Easy pharmacology- Peripheral nervous system

Peripheral nervous system

The peripheral nervous system (PNS) is the division of the nervous system that is outside the brain and spinal cord. It includes all the nerves and ganglia that lie outside the central nervous system (CNS). The PNS is responsible for connecting the CNS to the rest of the body, and it is divided into two main parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls voluntary movements and sensory information from the skin, while the autonomic nervous system controls involuntary functions such as heart rate, digestion, and glandular activity.

Autonomic nervous system

The autonomic nervous system (ANS) is a division of the peripheral nervous system that controls the functions of internal organs and glands. It is responsible for regulating the body's unconscious actions, such as heart rate, digestion, and glandular activity. The ANS is divided into two main branches: the sympathetic nervous system and the parasympathetic nervous system.

The sympathetic nervous system is responsible for the "fight or flight" response, which prepares the body for sudden physical activity or stress. This branch of the ANS increases heart rate, blood pressure, and blood sugar levels, and diverts blood flow away from nonessential functions such as digestion.

On the other hand, the parasympathetic nervous system is responsible for the "rest and digest" response, which returns the body to a state of calm after the sympathetic nervous system has been activated. This branch of the ANS decreases heart rate, blood pressure, and blood sugar levels, and increases blood flow to the digestive system.

The ANS works in a delicate balance with the sympathetic and parasympathetic systems working together to maintain homeostasis, or balance in the body.

Receptors

The adrenergic system is the part of the autonomic nervous system that is activated by the release of the hormone adrenaline (also called epinephrine) and noradrenaline (also called norepinephrine). The receptors for this system are called adrenergic receptors.

There are two main types of adrenergic receptors: alpha (α) receptors and beta (β) receptors.

• Alpha (α) receptors are further divided into two subtypes: α1 and α2. α1 receptors are found in smooth muscle, and when stimulated they cause vasoconstriction (narrowing of blood vessels) and increased muscle tone. α2 receptors are found in the brain and other organs, and when stimulated they inhibit the release of certain neurotransmitters, such as norepinephrine and acetylcholine.

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• Beta (β) receptors are also divided into two subtypes: β1 and β2. β1 receptors are found in the heart and kidneys and when stimulated they increase heart rate, contractility and renin secretion. β2 receptors are found in the smooth muscles of blood vessels, bronchi, and uterus, and when stimulated they cause vasodilation (widening of blood vessels) and bronchodilation.

Sympathomimetic and sympatholytic drugs

Sympathomimetic drugs are a class of drugs that mimic the effects of the sympathetic nervous system. They work by stimulating the adrenergic receptors in the body, which are responsible for the "fight or flight" response. Examples of sympathomimetic drugs include:

• Adrenaline (epinephrine): a naturally-occurring hormone that is used to treat severe allergic reactions (anaphylaxis) and cardiac arrest
• Dopamine: used to increase blood flow in shock and heart failure
• Isoproterenol: a synthetic drug used to treat asthma and heart failure
• Phenylephrine: used to raise blood pressure in hypotension and as a nasal decongestant

On the other hand, sympatholytic drugs are a class of drugs that inhibit the effects of the sympathetic nervous system. They work by blocking the adrenergic receptors in the body, which can decrease heart rate, blood pressure, and other physiological processes. Examples of sympatholytic drugs include:

• Propranolol: a beta-blocker used to treat hypertension, angina and heart arrhythmias
• Prazosin: an alpha-blocker used to treat hypertension and benign prostatic hyperplasia
• Clonidine: an alpha-2 agonist used to treat hypertension, ADHD and menopausal hot flashes
• Reserpine: an inhibitor of the vesicular monoamine transporter used to treat hypertension

In summary, sympathomimetic drugs mimic the effects of the sympathetic nervous system, while sympatholytic drugs inhibit the effects of the sympathetic nervous system. Both types of drugs are used to treat a variety of conditions, such as hypertension, heart failure, and asthma.

Parasympathomimetic and parasympatholytic drugs

Parasympathomimetic drugs are a class of drugs that mimic the effects of the parasympathetic nervous system. They work by stimulating the cholinergic receptors in the body, which are responsible for the "rest and digest" response. Examples of parasympathomimetic drugs include:

• Acetylcholine: a naturally-occurring neurotransmitter that is used to stimulate muscle contractions, including those of the heart and gastrointestinal tract.
• Pilocarpine: used to treat glaucoma and dry mouth
• Bethanechol: used to treat urinary retention and gastroparesis
• Carbachol: used as ophthalmologic and otic medication

On the other hand, parasympatholytic drugs are a class of drugs that inhibit the effects of the parasympathetic nervous system. They work by blocking the cholinergic receptors in the body, which can increase heart rate, blood pressure, and other physiological processes. Examples of parasympatholytic drugs include:

• Atropine: used to dilate the pupils, to treat certain types of poisonings, as well as to dry secretions in conditions such as bronchitis, and to inhibit gastric motility in certain gastrointestinal conditions
• Scopolamine: used to prevent motion sickness and as a preanesthetic medication
• Glycopyrrolate: used to reduce secretions in surgery and in respiratory conditions

In summary, parasympathomimetic drugs mimic the effects of the parasympathetic nervous system, while parasympatholytic drugs inhibit the effects of the parasympathetic nervous system. Both types of drugs are used to treat a variety of conditions, such as glaucoma, urinary retention, and motion sickness.

Neuromuscular blocking agent

A neuromuscular blocking agent (NMB) is a type of drug that is used to temporarily paralyze the muscles, typically for surgical or diagnostic procedures. These drugs work by inhibiting the transmission of nerve impulses at the neuromuscular junction, the point where nerve impulses are transmitted to muscle fibers. This causes the muscles to relax and become paralyzed, which allows for certain procedures to be performed without movement or muscle contraction.

Examples of neuromuscular blocking agents include:

• Succinylcholine: a depolarizing agent that causes rapid, complete paralysis of all skeletal muscles.
• Pancuronium, vecuronium, rocuronium, and atracurium: non-depolarizing agents that cause a more gradual onset and longer duration of paralysis.
• Cisatracurium, mivacurium, and pipecuronium: intermediate-acting non-depolarizing agents

These drugs are typically administered by injection, either intravenously or intramuscularly. They are used in various surgical and diagnostic procedures, such as general anesthesia, mechanical ventilation, and intubation. It is important to note that these drugs are controlled substance and should be used only by trained medical professionals.

Skeletal muscle relaxants

Skeletal muscle relaxants are a class of drugs that are used to relax the muscles and reduce muscle spasms, cramps, and pain. They work by inhibiting the release of the neurotransmitter acetylcholine at the neuromuscular junction, which reduces the ability of the nerve to activate the muscle. This leads to a decrease in muscle tone, which can help to relieve muscle spasms, cramps and pain.

Examples of skeletal muscle relaxants include:

• Baclofen: used to treat spasticity associated with conditions such as multiple sclerosis, spinal cord injuries, and cerebral palsy.
• Cyclobenzaprine: used to treat muscle spasms and pain associated with acute musculoskeletal conditions such as strains and sprains.
• Metaxalone: used to treat muscle pain and stiffness
• Carisoprodol: used to treat muscle pain and stiffness
• Tizanidine: used to treat muscle spasticity associated with conditions such as multiple sclerosis and spinal cord injuries.

These drugs are typically administered orally and they have different onset of action, duration of effect and can have different side effects. It is important to note that these drugs should be used only under the direction of a physician, as they can have side effects such as drowsiness, dizziness, and dry mouth, and may interact with other medications.