NPB: Introduction to Skeletal Muscle Part 2

Twitches, Motor Units and Contractile Strength

Muscle Twitch Facts

• A single AP can induce a twitch

  • small magnitude

  • short duration

Muscle Twitch

• doesn’t last very long

• not a functionally full contraction

T/F: a single action potential causes a sustained muscle contraction

False, a single action potential does not cause a sustained muscle contraction. This is because this is only a brief twitch. And it would need to be a large enough action potential and constant to be sustained muscle contraction

Why is a muscle twitch not a functionally full contraction?

because they only last like 100 msec

Onset of Tension

How long does it takes to relax

T/F: A single AP isn’t enough to induce a twitch

False, a single AP can induce a twitch, it will just be a short duration and small magnitude

Summation

twitches add up but still partly relax

Tetany

Allowing twitches to fuse together into a smooth sustained contraction and the overall sum of forces is larger than before

With twitches, summation refers to the summation of…

A) force
B) voltage
C) calcium release in the motor neuron
D) individual action potentials fusing together
E) acetylcholine levels in the synaptic cleft

A) force: this is force of like newtons or kg

What is occurring here?

There is a onset force and relaxation, but its not fully relaxing, then another force and not full relax over and over again with a new peak higher than the one before

What is this phenomenon?

Unfused Tetany

Unfused Tetany

Stimulus frequency is slow

Fused Tetanus

• Stimulating at higher frequency

• Don’t give muscle time to relax

• Long lasting, BUT only long lasting if there is a constant stimulus

T/F: Tetanus occurs when muscle twitches completely relax between action potentials

False, tetanus is the summation of twitches with no relaxing point because its constant stimulation

T/F: action potential frequency, controls the strength of muscle contraction

True, the actual potential frequency does control the muscle contraction because if there's a constant action potential, this would be fused tetanus with a higher frequency, and if there is spaced out action potential, this will have a lower frequency, and the twitches won't last very long as it will be short magnitude

T/F: Fused tetanus has lower frequency than unfused tetanus

False, fused tetanus has higher frequency because it is stimulating nonstop not giving the muscle time to relax

T/F: Relaxation occurs with fused/complete tetanus

False, no relaxation occurs because there is a nonstop stimulus

Fused tetanus have a ___(smaller/larger) contraction than unfused tetanus

larger

A __________ is the fusion of repeated muscle twitches that leads to a sustained contraction.

Tetany

During fused tetany...
A) the muscle is stimulated at low frequency

B) the latent period becomes longer in duration

C) twitches sum together to form a strong, smooth contraction

D) a weak, but smooth contraction occurs

C) twitches sum together to form a strong, smooth contraction

When more motor units are activated, this is called motor unit __________.

recruitment

The strength of a muscle contraction increases with more frequent __________.

action potentials

Why doesn’t one action potential create a strong contraction?

a single AP doesn't create a strong contraction because it causes only a brief twitch that ends quickly.

What’s the difference between summation and tetanus?

Summation = twitches add up but still partly relax
Tetanus = twitches fully fuse into a smooth, sustained contraction.

Motor neuron

control 5 muscle cells (in this example) = motor unit

Motor Unit

Arrangement of muscle cells & how it talks to the motor neuron and this overall arrangements is small

T/F: In all instances motor units will vary in size

True, in all instances motor units will vary in size

Small Motor Unit

• Single motor neuron connected to ≈ 20-100 muscle cells

• doesn’t generate as much force

Large Motor Unit

• Single motor neuron connected to ≈ 200-500 muscle cells

• generates lots of force

Compared to large motor units, small motor units...
A) are only found in small muscles

B) are as strong as large motor units

C) contain smaller muscle cells

D) contain fewer muscle cells

D) contain fewer muscle cells

What is an advantage to small m.u.?

fine detection/ motor activities like for eyes

What is an advantage to large m.u.?

• Having large buttocks for maintaining our posture when sitting

When would we have a mix of small and large motor units?

When we have muscles that do a mix of strong and weak forces

How do we increase our overall force generation (total amount of force a muscle produces when doing a task)?

We do this with a motor unit recruitment (the process of activating more motor units to increase the strength of a muscle contraction)

T/F: When we activate muscle, this is high frequency

True, when we activate muscle, this is high frequency because lots of action potentials coming down the motor neuron for a long period of time then doing a sustained contraction

if I pick up a pen in my activating small motor units or large motor units

If I pick up my pen, I'm activating small motor units because I don't need to activate every single motor unit for something super light

if I am picking up a TV and my activating large motor units or small motor unit?

I will be activating a large amount of motor units because this is something heavier so I need to activate more of the motor units