Pericardium

The pericardium is the sac, or pouch, which wraps the heart and the roots of the great blood vessels. It consists of two layers; an external parietal layer, which is the pericardium proper, and an internal visceral layer, which is called the epicardium. The epicardium snugly envelops the myocardium.

The pericardium proper is composed, in turn, of an internal serous sheet, and an external fibrous layer. This sac-like structure surrounding the heart is shaped like a beveled cone, whose lower base lies on the diaphragm, with the apex almost reaching the level of the angle of sternum. Like the heart, it extends in breath more to the left than to the right. The superior sternopericardial ligament attaches the anterior part of pericardium to the manubrium sterni (upper portion of sternum).

The base of the pericardium, or inferior part, is intimately fused with the central tendon of diaphragm. It extends slightly over the front left areas of the muscular part of diaphragm, to which it is attached by areolar tissue. The anterior part faces the anterior wall of thorax and is in relation with the body of sternum, intercostal spaces, and the left portion of the xiphoid process.

The lateral parts (right and left) of pericardium are the mediastinal parts. They adjoin the mediastinal pleura, which is connected to it also by areolar tissue.

Heart Autonomic Innervation

The heart autonomic innervation modulates cardiac function. Although it has its own independent conducting system, with the sinoatrial (SA) and atrioventricular (AV) nodes, the myocardium is also innervated by the peripheral nervous system through parasympathetic and sympathetic efferent fibers. The parasympathetic innervation is constituted by the vagus nerve branches, with the right vagus nerve branches innervating the SA node and the left vagus nerve the AV node, making the heart beat slower, decreasing the heart rate. Meanwhile, sympathetic nerve fibers to the heart is supplied by the stellate ganglion of the sympathetic trunk, making the heart beat faster when it is stimulated. (The stellate ganglion is made up of the inferior cervical and the first thoracic ganglion).

Important overlap can occur in the anatomical distribution of nerve fibers to the heart. Atrial myocardium is also innervated by vagal efferent fibers, while the ventricular heart muscle is only sparsely innervated by vagal efferent branches. On the other hand, sympathetic fibers are found throughout both atria and ventricles as well as in the SA and AV nodes. As mentioned in the above paragraph, vagal branches stimulation decreases heart rate, which is called negative chronotropy, as it also decreases contractility of heart muscle fibers, which is called negative inotropy. Vagal-regulated inotropic influences are moderate in the atria and relatively weak in the ventricles. Meanwhile, sympathetic presence is strong in both atria and ventricles.

There are also in the heart, vagal and sympathetic nerve fibers, which relay information from both stretch and pain receptors located in the myocardium. Stretch receptors send feedback information about blood volume and arterial pressure, while the pain receptors are usually activated during a coronary arterial branch ischemia, sending pain and discomfort information as feedback. This influences the electrical activity of the heart and can be seen in a cardiac cycle diagram.

Heart Sympathetic Innervation

The heart sympathetic innervation is carried out by nerve fibers which arise from neurons located in the medulla oblongata. Most of these fibers nerve cells are found in the rostral ventrolateral aspect of medulla. When the activity of these neurons increases, there is cardiac stimulation and systemic vasoconstriction. Here, sympathetic nerve cells have spontaneous action potential activity, resulting in tonic stimulation of the heart muscle and vasculature (coronary arteries). Thus, acute denervation of the heart and systemic blood vessels generally results in cardiac slowing and systemic vasodilation.

During the sympathetic innervation of the heart, the axons from the sympathetic neurons extend out of the medulla and run down the spinal cord to establish synapse within the intermediolateral cell column of spinal cord. Then they exit at specific thoracolumbar levels (T1-T2). Then these preganglionic fibers synapse within sympathetic paravertebral ganglia (cervical, stellate, and thoracolumbar sympathetic chain), which lie on each side of spinal column. They also establish synapse within prevertebral ganglia, such as the celiac, mesenteric, and inferior mesenteric ganglia.

Some sympathetic nerve fibers also extend to the adrenal glands where they make synapse there with local nerve cells. Meanwhile, postganglionic sympathetic fibers runs towards target organs, where they innervate arteries and veins. As they travel, they send small nerve branches towards the adventitia layer of blood vessels. Meanwhile, varicosities, which are tiny enlargements within the sympathetic nerve fibers, are the sites of neurotransmitter release.

Below, diagram/drawing of the heart sympathetic innervation, which is marked with the black straight and dotted lines and arrows arising form spinal cord and the paravertebral ganglionic chains that run parallel to the spinal column. The red line and arrow is the parasympathetic innervation carried out by the vagus nerve.

