General Chemical Organization of the ANS
Signals in the autonomic nervous system are transmitted chemically. Several types of neural pathways are distinguished based on the substance they utilize. The basic classification includes cholinergic and noradrenergic transmission, though serotonergic, dopaminergic, and purinergic mechanisms also exist.
The final response of the target organ is determined by the neurotransmitter-receptor combination. A single tissue may contain a whole set of various receptors, making the organ's response strictly specific.
Based on their localization, receptors are divided into:
- Postsynaptic — located directly on the cells of the effector organ.
- Presynaptic — located on the nerve terminal itself and operate via a feedback loop, controlling the amount of released neurotransmitter.
- Extrasynaptic — located in the pericellular space and respond to biologically active substances circulating in body fluids.
Signal Transmission in the Sympathetic and Parasympathetic Divisions
The pathway from the central nervous system to the effector organ in the ANS always consists of two neurons: preganglionic and postganglionic.
Parasympathetic Division Both neurons in this chain use the same neurotransmitter — acetylcholine.
- In the autonomic ganglion, acetylcholine is released by the first neuron and binds to nicotinic cholinergic receptors (nAChRs) on the second neuron.
- At the target organ level, the postganglionic fiber releases acetylcholine again, but here it acts on muscarinic cholinergic receptors (mAChRs).
Sympathetic Division Here, the neurotransmitter changes along the pathway.
- Transmission in the ganglion is completely identical to the parasympathetic division: acetylcholine is released, exciting the nAChRs of the second neuron.
- The terminal of the postganglionic fiber releases noradrenaline. It interacts with specific adrenoceptors ($\alpha$ and $\beta$ types) on the effector organ.
Classification of Major Receptors
Receptor systems are divided into two major groups based on their sensitivity to specific chemical agents.
Cholinoceptors Respond to acetylcholine. They are subdivided into:
- M-cholinoceptors (muscarinic) — the primary target of the parasympathetic system in effector organs.
- N-cholinoceptors (nicotinic) — localized in sympathetic and parasympathetic ganglia, ensuring excitation transmission between nerve cells.
Adrenoceptors Excited by noradrenaline and adrenaline. They include two main classes:
- $\alpha$-adrenoceptors ($\alpha_1$ and $\alpha_2$ subtypes).
- $\beta$-adrenoceptors ($\beta_1$ and $\beta_2$ subtypes).
Physiological Properties of Fibers and Ganglia
Autonomic neural structures differ markedly from somatic ones. Nerve fibers exhibit low excitability, prolonged absolute refractory period, and low lability (no more than 15 impulses/s). Impulse conduction velocity is extremely low: in preganglionic fibers (B group) it is 3–20 m/s, whereas in postganglionic fibers (C group) it drops to 0.5–3 m/s. The action potential duration is prolonged up to 150 ms due to pronounced after-phases.
Autonomic ganglia themselves act as complex integrative centers with low baseline activity. They exhibit spatial and temporal signal summation, as well as network processes: multiplication, convergence, and occlusion. Synaptic delay here ranges from 1 to 25 ms. A characteristic feature of ganglia is their high sensitivity to blood adrenaline, which acts as a modulator of synaptic transmission.
Briefly: answers to topical questions
Which neurotransmitters are released by postganglionic sympathetic fibers innervating sweat glands?
Postganglionic sympathetic fibers innervating sweat glands release the neurotransmitter acetylcholine.
This is a neurochemical exception, since the majority of postganglionic sympathetic fibers release noradrenaline. Such fibers are termed sympathetic cholinergic fibers. The acetylcholine they release interacts with M-cholinoceptors on the effector organ, ultimately leading to the activation of sweating.
What are the subtypes of muscarinic cholinoceptors (M1–M5) and where are they located?
Available materials do not provide a complete answer regarding all subtypes (M1–M5); only three of them are mentioned.
- M1 subtype — located on the postsynaptic membrane of the target organ.
- M2 subtype — localized on the presynaptic membrane, where it participates in the inhibitory autoregulation of acetylcholine release.
- M3 subtype — mentioned as one of the targets for non-selective blockers (atropine), although its precise localization is not described.
To avoid confusion regarding ganglia, remember the rule: "In all autonomic ganglia, Acetylcholine and Nicotine rule." Regardless of whether it is a sympathetic or parasympathetic pathway, the first synapse always utilizes acetylcholine and nAChRs.
Class and exam questions
Which neurotransmitter is released at the terminals of the sympathetic nervous system on effector organs?
Postganglionic sympathetic terminals release noradrenaline, which binds to $\alpha$- or $\beta$-adrenoceptors in tissues.
What happens upon sympathetic nerve transection?
The phenomenon of denervation supersensitivity will develop. Tissues deprived of neural control will become abnormally sensitive to biologically active substances (catecholamines) circulating in the blood.
Where are presynaptic receptors localized and what is their function?
They are located on the nerve terminal that releases the neurotransmitter. Their main function is to regulate the release of this neurotransmitter via a feedback mechanism.
What else to study on this topic
- Autonomic Reactivity and Autonomic Support of Activity
- Network phenomena in autonomic ganglia: convergence, occlusion, multiplication
- Specifics of action potential after-phases in autonomic nerve fibers
- Mechanism of spatial and temporal summation in ganglionic neurons
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Neurotransmitters and Receptors of the Autonomic Nervous System" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the «Autonomic Nervous System» section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.