(3) Lecture #15 - Structure and Function of the Skeletal Muscle System

Types of Muscle

Muscle	Control	Striated?	Function
Skeletal	Voluntary	Yes	Moves bones
Smooth	Involuntary	No	Moves fluids, regulates vessels
Cardiac	Involuntary	Yes	Pumps heart

Skeletal Muscle Structure

Muscle fiber → myofibril → myofilaments → sarcomere

Sarcomere

Smallest unit of contraction.

Inside sarcomere:

• Actin = thin filament

• Myosin = thick filament

• Myosin does NOT move

• Actin moves toward the center

Z Line

border between sarcomeres

Sliding Filament Theory is about what…

How contraction works.

Sliding Filament Theory

Steps:

1. Myosin binds actin

2. Myosin pulls actin inward

3. Sarcomere shortens

4. Muscle shortens

Troponin / Tropomyosin

what they do…

These control contraction.

Protein	Function
Troponin
Tropomyosin

Protein	Function
Troponin	binds calcium
Tropomyosin	blocks actin binding sites

What happens when calcium binds to troponin…

Troponin moves tropomyosin → actin site exposed → contraction occurs.

Calcium =

force

More calcium →

stronger contraction.

Neuromuscular Junction (NMJ)

function

Connection between motor neuron and muscle fiber.

Neuromuscular Junction (NMJ)

important features

1. Neurotransmitter

• Acetylcholine (ACh)

2. Receptor

• Nicotinic receptor

Neuromuscular Junction (NMJ)

Process:

1. Motor neuron fires

2. ACh released

3. ACh binds receptor

4. Action potential in muscle

5. Contraction begins

Motor Unit

Definition:

1 motor neuron + all muscle fibers it controls

Motor Unit

Examples

Eye muscles

• 1 neuron → few fibers

• precise control

Leg muscles

• 1 neuron → thousands of fibers

• powerful movement

Excitation–Contraction Coupling

Order:

1. Motor neuron fires

2. ACh released

3. ACh binds receptor

4. Action potential in muscle

5. Signal goes down T-tubules

1. Sarcoplasmic reticulum releases calcium

1. Calcium binds troponin

2. Actin + myosin bind

3. Muscle contracts

T-Tubules

Definition:

Channels that allow the action potential to travel deep inside muscle fibers.

T-Tubules

Purpose:

Make contraction fast and synchronized.

Sarcoplasmic Reticulum (SR)

Function:

• Stores calcium

• When muscle is stimulated → SR releases calcium.

Cross-Bridge Cycle

Steps:

1. Calcium exposes actin binding site

2. Myosin binds actin

1. Power stroke pulls actin

1. ATP causes myosin to detach

2. Cycle repeats

ATP is needed for:

• myosin detaching from actin

• power stroke cycle

• pumping calcium back into SR

• important

  ATP required for both contraction AND relaxation

Acetylcholinesterase

• Enzyme that breaks down acetylcholine

• If blocked:

→ continuous muscle contraction

Rigor Mortis

After death:

• calcium leaks into muscle

• myosin binds actin

• no ATP available

 Rigor Mortis

Result:

myosin cannot detach → muscles become stiff

Types of Contraction

1. Concentric

Muscle shortens

Example: lifting weight

2. Eccentric

Muscle lengthens while contracting

Example: lowering weight

3. Isometric

Muscle tension without length change

Example: holding weight still

Twitch

One nerve signal → one contraction

Summation

Signals come quickly → contractions add together

Tetanus

Very high frequency stimulation → sustained contraction

Who cam and cannot summuate contractions?

Skeletal muscle can summate contractions

Heart muscle cannot.

Muscle Strength Depends On

1. Number of fibers contracting

2. Frequency of stimulation

3. Motor unit recruitment

4. Muscle fiber length

 Length-Tension Relationship

Maximum force occurs when muscle is at optimal resting length.

Length-Tension Relationship

Too short →

Too long →

Too short → too much overlap

Too long → not enough overlap

Humans mostly grow muscle by:

• Hypertrophy

• Meaning:

  • Muscle fibers get larger, not more numerous.