Endocrine System

The endocrine system is, along with the nervous system, the main controller of bodily functions. Both systems are coordinated to carry out their functions and they control each other: the nervous system controls the production and secretion of hormones and some hormones can control the nervous system.

Both systems have, however, some relevant differences. The nervous system carries out its functions very quickly, in milliseconds. And it controls punctual or not very lasting actions. The endocrine system, on the other hand, carries out its functions more slowly, in seconds or even minutes. And it controls lasting actions, that can take minutes, such as the vasodilation promoted by some hormones, hours, such as the digestive process or even years, such as growth.

The endocrine system is the main regulator of the homeostasis and metabolism (both anabolism and catabolism). Its functionality is based on the secretion of chemical substances called hormones. They are produced and released by endocrine glands.

Locomotor System: Muscles

Muscular System

Muscles in the head

• Frontal: it moves the scalp forwards. It raises eyebrows.  It wrinkles the forehead skin.
• Occipital: it moves the scalp backwards. It is joined to the frontal muscle by the aponeurosis that covers the upper part of the cranium. 
• Nasal: there are two nasal bones. They wrinkles the nose.
• Buccinator: it inflates the cheeks.
• Orbicularis oculis: it closes the eyes.
• Orbicularis oris: it closes and presses the lips together. It can also push them forwards.
• Risorius: it pulls the lip commissure sideways. Like smiling
• Supercilii: it pulls the internal part of the eyebrows down. Like frowning.
• Zygomatic major: it moves the extremes of the lips upwards. Like laughing.
• Masseter: it closes the mouth raising the mandible.
• Temporal: it raises and retracts the mandible. If one of them relaxes and the other contracts, the mandible moves laterally.
• Levator palpebrae superioris: it raises the upper eyelid.
• Levator lavi superioris: it raises the upper lip.
• Digastric: it raises the hyoid bone and descends the mandible to open the mouth.
• Mentalis: it raises the central part of the lower lip. 

Locomotor System: Muscular Function

Introduction

Muscles are responsible for providing the bodily movements. There are also muscles that do not move bones, but provide involuntary movements of internal organs, such as the peristaltic movements of the intestine and the contraction of blood vessels. Finally, when the muscles contract they produce heat (consuming energy).

There are three different types of muscle: smooth, cardiac and skeletal. When we are talking about the locomotor system, however, we are only referring to skeletal muscles, that provide general movements of the skeleton.

The muscular system is an important part of our body. It is 40 % of our total weight.

Structure of muscular fibres

All the skeletal muscles are surrounded by a layer made up of connective tissue called epimysium. The muscle is divided into fascicles by a connective membrane called perimysium. The fascicles are made up of several cells called muscular fibre. Each muscular fibre is surrounded by a thin connective membrane called endomysium. These three membranes join at the edge of the muscle. After the fusion of these membranes, the connective tissue becomes richer in elastic and cartilaginous fibres, forming the tendon. The tendon firmly connects the muscle to the bone.

Skeletal muscle.

The muscle cells that make up the skeletal muscles, called myocytes, are cylindrical and extremely long. Indeed, they can be more than five centimetres long. They have many nuclei, even more than one hundred nuclei per cell.

Locomotor System: Joints

Joints (Articulations)

Joints are structures responsible for joining different bones. They support the weight of the body and allow the movement of bones. 

According to their movement, articulations can be classified as:

• Synarthrosis: they do not allow any movement. The joints of the cranial bones are the most typical examples.
• Amphiarthrosis: they allow slight movements. The joints of the vertebrae are the most typical examples. 
• Diarthrosis or synovial joints: they allow complete movements. The bones are linked by ligaments. And the part of the bones in contact with other bones are in covered by cartilage. The space between the cartilaginous pieces that cover the bones in the diarthrosis are filled with a liquid that prevent them from friction and it is called synovial fluid. 

According to the type of movement, the synovial articulation can be classified as:

Locomotor System: Skeleton

Skeleton

Introduction

The skeleton is the main system to support the body. It forms an internal hard structure that supports other organs, protecting them. Some delicate organs are enclosed in a sort of armour made of bones. The brain, for instance, is enclosed in the cranium and the lungs and heart are protected by the rib cage.

The skeletal muscles are attached to bones and the contraction of different muscles move the bones they are joined to, providing movement to the body.

The bones are, besides, the main reservoir for calcium in human body. When the calcium is required in the blood, it is extracted from the bones.

Finally, in the bone marrow, which can be found in the interior of large bones. The haematopoiesis process is carried out to produce blood cells.

