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By the end of this section, you will be able to:
  • Describe the components involved in a muscle contraction
  • Explain how muscles contract and relax
  • Describe the sliding filament model of muscle contraction

The sequence of events that result in the contraction of an individual muscle fiber begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize as positively charged sodium ions (Na + ) enter, triggering an action potential that spreads to the rest of the membrane will depolarize, including the T-tubules. This triggers the release of calcium ions (Ca ++ ) from storage in the sarcoplasmic reticulum (SR). The Ca ++ then initiates contraction, which is sustained by ATP ( [link] ). As long as Ca ++ ions remain in the sarcoplasm to bind to troponin, which keeps the actin-binding sites “unshielded,” and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin strands by myosin, the muscle fiber will continue to shorten to an anatomical limit.

Contraction of a muscle fiber

The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how the release of acetylcholine into the muscle cells leads to the release of calcium. The middle panel shows how calcium release activates troponin and leads to muscle contraction. The bottom panel shows an image of a muscle fiber being shortened and producing tension.
A cross-bridge forms between actin and the myosin heads triggering contraction. As long as Ca ++ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten.

Muscle contraction usually stops when signaling from the motor neuron ends, which repolarizes the sarcolemma and T-tubules, and closes the voltage-gated calcium channels in the SR. Ca ++ ions are then pumped back into the SR, which causes the tropomyosin to reshield (or re-cover) the binding sites on the actin strands. A muscle also can stop contracting when it runs out of ATP and becomes fatigued ( [link] ).

Relaxation of a muscle fiber

The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how calcium is being absorbed into the muscle fiber. This results in the relaxation of the thin and thick filaments as shown in the bottom panel.
Ca ++ ions are pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands. A muscle may also stop contracting when it runs out of ATP and becomes fatigued.

The release of calcium ions initiates muscle contractions. Watch this video to learn more about the role of calcium. (a) What are “T-tubules” and what is their role? (b) Please describe how actin-binding sites are made available for cross-bridging with myosin heads during contraction.

The molecular events of muscle fiber shortening occur within the fiber’s sarcomeres (see [link] ). The contraction of a striated muscle fiber occurs as the sarcomeres, linearly arranged within myofibrils, shorten as myosin heads pull on the actin filaments.

The region where thick and thin filaments overlap has a dense appearance, as there is little space between the filaments. This zone where thin and thick filaments overlap is very important to muscle contraction, as it is the site where filament movement starts. Thin filaments, anchored at their ends by the Z-discs, do not extend completely into the central region that only contains thick filaments, anchored at their bases at a spot called the M-line. A myofibril is composed of many sarcomeres running along its length; thus, myofibrils and muscle cells contract as the sarcomeres contract.

