24 Additional Structures of the Nervous System

Although we mostly think about the neurons that make up the brain and spinal cord as being the main characters of the nervous system, there are many other anatomical features that play important supporting roles. These are often non-neuronal structures that are still critically important in allowing the nervous system to do what it needs to do.

Meninges

The brain is a soft and delicate internal organ housed inside the skull. If there weren’t some protective buffer separating the soft brain matter from the rigid bone, the jelly-like brain would be smashed up against the inside of the skull and get injured as the head moves around.

The meninges are a series of protective membranes that minimize this kind of damage. They surround the brain and extend all the way down the spinal cord. Think of the meninges as an organic type of “bubble wrap” that encases a fragile nervous system.

There are three types of membranes that collectively make up the meninges. From the outermost to innermost layer, they are:

1. Dura mater. The dura is made of thick, fibrous material and can get to be 0.8 mm thick in the adult body (if you took a piece of printer paper and fold it in half four times, that should give you an idea of how thick the dura mater is in the adult human). The dura mater is physically attached to the inside of the skull with highly resilient connections found at the sutures between the plates of the cranium. The name originates from Latin meaning “tough mother”.
2. Arachnoid mater. The arachnoid mater is the middle layer of the meninges. The fibers are very delicate and resemble a spider web, which is where the name comes from. Within this space, there are protrusions that allow for CSF to drain into sinuses, which allow for recycling of soluble substances. Most of the CSF in the brain exists underneath this layer in the subarachnoid space.
3. Pia mater. The pia mater is the third layer of the meninges. It is very fragile, is in direct contact with the surface of the brain, and closely follows the sulci and gyri. The name means “pious mother”.

Brain Circulation

Like every other organ in the body, the brain requires oxygen and nutrients to function. In humans, this function is accomplished by the blood that is pumped around the body using a network of blood vessels called the circulatory system. The brain has a very high demand for oxygen and nutrients: at only 2% of total body weight, it receives about 15% of total cardiac output.

Stroke

Stroke is an extremely common, life-threatening medical condition that results in a loss of blood flow to the brain. According to 2016 statistics from the World Health Organization, stroke is the second-highest cause of death worldwide. The number one risk factor for stroke is high blood pressure. There are two common types of strokes that a person may experience.

More than 80% of all strokes are ischemic strokes (pronounced is-keemik), which happens when normal blood flow is interrupted, causing cell death by deprivation of oxygen and nutrients to brain tissue. Generally, this type of injury can happen when a blood clot forms, travels through the circulatory system, and gets lodged in a tiny brain blood vessel, thus, blocking the passage of blood.

The other 20% of strokes are hemorrhagic strokes, which result from a burst blood vessel that causes bleeding into the brain. The presence of uncontrolled blood inside the brain causes an increase in intracranial pressure, which can be lethal. Many brain cells may die since they cannot take up oxygen directly from the blood. Additionally, blood has dramatically different properties than the normal solution brain cells live in, and this can cause the neurons to trigger a self-destruction program. Generally, hemorrhagic stroke is more deadly than ischemic stroke.

Because the different blood vessels of the brain’s circulatory system are responsible for providing blood to specific areas of the brain, it is possible to diagnose the specific area where the stroke is happening based on the presentation of symptoms. For example, if the middle cerebral artery blood is occluded by an ischemic stroke, the left hemisphere motor cortex will lose blood flow. Because of the contralateral organization of the descending motor pathway, the patient may therefore present with paralysis or weakness in the right half of the body. It is vitally important to correctly diagnose and differentiate between the two types of strokes. An ischemic stroke may be treated with injection of a “clot-busting” drug, a substance that helps the body break down the offending blockage. However, these clot-busters could make the bleeding from a hemorrhagic stroke even worse.

Blood–Brain Barrier

It is important for oxygen and nutrients to pass from the blood into the brain tissue. Small blood vessels outside of the brain, such as the capillaries in the fingertips, have very thin walls—sometimes the width of a single cell—and are, therefore, highly permeable to gases. These vessels can either contain tiny holes or large protein structures that physically transport substances across the blood vessel. On the other hand, it is also advantageous to separate toxins and foreign pathogens in the bloodstream from getting into brain tissue.

