Scopolamine and
atropine have similar effects on the
peripheral nervous system. However, scopolamine has greater effects on the
central nervous system (CNS) than atropine due to its ability to cross the
blood–brain barrier. At higher-than-therapeutic doses, atropine and scopolamine cause CNS depression characterized by amnesia, fatigue, and reduction in
rapid eye movement sleep. Scopolamine (Hyoscine) has
anti-emetic activity and is, therefore, used to treat
motion sickness. Antimuscarinics are also used as anti-parkinsonian drugs. In
parkinsonism, there is imbalance between levels of
acetylcholine and
dopamine in the brain, involving both increased levels of acetylcholine and degeneration of
dopaminergic pathways (
nigrostriatal pathway). Thus, in parkinsonism there is decreased level of dopaminergic activity. One method of balancing the neurotransmitters is through blocking central cholinergic activity using muscarinic receptor antagonists. Atropine acts on the
M2 receptors of the heart and antagonizes the activity of acetylcholine. It causes
tachycardia by blocking vagal effects on the
sinoatrial node. Acetylcholine hyperpolarizes the sinoatrial node; this is overcome by MRAs, and thus they increase the heart rate. If atropine is given by intramuscular or subcutaneous injection, it causes initial
bradycardia. This is because when administered intramuscularly or subcutaneously atropine acts on presynaptic
M1 receptors (
autoreceptors). Uptake of acetylcholine in
axoplasm is prevented and the presynaptic nerve releases more acetylcholine into the
synapse, which initially causes bradycardia. In the
atrioventricular node, the
resting potential is lowered, which facilitates conduction. This is seen as a shortened PR-interval on an
electrocardiogram. It has an opposite effect on
blood pressure. Tachycardia and stimulation of the
vasomotor center causes an increase in blood pressure. But, due to feedback regulation of the vasomotor center, there is a fall in blood pressure due to
vasodilation. Important muscarinic antagonists include
atropine,
hyoscyamine,
hyoscine butylbromide and
hydrobromide,
ipratropium,
tropicamide,
cyclopentolate,
pirenzepine and
scopolamine. Muscarinic antagonists such as
ipratropium bromide can also be effective in treating
asthma, since
acetylcholine is known to cause
smooth muscle contraction, especially in the
bronchi. ==Comparison table==