The bones have three parts:

• Periosteum: it is the outermost layer that surrounds the bone. It is made up of connective tissue and it is related to the growth the bone thickness.
• Compact bone: it is the hard part of the bone and its main structural component. It is made up of a hard extracellular matrix, rich in collagen and calcium, and cells that maintain this matrix that are called osteocytes. Another less abundant type of cells are the osteoclasts, located in the interior part of the compact bone and responsible for destroying extracellular matrix in order to release calcium into the blood. The extracellular matrix is extremely ordered, forming cylindrical structures called osteons (also known as harversian systems). In the centre of the osteons there is canal where the blood vessels and nerves can be found. Each bone is, in its compact part, made up of thousands of parallel osteons.
• Spongy bone: it is in the interior of large bones. It is a complex network of tissue that forms a trabecular system, similar to a sponge. The interior of this trabecular system is the place where the stem cells responsible for producing blood cells are nested. Due to this, it is the place where the haematopoietic process occurs.

Locomotor System: Planes and Body Regions

Introduction

The locomotor system is responsible for sculpting our corporal structure, shaping our body, promoting the movements and protecting some delicate organs.

It is made up of two big parts: skeletal and muscular systems.

Planes and body regions

The different parts of the body

Just before studying the skeletal and muscular systems, we must describe the different body regions and the body planes. It makes finding or locating different anatomic parts, bones and regions easier.

The body planes are layers that cross the body. There are different planes according to how they transect the body. The three most relevant are:

• Frontal plane: it longitudinally crosses the body, dividing it into anterior and posterior parts.
• Sagittal plane: it is perpendicular to the frontal plane and divides the body into right and left parts.
• Transversal plane: it crosses the body transversally and divides the body into upper and lower parts. It is transversal to the sagittal and frontal planes. 

The Lymphatic System

It is made up of a fluid called lymph, the vessels that transport this liquid, called lymphatic vessels and a group of structures and organs related to the lymph.

The lymphatic system has three essential functions:

• Draining the interstitial liquid.
• Transporting fats from the digestive system to the blood.
• Defending our body. Lymph transport many leucocytes and some lymphatic organs and structures are related to the immune system.

There are two types of lymphatic vessels.

• Lymphatic capillaries: they are small, thin vessels, made of endothelial cells with bottle-like shape. This shape is related to the junction between successive cells, forming a valve that makes the lymph move only in one direction. These junctions, besides, can open and close to allow the interstitial liquids, the substances and the defensive cells to enter the lymphatic capillaries.
• Lymphatic vessels: they are vessels with thin walls and many valves that prevent lymph from flowing in the opposite direction. The lymphatic system doesn't have any organ to pump the lymph, so preventing the reflux is essential. The lymphatic vessels are formed when the capillaries join and, just like the blood vessels, they have anastomoses. The biggest lymph vessels are called lymphatic trunks. Through these vessels our body can transport between two and four litres of lymph. The lymph moves due to the peristaltic contractions of the muscular wall that can be found in the lymphatic vessels. Indeed, they can contract several times per minute. 

The big vessels, besides, run between skeletal muscles that press the vessels when they contract, draining the lymph and forcing it to move. This process is known as milking. All the blood vessels end up in to big trunks that are connected to the circulatory system in two points: the angle that form the left internal jugular vein and the left subclavian vein, and the angle that form the right internal jugular vein and the right subclavian vein.

Regarding the lymphatic organs, the most relevant ones are these:

• Lymphatic ganglions: they are oval in structure, with variable size, from one to twenty five centimetres in diameter, that are located along the lymphatic vessels. The human body has between six hundred and seven hundred ganglions, isolated or forming groups. Sometimes these groups are divided into superficial and deep groups. They are made up of an external capsule and an internal trabecular system. The capsule is made of dense connective tissue. The interior of the ganglion builds up lymphocytes and macrophages. The ganglions have two main functions. They can be used as reservoirs for lymph. The most relevant function, however, is the filtration of lymph, detecting invaders, such as viruses and bacteria that are trapped in the interior of the ganglion and attacked by the defensive cells. 
• Tonsils: they are an aggregation of lymphatic ganglions forming a ring that surrounds the pharynx. They protect the body against inhaled or ingested invaders.
• Spleen: this is an oval organ, around twelve centimetres long, located in the upper left part of the abdomen, between the diaphragm and the stomach. It is the place where lymphocytes B grow. It is also related to the destruction of damaged erythrocytes and lymphocytes. Blood can also be built up in this organ, being released it to the circulatory system when it is required.
• Thymus: bilobulated organ located in the upper part of the mediastinum. It is the place where the lymphocytes T grow. It is really active in children, becoming inactive in many adults.