The blood–brain barrier (BBB) is an anatomical adaptation that selectively transports substances necessary for normal biological function, while simultaneously excluding potentially harmful invaders from the brain. The BBB physically surrounds blood vessels in the brain. It is made up of endothelial cells and a type of glial cell called an astrocyte. The BBB is injured in a variety of medical disorders, ranging from stroke, epilepsy, and Alzheimer’s disease, just to name a few. It is still unknown what role the disruption of the BBB plays in brain disorders.

The exclusive nature of the BBB can be a double-edged sword. It is difficult to deliver a drug into the brain from the blood stream if that drug is unable to pass through the BBB. For example, the current gold standard pharmaceutical treatment for Parkinson’s disease is to increase the brain’s levels of dopamine. However, dopamine does not pass through the BBB. To get around this, physicians give the BBB-permeable substance L-DOPA, which the brain is able to convert into dopamine. Many other therapeutic drugs do not cross the BBB, so researchers are developing methods using electromagnetic fields to temporarily weaken the barrier, or surround the drugs in nanoparticles so small that the body cannot identify them as foreign.

Ventricles

There are hollow spaces within the brain called ventricles. The human brain has a total of four ventricles. The two very large, paired ventricles (one in each hemisphere) are the lateral ventricles. They are connected medially to the third ventricle, which extends to the posterior aspect of the brain. From here, the cerebral aqueduct that runs ventrally extends into the fourth ventricle before continuing into the central canal: a narrow space that runs all the way through the length of the spinal cord along the midline.

The entire ventricular system is interconnected. The ventricles are filled with a liquid called cerebrospinal fluid (CSF). CSF is basically a high-salt water solution. Because of the high osmolarity of CSF, it is a very buoyant solution. Like a fully grown person who can float easily on the surface of the extremely salty Dead Sea, CSF allows the brain to remain “floating” inside the skull. Without CSF, the brain weighs almost 1.5 kg (~3 lbs). Cells and blood vessels at the ventral base of the brain would be crushed under the weight of the brain itself. But when the brain is surrounded by CSF, it weighs less than 50 grams, almost two orders of magnitude lighter!

CSF is also found within the meninges that encase the brain. In fact, more than 80% of the CSF in the body exists in this space outside the brain. This liquid serves as a form of “cushioning” that protects the brain from rapid head movements. If it weren’t for this physical protection, the inertia of head movement may cause your brain to smash against the inside of the rigid skull if you move your head too quickly.

The CSF layer allows the head to withstand some sloshing of the brain, but a movement that is too abrupt can cause a traumatic brain injury. CSF can also function as a way to wash impurities out of the brain. The volume of CSF in the typical human body is about 150 mL, a little more than half a cup. Because there is frequent turnover of CSF, the material gets absorbed back into the body regularly. Each day, the body produces about half a liter of CSF, so the brain cycles through the entire volume a few times.

Hydrocephalus

Hydrocephalus, historically called “water on the brain”, is a common condition affecting the brain of about 1 in 200 newborns and a small number of adults. In patients with hydrocephalus, the volume of CSF increases, which elevates intracranial pressure, causing symptoms such as fever, stiff neck, headache, seizures, or altered mental status.

In adults, the skull is rigid and unmoving. But in newborns with hydrocephalus, the plates of the skull are not completely fused together. Often, these children will have a bulging parts on the skull and an expansion of the forehead.

Increased CSF volume can happen in a couple ways. The clearance of CSF may fail while production remains normal, or the entrance to the central canal in the spinal cord may be narrowed or blocked by a tumor, leading to an increase in the volume in the brain. A common treatment for hydrocephalus is to surgically implant a shunt (a hollow tube that runs from the ventricle down into the abdominal space) that allows for drainage, thus decreasing intracranial pressure.

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Attributions

Portions of this chapter were remixed and revised from the following sources:

1. Open Neuroscience Initiative by Austin Lim. The original work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.