NEUROSCIENCES – HISTOLOGY

The blood-brain barrier (BBB) is a highly selective barrier that protects the brain from toxins, pathogens, and fluctuations in systemic blood composition while allowing essential nutrients to pass through. It is composed of multiple structures working together to maintain CNS homeostasis.

Correct Answer: All of These

Why is “All of These” the Correct Answer?

The BBB is formed by several key components:

1. Tight Junctions Between Capillary Endothelial Cells

   • The capillary endothelial cells in the brain have tight junctions (zonula occludens), preventing the paracellular movement of substances.
   • These junctions force molecules to pass through the endothelial cells (transcellular transport) instead of between them, creating a highly selective barrier.

2. Pericytes Present Around the Capillaries

   • Pericytes are contractile cells that wrap around capillaries, regulating blood flow and barrier integrity.
   • They help control the permeability of the BBB and play a role in the immune response within the CNS.

3. The Impermeable, Non-Fenestrated, Basement Membrane of the Capillaries

   • The capillary basement membrane is continuous and non-fenestrated, acting as a mechanical and biochemical barrier that prevents large molecules and pathogens from entering the CNS.

4. Pseudopods of the Astrocytes

   • Astrocyte foot processes (pseudopods) surround the capillaries and release signals that maintain and regulate the integrity of the BBB.
   • They help in nutrient transport, repair, and modulation of neurovascular coupling.

Since all of these components collectively contribute to the BBB, the correct answer is “All of These.”

Why Are the Other Individual Options Incorrect on Their Own?

Each option represents an essential component of the BBB, but the barrier is not formed by just one of them alone—it is a combination of all these structures that provides the selective permeability characteristic of the BBB.

Schwann cells are glial cells that are responsible for the myelination of axons in the peripheral nervous system (PNS). Each Schwann cell wraps around a single axon segment, providing insulation and increasing the speed of nerve impulse conduction through saltatory conduction.

🔹 Key Features of Schwann Cells:

• Found in the PNS
• Myelinate one axon segment per Schwann cell
• Aid in nerve regeneration after injury

This makes the statement “They are produced in the peripheral nervous system” correct.

Why Others Are Incorrect?

❌ One Schwann cell covers 30 nerve fibers:

• Incorrect because each Schwann cell myelinates only one axon segment.
• In contrast, oligodendrocytes (CNS glial cells) can myelinate multiple axons.

❌ They are produced in the central nervous system:

• Incorrect because Schwann cells are in the PNS.
• Oligodendrocytes are the CNS myelinating cells.

❌ Multiple sclerosis is a demyelinating disorder that affects the Schwann cells:

• Incorrect because MS affects the CNS (oligodendrocytes), not Schwann cells.
• Guillain-Barré syndrome (GBS) is a demyelinating disease of the PNS, affecting Schwann cells.

❌ Schwann cells cover all cranial nerves:

• Incorrect because some cranial nerves (like the optic nerve) are myelinated by oligodendrocytes (since they are part of the CNS).
• Schwann cells only myelinate peripheral cranial nerves (e.g., CN III–XII).

Understanding the Cellular Composition of the Cerebellar Cortex

The cerebellar cortex consists of three layers:

1. Molecular Layer (Outer Layer)

   • Contains stellate cells and basket cells (both inhibitory interneurons).
   • Contains the dendrites of Purkinje cells and parallel fibers of granule cells.

2. Purkinje Layer (Middle Layer)

   • Contains Purkinje cells, which are the sole output neurons of the cerebellar cortex.

3. Granular Layer (Inner Layer)

   • Contains granule cells and Golgi cells.
   • Granule cells are the only excitatory neurons in the cerebellar cortex.

Why the Correct Answer is Right

✅ Pyramidal cells are NOT present in the cerebellar cortex.

• Pyramidal cells are excitatory neurons found in the cerebral cortex, particularly in the motor and association areas.
• They are NOT part of the cerebellar cortex, which is primarily composed of Purkinje cells and interneurons.

Why the Other Options Are Incorrect (These Cells Are Present in the Cerebellar Cortex)

• Stellate cells – Present in the molecular layer; inhibitory interneurons.
• Golgi cells – Present in the granular layer; inhibitory interneurons that regulate granule cells.
• Granule cells – Present in the granular layer; the only excitatory neurons in the cerebellar cortex.
• Basket cells – Present in the molecular layer; inhibitory interneurons that modulate Purkinje cells.

Explanation:

1. Cerebrum:
   • The cerebral cortex has 6 layers (also called the neocortex). These layers are:
     1. Molecular layer
     2. External granular layer
     3. External pyramidal layer
     4. Internal granular layer
     5. Internal pyramidal layer
     6. Multiform layer
2. Cerebellum:
   • The cerebellar cortex has 3 layers:
     1. Molecular layer (outermost)
     2. Purkinje cell layer (middle)
     3. Granular layer (innermost)

Why the Other Options Are Wrong:

1. 3 layers, 6 layers:
   • This is incorrect because the cerebrum has 6 layers, and the cerebellum has 3 layers.
2. 4 layers, 6 layers:
   • This is incorrect because the cerebrum has 6 layers, and the cerebellum has 3 layers.
3. 3 layers, 3 layers:
   • This is incorrect because the cerebrum has 6 layers, not 3.
4. 3 layers, 1 layer:
   • This is incorrect because the cerebrum has 6 layers, and the cerebellum has 3 layers.

The cerebral cortex contains several types of neurons, but Purkinje cells are not among them. Purkinje cells are found in the cerebellar cortex, not the cerebral cortex. Here’s a breakdown of the cell types mentioned:

1. Pyramidal cells: These are the most abundant neurons in the cerebral cortex and are involved in excitatory signaling.
2. Fusiform cells: These are spindle-shaped neurons found in the deepest layer of the cerebral cortex.
3. Betz cells: These are large pyramidal cells located in the primary motor cortex (layer V) and are involved in motor control.
4. Granule cells: These are small, densely packed neurons found in the granular layers of the cerebral cortex (e.g., layer IV).

