Lecture 23: Myosin and Muscle

actin networks at the front and back of the cell

Lamelopodia and filopodia at the leading edge, stress fibres at the back

what creates the different zones of Rho activity?

extracellular signalling, chemotactic gradient

what does myosin protein do?

generate force through a conformational change with phosphate release (ATP to ADP)

what are muscles made of?

billions of contractile bundles

bundle of muscle fibres → myofibril → sarcomere

what is a sarcomere?

basic unit of muscle contraction, made from tightly packed arrays of actin filaments and myosin filaments

what are Z disk, I band and A band?

Z disk → where myosin filaments extend from

A band → myosin bundles

I band → distance between Z disk and A band

what happens with muscle stimulation?

sarcomeres undergo rapid contraction (50 ms! 5-10 ms for period between signal and start of contraction, 40-45 ms for contraction phase from fully elongated to fully contracted)

Crossbridge cycle

1 ATP hydrolysis = 1 “power stroke”

1. ATP binds to myosin head and it is released from actin

2. Hydrolysis of ATP to ADP + Pi, head rotates to cocked state

3. Myosin head binds actin filament

4. “Power stroke” : release of Pi and elastic energy strains myosin, moving actin filament left

5. ADP released and repeat step 1

How is Rigor Mortis caused?

from failure of myosin to detach in the absence of ATP, causing stiffness

neck domain of lever arm

if the arm is longer, there will be a larger swing, moving actin faster

myosin force produced

~ 5 pN of force over a length of ~ 10 nm (50 pN nm of work)

design challenges that muscles face

1. prevention of continuous contraction

2. activation of contraction

3. Freezing the structure of the sarcomere (to hold steady)

what determines the structure of the sarcomere?

nebulin, titin, and capping proteins

what does tropomyosin do?

it blocks the binding site for myosin on the actin filament.

what happens to tropomyosin in the presence of Ca²⁺?

It rotates the tropomyosin to reveal the myosin-binding site of actin

How is muscle contraction activated?

From motor neurons that receive signals from the brain and transmit them to the muscle

what happens when motor neurons are stimulated?

a rapid spike in Ca²⁺ concentration in the muscle fibres

what does the sarcoplasmic reticulum do?

it stores Ca²⁺

What does troponin do?

it binds calcium and pulls tropomyosin out of the way, allowing myosin to bind

muscle contraction is stimulated by what?

presence of calcium