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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

life circle of RBC and the life circle of WBC.
Yemi Reply
RBC 120days
Zeph
RBC 120days and WBC 10-12days
sai
what is anatomy?
Md Reply
positive feedback mechanism
Sirimala Reply
what is immunology
Riya Reply
immunology is a branch of medicine that study's the body immune system
SAMUEL
Immunology This is the study of specific and non-specific resistance of the body against infection i.e. the study of the immune response of a host to a foreign substance, which includes study of various reactions which are induced in the body by introduction of a substance.
Kaluki
what is role of elimination need like fluid and also stools
Munmun Reply
bone
Vijay
what is joint pain
Vijay
is the physical suffering caused by illness or injury of the joint
malulu
pls can someone describe shock,types ,pathophysiology and treatment
Isaac
this is what I'm thinking "After taking out everything the body needs, the bowel then expels the leftover waste."
isaiah
I think elimination also helps in the continuation of the digestive system because if the unwanted fluids and stools does not come out of the system it can create a problem in the digestive. system resulting in diseases.
Martha
shock is a condition whereby the circulating system is unable to get enough blood and oxygen to vital organs like the brain,heart,eye,kidney and others.
Martha
causing depression of those organs.
Martha
there are 2 classification of shock. primary shock: this occurs immediately after injury due emotional stimulus or pain.example hearing a bad news,sudden obstruction of airway.sudden heart attack. secondary shock :it occurs when primary shock is delayed
Martha
types of shock syncope (faint) oligaemic or hyppvovaemic shock. Anaphylactic shock. neurogenic/ physical shock septic sock catdiogenic shock.
Martha
What is the difference between dna duplication and chromosomes duplication?
to help you identify the human body parts to help you live a healthy life the study of Anatomy helps one to work in any health sector
sophia Reply
okay.
what is the function of the mitochondrial in the cell
Vida Reply
define and explain the synovial membrane
Mahmudu Reply
What is cloning?
Jesam Reply
relationship between anatomy and physiology
Ranjeeta Reply
anatomy is the structure and physiology is the function
Isaac
the branches of physiology
Asiedu Reply
is single DNA arranged into 46 chromosomes
Vaishnavi Reply
don't know about it
Sachin
no it is duble strand or pair of chromosomes
Marta
how does muscle contraction work?
Matthew
no,it is arranged as 23 pairs chromosomes
Ajiola
what are the parts of a cell?
Noel Reply
cell body, nucleus, cytoplasm, endoplasmic reticulum (rough and smooth), Golgi apparatus, cell membrane and organelles.
Heather
cell membrane, cell wall,cytoplasm, nucleus, etc
Felix
explain how skeletal muscles work
Felix
they work voluntarily
Trina
46 chromosomes present in which part of human body
Anar
when twins born how both of them carry 46 chromosomes
Anar
In the nuclear membrane
wisdom
but thiere r many cells n definetely cells have many nuclear membrane
Anar
cytoplasm plasma membrane nucleus
Ajiola
nucleus cytoplasm epr spr mitochondria
sureshbabu
cell have many parts and it act as different function s
sureshbabu
lysosome, golge body, cytoplasm, smooth endoplasmic reticulum
Vida
what is sex with male and female!
Muhammad Reply
intercourse
Jessie
sexual intercourse
Jessie
for formation of new generation
Sunil
sex is a female and male body courtship, rubbing of penis and vagina which results in release of fluids (sperm) from male in to the vagina of the female know as ejaculation
CHUOL
sex is a body courtship, penis and vagina rubbing which results in release of fluids sperm)
CHUOL
how sure a u?
Pius
it's like copulation
Pius

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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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