muscles

The transverse (t-) tubule system of skeletal muscle is an important structure that:

• allows action potentials to propagate rapidly into all areas of the fibre

• allows a high frequency of action potentials to excite the muscle

• allows the fibre to be highly elongated

• is an important source of Ca²⁺ ions

• allows action potentials to be brief in skeletal muscle

Summation in cardiac muscle is prevented mainly due to:

• long inactivation of Na⁺ channels

• spontaneous depolarisation of the sinoatrial (SA) node

• slow Ca²⁺ influx

• delay in the atrioventricular (AV) node

• long relative refractory period

The electrical events of the atria and ventricles are isolated from each other:

• so that atrial contraction can finish before ventricular contraction begins

• so that both ventricles contract at the same time

• so that the ventricular contraction terminates during the refractory period of the Na⁺ channel

• so that the left and right side of the heart have the same cardiac output

• to prevent the back-flow of blood between the chambers

Which of the following events occurs during systole?

• Ca²⁺ concentration is low in the cytoplasm of the ventricular myocytes

• There is a delay of the action potential at the atrioventricular (AV) node

• There is depolarisation of the atria

• The pressure in the left ventricle exceeds that of the aorta

• Blood moves from the atria to the ventricle

Which of the following is CORRECT for pacemaker action potentials?

• They have a slow outward Na⁺ current

• They exhibit stable resting membrane potentials

• At threshold, Ca²⁺ influx occurs

• They depend on Na⁺ channels for the fast depolarisation phase

  
  

• Their rate cannot change

What will happen if the parasympathetic nervous system secretes acetylcholine onto the sinoatrial node of the heart?

• The funny current begins moving ions in the opposite direction

• The Na⁺ channels inactivate

• Blood will stop flowing through the chambers of the heart

• There will be an increase in K⁺ permeability across the plasma membrane of the sinoatrial node

• The action potential frequency will increase

A rapidly rising Ca²⁺ transient in a cell would:

• lead to cellular death

• saturate all of the cellular Ca²⁺ buffers

• allow Na⁺ channels to remain open

• rapidly initiate a cellular response

• cause the sarcoplasmic reticulum to fully deplete of Ca²⁺ ions

An ion channel is able to:

• move between the states of inactivation and reprimed without a change in membrane voltage

• translocate from one membrane to another in response to a change in membrane voltage

• change its conformation in response to a change in membrane voltage

• begin passing ions without changing its conformation

• remain open all the time

Cardiac muscle has special structures to ensure electrical and mechanical coupling. These are the:

• gap junctions and Na⁺ channels

• desmosomes and gap junctions

• Na⁺ channels and desmosomes

• plasma membrane and mitochondria

• myofibrils and sarcoplasmic reticulum

What does a voltage-gated Na⁺ channel require to move from its inactivated state to a reprimed state?

• Ca²⁺ channels to open

• phosphorylation of the channel

• another action potential to excite the cell

• repolarisation of the membrane

• the Na⁺ gradient across the membrane to change