Musculoskeletal system

Structure

Structurally, the skeletal muscles are composed of the skeletal muscle cells which are called the myocytes (muscle fibres, or myofibrils). Muscle fibers are specialized cells whose main feature is the ability to contract. They are elongated, cylindrical, multinucleated cells bounded by a cell membrane called sarcolemma. The cytoplasm of skeletal muscle fibers (sarcoplasm), contains contractile proteins called actin and myosin. These proteins are arranged into patterns, forming the units of contractile micro-apparatus called sarcomeres. 

Each muscle fiber is enclosed with a loose connective tissue sheath called endomysium. Multiple muscle fibers are grouped into muscle fascicles or muscle bundles, which are encompassed by their own connective tissue sheath called the perimysium. Ultimately, a group of muscle fascicles comprises a whole muscle belly which is externally enclosed by another connective tissue layer called the epimysium. This layer is continuous with yet another layer of connective tissue called the deep fascia of skeletal muscle, that separates the muscles from other tissues and organs. 

This structure gives the skeletal muscle tissue four main physiological properties:

• Excitability - the ability to detect the neural stimuli (action potential);
• Contractibility - the ability to contract in response to a neural stimulus;
• Extensibility - the ability of a muscle to be stretched without tearing; 
• Elasticity - the ability to return to its normal shape after being extended.

Learn everything about the skeletal muscle structure with our articles, video tutorials, quizzes and labelled diagrams.

Muscle contraction

The most important property of skeletal muscles is its ability to contract. Muscle contraction occurs as a result of the interaction of myofibrils inside the muscle cells. This process either shortens the muscle or increases its tension, generating a force that either facilitates or slows down a movement. 

There are two types of muscle contraction; isometric and isotonic. A muscle contraction is deemed as isometric if the length of the muscle does not change during the contraction, and isotonic if the tension remains unchanged while the length of the muscle changes. There are two types of isotonic contractions: 

• Concentric contraction, in which the muscle shortens due to generating enough force to overcome the imposed resistance. This type of contraction serves to facilitate any noticeable movement (e.g. lifting a barbell or walking on an incline).
• Eccentric contraction, in which the muscle stretches due to the resistance being greater than the force the muscle generates. During an eccentric contraction, the muscle maintains high tension. This type of contraction usually serves to slow down a movement (e.g. lowering a barbell or walking downhill).

The sequence of events that results in the contraction of a muscle cell begins as the nervous system generates a signal called the action potential. This signal travels through motor neurons to reach the neuromuscular junction, the site of contact between the motor nerve and the muscle. A group of muscle cells innervated by the branches of a single motor nerve is called the motor unit.

The incoming action potential from the motor nerve initiates the release of acetylcholine (ACh) from the nerve into the synaptic cleft, which is the space between the nerve ending and the sarcolemma. The ACh binds to the receptors on the sarcolemma and triggers a chemical reaction in the muscle cell. This involves the release of calcium ions from the sarcoplasmic reticulum, which in turn causes a rearrangement of contractile proteins within the muscle cell. The main proteins involved are actin and myosin, which in the presence of ATP, slide over each other and pull on the ends of each muscle cell together, causing a contraction. As the nerve signal diminishes, the chemical process reverses and the muscle relaxes.

Tendons

A tendon is a tough, flexible band of dense connective tissue that serves to attach skeletal muscles to bones. Tendons are found at the distal and proximal ends of muscles, binding them to the periosteum of bones at their proximal (origin) and distal attachment (insertion) on the bone. As muscles contract, the tendons transmit the mechanical force to the bones, pulling them and causing movement.

Being made of dense regular connective tissue, the tendons have an abundance of parallel collagen fibers, which provide them with high tensile strength (resistance to longitudinal force). The collagen fibers within a tendon are organized into fascicles, and individual fascicles are ensheathed by a thin layer of dense connective tissue called endotenon. In turn, groups of fascicles are ensheathed by a layer of dense irregular connective tissue called epitenon. Finally, the epitenon is encircled with a synovial sheath and attached to it by a delicate connective tissue band called mesotenon.

