musculoskeletal muscle chapt 19

Muscles - characteristics (4)

1. Used for movement (walking, running, etc)

2. Stimulated by motor neurons

3. Attached to bones by tendons

4. Most abundant tissue in the body

Muscle Fiber

Muscle cell, Multi-nucleated, With bundles of myofibrils

Multi-nucleated

Many nuclei because it’s formed from the fusion of many embryonic cells.

Myofibrils

Long, rod-like structures inside each muscle fiber made up of sarcomeres. Runs length-wise

Sarcomeres

Repeated contracting segments

T Tubules (transverse tubules)

Extensions of PM connect AP impulses to Sarcoplasmic Reticulum (SR)

Sarcoplasmic Reticulum (SR)

Special kind of smooth endoplasmic reticulum that stores and releases calcium ions (Ca²⁺) needed for contraction.

The SR is wrapped around the myofibrils like a net.

Thick Filament made of?

350 MYOSIN molecules

Thin Filament - Proteins(3)

1. Actin: 2 strands (contractile)

2. Tropomyosin

3. Troponin Complex

Sarcomere Characteristics (5)

1. Overlapping thick + thin filaments

2. Z-lines join sarcomeres (divide)

3. M lines - middle

4. Thin filament attach to Z line

5. Thick Filaments attach to M line

Sarcomere contraction

Distance between Z lines get smaller

Length of filaments do NOT change

Steps of Muscle Contraction (10)

1. Nerve Signal

   • Motor neuron sends an action potential (AP) to the muscle.

2. Neurotransmitter Release

   • Acetylcholine (ACh) is released at the neuromuscular junction.

   • ACh triggers a muscle action potential.

3. Calcium Release

   • AP travels down T-tubules to the sarcoplasmic reticulum (SR).

   • SR releases Ca²⁺ into the muscle cell.

4. Binding Site Exposure

   • Ca²⁺ binds to troponin, moving tropomyosin.

   • This exposes myosin-binding sites on actin.

5. Cross-Bridge Formation

   • Myosin heads bind to actin → cross-bridges form.

6. Power Stroke

   • Myosin pulls actin inward → sarcomere shortens.

   • ADP + Pi are released.

7. Cross-Bridge Detachment

   • ATP binds to myosin → cross-bridge breaks.

8. Reset

   • ATP is hydrolyzed → myosin head resets to high energy.

9. Cycle Repeats

   • As long as Ca²⁺ and ATP are present, steps 5–8 repeat.

10. Relaxation

• Ca²⁺ is pumped back into SR.

• Tropomyosin blocks binding sites → muscle relaxes.

Source of ATP for muscle contraction (2)

1. Creatine phosphate → 15 secs worth energy, transfers phosphate to ADP

2. Glycogen → broken down into glucose, stored in muscle fibers

Rigor mortis + what happens (2)

Stiffening after death

1. No ATP made - no new bridges formed

2. Ca2+ not returned to SR

Cardiac Muscle Characteristics (5)

1. Found in heart walls

2. Must contract all together

3. High Na⁺ permeability → makes it easier to start action potentials (APs).

4. Can initiate its own APs(spontaneously) — no nerve required

5. Intercalated Discs

Intercalated Discs + Characteristics (2)

Specialized connections/junctions between heart muscle cells.

1. Allow ions to flow directly from one cell to the next.

2. Enables APs to spread rapidly to all cells allowing them to contract in sync.

Smooth Muscle Characteristics (6)

1. Found in Digestive tract, bladder, blood vessels

2. Contractions are long and sustained

3. Not attached to bones

4. Not striated (not ordered)

5. No T tubules

6. SR not very developed