NEET MDS Lessons
Pharmacology
Structure of the CNS
The CNS is a highly complex tissue that controls all of the body activities and serves as a processing center that links the body to the outside world.
It is an assembly of interrelated “parts”and “systems”that regulate their own and each other’s activity.
1-Brain
2-Spinal cord
The brain is formed of 3 main parts:
I. The forebrain
• cerebrum
• thalamus
• hypothalamus
II. The midbrain
III. The hindbrain
• cerebellum
• pons
• medulla oblongata
Different Parts of the Different Parts of the CNS & their functions CNS & their functions
The cerebrum(cerebral hemispheres):
It constitutes the largest division of the brain.
The outer layer of the cerebrum is known as the “cerebral cortex”.
The cerebral cortex is divided into different functional areas:
1.Motorareas(voluntary movements)
2.Sensoryareas(sensation)
3.Associationareas(higher mental activities as consciousness, memory, and behavior).
Deep in the cerebral hemispheres are located the “basal ganglia” which include the “corpus striatum”& “substantianigra”.
The basal gangliaplay an important role in the control of “motor”activities
The thalamus:
It functions as a sensory integrating center for well-being and malaise.
It receives the sensory impulses from all parts of the body and relays them to specific areas of the cerebral cortex.
The hypothalamus:
It serves as a control center for the entire autonomic nervous system.
It regulates blood pressure, body temperature, water balance, metabolism, and secretions of the anterior pituitary gland.
The mid-brain:
It serves as a “bridge”area which connects the cerebrum to the cerebellum and pons.
It is concerned with “motor coordination”.
The cerebellum:
It plays an important role in maintaining the appropriate bodyposture& equilibrium.
The pons:
It bridges the cerebellum to the medulla oblongata.
The “locus ceruleus”is one of the important areas of the pons.
The medulla oblongata:
It serves as an organ of conduction for the passage of impulses between the brain and spinal cord.
It contains important centers:
• cardioinhibitory
• vasomotor
• respiratory
• vomiting(chemoreceptor trigger zone, CTZ).
The spinal cord:
It is a cylindrical mass of nerve cells that extends from the end of the medulla oblongata to the lower lumbar vertebrae.
Impulses flow from and to the brain through descending and ascending tracts of the spinal cord.
Anti-Histamines:
The effect of histamine can be opposed in three ways:
1. Physiological antagonism: by using a drug to oppose the effect (e.g adrenaline). Histamine constricts bronchi,
causes vasodilatation which increases capillary permeability. Adrenaline opposes this effect by a mechanism unrelated to histamine.
2. By preventing histamine from reaching its site of action (receptors), By competition with H1-H2 receptors (Drug antagonisms).
3. By preventing the release of histamine. (adrenal steroids and sodium-cromoglycate can suppress the effect on the tissues)
Types of Anti-histamine drugs
Selected H1 antagonist drugs
First-generation H1 receptor antagonists:
Chlorpheniramine (Histadin) & Dexchlorpheniramine
Diphenhydramine (Allermine)
Promethazine (Phenergan) - strong CNS depressants
Cyproheptadine (Periactin)
ACTION
These drugs bind to both central and peripheral H1 receptors and can cause CNS depression or stimulation.
- They usually cause CNS depression (drowsiness,sedation) with usual therapeutic doses
- Cause CNS stimulation (anxiety, agitation)
with excessive doses, especially in children.
They also have Anticholinergic effects (e.g. dry mouth, urinary retention, constipation, blurred vision).
Second-generation H1 receptor antagonists (non-sedating) agents
Terfenadine
Fexofenadine
Loratadine
Acravistine and Cetirizine
Astemizol
Action
They cause less CNS epression because they are selective for peripheral H1 receptors and do not cross the blood brain barrier.
Indications for use
The drugs can relieve symptoms but don’t relieve hypersensitivity.
1) Allergic rhinitis. Some relief of sneezing, rhinorrhea, nasal airway obstruction and conjunctivitis are with the use of antihistamine.
2) Anaphylaxis. Antihistamine is helpful in treating urticaria and pruritus.
3) Allergic conjunctivitis. This condition, which is characterized by redness, itching and tearing of the eyes.
4) Drug allergies. Antihistamines may be given to prevent or treat reactions to drugs (e.g, before a dignostic test that
uses an iodine preparation).
5) Transfusions of blood and blood products.
6) Dermatologic conditions. Antihistamines are the drug of choice for treatment of allergic contact dermatitis and
acute Urticaria. Urticaria often occurs because the skin has many mast cells to release histamine.
7) Miscellaneous. Some antihistamines are commonly used for non-allergic disorder such as motion sickness, nausea, vomiting, sleep, cough or add to cough mixtures.
Contraindication
hypersensitivity to the drugs, narrow-angle glaucoma, prostatic hypertroph, stenosing peptic ulcer, bladder neck obstruction, during pregnancy and lactating women
Adverse effects:
Drowsiness and sedation
Anticholinergic
Some antihistamines may cause dizziness, fatigue, hypotention, headache, epigastric distress and photosensitivity
Serious adverse reaction including cardiac arrest & death, have been reported in patients receiving high dose astemizole
H2-receptor antagonists
Cimetidine (Tagamate), Ranitidine (Zantac), Fomatidine, Nizatidine.
Mechanism of action
Numerous factors influence acid secretion by the stomach, including food, physiological condition and drugs. H2 receptor blockers reduce basal acid-secretion by about 95% and food stimulated acid-secretion by about 70%. Both conc. and vol. of H ions will decrease.
Pharmacokinetics:
1) They are all well absorbed after oral dose.
2) Antacids decrease their absorption in about 10-20%
Uses
Cimetidine - reduction of gastric secretion is beneficial, these are in main duodenal ulcer, benign gastric ulcer, stomach ulcer and reflux eosophagitis.
Rantidine -used as alternative for duodenal ulcer
Adverse effects:
headache, dizziness, constipation, diarrhoea, tiredness and muscular pain.