How to Read This Coordination and Motility Chapter

Movement requires both control and mechanics. The control story begins when a receptor detects change, continues through sensory input and integration, and ends with motor output to an effector. The mechanics story begins with force generated by actin-myosin interactions, passes through muscle, tendon, joint and skeleton, and becomes movement of a body part or locomotion of an organism. Strong explanations keep these stories connected without confusing an electrical signal with the force it ultimately triggers.

This chapter moves across scales more dramatically than most biology chapters. Calcium ions expose binding sites within a sarcomere measured in micrometres; many sarcomeres shorten within muscle fibres; recruited motor units increase the force of a whole muscle; antagonistic muscles rotate bones about joints; coordinated limb or fluke movements propel an animal. At every scale, structure constrains function.

Several pairs look deceptively similar: sensory and motor neurones, voluntary and involuntary control, movement and locomotion, exoskeleton and endoskeleton, tendon and ligament, hinge and ball-and-socket joint, actin and myosin, contraction and relaxation. Do not learn these as isolated definitions. Compare direction, location, material, attachment and consequence. A tendon and ligament are both collagen-rich connective tissues, for example, but one transmits muscle force to bone while the other stabilizes bone relative to bone.

The simulations in the core sections provide the chapter's visual spine, so this orientation deliberately has no separate simulation. Begin by predicting what must remain fixed when a variable changes: a stronger stimulus does not produce a larger action potential, actin and myosin filaments do not shorten during contraction, and a more forceful contraction does not change whether a joint is anatomically a hinge or ball-and-socket joint.

Section 9.1 develops nervous coordination, reflexes, nerve structure, the cerebellum and peristalsis. Section 9.2 distinguishes movement from locomotion and surveys movement in plants and sessile animals. Section 9.3 treats skeletons, synovial joints and antagonistic action. Section 9.4 examines skeletal muscle from myofibril to motor unit. Section 9.5 applies the system to locomotion, including the distinctive swimming adaptations of marine mammals.