Excitation and Contraction of Smooth Muscle

Question 1

Smooth muscle divided into two major types.
Name those 2

Answer 1

Multi-unit smooth muscle
Unitary (single-unit) smooth muscle

Question 2

Multi-Unit Smooth Muscle Characteristics

Answer 2

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

Question 3

Examples of multi-unit smooth muscle

Answer 3

ciliary and iris muscle of the eye, piloerector muscles of the skin.

Question 4

Unitary Smooth Muscle Characteristics

Answer 4

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.

Question 5

function of gap junctions

Answer 5

allow free flow of ions and the transmission of action potentials

Question 6

Examples of unitary smooth muscle

Answer 6

muscle of the GIT, bile ducts, ureters, uterus, and blood vessels.

Question 7

explain the Smooth Muscle Filament Organization

Answer 7

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.

Question 8

Smooth Muscle vs Skeletal Muscle Contraction
Explain the difference

Answer 8

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.

Question 9

Contraction of smooth muscle is stimulated by

Answer 9

increase in intracellular calcium ions.

Question 10

Increase intracellular calcium ions can be induced by

Answer 10

nerve stimulation, hormonal stimulation, stretch of the fiber, or changes in the chemical environment of the fiber.

Question 11

Sources of calcium ions:

Answer 11

Influx of calcium ions from the ECF .
Release of calcium ions from the sarcoplasmic reticulum.

Question 12

Calcium ions bind reversibly with? to form what?

Answer 12

calmodulin - Calmodulin-calcium complex

Question 13

Calmodulin-calcium complex activates what?

Answer 13

phosphorylating enzyme called myosin light chain kinase (MLCK).

Question 14

Active MLCK phosphorylate one of the light chains of each myosin head called?

Answer 14

regulatory chain

Question 15

The phosphorylation of the regulatory chain is required for the

Answer 15

attachment-detachment cycling of the myosin head with the actin filament to occur.

Question 16

Almost all the calcium ions for skeletal muscle contraction are release by

Answer 16

Sarcoplasmic reticulum

Question 17

Smooth muscles sarcoplasmic reticula are not fully developed and are fewer, thus most of the calcium ions are sourced from the

Answer 17

ECF

Question 18

Smooth muscle cells sarcoplasmic reticulum has small invaginations of the cell membrane, called ?

Answer 18

caveolae.

Question 19

The caveolae play a similar role as the ?

Answer 19

transverse tubule system of skeletal muscle.

Question 20

Action potential transmitted into the caveolae to excite what?

Answer 20

calcium ion release.

Question 21

Smooth muscle contraction is dependent on ECF calcium ion concentration, thus the force of contraction…. finish the sentence.

Answer 21

highly dependent on the ECF calcium ion concentration.

Question 22

For the smooth muscle to relax after it has contracted, the calcium ions must be removed from the ?

Answer 22

ICF

Question 23

Explain smooth muscle relaxation

Answer 23

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.

Question 24

Smooth muscle contraction can be stimulated to contract by ?

Answer 24

Nervous signals
Hormonal stimulation
Stretch of the muscle and or changes in the chemical environment of the smooth muscle fiber.

Question 25

Latch state/mechanism refers to what?

Answer 25

refers to tonic contraction during which force is maintained at low energy expenditure.

Question 26

which organs maintain prolonged tonic muscle contraction through latch mechanism.

Answer 26

Intestines, urinary bladder, gallbladder

Question 27

explain innervation of smooth muscles

Answer 27

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.

Question 28

Nerve terminals under neuromuscular junction of smooth muscle has what?
The function of that thing

Answer 28

Nerve terminals have multiple varicosities (not typical motor end plate)
Varicosities (vesicle) contain transmitter substance.

Question 29

Autonomic nerves secretes ?

Answer 29

acetylcholine and norepinephrine as transmitter substances.

Question 30

function of acetylcholine and norepinephrine

Answer 30

Acetylcholine and norepinephrine excite or inhibit smooth muscle.

Question 31

Inositol 1,4,5-triphosphate (IP3) gated calcium channels - found on the sarcoplasmic reticulum (SR) membrane.
explain role

Answer 31

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.

Question 32

explain role of hormones on smooth muscle contraction

Answer 32

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.

Question 33

Smooth muscle resting membrane potential is usually about

Answer 33

−50 to −60 millivolts.

Question 34

There two forms of action potentials that can occur in visceral smooth muscle –
Name those 2

Answer 34

Spike potentials (typical action potential).
Action potentials with plateaus (prolonged contraction).

Question 35

Prolonged plateau action potentials of some smooth muscle fibers occur as result of what?

Answer 35

Calcium channels open slowly
Calcium channels remain open much longer.

Question 36

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, —————–

Answer 36

A. extrinsic
B. slow wave rhythm
c. but slow waves can initiate action potentials.

Question 37

Excitation of visceral smooth muscle by muscle stretch
explain

Answer 37

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.

Question 38

Stretched induced smooth muscle contractions are important for the ?

Answer 38

peristaltic waves in the gut.

Question 39

In small smooth muscle cells of multi-unit smooth muscle contraction occur because of what?

Answer 39

local depolarization (also called the junctional potential).

Question 40

Small multi-unit smooth muscle fibres are too small to generate an action potential.
True or False

Answer 40

True

Question 41

Smooth muscle contraction responds to changes in local chemical conditions i.e., interstitial fluid.
True or false

Answer 41

True

Question 42

The smooth muscle responds to the following local chemical changes:

Answer 42

Depletion of oxygen in the local tissues causes smooth muscle relaxation = vasodilation.
Excess carbon dioxide causes vasodilation.
Increased hydrogen ion concentration causes vasodilation.