Heart Parasympathetic Innervation

The heart parasympathetic innervation is conducted by specialized nerve fibers that arise from the left branch of the vagus nerve (cranial nerve X), which is the longest of the twelve pair of cranial nerves. It belongs to the parasympathetic division of the peripheral nervous system. Innervation from these type of nerve fibers expands of the walls of arteries, dilating its diameter, and slow down the heart rate, and it happens when we are relaxed, free from stress. Its function is the opposite to that of the sympathetic innervation, which accelerates the heart beat when it is induced by adrenaline.

As it descends down the neck, the left branch of the vagus nerve runs down into the thorax between the left common carotid artery and the left subclavian artery. Having wound around the aortic arch, it gives off pulmonary branches that travel to the lungs to form the pulmonary plexus as well as the thoracic cardiac branches, which join the sympathetic nerve fibers that come from the sympathetic paravertebral ganglia to form the cardiac plexus. Having made their way into the heart, the parasympathetic fibers innervate the sinoatrial node, contributing to reducing the heart rate.

Cardiac Myocytes Metabolism

Cardiac myocytes metabolism plays a key role in the efficient contraction and relaxation of the heart muscle fiber. Every cell in the body uses fuel to produce energy so that it can function properly and divide. Unlike the rest of the body cells, the cardiac myocytes can employ four different types of fuel, with which their mitochondria generate adenosine triphosphate (ATP). These fuels are sugar (glucose), ketone bodies, fatty acid, and lactate, which are metabolically processed by the cells mitochondria to obtain the energy needed for the myocardium cells contraction. Red blood cells and liver hepatocytes can only use glucose, while nerve cells employs glucose and ketone bodies as fuel. Thus, the heart muscle cells are the most metabolically flexible of them all.

When the individual eats high amounts of carbohydrate in their meal, the heart muscle cells can adapt to burn the simplest form of carbs as fuel, which is glucose (sugar), to produce ATP through a process known as glycolysis to obtain pyruvate, which is further metabolized in the Creb's cycle of the mitochondria to produce ATP. However, when man is not eating carbohydrates (bread, potatoes, sugar, etc) but only meat, entrails, butter, and cheese, or when he is fasting, his heart myocytes can quickly and easily adapt to consume either fatty acids or ketone bodies, such as β-hydroxybuterate and acetoacetate, as sources of fuel to burn for the generation of energy. Ketone bodies is produced by the liver cells mitochondria from fatty acid, which derives from the saturated fat contained in the butter, meat, entrails, etc, man eats.

The fourth type of fuel the heart muscle cells employ is lactate, which a salt of lactic acid. Lactate can replace glucose as it becomes an alternative fuel during physical exercises, when concentrations of lactate in the bloodstream increase. Lactate is the byproduct of the skeletal muscle cells glycolysis. Although skeletal muscle fibers cannot use lactate as fuel (because is causes cramp), the myocardium cells can utilize it as fuel as long as they have enough oxygen available. This is so, because myocyte mitochondria can produce enough ATP for the myocardium contraction only in aerobic conditions (with plenty of oxygen).

Myocyte oxygen consumption increases sharply when the frequency of contraction (heart rate) and force of contraction are increased. Under these physiological circumstances, more oxygen must be delivered to the heart by the coronary arteries circulation to supply the myocytes metabolic demands. Biochemical messengers dilate the coronary blood vessels to allow additional blood flow and, with it, oxygen in greater amount.

Human Heart

The human heart is a hollow muscular organ, whose function is to pump blood, making it flow throughout a complex network of blood vessels (arteries, capillaries and veins) to provide the different body tissues and organs of the body with oxygen and nutrients, which are essential for life. This pumping function is autonomous as its muscle contractions does not depends on our cerebral cortex but on its own conducting system. The muscle of the heart is called myocardium as it is specialized striated muscle.

When filled with blood, it is about the size of a human fist and is located in the center of the chest, slightly to the left of your sternum (breastbone). As the body develops, the heart grows at the same rate as the fist. So an infant's heart and fist are about the same size at birth. The human heart is actually shaped like an upside-down pear. A double-layered membrane called the pericardium surrounds your heart like a sac. The outer layer of the pericardium surrounds the roots of your heart's major blood vessels and is attached by ligaments to your spinal column, diaphragm, and other parts of your body.

