14.1 Coordination and response

The mammalian nervous system as the body's high-speed coordination network — central nervous system (brain + spinal cord) plus peripheral nervous system (all nerves outside the CNS), with electrical impulses travelling along three types of neurones: sensory (input to CNS), relay (inside CNS), and motor (output to effectors). The reflex arc (stimulus → receptor → sensory → relay → motor → effector → response) bypasses the brain to give a fast, automatic, life-saving response. HL extension: synapse structure (vesicles, neurotransmitter, synaptic gap, receptor proteins), the four-step events (impulse arrival → vesicle release → diffusion → binding → new impulse), and one-way directionality.