AUTONOMIC NERVOUS SYSTEM-PART-V-Continued...

Ganglionic Blockers-Continued...

These agents are antagonizing or blocking acetylcholine to act on its neuronal nicotinic (Nn,)receptors at the ganglia.

As we all know the Nn receptor action of acetylcholine is necessary to release the respective transmitters of cholinergic and adrenergic postganglionic end to act on the respective target cells.If these ganglionic Nn receptors are blocked both the systems will be affected not at the same degree but in different degrees in different effector sites as follows:-

Heart-In heart normally parasympathetic (cholinergic) nervous system is prominent to keep the heartbeat in a proper peaceful rhythm. But by blocking the ganglia, the system would lose its prominence and the unopposed sympathetic(adrenergic)system would raise the heart rate leads to tachycardia, and arrhythmias and blood pressure would rise.

Blood vessels-The The arteries and veins are dilated to deliver more blood to the periphery as the predominant sympathetic(adrenergic)system loses its prominence.

Eye-The normally predominant parasympathetic system loses its hold and the unopposed sympathetic system produces cycloplegia and mydriasis (dilation of the pupil)

Digestive system-The The normally predominant parasympathetic nervous system loses its hold leads to unopposed sympathetic effects such as constipation, dry mouth, reduced gastric and pancreatic secretions.

The urinary system-The usual predominant parasympathetic system loses its hold leads to urinary retention 

Sweat glands-The usually predominant sympathetic (cholinergic) effect loses its hold to sweat block(hot flashes)

Classifications of Ganglionic Blockers

1. Depolarizing GBs-These are the agents such as nicotine first stimulating the Nn-receptor in the ganglia and depolarize it similar to acetylcholine. But with persisting prolonged depolarization the action potential is nullified and the receptor is further desensitized to acetylcholine followed by the ganglionic blockade.

That is why in the tobacco(nicotine) poisoning at first the heart rate is increased due to the stimulation of the sympathetic ganglia and adrenal medulla to release more catecholamines followed by the ganglionic blockade to counteract these effects

(e.g.) Nicotine and Labeling

These drugs have no therapeutical values.

2.Non-depolarizing GBs -These drugs directly block the receptors without prior stimulations or depolarizing effects.

(e.g) hexamethonium, mecamylamine, and trimetaphane.

Because of the severe toxicities and complications these drugs used very rarely in treatments.

 

AUTONOMIC NERVOUS SYSTEM-PART-V

GANGLIONIC BOCKING AGENTS

What is a ganglion? A ganglion is a singular form in number for which the plural term is ganglia is a small round shaped nerve node at which two nerve fibers one from the spinal cord towards the ganglion the preganglionic fiber and the other from the ganglion to the target tissues, the postganglionic fiber are snapped together with a small gap between the presynaptic and postsynaptic receptors. The above video may give a proper study of the ganglion. If you want a thorough about ganglia please watch the video.

AUTONOMIC NERVOUS SYSTEM-PART-IV

NEUROMUSCULAR BLOCKING AGENTS

These drugs block completely the neuromuscular cholinergic transmission by blocking at the nicotinic receptors of the somatic system. The somatic nervous system (SNS) is as we have already seen as a part of the Peripheral Nervous System(PNS) apart from ANS. Even though it is to some extent limits to a voluntary nervous system but beyond the limits, it may become autonomous.

The drugs which block the cholinergic transmissions at the neuromuscular junction causes muscle relaxation and are used as adjuvant drugs to produce anesthesia.

Classifications:-

1.Nondepolarizing blockers

2.Depolarizing blockers

A.Nondepolarizing blockers:-

1.Tubocurarine

2.Pancuronium 

3.Atracurium

4.Vecuronium

Among them, pancuronium has a longer duration of action.

They are not affecting all the muscles together but in order of first, the eye muscles followed by the facial muscles and at last the respiratory muscles.

Because of the poor intestinal absorption, these medicines are recommended by intravenous routes.

Side effects

They produce severe allergic reactions as they stimulate histamine release causes severe anaphylactic reactions, shock, hypotension, tachycardia followed by respiratory failure.

The above side effects can be effectively counteracted by administering a cholinergic agonist such as edrophonium, or neostigmine.

