Lecture 7: Skeletal Muscle Contraction

Myofibril

Contractile organelle within a muscle fibre

Myofilament

Inside of myofibril

Thick and thin filaments

Sarcomere

Repeating units of myofilaments

Organization

Myofiber, myofibril, myofilament

Aspects of sarcomere

A band

I band

H band

Z lines

M lines

Z line/disk

Marks the end of the sarcomere

Thick, dark line on electron micrograph

Z shaped line on diagram

Thick filaments

Found in the center of sarcomere

Darkish band/area on micrograph

As thick as the A band

M line

Marks the middle of the sarcomere

H band

Near the middle of the sarcomere where there is no overlap b/w thick and thin filaments. Only thick filaments

Skinny, light line on micrograph next to M line

Thin filaments

Start on the end of Z line going towards middle, intersecting with thick filaments

On micrograph, thick light area = thin filaments no overlap, darker = overlap

I band

No thick filament, only thin

Thin filaments structure

Twisted strands built from actin and other proteins

How are the proteins in a thin filament organized?

Actin (F and G): Organized in a double-helical fibre

Actinin: Found at the Z line; interconnects thin filaments

Tropomyosin and troponin: Wrap around the actin helix

What are thick filaments composed of

Myosin protein

Structure of myosin molecule and their role

Myosin tail

Free head: Binds to actin

Hinge: Moves the head for contraction

Titin

Fastens thick filament to Z line

When a muscle contracts, what happens to the sarcomere, distance between Z-lines, thin and thick filaments, A-band, I-band

Gets shorter/decreases in length

Distance between Z-lines decreases

Slides past each other

Stays the same

Gets shorter

What happens to myosin and actin filaments when contracting?

Slide past each other in a repeating cycle

When does an excited muscle begin to contract

After calcium levels increased and will continue to contract as long as calcium remains elevated

How does calcium play a role in contraction

Calcium interacts with troponin-tropomyosin complex of the thin filament, revealing the myosin-binding active site on actin

No Ca: Myosin head tries to bind to the binding site but it can’t because it’s covered by troponin-tropomyosin complex

Step 0 Myosin and actin interaction cycle

Calcium is released, binds to troponin, reveals active site

Step 1 and 2 Myosin and actin interaction cycle

1: Myosin head binds to actin, cross bridge forms

2: Myosin neck pivots, filaments slide (power stroke, contraction)

Step 3, 4, and 5 Myosin and actin interaction cycle

3: ADP is released from myosin head and diffuses away

4: ATP binds to myosin head, myosin detaches from actin, cross bridges detach

5: Myosin neck resets, ATP hydrolysis (Turn into ADP + P)

When is ATP used in muscle contraction

To reset the myosin neck

NOT while it is pulling on actin during the power stroke

What happens to the tendons during sliding filaments

Creates tension in the tendons

How is tension produced

Can only be produced by myosin heads that overlap with and can bind actin

Depends on calcium levels (more = more tension)

What is the amount of pull a thick filament can make on the thin filament dependant on?

How many of its myosin heads can grab onto actin (e.g. 6 heads grabbing on actin vs 4 heads grabbing on actin)

Under what conditions do muscles produce max tension

When they are excited while at an intermediate sarcomere length

What happens have you start with compressed sarcomeres?

As myosin and actin are interacting, there’s less space for them to move (i.e. slide more)

= Less tension

What happens have you start with stretched sarcomeres?

Myosin and actin are spread apart, no overlap = no interaction

Muscle twitch

Contractile/tension response of a muscle fibre in response to a single action potential (stimulus)

Muscle twitch contraction phases. How long does the start of the first phase to the end of the last phase take?

Latent period (muscle excitation)

Contraction phase (calcium buildup)

Relaxation phase (calcium removal)

~40 ms

Summation of tension

Repeatedly stimulating a single muscle fibre due to residual calcium build up

Calcium enters cytosol before all the original calcium is removed, revealing more active sites

Twitch vs summation

One stimulus vs multiple stimuli

Why does summation happen?

To get calcium out of sarcoplasmic → Needs transport and pump

Pumps move ions slower than an ion channel (RyR)

Tetanus

A muscle producing persistent max tension with little to no relaxation phase

Muscle fibres have a max level of tension production when all troponin sites are bound by calcium

Concentric contractions

Shortening

Occur when the external load is less than internally generated tension

Produce movement of a joint

External load (e.g. weights) on a muscle slows rate of contraction

Isometric contraction

Occur when muscle produces just enough tension to balance an external load

Stabilizes a joint

Internal tension = External force → Contraction w/o shortening

Eccentric contraction

Lengthening

Occur when external load is higher than internally generated tension

Slows down movement at a joint

Muscle is activated (produces tension), it gets longer