Flashcards in Lecture 8 b-smooth muscle Deck (20)
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1
Where is most smooth muscle found?
-walls of hollow organs and tubes
2
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
3
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
4
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
5
How are the thick and thin filaments arranged?
-slightly diagonally towards each other, diamond shaped lattice
6
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
7
Is there troponin in smooth muscle cells?
-no, there is tropomyosin but it doesn't block actin sites here
8
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)
9
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
10
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,
11
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
12
How are self excitable cells depolarised?
-pacemaker potentials
-slow wave potentials
13
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
14
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
15
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
16
Does smooth muscle retain tone?
-yes
-have enough Ca2+ to maintain a low level of tension/tone even in absence of AP
17
Is smooth muscle modified by the autonomic system?
-yes
-the single unit doesn't need to be excited by it but can be modified
18
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.
19
Is smooth muscle slower than skeletal?
yes, slow and economical
-rate of splitting ATP by myosin much slower
-slower rate of Ca2+ removal
20
