Excitation and Contraction of Smooth Muscle Flashcards
Smooth muscle divided into two major types.
Name those 2
Multi-unit smooth muscle
Unitary (single-unit) smooth muscle
Multi-Unit Smooth Muscle Characteristics
Multi-unit smooth muscle is composed of individual separated smooth muscle fibers.
Individual fiber operates independently.
Each fiber is innervated by a single nerve ending, similar to skeletal muscle fibers.
Each fiber can contract independently of the others
Examples of multi-unit smooth muscle
ciliary and iris muscle of the eye, piloerector muscles of the skin.
Unitary Smooth Muscle Characteristics
Composed of many of muscle fibers that contract as a single unit.
Unitary smooth muscle also called syncytial smooth muscle or visceral smooth muscle.
Unitary smooth muscle fibers can be arranged in sheets or bundles.
Fiber cell membranes are adherent to one another at multiple points.
Cell membranes are connected by gap junctions.
function of gap junctions
allow free flow of ions and the transmission of action potentials
Examples of unitary smooth muscle
muscle of the GIT, bile ducts, ureters, uterus, and blood vessels.
explain the Smooth Muscle Filament Organization
Smooth muscle contains both actin and myosin filaments but no troponin complex.
Actin filaments are attached to dense bodies.
Dense bodies play the same role as the Z disks in skeletal muscle.
Dense bodies are attached to cell membrane, and some are scattered inside the cell.
Smooth Muscle vs Skeletal Muscle Contraction
Explain the difference
Skeletal muscles can contract and relax rapidly whereas smooth muscle contraction is a prolonged tonic contraction that can last for hours or even days.
The cycling of the myosin cross-bridges is slow in smooth muscles.
Slow cycling is attributed to less ATPase activity in smooth muscle.
Overall utilization of ATP to energizes the movements of the cross-bridge heads is greatly reduced in smooth muscles.
Contraction of smooth muscle is stimulated by
increase in intracellular calcium ions.
Increase intracellular calcium ions can be induced by
nerve stimulation, hormonal stimulation, stretch of the fiber, or changes in the chemical environment of the fiber.
Sources of calcium ions:
Influx of calcium ions from the ECF .
Release of calcium ions from the sarcoplasmic reticulum.
Calcium ions bind reversibly with? to form what?
calmodulin - Calmodulin-calcium complex
Calmodulin-calcium complex activates what?
phosphorylating enzyme called myosin light chain kinase (MLCK).
Active MLCK phosphorylate one of the light chains of each myosin head called?
regulatory chain
The phosphorylation of the regulatory chain is required for the
attachment-detachment cycling of the myosin head with the actin filament to occur.
Almost all the calcium ions for skeletal muscle contraction are release by
Sarcoplasmic reticulum
Smooth muscles sarcoplasmic reticula are not fully developed and are fewer, thus most of the calcium ions are sourced from the
ECF
Smooth muscle cells sarcoplasmic reticulum has small invaginations of the cell membrane, called ?
caveolae.
The caveolae play a similar role as the ?
transverse tubule system of skeletal muscle.
Action potential transmitted into the caveolae to excite what?
calcium ion release.
Smooth muscle contraction is dependent on ECF calcium ion concentration, thus the force of contraction…. finish the sentence.
highly dependent on the ECF calcium ion concentration.
For the smooth muscle to relax after it has contracted, the calcium ions must be removed from the ?
ICF
Explain smooth muscle relaxation
Calcium pump transport calcium ions out of the ICF and back into the sarcoplasmic reticulum.
Calcium ion channels close.
Myosin phosphatase degrade the phosphate from the regulatory chain (myosin head) and stops cycling and contraction.
The calcium pump utilizes ATP and slow acting in comparison with the fast-acting sarcoplasmic reticulum pump found in skeletal muscle.
Smooth muscle contraction can be stimulated to contract by ?
Nervous signals
Hormonal stimulation
Stretch of the muscle and or changes in the chemical environment of the smooth muscle fiber.
Latch state/mechanism refers to what?
refers to tonic contraction during which force is maintained at low energy expenditure.
which organs maintain prolonged tonic muscle contraction through latch mechanism.
Intestines, urinary bladder, gallbladder
explain innervation of smooth muscles
Nerve fibres that innervate smooth muscle branch diffusely on top of a sheet of muscle fibres.
Diffuse junctions are not in direct contact with the smooth muscle fibre membranes.
Action potential conduction in the muscle mass or diffusion of the transmitter substance.
Nerve terminals under neuromuscular junction of smooth muscle has what?
The function of that thing
Nerve terminals have multiple varicosities (not typical motor end plate)
Varicosities (vesicle) contain transmitter substance.
Autonomic nerves secretes ?
acetylcholine and norepinephrine as transmitter substances.
function of acetylcholine and norepinephrine
Acetylcholine and norepinephrine excite or inhibit smooth muscle.
Inositol 1,4,5-triphosphate (IP3) gated calcium channels - found on the sarcoplasmic reticulum (SR) membrane.
explain role
Hormones or neurotransmitters binds on receptors on the sarcolemmal membrane.
Receptors are coupled via a G protein, to phospholipase C (PLC).
PLC catalyzes the hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3 and diacylglycerol (DAG).
IP3 then diffuses to the sarcoplasmic reticulum and opens calcium release channels.
explain role of hormones on smooth muscle contraction
Hormone can inhibit smooth muscle contraction if the membrane contains inhibitory receptors for the hormone rather than excitatory receptors.
Binding of the hormone on the receptor closes the sodium and calcium channels to prevent entry of these positive ions.
Inhibition can also occur if the normally closed potassium channels are opened, allowing positive potassium ions to diffuse out of the cell.
Hormone stimulate smooth muscle contraction - Binding on the receptors in the smooth muscle membrane open sodium or calcium ion channels and depolarize the membrane.
Smooth muscle resting membrane potential is usually about
−50 to −60 millivolts.
There two forms of action potentials that can occur in visceral smooth muscle –
Name those 2
Spike potentials (typical action potential).
Action potentials with plateaus (prolonged contraction).
Prolonged plateau action potentials of some smooth muscle fibers occur as result of what?
Calcium channels open slowly
Calcium channels remain open much longer.
A. Self-excitatory action potentials arise without an ———– stimulus.
B. Self-excitatory action potentials are associated with basic ———- of the membrane potential.
C. Slow waves (pacemaker waves) are not the action potentials, —————–
A. extrinsic
B. slow wave rhythm
c. but slow waves can initiate action potentials.
Excitation of visceral smooth muscle by muscle stretch
explain
Stretched visceral (unitary) smooth muscle can generate action potentials resulting in muscle contracting.
Stretched induce smooth muscle contraction occur due to:
Slow wave potentials
A decrease in negativity of the membrane potential caused by the stretch.
Stretched induced smooth muscle contractions are important for the ?
peristaltic waves in the gut.
In small smooth muscle cells of multi-unit smooth muscle contraction occur because of what?
local depolarization (also called the junctional potential).
Small multi-unit smooth muscle fibres are too small to generate an action potential.
True or False
True
Smooth muscle contraction responds to changes in local chemical conditions i.e., interstitial fluid.
True or false
True
The smooth muscle responds to the following local chemical changes:
Depletion of oxygen in the local tissues causes smooth muscle relaxation = vasodilation.
Excess carbon dioxide causes vasodilation.
Increased hydrogen ion concentration causes vasodilation.
