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Muscle contraction

This figure shows smooth muscle contraction. The left panel shows the structure of relaxed muscle and the right panel shows contracted muscle cells.
The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.

Although smooth muscle contraction relies on the presence of Ca ++ ions, smooth muscle fibers have a much smaller diameter than skeletal muscle cells. T-tubules are not required to reach the interior of the cell and therefore not necessary to transmit an action potential deep into the fiber. Smooth muscle fibers have a limited calcium-storing SR but have calcium channels in the sarcolemma (similar to cardiac muscle fibers) that open during the action potential along the sarcolemma. The influx of extracellular Ca ++ ions, which diffuse into the sarcoplasm to reach the calmodulin, accounts for most of the Ca ++ that triggers contraction of a smooth muscle cell.

Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca ++ ions back into the SR and out of the cell. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone. This remaining calcium keeps the muscle slightly contracted, which is important in certain tracts and around blood vessels.

Because most smooth muscles must function for long periods without rest, their power output is relatively low, but contractions can continue without using large amounts of energy. Some smooth muscle can also maintain contractions even as Ca ++ is removed and myosin kinase is inactivated/dephosphorylated. This can happen as a subset of cross-bridges between myosin heads and actin, called latch-bridges    , keep the thick and thin filaments linked together for a prolonged period, and without the need for ATP. This allows for the maintaining of muscle “tone” in smooth muscle that lines arterioles and other visceral organs with very little energy expenditure.

Smooth muscle is not under voluntary control; thus, it is called involuntary muscle. The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction (the stretch-relaxation response).

Axons of neurons in the ANS do not form the highly organized NMJs with smooth muscle, as seen between motor neurons and skeletal muscle fibers. Instead, there is a series of neurotransmitter-filled bulges called varicosities as an axon courses through smooth muscle, loosely forming motor units ( [link] ). A varicosity    releases neurotransmitters into the synaptic cleft. Also, visceral muscle in the walls of the hollow organs (except the heart) contains pacesetter cells. A pacesetter cell    can spontaneously trigger action potentials and contractions in the muscle.

Motor units

In this figure, the left panel shows a neuron with vesicles containing neurotransmitters. The right panel shows a bundle of smooth muscle cells with neurons wound around them.
A series of axon-like swelling, called varicosities or “boutons,” from autonomic neurons form motor units through the smooth muscle.

Smooth muscle is organized in two ways: as single-unit smooth muscle, which is much more common; and as multiunit smooth muscle. The two types have different locations in the body and have different characteristics. Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. This type of smooth muscle is found in the walls of all visceral organs except the heart (which has cardiac muscle in its walls), and so it is commonly called visceral muscle    . Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response    . This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction, but this is immediately followed by relaxation so that the organ does not empty its contents prematurely. This is important for hollow organs, such as the stomach or urinary bladder, which continuously expand as they fill. The smooth muscle around these organs also can maintain a muscle tone when the organ empties and shrinks, a feature that prevents “flabbiness” in the empty organ. In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body.

Multiunit smooth muscle cells rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated. Stimuli for multiunit smooth muscles come from autonomic nerves or hormones but not from stretching. This type of tissue is found around large blood vessels, in the respiratory airways, and in the eyes.

Hyperplasia in smooth muscle

Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size. Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia    . This can most evidently be observed in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers, and greatly increases the size of the myometrium.

Sections summary

Smooth muscle is found throughout the body around various organs and tracts. Smooth muscle cells have a single nucleus, and are spindle-shaped. Smooth muscle cells can undergo hyperplasia, mitotically dividing to produce new cells. The smooth cells are nonstriated, but their sarcoplasm is filled with actin and myosin, along with dense bodies in the sarcolemma to anchor the thin filaments and a network of intermediate filaments involved in pulling the sarcolemma toward the fiber’s middle, shortening it in the process. Ca ++ ions trigger contraction when they are released from SR and enter through opened voltage-gated calcium channels. Smooth muscle contraction is initiated when the Ca ++ binds to intracellular calmodulin, which then activates an enzyme called myosin kinase that phosphorylates myosin heads so they can form the cross-bridges with actin and then pull on the thin filaments. Smooth muscle can be stimulated by pacesetter cells, by the autonomic nervous system, by hormones, spontaneously, or by stretching. The fibers in some smooth muscle have latch-bridges, cross-bridges that cycle slowly without the need for ATP; these muscles can maintain low-level contractions for long periods. Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contractions so that the muscle contracts as a single unit. Single-unit smooth muscle in the walls of the viscera, called visceral muscle, has a stress-relaxation response that permits muscle to stretch, contract, and relax as the organ expands. Multiunit smooth muscle cells do not possess gap junctions, and contraction does not spread from one cell to the next.

