Regulation and Integration of the Body
Channel-Linked Receptors: Mechanism of Action
) are ligand-gated
ion channels that mediate direct neurotransmitter action. Tey
are composed of several protein subunits in a “rosette” around
a central pore. As the ligand binds to one (or more) receptor
subunits, the proteins change shape. Tis event opens the central
channel and allows ions to pass
. As a result, the
membrane potential of the target cell changes.
Channel-linked receptors are always located precisely op-
posite sites of neurotransmitter release, and their ion channels
open instantly upon ligand binding and remain open 1 ms or
less while the ligand is bound. At excitatory receptor sites (nico-
tinic ACh channels and receptors for glutamate, aspartate, and
A±P), the channel-linked receptors are cation channels that al-
low small cations (Na
) to pass, but Na
tributes most to membrane depolarization. Channel-linked
receptors that respond to GABA and glycine, and allow Cl
pass, mediate fast inhibition (hyperpolarization).
G Protein–Linked Receptors: Mechanism of Action
Unlike responses to neurotransmitter binding at channel-linked
receptors, which are immediate, simple, and brief, the activity
G protein–linked receptors
is indirect, complex,
slow (hundreds of milliseconds or more), and oFen prolonged—
ideal as a basis for some types of learning. Receptors in this class
In Chapter 3, we introduced the various receptors involved in cell
signaling. Now we are ready to pick up that thread again as we ex-
amine the action of receptors that bind neurotransmitters. ²or the
most part, neurotransmitter receptors are either channel-linked
receptors, which mediate fast synaptic transmission, or G protein–
linked receptors, which oversee slow synaptic responses.
Ion flow blocked
Direct neurotransmitter receptor mechanism:
These chemically gated ion channels
promote rapid synaptic transmission. In this case, ligand binding
directly opens the channel.
and activates receptor.
ATP to cAMP
cAMP changes membrane
permeability by opening or
closing ion channels.
Closed ion channel
Open ion channel
Recall from Chapter 3 that G protein
signaling mechanisms are like a
molecular relay race.
Indirect neurotransmitter receptor mechanism: G protein–linked
These receptors cause the formation of intracellular second messengers—cyclic
AMP (cAMP) in this example—that indirectly bring about the cell’s response.