The Musculoskeletal System: Bones, Muscles, and Joints

Posted on Nov 6, 2024 in Biology

The Musculoskeletal System

Bones

Bones are the main components of the skeleton. They are strong, hard, and tough organs composed of bone tissue, a specialized connective tissue with calcified extracellular components.

Chemical Composition:

  • 25% H2O
  • 45% mineral
  • 30% organic matter

Functions of Bone:

The musculoskeletal system causes the skeleton to serve as the structural framework of the body. It protects vital organs, allows movement, and is a deposit of hematopoietic organs. Bone marrow houses and forms the components of blood.

Classification According to Their Structure:

Compact Bone:

Composed of a solid mass, compact bone forms the outer layer of all bones of the body and most of the diaphysis of long bones. It provides protection and helps support long bones, allowing them to withstand the stress of weight-bearing.

A key difference is that compact bone has a concentric ring structure. Blood and lymph vessels and nerves from the periosteum penetrate the compact bone through perforating (Volkmann’s) canals. Blood vessels in these channels connect with blood vessels and nerves of the marrow cavity and the central (Haversian) canal. Central canals run longitudinally through the bone.

Around the canals are concentric rings of crystallized hard matrix called lamellae. Between the lamellae are small spaces called lacunae containing osteocytes. From the lacunae arise tiny tubes called canaliculi that are arranged radially in all directions and are occupied by extracellular fluid. Osteocytes are located inside the canaliculi. The canaliculi connect to each other and the lacunae, ultimately connecting with other central canals. Therefore, there is an intricate miniature canal system that covers the entire bone. This extensive network of canals provides many avenues for nutrients and oxygen to reach the osteocytes and for waste to be removed.

Osteocytes located in neighboring lacunae connect via canaliculi, facilitating the movement of materials between cells. Each central canal, with its adjacent lamellae, lacunae, osteocytes, and canaliculi, forms an osteon (Haversian system). Osteons are characteristic of adult compact bone.

Cancellous Bone (Trabecular Bone):

Spongy bone does not contain osteons. Instead, it has irregularly shaped plates called trabeculae that form a lattice-like structure, leaving holes filled with red bone marrow. Within the trabeculae are osteocytes. Blood vessels penetrate directly into the cancellous bone and allow the exchange of nutrients with the osteocytes. Cancellous bone is found in the epiphysis of long bones and inside other bones.

Classification According to Shape:

Long Bones:

Length dominates width and thickness. The ends are called epiphyses: the peripheral portion allows the insertion of muscles and joints. The diaphysis is the central, cylindrical, and hollow main portion. A cavity occupied by bone marrow, a fatty substance whose function is to generate red and white blood cells, is coated with compact bone tissue.

Short Bones:

Three dimensions (length, width, and depth) are nearly equal. The interior is spongy, and it has a thin, compact surface with high strength for weight-bearing.

Flat Bones:

Length and width are the dominant dimensions. They are covered by two compact plates that contain a spongy layer and protect noble organs.

Bone Cells:

Osteocytes:

Mature bone cells found in lacunae. Their function is to continue synthesizing the components necessary for maintaining the surrounding matrix.

Osteoblasts:

Cells involved in osteogenesis (bone formation and bone matrix production).

Osteoclasts:

Cells involved in bone resorption; they are the macrophages of the bone (involved in bone destruction).

Osteoprogenitor Cells:

They have the potential to differentiate into chondroblasts or osteoblasts. These persist into postnatal life and are found on almost every free surface of the bone. They are more active during the growth phase and also increase their activity during bone lesion repair.

Arthrology

Arthrology is the part of anatomy that deals with the study of joints.

Articulation: A set of soft and hard parts through which two or more bones are joined, considering them from an anatomical perspective. It is the point of contact between two bones or two cartilages of the skeleton.

Classification:

Joints can be classified into three groups: synovial, synarthrosis, and amphiarthrosis. This classification is based on the degree of movement allowed by the joint.

According to the Tissue That Connects Them:

Fibrous Joints:

No synovium. Bones are bound by tough, fibrous tissue with little movement, e.g., back, sacrum, and skull.

Cartilaginous Joints:

Bone and cartilage allow flexible movement, e.g., costal cartilage of the sternum.