Questions & Answers

coronary circulation ?
Juri Reply
Coronary circulation is the circulation of blood in the blood vessels that supply the heart muscle (myocardium). Coronary arteries supply oxygenated blood to the heart muscle, and cardiac veins drain away the blood once it has been deoxygenated. Because the rest of the body.
Riyaz
event of cardiac cycle
Juri Reply
hii
Chandan
whatisanatom
kaso Reply
anatomy is the scientific way of studying the body structure.
cynthia
the branch of science concerned with the bodily structure of humans, animals, and other living organisms, especially as revealed by dissection and the separation of parts.
Noor
what is means by LAPE and HAPE
Noor
guys i've question what occur when homeostasis balance mechanisms lost
cris Reply
Bladder is blank to the small intestine, what is the right directional term for that?
Julaika Reply
When peristaltic movement is correct.
Farid
what tissue that support body organ?
Sabrina Reply
the skeleton
veronica
Skeleton
GEBAH
skeleton
Juwita
skeleton
Farid
skeleton
Priya
skeleton
Emmanuel
skeleton
Noor
skeleton
BILAL
skeleton
hanuman
how many bones are there in the body
hanuman Reply
go uuu
Prasad
206
Prasad
206
Nelago
206
Esmeralda
206
Joey
baby me 360. and human body 206.found bones.....
Sneha
what is a tissue in Anatomy and physiology
Nelago
206 bones
Juwita
what is an example of a molecule that can directly pass through the phospholipid bilayer of the plasma membrane?
Kathy
206
Johara
how many bones are there in the hand and wrist
hanuman
200 tissue
hanuman
answer 206 bones
GEBAH
axial 80 appendicular 126 total bone is 206
Vineeta
wrist bone 8
Vineeta
baby bone 270 he is correct answer
hanuman
Jo carpus wrist bone h
Vineeta
206
Musonda
206 bones in human body.
Farid
What is Heart attack?
Farid
how can l understand Anatomy quickly?
Agness Reply
you can't!!!! Anatomy is easy but the physiology takes time to understand. Anatomy is simply systems and organs but you must take time to learn physiology to understand how the anatomy works
Rachael
alright.. so how can l at least memorize the information about physiology?
Agness
what name is given to the fluid that is drawn from the villi to the lymphatic vessels
margaret Reply
I think venous
Noor
lymph
Rachael
I also think it's lymph
Akhi
me too I think lymph
Esmeralda
ans Rhythmic contraction
GEBAH
what are the ten (10) rights for drug administration
GEBAH
importance of muscle in the anatomical structures.
Akomeng Reply
it permits the constant level of movement,and gives structure and shape to the body
Priya
what is assimilation
Asiimwe Reply
cellular level organism level chemical level organ system level tissue level organ level
Aleah Reply
, the current topic
Hilya
cellular level
Priya
tissue level
Priya
organ level
Priya
organ systems level
Priya
organism level
Priya
what is anatomy?
Aleah Reply
anatomy is the study of internal parts of body by disecting it.
Shweta
The study of the internal body parts and it's organs are referred as Anatomy.
Adarsh
hey guys am new here?
Annica Reply
undesrtanding the different part of a cell
Jean
***youtu.be/QXT4OVM4vXI this is a great video to help in understanding. It has helped me allot and learned about the distance between atom.
david
sperm cell and egg cell which one have more cell than the other
Urama Reply
sperm cell!
Longvwam
The sperm is _______ to the sternum
Jean
distal
Rachael
posterior
rOx

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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