What Is a Sinoatrial Node?

The sinoatrial node is the pacemaker with the highest automatic rhythm of the heart. It is located in the upper part of the right atrium, near the epicardium near the terminal groove of the right atrium and superior vena cava. It is horseshoe-shaped, with a total length of about 15mm, a width of about 5mm, and a thickness of about 2mm. The sinoatrial node contains many cells with automatic rhythms, called pacemaker cells. A fiber bundle from the sinoatrial node surrounds the superior vena cava, and the muscles distributed in the atrium are connected to the atrioventricular node. Normally, the sinoatrial node can send out impulses 60 to 100 times per minute, which are conducted along its system to all parts of the whole heart, governing the contraction and expansion of the heart muscle. If it occurs, the heart rate can change, which is called sinus arrhythmia. Clinically there are sinus tachycardia, sinus bradycardia, and sinus arrhythmia.

The sinoatrial node is the pacemaker with the highest automatic rhythm of the heart. It is located in the upper part of the right atrium, near the epicardium near the terminal groove of the right atrium and superior vena cava, and is horseshoe-shaped. The sinoatrial node contains many cells with automatic rhythms, called pacemakers. A fiber bundle from the sinoatrial node surrounds the superior vena cava, and the muscles distributed in the atrium are connected to the atrioventricular node. Normally, the sinoatrial node can send out impulses 60 to 100 times per minute, which are transmitted to the whole heart along its system, governing the contraction and expansion of the myocardium. If it occurs, the heart rate can change, which is called sinus arrhythmia. Clinically there are sinus tachycardia, sinus bradycardia, and sinus arrhythmia.

Chinese name

Sinoatrial node

Foreign name

Sinoatrial node

Types of

Body structure

Shape

Oval cylinder

Volume

15mm × 5mm × 1.5mm

Location

Right atrial adventitia

Make up

"P" cells

The morphological structure of the sinoatrial node:

The sinoatrial node, also called the sinus node, is the pacemaker of the heart in normal times. The sinoatrial node is located at the upper boundary of the sulcus at the junction of the superior vena cava and the right atrium. The long axis of the knot is parallel to the boundary groove, and its "head" position is slightly higher in front of it, reaching the junction of the boundary groove and right atrial appendage, and the "tail" position in the lower back is slightly lower. There are individual differences in the location of the sinoatrial node. Some can ride to the left of the junction of the right atrial appendage, while others are even lower right. The sinoatrial node is located in the atrial wall 1 mm below the epicardium, with no myocardial surface. The deep side of the sinoatrial node (except for the tip of the "tail", which is in contact with the endocardial tissue) is generally not adjacent to the endocardium, and the myocardium of the right atrium is often separated from the endocardium. Nerve fibers and nerve endings are often seen on the surface of the sinoatrial node.

The shape of the sinoatrial node is mostly pointed at both ends, with a thick fusiform or half-moon shape. But its shape is changeable, or stubby, or slender, or forked, or narrowed in the middle. The edges of the knot are irregular, and finger-shaped protrusions are emitted from the knot to the periphery.

The size of the sinoatrial node is 10 to 15 mm in length, 3 to 5 mm in the widest part, and 1 to 2 mm in the thickest part. The lower edge of the sinoatrial node is thick, triangular in cross section, pointed upward.

There is a thick sinus node artery in the center of the sinoatrial node, and the small muscle fibers surrounding the artery constitute the main body of the sinoatrial node. These small muscle fibers are clustered into clusters, scattered in a scaffold of a reticular structure woven by dense collagen fibers.

Tissue structure of human sinoatrial node:

