INTRODUCTION & SUBDIVISION OF THE NERVOUS SYSTEM

MENINGES, VENTRICLES & CSF

Divisions of the Nervous System

General Plan of the Nervous System

Neuron

Neurons (Functional Classifications & Synaptic Sequences)

The Central Nervous System (CNS)

• The Central Nervous System (CNS) has two major structures: the brain and the spinal cord.
• The brain is protected by the cranial cavity and the spinal cord is surrounded by the spinal cavity in the vertebral column.
• They are both covered by protective membranes called meninges.

The Brain

• Is semisolid in consistency and conforms to the shape of its container.
• Weighs approximately 1400 g in an adult.
• Male brain is on average slightly heavier than the female brain.
• Protected from the external environment by three barriers:
  • Skull
  • Meninges
  • Cerebrospinal fluid

Meninges

The brain and spinal cord are enclosed within three distinct membranous envelopes:

• Dura mater: Outermost, thickest, pachymeninx.
• Arachnoid mater: Intermediate avascular web-like leptomeninx.
• Pia mater: Delicate, highly vascular, adherent inner leptomeninx.

SPACES BETWEEN MENINGES

• Epidural space
• Subdural space
• Subarachnoid space

CEREBRAL DURAL VENOUS SINUSES

Endothelium-lined venous channels situated between the endosteal (periosteal) and meningeal layers of the dura mater, collecting blood from the brain and CSF from the subarachnoid space.

Spinal Cord Meninges

Ventricular system

• Ventricles are developmentally derived from the cavity of the neural tube.
• Lined throughout with ependyma and are filled with cerebrospinal fluid.
• They are:
  • Lateral ventricles (paired within cerebral hemispheres)
  • Third ventricle (midline cleft within diencephalon)
  • Fourth ventricle (cavity posterior to pons/medulla, anterior to cerebellum)

Lateral ventricles

• Two lateral ventricles and one is present in each cerebral hemisphere.
• Each is a roughly C-shaped cavity and includes a body, which occupies the parietal lobe.
• Anterior, posterior and inferior horns extend from the body into the frontal, occipital, and temporal lobes respectively.
• Communicate with the cavity of the 3rd ventricle through the interventricular foramen (of Monro), which lies in the medial wall of each ventricle.
• The choroid plexus of the lateral ventricles projects into each ventricle on its medial aspect.

A cast of the ventricular cavities

Third Ventricle

• A slit-like cleft between the two thalami.
• Communicates anteriorly with the lateral ventricles through the interventricular foramina (of Monro) and posteriorly with the 4th ventricle through cerebral aqueduct (of Sylvius).
• Blood supply of the choroid plexuses of the 3rd and lateral ventricles is derived from the choroidal branches of the internal carotid and basilar arteries.
• Venous blood drains into the internal cerebral veins.
• Cerebral aqueduct of Sylvius is a narrow channel about 1.8 cm long which connects the 3rd with the 4th ventricle.
• It is lined with ependyma and is surrounded by a layer of gray matter called the central gray (periaqueductal gray).

Fourth Ventricle

• A cavity situated anterior to the cerebellum and posterior to the pons and the superior half of the medulla oblongata.
• Continuous above with the cerebral aqueduct and below with central canal of the spinal cord.
• Possesses lateral boundaries, a roof and a rhomboid-shaped floor.
• Lateral boundaries are formed by the superior and inferior cerebellar peduncles.

Fourth Ventricle (Roof & Apertures)

• The tent-shaped roof projects into the cerebellum.
• Formed by the superior medullary velum above and the inferior medullary velum below.
• Inferior medullary velum is pierced in the midline by a large opening, the median aperture, the foramen of Magendie.

Fourth Ventricle (Floor / Rhomboid Fossa)

• Floor is formed by the posterior surface of the pons and the cranial half of the medulla oblongata.
• In the midline is the median sulcus.
• On each side of this sulcus is the median eminence, which is bounded laterally by sulcus limitans.
• Lateral to sulcus limitans, there is the vestibular area, beneath which lie the vestibular nuclei.
• The facial colliculus, produced by the fibres of the facial nerve looping over the abducens nucleus, lies at the inferior end of the medial eminence.

Fourth Ventricle (Internal Features & Vascularization)

• Strands of nerve fibers, the stria medullaris, emerge from the median sulcus and pass laterally to enter the inferior cerebellar peduncle.
• Inferior to the stria, lies the hypoglossal triangle formed by the underlying hypoglossal nucleus and lateral to this lies the vagal triangle produced by the underlying dorsal motor nucleus of the vagus.
• The choroidal plexus of the 4th ventricle is suspended from the interior half of the roof of the ventricle.
• The blood supply to the plexus is from the inferior cerebellar arteries (PICA and AICA).

Embryonic Brain Subdivisions, Adult Derivatives & Ventricles

Central Canal

• Central Canal of the spinal cord and medulla oblongata opens superiorly into the 4th ventricle.
• Inferiorly, it extends through the inferior half of the medulla oblongata and through the entire length of the spinal cord.
• In the conus medullaris of the spinal cord, it expands to form the terminal ventricle.
• The central canal is closed at its lower end.
• It is filled with CSF, and is lined with ependyma.
• The central canal is surrounded by gray matter, the gray commissure.
• There is no choroid plexus in the central canal.

Subarachnoid Space

• Is the interval between the arachnoid mater and pia mater and envelopes the brain and spinal cord.
• Filled with CSF and contains the large blood vessels of the brain.
• Inferiorly, extends beyond the lower end of the spinal cord and invests the cauda equina.
• Ends below at the level of the interval between the second and third sacral vertebrae.

