Download Microscopic Structure and Contraction of Muscles

3B.2 Muscular System
Microscopic Structure and
Contraction of Muscles
The structure of skeletal muscle
Muscle cells (muscle fibres) occur in bundles
surrounded by a layer of tough connective tissue
(perimysium).
The structure of skeletal muscle
• A skeletal muscle fibre is:
– cylindrical with many nuclei
– surrounded by a thin membrane, the sarcolemma
– cytoplasm is called sarcoplasm
– contains thread-like structures called myofibrils
Myofibrils
Myofibrils run the length of the fibre:
−where contraction takes place
−composed of myofilaments (actin and myosin)
Actin and Myosin
The movement of proteins actin and myosin enable
the cells to shorten – a sliding filament model
explains this.
Actin – thin
filaments
Myosin – thick
filaments
The sliding filament model
myosin heads form cross-bridges with binding sites
on the actin to pull the filaments across each other
Myosin head
forming cross
bridge
The Sarcomere
The Sarcomere
• Contractile unit of skeletal muscle
• A single sarcomere runs from Z-line to Z-line
Light band
gets shorter
during contraction
Dark
A Light
band I band
Z
M
Sarcomere
M line
Dark band
Remains same size
during contraction
sarcomere
Myosin filament
Actin filament
Z line
AHIZ Banding
•
•
•
•
Z-line: band of dense material
A-band: length of myosin filaments
I-band: distance between ends of myosin filaments
H-zone: distance between ends of actin filaments
• When the sarcomere contracts and shortens, the
I-band and H-zone decrease in length
Contraction of the sarcomere
Muscular System, Sliding Filament Theory
http://www.youtube.com/watch?v=EdHzKYDxrKc
Video – Muscle Structure and Function
Contraction Cycle
Regulation of Contraction - 1
• Calcium ions regulate contraction
• an increase in Ca2+ concentration in the sarcoplasm
starts the sliding process
• a decrease in Ca2+ turns it off
• contraction also requires energy in the form of ATP
• Glucose is the energy source for cellular respiration
Regulation of Contraction - 2
• during relaxation, the binding site on actin is
blocked (by a troponin-tropomyosin complex)
• Ca2+ binds to (and changes the shape of the
troponin-tropomyosin complex) and uncovers the
myosin binding sites on actin
• this allows the myosin to attach to the actin
Cycle of Contraction
1. release of Ca2+
2. attachment – binding of myosin head to actin
3. Power Stroke –myosin head rotates towards
centre of sarcomere, pulling along the actin
4. detachment – binding of ATP to myosin head
5. energy transfer – ATP breaks down to ADP + P,
reorients myosin head
Contraction Cycle
Sliding Filament Theory of Muscle
Contraction
http://www.youtube.com/watch?v=0kFmbrRJq4w
Neuromuscular Junction
• Junction between motor nerve and the muscle (also
known as motor end plate)
• Neurotransmitter acetylcholine is released from the
nerve ending
• Diffuses across gap between nerve and muscle fibre
(reaction time)
• Picked up by receptors on muscle fibre which
triggers a muscle action potential
• Calcium ions are released from sarcoplasmic
reticulum causing muscle contraction
• Acetylcholine then is broken down by acetylcholinesterase and recycled
Mechanism of Filament
Sliding
http://www.blackwellpu
blishing.com/matthews/
myosin.html
Human Perspectives 3A/3B
Chapter 14 – RQ 6-11