Autonomic nervous system synapses: neurotransmitters and receptors

Q: What types and subtypes of adrenergic receptors are located on the cells of effector organs?

Postsynaptic membranes of effector cells feature α1- and β1-adrenergic receptors. - α1-adrenergic receptors are located in the smooth muscles of arteries and veins, the radial muscle of the iris, the sphincters of the gastrointestinal tract and urinary bladder, the prostatic urethra, the prostate gland, the uterine myometrium, and the splenic capsule. - β1-adrenergic receptors are localized in the heart, and their stimulation causes tachycardia. - β-adrenergic receptors are also present in the myocardium.

Q: Which postganglionic sympathetic fibers are cholinergic (exceptions)?

An exception to the general rule is represented by certain postganglionic sympathetic fibers that release acetylcholine instead of noradrenaline. Such cholinergic fibers include: - Sweat gland fibers — mediate increased sweating. - Arrector pili muscle fibers — innervate piloerectors. - Vascular fibers — innervate blood vessels of the brain, heart, lungs, and active muscles. The release of acetylcholine in these vessels causes their dilation to provide anticipatory oxygen supply to tissues during mobilization.

Q: Which enzyme destroys acetylcholine in the synaptic cleft?

Acetylcholine in the synaptic cleft is degraded by the enzyme acetylcholinesterase. Inhibition of acetylcholinesterase prevents the hydrolysis of acetylcholine, leading to the accumulation of endogenous acetylcholine in the synaptic cleft, as well as the enhancement and prolongation of its effects.

Preganglionic level of signal transmission

At the stage of excitation transmission from the preganglionic nerve fiber to the postganglionic neuron, the mechanism of synaptic function is completely identical for both the sympathetic and parasympathetic divisions of the autonomic nervous system.

In these nerve ganglia, acetylcholine (ACh) acts as the universal chemical signal transmitter. Released from the endings of preganglionic fibers, this neurotransmitter crosses the synaptic cleft and binds to specific receptors on the membrane of the postganglionic neuron.

Both types of autonomic ganglia contain nicotinic acetylcholine receptors (nicotine-sensitive receptors). In a schematic representation of the nervous system structure, this interaction is characteristic of the parasympathetic ganglion synapse (designated as synapse A) and the sympathetic ganglion synapse (designated as synapse B). Thus, at the first stage of transmitting a command from the central nervous system to the periphery, the anatomical differences between the divisions do not affect the neurochemical essence of the process.

Postganglionic level and effector organs

Key differences in neurochemical impulse transmission begin to appear at the postganglionic level, i.e., directly at the site of contact between the nerve ending and the cells of the target organ (effector organ).

Parasympathetic nervous system (synapse B). The endings of postganglionic parasympathetic fibers continue to use acetylcholine as a neurotransmitter. However, on the cells of effector organs, this neurotransmitter interacts with a different type of receptor — muscarinic acetylcholine receptors (muscarine-sensitive).
Sympathetic nervous system (synapse E). The endings of postganglionic sympathetic fibers release a completely different neurotransmitter — norepinephrine (NE). The interaction occurs within the framework of adrenergic synapses.
Somatic nervous system (synapse G). For completeness, the innervation of skeletal muscle is also considered. In the neuromuscular junction of the somatic system, acetylcholine also serves as the neurotransmitter, but the receptor here is specific — the skeletal-type nicotinic acetylcholine receptor.

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Unique innervation of the adrenal glands

The adrenal medulla occupies a special place in the study of the autonomic nervous system, as from an anatomical and physiological point of view it is considered a modified sympathetic ganglion. This is due to the features of embryogenesis: chromaffin cells of the adrenal medulla have a direct affinity with sympathetic neurons.

Innervation of this zone is carried out directly by preganglionic nerve fibers, bypassing standard sympathetic ganglia. Signal transmission in this area (synapse D) occurs according to the classical ganglionic type:

Neurotransmitter: acetylcholine is released.
Receptors: nicotine-sensitive nicotinic acetylcholine receptors are located on the membrane of chromaffin cells.

