Chapter 14: Cellular Movement: Motility & Contractility

Eukaryotic motility is categorized into two primary systems: those utilizing microtubule tracks and those relying on microfilament networks. Microtubule-associated motors include kinesins, which generally transport cellular cargo toward the plus ends of filaments, and dyneins, which move toward the minus ends. These proteins are essential for internal processes like fast axonal transport in neurons and the dynamic organization of the endomembrane system. Beyond internal transport, microtubules form the structural core—the axoneme—of cilia and flagella. Within these appendages, axonemal dyneins generate the sliding forces necessary for swimming or moving the external environment past the cell. The chapter provides critical clinical context through conditions like Kartagener syndrome and primary ciliary dyskinesia, illustrating how molecular defects in these motor proteins can lead to organ reversal or chronic respiratory issues. Shifting to microfilaments, the text focuses on the diverse myosin family. Myosin II is identified as the fundamental motor for muscle contraction, meticulously organized within the repeating units of the muscle fiber known as sarcomeres. According to the sliding-filament model, muscle contraction occurs as thick myosin filaments pull thin actin filaments toward the center of the sarcomere, increasing overlap without changing the actual length of the individual filaments. This cyclic process—encompassing cross-bridge formation, the power stroke, and detachment—is fueled by the hydrolysis of ATP. Precise control over skeletal muscle is maintained by a calcium-sensitive switch involving the troponin-tropomyosin complex, which regulates access to actin binding sites based on electrical signals carried into the cell via T-tubules and the subsequent release of calcium from the sarcoplasmic reticulum. Cardiac muscle functions through a similar sarcomeric structure but is coordinated across the heart via electrical coupling at intercalated discs. In contrast, smooth muscle and non-muscle contractile systems lack this striation and instead utilize a slower regulatory pathway involving the protein calmodulin and the phosphorylation of myosin light chains. Finally, the chapter details microfilament-based motility in non-muscle cells, such as the crawling of fibroblasts through the extension of lamellipodia and filopodia, amoeboid movement governed by cytosolic gel-to-sol transitions, and chemotaxis, where cells migrate directionally in response to environmental chemical gradients.