Fourth Ventricle Floor Structures | UPSC Mains MEDICAL-SCIENCE-PAPER-I 2013

Describe the structures forming floor of fourth ventricle of brain.

How to Approach

This question requires a detailed anatomical description. The approach should be systematic, starting with the overall location and boundaries of the fourth ventricle, then detailing each structure contributing to its floor. Focus on identifying the specific anatomical features and their relationships. A diagrammatic representation (though not possible here) would be ideal in an exam setting. Mentioning the developmental origins can add depth. The answer should demonstrate a clear understanding of neuroanatomy.

Structures Forming the Floor of the Fourth Ventricle

The floor of the fourth ventricle is a diamond-shaped area exhibiting several distinct features. It can be divided into three parts: the superior medullary velum, the locus coeruleus, and the inferior medullary velum. These are formed by various structures, which we will detail below.

1. Superior Medullary Velum

The superior medullary velum forms the dorsal (roof) part of the fourth ventricle, but its caudal extension contributes to the upper portion of the floor. It is a thin layer of grey matter and is continuous with the cerebellar cortex via the folia of the cerebellum. It contains:

Subarachnoid space: A space filled with CSF.
Choroid plexus: A highly vascularized structure responsible for CSF production. It appears as a cauliflower-like mass.

2. Locus Coeruleus

Located in the dorsal pons, the locus coeruleus is a small, but critically important, nucleus. It is a major source of norepinephrine in the brain and plays a role in arousal, attention, and stress response. It appears as a dark blue streak in fresh brain specimens (hence the name 'coeruleus' meaning 'azure').

3. Inferior Medullary Velum

The inferior medullary velum is the primary component forming the floor of the fourth ventricle. It is a thin layer of grey matter and contains several important structures:

Pyramidal Decussation: This is a crucial landmark where the corticospinal tracts (responsible for voluntary movement) cross over to the opposite side.
Medial Longitudinal Fasciculus (MLF): A vertically oriented fiber tract involved in coordinating eye movements and head position.
Vestibular Nuclei: These nuclei receive input from the vestibular apparatus in the inner ear and are involved in maintaining balance and spatial orientation.
Cochlear Nuclei: These nuclei receive auditory information from the cochlea and are the first relay station for auditory processing.
Inferior Olivary Nucleus: Involved in motor learning and coordination.
Hypoglossal Nucleus: Controls the muscles of the tongue.
Vagal Nucleus: Controls parasympathetic functions of the viscera.
Dorsal Motor Nucleus of Vagus: Part of the vagal nucleus, specifically controlling motor functions.
Nucleus Ambiguus: Controls muscles of the pharynx, larynx, and esophagus.
Spinal Trigeminal Nucleus: Receives pain and temperature sensation from the face.

4. Foramen of Magendie & Foramina of Luschka

These foramina are openings in the roof of the fourth ventricle that allow CSF to exit into the subarachnoid space. The foramen of Magendie is a single opening located at the caudal end of the fourth ventricle, while the foramina of Luschka are paired openings located laterally.

Table summarizing key structures:

Structure	Location	Function
Locus Coeruleus	Dorsal Pons	Norepinephrine production, arousal, attention
Pyramidal Decussation	Inferior Medullary Velum	Crossing of corticospinal tracts
Vestibular Nuclei	Inferior Medullary Velum	Balance and spatial orientation
Hypoglossal Nucleus	Inferior Medullary Velum	Tongue muscle control

Additional Resources

Key Definitions

Cerebrospinal Fluid (CSF)

A clear bodily fluid found in the brain and spinal cord. It provides cushioning, buoyancy, and delivers nutrients while removing waste products.

Rhombic Lip

A dorsal thickening of the neural tube that gives rise to several structures in the hindbrain, including parts of the fourth ventricle floor and cranial nerve nuclei.

Key Statistics

Approximately 150 ml of CSF is produced daily, primarily by the choroid plexus.

Source: Guyton and Hall Textbook of Medical Physiology (Knowledge cutoff 2023)

The brainstem, including the region of the fourth ventricle, accounts for approximately 18% of the total brain volume.

Source: Netter's Atlas of Human Anatomy (Knowledge cutoff 2023)

Examples

Hydrocephalus

A condition characterized by an abnormal accumulation of CSF within the brain, often due to blockage of the ventricular system or impaired CSF absorption. This can lead to increased intracranial pressure and neurological damage.

Frequently Asked Questions

▶What happens if the foramina of Luschka and Magendie are blocked?

Blockage of these foramina can lead to non-communicating hydrocephalus, where CSF cannot flow from the ventricular system into the subarachnoid space, causing a buildup of pressure within the ventricles.