Anatomical Description

The heart has four cavities, which are called chambers; two atria and two ventricles. The two atria are not communicated but isolated from one another, with the two ventricles also being isolated from one another. Thus, the right atrium is communicated only with the right ventricle through the tricuspid valve, constituting the right side of the heart as only deoxygenated blood flows through them in its way to the lungs. Meanwhile the left atrium is connected to the left ventricle through the mitral valve as oxygen-rich blood flows through them, being pumped into every organ and tissue by the left ventricle contraction (systole).

The ventricles meet at the bottom of the heart to form a pointed base which slightly points toward the left side of your chest. The left ventricle contracts most forcefully, so you can best feel your heart pumping on the left side of your chest. A wall of muscle, called the septum, separates the right and left sides of the heart. The left ventricle is the largest and strongest chamber in your heart as its chamber walls are only about a half-inch thick, but they have enough force to push blood through the aortic valve and into your body.

Four valves controls the blood flow through your heart. 1) The mitral valve connects the left atrium with the left ventricle below it, allowing oxygen-rich blood from the lungs pass through. 2) The tricuspid valve connects the right atrium with the right ventricle. 3) The pulmonary valve controls blood flow from the right ventricle into the pulmonary arteries, which carry blood to your lungs to pick up oxygen. 4) The aortic valve opens the way for oxygen-rich blood to pass from the left ventricle into the aorta, which is the largest artery, delivering it to the rest of the body.

Electrical impulses that travels through nerves connected to the myocardium (heart muscle) cause the heart to contract. This electrical signal begins in the sinoatrial (SA) node, located at the top of the right atrium. The SA node is sometimes called the heart's natural pacemaker. An electrical impulse from this natural pacemaker travels through the muscle fibers of the atria and ventricles, causing them to contract. Although the SA node sends electrical impulses at a certain rate, the heart rate may still change depending on physical demands, stress, or hormonal factors.

From the moment of development through the moment of death, the heart pumps. The heart, therefore, has to be strong. The average heart's muscle, called cardiac muscle, contracts and relaxes about 70 to 80 times per minute without you ever having to think about it. As the cardiac muscle contracts it pushes blood through the chambers and into the vessels. The ventricles of the heart have two states: systole (contraction) and diastole (relaxation). During diastole blood fills the ventricles and during systole the blood is pushed out of the heart into the arteries.

Below, schematic image of the human heart, showing its four cavities and valves.

 

Left Atrioventricular Valve

The left atrioventricular valve is located in the inferior wall of the left atrium and it is attached to the atrioventricular orifice. It consists of two cusps, or flaps; an anterior cusp and a posterior one. This is the reason it is also called 'bicuspid' or 'mitral valve', because it resembles the top portion of a bishop miter. The two flaps project downwards into the cavity of left ventricle and close tightly during myocardial contraction (systole).

The right and left atrioventricular valve cusps are opened and closed by the cardiac muscle relaxation during diastole and the pressure of blood when the heart ventricles contract during systole, respectively. The chordae tendineae (tendinous chords) are attached to the edges of the flaps and to the papillary muscles, which are on the wall of ventricle. The function of the chordae tendineae and the papillary muscles is to prevent the cusps of the valve from collapsing and caving upwards by the blood pressure when the heart muscle contracts.

Function

The function of the mitral valve is to regulate the flow of oxygenated blood from the left atrium into the left ventricle. It blocks the blood flow during systole (contraction of myocardium) and opens to let it flow during diastole (relaxation). When the valve is damaged and does not work properly, there is mitral regurgitation, which is the backflow of blood from the left ventricle back into the atrium.

Below,  left aspect of open left side of heart. You can see both chambers with the left atrioventricular valve in between.

 

Right Atrioventricular Valve

The right atrioventricular valve is called 'tricuspid valve' because it consists of three cusps. These are triangular flaps which close and open simultaneously to regulate the flow of blood from the right atrium into the right ventricle during systole and diastole (cardiac muscle contraction and relaxation, respectively). Action: it prevents the backflow of blood from the cavity of the right ventricle into the cavity of the right atrium.

The right atrioventricular valve is composed of a medial cusp (cuspis septalis), an inferior cusp (cuspis posterior), and an anterior cusp (cuspis anterior). All three flaps project into the cavity of the right ventricle. The medial cusp is closest to the ventricular septum and is attached to the medial portion of the right atrioventricular orifice. The inferior cusp, on the other hand, is smaller and it is affixed to the postero-lateral periphery of the orifice. The anterior cusp is the smallest of the tree, and it is anchored to the anterior periphery of the atrioventricular orifice.