B.Depolarizing Blockers:-

Membrane depolarization usually occurs by the action of acetylcholine at its nicotinic receptors and the sodium channels are opened and rapid exchange of positively charged sodium ions from outside to inside the neuronal axon(lumen) through its channels to depolarize the muscle and when the threshold reached the acetylcholine is rapidly inactivated by the acetylcholinesterase and action potential occur followed by the rapid repolarization to sensitize our muscle for its normal functioning.

The depolarizing blockers such as Succinylecholine(Suxamethonium) is acting in a similar manner to acetylcholine by binding to the nicotinic receptor and causes depolarization. But as succinylcholine is not inactivated by the enzyme the depolarization is prolonged and goes beyond the threshold and the receptor is continuously desensitized to acetylcholine so that there is no or very weak action potential occur and the muscle is relaxed or paralyzed.

The duration of action is very short (3 to 6 min)

Unlike acetylcholine which is metabolized by the enzyme acetylcholinesterase, succinylcholine is metabolized by plasma cholinesterase.

It is used as an adjuvant in general anesthesia.

Since the neuromuscular blockers have effect only on nicotinic Nm subtype receptors and hence they have no ganglionic blocking effect at Nn subtype.

 

 

 

AUTONOMIC NERVOUS SYSTEM-PART-III

NEUROMUSCULAR BLOCKING AGENTS

These drugs block completely the neuromuscular cholinergic transmission by blocking at the nicotinic receptors of the somatic system. The somatic nervous system (SNS) is as we have already seen as a part of the Peripheral Nervous System(PNS) apart from ANS. Even though it is to some extent limits to a voluntary nervous system but beyond the limits, it may become autonomous.

The drugs which block the cholinergic transmissions at the neuromuscular junction causes muscle relaxation and are used as adjuvant drugs to produce anesthesia.

Classifications:-

1.Nondepolarizing blockers

2.Depolarizing blockers

A.Nondepolarizing blockers:-

1.Tubocurarine

2.Pancuronium 

3.Atracurium

4.Vecuronium

Among them, pancuronium has a longer duration of action.

They are not affecting all the muscles together but in order of first, the eye muscles followed by the facial muscles and at last the respiratory muscles.

Because of the poor intestinal absorption, these medicines are recommended by intravenous routes.

Side effects

They produce severe allergic reactions as they stimulate histamine release causes severe anaphylactic reactions, shock, hypotension, tachycardia followed by respiratory failure.

The above side effects can be effectively counteracted by administering a cholinergic agonist such as edrophonium, or neostigmine.

B.Depolarizing Blockers:-

Membrane depolarization usually occurs by the action of acetylcholine at its nicotinic receptors and the sodium channels are opened and rapid exchange of positively charged sodium ions from outside to inside the neuronal axon(lumen) through its channels to depolarize the muscle and when the threshold reached the acetylcholine is rapidly inactivated by the acetylcholinesterase and action potential occur followed by the rapid repolarization to sensitize our muscle for its normal functioning.

The depolarizing blockers such as Succinylecholine(Suxamethonium) is acting in a similar manner to acetylcholine by binding to the nicotinic receptor and causes depolarization. But as succinylcholine is not inactivated by the enzyme the depolarization is prolonged and goes beyond the threshold and the receptor is continuously desensitized to acetylcholine so that there is no or very weak action potential occur and the muscle is relaxed or paralyzed.

The duration of action is very short (3 to 6 min)

Unlike acetylcholine which is metabolized by the enzyme acetylcholinesterase, succinylcholine is metabolized by plasma cholinesterase.

It is used as an adjuvant in general anesthesia.

Since the neuromuscular blockers have effect only on nicotinic Nm subtype receptors and hence they have no ganglionic blocking effect at Nn subtype.

 

 

 

AUTONOMIC NERVOUS SYSTEM-PART-II-CONTINUED

INDIRECTLY ACTING CHOLINERGIC STIMULANTS(AGONISTS)

These are the drugs enhancing cholinergic activities indirectly by inhibiting the enzyme acetylcholinesterase (which is responsible for hydrolyzing and inactivating acetylcholine) and thereby enhancing the acetylcholine activities.