The result of stimulation of these receptors by acetylcholine is not the transmission of an impulse along a nerve fiber, but the massive release of hormones into the systemic circulation — epinephrine (Epi) and norepinephrine (NE).

Pharmacological classification of synapses

For clinical pharmacology, it is of paramount importance to divide all described synapses not by their anatomical localization, but exclusively by the type of released neurotransmitter. This allows for a systematic approach to choosing drugs that affect specific receptors.

Cholinergic synapses (where acetylcholine acts as the neurotransmitter) include the absolute majority of contacts:

Synapses in parasympathetic ganglia.
Synapses in sympathetic ganglia.
Contacts innervating the adrenal medulla.
Neuromuscular junctions of the somatic system (skeletal muscles).
Neuro-effector junctions of the parasympathetic system at target organs.

Adrenergic synapses (where norepinephrine acts as the neurotransmitter) are represented by only one category — these are sympathetic neuro-effector junctions at the level of the innervated organ.

Briefly: answers to topical questions

What types and subtypes of adrenergic receptors are located on the cells of effector organs?

Alpha-1 and beta-1 adrenergic receptors are located on the postsynaptic membrane of effector cells.

Alpha-1 adrenergic receptors are found in the smooth muscles of arteries and veins, the radial muscle of the iris, sphincters of the gastrointestinal tract and urinary bladder, the prostatic urethra, the prostate gland, the myometrium of the uterus, and the splenic capsule.
Beta-1 adrenergic receptors are localized in the heart; their stimulation causes tachycardia.
Beta adrenergic receptors are also present in the myocardium.

Which postganglionic sympathetic fibers are cholinergic (exceptions)?

An exception to the general rule is certain postganglionic sympathetic fibers that release acetylcholine instead of norepinephrine. Such cholinergic fibers include:

Sweat gland fibers — mediate increased sweating.
Arrector pili muscle fibers — innervate piloerectors.
Vascular fibers — innervate blood vessels of the brain, heart, lungs, and working muscles.

The release of acetylcholine in these vessels causes their dilation to proactively supply tissues with oxygen during mobilization.

Which enzyme destroys acetylcholine in the synaptic cleft?

Acetylcholine in the synaptic cleft is destroyed by the enzyme acetylcholinesterase. Inhibition of acetylcholinesterase prevents the hydrolysis of acetylcholine, leads to the accumulation of endogenous acetylcholine in the synaptic cleft, and enhances and prolongs its effects.

How to memorize

To avoid confusing neurotransmitters: absolutely all "relay stations" (ganglia and adrenal glands) and the entire parasympathetic system operate on Acetylcholine. And only the sympathetic system at the very finish line (at the effector organ) switches to Norepinephrine.

Class and exam questions

Which neurotransmitter operates in the ganglia of the autonomic nervous system?

At the preganglionic level in both sympathetic and parasympathetic ganglia, acetylcholine is released. It acts on nicotinic N-cholinergic receptors of the postganglionic neuron.

Why is the adrenal medulla considered part of the sympathetic system?

Chromaffin cells of the adrenal glands are embryologically related to sympathetic neurons. They are innervated by preganglionic fibers and, upon stimulation of nicotinic acetylcholine receptors by acetylcholine, release epinephrine and norepinephrine into the blood.

How does signal transmission differ in the neuromuscular junction?

The neuromuscular junction belongs to the somatic nervous system. Acetylcholine is also released there, but it interacts with specific skeletal-type nicotinic acetylcholine receptors.

According to what principle are synapses classified in pharmacology?

The main criterion for pharmacology is the type of released neurotransmitter. Based on this feature, all synapses are divided into two large groups: cholinergic (acetylcholine) and adrenergic (norepinephrine).

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What else to study on this topic

Mechanism of intracellular signal transmission via muscarinic acetylcholine receptors
Life cycle of norepinephrine: synthesis, release, and reuptake in an adrenergic synapse
Differences between adrenergic receptor subtypes (α1, β2) on effector organs
Structural and functional features of skeletal-type nicotinic acetylcholine receptors
The role of chromaffin cells in systemic body reactions

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Autonomic nervous system synapses