Force generation by the heart

Question 1

Myofibrils

Answer 1

Contractile units which contain alternating bands of thick (myosin) and thin (actin) filaments
Within each myofibril, actin and myosin are arranged into sarcomeres

Question 2

Desmosomes

Answer 2

Provide mechanical adhesion between adjacent cardiac cells

Question 3

Cross bridge formation (muscle contraction)

Answer 3

ATP and Ca2+ are required
Ca2+ binds to troponin causing a conformational change. This exposes the actin and myosin binding sites and thus cross bridge binding occurs.
This results in contraction as actin filaments slide on myosin filaments.

Question 4

Contraction and Ca2+ release in cardiac muscle

Answer 4

Extracellular Ca2+ is needed to stimulate the release of Ca2+ from the sarcoplasmic reticulum.
CICR - Ca2+ influx from plateau phase (phase 2) induces the release of more Ca2+ from the SR.

Question 5

Relaxation in cardiac muscle

Answer 5

During repolarisation (phase 3), Ca2+ channels close and Ca2+ influx ceases.
Ca2+ release from SR also ceases.
Ca2+ dissociates from troponin C and the cross bridges formed between actin and myosin break.
The result is relaxation.

Question 6

Refractory period

Answer 6

A period following an AP where it is not possible to produce another AP
Protective for the heart

Question 7

Stroke volume (SV)

Answer 7

Volume of blood ejected per ventricle per heart beat

SV = EDV-ESV

Question 8

Intrinsic control of SV

Answer 8

Changes in SV are due to changes in the diastolic length of myocardial fibres.
Determines the preload

Question 9

Frank Starling curve

Answer 9

Describes the relationship between venous return, EDV and SV.
The more the ventricle is filled with blood during diastole, the greater the volume of ejected blood will be during the resulting systolic contraction

Question 10

Afterload

Answer 10

Resistance into which the heart is pumping
Extra load is imposed after the heart has contracted
Force of contraction rises

Question 11

Extrinsic control of SV

Answer 11

Involves nerves and hormones

Sympathetic nerve stimulation - Increased force of contraction = +ve inotropic effect

Question 12

Sympathetic stimulation on ventricular contraction

Answer 12

Shifts frank starling curve to the left

Question 13

Parasympathetic nerves on ventricular contraction

Answer 13

Very little innervation of vagus in man

vagal stimulation has major influence on rate, not force, of contraction