Revision_Essay Sept2025

Front

What is the structural organization of the spinal cord?

Back

The spinal cord is a cylindrical structure extending from the medulla oblongata to the L1–L2 level. It is protected by the vertebrae, meninges, and cerebrospinal fluid.

 

Structurally, it has 31 segments, each giving rise to a pair of spinal nerves. The internal structure shows gray matter in the center (shaped like an “H”) surrounded by white matter.

 

-      Gray matter: contains neuron cell bodies — anterior horn (motor), posterior horn (sensory), and lateral horn (autonomic).

-      White matter: contains ascending (sensory) and descending (motor) tracts..

Front

What is the role of the spinal cord in reflex activity?

Back

The spinal cord acts as a reflex center, where sensory impulses enter through the dorsal root, are processed in interneurons, and motor responses exit via the ventral root — forming a reflex arc. This allows rapid, automatic responses without brain involvement.

Front

Main functions of the spinal cord

Back

The spinal cord performs several vital functions:

1. Sensory conduction: Carries sensory information from body receptors to the brain through ascending tracts.

2. Motor conduction: Transmits motor commands from the brain to muscles and glands through descending tracts.

3. Reflex activity: Acts as a reflex centre, producing quick, automatic responses (e.g., withdrawal reflex).

4. Autonomic control: Regulates involuntary functions like bladder control and blood vessel tone.

5. Coordination: Integrates sensory and motor information for balance and posture. Overall, the spinal cord serves as a communication link and control centre essential for movement and sensation.

Front

Describe the Monro–Kellie Doctrine and role in maintaining normal intracranial pressure.

Back

The Monro–Kellie Doctrine states that the cranial cavity is a fixed, rigid space containing three components:

-      Brain tissue (≈80%)

-      Blood (≈10%)

-      Cerebrospinal fluid (CSF) (≈10%)

 

Since the skull cannot expand, an increase in one component must be compensated by a decrease in another to keep ICP within the normal range (5–15 mmHg).

Example:

-      If blood volume rises due to vasodilation, CSF is displaced into the spinal canal or its absorption increases to balance pressure.

-      If compensation fails, ICP rises, reducing cerebral perfusion and risking brain herniation.

 

Thus, the Monro–Kellie Doctrine explains how ICP is maintained under normal and pathological conditions.

Front

What happens when intracranial pressure increases beyond normal limits

Back

Normal ICP is 5–15 mmHg. When ICP rises, cerebral perfusion pressure (CPP) decreases.
CPP = Mean Arterial Pressure – ICP.

 

Effects of increased ICP:

1. Reduced cerebral blood flow → ischemia and hypoxia.

2. Compression of brain stem and cranial nerves.

3. Shift of brain structures → herniation syndromes (e.g., uncal or tonsillar herniation).

4. Altered consciousness, vomiting, and papilledema.

5. Brain Death if ICP severely compromises brainstem function.

 

Early recognition and treatment are critical to prevent irreversible brain injury more oxygen and nutrients reach the active neurons

Front

Factors that affect drug absorption

Back

Drug absorption depends on how a drug moves from the site of administration into systemic circulation. Factors include:

1. Route of administration: Oral, intravenous, or intramuscular routes affect absorption speed.

2. Blood flow: Higher blood flow increases absorption.

3. Drug solubility: Lipid-soluble drugs are absorbed faster than water-soluble ones.

4. pH and ionization: Non-ionized forms cross membranes more easily.

5. Surface area: Large surface areas (e.g., intestines) increase absorption.

6. Presence of food: Some drugs are better absorbed on an empty stomach, others with food.

 

These factors determine how quickly and how much of a drug reaches the bloodstream.

Front

How drugs are excreted from the body

Back

Excretion removes drugs and their metabolites to end their action.

Main routes:

1.    Kidneys (urine): Primary route; filtration, secretion, and reabsorption determine drug removal.

2.    Bile and feces: Drugs can be eliminated through bile into the intestine.

3.    Lungs: Volatile drugs like anesthetics are exhaled.

4.    Sweat, saliva, and breast milk: Minor routes of elimination.

 

Factors affecting excretion: Kidney function, urine pH, and drug solubility. Poor excretion can cause drug accumulation and toxicity.

Front

Principle and mechanism of plasma exchange

Back

1.    Plasma exchange, or plasmapheresis, works on the principle of removing and replacing a patient’s plasma to eliminate harmful substances such as antibodies or toxins.

2.    Blood is withdrawn and separated into plasma and blood cells using a special machine. The plasma is discarded and replaced with donor plasma, albumin, or saline.

3.    This helps remove disease-causing components like autoantibodies, immune complexes, or abnormal proteins.

The main goal is to restore normal immune and metabolic balance, especially in autoimmune and hematologic conditions.

Front

Advantages and limitations of plasma exchange

Back

Advantages:

1.    Quickly removes harmful antibodies and toxins.

2.    Provides fast symptom improvement in autoimmune crises.

3.    Can be life-saving in severe conditions like Thrombotic Thrombocytopenic (TTP) or Guillain–Barre Syndrome (GBS).

 

Limitations:

1.    Temporary effect; antibodies may return.

2.    Requires specialized equipment and trained staff.

3.    Possible side effects like hypotension, infection, or allergic reaction.

Despite these limitations, plasma exchange remains an effective short-term therapy in many immune-mediated diseases.