In contrast, Purkinje cells are large, flask-shaped neurons found in the cerebellar cortex, where they play a key role in motor coordination.

Why the Other Cells Are Present in the Cerebral Cortex:

1. Pyramidal cells:
   • These are the principal neurons of the cerebral cortex and are involved in excitatory signaling.
2. Fusiform cells:
   • These neurons are found in layer VI of the cerebral cortex and are involved in corticothalamic communication.
3. Betz cells:
   • These are a type of pyramidal cell found in the primary motor cortex and are responsible for voluntary motor control.
4. Granule cells:
   • These are small neurons found in the granular layers (e.g., layer IV) of the cerebral cortex and are involved in sensory processing.

The histological structure of a peripheral nerve includes three layers of connective tissue:

1. Endoneurium: A thin layer of connective tissue surrounding individual nerve fibers (axons and their Schwann cells).
2. Perineurium: A dense, collagenous sheath that surrounds each fascicle (a bundle of nerve fibers). This layer provides structural support and acts as a diffusion barrier.
3. Epineurium: The outermost layer of connective tissue that surrounds the entire nerve, including multiple fascicles. It is thicker and less regular than the perineurium.

Thus, the statement that each fascicle is invested by a collagenous covering called the perineurium is correct.

Why the Other Options Are Wrong:

1. Epineurium is a thin, regular collagenous coat:
   • This is incorrect because the epineurium is thick and irregular, not thin and regular.
2. Around Schwann cells is a thin layer called the perineurium, consisting of reticular fibers:
   • This is incorrect because the perineurium surrounds fascicles, not individual Schwann cells. The layer around Schwann cells is the endoneurium, which contains reticular fibers.
3. Epineurium does not extend deeply to fill the space between fascicles:
   • This is incorrect because the epineurium does extend between fascicles, filling the space around them.
4. Axons and Schwann cells are not enclosed within layers of connective tissue:
   • This is incorrect because axons and Schwann cells are enclosed by the endoneurium, which is a layer of connective tissue.

Betz cells are large pyramidal neurons found in the primary motor cortex (Brodmann area 4). They are crucial for voluntary motor control as their axons form part of the corticospinal tract, which transmits motor signals from the brain to the spinal cord.

Key Facts about Betz Cells:

• Located in Layer V (5th layer) of the cerebral cortex – specifically in the internal pyramidal layer.
• Largest neurons in the CNS – they are giant pyramidal cells, with large somas and extensive dendritic trees.
• Axons contribute to the corticospinal tract, facilitating voluntary motor movement.

Why the Other Options Are Correct:

1. They are present in the 5th layer of the cerebral cortex → ✅ Correct
   • Betz cells are in Layer V (internal pyramidal layer), which is responsible for motor output.
   • This layer contains large pyramidal neurons, including Betz cells, that send descending motor commands to the spinal cord.
2. Their axons pass to the spinal cord as corticospinal fibers → ✅ Correct
   • Betz cells give rise to long axons that contribute to the corticospinal tract, which controls voluntary movement.
   • These fibers decussate (cross over) in the medulla (pyramidal decussation) before synapsing in the spinal cord.
3. Largest cells of the central nervous system → ✅ Correct
   • Betz cells are the largest neurons by soma size in the CNS, with extensive dendritic trees.
   • While Purkinje cells of the cerebellum have more dendritic branching, Betz cells have the largest cell bodies.
4. None of these → ❌ Incorrect
   • Since the statement about Betz cells being in the 6th layer is false, “None of these” is incorrect.

• Axons of peripheral nerves do NOT contain Nissl substance.
• Nissl substance (Nissl bodies) refers to the rough endoplasmic reticulum (RER) and ribosomes, which are abundant in the cell body (perikaryon) and dendrites but are absent in the axon.
• This is because protein synthesis does not occur in axons, as ribosomes and RER are needed for protein production, which is instead carried out in the cell body.

Why the Other Options Are Correct?

1. It may arise from axon hillock ✅
   • True, because axons originate from the axon hillock, which is a specialized region of the neuron’s cell body that lacks Nissl substance and is involved in action potential initiation.
2. It transmits action potential ✅
   • True, because the primary function of an axon is to transmit action potentials away from the neuron’s cell body to the next neuron or effector organ.
3. It may arise from the perikaryon (Soma) ✅
   • True, because in some neurons (e.g., unipolar neurons in the dorsal root ganglion), the axon can directly arise from the cell body (soma).
4. It may arise from a dendrite ✅
   • True, but rare. In some sensory neurons, particularly in the olfactory system, the axon may originate from a modified dendrite.

In the peripheral nervous system (PNS), myelination is carried out by Schwann cells, which wrap around a single axon to form a myelin sheath. This insulating layer enhances action potential conduction velocity by enabling saltatory conduction between Nodes of Ranvier.

• Each Schwann cell myelinates only one internode of one axon, unlike oligodendrocytes in the CNS, which can myelinate multiple axons.

Why the Other Options Are Incorrect?

1. Myelinating fibers have slow conduction velocity ❌
   • False, because myelination increases conduction velocity via saltatory conduction (action potentials jump between Nodes of Ranvier).
   • Unmyelinated fibers have slow conduction velocity.
2. It is also produced by fibroblasts as well ❌
   • False, because fibroblasts are involved in connective tissue support and extracellular matrix production, not myelination.
3. A Schwann cell can myelinate up to 30 axons ❌
   • False, because each Schwann cell myelinates only one internode of one axon in the PNS.
   • Oligodendrocytes in the CNS can myelinate multiple axons.
4. Oligodendrocytes are myelinating cells of the peripheral nervous system ❌
   • False, because oligodendrocytes myelinate axons in the CNS, not the PNS.
   • Schwann cells are exclusive to the PNS.

