Regulation of Cardiac Activity MCQ - Practice Questions with Answers

Heartbeat & Its Regulation

• Heartbeat refers to the rhythmic contraction and relaxation of the heart
• Each heart beat includes one systole and one diastole of the heart to distribute and receive blood to and from the body
• Heart beats are of two types i.e. neurogenic heart beat and myogenic heart beat

1. Neurogenic heartbeat- It is initiated by a nerve impulse coming from a nerve ganglion situated near the heart e.g. heart of some annelids and most arthropods
2. Myogenic heartbeat - it is initiated by a patch of modified heart muscles itself e.g. heart of molluscs, vertebrates including human beings

Origin of heartbeat

• The mammalian heart is myogenic i.e. it originates from a muscle
• A heart beat originates from the SA node lying in the wall of the right atrium near the opening of superior vena cava 

Conduction of heart beat

• The AV Node picks up the wave of contraction propagated by SA node.
• Later, the bundle of His and Purkinje fibres convey impulse of contraction from Av node to the myocardium of the ventricles. 

Regulation of heartbeat:

• There are two mechanisms to regulate the rate of heart beat 

1. Neural regulation 

• The cardiac centres of the body are placed in the medulla oblongata of the brain
• It is formed of 2 parts i.e. cardio-inhibitor and cardiac-accelerator 
• Cardio-inhibiting  part reduces the rate of heart beat while cardio-accelerating part increases the rate of heart beat
• Cardio inhibitor is connected to the heart with the help of vagus nerve
• Cardio accelerator is connected to the heart via sympathetic nerve fibres.
• There are Sensory fibres that extend from the receptors (in the superior vena cava, aorta, and carotid sinuses)  to the cardiovascular centres in the medulla oblongata
• The impulses from aorta and carotid sinus decrease the heart rate 
• The impulses from vena cava increase the heart rate

2. Hormonal regulation 

• The medulla of adrenal glands secrete two hormones namely adrenaline and noradrenaline 
• These two hormones tend to affect the heart rate
• The function of noradrenaline is to increase the heart rate under normal conditions and that of adrenaline is to perform the same function during an emergency.
• Noradrenaline and adrenaline can directly affect the SA Node 
• Thyroxine also increases the oxidative metabolism of the cells. This requires more oxygen and thus heart rate increases
• Hormone thyroxine is secreted by thyroid gland
   

Blood Pressure and Regulation of cardiac activity 

Regulation of cardiac activity is done at neural level and chemical level

Nervous regulation

• Sympathetic and parasympathetic nervous system regulates the cardiac activity
• Sympathetic nervous system accelerates the heart beat, constricts arteries raising the blood pressure
• Parasympathetic nervous system reduces the heart beats, dilates arteries thus,lowering the blood pressure.
• The changes in arterial pressure is can be measured through baroreceptors

Baro-receptors

• These are the endings of the nerves that lie in the walls of the arteries. 
• Baroreceptors are abundantly found in the carotid sinus and the wall of aortic arch
• A rise in arterial pressure stretches the baroreceptors causing them to transmit signals into the CNS.
• In response to these signals ‘feedback’ signals are then sent back via autonomic nervous system to the circulation to reduce the arterial pressure to the normal level
• The signals are transmitted from each carotid sinus through very small Hering’s nerve to the glossopharyngeal nerve and then to the tractus solitarius in the medulla oblongata of the brain
• Signals from aortic arch transmit through the vagus nerve to the tractus solitarius in the medulla oblongata of the brain

Blood Pressure 

• It is the pressure of circulating blood against the walls of arteries
• Hence it is regarded as Arterial blood pressure
• It is recorded as Systolic Blood Pressure or SBP (maximum pressure during contraction of heart) over Diastolic Blood Pressure or DBP (minimum pressure during diastole)
• It is measured in millimeters of mercury (mmHg)
• Normal BP is 120/80 mmHg
• The instrument used to measure BP is Sphygmomanometer
• Pulse Pressure : It is the difference between SBP and DBP. Thus normal value is 120-80= 40 mmHg
• Mean Arterial BP : DBP + 1/3 Pulse Pressure = 93 mmHg
• BP above 140/90 is regarded as High BP or Hypertension condition (many other factors are assessed too