Introduction

Sumit Karia MD Ashley Davidoff MD

Introduction

If one were to show the non inquisitive person the brain of a calf, he or she might ask – “What is that?” The easiest answer would be – “It is the brainof a calf,” and that could suffice as an answer. If on the other hand the same brain was shown to a curious minded, or analytically minded person, they may counter with “How do you know?” or “Why is it a brain?” The answer, based on the knowledge of the structure would include describing its unique and characteristic size, shape, position, and character. We may also speak to its parts, and its relationships to other structures its connections to other biological structures, how it changes with time and different cyclical events. The manner with which it functions in vivo would reflect its physiological characteristics.

The descriptors we used above are universal descriptors that can be applied to all biological structures whether it be the brain of a calf, the myocyte of the heart, or the pancreas of a mountain rat. We can use these descriptors to describe the cell, a tissue, an organ in both health and disease. We can also use these descriptors for clinical findings such as an enlarged liver felt in the right upper quadrant that feels hard, nodular, soft, or rubbery.

In this program our educational goal is to start with simple basic principles, and slowly advance to greater complexity. Thus far we started with the word brain, and advanced to the 5 principles that are used in medicine that includes the most basic knowledge of its structure, function, the diseases that affect it, the way we diagnose disease and the basic approach to treatment.

We are moving one step forward to advance the knowledge of the structure of the brain by writing a paragraph (or two) on the structural descriptors of the brain.

A Paragraph on the Structure of the Brain

The brain is a vital organ and is one of the largest organs in the body. It weighs about 1300-1400grams in the adult. Dimensions are approximately 17 cm in longitudinal size, 14 cm in transverse and 13 cm in height, in the adult male. In the adult female, each of these dimensions have about less 1 cm.It reaches its adult size at about 18 years is slightly larger in the male than the female, and begins to decline in size after 50 years. The size of the brain as expected has no correlation to its quality.

The shape of the brain is complex. From the external view, and looking at it either from the side or from above it is generally considered to be an oval or ovoid with the greater axis oriented antero-posteriorly, and the wider portion being posterior. The external face, convex, is in relation with the bony cranium. The sulci and gyri at once change the concept of a simple ovoid to an amazigly complex surface shape. The surface gyri and sulci are the most characteristic feature of the brain.

Characteristic Shapes of the Surface of the Brain

The shape of the brain is in general an oval, but sulci and gyri are the most unique aspects that define the shape of the brain surface.

The shapes, particulalrly the shapes of the forebrain, become more complex in the sagittal cuts of the brain. They consist of a series of inverted c-shaped structures starting from the outer cortex and ending in the thalamus after a series of consecutive rings.

Shapes in the Sagittal View

The shape of the brain in general is an ovoid. As one goes beyond the surface in the sagittal plane, the brain can be viewed as a series of inverted c shaped structures.

The brain is the most cranially positioned of the organs. It is both structurally and functionally the crowning glory of the body. It lies relatively snugly within the protective cranial vault and is divided between the three fossae; anterior cranial fossa, middle cranial fossa and posterior cranial fossa.

The Cranial Position of the Brain

Snug in the Calvarium

This sagittal MRI of the brain has ben artistically rendered, and reflects more detail of the anatomy of the forebrain, midbrain and hindbrain within the cranium.

Image Courtesy Philips medical Systems Davidoff art 92170b06b01.8s

The character of the brain is not easily defined but it has a light creamy color, and feels like jello, or soft butter. Its character is best exposed in vivo by its MRI characteristics, and even better under the microscope with specific stains.

Defining the parts of the brain is the most challenging task from the structural perspective superceded only by the task of assigning functions to these parts. For the sake of simplicity, we are going to start by using the a global calssification of forebrain, midbrain and hind brain.

Forebrain Midbrain and Hindbrain

The T1 weighted MRI taken in sagittal projection reflects the 3 basic parts of the brain; forebrain (torquoise) midbrain (lime green) and hind brain (salmon pink)

The brain is connected and related to every part of the body. The connections of the brain to the rest of the bodyare via its neurovascular bundles, and neuroendocrine axis.

The nerves of the somatic system and autonomic nervous system enable the brain to control the sensory and motor functions of the muscles, glands, and all the organs. Through the vascular sytem and pituitay gland it commands its control over the endocrine system.

The blood supply is via the carotid and vertebral systems, and is protected by the unique circle of Willis which enables cross over of the two circulations, as well cross over from the one side of the brain to the other side. It consumes 20% of the blood supply to the body.

Venous drainage is via the simnuses in the brain and then into the jugular system and finally into the superior vena cava.

The brain does not have a distinct lymphatic system but contains a unique cerebrospinal fluid that circulates within the internal ventricular system and the surface of the brain.

The brain develops embryologically from ectoderm to form the neural plate, which evolves into the neural tube. The large evolving mass has limited space and hence it has to fold on itself to accomodate the necessary volume. The brain ages like all the other organs but the manifestation of the aging process of such a vital organ becomes readily apparrent and sometimes devastating as evidenced by people with Alzheimers disease.

So why does a brain look like a brain?

The most characteristic view of the brain is the outside view looking either from the top or from the side (lateral view). In these views the outstanding features are the folds of gyri and the clefts of sulci.

External Appearance of the Brain

The anatomic specimens show the brain as viwed from above (a) and as viewed from the side (b) Looking at the brain from above, it has, in its midline, a deep fissure that divides the organ in 2 halves called hemispheres. The fissure is called the interhemispheric fissure. The surface of the hemispheres is known as cortex. It presents as a system of irregular prominences (called convolutions or gyri) that circumscribe depressions (called fissures or sulci) which are somewhat profound and full of twists and turns. The left side of the brain is covered by its meninges and the convolutions are therefore not as distinct. When viewed from the side the forebrain is distinct different with sulci and gyri from the visualised lower parts of the brain. Only parts of the hindbrain are seen.

Source unknown Modified by Davidoff MD 52981b05.8s

Great simulators of the brain have included the famous brain coral and the walnut.

Why is this Structure not a Human Brain?

The brain coral has folds that are reminiscent of the gyri and sulci of the brain. The structure may well confound the student ignorant of the finer detail of brain anatomy. The hard character, and its position in the bottom of the sea are totally uncharacteristic of human brain. The smoothness of roundness of the gyri on the brain specimen are very unlike the pattern on the coral. The orientation and shape of individual gyri are so characerttsic they are named and easily recognized by the expert, whereas the pattern on the coral is random.

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Why is this Structure not a Brain?

This image of a walnut has a few features that are reminiscent of a brain, including the symmetric hemispheric components and the undulations of the surface. However in reality it is about 1.8cms long, and only 5-8mms high, and is hard.