Muscles

Question 1

What is the sarcolemma and what is its function?

Answer 1

Is the cell membrane
Which surrounds the sarcoplasm - cytoplasm of fibre
Contains many organelles as seen in other cells
Has an abundance of 02 binding Protein myoglobin
Punctuated by openings =. Transverse tubules - t tubules
Which are narrow tubes that extend into the sarcoplasm at right angles
Filled with extracellular fluid.

Question 2

What is the sarcoplasm/reticulum ?

Answer 2

The cytoplasm and the endoplasmic reticulum

Question 3

What are sarcosomes?

Answer 3

Mitochondria

Question 4

What are myo fibrils?

Answer 4

Cylindrical structures within muscle fibre
Are bundles of protein filament = myo filaments
Of two types: actin, myosin
At each end of fibre myo fibrils are anchored to inner surface of the sarcolema
When myo fibrils shorten - muscle shortens = contracts

Question 5

Describe the contraction of the skeletal muscle

Answer 5

Under voluntary control
Each muscle cell is innervate by a branch of motor neuron
Action potentials are gated along neuron leading to release of ach at neuromuscular junction
Depolarisation of motor end plate + initiation of ap in muscle fiber
These actions are called - excitation- contraction coupling

Question 6

Describe muscle filaments

Answer 6

Each muscle fibre behaves as a single unit, is multinucleate
Contains myo fibrils
Which are surrounded by sarcoplasmic reticulum and are invaginated by t tubules
Arranged in sacromers
Repeating unit of sacromeres= unique banding pattern in skeletal/cardiac muscle -striated

Question 7

Describe thick filaments

Answer 7

They comprise large molecular weight protein called myosin
Which has 6 polypeptide chains : 1 pair of heavy, 2 pairs of light
Most heavy myosin has a helix structure in which the two chains coil around each other to form tail of myosin mol
Four light chains and n terminus of heavy chains form two globular heads on MM
Which have actin binding site, for cross bridge formation- site that binds/hydrolyses atp- myosin atpase

Question 8

Describe thin filaments

Answer 8

Composed of. 3 proteins
Actin, tropomyosin, TROPONIN
Actin- globular protein= in this form is called G actin
In thin filaments G actin is polymerised into two strands to form filamentous f actin
Actin has myosin binding sites
At rest- they are covered by tropomyosin
So that a and myosin cannot interact

Question 9

What is tropomyosin

Answer 9

Filamentous protein - runs along grove of each twisted actin filament
At rest- function is to block myosin-binding sites on actin
Must be moved out of way for contraction to occur

Question 10

What is troponin?

Answer 10

Complex of 3 globular proteins troponin I, T,C
Located at regular intervals along tropomyosin filaments
TT attactched troponin complex to tropomyosin
I- inhibition - along with tropomyosin- inhibit interaction of actin/myosin by covering binding site on actin
C- ca2 binding protein- role in intimation of contraction
When ca accumulates intracellularly it binds to tc = producing conformational change in troponin complex
Which moves tropomyosin out of way

Question 11

What is the scaromere?

Answer 11

Is the basic contractile unit
Delineated by z disks
Each sacromeres has: full A band and one half of two I bands either side of a bands

Question 12

Describe the a bands

Answer 12

Located in centre of saromere contain thick myosin filaments which appear dark under polarised light
Thick./thin filaments overlap in a band= sites for potential cross bridge formation

Question 13

DESCRIBE i bands

Answer 13

Located either side of a band
Light under p light
Contain thin actin f, intermediate f proteins and z disks
NO THICK F

Question 14

Describe z disks

Answer 14

Darkly staining structures that run down the middle of each I band which delineates the ends of each sacromere

Question 15

What is bare zone?

Answer 15

Located in centre of scaromere]- no thin f
No overlap of thick/thin f here
No cross bridge formation in this region

Question 16

What is the m line?

Answer 16

Bisects the bare zone- contains darkly staining proteins that link central portions of thick filaments together

Question 17

What are the roles of cytoskeletal proteins

Answer 17

Ensure that thick/thin f are aligned correctly at proper distances with respect to each other
There are two types longitudinal , transverse

Question 18

Describe longitudinal cytoskeletalproteins

Answer 18

Include two large proteins - titan, nebulin
Titin- associated with thick f, is a large molecular weight protein that extends from m lines to z disks.
Part of the molecule passes through thick f- the rest of mol which is either elastic or springlike is anchored to z disk
As length of sarcomere changes- so does elastic portion of titin mol
T Also helps to centre thick f in sacromere
Nebulin- associated with thin filaments. Single molecule of n extends from one thin f to another.
Serves as molecular ruler- setting lengths of thin f during assembly
A- actin anchors thin f to z disk

Question 19

Describe transverse cytoskeletal proteins

Answer 19

Link thick/thin f forming scaffold - for myofibrils and linking scaromeres of adjacent myofibrils
System of intermediate f holds myof together side by side- entire array of myof is anchored to cell membrane by actin binding filament dystrophin.

Question 20

Describe t tubules

Answer 20

Extensive network of sacrollemal membrane that invaginate into deep muscle fiber
They are responsible for carrying out depolarisation from ap at muscle cell surface to interior of fibre
T tubules make contact with internal cisternae of SR + contain voltage sensitive protein - dehydro-pyridine receptor

Question 21

Describe the sr

Answer 21

Internal tubular structure
Site of storage/release of ca for excitation- contraction coupling
Terminal cisternae of sr make contact with t tubule in triad arrangement
Contains ca2 release channel- ryanodine receptor
Ca accumulates in sr by action of ca atpase Serca in sr membrane
Serca plumbs ca from icf of muscle fibre into interior of sr, keeping ic-con of ca low when muscle fiber- at rest
With sr- ca is bound to calequestrin - low affinity high capacity ca binding protein which maintains low ca con in sr
Therefore large amounts of ca can be stored in sr whilst intrasarcoplasmic reticulum ca con remains low