The skeleton is made up of bones with the main roles of support and protect the body, as well as allowing the movement of vertebrate animals. Striated skeletal muscles are usually associated with bones and cartilage, although they can also exist as free muscles. Muscles are attached to bones through tendons. Movement is produced by muscle contraction and relaxation. We are going to include bones, tendons, and skeletal muscles as components of the motor system.
Skeleton
The skeleton consists of bones (Figure 1). It performs three main functions: supporting body structures, protecting vital organs, and forming stiff structures (long or short bones) that the muscles can move in order to move parts of the body or the entire body. The skeleton is also a store for calcium minerals, blood cells, and an energy reservoir in the form of fat in adipocytes.
Mechanical support. The skeleton is a scaffold for supporting and organizing the body's structures and organs. Without the skeleton, the rest of the body would collapse like soft dough. For instance, the brain is a very soft tissue that will spill out if not encased by the skull. The viscera are encased within the thoracic compartment and are supported by the spine. Muscles, although more consistent, also keep their position and organization through their attachment to the bones. The shape and size of our body largely depend on our bones. For instance, consider the length of the limbs and their components.
Protection. The animal body contains many delicate organs, such as the nervous system (encephalon and spinal cord), the lungs, and the heart. These organs need to be protected against strokes and mechanical insults. The nervous system is protected by the skull and vertebrae, whereas the ribs and breastbone protect the lungs and the heart.
Movement. The coordinated action of muscles and bones in our limbs allows us to go from one place to another. They are also necessary for the movement of body parts, like the head, fingers, and for making postural adjustments. The bones act as levers connected through tendons to the muscles, which provide the power for movement.
Calcium storage. Calcium is found in the extracellular matrix of bones in the form of hydroxyapatite mineral. When the mineral is dissolved, the calcium is freed and can enter the bloodstream to reach whatever part of the body.
Blood cell production. In the spongy bone, many hematopoietic cells are found within bone cavities (lacunae). They are stem cells that proliferate and differentiate into blood cells. The bone marrow includes the hematopoietic stem cells as well as all cell types found in the bone cavities.
There are 206 bones in the human body (300 in babies, as some bones fuse during development). Bones are living structures containing blood vessels and nerves, undergoing continuous renewal through the activity of bone cells: osteoblasts, osteoclasts, and osteocytes. As mentioned before, various cell types are found in the bone marrow. A typical bone consists of an outer layer of compact bone, where the spongy bone is found within the bone structure.
Bones can be classified in different ways. Regarding their position, there are axial and appendicular bones. The axial skeleton is found along the middle line of the body, consisting of 80 bones that include the skull, the hyoid bone, auditory ossicles, ribs, the breastbone, and vertebrae. The appendicular skeleton, with 126 bones, includes the fore and hind limbs, pelvis, and shoulders. In addition to their location, bones can also be classified based on their shape, the bones are classified into large, short, flat, irregular, sesamoid, and sutural bones.
Long bones mostly consist of compact bone tissue and a lower proportion of bone marrow. These bones are primarily found in the limbs, providing structural support for the body and allow movement. The long bones of the forelimbs are the humerus, ulna, radius, metacarpals, and phalanges, while the femur, tibia, fibula, metatarsals, and phalanges form the hind limbs. Each long bone is divided into diaphysis and epiphysis.
The diaphysis is the shaft of the long bones, representing the central region. It is a tubular structure compose of bone tissue containing bone marrow within its lacunae. The wall of the diaphysis is formed of dense and hard compact bone. A significant amount of adipose tissue is found in the so-called yellow bone marrow, located in the inner part of the diaphysis.
The epiphysis forms the ends of long bones. In each bone, the epiphysis shows a morphology that is adapted to accommodate the next bone. The epiphysis is a component of the joints. The wall of the epiphysis is made up of compact bone, while the inner region consists of spongy bone, which contains a large amount of bone marrow responsible for the production of erythrocytes, and it is highly supplied by the circulatory system. A structure called an "epiphyseal plate" is observed in the epiphysis. This plate is essential for the growth of new bone, contributing to the elongation of the bone during its development. The epiphysial plate is also known as the growth plate.
Short bones are wider than they are long. Examples of these include the bones of the wrist and the ankle. These bones lack a diaphysis. They consist of spongy bone surrounded by compact bone. Bone marrow is found within them. There are 32 short bones in humans.
Flat bones possess a thin and curved form. They consist of one to several layers of compact bone encasing an central layer of spongy bone. Flat bones, like the skull and sternum, protect against mechanical damage.
Sesamoids are bones embedded in tendons. These bones protect the tendons.
Irregular bones have shapes that do not fit into the previous mentioned bone categories. They include vertebrae, facial bones, and pelvic bones. They usually perform a protective function.
Muscles
Muscles provide the power for bodily movements or for maintaining posture. There are about 650 muscles in the human body (Figure 2). Together with bones, muscles contribute to the shape of the body and are about half of the total weight of the body. Most muscles that allow movement are composed of striated skeletal muscle cells, connective tissue, blood vessels, and nerves, connected to the bone through tendons. However, some muscles are directly attached to the bone. Some muscles, such as the tongue and specific facial muscle responsible for expressions, produce movement without being connected to any bone. The digestive tract is moved and the diameter of the arteries changes by the contraction of smooth muscle cells. In the same way, the heartbeat is an independent movement produced by the cardiac muscle cells. We will deal with the muscles associated with bones.