Synovial Joints (Diarthroses):

Allow large movements. They are characterized by the presence of a synovial membrane. In a synovial joint, the following must be considered: articular cartilage surrounding the two joint surfaces (with a very smooth, polished outer face and a rougher inner face); the extension of these cartilages is determined by the amplitude of movements the joint makes. Synovial joints are classified as follows:

  • Ball-and-Socket Joint: The articular surfaces are spherical, with one concave and one convex face. They perform all movements, e.g., glenohumeral joint.
  • Condyloid Joint: An elongated articular surface is convex, and the other is concave. They perform all movements except rotation, e.g., the knee.
  • Hinge Joint: The articular surface is a pulley or trochlea with two facets separated by a ridge, allowing flexion and extension, e.g., the elbow.
  • Saddle Joint: The articular surfaces are concave in one direction and convex in the other, fitting perfectly. They allow all movements with small amplitude, except rotation, e.g., the calcaneocuboid joint.
  • Pivot Joint: The articular surface is a ring and an axis. They allow rotation movements, e.g., the radioulnar joint.

According to the Degree of Motion:

Synarthrosis (Sutures):

These joints do not have any kind of movement, or very limited movement. The bones are united intimately by their edges. They are characteristic of the bones of the skull and face. Sutures are classified as follows:

  • Serrate Suture: When both surfaces are represented by teeth that fit perfectly.
  • Squamous Suture: Beveled or overlapping edges.
  • Plane Suture: The bones are connected by more or less straight edges.
  • Schindylesis: One of the bones has a ridge that fits into a groove of another bone.
Amphiarthrosis:

Occupies an intermediate position between synovial and synarthrosis joints. They have limited movement, no joint cavity, and no synovium, but they do have articular surfaces that are flat or nearly flat, peripheral articular cartilage, and ligaments. Intervertebral discs are considered fibrous amphiarthroses.

Diarthrosis:

These are the most numerous and mobile joints. They are characterized by the presence of a joint cavity lined by a synovial membrane. In a synovial joint, the following must be considered: articular cartilage surrounding the two joint surfaces (with a very smooth, polished outer face and a rougher inner face); the extension of these cartilages is determined by the amplitude of movements the joint makes.

Movements:

  • Flexion
  • Extension
  • Abduction (bone moves away from the midline)
  • Adduction (bone moves toward the midline)
  • Rotation
  • Pronation (forearm rotated so the palm faces backward)
  • Supination (forearm rotated so the palm faces forward)

Muscles

Sarcomere:

The anatomical and functional unit of muscle. It consists of actin and myosin.

Muscle Contraction:

It consists of the sliding of actin filaments over myosin filaments in the sarcomere. Actin filaments (thin filaments) originate from the Z discs, where there is actinin (an actin-binding protein) and titin. Titin is an elastic protein (the largest in the body) with two functions: it keeps myosin in place, and because it has an elastic portion, it acts as a spring, restoring the length of the fibril after muscle contraction.

Functions of the Muscular System:

  • Movement: Allows skeletal muscles to move.
  • Stability: Maintains the stability and shape of the body.
  • Control: In vertebrates, it is controlled by the nervous system, although some muscles, such as the heart, can function autonomously.
  • Circulation: Responsible for the movement of blood.
  • Internal Organ Activity: Responsible for making all of our organs perform their functions, helping other systems.
  • Posture: Controls the positions the body makes at rest.
  • Heat Production: Muscle contractions generate heat energy.
  • Body Shape: Muscles and tendons give the body its typical appearance.
  • Protection: Muscles protect the proper functioning of the digestive tract and vital organs. They also stimulate blood and lymph vessels.

Muscle Twitching:

Experimentally demonstrated that a muscle responds to excitation with a short contraction called a muscle twitch. It can be decomposed into three phases:

  1. Latency Period: From the application of the stimulus until the muscle responds (0.01 sec).
  2. Contraction Period: Sarcomere shortening (0.04 sec).
  3. Relaxation Period: The muscle relaxes and returns to its initial length (0.05 sec).

If a muscle is subjected to a series of stimuli with high intensity (3 to 5), there is an initial treppe or staircase effect, where the contractions gradually increase in strength, followed by a plateau phase where the contractions are equal in strength, and finally, a fatigue phase where the contractions gradually decrease in strength. The muscle is unable to recover its initial length (contracture). The ATP deficit increases and cannot regenerate quickly. The muscle can relax with the supply of oxygen, glucose, and ATP. When the muscle is dying, it enters a state of rigor mortis due to ATP loss, and the actin-myosin bridges cannot be separated.