HUBS1403 Lecture Notes - Lecture 33: Muscle Spindle, Motor Neuron, Reuptake
8 Jun 2018
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Mechanisms of Muscle Contraction
Sarcomere - Basic Contractile Unit in Skeletal Muscle
Components
Component
Description
Z discs
Narrow, plate-shaped regions of dense material that separate 1 sarcomere from the
next
A band
Dark, middle part of sarcomere that extends entire length of thick filaments and
includes those parts of thin filaments that overlap thick filaments
I band
Lighter, less dense area of sarcomere that contains remainder of thin filaments, A Z
disc passes through centre of each I band
H zone
Narrow region in centre of each A band that contains thick filaments but no thin
filaments
M line
Region in centre of H zone that contains proteins that hold thick filaments together
at centre of sarcomere
Actin/Myosin Interaction - Cross bridge Cycling
• Myosin binds to actin forming a cross bridge between the thick and thin filaments
• Actin and myosin are proteins
• Actin molecules joins up to form actin polymer
• Energy comes from ATP
Crossbridge Cycle
The cycle of the pulling in of the actin across the myosin and then the letting go - muscle contraction
Resting
and
Step 1
Resting
• Troponin holds tropomyosin out of actin grooves
• Tropomyosin covers up the actin active site
• Typical formation in resting muscle
• Myosin is 'cocked' in high energy conformation waiting for something to release it to
expend the energy
Then: step 1:
• CA2+ binds to troponin, allows tropomyosin to sink into actin grooves
• Binding sites for myosin exposed o actin
Step 2
& 3
Step 2:
• Myosin heads attach to actin crossbridges
• ADP and P released and myosin head pivots; powerstroke
Step 3:
• When myosin binds it released energy that was stored in it from breakdown of ATP to ADP
and P
• Until ATP binds it can't let go of actin
Step 4
& 5
• ATP binds to myosin head, link to actin broken
• ATP splits, energy 'cocks' crossbridgge head, ADP and P attach to myosin head
• Ready to go again if Ca2+ levels still high and actin sites available for myosin head to bind to
• ATP binds here and releases the myosin
Document Summary
Sarcomere - basic contractile unit in skeletal muscle. Narrow, plate-shaped regions of dense material that separate 1 sarcomere from the next. Dark, middle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments. Lighter, less dense area of sarcomere that contains remainder of thin filaments, a z disc passes through centre of each i band. Narrow region in centre of each a band that contains thick filaments but no thin filaments. Region in centre of h zone that contains proteins that hold thick filaments together at centre of sarcomere. Actin/myosin interaction - cross bridge cycling: myosin binds to actin forming a cross bridge between the thick and thin filaments, actin and myosin are proteins, actin molecules joins up to form actin polymer, energy comes from atp. The cycle of the pulling in of the actin across the myosin and then the letting go - muscle contraction.
