The Ventricular System

Cerebrospinal fluid (CSF)

Is an ultrafiltrate of plasma and functions to support the brain cells, prevent mechanical damage to the brain, and eliminate waste products

What does CSF carry?

It acts as an alternate circulation system within the brain carrying neuropeptides secreted in one brain region to another

Where does CSF flow?

CSF flows from the ventricular system into the sub-arachnoid space, where it circulates around the brain and spinal cord by pulsatile flow and is reabsorbed into the blood stream

Ventricular system

Is a set of structures containing CSF in the brain and is continuous with the central canal of the spinal cord

• Includes 4 ventricles in the brain that form a continuous space

4 ventricles

• Paired ventricles- lie within cerebral hemispheres

• Third ventricles- lies in the midline between 2 halves of thalamus

• Fourth ventricle- lies in dorsal end of midbrain, rostral end of pons

Interventricular foramina

Allows lateral ventricles and the third ventricle to communicate

Cerebral aqueduct

Allows communication between the third and fourth ventricle

Lateral recess

Communication between fourth ventricle and subarachnoid space

The ventricular system drains through

The subarachnoid space-veins-heart

Choroid plexus

CSF is mainly secreted by the choroid plexus (a highly vascularized tissue that partially lines the ventricles) of the lateral, third, and fourth ventricles, small amounts are derived directly from brain extracellular fluid

The functional unit of the choroid plexus

Is composed of a capillary enveloped layer of differentiated ependymal epithelium- a subtype of glial cell

Production and absorption of CSF

• The choroid plexus produces CSF

  • Produced at 400-500 mL/day

• Subarachnoid villi: cells that absorb the CSF

  • Reabsorbs at 400-500 mL/day

CSF resembles plasma, but differs in that

• It is normally clear, colorless fluid

• Has much less protein

• Produced at a relatively constant rate

• Reabsorbed in the same rate through the arachnoid villi - spiderlike structure

Hydrocephalus

A medical condition in which there is an abnormal accumulation of CSF in the ventricles. Occurs when the rate of production and absorption of CSF is not equal

• This may cause increased intracranial pressure inside the skull

• Leads to progressive enlargement of the head, convulsion, hearing loss, cognitive impairment, and death

3 causes of hydrocephalus

• Excess production compared to absorption

• Normal production but decreased absorption

• Blockage of CSF circulation between sites of production and absorption

Blood-brain barrier (BBB)

Is a separation of circulating blood from the brain (barrier between arteries and brain/nuclei

• In most parts of the body, except in the brain, most molecules can pass through the capillary walls into the extracellular space of the surrounding tissue (usually no barrier). Usually they are 10 nm spaces. This interphase is absent in the brain. Tight junctions exist in all brain capillaries that normally do not exist in normal circulation. 

The BBB results from

This "barrier" in the brain results from the selectivity of the tight junctions between endothelial cells in CNS vessels that restricts the passage of solutes.  At the interface between blood and brain, endothelial cells are stitched together by tight junctions - transmembrane proteins​

BB is formed by

• Endothelial cells of the capillary wall

• Astrocyte end-feet ensheathing the capillary and

• Pericytes (vascular smooth muscle cells) embedded in the capillary basement membrane​

The BB restricts diffusion of

Microscopic objects, like bacteria, and large molecules into the brain, while allowing the diffusion of small molecules like oxygen, ethanol, steroid hormones, etc

• Some molecules (glucose and amino acids) can pass thru specific transport systems​

• Has great clinical significance:​ prevents many useful drugs like antibiotics from entering the brain​

Blood-CSF Barrier (B-CSFB)

Separates CSF from blood tissue.

• CSF is the protective fluid circulating around the brain. ​It protects the brain from external injury and shock and​provides a metabolic function by regulating entry and exit ​of nutrients and waste products

The Blood CSF barrier is made of

• Mostly made of the choroid plexus which has ependymal cells- ependymal cells are connected by very tight junctions

• Capillary endothelial cells

• Basal membrane ​

CSF-brain interphase

• Communicates and allows for the passage of molecules​

• Ependymal cells (ciliated simple cuboidal epithelium) line the ventricular walls and allow for communication​

• ​No barrier occurs here: macromolecules can flow freely between CSF and intercellular space of CNS