ANS Smooth muscle pt2 and Cardiac Muscle

What are the calcium sources for smooth muscel?

1. Extracellular fluid

2. and sarcoplasmic reticulum.

What is the calcium sensitive protein in smooth muscles? 

Calmodulin

What are dense bodies in smooth muscle?

They serve as anchoring points for actin filaments

Single unit smooth muscle cells

connected by gap junctions and cells contract as a single unit.

A cluster of muscle cells that has ONE neurojunction

Ex: Small intestines

Multi-unit smooth muscle cells

are not electrically linked and each cell must be stimulated independently. 

Every cell has its own neural junction

Ex: eye

Actin in smooth muscle:

more plentiful: 10-15 actin : 1 myosin

Associated with tropomyosin and calmodulin 

Myosin in smooth muscle

Filements are longer 

entire surface of filament covered with myosin heads 

Sarcoplasmic reticulum in smooth muscle

Amount of SR varies and is less organized

No T-Tubules but caveolae 

Extensive cytoskeleton in smooth muscle

intermediated filaments and dense bodies, Using it a lot more

What are the 4 ways in which excitation happens?

1. Autonomic neurotransmitters and hormones. Released by autonomic neurons. 

   1. Autonomic neurotransmitter: (can be acetylcholine) binds on smooth muscle cells and Na channel opens.

   2. Hormone: Hormone, like oxytocin, binds to its hormone receptor on smooth muscle cells, causing turbulence opening Na channel. 

2. Mechanical Stretch: After eating, the stretched stomach opens Na channel, (can be urination in the bladder. 

3. Slow electrical activity: Ionic fluctuations in the ECF. Changes in the ECF. An acidic molecule made after a meal or other process, the acidic molecule made in the stomach is absorbed into the blood supply, and now all cells are bathing in it. The acidity is increasing. 

4. Paracrine signals:

   1. Histamine constricts the smooth muscle of the airways. 

   2. Nitric oxide relaxes the smooth muscle of blood vessels. 

Smooth Muscle contraction

1. Calcium increases

   • Ca²⁺ comes from the SR and ECF. 

2. Calcium binds to calmodulin

   • Forms a Ca²⁺–calmodulin complex.

3. Activates MLCK

   • The complex activates MLCK (myosin light chain kinase).

   • MLCK uses ATP to add a phosphate (PO₄) to light chains in myosin heads.

4. Myosin activates

   • Phosphorylated myosin can now break down ATP and bind to actin.

5. Contraction

   • ATP is used for myosin to pull on actin (the power stroke).

Relaxation of smooth muscle:

• Repolarization

  • The cell membrane returns to its resting state (inside becomes more negative again).

  • This reduces calcium channel activity.

• Calcium removal

  • Ca²⁺ is pumped out of the cytosol:

    • Back into the SR (sarcoplasmic reticulum)

    • Out to the ECF (extracellular fluid)

  • This movement goes against the concentration gradient, so it requires ATP.

• Myosin dephosphorylation

  • The enzyme MLCP (myosin light chain phosphatase) removes the phosphate (PO₄) from the myosin head.

  • This inactivates myosin ATPase, stopping cross-bridge cycling.

• Return to resting state

  • Dense bodies (which anchor actin filaments) return to their normal position as the muscle relaxes.

  • The cell returns to its resting length and tone.

cardiac Muscle is an intermediate Muscle 

Like skeletal:

• Striated

• sarcomere structue

• elastically linked to one another

• has tweo sources of calcium

• under autonomic as swll as hormonal control

Unlike skeleral muscel

• Muscle fibers are shorter

• may be branched

• have single nucleus

• Gap junctions in intercalated disks and desmosomes 

Cardiac Muscle: excitations

Pacemaker cells (myocardial cells) automatically opens Na+ channels 

Autonomic NT regulated the rate and rhythm of depolarization 

Cardiac Muscle: contration

2 sources of Ca++: Sarcoplasmic reticulum and sarcolemma sodium Na+.

Ca+ binds to troponin… same as skeleton 

H-zone disappears, fully contracted sarcomers

Cardiac Muscle: relaxastion

1. repolarizations

2. Return Ca++ back to SR and ECF, Ca++ declining 

3. Tropomyosin back on myosin off