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organization of neuronal networks can throughout the central nervous system.
Neurons, receptors, and synapses using
contribute to the processing and integra­
VetBooks.ir tion of information. For example, a single ACh are said to be cholinergic. Two general
classes of cholinergic receptors, nicotinic
neuron can be simultaneously stimulated
by excitatory and inhibitory neurotrans­ and muscarinic, are found at cholinergic
mitters from different presynaptic neurons synapses. Nicotinic receptors (often
(a converging network). The property of abbreviated nAChR) are ligand‐gated ion
spatial summation permits the neuron channels, meaning that they open an ion
receiving these converging inputs to inte­ channel when they bind their neurotrans­
grate the different stimuli and respond mitter (the “ligand”). On binding of ACh,
appropriately. the nAChR is always excitatory to the post­
synaptic membrane. Muscarinic receptors
(mAChR) are coupled to a variety of sec­
Neurotransmitters ond messenger proteins within the cell
and depending on the subtype may be
Most neurotransmitters can be classified excitatory or inhibitory. In cholinergic syn­
as “small molecules” (of which several apses, the enzyme acetylcholinesterase is
important molecules are modified amino responsible for rapidly degrading ACh and
acids) or polypeptides. While as many as thus terminating its effect.
a hundred molecules have been shown to Norepinephrine is the neurotransmit­
function as neurotransmitters, a small ter used by many peripheral neurons in the
number are especially prevalent through­ sympathetic division of the autonomic
out the nervous system and should receive nervous system (ANS) and at synapses at
individual consideration (Table 11‐1). particular sites within the CNS. Presynaptic
Acetylcholine or ACh is the neurotrans­ neurons and synapses using norepineph­
mitter released at the neuromuscular rine are described as adrenergic, and this
junction on skeletal muscle and by some term is also applied to cell membrane
peripheral neurons of the autonomic nerv­ receptors that bind norepinephrine. The
ous system and is found at many synapses term adrenergic has been applied to these

Table 11-1. Common Neurotransmitters and Their Receptors
Neurotransmitter Where Found Receptor(s) Effect
Acetylcholine Neuromuscular junction nAChR Excitation
(ACh) Autonomic ganglia nAChR Excitation
Parasympathetic mAChR Excitation or inhibition
postganglionic neurons (depending on receptor type)
CNS Both nAChR and Excitation or inhibition
mAChR (depending on receptor type)
Norepinephrine Sympathetic Adrenergic receptors Excitation or inhibition
postganglionic neurons (depending on receptor type)
CNS Adrenergic receptors Excitation or inhibition
(depending on receptor type)
Dopamine CNS D1 receptors Excitation
D2 receptors Inhibition
GABA CNS GABA receptor Inhibition
+
Glutamate CNS AMPA receptor Excitatory (Na influx)
NMDA receptor Excitatory (Na and Ca influx)
+
2+
CNS = central nervous system; GABA = γ‐Aminobutyric acid; mAChR = muscarinic acetylcholine receptors;
nAChR = nicotinic acetylcholine receptors.