Tendon Anatomy Basics
Demystifying Tendons: A Primer on Structure, Function, and Fluoroquinolone Impact (with Citations)
This blog post is the first in a series that delves into the world of tendons, exploring their intricate structure, their essential function, and how they can be impacted by fluoroquinolone antibiotics. Before we can discuss the effects of these antibiotics, it's crucial to establish a foundational understanding of what tendons are and how they work.
What Exactly Are Tendons?
Tendons are the tough, fibrous connective tissues that link muscles to bones, enabling movement. In anatomical illustrations, they are often depicted in white, contrasting with the red muscles they serve. (1)
Tendon Basics
Every muscle has tendons at both ends. The insertion is the point where the tendon attaches to the bone that moves during muscle contraction. Conversely, the origin is where the tendon attaches to the stationary bone. (1)
Tendon Composition: The Collagen Connection
Tendons are primarily composed of strong, tensile collagen type I fibers aligned in the direction of muscle contraction. (1) These fibers provide resilience and resistance to tearing. Interspersed between the collagen fibers are elastic fibers and specialized tendon cells known as tenocytes. (1, 2)
A Closer Look at Collagen:
Collagen, a protein vital for connective tissue integrity, is composed of three intertwined peptide chains. Fibroblasts are the primary cells responsible for collagen production. (1) Interestingly, there are approximately 40 different types of collagen in the human body, each with a unique amino acid composition and sequence tailored to its specific function. (1)
Type I collagen, the most abundant type, is found in skin, bones, tendons, ligaments, and intervertebral discs. (1)
Type II collagen is predominantly found in cartilage. (1)
Type III collagen is a major component of blood vessels and various organs. (1)
Collagen synthesis basics
The Importance of Collagen Synthesis:
Maintaining the balance of collagen synthesis and degradation is crucial for tendon health. Factors like starvation, immobility, rheumatoid arthritis, and even weightlessness can accelerate collagen degradation. (1) Conversely, controlled exercise can stimulate collagen synthesis and promote tendon health. (3)
Key Anatomical Features of Tendons
Two critical structures in a tendon are:
Myotendinous Junction: This is the transitional zone where the tendon fibers intertwine with muscle fibers. It is often a weak point in the muscle-tendon unit and a common site of injury. (1, 5)
Tendon-Bone Junction: This is where the tendon anchors to the bone, often composed of fibrocartilage. It's the site where tendon growth originates. (1)
Tendon Sheaths: Minimizing Friction
Some tendons, particularly those in the hands, fingers, feet, and toes, are enveloped by sheaths. These sheaths are fluid-filled structures that reduce friction during movement. (1, 4)
Blood Supply and Innervation
Tendons receive blood supply both intrinsically (from within) and extrinsically (from surrounding tissues). (4) The blood supply can diminish with age and increased mechanical stress. (4)
Tendons also have a network of nerves, including:
Mechanoreceptors: These specialized nerve endings detect changes in pressure and tension. (4)
Nociceptors: These transmit pain signals. (4)
Sympathetic and Parasympathetic Nerves: These are components of the autonomic nervous system. (4)
Tendon Complexity and Individuality
Tendons are complex structures with varying anatomy depending on their location and function. This complexity underscores the importance of individualized assessment and treatment for tendon disorders. Sometimes the nerves around the tendons can also get irritated from the tension. See more about the nerves here.
In the next blog post in this series, we will examine the epidemiology of fluoroquinolone-associated tendinopathies and explore important definitions and classifications. Stay tuned!
References:
Benjamin M, Kaiser E, Milz S. Structure-function relationships in tendons: a review. J Anat. 2008;212(3):211-228. doi:10.1111/j.1469-7580.2008.00864.x
Kannus P. Structure of the tendon connective tissue. Scand J Med Sci Sports. 2000;10(6):312-320. doi:10.1034/j.1600-0838.2000.010006312.x
Kjaer M. Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading. Physiol Rev. 2004;84(2):649-698. doi:10.1152/physrev.00031.2003
Sharma P, Maffulli N. Tendon injury and tendinopathy: healing and repair. J Bone Joint Surg Am. 2005;87(1):187-202.
Riley G. The pathogenesis of tendinopathy. A molecular perspective. Rheumatology (Oxford). 2004;43(2):131-142. doi:10.1093/rheumatology/keh070
