Processes important for life such as sexual arousal, salivation, lacrimation (tearing), urination, digestion, defecation (passing stool) - remembered by the mnemonic SSLUDD - are largely regulated automatically and involuntarily by the parasympathetic nervous system (PSNS).
The drug atropine blocks some of the activities of the PSNS by interacting with muscarinic receptors, which are the main receptors targeted in tissues regulated by the PSNS.
Atropine overdose causes hot dry skin due to reduced sweating, red skin due to blood vessel dilatation, dryness from reduced tearing and sweating, visual disturbance because of pupillary dilatation as well as inability to focus and confusion due to effects on the brain. These changes may be summarized as, ‘hot as a hare, red as a beet, dry as a bone, blind as a bat and mad as a hatter’.
Control of the aforementioned processes and others also involves the sympathetic nervous system (SNS) which along with the PSNS by and large constitutes the autonomic nervous system.
Ramjan KA, Williams AJ, Isbister GK, Elliot EJ: 'Red as a beet and blind as a bat' Anticholinergic delirium in adolescents: lessons for the paediatrician. J Paediatr Child Health, 2007 43(11):779-80. Go to reference
Autonomic nervous system: PSNS = blue, SNS = red