3 Structure and Function of the Musculoskeletal System

      A Systems View of the Musculoskeletal System

      The musculoskeletal system is composed of a variety of specialized forms of tissues, including skeletal muscles, tendons, bones, joints, ligaments, and associated connective tissues. It also includes the nerves and blood vessels that bring innervation and blood supply to these structures. These tissues work together as a system, with skeletal muscles contracting as a result of nervous impulses leading to movement through force exertion on tendons, which then pull on bones. Most muscles cross one or more joints (producing movement in the joints that they cross) before attaching to the articulating bones that form a joint. With muscle contraction, one articulating bone is drawn toward the other. One bone is typically held in its original position, because other muscles stabilize it or contract in the opposing direction, or because the structure of the bone or joint makes it less movable. Ligaments provide stability to these joints. In this chapter, we will discuss the basic structure and function of each of these tissues, except for the basic structure and function nerves, which will be discussed in Chapter 4. Repair properties of each of these tissues will be discussed in Chapter 11.

Connective Tissues: General Overview

      Connective tissues are the most abundant tissues in the body. They envelop, protect, support, and separate structures as well as bind structures together. Connective tissues also act as a cushion between tissues, provide a pathway for nerves and blood vessels into and out of individual tissues, and more. Most connective tissues have a rich nerve supply and blood supply (except for cartilage which is aneural and avascular, and tendons which have a scanty blood supply). Special connective tissue types include cartilage, osseous (bone), and vascular (blood). The matrixes of these connective tissues differ, with some being fluid (blood), semifluid, gelatinous, fibrous (tendon), or calcified (bone). In cartilage, for example, the matrix is firm but pliable, while in bone, the matrix is considerably harder but not pliable.

      General Connective Tissue Structure

      Cells

Fibroblasts are common support cells in connective tissues. They are mesenchymal pluripotent cells with great heterogeneity in their subtypes. Fibroblasts are key producers of collagen and are heavily involved in the continuous slow turnover of a tissue’s extracellular matrix. When exposed to mechanical or physiological stress, fibroblasts adapt to their environment and have the ability to respond to and send local signals (e.g., cytokines and growth factors) (Dick, Miao, & Limaiem, 2020). Fibroblasts can also transform their phenotype into several other cell types for wound healing and replacement of damaged tissue (Dick et al., 2020). Other types of cells within connective tissues include adipocytes, macrophages (phagocytic immune cells), and plasma cells (cells that give rise to antibodies for immune defense). There are also mast cells that produce histamine and serotonin (chemicals that dilate small blood vessels) and heparin (an anticoagulant).

      Extracellular matrix

      The extracellular matrix of connective tissues consists of ground substance and fibers and plays a critical role as a scaffold within which tissues organize. The ground substance is the component of the matrix located between the cells and the fibers. It is typically a semifluid gel medium that serves for the passage of molecules through connective tissues and for the exchange of metabolites with the circulatory system (Young & Heath, 2000). It contains a mixture of long unbranched polysaccharide chains and water stabilized by glycosaminoglycans (GAGs), proteoglycans, and glycoproteins. The glycoproteins include structural (fibrillin and fibronectin), nonfilamentous (laminin, entactin, and tenascin), and integrin proteins (cell adhesion molecules). The typical gel‐like property of the extracellular matrix is imparted by the large volume of these globular complexes and their hydrophilic nature (their highly charged side chains attract large volumes of water and ions), which, when combined, produces the characteristic turgor of the ground substance. The mechanical properties of a tissue’s particular ground substance depend on the reinforcing fibrous proteins (collagen subtypes or elastin) or minerals (bone) to which these aggregates and other components are bound.

A number of types of fibrous proteins can be found in the ground substance of connective tissue. Collagen type I is the primary collagen in the dermis of skin (Figure 3.2), tendons, ligaments, and bone. It is formed by tropocollagen triple chains into larger fibrils (Iannarone, Cruz, Hilliard, & Barbe, 2019) that are visible under light microscopy. In contrast, collagen type II is the main collagen of hyaline cartilage and consists of fine filaments that cannot be visualized under light microscopy. Collagen type III is the major component of the fine reticular network within the bone marrow. It is also the first type of collagen deposited into