Lab 7: EMG

What are the main characteristics of skeletal muscle?

It can extend and stretch

It is capable of responding to action potential to contract

What is an Action Potential?

A rolling wave of depolarization followed by repolarization that travels along the cell membrane, allowing an electrical signal to propagate down a neuron.

List the Structures of Skeletal Muscle from Largest to Smallest

Muscle

Fascicle

Fibers / Cells

Myofibril

Sarcomere

Myofilament

What is a Fascicle?

“Strands” of muscle that can be seen by the naked eye

Surrounded by the perimysium

What is a Myofibril?

Made up of repeating sarcomeres that are end to end

Epimysium vs Perimysium vs Endomysium

All are types of Connective Tissue

Epimysium: Surrounds the whole skeletal muscle organ

Perimysium: Surrounds Fascicles

Endomysium: Surrounds Muscle Fibers / Cells

What is a Sarcomere?

The basic unit of muscular contraction (sliding filament model)

Goes from Z-Disc to Z-Disc

Contains Z-Disc, M-Line, A-Band, I-Band, and H-Zone

Z-Disc

- marks the beginning and end of a sarcomere
- anchors actin
- moves closer during contraction

M-Line

- in the center of the sarcomere
- anchors myosin

A-Band

- dark in color
- the entire length of myosin filaments, including overlap
- does not change in length during contraction

I-Band

- light in color
- contains ONLY actin
- shortens during contraction

I am thin

H-Zone

- the zone of ONLY myosin
- disappears at complete contraction

Actin vs Myosin

Actin
- thin
- slides during contraction

Myosin
- thick
- does not move during contraction

Neuromuscular Junction (NMJ)

Where a motor neuron communicates with a muscle fiber to tell it to contract

Steps of Skeletal Muscle Contraction

1. Action potential moves down to an axon terminal

2. Ca²⁺ enters the axon terminal and signals Acetylcholine to be released

3. Acetylcholine binds to ligand-gated Na⁺ channels which opens them and allows Na⁺ into the muscle cell

4. Na⁺ makes the cell more positive. If threshold voltage is reached, Na⁺ channels open and the muscle cell is depolarized.

5. K⁺ leaves the muscle cell, which depolarizes it.

6. The electrical signal travels down the membrane and T-tubules which releases Ca²⁺ and the muscle contracts.

Phases of a Muscle Twitch

Lag Phase (0-5ms)

- electrical impulse is traveling before contraction

Contraction (100ms)

- myofilaments slide across one another and the muscle fiber shortens
- actin moves, myosin does not

Relaxation (100ms)

- fibers return to resting state

Wave Summation and Tetanus

Twitch

- muscle responds to a signal then resets to baseline

Wave Summation

- muscle receives another signal before returning to baseline
- basically the twitches sum together

Tetanus

Unfused / Incomplete —> rapid, repeated twitches that continually sum with very short rests between them
Fused / Complete —> so rapid that the muscle has NO time to rest so signal looks like one big signal not like a bunch of individual waves

What is a Motor Unit?

A motor neuron and all the fibers it control

Motor Neurons control at least 5 fibers

Size Recruitment Principle

Smaller motor units are recruited first and are useful for smaller actions like sewing, while larger units are recruited last and are more useful for explosive movement like jumping.

Subthreshold vs Threshold Stimulus

Subthreshold Stimulus: The signal is too small for any units to respond

Threshold Stimulus: A single motor unit responds

3 Ways of Generating ATP for Muscle Activity

Creatine Phosphate
- 1 ATP
- very fast
- only lasts 10 seconds

Anaerobic Glycolysis
- 2 ATP
- very fast
- very inefficient
- energy for 40 seconds

Aerobic Respiration
- 32 ATP
- major energy source for most activities
- slow, but very efficient