Control and coordination
Nerve impulses and synapses
At rest (about −70 mV), the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in; the membrane is more permeable to K⁺, which leaks out. A stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in: depolarisation to about +40 mV. Na⁺ channels close and K⁺ channels open: repolarisation, overshooting (hyperpolarisation). The refractory period ensures one-way, discrete impulses. Action potentials are all-or-nothing; stimulus strength is coded by frequency. Myelin (Schwann cells) insulates the axon so impulses jump between nodes of Ranvier (saltatory conduction), much faster. Speed also rises with axon diameter and temperature. At a cholinergic synapse, Ca²⁺ enters the presynaptic knob, vesicles release acetylcholine by exocytosis; ACh diffuses and binds receptors, opening Na⁺ channels; acetylcholinesterase breaks it down. Synapses allow summation, one-way transmission and inhibition.
≈ 55 minMuscle contraction and plant responses
Skeletal muscle fibres contain myofibrils made of repeating sarcomeres (Z line to Z line): thin actin and thick myosin filaments overlap. At the neuromuscular junction, ACh depolarises the sarcolemma; the impulse travels down T-tubules; the sarcoplasmic reticulum releases Ca²⁺. Ca²⁺ binds troponin, moving tropomyosin off the binding sites on actin. Myosin heads bind (cross-bridges), perform the power stroke (releasing ADP + Pᵢ), then ATP binds to detach them and is hydrolysed to recock the head. Actin slides over myosin: the I band and H zone shorten; the A band stays the same. Phosphocreatine regenerates ATP rapidly. Plants: auxin (IAA) causes cell elongation (acid growth) and phototropism; gibberellin causes stem elongation and stimulates amylase in seed germination; abscisic acid closes stomata in drought; the Venus fly trap closes by rapid changes in turgor after two hair touches.
≈ 55 min