The three cusps of this valve shuts tight, checking the downward blood flow, during each myocardial contraction (systole), and open to let it flow again during muscle relaxation (diastole). The three flaps opens up during heart muscle relaxation (diastole) and they close tightly during cardiac muscle contraction (systole), due to the blood pressure. The chordae tendineae, which are string-like tendinous chords attached to the edges of the cusps, prevent them from caving upwards into the atrium during ventricular contraction. The papillary muscles are located on the wall of the ventricle.

The atrioventricular valve action of checking the blood flow tight is to avoid regurgitation, which is the backflow of deoxygenated blood from the right ventricle back into the right atrium. This can happen sometimes when the valve is damaged and there is valvulary insufficiency.

Below, longitudinal cut of the right side of heart (side view), showing the right atrium, right ventricle and the tricuspid valve.

Venae Cordis Minimae

The venae cordis minimae (smallest cardiac veins) are the small veins of the heart proper. They drain deoxygenated blood from the capillary network in the heart wall, especially those in the myocardium. They open through the foramina venarum minimarum (small venous orifices in the endocardium) on the atrial septum and the lower parts of the lateral and anterior walls of the atrium.

There are more venae cordis minimae in the right atrium than in the other three chambers. Their openings are found in the endocardium. They allow these tiny veins to return blood into the heart cavities. Thus, the smallest cardiac veins are an alternative venous drainage system of the myocardium.

Endocardium

The endocardium is the inner layer of the wall of the heart. It lines the four chambers of this organ. It is a thin sheet consisting of collagen, elastic fibers, smooth-muscle cells, and endothelium. The endothelial layer of the endocardium faces the cavities of the heart and lie on the smooth muscle layer.

Strictly histologically speaking, the endocardium is the continuation of the inner coat of the blood vessels that arise from the heart; that is to say, the aorta, the pulmonary trunk, vena cava, and pulmonary artery. The endocardium is thicker in the atria than in the ventricles, and it is thickest in the left atrium, and it is thinner in the areas where it covers the papillary muscles, with the cordae tendineae and the trabeculae carneae.

In the thinnest areas of the atrial walls, where there are spaces in the myocardium, the endocardium comes into contact and it even fuses with the epicardium (the heart outer layer. In the regions of the fibrous rings and atrioventricular, aortic and pulmonary orifices, the endocardium forms folds.

Left Ventricle

The left ventricle of the heart has an elongated shape and it is slightly larger than the right. From an anterior view perspective, it is located to the left, to the back, and downwards in relation to the other three cavities. Its narrow and anterior-inferior part corresponds to the apex of the heart. The boundary between the right and left ventricle on the surface of the heart correlates with the anterior and inferior interventricular grooves.

Although the cavity of the left ventricle is narrower than that of the right, it is slightly larger. On transverse section, the chamber of this ventricle is slit-like at the apex but it gradually becomes oval nearer to the base on top. The posterior-superior portion of this cavity communicates with the left atrium by means of the left atrioventricular orifice in whose circumference the left atrioventricular (mitral) valve is attached. This valve is made up of two cusps which projects downwards into the cavity.

The interventricular septum separates the left from the right ventricle. The rest of the myocardium on the left ventricle is much thicker than on the right ventricle. This is due to the fact that it has to pump a larger volume of blood than the right does. The right has to pump it only into the lungs, while the left ventricle has to pump to the rest of the body, from the head to the feet.

The left ventricle receives oxygen-rich blood from the left atrium via the mitral valve. This oxygenated blood flows into the left atrium from the lungs through the pulmonary vein. During systolic contraction, the left ventricle sends this oxygen-rich blood to the body through the aortic valve and aortic artery.

Below, the heart cut longitudinally to exhibit the four chambers (cavities) of the heart. Notice that the left ventricle cardiac muscle wall is much thicker than on the right and it is also larger, especially in the posterior portion.

 

Myocardium

The myocardium is the cardiac muscle, which forms the middle and the thickest layer of the heart wall. The rhythmic contraction of its fibers makes it possible the constant blood circulation throughout our bodies to supply every tissue and organ with oxygen and nutrients. Thus, the cardiac muscle is the key player and the essential part of the heart function, which is to pump blood and keep us alive. (The other two layers of the heart wall are the epicardium and the endocardium).

Like the skeletal muscle, the myocardium is striated muscle tissue. However, the heart fibers are joined together end to end and are cylinder-shaped. They are also arranged in a different fashion and their contraction is not voluntary but automatic and independent of our will. This functional autonomy is carried out by highly specialized cardiac fiber cells organized around the sinoatrial node, the atrioventricular node, the bundle of His, and the Purkinje fibers, with the first one initiating the chemical-electrical impulses that spread throughout the other three structures, contracting the myocardium. These impulses, in turn, are reinforced by sympathetic and parasympathetic nerve fibers, which arise from the autonomic nervous system and form the cardiac plexus.

