cardiac muscles

Question 1

Cardiac Muscle Properties

Answer 1

Small branched cells connected by intercalated discs
Striated appearance under the microscope
Fundamental unit is sarcomere of actin/myosin arrangement
Troponin is the calcium sensing protein in cardiac muscle
Involuntary muscle

Question 2

The Sliding Filament Mechanism

Answer 2

Contraction means the development of force rather than shortening
Myofilaments remain the same length, however they overlap to a greater extent

Question 3

Cardiac Muscle Cells

Answer 3

Interconnecting, electrically coupled cells: act in a syncytium
Cardiac cells separated from each other but are electrically coupled
1% of cardiac cells are non-contractile

Question 4

Intercalated Discs

Answer 4

3 components

1. Interdigitating folds:
   in folds at the end of the cells that increase the surface of area of cell-cell connection
2. Mechanical Junctions:
   Two types of junctions - the fascia adherens and desmosomes.

Adheren Junctions
Couple the membrane to the contractile apparatus (actin cytoskeleton) of the cardiomyocytes
Aid the transmission of contractile force from one myocyte to the next

Desmosomes
Proteins that provide strong structural support between cardiomyocytes
Essential to ensure that cardiomyocytes can with stand contractile forces of the heart

3. Gap Junctions:

Electrical coupling - directly couple the cytosol of neighbouring cardiomyocytes and provide a pathway of low resistance pores

Question 5

Gap Junctions

Answer 5

A hexamer composed of connexins provided by each cell
Gap junction hexamers (6 subunits) provided by adjacent cells to form a gap junction with 12 subunits connecting the cells
Present in intercalated discs
The gap junction serves as a low-resistance pathway between cells
Cell-cell conductance - electrical coupling
5 – 500 gap junctions represent a gap junction plaque

Question 6

Blood supply in cardiac muscle

Answer 6

Requires a continuous supply of oxygen
Rich capillary supply
Short diffusion distances for oxygen and waste products

Question 7

Pacemaker potentials

Answer 7

Pacemaker potential: spontaneous gradual depolarization

SA node does not have a steady resting potential but slowly depolarizes

ION CONDUCTING MECHANISMS

1. K+ channels that open during the repolarization phase then start to close at negative potentials. 

2. The funny channel (F-current)  =  a sodium permeable ion channel that opens at negative potentials (unlike most voltage gated ion channels).   The funny channels depolarize the membrane.  

3. T-type calcium channels –voltage gated calcium channels that contribute to the final depolarizing boost of the pacemaker potential.

Question 8

Cardiac action potential

Answer 8

Resting membrane potential close to -90 mV

Action Potential =

Fast depolarizing phase due to voltage gated sodium channels opening.
Voltage gated sodium channels rapidly inactive

The membrane depolarization activates L-type voltage gated calcium channels (L=long lasting)
Modified versions of DHP receptors (act as voltage sensors in skeletal muscle)
Sustained depolarization is observed (plateau phase)

Repolarization occurs as the L-type calcium channels will close and voltage gated potassium channels open after a delay

Question 9

Excitation contraction coupling

Answer 9

Actin myosin based contraction

Fundamental unit is sarcomere

Calcium initiates contraction

Regulated by troponin – tropomyosin complex

L type calcium channel opening during the cardiac action potential
Small amount of calcium enters the cell and this triggers the opening of the ryanodine receptor calcium channel in the. SR membrane
Calcium is released from the SR and binds to troponin to trigger cross bring cycling
Ca-ATPase pumps (SERCA) return calcium back into the sarcoplasmic reticulum
Na+/Ca2+ Exchanger removes calcium out of the cell
Membrane is repolarized by potassium channels at the end of the action potential

Question 10

Cardiac Troponins

Answer 10

Cardiac troponins are released into the blood when heart muscle is damaged

Used to detect myocardial infarction

Question 11

Five key points from this lecture

Answer 11

Cardiac muscle is composed of branched interconnecting cardiomyocytes
Cells have a striated appearance reflecting the organized actin-myosin arrangement
Cells are electrically connected via the intercalated disc
Two types of electrically activity: pace maker potential and prolonged cardiac action potential
Heart rate is regulated by catecholamines