Brain Surgery Information:

PLEASE NOTE:

• If you are looking for information regarding a specific brain condition, please visit the Contents page or, alternatively, use the custom search engine by Google on the What's New page.
• If you would like to obtain a downloadable e-Book version or an official hard copy book version pertaining to the information and illustrations contained in all 25 chapters, click here.

CHAPTER 2: ABOUT THE BRAIN

Organization of the brain

The word cerebral refers to the brain, or cerebrum. The brain and spinal cord constitute the central nervous system (CNS). The brain is encased by a thin but tough leathery sheath or membrane known as the dura. Just deep to the dura is a spidery and normally watertight membrane called the arachnoid. Under this lies the thin pia, which physically coats the brain’s  outermost layer. The dura, arachnoid and pia are referred to as meninges, in that they are the three coverings of the brain (Figure 1). The arachnoid membrane houses the fluid surrounding the brain. This fluid, referred to as cerebrospinal fluid (CSF), lies in the subarachnoid space (SAS; Figures 2 and 3).

Figure 1 (above). Coverings of the brain.

CSF is made by small, wispy structures deep in the brain collectively called the choroid plexus. In adults, approximately 20 milliliters of CSF are produced each hour, and the total volume of CSF in and around the brain and spinal cord is about 125 milliliters. CSF circulates through spaces in the brain called ventricles, which are somewhat akin to the heart’s various chambers. It exits the brain at various openings, and then circulates in the SAS both under and over the brain’s outer curved surfaces or convexities. CSF is normally absorbed by arachnoid granulations which are structures that lie mainly in the midline over the convexities (Figure 2).

Figure 2 (above). CSF production and absorption.

The brain surface is referred to as the cortex, which means bark as in tree bark. The cortex is a few millimeters thick. The very outermost part of the cortex is referred to as the pial surface of the brain. In order to accommodate more of the brain’s nerve cells, the cortex has many folds in it, and these are its gyri, each one being a gyrus. Between the gyri lie valleys or sulci, each one being a sulcus. In the sulci are found small blood vessels, referred to as pial vessels (Figure 3).

Figure 3 (above). Brain surface.

Brain parts are often classified as forebrain, which is the larger upper part of the brain, and hindbrain, the smaller under back part of the brain (Figure 4). The forebrain is divided into lobes which include frontal, parietal, temporal and occipital (Figure 5), and also insular (Figure 6) and limbic. These are described below. The two halves of the forebrain are called hemispheres (Figure 6). Interconnecting the left and right cerebral hemispheres is a dense collection of nerve fibers collectively called the corpus callosum (Figure 7). The hindbrain is comprised of the cerebellum, or little brain, and the brainstem (Figure 7). The cerebellum looks like a smaller and more compact version of the forebrain, and in a brain model it appears to hang off the back undersurface of the brain, immediately behind and around portions of the brainstem (Figure 7). The brainstem is made up of, from above to below in approximate thirds, the midbrain, pons, and medulla oblongata (Figure 7). From the brainstem arise cranial nerves. These are paired nerve bundles that supply many structures in the head and neck (Figure 7).

Figure 4 (above). Main subdivisions of the brain.

Figure 5 (above). Lobes of the forebrain.

Figure 6 (above). Forebrain hemispheres and insular lobe.

Figure 7 (above). Corpus callosum and hindbrain.

The brain and its dura are housed in a rigid box referred to as the skull or cranium. There are many small openings in the base of the skull through which small blood vessels and cranial nerves pass. However, the main opening in the skull is at its base towards the back, where the bottom of the brainstem or medulla becomes the upper part of the spinal cord (Figure 7). This opening is called the foramen magnum and in adults is only three to four centimeters in diameter.

At any one time, the brain receives 20% of the heart’s blood output. Regarding the organization of the brain’s blood supply, there are four main pipes or trunks that enter into the brain from the neck, namely, the two internal carotid arteries at the front under surface of the brain and the two vertebral arteries at the back under surface of the brain. From the ends of these trunks arises a ring of arteries that encircles the under surface of the brain. This ring is known as the circle of Willis (Figure 8). In approximately 20-25% of persons, this “ring” is in fact not a complete circle, a finding regarded as a normal variation of brain vessel anatomy.

Figure 8 (above). Circle of Willis.

Arteries of the circle of Willis eventually give rise to pial arteries which course over the brain’s convexities (Figure 3). From the pial arteries many smaller arterioles take off, usually at right angles to their parent vessels, and perforate into the brain’s deeper substance. These end in capillaries, which then drain into venules and then larger veins, which then make their way into very high-volume, low-pressure venous systems known as dural venous sinuses. These high throughput venous channels will eventually empty into the neck’s internal jugular veins on their way back to the heart's right atrium.

