10 - Skeletal muscle

Question 1

What defines skeletal muscle?

Answer 1

Under conscious control and connected to skeletal system by tendons (tongue exception).

Question 2

Organisation of a skeletal muscle

Answer 2

• A muscle fibre contains many MYOFIBRILS

* Myofibrils are comprised of many MYOFILAMENTS, (thick and thin filaments) arranged into SARCOMERES.

Question 3

What are the 6 functions of the skeletal muscle?

Answer 3

1) Pull on bones to make skeleton move.
2) Maintain posture.
3) Support for soft tissues.
4) Control of openings and exits.
5) Maintenance of body temperature
6) Nutrient reserve

Question 4

What is a skeletal muscle fibre?

Answer 4

Several myoblast cells fuse early in development to form the muscle fibre.
Multi-nucleated - – nuclei located under sarcolemma (plasmamembrane).
Long and full muscle length.
Striated appearance - due to aligned sarcomeres
.

Question 5

Myofibrils

Answer 5

• Myofibrils consist of THICK and THIN myofilaments arranged into SARCOMERES (about 10,000 arranged end-to-end).
• Thin filaments = ACTIN
• Thick filaments = MYOSIN

Question 6

What is the sarcoplasmic reticulum?

Answer 6

What the myofibrils are surrounded by, storing ca ions.

They have T tubules that transfer the action potential to the myofibrils.

Question 7

A sarcomere

Answer 7

M-line = middle line – attaches thick filaments together
A band = dark bands, contain myosin +/- actin
I band = light bands, contain only actin
H band = contains only myosin
Z line (Z disc)=joins adjacent sarcomeres. 
Zone of overlap = region where actin and myosin overlap

Question 8

Thick filaments

Answer 8

THICK FILAMENTS are made of MYOSIN.
Many myosin molecules wound = thick filament
Titin (prevents overExtension of sarcomere)
Contraction- thick filaments interact with thin filaments.

Question 9

Thin filaments

Answer 9

THIN FILAMENTS made of ACTIN.
When relaxed, the myosin binding site on actin is covered by tropomyosin.
When stimulated to contract, calcium enters the muscle.
Calcium binds to troponin which causes tropomyosin to uncover myosin binding sites on actin.

Question 10

What is the sliding filament theory?

Answer 10

Theory that actin filaments slide toward each other during muscle contraction, while the myosin filaments are still.
Shortens the sarcomere.

Question 11

What is the full sliding filament mechanism?

Answer 11

1) Relaxed so binding site is covered. ATP is hydrolysed to ADP and P on myosin head and enters the high energy state.
2) Action potential causes ca release. Ca bind to troponin and tropomyosin moves and uncovers the myosin binding site.
3) Myosin-actin cross bridge forms.
4) Stored energy is released and myosin head pivots.
5) Cross bridge breaks as ATP binds to myosin head.

Question 12

What happens on the molecular level?

Answer 12

• Muscle movement activates motor neurone
• Action potential travels to muscle fibre
• Acetyl choline is released from the synaptic terminal.
• Ach binds to receptors on motor end plate of sarcolemma.
• Muscle fibre is depolarised.
• T-tubules transfer action potential to all the myofibrils
• Ca released from sarcoplasmic reticulum
• Ca binds to troponin enabling myosin-actin binding.
• Contraction occurs

Question 13

How does the contraction cycle end?

Answer 13

• Acetyl choline broken down in synaptic cleft.
• Calcium no longer released from sarcoplasmic reticulum.
• Calcium ions are pumped from the cytoplasm, back into the sarcoplasmic reticulum.
• Calcium detaches from the troponin complex on actin, so tropomyosin covers active sites, preventing myosin binding.