Muscles: Smooth Muscle Flashcards
Is smooth muscle voluntary
no - it is involuntary
Where is smooth muscle found?
Found in the wall of internal organs (gut, blood vessels, airways, bladder, reproductive organs, iris etc)
Structural features of smooth muscle
Spindle shaped, uninucleated cell, Not striated
Width of smooth muscle
5 µm
Length of smooth muscle
100-400 µm
Type of junction in single unit smooth muscles
gap junction so that they can act in unison (are electrically coupled)
Where are a single unit smooth muscle cells located
Found in most blood vessels and hollow organs (respiratory, digestive, urinary and reproductive tracts)
What is a multi-unit smooth muscle
tissue made of discrete bundles of independent cells which are densely innervated and contract only in response to its innervation
Examples of multi-unit smooth muscle
vas deferens, iris, piloerection
Function of inner “unitary” smooth muscle
It is circular and contracts = they narrow and mush up food
Function of outer “unitary” smooth muscle
It is longitudinal and contracts = making food move down tube
Does smooth muscle contain T-tubules
no
What does smooth muscle contain rather than T-tubules
caveolae
What is the function of caveolae
they are “dips” that increase smooth muscles SA to get more AP
What is the function of dense bodies in smooth muscle
Dense bodies act like z-lines to “anchor” actin to sarcolemma
Does unitary smooth muscle contain gap junctions
yes - to electrically connect cells
What is the sarcoplasmic reticulum like in smooth muscle?
it is under/poorly developed (has small volume)
Does smooth muscle contain striaitons?
No
Does smooth muscle contain actin and myosin filaments
Yes
What does decreased organisation of myosin and actin allow smooth muscle to do?
It allows for greater shortening (contract more) and can operate over a large range of lengths (60 - 75% shortening possible)
What initiates contraction in smooth muscle
voltage gated Ca2+ channels (relatively few Na+ channels)
What triggers the opening of voltage gated Ca2+ channels
increase in intracellular Ca
The three types of initiation of smooth muscle
neural, hormonal or spontaneous (myogenic)
Smooth muscle calcium source
Extracellular and SR Regulation via voltage, hormones, neurotransmitters and specific ions
Why is smooth muscle initiation of contraction a slower process
because it is enzyme regulated: energy conserving as smooth muscle doesn’t need to be pumping all the time
What occurs in Stage 1 of the initiation of smooth muscle contraction: Increased intracellular Ca2+
Ca2+ enter the cytosol from the Extracellular fluid (ECF) via voltage- dependent or voltage- independent Ca2+ channels, or from the scant SR
What occurs in Stage 2 of the initiation of smooth muscle contraction: Calmodulin (CaM) activation
Ca2+ binds to and activates calmodulin (instead of troponin)
What occurs in Stage 3 of the initiation of smooth muscle contraction: Activation of kinase
The activated calmodulin then activates myosin light chain kinase (MLCK). MLCK is an enzyme
What occurs in Stage 4 of the initiation of smooth muscle contraction: Phosphorylation of myosin
MLCK activates the myosin by phosphorylating it, which in turn activates the myosin ATPases.
What occurs in Stage 5 of the initiation of smooth muscle contraction: Cross bridge cycle
Activated myosin forms cross bridges with actin of the thin filaments and shortening begins in the usual fashion.
Contraction occurs by the same actin-myosin interaction as in striated muscle (sliding filament) , but REGULATION IS MYOSIN (NOT ACTIN) BASED. Turning on switch is the phosphorylation of the myosin head. (maximum rate of crossbridge formation is low = slow contractions)
When is MLCK active
MLCK is active only in the presence of a small Ca-binding protein, calmodulin (and only when it has Ca bound)
Regulatory protein in smooth muscle contraction
In smooth muscle the reg protein is calmodulin, troponin complex is absent.
What has to happen for myosin to hydrolyse ATP
Myosin does not hydrolyse ATP (to become energized) unless it is first phosphorylated (on the regulatory light chain, LC20 located on the neck of the myosin)
When does a smooth muscle contraction end
Contraction ends when a myosin light chain phosphatase dephosphorylates the myosin light chain
What removes Ca2+ from the intracellular space
Ca-ATPase in cytoplasm membrane is the primary mechanism for reducing intracellular Ca++
What is Myosin Phosphatase (MLCP)
an enzyme that removes phosphate from myosin head (usually from Ca levels decreasing)
The function of enzymes in smooth muscle contraction
Balance of two enzymes (putting/removing Ca) causes contraction (which is dominant)
Specific activity of each enzyme in smooth muscle contraction
↑ MLCK activity (Ca2+ regulated) will favour contraction
↑ MLCP activity will favour relaxation
When intracellular Ca2+ drop MLCP activity will dominate
what MUST be present for contraction to occur
phosphorylation of the myosin head
What are diffuse junctions
When autonomic nerve fibres branch and touch the underlying smooth muscle fibres
What do varicosities do in autonomic nerve fibres
they release their neurotransmitters into a wide synaptic cleft (a diffuse junction)
What happens when a smooth muscle is stretched
Initially contract, effectively resisting the stretch (e.g. blood vessels trying to maintain blood flow constant) -stretch activated calcium channels.
But over time it slowly relaxes, adapting to the change in length (e.g. gut) - via calcium-dependent K+ channels, hyperpolarizing the membrane potential
