7.2 Drugs, Pain and Consciousness

A psychoactive substance can change transmitter synthesis, vesicle release, receptor binding, enzymatic breakdown or reuptake. An agonist activates a receptor; an antagonist binds without producing the normal response and blocks an agonist. Neither term means globally stimulating or depressing: an antagonist at an inhibitory receptor may increase network activity, while an agonist at that receptor may suppress it.

Nicotine activates nicotinic acetylcholine receptors and is not removed by acetylcholinesterase in the same way as acetylcholine, altering reward pathways and producing dependence. Neonicotinoid insecticides target related receptors in insects and can disrupt synaptic transmission; environmental exposure of non-target insects, including pollinators, is therefore a concern. Botulinum toxin acts differently by preventing acetylcholine vesicle release, causing flaccid paralysis at affected neuromuscular junctions.

Cocaine blocks dopamine reuptake transporters, so dopamine persists in particular synapses. Benzodiazepines enhance signalling through GABA(_A) receptors, increasing inhibitory effects. Alcohol has multiple molecular targets, including enhancement of GABA-mediated inhibition and suppression of glutamatergic excitation, which contributes to impaired coordination, judgement and memory. THC acts through cannabinoid receptors and alters transmitter release in circuits involved in memory, movement and perception.

Repeated drug exposure can change receptor abundance, transmitter release, gene expression and learned associations. Tolerance means a larger dose is needed for the same effect; dependence means nervous-system adaptation produces withdrawal when the substance is removed. Addiction includes persistent, harmful use and impaired control. These ideas overlap but are not synonyms, and vulnerability reflects biological, psychological and social factors.

Free nerve endings in skin and other tissues contain channels activated by potentially damaging mechanical force, extreme temperature or chemicals. Capsaicin, for example, activates a channel also sensitive to damaging heat. Tissue injury releases mediators that can lower nociceptor threshold, making the region more sensitive. When generator potential reaches threshold, action potentials travel through sensory neurons to the spinal cord and brain.

Nociception is neural encoding of harmful stimuli; pain is the conscious experience produced when the brain interprets activity in context. They are related but not identical. Reflex withdrawal can begin through spinal circuits before conscious pain, and pain can be shaped by attention, expectation, memory and descending control. Endogenous opioid peptides can reduce transmission within pain pathways.

Speech, memory, choice and awareness depend on coordinated activity among many regions. A single oscilloscope trace can reveal a neuron's voltage but cannot by itself show a thought. Evidence comes from combining electrical recordings, imaging, behavioural reports, lesions and pharmacological interventions. The systems account does not replace molecular mechanisms; it connects them across scales.