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Flashcards in Lecture 8 b-smooth muscle Deck (20)
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Where is most smooth muscle found?

-walls of hollow organs and tubes


What do smooth muscle cells look like?

-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?

-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?

-have enough Ca2+ to maintain a low level of tension/tone even in absence of AP


Is smooth muscle modified by the autonomic system?

-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