The cardiac muscle consists of a dense aggregation of muscle cells, which are called cardiomyocytes. These elongated muscle fibers which are arranged longitudinally and transversally (because they branch out diagonally). They are attached to one another, end-to-end, by a specialized proteins called intercalated disks, with fascia adherens playing an important role in holding the muscle fibers together.

The myocardium surrounding the two atria is composed of two muscular layers; a superficial and a deep one. The thickest part of the cardiac muscle is the portion surrounding the two ventricles, especially the left one. This is called ventricular myocardium, which, along with the interventricular septum, is made up of three muscular layers. The deep layer is formed by bundles running upwards from the apex; the middle layer, in contrast, consists of circular muscle bundles that surround the ventricles; the outer layer, on the other hand, is relatively thin and consists of oblique rounded and flattened bundles.

The reason for the myocardium to be thicker on the ventricles is due to the fact that it makes a stronger exertion (with all the pumping power) at each ventricular contraction during systole to drive the blood to the lungs (right ventricle) and to the rest of the body (left ventricle).

Below, the myocardium of the heart, with the three layers exposed in the area around the ventricles.

Anterior aspect of human heart, which has been stripped of the pericardium to expose the myocardium.

Right Ventricle

The right ventricle is one of the four chambers (cavities) of the heart. It lies in the lower right portion of this organ, just below the right atrium. It is separated from the left ventricle by the interventricular septum, which is a muscular wall dividing and isolating the two lower chambers from one another. The right ventricle receives deoxygenated blood from the right atrium, with which it is directly communicated through the tricuspid valve.

Anatomical Description

The right ventricle is shaped like a three-sided pyramid, whose base faces upwards, towards the right atrium. The apex, on the other hand, is directed downwards and to the left. The anterior wall of this lower chamber bulges forwards, while its posterior wall is flat. Meanwhile, its medial wall, which is the interventricular septum, bulges into its interior (inwardly), thus being slightly convex from the right perspective.

Function

Like the left, the right ventricle function is to pump blood upon each systolic contraction of the heart. However, there is a difference, because the left lower chamber pumps oxygenated blood that comes from the lung into the rest of the body to supply every tissue and organ with oxygen, while the right ventricle pumps deoxygenated blood into the lungs. This deoxygenated blood exits the right ventricle through the semilunar valve into the pulmonary artery.

Below, image of human heart, showing the left and right ventricle.

Most Venomous Snakes

The most venomous snakes are found world-wide and they belong to two different families of serpents; the Elapidae (elapids) and the Viperidae (vipers). The main difference lies in the type of venom they inject into the subcutaneous and muscular tissue when they bite. The elapids produce neurotoxic venom, which means it attacks the nervous system, affecting the nerve impulses, while the vipers inject cytotoxic and hemotoxic venom, which means that it destroys body tissues.

The neurotoxic venom of elapids causes paralysis, with the patient usually dying from respiratory failure as the the diaphragm and the intercostal (rib) muscles cannot be expanded and contracted. The cytotoxic venom of vipers causes necrosis and extreme fever and pain.
Most venomous snakes

1- King Cobra (Family: Elapidae. Genus: Ophiophagus). Also known as hamadryad, it is found in India and other countries of Southeast Asia. It features an extremely potent venom and the side hoods when it is provoked and feels cornered.

2- Black Mamba (Family: Elapidae. Genus: Dendroaspis). It is from Central and Southern Africa. Although its venom is not as potent as the King Cobra, it is aggressive and its fangs are located well forward in its upper jaw, enabling it to bite bigger body parts, such as the thigh, and inject larger doses of neurotoxic venom.

3- Taipan (Family: Elapidae. Genus: Oxyuranus). It is endemic to Australia. Although some snake specialists consider it to be the most venomous, it is not as aggressive as the black mamba and the cobras from Asia.

4- Bitis Gabonica (Family: Viperidae. Genus: Bitis). It is also known as Gaboon viper, its natural habitats are the African savannas of Central Africa, from Ghana to Mozambique on the eastern coast. It is considered to be among the most venomous. It has a "dry-leaves" skin pattern, which enables it to blend in perfectly in a dry vegetation background. If you get bitten, it is an almost sure death, if you do not receive urgent medical treatment.

