physiology 1 lecture 17 skeletal muscle fiber anatomy

PHYSIOLOGY 1PHYSIOLOGY 1

LECTURE 17LECTURE 17

SKELETAL MUSCLESKELETAL MUSCLE

FIBER ANATOMYFIBER ANATOMY

SKELETAL MUSCLESKELETAL MUSCLEFIBER ANATOMYFIBER ANATOMY

OBJECTIVES:OBJECTIVES: The student should; The student should;

1. Learn the basics of skeletal muscle 1. Learn the basics of skeletal muscle anatomyanatomy

2. Learn the relationship of the 2. Learn the relationship of the contractile proteins to each other within contractile proteins to each other within the sarcomerethe sarcomere

3. Learn the basic process of skeletal 3. Learn the basic process of skeletal muscle excitation - contraction - couplingmuscle excitation - contraction - coupling

4. Learn how skeletal muscle terminates 4. Learn how skeletal muscle terminates contraction.contraction.


The act of skeletal muscle fiber The act of skeletal muscle fiber contraction is a complex series of contraction is a complex series of nerve muscle interactions which nerve muscle interactions which lead to actin myosin crossbridge lead to actin myosin crossbridge cycling and sarcomere shortening cycling and sarcomere shortening thereby shortening the muscle and thereby shortening the muscle and generating movement.generating movement.


A skeletal muscle fiber consists of the tendons at A skeletal muscle fiber consists of the tendons at each end of the cell body which is connected to each end of the cell body which is connected to both each other and the cell body by the both each other and the cell body by the endomysium layer of connective tissue.endomysium layer of connective tissue.

1. Tendons - Elastic elements similar to rubber 1. Tendons - Elastic elements similar to rubber bands connect the muscle cell to the bone - bands connect the muscle cell to the bone - Composed of collagen and elastin which merges Composed of collagen and elastin which merges with the endomysiumwith the endomysium

2. Muscle body - The skeletal muscle cell is one 2. Muscle body - The skeletal muscle cell is one of the largest in of the body. These are of the largest in of the body. These are multinucleated cells and appear banded due to multinucleated cells and appear banded due to the sarcomere contractile protein structure under the sarcomere contractile protein structure under a microscope.a microscope.


Internal Skeletal Muscle FiberInternal Skeletal Muscle Fiber– 1. Myofibril - Series of sarcomeres 1. Myofibril - Series of sarcomeres

running the length of the fiberrunning the length of the fiber– 2. T-tubules 2. T-tubules – 3. Sarcoplasmic reticulum3. Sarcoplasmic reticulum– 4. Sarcomere4. Sarcomere– 5. Multinucleated5. Multinucleated– 6. Mitochondria 6. Mitochondria

SKELETAL MUSCLESKELETAL MUSCLEInternal FiberInternal Fiber

Myofibril Myofibril -- A series of A series of

sarcomeres sarcomeres running the running the length of the length of the skeletal muscle skeletal muscle cell.cell.

SKELETAL MUSCLESKELETAL MUSCLEInternal FiberInternal Fiber

T-Tubules - 2 per T-Tubules - 2 per sarcomere - These sarcomere - These are invaginations of are invaginations of the cells outer the cells outer membrane - they membrane - they also bring the AP into also bring the AP into close approximation close approximation to the SRto the SR

3. SR - Ca++ 3. SR - Ca++ storagestorage

SKELETAL MUSCLESKELETAL MUSCLESarcomere Protein ComponentsSarcomere Protein Components

The Sarcomere is composed of two The Sarcomere is composed of two units the thick fibers (Myosin) and units the thick fibers (Myosin) and the thin fibers (Actin-Troponin-the thin fibers (Actin-Troponin-Tropomyosin complex)Tropomyosin complex)


The Thin FiberThe Thin Fiber - - 1. The thin fiber is 1. The thin fiber is

composed of a composed of a protein monomer protein monomer forming a polymer of forming a polymer of g-Actin subunits. g-Actin subunits.

2. Tropomyosin a 2. Tropomyosin a long thin fiber like long thin fiber like protein covering 7 protein covering 7 actin monomers. actin monomers.


Tropin - Composed Tropin - Composed of three globular of three globular proteins proteins

a. Protein I binds a. Protein I binds to the actin fiberto the actin fiber

b. Protein C binds b. Protein C binds calcium (4)calcium (4)

c. Protein T binds c. Protein T binds to tropomyosinto tropomyosin


Action of Tropin - Action of Tropin - When Ca++ is bound When Ca++ is bound to the C protein the to the C protein the tropin molecule bends tropin molecule bends inward forcing the inward forcing the tropomyosin to move tropomyosin to move clear of the myosin clear of the myosin head binding sites on head binding sites on actin actin


The Thick FiberThe Thick Fiber - - The thick fiber is composed The thick fiber is composed

of the protein myosin - of the protein myosin - Myosin is composed of six Myosin is composed of six (6) protein sub units - Two (6) protein sub units - Two long thin units twisted long thin units twisted together with a globular together with a globular head and four light chain head and four light chain regulatory units. The regulatory units. The myosin molecules adhere myosin molecules adhere to one another forming a to one another forming a long fiber with the heads on long fiber with the heads on the outsidethe outside

SKELETAL MUSCLESKELETAL MUSCLESarcomere StructureSarcomere Structure

Z - Line or Disk - Protein structure which Z - Line or Disk - Protein structure which supports the actin fiberssupports the actin fibers