Subarachnoid Cisterns

• In certain locations around the base of the brain, the subarachnoid does not closely follow the surface of the brain so that the subarachnoid space expands to form cisterns.
• The cerebellomedullary cistern (cisterna magna) lies between the cerebellum and the medulla oblongata.
• The pontine cistern lies on the anterior surface of the pons.
• The Interpeduncular cistern lies on the anterior surface of the midbrain between the crura cerebri.

Extensions of the subarachnoid space

• Around the optic nerve to the back of the eyeball where the arachnoid mater and pia mater fuse with the sclera.
• The central artery and vein of the retina cross this extension of the subarachnoid space to enter with the optic nerve and they may be compressed in patients with raised CSF pressure.
• Around the other cranial and spinal nerves.
• Around the arteries and veins of the brain and spinal cord at points where they penetrate the nervous tissue (forming perivascular Virchow-Robin spaces).

Cerebrospinal Fluid

Cerebrospinal fluid

• A clear, colorless liquid that fills the ventricles of the brain and bathes the external surface of the brain and spinal cord.
• Formed from the choroid plexus within the ventricles and circulates through the three openings in the roof of the fourth ventricle to reach the subarachnoid space.
• Produced continuously at a rate of about 0.5 ml per minute and with a total volume of about 30 ml (infant volume; adult total volume is approximately 140–150 ml): this corresponds to a turnover time of about 5 hours.

Formation of CSF

• Formed in the choroid plexuses of the lateral, third, and fourth ventricles; some originates as tissue fluid in the brain substance.
• The choroid plexuses have a folded surface and consist of a core of vascular connective tissue covered with cuboidal epithelium of the ependyma.
• The blood of the capillaries are separated from the ventricular lumen by fenestrated endothelium, a basement membrane and the surface epithelium. The ependymal cells of the choroid plexuses actively secrete the CSF.

Composition

• CSF has a specific gravity of about 1.007.
• Possesses in solution, inorganic salts similar to those in the blood plasma.
• Glucose content is about half that of blood and there is only a trace of protein.
• A few cells are present and these are lymphocytes.
• Normal lymphocyte count is 0 to 3 cells per cu mm.
• In the lateral recumbent position, CSF pressure, as measured by lumbar puncture, is about 60 to 150 mm of water.
• This pressure may be easily raised by straining, coughing or compressing the internal jugular veins in the neck (Queckenstedt test).

Composition Summary

• Contains not more than 5 lymphocytes/μl and is usually sterile.

Functions of the CSF

• Serves as a protective cushion between the central nervous system and the surrounding bones.
• Because the density of the brain is only slightly greater than that of the CSF, it provides mechanical buoyancy and support for the brain (reducing effective net weight of brain from ~1400 g to ~50 g).
• The close relationship of the fluid to the nervous tissue and the blood enables it to serve as a reservoir and assist in the regulation of the contents of the skull (Monro-Kellie doctrine).
• Is an ideal physiological substrate and probably plays an active part in the nourishment of the nervous tissue;
• Assists in the removal of products of neuronal metabolism.
• The secretions of the pineal gland possibly influence the activities of the pituitary gland by circulating through the CSF in the third ventricle.

Circulation of CSF

• Passes from the lateral ventricles into the 3rd ventricle through the interventricular foramina (of Monro).
• Then passes into the 4th ventricle through the cerebral aqueduct.
• The circulation is aided by the arterial pulsations of the choroid plexuses and the cilia on the ependymal cells lining the ventricles.
• From the 4th ventricle, the fluid passes through the median aperture (foramen of Magendie) and the lateral foramina of Luschka of the lateral recesses of the 4th ventricle and enters the subarachnoid space.
• The fluid then flows superiorly through the interval in the tentorium cerebelli to reach the inferior surface of the cerebrum.
• It now moves superiorly over the lateral aspect of each cerebral hemisphere.
• Some of the CSF moves inferiorly in the subarachnoid space around the spinal cord and cauda equina.
• The pulsations of the cerebral and spinal arteries and the movements of the vertebral column facilitate this flow of fluid.

Absorption of CSF

• CSF is absorbed into the arachnoid villi that project into the dural venous sinuses, especially the superior sagittal sinus.
• The arachnoid villi are grouped together to form arachnoid granulations (Pacchionian bodies). Each arachnoid villus is a diverticulum of the subarachnoid space that pierces the dura mater.
• Absorption of CSF into the venous sinuses occurs when the CSF pressure exceeds that in the sinus (acts as a one-way pressure-dependent valve).
• Studies of the arachnoid villi indicate that fine tubules lined with endothelium permit a direct flow of fluid from the subarachnoid space into the lumen of the venous sinuses.

Blood – Brain Barrier

Blood – Brain Barrier Absence (Circumventricular Organs)

In select specialized neural midline regions known as Circumventricular Organs (CVOs), the blood-brain barrier is absent because the capillary endothelium possesses fenestrations, allowing direct neuroendocrine sampling or hormone secretion into the bloodstream.

Blood – Brain Barrier (Permeability Characteristics)

• Gases and water pass readily through the barrier (rapid lipid diffusion of O₂, CO₂, and aquaporin channels).
• Glucose and electrolytes pass more slowly (require specialized facilitative carrier-mediated transporters like GLUT-1 and ion channels).
• The barrier is almost impermeable to plasma proteins and other large organic molecules.
• Compounds with molecular weights of about 60,000 and higher remain within the blood circulatory system.

Clinical Notes