L15 Cardiac Muscle

What are key structural features of cardiac muscle cells? (4)

• Single nucleus per cell

• Branched cells

• Many mitochondria

• Joined by intercalated disks

• Less organized sarcomeres

What do intercalated disks contain? (2)

Desmosomes and gap junctions.

What is the function of desmosomes in cardiac muscle?

Connect intermediate filaments and transmit mechanical force between cells

What is the function of gap junctions in cardiac muscle?

Electrically couple cells — allow APs to pass directly cell-to-cell.

What do tight junctions and adherens do

stop paracellular transport

Transmit force and connect actin filaments

What acts as the main pacemaker of the heart?

The sinoatrial (SA) node. it generates rhythmicity

How long is ventricular AP and what is its resting potential

250ms and resting potential -90mV

What do the purjunkie fibers and bundle of hiss do

They are cardiac tissues specialized for transmission of AP. They transmit electrical signals from the ATV to other parts of the heart

Transmission of AP from the ATV 

Explain pacemaker activity/AP step by step (8)

1. Pacemakers don’t have a stable resting membrane potential

2. Depolarization to threshold -40mV opens VGCC L type channel

3. Calcium influx in pacemaker cells

4. Repolarization occurs from delayed closing of L type channels and opening of Kv channels

5. Potassium moves out causing membrane potential to be more negative 

6. Closure of Kv channels reduce potassium permeability causing depolarization

7. repolarization opens the funny channel (HCN- hyperpolarization cyclic nucleotide channels)

8. Sodium comes in via the funny current If making the cell more positive

What initiates the pacemaker potential?

Decrease in K⁺ permeability, Na⁺ influx through the funny current (Iₑf), and slight Ca²⁺ influx.

Explain ventricular AP step by step

1. Cell has a very negative resting potential -90mV

2. Nav channels open resulting in quick sodium influx making the cell more positive

3. Nav channels closing causes a slight repolarization (down tick)

4. L type calcium channels open causing a plateau as its balanced by potassium efflux

5. Closing of sodium channels and opening of potassium voltage channels repolarizes cells

Explain excitation contraction coupling of cardiac muscle (4)

1. Calcium enters cell through L type channels causing calcium induced calcium release of calcium store in the sarcoplasmic reticulum (CICR

2. Calcium is released through ryanodine channels which are a calcium gated calcium channels

3. Calcium then binds troponin, moves tropomyosin so actin myosin cross bridges can form

4. Calcium is removed by SERCA back into the SR, plasma membrane calcium ATPase and sodium calcium exchanger

How does the cardiac length–tension curve differ from skeletal?

• Active force curve is narrower.

• Passive force curve increases steeply.

• Relaxed cardiac muscle is stiffer due to more connective tissue.

What is the Frank Starling law

Increased cardiac filling (preload) stretches fibers → improved actin–myosin overlap → stronger contraction → increased stroke volume.

What neurotransmitter increases heart rate?

What neurotransmitter decreases heart rate?

Noradrenaline (NA).

Acetylcholine (ACh).

How does noradrenaline increase HR? (4)

• NA binds to β₁-adrenergic receptors activating Gs which increases cAMP)

• Decreases K⁺ permeability at rest

• Increases funny current (Iₑf)

• Depolarizes pacemaker cells faster → quicker threshold → faster HR

How does ACh slow heart rate?

• ACh: M₂-muscarinic receptors (Gi → ↓cAMP)

• Increases K⁺ permeability (hyperpolarization)

• Decreases funny current (Iₑf)

• Slower depolarization to threshold

What is inotropy?

What increases it? 

The force of cardiac contraction.

• Sympathetic stimulation (NA, adrenaline)

• Digitalis

• Increased extracellular Ca²⁺

What is chronotropic

Increase in heart rate

How do β₁-adrenergic receptors increase inotropy?

• Activate Gs → ↑cAMP → activate PKA (Protein Kinase A)

• PKA phosphorylates L-type Ca²⁺ channels → more Ca²⁺ entry

• PKA increases SERCA activity → more Ca²⁺ in SR

• More Ca²⁺ release from RyR → stronger contraction

What are β₁-adrenergic receptors

G protein coupled receptors linked to Gs

How does ACH have negative inotropic actions 

It activates m2-muscurinic receptors linked to Gi which inhibit PKA preventing calcium release