I - Band - Overlaps two sarcomeres the Z disk is I - Band - Overlaps two sarcomeres the Z disk is center and contains only actin fiberscenter and contains only actin fibers

A - Band - Delineates the Myosin fibers, does not A - Band - Delineates the Myosin fibers, does not change length during muscle contractionchange length during muscle contraction

H - Zone - Contains myosin fibers only centered H - Zone - Contains myosin fibers only centered on M-Line or Diskon M-Line or Disk

M - Line or Disk - The central protein structure of M - Line or Disk - The central protein structure of the sarcomere which supports the myosin fibersthe sarcomere which supports the myosin fibers

SKELETAL MUSCLESKELETAL MUSCLESarcomere StructureSarcomere Structure

SKELETAL MUSCLESKELETAL MUSCLEPropagation of Action Propagation of Action

PotentialPotential

Once an action potential is generated in the Once an action potential is generated in the skeletal muscle fiber it is propagated over the skeletal muscle fiber it is propagated over the entire sarcolemma and down the T-tubules entire sarcolemma and down the T-tubules (Invaginations of the sarcolemma). The (Invaginations of the sarcolemma). The passage of the AP through the T-tubules passage of the AP through the T-tubules activates voltage sensitive proteins. These activates voltage sensitive proteins. These withdraw into the T-tubules opening calcium withdraw into the T-tubules opening calcium channels in the sarcoplasmic reticulum channels in the sarcoplasmic reticulum causing a major release ofcausing a major release of calcium ion into calcium ion into the sarcoplasm. Similar to pulling a cork from the sarcoplasm. Similar to pulling a cork from a bottle.a bottle.


PotentialPotential


PotentialPotential

The passage of the The passage of the action potential action potential causes the pillars to causes the pillars to withdraw from their withdraw from their plugging position plugging position which blocks the which blocks the sarcoplastic sarcoplastic reticulum’s calcium reticulum’s calcium channels. Calcium channels. Calcium release is about 1000 release is about 1000 fold.fold.


PotentialPotential

3. Calcium release into the 3. Calcium release into the sarcoplasm is about 1000 fold or - sarcoplasm is about 1000 fold or - Ca++ concentration goes from 10Ca++ concentration goes from 10-7-7 to 10 to 10 -4.-4.

SKELETAL MUSCLESKELETAL MUSCLEEXCITATION - CONTRACTION - COUPLINGEXCITATION - CONTRACTION - COUPLING

The molecular process of The molecular process of excitation - contraction - coupling excitation - contraction - coupling (or crossbridge cycling) is the basis (or crossbridge cycling) is the basis of current thought on how of current thought on how sarcomere contraction occurs.sarcomere contraction occurs.


A. Four CaA. Four Ca++++ ions bind to Tropin C ions bind to Tropin C B. Shape change in Tropin moves B. Shape change in Tropin moves

Tropomyosin away from myosin Tropomyosin away from myosin head binding site on actinhead binding site on actin

C. Myosin head + ADP - P binds to C. Myosin head + ADP - P binds to actin at binding siteactin at binding site

D. ADP - P is released from the D. ADP - P is released from the Myosin headMyosin head


E. The release of ADP and the P E. The release of ADP and the P group cause a conformational group cause a conformational change in myosin resulting in change in myosin resulting in flexure of the myosin head from flexure of the myosin head from it’s original 90it’s original 90oo position to an angle position to an angle of about 45of about 45oo moving the actin fiber moving the actin fiber inward about 10 nm.inward about 10 nm.


F. ATP binds to the myosin head F. ATP binds to the myosin head causing a conformational change which causing a conformational change which causes the myosin head to release the causes the myosin head to release the actin fiber.actin fiber.

G. The MyosinATPase splits the ATP G. The MyosinATPase splits the ATP into ADP - P causing the last into ADP - P causing the last conformational change resulting in the conformational change resulting in the return of the myosin head to it’s 90o return of the myosin head to it’s 90o position ready to repeat the cycle.position ready to repeat the cycle.

SKELETAL MUSCLESKELETAL MUSCLETermination of ContractionTermination of Contraction

The contraction is terminated The contraction is terminated when the cytosolic Cawhen the cytosolic Ca++++ concentration levels fall back to concentration levels fall back to normal 10normal 10-7-7 molar. This is molar. This is accomplished by a Sarcoplasmic accomplished by a Sarcoplasmic Reticular CaReticular Ca++++ATPase which ATPase which transports two Catransports two Ca++++ ions into the ions into the SR for every ATP burned.SR for every ATP burned.

SKELETAL MUSCLESKELETAL MUSCLETermination of ContractionTermination of Contraction

SUMMARYSUMMARY 1. What are the basics of skeletal muscle 1. What are the basics of skeletal muscle

anatomy?anatomy? 2. What is the relationship of the 2. What is the relationship of the

contractile proteins to each other within contractile proteins to each other within the sarcomere?the sarcomere?

3. What is the basic process of skeletal 3. What is the basic process of skeletal muscle excitation - contraction – muscle excitation - contraction – coupling?coupling?

4. How does skeletal muscle terminate 4. How does skeletal muscle terminate contraction?contraction?

physiology 1 lecture 17 skeletal muscle fiber anatomy

Documents

skeletal muscle cell

protein myosin myosin

c protein

muscle body

protein monomer

protein sub units

protein t binds

disk protein structure