Structure of the brain

The average adult brain weighs approximately 1350 grams, or 3 pounds. It is comprised of tens of billions of cells. The vast majority of these cells are in fact not neurons or nerve cells, but are rather glia or supporting cells. Supporting cells of the brain include astrocytes, oligodendrocytes, microglia, ependymal cells, and others. At a microscopic level, neurons receive information at their tree-like dendrites, where throughput occurs at specialized structures called synapses. These specialized connections allow neurons to communicate by passing tiny electrical signals from one neuron to others (Figure 9).

Figure 9 (above). Neuron.

Oligodendrocytes in the brain wrap thinly layered fatty sheaths of myelin around axons which are the long electrically conductive parts of neurons. The myelin helps in maintaining the speed of electrical conduction throughout the CNS (Figure 9). The gray matter of the brain is basically its cortex and the deeper islands of neurons which form its deep nuclei such as the basal ganglia, thalamus, brainstem cranial nerve nuclei, and so forth. The white matter of the brain, which is the majority of the brain, is essentially all the myelinated pathways or tracts running to and from the cortex, deep nuclei, brainstem and spinal cord (Figure 3). The white matter also includes the corpus callosum.

Functions of the brain

In essence, the left side of the brain controls the right side of the body, while the right side of the brain controls the left side of the body. For the vast majority of the population, the left hemisphere is the dominant hemisphere. Dominance here refers to the fact that this half of the brain is responsible for key aspects of the individual’s language, memory, and higher “cognitive” functioning. While the dominant hemisphere is the left hemisphere for virtually all right-handed individuals, it may be the right hemisphere for some, but not all, left-handed individuals. The overwhelming majority of the population is right-handed and therefore left-brain-dominant.

The various lobes of the forebrain each subserve unique functions, but are also complexly interconnected (Figure 10):

·  Frontal lobe: This is located at the upper front and midpart of the forebrain in each hemisphere, and is concerned with personality and behavior, judgment, attention, problem solving and movement planning. The primary motor cortex for control of the opposite side’s movements from top to toe is also part of the frontal lobe. On the dominant side, which is typically the left, it also houses a critical area for speech or “expressive” language function known as Broca’s area.

·  Parietal lobe: This is located at the upper back and midpart of the forebrain in each hemisphere, and is concerned with the opposite side’s body part sensation through the primary sensory cortex it houses. On usually the left side, it also contributes to the ability of an individual to carry out calculations, to write, and to determine left from right. On the right side, it contributes to a person’s ability to carry out certain constructional and dressing tasks, and to orient himself or herself in space. On the dominant side, it houses a critical area for understanding spoken language known as Wernicke’s area.

·  Temporal lobe: This is located at the lower midpart of the forebrain in each hemisphere, and is concerned with memory and learning, the sense of smell, and sound processing. The right temporal lobe may be more involved in visual memory, say, for faces, places, and pictures, while the left may be more involved in verbal memory, say, for names and words.

·  Occipital lobe: This is located at the back of the forebrain and partly draped over the cerebellum in each hemisphere, and is responsible for processing vision. That is, actual recognition of what is seen, and associating what is seen with smells, colors, shapes, and so forth, are the critical functions of the occipital lobe.

·  Limbic lobe: This is configured in a c-shaped manner, with the “c” cupped forward, and is located in the deeper and more inward or medial parts of each hemisphere. It is responsible for modulating a variety of emotions, including fear, rage, pleasure, feeding, laughing, and so forth. It may also play a role in motivation.

·  Insular lobe: Like the limbic lobe, the insular lobe or insula is also hidden from view. In both hemispheres, it is an island of brain found deep to an area called the Sylvian fissure, where the frontal, parietal, and temporal lobes meet on the outside. The insular lobe’s functions are complex and not well understood. It plays a role in taste perception, but also integrates a variety of functions spanning movement, balance, and sensation.

·  Deep nuclei: These deep islands of nerve cells in the forebrain include the thalamus, subthalamus, caudate, putamen and globus pallidus. They collectively constitute the basal ganglia. These are responsible for the gating of movement and/or sensation for the opposite side of the body.

Figure 10 (above). Forebrain functions.

Regarding functions of the hindbrain (Figure 11):

·  The cerebellum: This structure plays a central role in balance and coordination of movements.

·  The brainstem and its many cranial nerve nuclei: These structures subserve many functions including eye movement, facial sensation and expression, chewing and swallowing, speech, hearing and balance, and contribute to head turning. Note that two cranial nerves that do not arise from the brainstem are the first and second cranial nerves, or olfactory and optic nerves, which serve the senses of smell and vision, respectively. The brainstem is also a conduit for the many long motor and sensory tracts passing to and from the brain and spinal cord. Additionally, it has major centers for the control of heart rate and breathing. That is, it houses a key cardiorespiratory center, in addition to centers thought to play a role in general brain activation.

Figure 11 (above). Hindbrain functions.