The myelin sheath in the peripheral nervous system (PNS) is produced by Schwann cells, which wrap around axons to increase conduction velocity by allowing saltatory conduction between Nodes of Ranvier.

• In the PNS, each Schwann cell myelinates only one internode of one axon.
• In the central nervous system (CNS), oligodendrocytes myelinate multiple axons at once.

Why the Other Options Are Incorrect?

1. In the central nervous system, myelinating cells myelinate several internodes of different axons ❌
   • True for CNS, but the question asks about the peripheral nerve.
   • Oligodendrocytes, not Schwann cells, myelinate multiple axons in the CNS.
2. It is produced by oligodendrocytes ❌
   • Oligodendrocytes produce myelin in the CNS, not the PNS.
3. It is produced by microglia ❌
   • Microglia are immune cells of the CNS, involved in phagocytosis and inflammation, not myelination.
4. In the peripheral nervous system, myelinating cells myelinate only one internode ❌
   • This is a tricky option because each Schwann cell myelinates only one internode.
   • However, the phrase “myelinating cells” might imply multiple Schwann cells working together, which could be misleading.
   • The most precise and correct answer remains: “It is produced by Schwann cells.”

The cerebellar cortex has three distinct layers, each containing specific neuronal cell types:

1. Molecular Layer (Outer Layer)
   • Contains Basket cells ✅
   • Contains Stellate cells ✅ (Incorrect option because Stellate cells are in this layer, not the granular layer)
   • Contains Purkinje cell dendrites and parallel fibers
2. Purkinje Layer (Middle Layer)
   • Contains Purkinje cells ✅ (Correct)
   • Purkinje cells are the only output neurons of the cerebellar cortex.
3. Granular Layer (Inner Layer)
   • Contains Granule cells ✅ (Correct)
   • Contains Golgi cells ✅ (Correct)
   • Granule cells excite Purkinje cells via parallel fibers

Why the Other Options Are Correct?

1. Purkinje cells in the middle Purkinje layer ✅
   • True. Purkinje cells form a single layer in the cerebellar cortex and provide inhibitory output to deep cerebellar nuclei.
2. Granule cells in the inner granular layer ✅
   • True. Granule cells are small excitatory neurons in the granular layer and give rise to parallel fibers in the molecular layer.
3. Basket cells in the outer molecular layer ✅
   • True. Basket cells are inhibitory interneurons in the molecular layer, synapsing onto Purkinje cell bodies.
4. Golgi cells in the inner granular layer ✅
   • True. Golgi cells are inhibitory interneurons in the granular layer, regulating granule cell activity.

• Neurons are the most sensitive cells in the central nervous system (CNS) because they have high metabolic demands and rely on continuous oxygen and glucose supply.
• Unlike glial cells, neurons have limited regenerative capacity and are highly susceptible to ischemia, hypoxia, toxins, and metabolic disturbances.
• Neuronal death due to ischemia (stroke) or neurodegenerative diseases (e.g., Alzheimer’s, Parkinson’s) often leads to permanent loss of function.

Why the Other Options Are Wrong:

1. Renshaw Cells ❌
   • These are inhibitory interneurons in the spinal cord, regulating motor neuron activity via glycine release.
   • While important for motor control, they are not the most sensitive CNS cells.
2. Astrocytes ❌
   • Astrocytes play a key role in neuroprotection, metabolism, and repair.
   • They are more resistant to damage than neurons and often proliferate in response to injury (reactive gliosis).
3. Microglial Cells ❌
   • Microglia are immune cells of the CNS, responsible for phagocytosis and inflammation.
   • They are more resilient than neurons and become activated in response to injury or infection.
4. Schwann Cells ❌
   • Schwann cells belong to the peripheral nervous system (PNS) and form myelin around peripheral nerves.
   • They are not CNS cells, making this answer incorrect.

• The gracile tubercle and cuneate tubercle are surface landmarks found on the dorsal (posterior) aspect of the medulla oblongata.
• These structures contain the gracile nucleus and cuneate nucleus, which are part of the dorsal column–medial lemniscus (DCML) pathway, responsible for fine touch, vibration, and proprioception.

Function of Each Tubercle:

✔ Gracile tubercle → Contains the gracile nucleus, which processes sensory information from the lower limbs.
✔ Cuneate tubercle → Contains the cuneate nucleus, which processes sensory information from the upper limbs.

Why the Other Options Are Wrong:

1. Pons ❌
   • The pons contains the pontine nuclei, cranial nerve nuclei, and transverse pontine fibers but does not have gracile or cuneate tubercles.
2. Cerebellum ❌
   • The cerebellum is involved in coordination and balance, and it lacks gracile and cuneate tubercles.
3. Spinal Cord ❌
   • The gracile and cuneate fasciculi (tracts carrying sensory information) are present in the spinal cord, but the tubercles (containing the nuclei) are only in the medulla.
4. Midbrain ❌
   • The midbrain contains structures like the superior and inferior colliculi, cerebral peduncles, and substantia nigra, but no gracile or cuneate tubercles.

• The blood-brain barrier (BBB) is a selective barrier that protects the brain from harmful substances while allowing the passage of essential nutrients.
• It is formed by three main components:
  1. Endothelial cells of brain capillaries → Have tight junctions that prevent free diffusion.
  2. Astrocyte foot processes (astrocytic end-feet) → Provide structural support and regulate permeability.
  3. Pericytes → Help maintain the integrity of the barrier.

Function of Astrocytes in the BBB:

✔ Regulate ion balance in the brain.
✔ Influence endothelial tight junctions to maintain barrier integrity.
✔ Metabolize and detoxify neurotransmitters.

