Corazón II: Cardiac Muscle Layers

• Lesson 16 covers the heart (Corazón II)

Cardiac Muscle

• The auricle musculature has both superficial and deep layers
• The superficial layer's muscle fibers extend from front to back in an arc shape. Also runs horizontally between both auricles
• The deep layer is independent for each auricle
• Circular muscle fascicles are present at the exit of the vessels
• The ventricular musculature consists of 3 layers: an external subepicardial with longitudinal fibers, a middle with annular fibers, and an internal subendocardial with longitudinal orientation

Subepicardial Layer

• The fibers from the external subepicardial layer originate from the cardiac skeleton, mainly the fibrous trigones
• Fibers coil in a screw-like manner to the left and towards the ventricle's apex
• Fibers form the cardiac apex (vortex), merging with the fibers from the other layers

Tricuspid Valve

• The tricuspid valve is located on the right side of the heart
• When the right auricle wall contracts, blood passes the tricuspid valve into the right ventricle
• An annulus fibrosus forms the valve, attaching to the fibrous trigone
• This fibrous ring supports three cusps: anterior (a), posterior (p), and septal (s)

Mitral Valve

• The mitral valve (bicuspid) is on the left side
• The valve connects the left auricle to the left ventricle and uses an annulus fibrosus
• There are two powerful cusps: anterior (ai, anterointernal) and posterior (pe, posteroexternal)

Semilunar Valves

• The aortic and pulmonary valves are semilunar
• They attach to the fibrous trigone and are semilunar valves formed by three cusps
• The aortic valve has a left, right, and posterior cusp
• The pulmonary valve has anterior, left, and right cusps

Cardiac Vasculature

• Cardiac vascularization includes the left and right coronary arteries and their branches, cardiac veins, and the coronary sinus

Coronary Arteries

• The coronary arteries originate from the sinuses of Valsalva
• The right coronary artery (CD) and left coronary artery (CI) both have auricular and ventricular branches
• The left coronary artery (CI) branches into the circumflex artery (c) and the anterior interventricular artery (iva)

Cardiac Veins

• The coronary veins include the great cardiac vein (CM), small cardiac vein (cm), and anterior cardiac veins (ca)
• Other veins include the great cardiac vein (1), coronary sinus (2), oblique vein (3), middle cardiac vein (4 interventricular posterior), and small cardiac vein (5)

Coronary Artery Variations

• Right dominance presents with a short anterior interventricular artery. The right coronary artery passes the crux and goes to the posterior side of the left ventricle
• Left dominance involves the posterior interventricular branch arising from the circumflex branch of the left coronary artery

Cardiac Excitoconduction System

• The cardiac excitoconduction system ensures an automatic heartbeat and is under the control of the vegetative nervous system
• 99% of cardiac muscle cells can contract
• The remaining 1% consists of undifferentiated muscle cells capable of autoexcitation
• Tissue transmits the excitation to neighboring cells rapidly
• The system spontaneously produces rhythmic local stimuli that excite the remaining cardiac musculature
• The cardiac excitoconduction system comprises the sinusal node, the atrioventricular node, the Bundle of His, and Purkinje fibers

Sinus Node

• The sinusal or sinoatrial node (of Keith and Flack) measures 2 to 2.5 cm
• It is located in the right auricle on the anterior side of the superior vena cava's outlet, between the vena cava and the terminal crest
• Generates an excitation frequency of about 70 impulses per minute (sinus rhythm), hence the name "cardiac pacemaker"

Atrioventricular Node

• The auriculoventricular node (Aschoff-Tawara) is smaller than the sinusal node
• It is located in the posteroinferior region of the right auricle near the coronary sinus outlet, close to the septum and above the septal cusp of the tricuspid valve
• The excitation frequency is 40 impulses per minute

Bundle of His

• The Bundle of His continues from the auriculoventricular node
• It crosses the cardiac trigone and runs through the membranous part of the interventricular septum
• On musculature arrival, the bundle divides into the right and left branches
• The approximate rhythm is 20 impulses per minute, causing ventricular bradycardia

Right Branch

• The right branch of the Bundle of His rests on the right side of the septum and continues to the apex of the right ventricle
• From the septum, a branch passes through the moderator band to the anterior papillary muscle
• The remains divide by the walls of the right ventricle and form the subendocardial Purkinje fibers

Left branch

• The left branch of the Bundle of His passes to the left side of the septum and descends toward the left ventricle's apex
• It divides into fascicles that distribute along the interventricular septum's wall, the left ventricle, and papillary muscles as subendocardial Purkinje branches

Heart Innervation

• The heart is innervated by fibers originating from T1 to T4 spinal segments (anterior rami/roots) via preganglionic fibers
• These fibers pass through the white communicating branch to the ganglionic chain where synapse occurs at the superior, middle, and inferior cervical ganglia
• The inferior cervical ganglion joins the first thoracic ganglion, forming the stellate ganglion
• Postganglionic sympathetic cardiac nerves from the superior, middle, and inferior cervical ganglia exist
• Thoracic sympathetic cardiac nerves from T1-T4 thoracic ganglia also contain postganglionic fibers

Sympathetic

• Sympathetic stimulation accelerates the heart rate, leading to tachycardia
• Increases contraction force
• The cardiac output increases
• Coronary arteries dilate

Parasympathetic

• Parasympathetic activity decreases heart rate/frequency
• Reduces contraction force
• Constricts coronary arteries
• The first neuron is located in the bulb raquideo
• Preganglionic fibers branch off from the inferior ganglion of the vagus nerve to the cardiac cervical nerves and recurrent laryngeal nerve (superior and inferior)
• The thoracic vagal cardiac nerve branches off from the vagus nerve upon entering the thorax
• Synapses occur in intrinsic and superior/inferior cardiac ganglia
• The resulting cardiac plexus of nerves is formed by sympathetic and vagal (parasympathetic) nerves

Heart Sensations

• The heart is not sensitive to touch, cutting, heat, or cold
• Ischemia in the myocardium stimulates the nerve endings that transmit pain
• The nerves carrying painful information from the thoracic viscera are the cardiac sympathetic nerves
• Most afferent fibers from the pericardium travel with the phrenic nerve, although those that collect sensation from the visceral layer of the serous pericardium travel with the cardiac plexus