Physiology of Smooth Muscle Flashcards
What are the two types of smooth muscle cells?
- Multi-unit- cells function independently
-
Single- Unit- cells function as a single unit
- Most abundant
Which organs/structures a made up of multi-unit smooth muscle?
- Piloerector
- Vas deferens
- Iris of eye
- Ciliary body of eye
Which organs/structures are made up of single-unit smooth muscle?
- Gastrointestinal tract
- Blood vessels
- Urinary bladder
- Respiratory tract
*Also found within the walls of holow organs, except the heart
Single unit smooth muscle is electrically coupled by _________.
Gap junctions
NOTE: This allows for stimulation of one cell to be followed by stimulation of adjacent cells
Both multi-unit and single- unit smooth muscle cells are innervated by _________.
ANS
REMEMBER: Smooth muscle is involuntary
What is peristalsis?
the involuntary constriction and relaxation of the muscles of the intestine or another canal, creating wave-like movements that push the contents of the canal forward.
What type of potentials are exhibited by single-unit smooth muscle?
Slow wave potentions
Spontaneous action potentials (pace maker cells)
What are other names for single-unit smooth muscle?
Unitary
Visceral
What are the two layers of unitary smooth muscle? What does contraction of each type lead to?
-
Longitudinal layer
- Contraction leads to dilation of lumen and shortening of organ
-
Circular layer
- Contraction leads to constriction of lumen and elongation of organ
NOTE: The stomach has an oblique third layer of smooth muscle
Which unitary smooth muscle is being described below?
Long axis of cell runs parallel of long axis of organ
Longitudinal
Which unitary smooth muscle is being described below?
Long axis of cell circles the lumen or circumference of the organ
Circular
Circular and longitudinal layers of smooth muscle allow for ___________.
Peristalsis
In what ways does smooth muscle differ from skeletal muscle?
- Spindle-shaped fibers-thin and short compared with skeletal muscle fibers
- Single nucleus
- Less structured SR
- Pouchlike infoldings (caveolae) of sacolemma sequester Ca2+ from outside cell
- NO sacromeres, myofibrils, or T tubules
Pouch-like infoldings called __________ of sarcolemma sequester Ca 2+ from outside cell.
Caveolae
How does the myosin-actin relationship differ in smooth muscle from skeletal muscle?
- Actin and myosin filaments arranged non-uniformly
- Myosin heads run the entire length of the myosin filament
- Myosin heads are oriented in opposing directions on each side
- Many more actin filaments per myosin filament
- NO troponin
There is no troponin in smooth muscle. What binds Ca2+?
Calmodulin
What are the components of the microscopic structure of smooth muscle?
- Intermediate filaments
- Dense bodies
- Gap Junctions
- Caveolae
- Nucleus
What are the characteristics of the intermediate filaments of smooth muscle?
- Noncontractie fibers (desmin or vimentin)
- Lattice like arrangement
Dense bodies of smooth muscle are analogous to which structure of skeletal muscles?
Z-bands
Dense bodies are attached to _______ and __________ and bind to __________.
Intermediate filaments; actin filament; endomysium
NOTE: Dense bodies contain alpha-actinin, which is also in z-band of skeletal muscle
Which smooth muscle is being described below?
- Contracts only periodically
- Action potential stimulation
Phasic
Which smooth muscle is being described below?
- Maintians a state of contraction
- Chemically stimulated without APs
Tonic
Which structures are normally contracted?
Sphintors
Which structures are normally and partially cotnracted?
Blood vessels and airways
Which structures are phasically active?
Stomach
Intestines
Which structures are normally relaxed?
Esophagus
Urinary bladder
True or false. Both skeletal and smooth muscles have neuromuscular junctions.
FALSE
*There is no NMJ in smooth muscle
Smooth muscle is innervated by autonomic nerve fibers at _______ junctions.
Diffuse
___________ of nerve fibers store and release neurotransmitters into diffuse junctions.
Varicosities
*Varicosities are bulbous swellings
Response of a smooth muscle depends on what two factors?
Neurotransmitter released
Type of receptor molecules (i.e. stretch)
Where do varicosities release their neurotransmitters?
Diffuse junction
Does neural stimulation of smooth muscle produce a graded potential or action potential?
Both!
Either type of potential can be produced
Some smooth muscle cells have no nerve supply. These cells are stimulated by hormones and chemicals. Explain the mechanism.
Cells depolarize spontaneously or in response to chemical stimuli that bind to G protein-linked receptors
NOTE: Chemical factors include hormones
Acetylcholine is released by __________ neurons.
