Receptors for Neurotransmitters (NTs)
Having understood the cholinergic and adrenergic systems, their role in the autonomic system is relatively simple to understand. All preganglionic fibers, both sympathetic and parasympathetic, release ACh. All ganglionic neurons—the targets of these preganglionic fibers—have nicotinic receptors in their cell membranes. The nicotinic receptor is a ligand-gated cation channel that results in depolarization of the postsynaptic membrane. The postganglionic parasympathetic fibers also release ACh, but the receptors on their targets are muscarinic receptors, which are G protein–coupled receptors and do not exclusively cause depolarization of the postsynaptic membrane. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh. (see Table 2).
Return to Parasympathetic Neurons below.
One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the adrenal medulla, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibers, but the target is not a ganglion; instead the targeted adrenal medulla acts as a modified sympathetic ganglion. The adrenal medulla releases signaling molecules (NE) into the bloodstream, rather than using postganglionic fibers to communicate with effectors. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells. These cells develop along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion.
Watch the Fight and Flight Responses video (opens in new window) to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body's fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body's reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. In the video Paul Andersen explains how epinephrine is responsible for changes in chemistry of our body associated with the fight or flight response.
What two changes does adrenaline bring about to help give energy to the skeletal muscle response?
Helpful Study Fact 1: The rule is that all organs have dual innervation which means that they receive input from both sympathetic and parasympathetic branches. But there are 2 exceptions to this rule. Sweat glands and blood vessels are only innervated by the sympathetic division and under tonic versus antagonistic control. This means that instead of being turned "on" by the sympathetic system and turned "off" by the parasympathetic system, the sympathetic system releases a lot of neurotransmitter to get a large response or very little for a small response. Think dimmer switch versus a traditional light switch. This only applies to the 2 exceptions to the rule.
Helpful Study Fact 2: The rule is that the sympathetic division utilizes NE at the effector junctions; but there is an exception to this rule. The sympathetic division releases ACh instead of NE onto sweat glands. So sweat glands are tonically controlled by ACh released by the sympathetic system. Making it the exception to the exception of the rule.
