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Autonomous nervous system

Autonomous nervous system (ANS) is divided into sympathetic (adrenergic) and parasympathetic (cholinergic) divisions. Autonomous nervous system maintains homeostasis by integrating with sensory, somatomotor, endocrinal, metabolic, immunological and emotional activities. The limbic system and hypothalamus coordinate autonomic reactions with emotions. Anteromedial and posterolateral nuclei of hypothalamus are associated with cholinergic and adrenergic activities respectively.

Vital centres in the medulla, e.g. vagal, vasomotor, vomiting, respiratory and cough centres play an important role in the coordination. The Autonomous nervous system has a property to regulate organ function automatically, largely independent of consciousness and has voluntary control (Autos—self, nomos—regulating). The characteristic features of autonomic nerves are:

  • They innervate smooth muscles, heart, blood vessels, and exocrine glands.
  • Their preganglionic neurone is myeiinated while postganglionic fibres are nonmyelinated.
  • The effector cells do not undergo atrophy on nerve damage and can maintain independent activity.
  • The chemical transmitter is acetylcholine or noradrenaline depending on the site of release.
  • The two divisions of Autonomous nervous system remain in a state of dynamic equilibrium to maintain homeostasis.

Parasympathetic system participates in conserving the body energy while sympathetic system participates in utilizing the body energy.

Autonomous nervous system consists of afferent and efferent fibres with central connections for their integration. Autonomic afferent fibres conduct impulses from the peripheral organs to the central nervous system. They are non-myelinated and run in the same nerve bundles as the efferent fibres. Autonomic efferent fibres conduct impulses from the central nervous system to the periphery. They are of two types of Autonomous nervous system, i.e. cholinergic and adrenergic. They differ from each other and important differences are given below:

Cholinergic Fibres

  • They originate from central nervous system (craniosacral outflow: Cranial—3, 7, 9, 10, 11 and sacral—2, 3,4).
  • Their preganglionic fibres are long and myeiinated.
  • The ganglia are located very close to the tissue, innervated.
  • Their postganglionic fibres are short and non-myelinated.
  • On stimulation, resultant reaction is localized and they produce changes that tend to conserve energy.

The neurotransmitter at preganglionic and postganglionic neurones is acetylcholine.

Adrenergic Fibres

  • They also originate from central nervous system (thoracolumbar outflow: T1 to T12, L1toL3).
  • Their postganglionic fibres are long and non-myelinated (except adrenal medulla).

Their ganglia are located away from organ as under:

a. A pair of paravertebral ganglion chains.

b. Prevertebral ganglia

c. Adrenal medulla

d. Terminal ganglia to urinary bladder and rectum.

On stimulation, they produce diffused or generalized reaction and they prepare the body for active exertion (fight or flight). The neurotransmitter at preganglionic fibres is acetylcholine, while mainly noradrenaline at postganglionic neuron except those which innervate sweat glands and some blood vessels in skeletal muscles where acetylcholine is neurotransmitter.

Cholinergic Receptors

The actions of acetylcholine on visceral effectors resemble the actions of naturally occurring plant alkaloid muscarine. Therefore, these receptors are called muscarinic receptors, which are blocked by atropine. The cholinergic receptors at autonomic ganglia (sympathetic and parasympathetic) and neuromuscular junction are known as nicotinic receptors because the responses on these receptors resemble with the effects of naturally occurring alkaloid nicotine. Hexamethonium b1ocks & d-tubocurarine blocks these receptors in the skeletal muscle. Recently the cholinergic receptors have been classified into subtypes as under:

1. M1 (neural):

  • Distributed in autonomic ganglia, myentric plexus of stomach, presynaptic and postganglionic parasympathetic nerve to SA node and cerebral cortex.
  • They are excitatory in nature.
  • They stimulate membrane phospholipase C-Ca Phosphoinositide system.
  • They induce gastric acid secretion and increase gastrointestinal motility.
  • Oxotremorine, acetylcholine and carbachol are agonists and pirenzepine, telenzepine, atropine and dicyclomine are antagonists.

2. M2 (cardiac):

  • Distributed in myocardium and smooth muscles.
  • They produce inhibitory effect by inhibiting adenylate cyclase and activating K channels.
  • They produce negative chronotropic and inotropic effects.
  • Acetylcholine and carbachol are agonists while atropine, gallamine and methoetramine are antagonists.

