Lecture 5: Skeletal vs. Cardiac (Striated Muscle)

Question 1

How does skeletal muscle work?

Answer 1

• each muscle has two points of attachment (typically on the skeleton): origin and insertion
• contraction of the muscle pulls on the site of insertion, moving that element of the skeleton

Question 2

How does cardiac muscle work?

Answer 2

• muscle surrounds the hollow chambers of the heart
• upon contraction, it generates pressure within the chamber it surrounds, which results in movement of blood

Question 3

What are some similarities between skeletal and cardiac muscle? (8)

Answer 3

• striated
• actin and myosin
• activated by Ca2+
• ATP used for cross-bridge formation
• length-tension relationship
• abundance of mitochondria and myoglobin
• t-tubules and sarcoplasmic reticulum
• same basic proteins involved, but with heart-specific isoforms

Question 4

How do skeletal and cardiac muscle differ in the name of their cells?

Answer 4

• skeletal: skeletal muscle cell = myofibre
• cardiac: cardiac muscle cell = cardiomyocyte

Question 5

How do skeletal and cardiac muscle differ in myofibril length?

Answer 5

• skeletal: usually the length of the entire muscle
• cardiac: approximately 100 sarcomeres

Question 6

How do skeletal and cardiac muscle differ in cell length?

Answer 6

• skeletal: varies depending on length of muscle, but usually the length of the entire muscle
• cardiac: approximately 0.2 mm in mammals

Question 7

How do skeletal and cardiac muscle differ in the number and location of nuclei?

Answer 7

• skeletal: multinucleate – located in cell periphery
• cardiac: mononucleate (or binucleate) – located at centre of cell

Question 8

How do skeletal and cardiac muscle differ in the arrangement of contractile units?

Answer 8

• skeletal: linear along long-axis of muscle
• cardiac: branching network – cardiomyocytes are connected end-to-end by intercalated disks, which contains gap junctions through which APs are propagated to adjacent cells

Question 9

How do skeletal and cardiac muscle differ in the type of innervation?

Answer 9

• skeletal: somatic
• cardiac: autonomic

Question 10

How do skeletal and cardiac muscle differ in the type of excitation?

Answer 10

• skeletal: neurogenic – neural input needed for contraction
• cardiac: myogenic – cardiomyocytes contract in response to input from other muscle cells

Question 11

How do skeletal and cardiac muscle differ in EC-coupling?

Answer 11

• skeletal: depolarization-induced Ca2+ release
• cardiac: Ca2+-induced Ca2+ release

Question 12

What are the differences between skeletal and cardiac muscle? (8)

Answer 12

• cell name
• myofibril length
• cell length
• nuclei
• arrangement of contractile units
• innervation
• excitation
• EC-coupling

Question 13

What are pacemaker cells?

Answer 13

specialized myocytes located in specific nodes that depolarize spontaneously

Question 14

Do pacemaker cells have stable or unstable resting membrane potentials? Why?

Answer 14

unstable resting membrane potential, partly due to f-channels (permeable to both Na+ and K+)

Question 15

Describe how an action potential occurs in cardiac muscle.

Answer 15

1. opening of voltage-gated Na+ channels causes depolarization
2. opening of voltage-gated K+ channels causes a small repolarization
3. opening of voltage-gated Ca2+ channels then causes a plateau phase, which extends the duration of sarcolemma depolarization
4. Ca2+ channels close and other K+ channels open, resulting in repolarization

Question 16

How do APs differ in skeletal and cardiac muscle?

Answer 16

effective refractory period is much longer in cardiac muscle compared to skeletal muscle

• in cardiac muscle, Ca2+ channels remain open for longer, producing the relatively long plateau of the cardiac muscle AP – lag phase

Question 17

Does summation occur in cardiac muscle?

Answer 17

when cardiac muscle is stimulated at higher frequency, a point is reached where stimulation occurs while the AP is in the refractory period

• contractions may or may not occur
• normal frequency is lost (arrhythmia)

Question 18

What are myogenic hearts?

Answer 18

cardiomyocyte contracts in response to input from adjacent muscle cells

• intrinsic rhythm of heart contraction is set by specialized muscle cells
• found in animals with closed circulatory systems – acts as pressure pump
• ie. many chordates including vertebrates and urochordates, many molluscs

Question 19

What are neurogenic hearts?

Answer 19

beating rhythm determined by neural input

• found in animals with an open circulatory system
• simple tubular or sac-like structure – acts like suction pump
• ie. many arthropods including crustaceans and insects, many annelids