Why the Other Options Are Wrong:

1. Neurons ❌
   • Neurons do not form the BBB but rely on it for protection and homeostasis.
2. Smooth Muscle Cells ❌
   • Found in blood vessels, but not involved in the BBB structure.
3. Hepatocytes ❌
   • Liver cells, not related to the brain or BBB.
4. Osteocytes ❌
   • Bone cells, completely unrelated to the blood-brain barrier.

• The posterior (dorsal) gray column of the spinal cord contains sensory nuclei involved in processing pain, temperature, proprioception, and visceral sensations.
• The phrenic nucleus, however, is located in the anterior (ventral) gray column and is responsible for motor control of the diaphragm.
• The phrenic nucleus is found in the C3-C5 spinal cord segments and gives rise to the phrenic nerve, which controls respiration.

Why the Other Options Are Correct (Present in the Posterior Gray Column):

1. Substantia Gelatinosa ✅
   • Located in the posterior horn; processes pain and temperature signals.
2. Visceral Afferent Nucleus ✅
   • Located in the posterior horn; receives visceral sensory input from internal organs.
3. Nucleus Proprius ✅
   • Located in the posterior horn; processes touch, proprioception, and pressure sensations.
4. Nucleus Dorsalis (Clarke’s Column) ✅
   • Located in the posterior horn (T1-L2 levels); transmits proprioceptive information to the cerebellum via the dorsal spinocerebellar tract.

• Veins of the brain have several unique features compared to veins in other parts of the body.
• They do not pierce the endosteal layer of dura mater, because:
  • The endosteal layer of the dura mater is tightly adherent to the inner surface of the skull.
  • Brain veins drain into the dural venous sinuses, which are located between the endosteal and meningeal layers of dura.
  • Instead, bridging veins pass through the arachnoid mater and meningeal dura before draining into the venous sinuses.

Why the Other Options Are Correct (Features of Brain Veins):

1. No Valves ✅
   • Brain veins lack valves, allowing bidirectional blood flow, which can contribute to the spread of infections (e.g., from the face to the brain via the cavernous sinus).
2. Pierce Arachnoid Mater ✅
   • Cerebral veins must pass through the arachnoid mater to reach the dural venous sinuses.
3. No Muscular Tissue ✅
   • Unlike systemic veins, brain veins have thin walls with no smooth muscle, making them less contractile.
4. Lie in Subarachnoid Space ✅
   • The superficial cerebral veins course through the subarachnoid space before draining into the venous sinuses.

• Bipolar neurons have one axon and one dendrite emerging from opposite poles of a spindle-shaped cell body.
• These neurons are specialized sensory neurons found in:
  • Retina (in the eye)
  • Olfactory epithelium (smell receptors)
  • Cochlear and vestibular ganglia (hearing and balance in the inner ear)

Why the Other Options Are Wrong:

1. Purkinje cells of the cerebral cortex ❌
   • Incorrect, because Purkinje cells are multipolar neurons (having multiple dendrites and a single axon).
   • They are large inhibitory neurons found in the cerebellar cortex, not the cerebral cortex.
2. Dorsal root ganglion of the spinal nerves ❌
   • Incorrect, because neurons in the dorsal root ganglia are pseudounipolar, not bipolar.
   • They have a single process that splits into two branches (one going to the periphery, the other to the spinal cord).
3. Sensory ganglia of cranial nerves ❌
   • Incorrect, because sensory neurons in cranial nerve ganglia (e.g., trigeminal ganglion) are also pseudounipolar, not bipolar.
4. Mesencephalic nucleus of the trigeminal nerve ❌
   • Incorrect, because neurons in the mesencephalic nucleus are pseudounipolar, not bipolar.
   • These neurons relay proprioceptive information from the muscles of mastication.

• The cerebral cortex is classified into three main types based on histological layers:
  1. Neocortex (Isocortex) → Six layers (Most of the cerebral cortex)
  2. Paleocortex → 3-5 layers (Olfactory structures)
  3. Archicortex → Three layers (Hippocampus)
• Not all areas of the cortex have six layers! The paleocortex and archicortex have fewer layers, making the statement incorrect.

Why the Other Options Are Correct:

1. “Neocortex shows alternate dark and light bands on histological appearance” ✅ Correct
   • The neocortex (which covers 90% of the human cortex) has six layers, and certain areas, like the striate cortex (primary visual cortex), exhibit banding patterns (stria of Gennari).
2. “Paleocortex is heterogeneous” ✅ Correct
   • The paleocortex (olfactory cortex) has variable layers (3–5 layers), making it heterogeneous.
3. “Isocortex is homogenetically typical” ✅ Correct
   • The isocortex (neocortex) is homogeneous across most regions, maintaining a six-layered structure throughout.
4. “Archicortex has heterogeneous, three layers” ✅ Correct
   • The archicortex (e.g., hippocampus) has three layers and is structurally heterogeneous.

• Purkinje cells are large, flask-shaped neurons located in the middle (Purkinje cell) layer of the cerebellar cortex.
• They have highly branched dendrites that extend into the molecular layer and receive input from granule cells via parallel fibers.
• Purkinje cells are inhibitory output neurons, sending GABAergic projections to the deep cerebellar nuclei.
• They do not stain well with routine H&E staining and require special techniques like silver impregnation or Golgi staining for clear visualization.

Why the Other Options Are Correct (Features of Purkinje Cells):

1. Arranged as single row between molecular and granular layer: ✅ Correct
   • Purkinje cells are organized in a single layer between the molecular layer (above) and granular layer (below).
2. Having branching dendrites that ascend into the molecular layer: ✅ Correct
   • Their extensive dendritic tree extends into the molecular layer, where they receive numerous synapses.
3. Largest output neuron of cerebellar cortex: ✅ Correct
   • Purkinje cells are the sole output neurons of the cerebellar cortex, inhibiting the deep cerebellar nuclei.
4. Collateral axon that enters the white matter: ✅ Correct
   • Purkinje cell axons project into the cerebellar white matter and synapse onto deep cerebellar nuclei.

