Lecture 8 b-smooth muscle Flashcards
(20 cards)
Where is most smooth muscle found?
-walls of hollow organs and tubes
What do smooth muscle cells look like?
- elongated
- spindle shaped
- single nucleus
- smaller than skeletal muscle cells
- single cell doesn’t extend the whole length of a muscle (unlike skeletal)
- groups of cells arranged into sheets
What are the 3 type of filaments in smooth muscle?
- thick myosin filaments(longer than the ones in skeletal muscle)
- thin actin filaments: have tropomyosin but no troponin
- filaments of intermediate size, unique to smooth muscle -don’t serve directly in contractile process but are cytoskeletal components of the framework supporting the cell
Is smooth muscle striated?
no
- don’t form myofibrils, not arranged in sarcomere pattern
- no Z lines but has dense bodies containing the same protein as in Z line
How are the thick and thin filaments arranged?
-slightly diagonally towards each other, diamond shaped lattice
Smooth muscle contraction?
- when thin filament moves past thick= the cell shortens and bulges= becomes wider
- unlike skeletal:cross bridges along the whole filament no central region without!
- thus thin filaments can be pulled for longer distances
- also half of thin filaments always pulled in one and the other half in the other direction
Is there troponin in smooth muscle cells?
-no, there is tropomyosin but it doesn’t block actin sites here
What are light chains?
lightweight chains of proteins attached to the heads of myosin molecules
- crucial regulatory function
- myosin can interact with actin only when light chain is phospohorylated (phosphate from ATP added)
How is light chain phosphorylated?
- excitation= increase in Ca2+
- Ca2+ acts as a messenger, binds with Calmodulin
- Ca2+-calmodulin complex bind to and activate myosin light chain kinase (MLC kinase)
- MLC kinase phosphorylates the light chain
Describe multiunit smooth muscle.
-multiple discrete unit functioning independently
-must be stimulated by nerves to contract (neurogenic)
-phasic, contracts only when neurally stimulated
-supplied by autonomic nervous system (involuntary)
-large blood vessels, large airways to the lungs,
in the muscle of the eye, iris, base of hair follicles,
Describe single unit smooth muscle.
- most of smooth muscle
- mostly in walls of hollow organs(digestive, urinary tracts etc.)
- become excited and contracts as a single unit
- linked by gap junctions (electrically)
- when an AP occurs anywhere near, quickly propagated by the gap junctions then operate as one unit= functional syncytium
- myogenic= self-excitable, doesn’t need nerve stimulation
- peristalsis, contraction of uterus
How are self excitable cells depolarised?
- pacemaker potentials
- slow wave potentials
How are self excitable cells depolarised using pacemaker potentials?
- membrane depolarises on its own because of shifts in passive ionic fluxes accompanying automatic changes in channel permeability
- reaches threshold= AP
- then repeats again and again
How are self excitable cells depolarised using slow wave potentials?
- gradually alternating hyperpolarising and depolarising swings in potential caused by automatic cyclic changes in the rate at which sodium ions are actively transported across the membrane
- threshold is not always reached
Self-excitable cells.
- don’t contract
- only a small portion of the cells in the single unit smooth muscle
- usually clustered together in one location
Does smooth muscle retain tone?
- yes
- have enough Ca2+ to maintain a low level of tension/tone even in absence of AP
Is smooth muscle modified by the autonomic system?
- yes
- the single unit doesn’t need to be excited by it but can be modified
What is a stress relaxation response?
-when muscle is suddenly stretched it initially increases its tension and then inherently relaxes back to the tension level before the stretch
= means it can develop tension even when very stretched= important for food movement in digestion etc.
Is smooth muscle slower than skeletal?
yes, slow and economical
- rate of splitting ATP by myosin much slower
- slower rate of Ca2+ removal
What is a latch state?
-cross bridges latch onto the thin filaments for minutes to hours= smooth muscle can maintain tension with low ATP consumption