There are two types of enzyme inhibitors as follows:-

A.IRREVERSIBLE INHIBITORS(PESTICIDE POISONING)

The followings are a few examples:-

1.Isoflurophate

2.Echothiophate

3.Parathion

All the above examples belong to a chemical group of organophosphates the pesticide poisons. Most of the pesticides contain organophosphates and are used for suicidal self-poisoning or careless accidental poisoning. In the olden days, these compounds are used in wars as nerve gases.

Organophosphates are very powerful and cause serious side effects as they irreversibly inactivate the enzyme acetylcholinesterase by forming covalent bonds. This will cause prolonged and overactivity of acetylcholine causes serious cholinergic crisis such as

1.Respiratory depression(Bronchoconstriction and increased secretions)

2.Bradycardia(depressed heart rate)

3.Diarrhea

4.Enuresis(Involuntary urination)

5.Over sweating

6.Blurred vision(Miosis)

and all the already elucidated DUMBELS cholinergic crisis leads to death.

These covalent bonds are mostly irreversible yet if a cholinesterase reactivator such as pralidoxime is given immediately along with a muscarinic antagonist like atropine in order to avoid phosphorylation of the enzyme irreversibly. Pralidoxime act immediately at bonding reaction between the active site of the enzyme and the phosphate radical of the drug. This action prevents the aging of the bond to become irreversible and the phosphate radical would easily be removed from the enzyme which becomes active again.

Treatments of organophosphates pesticide poisoning need immediate hospitalization with blood samples for assays to estimate the level of inactivation of the enzyme. The loss of fluids and electrolytes due to diarrhea should be compensated. Respiration should be monitored and corrected by giving appropriate respiratory stimulants.

Atropine should be given according to the doctor's advice along with the enzyme reactivators.

Uses

Very rarely these compounds are used in glaucoma, accommodative esotropia (Crossed Eye) 

B.REVERSIBLE INHIBITORS

1.Edrophonium (short-acting)

2.Physostigmine

3.Neostigmine

4.Pyridostigmine

All the above are natural and synthetic alkaloids. Physostigmine occurs naturally in Calabar beans(Physostigma venenosum)

Unlike organophosphates, these compounds have not inactivated the enzyme irreversibly. They do not form covalent bonds with the enzyme. Hence they have some therapeutic values.

A.Physostigmine

1.Second choice of drug in glaucoma after pilocarpine

2.Used to counteract an overdose of atropine, phenothiazines, and tricyclic antidepressants.

3.Used in intestinal atony.

Side effects:-

Physostigmine can cross CNS as it is a tertiary amine(unionized), can cause convulsions. Other effects are similar to DUMBELS

B.Neostigmine

Neostigmine is a synthetic alkaloid similar to physostigmine but it will not enter into CNS by crossing the blood-brain barrier as it is a quaternary carbamate compound forms highly polarized hydrophilic positive quaternary ammonium ion.

Use

1. This is the drug of choice in the treatment of myasthenia gravis the neuromuscular disease in which muscle weakness and fatigue manifest.

2.Treating urinary retention 

3.Treating paralysis of the small intestine(paralytic ileus)

4.The antidote for tubocurarine poisoning.

Neostigmine is acting a short course of action when compared with physostigmine usually of  2 to 4 hours

Side effects:-

DUMBELS

C.Edrophonium 

Edrophonium is similar to neostigmine and its a course of action is very shorter than neostigmine such as 5 to 15 minutes.

Because of this edrophonium is not used in maintenance therapy but can be as a diagnostic agent to differentiate myasthenia gravis from the cholinergic crisis. Both conditions can bring muscle weakness but administration of edrophonium may improve myasthenia gravis but worsens the cholinergic crisis.

Side effects:-

DUMBELS

C.Pyridostigmine    

Its duration of action is 3 to 6 hours.

Used conveniently in myasthenia gravis

Side effects:-

DUMBELS 

 

 

 

 

 

AUTONOMIC NERVOUS SYSTEM-PART-II

CHOLINERGIC STIMULATION

In the ANS acetylcholine is the major autonomic nerve transmitter in all the preganglionic fibers and the parasympathetic postganglionic fibers and the sympathetic postganglionic fibers supplied to the sweat glands.