• Oligodendroglia (Oligodendrocytes) are specialized glial cells of the central nervous system (CNS) responsible for myelination of axons.
• A single oligodendrocyte can myelinate multiple axons (typically 30–50 internodes), unlike Schwann cells, which only myelinate one axon per cell in the PNS.

Why the Other Options Are Wrong:

1. Helps in forming pia meningeal layer: ❌
   • The pia mater is part of the meninges, which arise from mesoderm, not neuroglia. Oligodendrocytes do not contribute to meningeal formation.
2. Supporting cells of PNS: ❌
   • Oligodendrocytes function only in the CNS.
   • Schwann cells, not oligodendrocytes, provide myelination in the peripheral nervous system (PNS).
3. Helps in blood-brain barrier formation/maintenance: ❌
   • Astrocytes, not oligodendrocytes, contribute to the blood-brain barrier (BBB) by forming the glial end-feet around blood vessels.
4. Phagocytic cells: ❌
   • Microglia, not oligodendrocytes, are the resident macrophages of the CNS and have phagocytic activity.

• Neuroglial cells (glial cells) are supporting cells of the CNS that provide structural, metabolic, and immune support to neurons.
• They include astrocytes, oligodendrocytes, microglia, and ependymal cells.
• Unlike neurons, most neuroglial cells do not stain well with routine H&E staining because they have small nuclei and minimal cytoplasm.
• Special stains like GFAP (glial fibrillary acidic protein) for astrocytes and silver or immunohistochemical stains are needed for better visualization.

Why the Other Options Are Correct:

1. Capable to divide through life: ✅ Correct
   • Unlike neurons, glial cells retain the ability to divide throughout life. This is why gliomas (brain tumors) are common, but primary neuronal tumors are rare.
2. They are non-neuronal cells of CNS: ✅ Correct
   • Glial cells are not neurons; they provide support and insulation to neurons.
3. Arise mostly from neuroectoderm: ✅ Correct
   • Most neuroglial cells (astrocytes, oligodendrocytes, and ependymal cells) arise from the neuroectoderm. Microglia, however, arise from the mesoderm (monocyte lineage).
4. They are five to ten times more abundant than neurons: ✅ Correct
   • Glial cells vastly outnumber neurons, with a ratio of approximately 5:1 to 10:1 in the CNS.

Explanation:

• Multipolar neurons are motor neurons and interneurons, primarily found in the ventral horn of the spinal cord, motor nuclei of cranial nerves, and throughout the central nervous system (CNS).
• Sensory neurons (found in the dorsal root ganglia and dorsal nuclei) are typically pseudounipolar, not multipolar.
• Therefore, the pairing “Multipolar Neurons – sensory and dorsal nuclei” is incorrect.

Why the Other Options Are Correct:

✔ 1. “Bipolar neurons – special senses” – Correct

• Bipolar neurons are found in special sensory pathways, including:
  • Retina (vision)
  • Olfactory epithelium (smell)
  • Vestibulocochlear system (hearing and balance)

✔ 2. “Betz cell – motor activity” – Correct

• Betz cells are large pyramidal neurons found in layer V of the primary motor cortex (Brodmann area 4).
• They are responsible for motor control by sending impulses via the corticospinal tract to the spinal cord.

✔ 3. “Pseudo unipolar – dorsal root ganglia” – Correct

• Pseudounipolar neurons are sensory neurons found in the dorsal root ganglia (DRG).
• They have a single process that splits into a central and peripheral branch, carrying sensory input to the CNS.

✔ 4. “Purkinje cells – cerebellar cortex” – Correct

• Purkinje cells are large inhibitory neurons located in the cerebellar cortex.
• They play a crucial role in coordinating movement by inhibiting deep cerebellar nuclei.

Red neurons are a hallmark of acute neuronal injury, typically due to hypoxic-ischemic damage (e.g., stroke, global ischemia). They appear within 12–24 hours after injury and are characterized by:

• Shrinkage of the cell body
• Loss of Nissl substance (which normally contains rough endoplasmic reticulum for protein synthesis)
• Pyknosis of the nucleus (nuclear condensation and darkening)
• Intensely eosinophilic (bright red) cytoplasm, giving them their name

Why the Other Options Are Wrong:

1. Abnormal vacuolization of neuron – This is more characteristic of spongiform encephalopathies (e.g., Creutzfeldt-Jakob disease), not red neurons.
2. Structural loss along with functional loss of the neuron – While red neurons indicate functional loss, this option is too vague. The key histological features are cytoplasmic eosinophilia, nuclear pyknosis, and Nissl substance loss.
3. Reactive astrogliosis – This occurs as part of the chronic response to injury, where astrocytes proliferate and form a glial scar. It does not describe the acute phase of red neurons.
4. Neurons having bright pink cytoplasm with large nucleus and numerous stout ramifying processes – This describes reactive astrocytes (not red neurons), which occur in conditions like gliosis or neurodegenerative diseases.

The white matter of the spinal cord consists primarily of myelinated axons that form ascending (sensory) and descending (motor) tracts. The myelin sheath, produced by oligodendrocytes, gives the white matter its characteristic pale appearance and is crucial for fast signal transmission via saltatory conduction.

Why the Other Options Are Wrong:

1. Perikarya (Neuron cell bodies) – These are mainly found in the gray matter, not white matter. The gray matter contains neuron somas, such as those in the anterior horn (motor neurons) and posterior horn (sensory neurons).
2. Dendrites – While dendrites are found in gray matter where synapses occur, they are not a major component of white matter, which consists mostly of axons.
3. Neuroglia – While glial cells (astrocytes, oligodendrocytes, and microglia) are present in white matter, they are not the primary component. The main function of white matter is axonal conduction, not glial support.
4. Blood vessels – Although white matter contains blood vessels to supply oxygen and nutrients, they are not the defining structural component of white matter.

