PSK 4U Unit 4, Day 3

Mrs. Petrin

Inside a Muscle Fibre... How it

Contracts!

Recall: What is a motor unit?

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?

1. Fine motor skills such as writing or tying your shoes

2. Gross motor skills like walking or raising your hand

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)

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.

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

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!

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

Inside Anatomy of Skeletal Muscle

epimysium

fascicle

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…)

Actin Myosin

So... how do muscle fibres shorten?

Demonstration: Tug of War

Task:

1. 4-6 volunteers

2. Two group members hold each end of the noodle

3. 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

Sliding Filament Theory Video : https://www.youtube.com/watch?v=hr1M4SaF1D4

Key Words:

Acetylcholine

Calcium

Sarcoplasmic reticulum

Myosin

Actin

Troponin

Tropomyosin

Adenosine Triphosphate (ATP)

Sliding Filament Theory

Steps:

1. Message is sent down axon

2. Axon terminal releases Ach

3. Ach receptors on sarcolemma receive Ach

4. This stimulates the release of Calcium ions inside the

muscle fibre (from sarcoplasmic reticulum)

5. Calcium binds to troponin (on tropomyosin)

6. Tropomyosin (wrapped around actin) swivels off

Sliding Filament Theory

7. This shows myosin binding sites

8. Myosin binds through the energy of ATP

9. Myosin attach, rotate, detach and attach again to “grab”

actin and pull towards its centre – the powerstroke

10. When calcium is depleted, when ATP is gone, the muscle

fibre passively returns to its resting state