5- Tiger Snake (Family: Viperidae. Genus: Notechis). An extremely venomous snake from Australia and the island of Tasmania.

6- Rattlesnakes (Family: Viperidae. Genus: Crotalus). They are found in North, Central, and South America, especially in the dry regions of the United States and Mexico. There are several species of rattlesnakes, such as the timber, the diamondback, and the horned rattlesnake.

Below, the King Cobra has the most potent neurotoxic venom in the world as death is certain in less than one hour after being bitten.

 

Yellow-Bellied Sea Snake

The yellow-bellied sea snake (Pelamis platurus) is an aquatic serpent (legless squamate reptile) found in Southeast Asia. It drifts in surface sea current, hunting small fish which shelter amid floating debris. The powerful venom causes paralysis and death of prey. This snake belongs to the Elapidae family, which means it has large hollow front fangs as it glands secrete neurotoxin venom.

Physical Characteristics

The yellow-bellied sea snake can grow to measure 70 cm in length (more than two feet). Its belly and the lower half of its body is bright yellow and its back is black or dark grey, with smooth scales and the head large symmetrical scales. Its fangs are large and located in front of upper jaw.

Natural Habitats

The yellow-bellied sea snake can be seen in the East Indies (Indonesia), the southern tip of India, and the southern, East African coastal waters. Vagrants are washed south in the Agulhas current from tropical seas around Madagascar. From 3 to 5 young are born in the surface waves at sea.

Below, photograph of a yellow-bellied sea snake on the coast of southeastern Africa.

 

Shield-Nosed Cobra

The shield-nosed cobra is a venomous snake which belongs to the Elapidae family. Its natural habitat is the southern portion of Africa, south of Uganda and Tanzania. Being less brightly colored than the coral snake, it is usually brown and black, with a white throat band. Its scales are arranged between 21 and 25 rows at mid body. All along its belly, the scales are much larger than those located on the rest of the body.

The shield-nosed cobra is short, when compared to other elapid snakes. The adult measures between 50 and 60 cm in length. When it feels cornered, it rears up and spread its narrow hood, huffing and puffing a lot. However, it does not always bite, for sometimes it strike with a closed mouth. It lays up to 12 eggs in summer time. It burrows in loose sand and feeds on rodents and frogs.

Below, the shield nosed cobra of Africa.

Extensor Carpi Radialis Longus

The extensor carpi radialis longus is a spindle-shaped muscle. It has a narrow tendon, which is much longer than its belly. The upper portion of this muscle is covered by the brachioradialis, while its distal part is crossed over by the abductor pollicis longus and the extensor pollicis brevis muscle.

The extensor carpi radialis longus muscle arises from the lateral epicondyle of humerus, and the lateral intermuscular septum of upper arm. Then it extends downwards all along the lateral border of forearm, lying between the brachioradialis and the extensor carpi radialis brevis muscle. In the third distal portion of forearm, it becomes a narrow tendon, which runs under the extensor retinaculum. It is inserted into the base of the dorsal surface of the second metacarpal bone (of index finger side).

Action/Function 

It extends and twists the wrists radially sideways. It is also a weak flexor of the elbow-joint, assisting the brachialis muscle. It also extends the hand at the wrist joint, aiding in its abduction.

Innervation

It is innervated by the radial nerve (C5-C6-C7), which springs from the brachial plexus.

Blood Supply

The extensor carpi radialis longus muscle receives oxygenated blood from branches originating from the radial collateral and recurrent artery.

Below, two schematic pictures showing the two radialis muscles of right forearm.

 

Extensor Carpi Radialis Brevis

The extensor carpi radialis brevis is a muscle of the forearm which belongs to the lateral group of extensor muscles. It lies on the radial and lateral aspect of forearm. It is slightly and proximally covered by the extensor carpi radialis longus; distally, it is crossed over by the abductor pollicis longus and extensor pollicis brevis muscle.

The extensor carpi radialis brevis muscle arises from the lateral humeral epicondyle and the collateral and annular ligaments. Then it runs downwards along the length of forearm, ending up in a tendon, which travels through the synovial sheath of the tendons of the radial extensors of wrist. It is inserted into the base of the third metacarpal bone.

Action

This muscle extends the hand at the wrist joint, abducting it a little.

Nerve Supply

The extensor carpi radialis brevis is innervated by branches of the radial nerve (C5, C6, C7).

Blood Supply

It receives oxygen-rich blood from the deep brachial (profunda brachii) and the recurrent radial artery.