The neuropil of the spinal cord is most abundant in the gray matter. Neuropil refers to the dense network of unmyelinated axons, dendrites, and glial cell processes that surround neuronal cell bodies. It is the primary site for synaptic connections and neuronal processing.

Key Features of Neuropil in the Gray Matter:

• Found between neurons, serving as a site for synapses and interneuronal communication.
• Composed of unmyelinated axons, dendrites, and glial cells, but few neuronal cell bodies.
• Plays a role in sensory processing, reflex activity, and integration of motor responses.
• Most abundant in the dorsal horn, where sensory processing occurs.

Why the Other Options Are Wrong:

1. Perikarya (✗) [Neuronal Cell Bodies, Not Neuropil]
   • Perikarya (soma) refer to neuron cell bodies, which are surrounded by neuropil but do not constitute neuropil themselves.
2. Synapsis (✗) [Synapses Are Part of Neuropil, But Not Its Main Component]
   • Synapses occur within the neuropil, but neuropil is not made exclusively of synapses.
   • Neuropil includes dendrites, unmyelinated axons, and glial processes, not just synaptic junctions.
3. Neuroglia (✗) [Glial Cells Are Present, But Not the Main Component]
   • Neuroglia (astrocytes, oligodendrocytes, microglia) support the neuropil but do not make up the majority of its structure.
4. Unmyelinated Axons (✗) [Present, But Not the Sole Component of Neuropil]
   • Neuropil contains unmyelinated axons, but also dendrites and glial cells.
   • The term neuropil refers to all synaptic connections and their supporting structures, not just axons.

In the central nervous system (CNS), the myelin sheath is produced by oligodendrocytes. Myelin is a fatty insulating layer that wraps around axons, increasing the speed of nerve impulse conduction by enabling saltatory conduction between the nodes of Ranvier.

Each oligodendrocyte can myelinate multiple axons, unlike Schwann cells, which myelinate only one axon in the peripheral nervous system (PNS).

Why the Other Options Are Wrong:

1. Schwann cells – These produce myelin in the peripheral nervous system (PNS), not the CNS.
2. Astrocytes – These are support cells in the CNS that help in blood-brain barrier maintenance, nutrient transport, and repair, but they do not produce myelin.
3. Microglia – These are the resident immune cells of the CNS, acting as macrophages to remove debris and pathogens. They have no role in myelination.
4. Ependymal cells – These line the ventricles of the brain and the central canal of the spinal cord, producing cerebrospinal fluid (CSF), but they do not form myelin.

The glomerulus in the cerebellum is a synaptic structure found in the granular layer, not in the molecular layer. It is a key site where mossy fibers synapse with granule cell dendrites and are modulated by Golgi cell inhibition.

Why the other options are correct:

1. Mossy fibers synapse here (Correct):
   • Mossy fibers bring afferent excitatory input from the spinal cord, brainstem, and cerebral cortex and synapse in the cerebellar glomerulus.
2. Golgi cells are present here (Correct):
   • Golgi cells, which are inhibitory interneurons, synapse in the glomerulus and regulate granule cell excitation through GABAergic inhibition.
3. Present in granular layer (Correct):
   • The cerebellar glomerulus is located in the granular layer, where it serves as a hub for neuronal synapses and signal integration.
4. None of these (Incorrect):
   • There is a definite incorrect statement in the options (glomerulus being in the molecular layer), so this option is not the right choice.

Rosenthal fibers are elongated, brightly eosinophilic protein aggregates that accumulate within astrocyte processes in conditions involving chronic gliosis. These fibers are composed of:

• Glial fibrillary acidic protein (GFAP)
• Ubiquitin
• Heat shock proteins (HSPs)

They are commonly seen in:

• Chronic gliosis (reactive astrocytosis in response to brain injury)
• Pilocytic astrocytomas (a type of low-grade brain tumor)
• Alexander disease (a rare leukodystrophy affecting astrocytes)

Why the other options are incorrect:

1. Nissl bodies:
   • Nissl bodies are clusters of rough endoplasmic reticulum (RER) and ribosomes found in neurons, not astrocytes.
   • They are involved in protein synthesis, not gliosis.
2. None of these:
   • Incorrect, because Rosenthal fibers are a well-documented histopathological feature of chronic gliosis.
3. Microglial granules:
   • Microglia are the immune cells of the CNS, and they form granular bodies in response to infection, neurodegeneration, or injury, but these are not associated with astrocytes or gliosis.
4. Fibrillary astrocytes:
   • Fibrillary astrocytes are a type of reactive astrocyte seen in gliosis, but they do not refer to the eosinophilic inclusions seen in Rosenthal fibers.

Multipolar neurons are not primarily involved in sensory function or dorsal nuclei. Instead, they are mostly found in motor pathways and interneurons, such as:

• Ventral horn of the spinal cord (motor neurons).
• Autonomic nervous system (sympathetic and parasympathetic neurons).
• Cerebral cortex and cerebellum (interneurons).

Sensory neurons in the dorsal root ganglia are pseudo-unipolar, not multipolar.

Why the other options are correctly matched:

1. Bipolar neurons – special senses (Correct):
   • Bipolar neurons are found in sensory systems requiring highly specific signal transmission, such as:
     • Retina (vision)
     • Olfactory epithelium (smell)
     • Vestibular and auditory systems (balance and hearing)
2. Betz cell – motor activity (Correct):
   • Betz cells are large pyramidal neurons located in the primary motor cortex (layer V).
   • They play a critical role in voluntary movement by sending signals through the corticospinal tract.
3. Pseudo-unipolar – dorsal root ganglia (Correct):
   • Pseudo-unipolar neurons have a single axon that splits into two branches and are responsible for transmitting sensory information.
   • They are found in dorsal root ganglia, which are involved in relaying sensory input from the periphery to the CNS.
4. Purkinje cells – cerebellar cortex (Correct):
   • Purkinje cells are large, inhibitory neurons in the cerebellar cortex.
   • They play a role in fine-tuning motor coordination by sending inhibitory signals to the deep cerebellar nuclei.

