Fig. 5. Glutamatergic and cholinergic pathways to cardiac parasympathetic neurons. Stimulation of the nucleus tractus solitarius (NTS) evokes a glutamatergic pathway that activates non–N -methyl-d-aspartate (NMDA) postsynaptic currents in cardiac parasympathetic neurons. 2This pathway may constitute the essential link between increases in blood pressure and afferent baroreceptor activity, which activates neurons in the NTS, and the reflex compensatory decrease in heart rate caused by increases in efferent cardioinhibitory cardiac parasympathetic activity. Acetylcholine (Ach) is likely involved in mediating cardiorespiratory interactions and excites cardiac parasympathetic neurons via three mechanisms: activating a direct ligand-gated postsynaptic nicotinic receptor, enhancing postsynaptic non-NMDA currents, and presynaptically (via α7subunit–containing nicotinic receptors) facilitating transmitter release. 10,11At least one cholinergic pathway to cardiac parasympathetic neurons originates from superior laryngeal neurons. 21Ketamine inhibits both the postsynaptic nicotinic receptors that evoke an inward current, as well as the presynaptic nicotinic receptors that facilitate glutamatergic neurotransmission.