How are Tacrine, Rivastigmine and Galantamine as anti-cholinesterases?

In patients with loss of cholinergic neurons like Alzheimers, anticholinesterases are used to remedy the loss of cognitive function, Tacrine first one available but replaced due to hepatotoxicity. The others cannot stop progression of the disease but delay it. GI distress is primary adverse effect (Vomiting and nausea)