The substantia gelatinosa is a gray matter structure located in the dorsal horn of the spinal cord (mainly in Rexed lamina II). It plays a crucial role in modulating pain perception by processing nociceptive (pain) signals before they are transmitted to the brain via the spinothalamic tract. It is also involved in the gate control theory of pain, which regulates pain signals through inhibitory interneurons.

Why the other options are incorrect:

1. Clark’s nucleus (Nucleus dorsalis):
   • This nucleus is found in Rexed lamina VII and is involved in proprioception, not pain perception. It receives sensory input from muscle spindles and Golgi tendon organs and sends information to the cerebellum via the dorsal spinocerebellar tract.
2. Nucleus proprius:
   • The nucleus proprius (found in Rexed lamina IV-VI) is involved in processing general sensory information (e.g., touch and proprioception), but it is not the primary structure for pain perception.
3. Visceral afferent nucleus:
   • This nucleus is involved in visceral sensation, such as autonomic functions, but not pain perception from somatic sources.
4. Nucleus dorsalis (Clark’s column):
   • Another name for Clark’s nucleus, which is involved in proprioception rather than pain processing.

Nissl substance (or Nissl bodies) consists of rough endoplasmic reticulum (RER) and ribosomes, which are responsible for protein synthesis in neurons. It appears as basophilic granules under the microscope due to its high RNA content (from ribosomes). The presence of abundant Nissl substance is characteristic of large, metabolically active neurons that produce large amounts of neurotransmitters and proteins.

Why the other options are incorrect:

1. Lysosome:
   • Lysosomes are involved in intracellular digestion and waste removal but are not a major component of Nissl substance.
2. Golgi complex:
   • The Golgi complex is involved in protein modification and packaging, but it does not stain basophilic like the Nissl substance and is usually found in a different part of the neuron.
3. Mitochondrion:
   • Mitochondria provide energy (ATP) for cellular processes but do not contribute to the basophilic staining of Nissl substance.
4. Smooth endoplasmic reticulum (SER):
   • SER is involved in lipid synthesis and calcium storage but lacks ribosomes, so it does not stain with Nissl stain.

Astrocytes are glial cells in the central nervous system (CNS) that secrete nerve growth factors (NGFs). These growth factors support neuronal survival, differentiation, and repair after injury. Astrocytes also help in maintaining the blood-brain barrier, regulating neurotransmitter levels, and providing metabolic support to neurons.

Why the other options are incorrect:

1. Microglial cells:
   • Microglia are the immune cells of the CNS and primarily function in phagocytosis and neuroinflammation. They do not play a major role in secreting nerve growth factors.
2. Oligodendrocytes:
   • Oligodendrocytes are responsible for myelination in the CNS, not the secretion of nerve growth factors.
3. Ependymal cells:
   • Ependymal cells line the ventricles of the brain and produce cerebrospinal fluid (CSF), but they do not secrete nerve growth factors.
4. Schwann cells:
   • Schwann cells provide myelination in the peripheral nervous system (PNS) and do secrete some neurotrophic factors, but in the CNS, astrocytes are the primary source of NGFs.

Blood vessels do run in the endoneurium, but they are very small and form a capillary network that supplies individual nerve fibers. However, the primary blood supply to nerves comes from larger vessels located in the epineurium. The endoneurial capillaries are part of the blood-nerve barrier, which helps maintain the microenvironment around nerve fibers.

Why the other statements are incorrect:

1. The perineurium surrounds many nerve bundles into one nerve fiber (called the nerve proper):
   • This is incorrect because the perineurium surrounds individual fascicles (bundles of nerve fibers), not the entire nerve. The epineurium surrounds the entire nerve (nerve proper).
2. The toughest part is the perineurium:
   • This is actually correct. The perineurium is the toughest layer due to its dense connective tissue structure, which provides mechanical strength and acts as a diffusion barrier.
3. The epineurium encloses a single nerve fiber and adheres it to other nerve fibers to form a nerve bundle:
   • This is incorrect because the epineurium surrounds the entire nerve (multiple fascicles), not individual nerve fibers. The endoneurium encloses single nerve fibers.
4. The endoneurium surrounds the nerve bundles:
   • This is incorrect because the endoneurium surrounds individual nerve fibers, not nerve bundles. The perineurium surrounds nerve bundles (fascicles).

The cerebellar cortex does not contain pyramidal cells. Pyramidal cells are typically found in the cerebral cortex, particularly in the motor and sensory areas, where they play a key role in transmitting motor commands. In contrast, the cerebellar cortex has different types of neurons that are specialized for motor coordination and processing, including granule cells, stellate cells, Golgi cells, and basket cells.

Here’s why the other options are present in the cerebellar cortex:

1. Granule cells: These are the most numerous cells in the cerebellar cortex. They play a key role in transmitting information from the mossy fibers to the Purkinje cells.
2. Stellate cells: These are inhibitory interneurons in the cerebellar cortex that help regulate the activity of Purkinje cells.
3. Golgi cells: These are also inhibitory interneurons in the cerebellar cortex, playing a role in modulating the activity of granule cells.
4. Basket cells: These are inhibitory interneurons in the cerebellar cortex that have a key role in controlling the output of Purkinje cells.