Below, the extensor muscles of the forearm, showing the extensor carpi radialis brevis.

 

Extensor Carpi Ulnaris

The extensor carpi ulnaris muscle lies on the medial border of the posterior surface of forearm (on the little finger side). It has a long, spindle-shaped belly. It originates from the lateral epicondyle of humerus, the posterior border of ulna, and the capsule of the elbow-joint.

From its points of origin, the extensor carpi ulnaris extends down, slightly obliquely, as it crosses over to the ulnar side of forearm. When it reaches the distal portion of ulna, it becomes a tendon that runs under the extensor retinaculum of wrist. Finally, it is inserted into the base of the dorsal surface of the fifth metacarpal bone by the synovial sheath of the extensor carpi ulnaris tendon.

Action

It abducts the hand to the ulnar side. It also extends it at the wrist-joint.

Innervation

This spindle-shaped muscle is innervated by the radial nerve (C5-C6-C7).

Blood Supply

The extensor carpi ulnaris muscle receives oxygen-rich blood from the posterior interosseous artery.

Image of posterior compartment of forearm, showing the extensor muscles

Extensor Digiti Minimi

The extensor digiti minimi is a superficial muscle which lies on the posterior (dorsal) aspect of forearm. It is long, thin, and spindle-shaped, being located directly under the skin. It arises from the lateral epicondyle of humerus, the lateral ligament of elbow, and the antebrachial fascia. Then it stretches downwards and slightly obliquely, switching over to the ulnar aspect of forearm (little finger side).

In the distal portion of ulna, it becomes a tendon, which gets lodged in the synovial sheath of the extensor digiti minimi tendon. Once it has left this sheath, it joins the extensor digitorum tendon to be inserted into the base of the distal phalanx of little finger.

Action/Function

It extends the little finger.

Innervation

It is supplied by the radial nerve (C5-C6-C7-C8).

Blood Supply

The extensor digiti minimi muscle gets oxygenated blood from branches of the posterior interosseous artery, which arises from the common interosseous.

Below, superficial muscles of posterior compartment of right forearm, with the extensor digiti minimi clearly labeled.

Extensor Indicis

The extensor indicis is a deep muscle of human forearm, which lies on its posterior side, underneath the extensor digitorum. It has a narrow, elongated, and spindle-shaped belly.

The extensor indicis muscle arises from the lower third of dorsal (posterior) surface of the ulna. Stretching downwards and slightly obliquely, the it ends up as a long tendon that runs under the extensor retinaculum. Together with the extensor digitorum tendon, it passes through the synovial sheath. Finally, it is inserted in the tendinous expansion of dorsal surface of index finger.

Action

This long spindle-shaped muscle extends straight the index finger, as when we point at something or to give a sign.

Innervation

The extensor indicis muscle is innervated by a branch of the radial nerve (C6-C7-C8).

Blood Supply

It receives oxygenated blood from the posterior and anterior interosseous artery.

Below, schematic image that shows the extensor indicis and other extensor muscles of forearm

 

Extensor Digitorum Muscle

The extensor digitorum muscle is a long bipennate muscle which is located near the medial border of posterior side of forearm. Having a spindle-shaped belly, it arises from the lateral epicondyle of humerus, the capsule of the elbow-joint, and the antebrachial fascia. Then the extensor digitorum runs downwards, along the full length of forearm. Being part of the superficial layer of muscles, it lies directly under the skin, stretching over the supinator, the abductor pollicis longus, and extensor pollicis longus muscle.

Before it reaches the distal third portion of forearm, the belly of extensor digitorum divides into four tendons which travel under the extensor retinaculum. Then these tendons are enclosed, together with the extensor indicis tendon, in a common synovial sheath. Finally, having reached the metacarpal bone, each one of the tendons ends in a tendinous expansion which fuses with the capsule of the metacarpophalangeal joint. Each one of the tendinous expansion in turn separates into three slips, with the lateral two being inserted into the base of the distal phalanx and the middle one into the base of the middle phalanx.

Action

This bipennate muscle of forearm extend the four fingers and assists the extension of hand at the wrist.

Innervation

The extensor digitorum muscle is innervated by the posterior interosseous branch of the radial nerve (C6-C7-C8).

Blood Supply

This muscle is supplied by branches of the posterior interosseous artery, which is arises from the common interosseous and this, from the ulnar.

Below, the superficial muscles of right forearm, showing the extensor muscles

 

Flexor Pollicis Longus

The flexor pollicis longus is one of several muscles of the human thumb. It is located on the lateral border of anterior side of forearm. It is long and unipennate (having a feather-like form on one side).

