Cardiovascular and Nervous System

It is a decrease in pulse rate below 60 beats/minutes. At this rate, the heart is not able to pump enough oxygen rich blood to our body during normal activity or exercise. Physiologically, it is seen in athletes and during sleep. Pathologically, it is observed in myxoedema and in heart blocks.

Myogenic heart contracts without having a message from the brain as in the pacemaker. In the human heart, contraction is initiated by a special modified heart muscle known as sinoatrial node. Since the heart beat is initiated by the SA node and the impulse of contraction originates in the heart itself, the human heart is termed myogenic. The heart of vertebrates and molluscs are also myogenic.

Sino atrial node is a specialised cardiac tissue present at the junction of superior vena cava and right atrium. It generates the maximum number of impulses (70 to 80 per minute) and sets the pace for the heart. Hence, it is called pacemaker of the heart. The cardiac impulse originates at the SA node.

The auricle (also called atrial appendage) is a small, pouch-like diverticulum that projects from each atrium of the human heart. It slightly increases the volume of the atrium. The apex is the pointed bottom of the heart, the base is the top where great vessels attach, and pericardial reflection refers to where the pericardium folds. The auricle is the correct anatomical structure described in the question.

The pacemaker (sinoatrial node or SA node) generates rhythmic electrical impulses that coordinate cardiac muscle contraction. Without the SA node, cardiac muscle cells would contract independently and asynchronously, leading to uncoordinated, ineffective heart pumping. The heart would lose its rhythmic, synchronized beating pattern.

The frog heart is myogenic, meaning it generates its own electrical impulses independent of nervous system input. When removed from the body and kept moist in physiological saline, the excised heart continues beating autonomously for extended periods due to this intrinsic automaticity. Keeping it moist is essential for maintaining cellular function.

The vagus nerve (cranial nerve X) is the primary component of the parasympathetic nervous system's influence on the heart. When stimulated, it releases acetylcholine at the SA node, which decreases the rate of depolarization and slows the heart rate. This is part of the 'rest and digest' parasympathetic response.

The vagus nerve is part of the parasympathetic nervous system, which releases acetylcholine to slow heart rate. This is why vagal stimulation decreases heartbeat, known as vagal bradycardia.

Fish have a 2-chambered heart with a single atrium and a single muscular ventricle. 

During depolarisation, the impulses are carried from endocardium to epicardium.

Myocardium is made up of cardiac cells, which function in autorhythmicity. Some cardiac muscle cells are self-excitable, allowing for rhythmic waves of contraction to adjacent cells throughout the heart.

This is not true. Cardiac cells are spindle-shaped.

Endocardium serves as a kind of blood-heart barrier and thus, controlling the ionic composition of the extracellular fluid in which the cardiomyocytes bathe.

Cardiac tamponade is relieved by pericardiectomy. It is caused due to excess fluid in pericardial cavity.

Visceral pericardium is fused with epicardium. The epicardium layer is immediately outside of the heart muscle proper.