The musculoskeletal system is composed of two systems – the muscular system and the skeletal system – but is commonly referred to as 'musculoskeletal' because of the main common functions of the said two systems, which are, movement and support.
The musculoskeletal system is made up of hard and soft tissues. The hard tissue includes bones and cartilages (articular cartilages), while the soft tissues are the muscles, tendons, synovial membranes, joints capsule and ligaments.
Primarily, the roles of the musculoskeletal system are movement and support, but the system also performs the following functions:
- Protection of vital structures
- Provision of body forms
- Storage of salts (e.g., calcium)
- Formation and supply of new blood cells
Essentially the skeletal part of the system pertains to the arrangement of bones, and how they join to one another to form joints which permit and limit specific movements. This part also outlines the factors that influence stability of some of those joints. For example, a joint with good bony congruence (joint with bones fitting well together) is most likely to be more stable than one with poor bony congruence. On the other hand, the muscular portion of the musculoskeletal system primarily describes the movements produced at joints, which as a basic principle, is based on the location of a muscle in relation to the joint and attachment to bones forming the joint. For example, a muscle lying anterior to two or more bones, and also crossing the joint formed by those bones anteriorly will produce the movement – “flexion” at that joint when it is contracted.
Muscles are the largest soft tissues of the musculoskeletal system. The muscle cells - muscle fibres - produce contractions that move body parts, including internal organs. Associated connective tissue binds muscle fibres into fascicles or bundles, and these associated connective tissues also convey nerve fibres and blood vessels (capillaries) to the muscle cells.
- Production of movement
- Support of the body
- Stability of joints
- Production of body heat
- Provision of form to the body
Muscles can be grouped into the following three types:
- Skeletal muscle, which move bones and other structures (e.g., the eyes)
- Cardiac muscle, which forms most of the walls of the heart and adjacent great vessels, such as the aorta
- Smooth (Visceral) muscle, which forms part of the walls of most vessels and hollow organs, move substances through viscera such as the intestine, and controls movement through blood vessels
However, the basic histological classification of muscles is into two types:
Based on this classification, skeletal and cardiac muscles are grouped as striated muscles, while the visceral muscle is non-striated. This structural characteristic (striation) is due to the way the filaments of actin and myosin are arranged in each of the classes of muscles.
Tendons and Ligaments
A tendon is a tough, flexible band of fibrous connective tissue that connects muscles to bones. The extracellular connective tissue between muscle fibres binds to tendons at the distal and proximal ends, and the tendon binds to the periosteum of bones at the muscle’s proximal attachment to bone (origin) and distal attachment (insertion). As muscle contracts, tendon transmits the force to the bones, pulling on them and causing movement.
Tendons and ligaments are made of dense fibrous connective tissue (DFCT) which has an abundance of collagen fibre bundles arranged in parallel, creating a high tensile strength (resistance to longitudinal force). Tendons and ligaments appear white because their fibrous connective tissues are made up of collagen, and collagen is white. They also appear white because they have very poor blood supply. Nutrients reach those structures by diffusion.
Tendons are generally rounded cords and thick. Ligaments are flatter in shape than tendons and attach a bone to another bone. They have more elastic fibres and thus are slightly stretchier than tendons.
Joint Capsule and Synovial Membrane
Synovial membranes line the synovial cavity and secrete synovial fluid that lubricates most joints where they are found (synovial joints) in order to reduce friction. The fluid secreted by a synovial membrane also serve as a source of nutrients for tendons and ligaments, as well as to articular cartilages.
Joint capsules are very strong and surround a joint, particularly synovial joints. They are composed of dense fibrous connective tissue. Ligaments are usually found by thickenings of the joint capsule, for example, the medial and lateral thickenings of the joint capsule at the knee joint, forms the medial and lateral collateral ligaments of the knee joint.
This system is composed of bones and cartilages, and makes up the hard tissue of the musculoskeletal system. Functions of the skeletal system include:
- support for the body
- shock absorption
- storage for salts
- production of blood cells
- production of vital organs
- mechanical basis for movement
The skeletal system consists of two main parts, the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones of the head, neck and trunk. The appendicular skeleton consists of the bones of the limbs, including those forming the pectoral and pelvic girdles.
Hard tissues of the musculoskeletal system will be discussed further under the headings – bones, cartilages, and joints.
Bones are made up of a superficial layer, compact bone, and a deeper layer of spongy bone, except where the latter is replaced by a medullary (marrow) cavity. Within this cavity in the adult bone, and between the spicules of spongy bones, blood cells are formed.
A typical bone (especially long bones) has a head, neck and body or shaft. It also possesses some markings and formations that gives passage and attachments to soft tissues like ligaments and tendons. Some of those features (markings) include:
- Condyle – rounded articular area (e.g. lateral femoral condyle)
- Crest – ridge of bone (e.g. iliac crest)
- Epicondyle – eminence superior to a condyle
- Facet – smooth, flat area, usually covered with cartilage
- Foramen – passage through a bone
Classification of Bones
Bones can be classified according to their shapes as follows:
- Long bones - These are bones longer than they are wide. They are tubular (e.g. the humerus in the arm)
- Short bones - These bones are roughly cube-like or round. Examples include the tarsals and carpals
- Flat bones - these types of bones are mostly thin, flattened and usually curved. They mostly serve protective functions. Examples include most of the skull bones – protecting the brain, and the ribs – protecting the thoracic viscera
- Irregular bones - These are bones that do not fit into any of the other types of bones. Generally, irregular bones will have a foramen through them. A very good example is the hip bone
Cartilages line the articulating surfaces of bones. Thus, cartilages are usually found deep within a joint. They are great for weight bearing and are extremely slippery to reduce the friction inside a joint (movable joint like synovial joint). Synovial joints possess hyaline cartilage.
Joints are formed where two or more bones meet. They promote movements of body parts, however, movement is not a necessary attribute of a joint as some joints do not move, e.g. joints between bones of the skull. The integrity or stability of a joint is guaranteed by several factors including the bony congruence (fit of bones), and other structures which cross the joint.
Joints can be classified broadly by the connective tissues found between the bone ends. The following are three categories based on their structures:
- Fibrous joints - The tissues between the bone ends of this class of joints is dense fibrous connective tissue (DFCT). The bones are held together firmly and the joint allows little or no movement. Examples are the sutures of the skull
- Cartilaginous joints - The tissue between the bones forming this category of joints is cartilage. The bones are firmly held together by these, but movement is also allowed. An example is the joints between the bodies of the vertebrae
- Synovial joints - These joints have the potential to allow movement through a wide range – far more than fibrous or cartilaginous joints. There are no direct bone to bone attachment by tissues at the bone ends, instead they are held together by a connective tissue sleeve – the joint capsule, attached at the margins of the joint.
There is a potential space called synovial cavity between the bone ends, allowing easy movements. Cartilage is also found on the articular surfaces of the bones. Synovial membrane is found in this joint, and covers structures within the synovial cavity other than those covered with articular cartilage. Hence, synovial membrane is the inner lining of the joint capsule.
Other classification of joints includes:
Based on Axis:
- Uni-axial joints
- Bi-axial joints
- Multi-axial joints
Based on Shape (Synovial joints):