Parasympathetic
Acetylcholine binds to ____________ recpetors in smooth muscle cells and cause smooth muscle _________ (contraction/relaxation).
M3 muscarininc; contraction
What is the mechanism by which acetycholine causes smooth muscle contraction?
- Acetycholine binds to muscarinic receptor (which is a G-protein) causing a conformational change of the receptor
- Gq- protein binds to and activates phospholipase C
- Phospholipase C metabolizes PIP into IP3 and diacylglcerol (DAG)
- IP3 diffuses into sarcoplasmic reticulum and binds to specialized Ca2+ channels
- Ca2+ is released
Epinephrine is released from _________.
Adrenal medulla
Norepinephrine is released from ________ neurons.
Sympathetic
What are the two major kinds of receptors to which norepinephrine binds? What action does each receptor produce?
alpha-receptors
- Vasoconstriction
- Iris dilation
- Intestinal relaxation
- Intestinal sphincter contraction
- Bladder sphinctor contraction
beta- receptors
- Vasodilation
- Cardioacceleration
- Interstinal relaxation
- Uterus relaxation
- Bronchiodilation
From where is nitric oxide released?
- Neurons of the intrinsic nervous system
- CNS neurons
- Endothelial cells
Nitric oxide causes ________ (relaxation/contraction) of smooth muscle.
Relaxation
Nitric oxide binds to ___________.
Guanylyl cyclase
*This activates the production of cGMP from guanosine triphosphate
There are 3 paracrines that affect smooth muscle. What are they and what action do they cause?
Prostacyclin
- Causes relaxation of smooth muscle
Endothelium-derived hyperpolarizing factor (EDHF)
- Causes relaxation of vascular smooth muscle
Endothelin (ET)
- Causes contraction of vascular smooth muscle
By what mechanism does prostacyclin act on smooth muscle?
- Released by endothelial cells
- Binds to a cell surface receptor (IP) that leads to the production of cAMP
- cAMP tends to causes relaxation of smooth muscle
By what mechanism does EDHF act on smooth muscle?
- Potassium- dependent mechanism
- Causes relaxation of vascular smooth muscle
By what mechanism does endothelin act on smooth muscle?
- Peptides formed in the endothelial cells
- G-protein coupled receptors
- Causes contraction of vascular smooth muscle
What is unique about the way Ca2+ is obtained in smooth muscle?
ca2+ is obtained from the SR and the extracellular space
in smooth muscle, actin and myosin interact by ________________ mechanism.
Sliding filament
NOTE: ATP energizes sliding process and utilizes aerobic respiration
What is the final trigger of smooth muscle contraction?
Intracellular Ca2+
What are the steps to smooth muscle contraction?
-
Calcium ions enter the cytosol from the ECF via voltage-dependent and voltage- independent Ca2+ channels or from the scant SR
* Calcium- induced calcium release
2. Calcium binds to and activates calmodulin
- Activated calmodulin activates the myosin light chain kinase enzymes (MLCK)
- The activated MLCK enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases
- Activated myosin has high affinity for actin and forms cross bridges with actin filaments. Shortening begins.
NOTE: Slow to contract and relax but maintains tension for prolonged periods with little energy cost
Because smooth muscles maintain tension for prolonged periods, they require action for relaxtion. What actions are required?
- Active transport of calcium into SR and ECF
- Calcium detachment from calmodulin
- Inactivation of MLCK
- Myosin light chain phosphatase enzyme dephosphorylates myosin, which reduces myosin ATPase activity.
___________ dephosphorylates myosin.
Myosin light chain phosphatase enzyme (MLCP)
*This reduces myosin ATPase activity
At end of the power stoke myosin and actin remain latched together until __________ binds.
ATP
NOTE: Unphosphorylated myosin has a weak affinity for ATP
Abrupt streches may cause muscle to _________, but continuous slow strectches may allow the muscles to _________.
Cotnract; extend
Some cells contain stretch activated channels. The cell responds to stretch only briefly, then channels close. By what mechanism does this occur?
- As Ca2+ is pumped out of cell, muscle relaxes and adapts to new length
- This enables organs such as stomach and bladder to temporarily store increasing volume of contents
Compare contraction and stretching capacity of smooth and skeletal muscle.
Smooth muscle
- Can contract to half its resting length
- Can eb stretched to twice its resting length
Skeletal muscle
- Contracts maximally to 70% of its resting length
- Stretches up to 30% beyond its resting length
Hyperplasia
the enlargement of an organ or tissue caused by an increase in the reproduction rate of its cells
*Smooth muscles have the capacity for hyperplasia
