INTERRUPTION OF NEURONAL TRANSMISSION BY INHALED ANESTHETICS
Peripheral Receptors
The action of inhaled anesthetics cannot be explained by depression
of peripheral receptors. Anesthetizing concentrations of clinical agents do not
alter cutaneous receptor responses to touch or movement of hair and can even
Figure 4-4
The effects of halothane (left)
and isoflurane (right) on three excitatory synaptic
pathways in the rat hippocampal slice. The potential recordings were produced from
stimulation of three discrete regions of the hippocampal slice: the stratum radiatum
(RAD), the stratum oriens (OR), and the perforant path (PP) fibers. Each superimposed
series of records shows control (C) responses before (solid
line) and after washout (dotted line)
together with two or three concentrations of anesthetic (numbers refer to volume
percent). The horizontal calibration bar represents 20 msec, and the vertical calibration
bar represents 2.0 mV. These studies demonstrate that the ability of inhaled agents
to depress or enhance neuronal excitability depends on the anesthetic, the anesthetic
concentration, and the particular brain region examined. (From Maclver MB,
Roth SH: Inhalation anaesthetics exhibit pathway-specific and differential actions
on hippocampal synaptic responses in vitro. Br J Anaesth 60:680, 1988.)
Figure 4-5
Decerebration does not alter anesthetic requirement,
as measured by the minimum alveolar concentration (MAC). Removal of brain tissue
rostral to the heavy black line does not alter isoflurane
MAC in rats. PAG, periaqueductal gray; RF, reticular formation; RN, red nucleus;
Thl, thalamus. (Adapted from Rampil IJ, Mason P, Singh H: Anesthetic potency
[MAC] is independent of forebrain structures in the rat. Anesthesiology 78:707,
1993.)
sensitize and promote excitation of nociceptors in mammalian A and C fibers.[32]
Moreover, a selective perfusion technique allowing for low (about 0.2% atm) isoflurane
concentration at the site of peripheral noxious stimulus in dogs while maintaining
higher concentrations in the rest of the body results in no change in the isoflurane
MAC and demonstrates that the peripheral effects of isoflurane do not influence the
response to a noxious stimulus.[33]
In humans,
peripheral nerve conduction is unaltered by the concentrations of desflurane needed
for surgical anesthesia.[25]