It consists of a body of small muscle fibers surrounding the sinoatrial node artery. These tiny muscle fibers aggregate into grape-like shapes and are scattered in a stent of a reticular structure woven by dense collagen fibers. There are four types of cells in the sinoatrial node: P cells, transitional cells or transitional cells, ordinary cardiomyocytes, and Purkinje cells. There are still sympathetic and parasympathetic nerve fibers in the nodule. Most types of cells in the sinoatrial node have the characteristics of Purkinje fibers. The smaller sinoatrial node cells are intertwined and can persist with some large Purkinje cells. Most Purkinje cells and smaller nodule cells form a fiber bundle that leaves the sinoatrial node and enters the atrium to participate in the composition of the nodular bundle. Knot cells are mainly located in the central part of the knot, pale, because they have a pacing effect, they are pacing cells, also known as P cells. Physiology suggests that nodule cells are the site of pacing impulses. The peripheral transitional cells of the nodule cells are connected between the nodule cells and general myocardial cells. Transitional cells are now considered to be conductive cells within the sinoatrial node. Normally, the sinoatrial node is the most autonomous in the heart tissue. Therefore, the rhythm of the heartbeat is controlled by the sinoatrial node, and the heart beat formed by the impulses issued by the sinoatrial node is called sinus rhythm. Myocardial cells are conductive, and the sinus node is slower in conductivity, which has its special self-protection characteristics. That is, the electric pulses generated by the nodule can be easily transmitted from the nodule cluster, but other stimulation signals are prevented from entering the nodule cell group, so that the function of the sinoatrial node is not easily disturbed by external signals, so as to maintain its autonomic stability. Since the sinoatrial node arteries are disproportionately large and pass through the center of the sinoatrial node, the frequency of the sinus node's impulses may be regulated by the sinus node arterial pulse stimulation. Each time the heart contracts, while the blood enters the coronary arteries, the caliber of the sinoatrial node arteries and the blood flow velocity in the caliber sometimes change accordingly, pulling the collagen fiber network in the node to stimulate the nodule cell population in the mesh. To affect and regulate the frequency of discharge of nodule cells.

Sinus node anatomy:

(A) sinoatrial node:

The sinoatrial node is the pacemaker for cardiac rhythmic activity and also controls the rhythm of normal heart activity. It is oblong in shape and is located in the subepithelial layer (ie above the boundary groove) at the junction of the superior vena cava and the right atrium. It consists of a mixture of nodules and dense connective tissue. There is a nodular artery in the center of the nodule, and there are adrenergic and cholinergic nerves in the nodule. There are two main types of cells in the nodule.

1. The pacemaker cells (P cells) are small, spindle-shaped or polygonal, intertwined into a net, and embedded in connective tissue. The cytoplasm contains more glycogen and a small amount of myofilaments and swallowing vesicles, and there are fewer other organelles. This type of cell is the site where the heart's pacing impulses occur, and it quickly transmits the impulses to the atrial muscles, causing atrial contractions. At the same time, the impulse is transmitted to the atrioventricular node. Such cells are also present in the atrioventricular node, but their number is small.

2. Transitional cells (excessive cells) are the connecting cells between pacing cells and cardiomyocytes. Its morphological structure lies between pacemaker cells and cardiomyocytes, so it is called transitional cells. The cytoplasm contains more myofilaments, but is shorter and thinner than normal myocardial cells.

(B) the heart:

The heart is the power organ of the vascular system, located in the chest cavity, between the two lungs, like a slightly flat front and back cone. The heart is divided into four chambers, the left and right atrium and the left and right ventricle. The atrial septum and the septum divide the heart into left and right halves, and each half is divided into the upper atrium and the lower ventricle by the atrioventricular opening and surrounding valves. The left atrium receives blood from the left and right lungs, which is arterial blood. The left ventricle receives blood from the left atrium and presses it into the aorta when it contracts, pushing the large circulation. The right atrium receives venous blood that flows back to the heart throughout the body, constricting the blood into the right ventricle. When the right ventricle contracts, blood is pushed into the pulmonary veins. This shows that the heart is a blood pumping organ. Cardiac output (the amount of blood ejected by the heart per minute) is an important indicator of pumping function. The average heartbeat of healthy adults is 75 beats per minute, and the calculated cardiac output is about 51. During strenuous exercise, the cardiac output can increase 5 to 6 times of that during quietness. It can be seen that the pumping function of the heart has a certain reserve. The heart has a structure that conducts excitement and regulates the rhythmic beat, called the heart conduction beam. The cardiac conduction bundle is composed of specialized myocardial fibers, including the sinoatrial node, the atrioventricular node, the atrioventricular bundle, and the left and right bundle branches. The sinoatrial node is the pacemaker of the heartbeat. One contraction and relaxation of the heart is called the cardiac cycle. During the cardiac cycle, blood flows in a certain direction due to changes in pressure and volume in the various chambers of the heart, and the opening and closing of valves. Coupled with the role of the cardiac conduction system, the heart contracts and relaxes rhythmically throughout life, which promotes and maintains continuous blood circulation and ensures blood supply in the body.