Acetylcholine is acting on the nicotinic receptors in the preganglionic level of both sympathetic and parasympathetic systems, and muscarinic receptors at the postganglionic level.

This can be detailed as follows:-

1.Preganglionic fibers of autonomic ganglia (Nn)

2.Preganglionic fibers that supplied to the adrenal medulla (Nn)

3.Postganglionic fibers of the parasympathetic system (M)

4.Postganglionic fibers of the sympathetic system that supplied to the sweat glands(M)

Nn means the nicotinic receptor at the neuronal level the name earned because it was first identified by nicotine.

M means the muscarinic receptor because it was first identified by using muscarine.

Here we deal with drugs that mimic acetylcholine's cholinergic stimulant actions.

These drugs are known as cholinomimetics and are categorized as follows:-

1.Direct Acting Stimulants (Agonists)

These are acting by chemically binding with the acetylcholine's nicotinic and muscarinic receptors in the body.

a.Acetylcholine

This is the natural endogenous prototype drug secreted within our body and affects almost every system in our body. This can be summarised as follows:-

Cardiovascular system In this system if acetylcholine dominates by acting on its muscarinic receptors in the parasympathetic postganglionic level as in rest and sleep, it decreases heart rate, B.P., and contractility.

Digestive system Increases intestinal motility

Urinary system Increases contractility of the bladder 

Pulmonary system It increases pulmonary secretions.

The eye In the eyes it causes pupillary constriction (miosis) 

Periphery It causes contraction of the muscles by its somatic nicotinic receptors(Nm) at the neuromuscular junction

Endocrine system It causes the release of adrenaline from the adrenal medulla by its nicotinic action.

As a medicine, this drug is very rarely used to produce miosis in the eyes and its use is very limited as it has widespread unwanted actions and is rapidly destroyed by acetylcholinesterase and eliminated by the body.

Side effects

Diarrhea and Decreased BP

Urination

Miosis

Bronchial secretions and Bradycardia

Excitation of the bone muscles

Lacrimation

Salivation

The side effects can be easily memorized by the anagram 'DUMBELS'

b.Bethanechol 

It is chemically a carbamic acid ester.

It acts mainly on muscarinic receptors

Therapeutically it can be used to relieve constipation and urinary retention.

Side effects

Similar to that of acetylcholine

c.Carbachol

It is also similar to bethanechol a carbamic ester but because of more side effects due to nicotinic effects the drug is very rarely used to produce miosis during eye surgery and in glaucoma

Acting on both muscarinic and nicotinic receptors

Side effects similar to that of acetylcholine

d.Pilocarpine

Pilocarpine is an alkaloid from the leaves of typical south American shrubs from the genus Piocarpus

Causes,

miosis

decreased heart rate

bronchial contractions

increase salivary, lacrimal and sweat secretions

The drug is unaffected by the enzyme acetylcholinesterase and hence yield prolonged action.

This is the drug of choice for the treatment of glaucoma

It has the advantage of acting primarily on muscarinic receptors only.

Side effects

Similar to acetylcholine but in addition to that it can enter the CNS to give effects like migraine,delusions, dizziness, etc.

e.Methacholine

Because of its short duration of action, it can be used for diagnosing asthma and bronchial hyperactivity.

Acting mainly on muscarinic receptors.

Side effects are due to generalized cholinergic stimulation.

-Continued (Indirect Acting Agonists)next post. 

 

 

AUTONOMIC NERVOUS SYSTEM-PART-1

AUTONOMIC NERVOUS SYSTEM(ANS)-OVER VIEW

In general, our nervous system is divided into two branches such as 1.Central Nervous System(CNS) which is centered at the brain systems and highly protected by the blood-brain barrier and skull.

2. Peripheral Nervous System(PNS) which is centered outside the brain midbrain, and spinal cord and is not much protected as CNS. Hence PNS is highly prone to be injured by toxins and mechanical means.

PNS is acting as a mediator between the end organs with the CNS.

The PNS  is again divided into two such as 1. The Somatic nervous system supplied to the skeletal muscles to express our voluntary desired movements to the brain by sending and receiving back and forth the impulses and commands from the brain accordingly.