Learn more about the microstructure of tendon in this study unit:

Bones

Bones are rigid structures made of calcified dense connective tissue. Bone tissue is composed of a mineralized bone matrix that consists of type 1 collagen fibers dispersed throughout the ground substance. The cellular component of the bones is represented by three types of specialized bone cells called osteocytes, osteoblasts and osteoclasts. 

The bones consist of two distinct layers that differ in histological appearance and characteristics; 

• Compact (cortical) bone is the outer much denser layer of the bone which gives it its smooth, white, and solid appearance. The outer surface of the compact bone is covered with a layer of dense connective tissue called the periosteum. On its inner surface, the compact bone is covered with endosteum, which is the boundary between the compact and spongy bones.
• Spongy (cancellous) bone is the deep airy layer of the bone. Unlike the compact bone, spongy bone is highly vascularized and more metabolically active. It is typically found within the ends of long bones and in the vertebrae. In certain bones, like the hip bone, sternum or femur, the central part of spongy bone houses the bone marrow, which is the site of hematopoiesis in the adult.

Types of bones

Bones can be classified according to their shapes as follows:

• Long bones have a tubular shape, with a longer longitudinal and a shorter transverse diameter. They are composed mostly of compact bone, while the spongy bone and bony marrow fill the ends of the bones. Examples of long bones include the humerus, ulna, tibia and clavicle. 
• Short bones have a roughly cuboid or round shape, and only contain a thin layer of compact bone surrounding the spongy bone. Examples include the tarsal and carpal bones.
• Flat bones are mostly thin, flattened and usually curved. They contain two parallel layers of compact bones surrounding a layer of spongy bone. Examples include most of the skull bones, scapula, sternum and sacrum.
• Sesamoid bones are small, rounded unique types of bones that are embedded in muscle tendons where the tendon passes over a joint. The largest sesamoid bone in the body is the patella, but several other smaller sesamoid bones can be found in the hand and foot, usually in close proximity to the joints.
• Irregular bones do not fit into any of the other categories. Generally, irregular bones contain foramina through which soft tissue and neurovascular structures pass. Examples include the vertebrae, hip bone and some bones of the skull.

Wondering how to cut time in learning the bones of the body? Try our skeletal system quizzes!

A typical long bone consists of a long shaft (diaphysis) that extends into a neck (metaphysis) and head (epiphysis) on its proximal and distal ends. It also features various markings and formations that give passage to neurovascular structures, as well as the attachment sites to the ligaments and tendons. Some of those features include:

• Sulcus – a shallow groove on the bone surface (e.g. radial sulcus of humerus)
• Condyle – rounded articular area (e.g. lateral condyle of tibia)
• Epicondyle – eminence superior to a condyle (medial epicondyle of femur)
• Crest – ridge of bone (e.g. iliac crest)
• Facet – smooth, flat area, usually covered with cartilage (e.g. articular facet on vertebrae)
• Foramen – passage through a bone (e.g. foramen magnum on the occipital bone)

Cartilage

Cartilage is a flexible connective tissue found in multiple organ systems of the body. Cartilage is composed of specialized cells called chondrocytes, collagen fibers and abundant ground substance rich in proteoglycan and elastin fibers.

Cartilage is classified into the following types based on its composition:

• Hyaline cartilage is composed of type II collagen and an abundance of ground substance, which gives it a glossy appearance. It is the most abundant type of cartilage found in joints (articular cartilage), as well as the nose, larynx, trachea and ribs.
• Elastic cartilage is similar to hyaline cartilage but contains more elastic fibers. It is found in structures such as the pinna of the ear, auditory tube and epiglottis.
• Fibrocartilage is composed of plenty of collagen fibers type I and a smaller amount of ground substance. Examples of fibrocartilage include intervertebral discs, pubic and other symphyses.

The musculoskeletal system specifically contains articular cartilage, a type of cartilage that lines the articulating surfaces of bones. The articular cartilage provides congruence to the articulating bones and allows them to bear weight and glide over each other with very little friction. 

