ca2+
What are the steps of contraction of smooth muscle
- Ca2+ enter cytosol from extracellular fluid via voltage dependent or independent Ca2+ channels or from scant SR
- Ca2+ binds to and activates Calmodulin- (no troponin)
- Activated calmodulin then activates myosin light chain kinase- enzyme that works on myosin
- Myosin light chain kinase phosphorylates the light chain LC20 on the neck of myosin enabling Myosin ATPases to hydrolyse ATP and start cross bridge cycle
What is the speed of contractions and how does sm relax
Contractions quite slow as maximum cross bridge cycling rate is low. Relaxation is brought by Myosin light chain phosphatase that dephosphorylates the light chains. As it is enzymatic processes it is slow to stop and start-> doesn’t require as much energy to contract for longer as it has another process needed to stop it. Ca-ATPase in cytoplasm membrane primary mechanism for reducing intracellular Ca2+
When intracellular Ca2+ drop which enzyme activity will dominate
Myosin light chain phosphatase as MLCK is activated through Ca2+
What do things modulating smooth muscle contraction focus on
Cytosol Ca2+, MLCK or MLCP (incl NO)
Describe the innervation of smooth muscle
Para and sympathetic autonomic nerve fibres branch and form diffues junctions with underlying smooth muscle fibres. Varicosities in the terminal axons contain neurotransmitter that is secreted into the matrix coating and diffuses to more than one cells - casual modulator
What is the response to stretch in a Smooth muscle
When you first stretch it will initially contract to resist the stretch. initially stretch activated Ca2+ channels. Over time slowly relaxes adapting to the change in length via Calcium dependent K+ channels, hyperpolarising the membrane potential
What is the source of calcium for smooth muscle
interstitial fluid + SR via IP3
