(18.4) Cardiac Muscle Fibers

What cells are present in the cardiac muscle fibers?

Cardiac muscle cells 

List two kinds of myocytes of the heart

1. Contractile cells → responsible for contraction

2. Pacemaker cells → non-contractile cells that spontaneously depolarize

Explain Structure and Function of Cardiac Muscle Cells

• Striated, Short, Branched, Fat, Interconnected

• Contain numerous large mitochondria (25-35% of cell volume)

  • That afford resistance to fatigue 

• Rest of volume composed of sarcomeres 

  1. Z discs

  2. A bands 

  3. I bands 

• T tubules are wider, but less numerous

  • Enter cell only once at Z disc  

• SR simpler than in skeletal muscle → no triads 

• Intercalated discs are connecting junction between cardiac cells   

Role of Intercalated Discs 

• Connecting junctions between cardiac cells that contain:

  1. Gap junctions

Role of Gap Junctions

Allows heart to be functional syncytium → a single coordinated unit

Explain Functional Syncytium

• A group of cells that behaves as a single coordinated unit

• EX: cardiac muscle cells that contract together because they are electrically coupled by gap junctions

Structure and Function of Endomysium

Intercellular space between cells has connective tissue matrix → Endomysium

• STRUCTURE

  • Contains numerous capillaries

• FUNCTION

  • Connects cardiac muscle to cardiac skeleton, giving cells something to pull against

Describe cardiac tissue characteristics

1. Striated

2. Short 

3. Branched 

4. Fat 

5. Interconnected 

Similarity or Difference in Cardiac vs Skeletal Muscle: Structure ?

CARDIAC MUSCLE 

• Striated 

• Short 

• Branched 

• One or two nuceli per cell 

SKELETAL MUSCLE 

• Striated 

• Long

• Cylindrical 

• Multinucleate

Similarity or Difference in Cardiac vs Skeletal Muscle: Are Contractile Tissues?

SIMILAR → BOTH SKELETAL & CARDIAC MSUCLE are contractle tissues 

• Muscle contraction is preceded by depolarizing action potential 

• Depolarization wave travels down T tubules → causes sarcoplasmic reticulum (SR) to release Ca²⁺ 

• Excitation-coupling occurs → Ca2+ binds troponin causing myofilaments to slide in BOTH skeletal and cardiac muscle cells

Similarity or Difference in Cardiac vs Skeletal Muscle: Ca2+ binds to troponin?

SIMILAR → Exication-contraction coupling occurs in BOTH SKELETAL & CARDIAC MUSCLE

• Ca2+ binds troponin causing filaments to slide

Similarity or Difference in Cardiac vs Skeletal Muscle: Gap Junctions between cells ?

CARDIAC MUSCLE 

• Yes

SKELETAL MUSCLE 

• No

Similarity or Difference in Cardiac vs Skeletal Muscle: Contracts as a unit ?

CARDIAC MUSCLE 

• Yes

  • All cardiomyocytes contact as a unit (gap junctions create a functional syncytium) or none contract

SKELETAL MUSCLE 

• No

  • Motor units must be stimulated individually

Similarity or Difference in Cardiac vs Skeletal Muscle: T tubules?

CARDIAC MUSCLE 

• Fewer, wider 

SKELETAL MUSCLE 

• Abundant 

Similarity or Difference in Cardiac vs Skeletal Muscle: Sarcoplasmic reticulum?

CARDIAC MUSCLE 

• Less elaborate  

•  No terminal cisterns (triads)

SKELETAL MUSCLE 

• Elaborate 

• Has terminal cisterns 

Similarity or Difference in Cardiac vs Skeletal Muscle: Source of Ca2+ for contraction?

CARDIAC MUSCLE 

• Sarocplasmic reticulum AND extracellular fluid 

  • Influx of Ca²⁺ from ECF triggers Ca²⁺ release from SR

  • Depolarization opens slow Ca²⁺ channels in sarcolemma, allowing Ca²⁺ to enter cell

  • Extracellular Ca²⁺ then causes SR to release its intracellular Ca²⁺

SKELETAL MUSCLE 

• Sacroplasmic reticulum ONLY  

Similarity or Difference in Cardiac vs Skeletal Muscle: Contractile cells?

YES → Contractile cells: responsible for contraction 

• Muscle fibers → contains myofibrils composed of organized sarcomeres (thick myosin filaments & thin actin filaments) 

Similarity or Difference in Cardiac vs Skeletal Muscle: Pacemaker cells present?

CARDIAC MUSCLE 

• Yes → Pacemaker cells: non-contractile cells taht spontaneously depolarize 

  • Initiate depolarization of entire heart 

  • Do not need nervous system stimulate 

SKELETAL MUSCLE 

• No → Needs the nervous system stimulation 

Similarity or Difference in Cardiac vs Skeletal Muscle: Tetanus possible?

CARDIAC MUSCLE 

• No → Longer refractory period

  • Absolute refractory period is almost as long as contraction itself → prevents tetanic contractions

  • Allows heart to relax and fill as needed to be an efficient pump

SKELETAL MUSCLE 

• Yes 

Similarity or Difference in Cardiac vs Skeletal Muscle: Supply of ATP?

CARDIAC MUSCLE 

• Aerobic ONLY → more mitochondria

  • Greater dependence on O² → cannot function without O²

  • More adaptable to other fuels: lactic acid, but MUST have O²

SKELETAL MUSCLE 

• Aerobic AND Anaerobic → fewer mitochondria  

  • Can go through fermentation when O² not present

Key Differences between Skeletal and Cardiac Muscle

What makes the heart a functional unit

1. Gap junctions

   • Allows heart to be a functional syncytium 

2. Endomysium

   • Connective tissue matrix that connects cardiac muscle to cardiac skeleton giving cells something to pull against 

3.  Pacemaker cells

   • Non-contracle cells that spontaneously depolarize; does not need nervous system stimulation

4. All cardiomyocytes contract as a unit

   • Ensures effective pumping action

5. Longer refractory period

   • Allows heart to relax and fill as needed