Cytoskeleton

A cell may look like a bag of water, but inside it is a dense web of protein fibers. This web is the cytoskeleton. Just as the human skeleton supports the body, the cytoskeleton maintains the cell’s shape. Unlike bones, however, the cytoskeleton constantly changes form as it is assembled and disassembled. This is what allows cells to become elongated or round, or to move toward a wound.

The cytoskeleton is broadly divided into three types of fibers according to thickness.

• Actin filaments (microfilaments) These are the thinnest fibers. Located just inside the cell membrane, they help control the cell’s shape and movement. Actin also interacts with myosin to generate a pulling force during muscle contraction.
• Intermediate filaments These tough, strong, rope-like fibers help keep cells from tearing when pulled. Keratin in skin cells is a typical example.
• Microtubules These hollow, tube-shaped structures are the thickest and most rigid. They serve as railway tracks for transporting materials within the cell and also form the spindle fibers that pull chromosomes apart when a cell divides into two.

These railway tracks are even more important in cells such as nerve cells, which can reach 1 meter in length. For materials made in the cell body to reach the far end, protein carriers must travel along the microtubules.

The cytoskeleton is interesting in relation to manual therapy because it provides a pathway for transmitting external forces into the cell. Cells attach to the surrounding extracellular matrix—a network of fibers such as collagen—through proteins called integrins. Integrins span the cell membrane, with their opposite ends connected to the cytoskeleton. When tissue is stretched or compressed, the force therefore passes through integrins and along the cytoskeleton to the area near the cell nucleus.

This process is called mechanotransduction. A cell subjected to force does more than simply deform: it adjusts which proteins it makes more or less of. Bones becoming stronger with repeated loading and unused muscles becoming thinner are also the result of these signals accumulating. Cell biologist Donald E. Ingber explained this phenomenon through the tensegrity model, in which cells maintain their shape through a balance between taut cables and rigid struts.

Books introducing Integrative Manual Therapy (IMT) highlight that forces applied to cells can even change gene expression, using this as a theoretical basis for connective tissue manual therapy. From this perspective, gently stretching and releasing fascia does more than change tissue length: it changes the signals received by cells, including fibroblasts. This is an attempt to explain, at the cellular level, why Integrative Manual Therapy and the Three-planar fascial fulcrum approach apply gentle, sustained pressure to tissues.

Of course, research is still ongoing into exactly what changes hands-on work produces inside cells. However, the fact that forces alter cellular activity through the cytoskeleton is itself well established in cell biology.

• Movement stimulates cells. The moderate forces produced by walking, stretching, and strength training provide growth and repair signals to cells in muscles, tendons, and ligaments. The feeling that tissues are softer after stretching can be understood as involving these cellular responses as well.
• Both too little movement and too much strain change the signals. An arm becoming thinner and stiffer after a few weeks in a cast is the result of interrupted mechanical signals, while excessive repetition places an accumulating burden on tissues. Moderate, consistent movement provides the best stimulus.
• Cells need rest, too. A cytoskeleton altered by force is reassembled during recovery. This is also one reason for allowing rest between exercise sessions.

Think about what happens when you get a wound. As fibroblasts beneath the skin move toward the wound, actin is newly assembled at the front of each cell, pushing the cell membrane outward, while it is disassembled at the rear, allowing the cell to move forward. Inside the cell, materials needed to make collagen are transported along microtubules. In this way, the three types of cytoskeletal fibers work simultaneously as the wound fills in. Behind the visible changes we recognize as healing are these movements at the cellular level.