This the flexor pollicis longus arises from the upper two-third of the anterior surface of radius and interosseous membrane as well as from the medial epicondyle of humerus. Then it runs down the forearm, parallel to the flexor digitorum profundus, as it becomes a long tendon which travels through the carpal tunnel. Next it is invested by the synovial sheath of the flexor pollicis longus tendon to finally be inserted into the base of the distal phalanx of thumb.

Action

Upon contraction, this long muscle of forearm flexes the distal phalanx of thumb, pulling it towards the palm of hand.

Innervation

The flexor pollicis longus is innervated by the median nerve (C6-C8).

Blood Supply

It receives oxygen-rich blood from muscular branches of the radial, ulnar and anterior interosseous artery.

Below, picture of third-layer muscles of forearm: flexor pollicis longus and flexor digitorum profundus muscle

Flexor Carpi Ulnaris

The flexor carpi ulnaris is a first-layer muscle which occupies the medial border of forearm (on the little finger's side). It has a long belly and a rather thick tendon. It arises by two heads; its humeral head originates from the medial epicondyle of humerus and the intermuscular septum, while its ulnar head springs from olecranon of ulna and the two upper third of superficial dorsal fascia of forearm. (An intermuscular septum is an aponeurotic sheet of connective tissue diving two muscles).

After emerging from its two points of origin, the flexor carpi ulnaris muscle travels downwards, superficially along the medial border of forearm, parallel to ulna. Then it tapers to a tendon which runs under the flexor retinaculum to be inserted into the pisiform of carpus (the small bones that make up the wrist). However, some of the fibers of the flexor carpi ulnaris keep stretching forward together with the pisometacarpal and pisohamate ligaments, which are in turn inserted into the hamate and the fifth metacarpal bone.

Action

This long muscle of forearm flexes the wrist (and hand) inwardly, towards the ulna.

Innervation

It is supplied by collateral nervous fibers originating from the ulnar nerve (C7-C8-T1).

Blood Supply

This long muscle receives oxygenated blood from the superior ulnar collateral, supratrochlear brachial, and ulnar artery.

Below, schematic drawing of the superficial anterior muscles of forearm, with the flexor carpi ulnaris extending along its medial border.

Flexor Digitorum Profundus

The flexor digitorum profundus (deep flexor of fingers) is a longitudinal muscle located deep in the forearm, underneath the flexor digitorum superficialis muscle. Its upper end is broader and thicker than its lower end, arising from the anterior and medial surface of the ulna. Running down along this bone, the flexor digitorum profundus ends up in four long tendons that travel through the carpal tunnel into the hand. These tendons extend along the four medial fingers.

Action/Function

The flexor digitorum profundus closes the four fingers, flexing the proximal and distal interphalangeal joints. It also flexes the metacarpophalangeal joints. Although it is situated in the forearm, it is an extrinsic (external) muscle of the hand, because its contractions act on the hand, closing the fingers and flexing in unison with flexor digitorum superficialis and the flexor pollicis longus of the thumb, when we want to grip or grasp something.

Innervation

The lateral half of the flexor digitorum profundus is innervated by the anterior interosseous nerve, which in turn originates from the median nerve. The medial half of this muscle is supplied by the ulnar nerve.

Blood supply

This forearm muscle of the hand receives oxygen-rich blood from the side branches of the ulnar artery (common interosseous arteries).

Below, drawing/diagram of deep muscles of forearm, showing the flexor digitorum profundus

 

Flexor Digitorum Superficialis

The flexor digitorum superficialis is a second layer muscle of the forearm. It is covered by the palmaris longus, the flexor carpi radialis, and the pronator teres muscle. It originates by two heads: the humero-ulnar head arises from the medial epicondyle of the humerus and the coronoid process of the ulna, while the radial head arises from the proximal palmar surface of radius. The two heads meet to form a common belly that extends downwards along the length of the forearm as it gets narrow, ending up in four tendons. These tendons run into the hand through the carpal tunnel as they fan out to be inserted into the middle phalanges of fingers. This forearm muscle is also called flexor digitorum sublimis.

Action/Function

It flexes the middle phalanges of all four fingers: index, middle, ring, and little, closing them.

Innervation

The flexor digitorum superficialis is innervated by branches of the median nerve, which arises from the brachial plexus (C7, C8, T1).

Blood Supply

It receives oxygenated blood from collateral branches from the radial and ulnar artery.

Below, schematic drawing of the muscles of forearm, second layer, with the flexor digitorum superficialis.