Excitable tissue, Muscle lecture: one

Question 1

Describe in generic terms skeletal muscle (lecture one slide 3):

(what controls it and appearance)

Answer 1

• Under voluntary control
• Striated
• Multiple PERIPHERAL nuclei
• SINGLE ling cylindrical cells

Question 2

Describe in generic terms cardiac muscle (lecture one slide 4):

(what controls it and appearance)

Answer 2

• Myogenic (electrical potential originates from heart itself) (Involuntary control)
• striated
• BRANCHED CELLS (ONLY 1-3 CENTRAL nuclei)
• Connected via INTERCALATED DISCS

Question 3

Describe in generic terms smooth muscle (lecture one slide 5):

(what controls it and appearance)

Answer 3

• Involuntary
• Found in the wall of internal organs (gut, blood vessels etc)
• Spindle shaped, uninucleated cells
• Not striated

Question 4

What are myocytes composed of?

Answer 4

A myocyte is an individual muscle fibre

• Myocytes contain myofibrils
• Myofibrils contain highly organised contractile filaments

Question 5

How do muscles attach to bone?

Answer 5

Tendons

Question 6

How big can a myocyte get to?

Answer 6

35cm and 0.1cm wide

Question 7

What are the thin and thick filaments of myocytes?

Answer 7

Thin = Actin filament 
Thicc = Myosin filament

Question 8

Describe the microscopic structure of the myofibril referring to thick filament, thin filament, H zone, Z disc, M line

Answer 8

• Thick filaments: run the entire length of an A band
• Thin filaments: run the length of the I band and partway into the A band
• Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
• H zone: lighter mid-region where filaments do not overlap
• M line: line of protein myomesin that holds adjacent thick filaments together

Question 9

What membrane envelopes a muscle fibre?

Answer 9

Sarcolemma membrane envelopes collections of myofibrils

Question 10

What are t tubules?

Answer 10

•T-tubules - deep invaginations continuous with the sarcolemma (cell membrane) and circle each sarcomere at each of the junctions of the A and I bands. Allows action potentials to be carried deep within the muscle cell (contain the calcium channels for Ca to enter the cell and cause Ca induced Ca release from the SR)

Question 11

Whats the SR?

Answer 11

•Sarcoplasma reticulum (SR) – The calcium storage site. The terminal cisternae of the SR lie close to the T- tubules.

Question 12

What pumps Ca back into the SR?

Answer 12

Ca ATPase pumps i.e SERCA

Question 13

Describe the structure of the thick filament:

• Composition
• Structure
• function
• polarity
• protein anchor

Answer 13

• Composed of Myosin
• Each myosin has 2 subunits each with a globular head and a tail, the two tails intertwine to form a helix
• The heads have a binding site for actin
• The head is an enzyme that hydrolyses ATP (an ATPase)
• Arranged in a polarised fashion: i.e. with the myosin heads projection away from the M-line.
• Titin anchors the thick filament to the Z-line

Question 14

Describe the structure of thin filaments:

• Composition
• Structure
• Regulatory proteins

Answer 14

• Composed primarily of globular actin proteins
• The filaments are composed of a double stranded helical actin chain (polymers).
• Troponin and tropomyosin are regulatory proteins associated with actin in skeletal and cardiac muscle

Question 15

What regulates troponin?

Answer 15

Cytoplasmic [intracellular] Ca levels

Question 16

What does tropomyosin do?

Answer 16

Interacts with the myosin binding site

Question 17

What is the sliding filament theory of contraction?

Answer 17

The sarcomere shortens as the thin filaments are pulled over the thick filaments:
• The Z-line is pulled toward the M-line
• The I band and H zone become narrower

Question 18

What are the four major steps of the cross bridge cycle?

Answer 18

1. Cross bridge formation
2. Power stroke
3. Detatchment
4. Energisation of the myosin head

Question 19

Describe step one of the cross bridge cycle; Cross bridge formation

Answer 19

• Myosin binds to the actin binding site to form a cross bridge

BUT

This can only occur in the presence of Ca which binds to troponin, causing a conformational change of tropomyosin exposing the myosin binding site

Question 20

Describe step two the power stroke of the cross bridge cycle:

Answer 20

• ADP is released
• The myosin head rotates
to its low energy state (about 45° to the actin) pulling with it the thin filament
• The result is shortening of the sarcomere.

Question 21

Describe step three (detatchment) of the cross bridge cycle:

Answer 21

• A new ATP molecule binds to the myosin
• The actin-myosin bind is weakened and the myosin detaches
• (Note: No ATP = no detachment)

Question 22

Describe step four (energisation of the myosin head) of the cross bridge cycle:

Answer 22

• Myosin head hydrolyzes the ATP to ADP + Pi (NB this is the ADP released in the subsequent step two)
• The myosin head moves back to its “high energy (cocked)” confirmation (about 90° to the actin)

Question 23

Why is Ca soooo important in the cross bridge cycle?

Answer 23

• Ca provides the“on” switch for cross-bridge cycle to begin.
• When Ca binds troponin the tropomyosin moves to expose the myosin binding sites on actin
• The cross-bridge cycle will continue as long as calcium levels remain above the critical threshold (0.001-0.01 mM)

Question 24

How is Ca regulated?

Answer 24

• In skeletal muscle opening of calcium channels in the SR allows the movement of calcium ions into the cytosol.
• Active transport pumps (Ca2+ ATPase (SERCA) are constantly moving Ca2+ from the cytoplasm back into the sarcoplasmic reticulum (SR)

Question 25

What is isotonic contraction?

Answer 25

Isotonic:

• Shortening
• Tension constant (iso = no change, tonic = tension)
• Velocity variable

Question 26

What is isometric contraction:

Answer 26

Isometric

• No shortening
• Length constant (iso =no change, metric = length)
• Tension variable

Question 27

Describe again the structure of the myofibril (image attatched)

Answer 27