Excretion, Locomotion and Neural Control and Coordination
What the exam tests
Excretion, locomotion, and neural control and coordination cover kidney function, urine formation, osmoregulation, muscle contraction, the skeleton, neurons, nerve impulses, synapses, and the reflex arc.
Core rules
- The nephron forms urine by glomerular filtration, tubular reabsorption, and tubular secretion. The countercurrent mechanism in the loop of Henle concentrates urine. ADH increases water reabsorption; aldosterone increases Na+ reabsorption.
- Skeletal muscle contraction follows the sliding filament theory: actin slides over myosin; Ca2+ binds troponin, exposing binding sites; ATP powers the cross-bridge cycle.
- Neurons maintain a resting potential (inside negative) via Na+/K+ pumps. An action potential involves Na+ influx (depolarization) and K+ efflux (repolarization). Myelinated axons conduct saltatory conduction.
- The reflex arc includes receptor, sensory neuron, interneuron, motor neuron, and effector.
Worked example
Inulin is filtered freely and not reabsorbed or secreted. If plasma inulin = 1 mg/mL, urine inulin = 100 mg/mL, and urine flow = 1.25 mL/min, then GFR = (urine inulin × urine flow) / plasma inulin = (100 × 1.25) / 1 = 125 mL/min. Reasoning: inulin clearance equals GFR because the kidney neither adds nor removes inulin.
Traps
Do not confuse filtration (glomerulus) with reabsorption (tubules). ADH reduces urine volume; aldosterone affects Na+ balance. In muscle contraction, ATP is needed for both cross-bridge cycling and Ca2+ reuptake. In a reflex arc, the sensory neuron enters the dorsal root; the motor neuron exits the ventral root.