The skeletal striated muscle cells form the skeletal muscles of our body. These cells are also called muscle fibers or myocytes. In addition to muscle cells, muscles are also made up of blood vessels, nerves, and connective tissue. Muscle cells are packaged into fascicles, and several fascicles form a muscle (Figure 3). Each muscle cell is wrapped by a layer of connective tissue called the endomysium; each fascicle is enveloped by the perimysium, which is a layer of dense connective tissue; and the epimysium is the connective sheath that covers the entire muscle. The blood vessels and nerve fibers enter the muscle and branch many times through these connective layers. The nerves control the muscle contraction. Skeletal muscles produce voluntary movement by shortening the muscle cells.
The numerous muscles that form the body of the animals can be classified according to their position. The axial muscles are those related to the spine and tail, and the appendicular muscles are involved in limb movements. A few muscles are the branchiomeric muscles, which are related to the jaws and the hyoid apparatus.
Tendons
Tendons connect muscles with bones, so they are intermediaries between these two components of the motor system. Tendons are also involved in stabilizing joints, such as in the knees and back. Regular, dense connective tissue is the main component of tendons. The fibers of the extracellular matrix, mostly thick collagen fibers, are oriented parallel to the axis of the mechanical strength, which confers great resistance to stretching.
Tendons are primarily composed of dense, regular connective tissue with thick bundles of collagen fibers and cells known as tenocytes. Tendons are enveloped by a sheath of connective tissue called the peritenon. This layer is in contact with the collagen bundles of the dense regular connective tissue and extends inward, forming walls of tissue that carry blood vessels and nerves. There is an outer layer, known as the paratenon, which covers the whole tendon surface. This layer is in contact with the tendinous sheath (vaina tendinis) and is continuous with the perimysium of the muscle. The epitenon facilitates the movement of the tendon and reducing friction with adjacent tissues. It is made up of an inner region containing capillaries and nerves, which is covered by a layer of simple squamous epithelium that resembles mesothelium. The outermost layer (epitenon or synovial layer) is a highly fibrous layer. A liquid solution, similar to the synovial fluid, is found between these two layers.
The attachment point of the tendon to the bone is known as the myotendinous junction, which is a complex structure. At the end of the skeletal muscle cells, the plasma membrane deeply folds, and tendon collagen bundles are inserted in these invaginations. On the bone side, the connective tissue of the tendon integrates with the bone matrix, with a transition of fibrous cartilage that gradually mineralizes towards the bone. The tendon collagen fibers enter the bone by crossing the periosteum, forming the so-called Sharpey's fibers. Macroscopically, the bone surface is rough, with small pores at the insertion point.
Ligaments are dense connective tissue that link bones together at the joints. They are fibrous structures that attach their ends to bones, therefore maintaining the structural integrity of joints. However, the so-called yellow ligaments are composed of elastic fibers and connect the upper parts of the vertebral arch.
Joints
Bones associate to form joints, facilitating bodily movements. Cartilage, ligaments, and lubricant liquid are also joint components. According to the movement they are able to allow, there are various types of joints.
JJoints are named based on the bones or part of the bones that form the joint. For instance, the radioulnar joint, the temporomandibular joint, the femorotibial joint, and so on. Structurally, joints are classified considering the more abundant tissue present: fibrous, cartilaginous, and synovial joints. Functionally, joints can be classified into those that do not permit movement (synarthrosis), those that allow limited move (amphiarthrosis), and those that enable high mobility (diarthrosis). While anatomical texts typically present the structural classification, the functional terminology is more commonly employed, including those joints with low mobility or those that are immobile. We will follow the functional terminology.
Synarthrosis
Since movement is not the primarely function of synarthrosis, the tissues that form this type of joint are mostly connective and cartilaginous tissues. Its function is to keep together parts of the skeleton or give support to the growth of the skeletal components involved in the joint.
Syndemosis is a type of synarthrosis that has connective tissue connecting the bones. Ligaments (dense connective tissue) maintain the cohesion of these joints. The sutures between the skull bones are examples of syndemosis. In newborns, there is a membrane called the fontanelle that provides flexibility to the bones as the head develops. Later, the fontanelle is closed, and the connective tissue of the sutures becomes ossified. In this point, it is renamed to synostosis. Other examples of syndemosis include some large bones, such as the humeroradial joint, which allows a limited movement between the radius and the humerus, and the attachment of teeth to the alveoli of the jaws by the periodontal ligament, which allows a slight movement of teeth during chewing.
Synchondrosis contains hyaline cartilage between the bones. The degree of mobility of bones in these joints is minimal, or not allowed at all. This type of joint may be transient, observed in the epiphyseal plates located between the epiphysis and diaphysis, facilitating the growth of long bones. The epiphyseal plates are ossified during puberty, and they are then called synostoses. Another example includes the joints connecting the ribs and the breastbone, which enable a slight movement between these bones. However, during development, only the connection between the first rib and breastbone remains. The rest of these joints, while still maintaining the cartilage, are transformed into synovial joints to allow movement during breathing.
In symphyseal joints, the cartilage linking the bones is fibrocartilage. Examples include the intervertebral discs and the attachment of the pubis bones. Slight mobility is allowed due to the flexibility of the fibrocartilage.
Diarthoroses
Diarthroses are the "true" joints, also known as synovial joints. The bones are separated from each other, allowing movement. The surface of the bones in these joints is covered by articular cartilage, a type of hyaline cartilage. There is no physical contact between the surfaces of the bones as the synovial liquid fills the joint cavity. Fibrocartilage is found in some of these joints, between the articular cartilages, forming joint discs, menisci, and joint labrum. Examples of articular disks include the temporomandibular and sternoclavicular joints. The disks separate the joint cavity into two independent cavities. In contrast, fibrocartilage does not form closed structures in the meniscus of the knee and elbow joints. The function of these disks and menisci is to absorb mechanical loads. The labrum, found in the shoulder and hip joints, serves to widen the articular surface for protection against fractures.
Endocrine