3. M3 (glandular):

  • Distributed in smooth muscles and exocrine glands.
  • They are excitatory in nature and the mechanism of action is same as that of M1 receptors.
  • They cause contraction of smooth muscles and secretion of exocrine glands.
  • Acetylcholine and carbachol are agonists while atropine, difenidol and hexahydrosila are antagonists.

4. ACh-NM (N1):

  • Distributed in neuromuscular junction.
  • They cause contraction of skeletal muscle by opening up of cationic channels (Nat Kj, resulting in depolarization on
  • Acetylcholine, nicotine and phenyl trimethylammonium are agonists while d-tubocurarine and alpha-bungarotoxin are antagonists.

5. ACh-NN (N):

  • Distributed in autonomic ganglia, suprarenal medulla, and CNS.
  • On stimulation at various sites, they produce transmission of impulse in ANG, release of catecholamine and excitation or inhibition of CNS by opening up of cationic channels (Nat, K, Caj resulting in depolarization.

Nicotine, ACh, dimethyl-phenyl piperazinium are agonists while hexamethonium and trimetaphan are antagonists.

Adrenergic Receptors

Norepinephrine is the neurotransmitter at the sympathetic postganglionic nerve terminals that innervate visceral effectors. Based on the nature and physiological response obtained, adrenoceptors are classified as alpha (a1 and a2) and beta (I and l2) receptors. Generally, alpha receptors are excitatory (vasoconstriction and beta receptors are inhibitory (vasodilatation) in nature with the exception of heart (1- stimulatory) and visceral smooth muscles (alpha and beta receptors are inhibitory).

Transmission in Autonomous nervous system: Two chemical transmitters have been established as neurotransmitters in the Autonomous nervous system. These are acetylcholine and norepinephrine. Both the transmitters are synthesized primarily in the nerve terminals and stored in the synaptic or varicosity vesicles until released by a nerve impulse.

TO REMEMBER

1. Autonomic nervous system (Autonomous nervous system) maintains homeostasis by integrating with sensory, somatomotor, endocrinal, metabolic, immunological and emotional activities. It is divided into sympathetic (adrenergic) and parasympathetic (cholinergic) divisions.

2. Autonomous nervous system consists of afferent (conducts impulses from periphery to CNS) and efferent (conducts impulses from CNS to periphery) fibers. Efferent fibers maybe cholinergic or adrenergic.

3. Cholinergic fibres have long and myelinated preganglionic fibres while short arid non-myelinated postganglionic fibres. The neurotransmitter at preganglionic and postganglionic neuron is acetyleholine. The ganglia are located very close to the tissue, innervated.

4. Adrenergic fibres have long and nonmyelinated postganglionic fibres (except adrenal medulla) and their ganglia are located away from organ. The neurotransmitter at preganglionic fibres is acetylcholine, while mainly noradrenaline at postganglionic neuron except those which innervate sweat glands and some blood vessels in skeletal muscles where it is acetyicholine.

5. Cholinergic receptors are divided into muscarinic (M1, M, and M3 subtypes) and nicotinic (ACh-NM (N1) and AChNN (N2) subtypes) receptors.

6. Muscarinic receptors (M1, M2 and M3) are blocked by atropine.

7. ACh-NM (N1) nicotinic receptors are blocked by d-tubocurarine, while AChNN (N2) nicotinic receptors are blocked by hexamethonium and trimetaphan.

8. Adrenergic receptors are classified as alpha (c and c) and beta (f and [2) receptors. Generally, alpha receptors are excitation and beta receptors are inhibitory with the exception of beta-i receptors of heart (stimulatory)and alpha and beta receptors of visceral smooth muscles (inhibitory).

9. Norepinephrine is the neurotransmitter at the sympathetic postganglionic nerve terminals.

10. Both the neurotransmitters (ACh and NE) are synthesized primarily in the nerve terminals and stored in the synaptic or varicosity vesicies until released by a nerve impulse.

About Dr. Muna

Dr. Muna Taqi is a Dental surgeon from India who has more than 10 years of experience in the field of Oral & Maxillofacial surgery, Endodontics, & Pedodontics. She has worked in multinational medical corporates in Middle East and is also a consultant dental surgeon for many. She has authored many articles for medical journals & websites and is a consultant dental expert for Healthdrip.

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