We will focus on three components of autonomic nervous system physiology. We will be examining the neurotransmitters, fiber types, and receptor types of both the parasympathetic and sympathetic divisions of the autonomic nervous system.
Neurotransmitters and fiber type:
The ANS utilizes two different neurotransmitters primarily, these neurotransmitters are acetylcholine and norepinephrine.
Parasympathetic neurons release acetylcholine. This is the same neurotransmitter released by the motor neurons of the somatic motor system. Unlike the somatic motor system however, the acetylcholine released by the parasympathetic system can be either excitatory or inhibitory based on the receptors found in the target tissue.
Because both the pre-ganglionic and post-ganglionic neurons release acetylcholine, we can say that parasympathetic fibers are cholinergic. Cholinergic fibers are nerve fibers that release acetylcholine.
Two groups of receptor types are found in parasympathetic target tissues, muscarinic receptors, and nicotinic receptors. These receptors are name for two drugs that are capable of binding to these receptors, nicotine and muscarine.
Nicotinic receptors: When ACh binds to a nicotinic receptor the effect is always excitatory. This explains why these receptors are found in skeletal muscle motor end plates, and on the dendrites of all autonomic post-ganglionic neurons.
Muscarinic receptors: Found on parasympathetic (and some sympathetic) target organs. When ACh binds here the effect can be either excitatory or inhibitory based on the specific subset of muscarinic receptor class.
Neurotransmitters and Fiber Type:
The sympathetic division’s neurons release both acetylcholine and norepinephrine. To be more specific, sympathetic preganglionic neurons release acetylcholine, and postganglionic neurons release norepinephrine (mostly). Some sympathetic post ganglionic neurons release acetylcholine however (specifically those that innervate sweat glands.
This means that sympathetic pre-ganglionic neurons are cholinergic fibers, and most sympathetic post-ganglionic neurons are adrenergic fibers.
Remember that the effects of acetylcholine and norepinephrine are not always excitatory or inhibitory. The effects of these two neurotransmitters will depend on the receptor to which they bind.
Again, in the sympathetic division of the ANS we see that there are two classes of receptor, alpha and beta. These classes of receptors can be further divided into subcategories. Whether the norepinephrine or acetylcholine released from the sympathetic postganglionic fiber will be excitatory or inhibitory will depend on which specific class of receptor is found in the target tissue. This explains why norepinephrine released from the sympathetic division can cause cardiac muscle to contract more quickly, while causing smooth muscle in the lungs to relax.