2. Autonomous Nervous System is an involuntary control of smooth muscles such as the heart, lungs, and liver etc.etc.

The system is connected to the CNS in two different physiological networks known as the sympathetic and parasympathetic nervous systems.

The autonomic nervous system is defined as a collection of nuclei, cell bodies, nerves ganglia, and plexuses that provides afferent and efferent nerves supplied to the smooth muscles and visceral organs that carries commands from the brain and stimuli or impulses to the brain by the end involuntary smooth muscles and organs.75% of the ANS nerve networks are linked with the CNS through the medulla oblongata.

 The autonomic nervous system is so important as it regulates involuntary functions such as blood pressure, heart rate, and the digestive system.
As in the diagram above the ANS is divided anatomically into two major components as follows:-
1.Parasympathetic Nervous System in which there is a long preganglionic fiber that snaps with a short postganglionic fiber at the ganglion situated somewhere nearby the target organ as in the diagram shown above. The preganglionic fibers originate from cranial nerve nuclei III, VII, IX, and X as well as the III and IV sacral spinal roots and this system is known as craniosacral outflow
2.Sympathetic Nervous System in which there are short preganglionic fibers that snap with the long postganglionic fibers at the ganglia to form a sympathetic ganglionic chain adjacent to the spinal cord. All preganglionic fibers of the sympathetic nervous system originate in the thoracolumbar portion of the spinal cord and it is known as thoracolumbar outflow
Pharmacologically acetylcholine is the nerve transmitter in the preganglionic fibers of both the systems and postganglionic fibers of the parasympathetic nervous system.

Norepinephrine is the major nerve transmitter at the postganglionic nerves in the sympathetic nervous system except the postganglionic sympathetic nerve supply in which acetylcholine is the nerve transmitter. 
Responses of the effector organs to the autonomic nervous system can give us an outline of predicting various drugs that mimic or antagonize the actions of these nerves.
The two divisions of the ANS are generally antagonizing each other in some sites like the heart and digestive system and synergizing in some sites such as in sex organs.

General Functions Of ANS

Generally, the combined functions of the ANS are important in regulating the activities of vital organs which are functioning involuntarily below the level of consciousness. Thus respiration, circulation, digestion, body temperature, metabolism, sweating, and endocrine secretions of hormones are all regulated and controlled in part or entirely by ANS.

Specific Functions Of ANS

The specific functions of the two divisions of the ANS can be emphasized as follows:-

1. The sympathetic nervous system is the predominant one in normal situations as the majority of the combined nerve supply to various parts of our body belongs to this anatomical division of ANS. This nervous system is active even at rest but becomes dominant during stress by its adrenergic postganglionic activity. For example, if you feel danger or angry, or any other mind provoking thoughts, your heart rate increases, blood pressure rises, eyes dilate, blood sugar rises, lungs expand with more oxygen demand, and the face and parts of the body reddened as the blood flow shifts from the skin to skeletal muscle in order to get ready for a fight or flight.

The effector organs are responding to sympathetic(adrenergic) stimulations by the following receptors such as alpha-1,alpha-2;

beta-1,beta-2;and dopamine receptors.

In contrast, the parasympathetic nervous system which is not much innervated in our body is predominant at rest, sleep, and the situation of calmness. It is a nervous system of peace and tranquil. Stimulation of this nervous system in which the postganglionic fibers are 100% cholinergic slows heart rate, lower blood pressure, increase intestinal motility, constrict the pupil(miotic) and empty the urinary bladder. In general, this is a peace and digestive system.

The effector organs respond to this system(Cholinergic) at the receptors such as nicotinic(Nm, Nn) and muscarinic(m-1 to m-5)

The two systems are working and active all the time. Their action is antagonized to each other according to the situations. For example, the heart rate is normally under the influence of cholinergic. But when the situation gets tense the heart rate goes under the influence of the adrenergic sympathetic nervous system.

Nerve Transmitters

Apart from the anatomical sympathetic division of ANS, a pharmacological division of ANS is more convenient to understand their activities by means of the autonomical transmitters as follows:-

1.Cholinergic Nervous System in which the transmitter at the target organ is Acetylcholine

2.Adrenergic Nervous System in which the transmitter at the target organ is Norepinephrine.

The calcium ion is required to release the transmitters from their storage vesicles.