Questions & Answers

What kind of discussion
horyaal Reply
what is the function of the placenta
Nchimunya Reply
The placenta acts to provide oxygen and nutrientsto the fetus, whilst removing carbon dioxide and other waste products.
Moha
Also it's the barrier through which the mother is connected to the fetus.
Samuel
I want to discuss... atherosclerosis.., everything about it, about to treatment n prevention at age 50 +
Doctors
Please participate in discussion
Doctors
ok let's discuss now
Moha
first define the word artherosclerosis
Moha
a disease of the arteries characterized by the deposition of fatty material on their inner walls.
tabe
hardening of the arteries, due to fats..
jenelyn
Q = which type of fat utilized for this.., I.e LDL, HDL, TG, VLDL...?
Doctors
Atherosclerosis is a condition of deposition of plaque inside the artries
Arvind
Plaque include such as fat, chalestrol, calcium etc
Arvind
Thank you kumar...., is there any way that we can protect these plaques without any medicines., I. e exercises n food stuffs
Doctors
go to the gym
Moha
what is chylomicrons?
Moha
how can plaque buildup in The angina or vessels ?
Moha
Atherosclerosis is a condition of deposition of plaque inside the artries
Doctors
atherosclerosis can also be caused by tortuousness of arteries with old age as contributing factor.
edward
You tube Prof Fink...he is an amazing lecturer and does a brilliant job on arteriosclerosis
Jacqueline
what are cell
Chigozie Reply
The basic structural and functional unit of any living thing. Each cell is a small container of chemicals and water wrapped in a membrane. 
Yusuf
cell is the structural and functional basic unit of life
Zaid
So every living thing was Created From a Cell
Anthony
from a fusion of two cells , the sperm and the egg
Shikoh
What is the only bone that doesn't have any articulation?
Jimmy
that magical fusion of cells
OBED
why body immune system attack and destroy the body own cell during type 1 diabetes?
Sanamacha Reply
It's an autoimmune disease... targeting the pancreas
Claudia
what are the three genetic defects of pregnancy?
Belinda Reply
structure of fallopian tubes
Akash Reply
what z the role played by the transport system
zinitha Reply
how fats are digested in the human body
Nabukwasi Reply
Fat digestion begins in the stomach but some argue in that it starts in the mouth. Reason is because the sublingual gland secretes an enzyme called lingual lipase. However, this enzyme is not activated until it comes into contact with gastric fluids (HCl). In the stomach, HCl breaks down the lipid..
Jeremiah
due to body heat
Mule
into smaller molecules. Going from a triglyceride and a fatty acid to a monoglyceride and a a fatty acid no longer bound to one another. This is known as lipolysis.
Jeremiah
After lipolysis in the stomach from gastric and lingual lipase, an acidic chyme is produced after stomach churning the bolus. The chyme exits the stomach at the pyloric sphincter and enters the first section of the small intestine known as the duodenum.
Jeremiah
in the duodenum. An alkaline mucus from goblet cells neutralizes the acidic chyme to prevent acid burns. After that, the pancreas and gallbladder secrete a number of enzymes to continue lipolysis. Bile from the gallbladder enters the duodenum via common bile duct. The acinar cells in the pancreas...
Jeremiah
secretes pancreatic lipase after enteroendocrine cells in the duodenum secrete a stimulator hormone called CCK. Cck stimulates bile synthesis and secretion as well as pancreatic lipase.
Jeremiah
Bile emuslifies the lipid, allowing the lipases to continue lipolysis
Jeremiah
this breakdown continues until it reaches the jejunum of the small intestines. At this point, the lipid has been broken down small enough to absorbed into the blood stream. So villi in the jejunum, absorb the contents.
Jeremiah
ileum, the last small intestine region, absorbs anything that wasn't absorbed previously. Like minerals, vitamins, bile salts, water soluble material. Villi here complete that task. Fatty acid and glycerol however, are absorbed by lacteals. small lymph vessels. And are transported to the liver.
Jeremiah
That concludes lipid digestion. Anything else that remains is deficated after it travels through the large intestines.
Jeremiah
parents with blood group AB & 0,,,what will b the blood group of their offspring
imran
what are the different branches of anatomy
Nabukwasi
hopefully that helped.
Jeremiah
which ion is low of blood level?
Ezra Reply
what is coagulation?
feng Reply
liquid turning to solid... blood clots.
Kristy
coagulation : liquid blood into blood clots caused with a coagulant.
jaime
when the blood turn from liquid form to solid
June
it said to coagulate by the action of active plasma protein called *fibrin*
Hassan
I.e liquid inform of blood when to solid
Hassan
it is the process by which blood becomes more viscous or becomes thick
CHRISTOPHER
cloting of blood cells
Kabange
clot of blood
Moses
the process of forming semi solid lumps in a liquid
rida
conversion of blood to solid state
Ezra
semi solid., rather than solid form
Doctors
what is pivot functioned
Ever
to less thefriction
Hirsi
how to calculate the micrograph
Ampong Reply
it can be used to knw the disease condition
Akbar Reply
which gland secret tears
Opoku
lacrimal glands
Diego
explain the blood supply to the brain
Brenda Reply
There are two paired arteries which are responsible for the blood supply to the brain; the vertebral arteries, and the internal carotid arteries. These arteries arise in the neck, and ascend to the cranium.
Sabrina
two arteries main vertebral arteries & internal carotid artery
Akbar
Is tissue the same as cells?
Seyram Reply
yes tissues are made up of cells
KJ
nope
Francis
a tissue is made by cells which release the components and rule the built of a specific extracellular matrix
Manuel
OK thanks very much
Seyram
ya
Ananthapadmanabha
what is the control system
zinitha
which system
Brenda
Nervous system is just one control system in the body. Endocrine system is the other.
Sabrina
1.study of anatomy n physiology helps us to understand how internal organs of our body function 2.it helps us to know the locations of the bones in the body and its functions 3.it helps us to know the functions of the micro (smaller) body structures like cell n tissue and its functions
paul Reply

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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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