1. Inside a Muscle Fibre... How it Contracts!
PSK 4U Unit 4, Day 3
Mrs. Petrin

2. Recall: What is a motor unit? Activity Some motor units are large!
Motor neuron + axon + muscle fibre
Activity
Some motor units are large!
Some motor units are small!
What types of movement do you think are produced?
Fine motor skills such as writing or tying your shoes
Gross motor skills like walking or raising your hand

3. Neuromuscular Junction
Junction point (synapse) between the nervous and muscular systems. A chemical neurotransmitter is released (Ach - acetylcholine)and is detected by receptors on surface of muscle fibres. This results in muscle contraction (more on that later)

4. The “All-or-None” Principle
One motor unit can stimulate several muscle fibres.
When a motor unit is stimulated to contract, it will do so to it’s fullest potential (maximum)
Whether a motor unit consists of 10 muscle fibres or 800, either all the fibres will contract or none will contract – ALL or NONE principle.

5. Activity: All or None Get into groups of 3 (small motor unit)
Some of you get into groups of 6 (large motor unit)
Appoint a neuron (the boss)
I will be the brain
When I tell the neurons, the neurons will tell you to blow on the ping pong ball
You must ALL blow on the ping pong ball!
The only person who can direct you is the neuron, who must receive their signal from me!
OBJECTIVE: get ping pong ball to target

6. Activity: All or None What was your strategy?
In the beginning… you would have been using GROSS motor skills (large muscle groups, large motor units)
As you approached the target, you would need to adjust
As you fine-tuned your movement, you would have been using FINE motor skills (small muscle groups)
In any case, the all or none principle is demonstrated by ALL of you blowing on the ball!

7. Inside Anatomy of Skeletal Muscle
The endomysium (sheath of connective tissue) surrounds each individual muscle fibre (aluminum foil from yesterday)
Beneath the endomysium is the sarcolemma, a plasma membrane containing the muscle cell’s cytoplasm (sarcoplasm)
The endomysium is surrounded by a rich blood supply and, therefore, a good system for O2 and CO2 removal
Nerves also innervate the muscle at neuromuscular junctions

8. Inside Anatomy of Skeletal Muscle
epimysium
fascicle

9. Inside Anatomy of Skeletal Muscle
Thread like structures, myofibrils (pieces of string yesterday), run the length of the muscle fibres
Within the myofibril are myofilaments:
Thin (actin) and thick (myosin) protein filaments
Actin and myosin are contained within sarcomeres (more about this later…)

10. Myosin
Actin

11. So... how do muscle fibres shorten?
Demonstration: Tug of War
Task:
4-6 volunteers
Two group members hold each end of the noodle
Remaining group members stand behind in single file
Listen for my instructions:
Each end pulls with equal force.
Each end approaches the middle of the noodle, hand over hand, taking a step each time toward the centre
From the middle, loosen your grip and slide back

12. Sliding Filament Theory
Video :
Key Words:
Acetylcholine
Calcium
Sarcoplasmic reticulum
Myosin
Actin
Troponin
Tropomyosin
Adenosine Triphosphate (ATP)

13. Sliding Filament Theory
Steps:
Message is sent down axon
Axon terminal releases Ach
Ach receptors on sarcolemma receive Ach
This stimulates the release of Calcium ions inside the muscle fibre (from sarcoplasmic reticulum)
Calcium binds to troponin (on tropomyosin)
Tropomyosin (wrapped around actin) swivels off

14. Sliding Filament Theory
This shows myosin binding sites
Myosin binds through the energy of ATP
Myosin attach, rotate, detach and attach again to “grab” actin and pull towards its centre – the powerstroke
When calcium is depleted, when ATP is gone, the muscle fibre passively returns to its resting state