1. Musculoskeletal System - Muscles
Types
Micro-anatomy
Excitation
Excitation-Contraction Coupling
Contraction
Properties of Whole Muscles
Red vs. White Meat

2. Muscle Tissue Types All muscle cells contract and relax. 3 types:
Skeletal
Connected to bones
Ex) biceps
Striated
Smooth
Tapered
Ex) stomach
“involuntary”
Cardiac
Only present in the heart
Have specialized junctions to propagate contraction.

3. Micro-Anatomy Muscle fiber = single muscle cell
Muscle = many muscle fibers stuck together
Tendons = link muscles to bones

4. Micro-Anatomy Myofibril Sarcomere = smallest unit of contraction
Bundle of filaments within a muscle fiber
Give the cell a striated appearance.
One unit of these repeating fiber striations is called a sarcomere.
Sarcomere = smallest unit of contraction

5. Micro- Anatomy - Sarcomere
Z-line:
dark zig-zag band
marks the end of a sarcomere
Actin (thin filaments):
looks like two stranded pearls
other proteins cover its grooves
Myosin (thick filaments)
bundled
each has a long tail and a double head
all pull on actin in the same direction

6. Micro- Anatomy Tropomyosin Troponin
A protein that covers myosin binding sites on actin.
When on, a muscle cannot contract.
Troponin
regulatory protein bound to tropomyosin
When calcium binds to it, it pulls tropomyosin off the actin filaments

7. Micro-Anatomy T-tubules Sarcoplasmic Reticulum
extensions of the cell membrane that move into the cell and surround each myofibril
Sarcoplasmic
Reticulum
A membrane like the
ER that stores Calcium
and surrounds each
myofibril

8. Excitation of a Muscle Fiber
Motor neurons synapse on muscle fibers and “tell” them when to contract.
The muscle fiber membrane depolarizes and the action potential spreads over the muscle fiber and into the T-tubules

9. Excitation-Contraction Coupling
Depolarization of T-tubule causes SR to release Calcium

10. Excitation-Contraction Coupling
Ca+2 binds to troponin
Troponin moves tropomyosin out of the way.
Note:
When nervous signal stops, the SR takes up the Ca+2 (uses ATP)
When Ca+2 is gone, troponin/tropomyosin complex covers the binding sites again.

11. Contraction Called the sliding filament model
A cross-bridge forms when a myosin head attached to a binding site on actin.
Cyclic
Powered by ATP energy
ATP split causes myosin to bind to actin.
Energy release causes power stroke – myosin bends.
Pulls actin filaments towards the center of the sarcomere.
New ATP causes myosin to detach.
Cycle begins again, a bit closer to the z-line.
A single contraction is many power strokes.
No ATP = rigor mortis

12. Contraction (Again)

13. Sequence of Events Between a Motor Neuron Action Potential & Skeletal Muscle Fiber Contraction

14. Properties of Whole Muscles
Muscle tension
mechanical force the muscle exerts on an object
opposed by weight or gravity
3 scenarios:
Isometric
no sarcomere shortening
muscle tension = load
ex) holding a book out
Isotonic
sarcomere shortening; lifting a load
muscle tension > load
ex) lifting the book
Lengthening
sarcomere lengthens while holding a load
muscle tension < load
ex) putting down the book
Isotonic
Lengthening

15. Properties of Whole Muscles

16. Properties of Whole Muscles
Motor unit:
A neuron + all the muscle cells it controls
Muscle twitch
Electrical stimulus of a motor unit
All-or-nothing
The greater the force on an object, the more muscle units that are recruited.

17. Properties of Whole Muscles
Treppe
Constant stimulation causes muscle to “warm up” and strength of contraction increases.
Due to increasing free Ca+2
Temporal Summation:
Stimulus is added before complete relaxation of a motor unit.
Additional influx of Ca+2
Tetany:
Stimulus so common that contractions fuse into continuous contraction.
Most common in movements of the body

18. Red vs. White Meat Red muscle White muscle Slow-twitch
Slow to engage but long lasting
High blood flow, mitochondria, and myoglobin = high ATP
Good for distance sports
White muscle
Fast-twitch
Rapid and powerful
Not sustainable
Low mitochondria and myoglobin
Good for sprinter