The correct statement regarding oligodendroglia is: They can myelinate more than one axon per cell. Oligodendrocytes are glial cells found in the central nervous system (CNS) that produce myelin sheaths to insulate axons. A single oligodendrocyte can extend processes to myelinate multiple axons, unlike Schwann cells in the peripheral nervous system (PNS), which myelinate only one axon per cell.

Why the other options are wrong:

1. Supporting cells of PNS:
   • This is incorrect because oligodendroglia are found in the CNS, not the PNS. The supporting cells of the PNS are Schwann cells and satellite cells.
2. Phagocytic cells:
   • This is incorrect because phagocytic cells in the CNS are microglia, not oligodendroglia. Microglia are the immune cells of the CNS and act as scavengers.
3. Helps in forming pia meningeal layer:
   • This is incorrect because the pia mater (the innermost meningeal layer) is formed by specialized cells called meningeal cells, not oligodendroglia.
4. Helps in blood-brain barrier formation/maintenance:
   • This is incorrect because the blood-brain barrier is primarily maintained by astrocytes, not oligodendroglia. Astrocytes have endfeet that surround blood vessels and help regulate the barrier.

The Purkinje cells are not found in the molecular layer of the cerebellar cortex. Instead, the cell bodies of Purkinje cells are located in the Purkinje cell layer, which lies between the molecular layer and the granular layer. However, their dendrites extend into the molecular layer.

Why the other options are present in the molecular layer:

1. Stellate cells:
   • Stellate cells are interneurons located in the molecular layer. They receive input from parallel fibers and synapse on Purkinje cell dendrites.
2. Neuroglia:
   • Neuroglia (glial cells) are present throughout the cerebellar cortex, including the molecular layer, where they provide structural and functional support.
3. Basket cells:
   • Basket cells are interneurons located in the molecular layer. They receive input from parallel fibers and synapse on Purkinje cell bodies.
4. Parallel fibers of granular cell axons:
   • The axons of granular cells ascend into the molecular layer, where they bifurcate to form parallel fibers. These parallel fibers run horizontally and synapse with Purkinje cell dendrites, stellate cells, and basket cells.

The white matter of the central nervous system (CNS) is primarily composed of:
✅ Myelinated axons → These transmit nerve signals over long distances.
✅ Glial cells → Mainly oligodendrocytes, which produce myelin, and astrocytes, which support neuronal function.

White matter appears white due to the high lipid content of myelin, which insulates axons and speeds up electrical transmission.

White Matter vs. Gray Matter:

• White matter → Contains myelinated axons and glial cells but few or no cell bodies.
• Gray matter → Contains neuronal cell bodies, dendrites, and synapses (e.g., cerebral cortex, basal ganglia).

Why not the other options?

• Cell bodies only → Cell bodies are found in gray matter, not white matter.
• Axons and dendrites → Dendrites are mostly in gray matter, where synapses occur.
• Cell bodies and dendrites → This describes gray matter, not white matter.
• Axons only → White matter also contains glial cells, particularly oligodendrocytes.

Thus, the correct answer is “Axons and glial cells,” as these are the main components of white matter in the CNS.

Purkinje cells are flask-shaped neurons located in a single row within the Purkinje layer of the cerebellar cortex, which is the middle layer. These cells are large and have extensive dendritic arbors that extend into the molecular layer. They serve as the sole output of the cerebellar cortex, projecting inhibitory signals via GABA to the deep cerebellar nuclei.

Other options:

1. Basket cells: Found in the molecular layer and provide inhibitory input to Purkinje cells.
2. Granular cells: Found in the granular layer and are small, densely packed excitatory neurons.
3. Stellate cells: Located in the molecular layer and synapse with Purkinje cells, providing inhibitory input.
4. Horizontal cells: Typically found in the cerebral cortex, not the cerebellar cortex.

The molecular layer (Layer I) of the cerebral cortex is the outermost layer. It contains few neurons and is predominantly composed of horizontal cells of Cajal, along with dendritic and axonal processes. These horizontal cells are rare and involved in modulating cortical activity.

Other layers:

1. Inner granular layer (Layer IV): Contains densely packed granular (stellate) cells and is involved in sensory input.
2. Internal pyramidal layer (Layer V): Contains large pyramidal cells responsible for motor output.
3. Multiform layer (Layer VI): Contains a mix of neuron types, including fusiform cells, and connects the cortex to subcortical structures.
4. External granular layer (Layer II): Contains small granular and pyramidal cells involved in intracortical communication.

The molecular layer of the cerebellar cortex is the outermost layer and serves as a synaptic zone. It is primarily composed of:

• Dendritic arborizations of Purkinje cells, which form extensive branching.
• Axons of granule cells (parallel fibers) that synapse on the dendrites of Purkinje cells.
• Interneurons, such as basket cells and stellate cells, which modulate Purkinje cell activity.

This layer is where most of the synaptic interactions within the cerebellar cortex occur, contributing to the integration of sensory and motor signals.

Why not the other options?

1. Pyramidal cell layer: Pyramidal cells are found in the cerebral cortex, not the cerebellar cortex.
2. Thalamus: A relay center in the diencephalon, unrelated to the cerebellar cortex.
3. Purkinje cell layer: The middle layer of the cerebellar cortex; it contains the soma of Purkinje cells, not their dendritic arborizations.
4. Granular cell layer: The innermost layer, composed of granule cells and their axons, which project into the molecular layer as parallel fibers.

The basket-like nest is formed around the soma of Purkinje cells by the axons of basket cells within the cerebellar cortex.

Why not the other options?

1. Stellate cells: Inhibitory interneurons in the molecular layer, but they synapse on the dendrites of Purkinje cells, not around their soma.
2. Granule cells: Excitatory neurons in the granular layer; their axons (parallel fibers) synapse on the dendrites of Purkinje cells, not their soma.
3. Pyramidal cells: Found in the cerebral cortex, not the cerebellum, and thus unrelated to this structure.