Neurotransmitters of the Peripheral Nervous System
In the synapses of the peripheral nervous system (PNS), two main neurotransmitters play a key role: acetylcholine and norepinephrine.
The effects of these substances strictly depend on where they are released and which receptors they bind to. For example, in the somatic nervous system, acetylcholine is exclusively an excitatory neurotransmitter. However, in neuroeffector synapses of the autonomic nervous system (ANS), both acetylcholine and norepinephrine can cause either excitation or inhibition—the final outcome depends entirely on the physiological properties of the postsynaptic receptors.
Localization of Acetylcholine Release:
- Neuromuscular junctions of the somatic nervous system.
- Ganglionic synapses of the entire autonomic nervous system.
- Neuroeffector synapses of the parasympathetic division.
- Neuroeffector synapses of the sympathetic division, but only in sweat glands and blood vessels of skeletal muscle.
Localization of Norepinephrine Release:
- The majority of neuroeffector synapses in the sympathetic nervous system.
Neurotransmitters of the Central Nervous System
In the CNS, neurotransmitters perform more complex tasks and are divided into three functional groups:
- Excitatory Neurotransmitters. The primary representative is glutamate. It is the main excitatory agent, responsible for transmitting the vast majority (up to 80%) of signals in the central nervous system.
- Regulatory Neurotransmitters. Their task is to adjust neuronal excitability, determine the level of arousal, and shape the motivational-emotional background.
- Acetylcholine maintains the baseline tone of nerve centers regardless of whether the body is asleep or awake.
- Norepinephrine additionally activates centers during stress and severe tension.
- Dopamine is critically important for regulating movement and maintaining normal muscle tone.
- Serotonin acts as a safety valve, keeping excessive excitation within necessary limits.
- Inhibitory Neurotransmitters. These include GABA (gamma-aminobutyric acid) and glycine. They can selectively and temporarily block the passage of nerve impulses, adapting to the current functional state of the body.
Cotransmitters and Neuromodulators
In addition to the primary transmitter, presynaptic terminals often release additional substances—cotransmitters. Their main function is the fine-tuning (modulation) of postsynaptic membrane sensitivity.
- In adrenergic synapses, cotransmitters can include dopamine, ATP, gonadotropin, and neuropeptide Y.
- In cholinergic synapses, this role is performed by vasoactive intestinal polypeptide (VIP).
Neuromodulators are also distinguished as biologically active compounds that modulate the potency of primary neurotransmitters:
- Histamine increases the responsiveness of nerve centers to basic neurotransmitters.
- Substance P is directly involved in conducting pain and emotional excitation, variously modulating synaptic transmission.
- Enkephalins and endorphins are a crucial part of the antinociceptive system. They effectively inhibit the release of primary neurotransmitters during intense pain excitation.
Receptor Classification
Postsynaptic membrane receptors are classified based on their sensitivity to specific chemical substances. The CNS contains virtually all types of receptors, whereas the periphery exhibits strict specialization.
| Receptor Type | Activated By | Main Localization (PNS) |
|---|---|---|
| N-cholinergic receptors (nicotinic) | Tobacco alkaloid — nicotine (in low doses) | Neuromuscular junctions, autonomic ganglia, sympathetic cholinergic synapses |
| M-cholinergic receptors (muscarinic) | Crustacean and fungal toxin — muscarine | Neuroeffector synapses of the parasympathetic nervous system |
| Adrenoceptors ($\alpha$, $\beta_1$, $\beta_2$) | Norepinephrine | Neuroeffector synapses of the sympathetic nervous system |