Joints

Each bone of the musculoskeletal system is connected to one or more bones via a joint. Joints provide a fulcrum to the bones, on which they pivot and thereby allow movements of body parts. However, movement is not a necessary attribute of a joint as some joints do not move, such as joints between the bones of the skull. The integrity or stability of a joint is provided by several factors including the bony congruence and structures that cross the joint, such as tendons and ligaments.

Based on the type of tissue that holds the neighboring bones together and the range of motion they exhibit, joints can be classified into the following:

• Synovial joints are freely mobile joints in which the bones are not in direct contact, but are separated by a potential space called the synovial cavity. The synovial cavity is lined by a synovial membrane that secretes the synovial fluid which nourishes and lubricates the articulating surfaces in order to reduce friction. The articulating bones in most synovial joints are lined with hyaline cartilage. These joints usually have a wide range of motion, which is defined by the joint capsule, the supporting ligaments and muscles that cross the joint. Examples of synovial joints include the knee, shoulder, sternoclavicular and elbow joints.
• Fibrous joints are the articulations in which the bones are connected by dense fibrous connective tissue. The bones in fibrous joints are firmly held together so that the joint allows negligible movement. Fibrous joints are found between the cranial sutures, the distal tibiofibular and cuboideonavicular joints.
• Cartilaginous joints are articulations in which the bones are connected by cartilage. The bones have a range of motion between synovial and fibrous joints. Cartilaginous joints are subdivided into synchondrosis (e.g. costochondral joints) and symphysis joints (e.g. pubic symphysis).

According to the movements they allow and/or the shape of their articulating surface, the synovial joints can be further subdivided into 6 major types:

• Ball and socket joints (e.g. hip joint)
• Condyloid joints (e.g. metacarpophalangeal joint)
• Hinge joints (e.g. elbow joint)
• Pivot joints (e.g. atlanto-axial joint)
• Saddle joints (e.g. carpometacarpal joint)
• Plane joints (e.g. acromioclavicular joint)

Ligaments

Ligaments are fibrous bands made of dense regular connective tissue which are similar in structure to tendons. Unlike the tendons that connect muscles to bone, the ligaments connect bone to bone. Besides the musculoskeletal system, the ligaments are also found in many other parts of the body, where they usually stabilize and hold internal organs in place and transmit neurovascular structures.

In the musculoskeletal system, ligaments stabilize the articulating bones and reinforce the joints. Depending on their anatomic position relative to the joint capsule, ligaments are classified into:

• Capsular ligaments are essentially thickenings of the joint capsule that form either elongated bands or triangular structures. These ligaments serve to reinforce the integrity of the joint capsule. An example of the capsular ligament is the iliofemoral ligament of the hip joint.
• Intracapsular ligaments are the ligaments that lie internal to the joint capsule. These ligaments reinforce the connection of the articulating surfaces of the joint, but allow a far wider range of motion than other ligaments. Examples include anterior and posterior cruciate ligament of the knee joint.
• Extracapsular ligaments are ligaments that lie outside the joint capsule. These ligaments provide the most stability to the articulating bones, and are important for preventing dislocations. Extracapsular ligaments can lie in close proximity (e.g. medial collateral ligament of the ankle joint) or a bit further from the joint capsule (vertebral ligaments).

Bursae

Bursae are small sac-like outpouchings of the joint cavity lined by synovial membrane. They are found around the joints, providing cushioning of the associated bones, tendons and muscles and reducing friction between adjacent structures.

The majority of synovial bursae are located near the large joints of the arms and legs. For example, one of the bursae of the knee joint is the suprapatellar bursa, found superior to the patella, between the femur and the tendon of the quadriceps femoris muscle. The suprapatellar bursa allows for these structures to slide over each other without friction during flexion and extension of the knee joint.

To improve your understanding of skeletal system terminology, take a closer look at some commonly used roots, prefixes and suffixes related to the skeletal system in the video below.

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