Lab 1: Topography of the Brain

Lab Summary

The purpose of this first lab is to begin developing your vocabulary for neural structures and your capacity to understand the 3-dimensional organization of the brain. This is a necessary step before learning the details of structure and function that will be presented in subsequent lectures and labs. In particular, this lab focuses on: 

SECTION 1: Planes of the Brain 

SECTION 2: External morphology 

SECTION 3: Blood supply 

SECTION 4: Medial Surface and the Ventricular System 

SECTION 5: Special dissections 

SECTION 6: Review

Planes of the Brain

In today’s lab you will examine intact human brains, brains sectioned along different axes. In this and future labs you should be sure to identify and locate all bolded terms in this manual. Hovering over them will reveal a definition.

Many of the images you will be examining in this and the later labs will be MRI, CT, or SPECT/PET images. Review the Primer for Interpreting Brain Images to ensure that you are familiar with the visual representation of tissues in these sections. As you read through each lab you will find references to INTERACTIVE widgets – these contain exercises and virtual slides (requires VPN if off-campus). At the end of each chapter is a Review section to help you assess your understanding.

Figure 1.1A-B illustrate the planes of section and directional terms as they apply to the human brain and spinal cord, respectively. Identify the frontal, horizontal and sagittal planes of section on your fresh brain specimens.

In MRI and CT scans the axial view is the same as the horizontal plane and is oriented perpendicular to the long axis of the brainstem and spinal cord. Discuss how the orientations of dorsal, ventral, caudal and rostral differ for the cerebrum and brain stem? Why do these orientations differ for the forebrain compared to the brain stem and spinal cord? How do these terms relate to the superior, inferior, anterior and posterior orientations? 

A useful supplement to the material in this lab manual is the Functional Neuroanatomy website produced by Dr. Claudia Krebs and Monika Fejtek through the University of British Columbia. The link is https://www.neuroanatomy.ca/index.html.

External Morphology

The Meninges 

Some of the whole brains may still have a part of the most exterior meningeal layer, the dura, attached (Figure 1.2). All will have the delicate arachnoid and pial layers, collectively known as the leptomeninges. The arachnoid stretches over the surface of the brain, does not enter into the sulci and is continuous with the arachnoid and arachnoid spaces of the spinal cord. In contrast, the pia closely adheres to surface of all grooves, sulci and elevations of the brain and spinal cord. 

The subarachnoid space between the arachnoid and the pia, contains cerebrospinal fluid (CSF) and blood vessels. This space is connected to the fourth ventricle by the medial foramen and the two lateral foramina. Through these foramina CSF leaves the ventricular system and enters the subarachnoid spaces.

The subarachnoid space is not uniform and is enlarged in areas where the brain and spinal cord draw away from the skull and spinal vertebrae. These CSF filled spaces are termed cisterns. Figure 1.3 gives some examples of cisterns you should be familiar with: interpeduncular; superior (quadrigeminal); cistern magna (cerebellomedullary); and pontine (prepontine). When reviewing the spinal cord you will also hear about the lumbar cistern which is caudal to the end of the spinal cord (not shown in figure 1.3). Identify the positions of these cisterns in relation to the whole and sagittal brain specimens. Bleeding (hemorrhage) into a particular cistern, as is commonly associated with trauma or rupture of an aneurysm, may produce pathological signs and symptoms related to structures or nerves in, or around, the cistern. 

The Five divisions of the brain retained from embryonic development (Figure 1.4) 

1. Myelencephalon (medulla) 

Rostral to the area of transition between the spinal cord and the brain is an enlarged area, the medulla. The dorsal surface of the medulla forms a diamond shaped depression, the floor of the fourth ventricle (Figure 1.5A). Use your sagittal preserved brain specimen and Sagittal MRI image (Figure 1.6A) to identify the area of the medulla. This area is normally covered by meninges, in vivo. The overlying cerebellum obscures much of the fourth ventricle in a whole brain specimen but can be viewed by slightly spreading (but not tearing!) the cerebellum and medulla apart. View this area to examine the continuity between the fourth and third ventricles via the cerebral aqueduct of the midbrain (Figure 1.6B). The choroid plexus is formed in the ventricles by the growth of blood vessels into the ventricular space; as the blood vessels grow, the pia and ependymal roof plate are pushed inward and invaginate into the ventricular space, separating the vascular system from direct contact with neural surfaces. The epithelial cells of the choroid plexus produce and secrete CSF into the ventricular spaces. 

2. Metencephalon (pons and cerebellum) 

On the dorsal surface of the brainstem, the boundary between the medulla and pons is apparent as the widest part of the fourth ventricle, the level of the lateral recesses. Immediately rostral to the lateral recesses are the middle cerebellar peduncles. The middle cerebellar peduncles are formed by axons of pontine nuclei projecting to regions of the cerebellum. 

On the ventral surface of the metencephalon (Figure 1.5B) the body of the pons is easily delimited from the medulla and the midbrain by its bulging band of transversely oriented fibers. Identify this area and the surrounding landmark structures on your preserved brain specimens, the sagittal MRI image and on the Mid-Sagittal Brain Image (Figure 1.6A-C). 

3. Mesencephalon (midbrain) 

The dorsal surface of the mesencephalon (Figure 1.5A) is distinguished by two pairs of rounded bodies, the inferior and the superior colliculi (corpora quadrigemina), that are related to auditory and visual information processing, respectively. 

On the ventral surface are the massive cerebral peduncles, formed by axons of neurons of the cerebral cortex that terminate in nuclei of the brain stem and spinal cord. The majority of axons within the cerebral peduncles carry information to motor areas of the brain stem and spinal cord and are involved in the initiation or regulation of voluntary movements. The space between the two peduncles is the interpeduncular fossa and is the location of the interpeduncular cistern. 

4. Diencephalon (thalamus, hypothalamus, epithalamus, subthalamus) 

The dorsal surface of the diencephalon is hidden by the corpus callosum, and the lateral surfaces by the cerebral hemispheres. 

On the ventral surface, portions of the hypothalamus are visible: the infundibulum, to which the pituitary (hypophysis) was attached; the tuber cinereum, the slight bulge caudal to the infundibulum, and the paired mammillary bodies. These landmarks overly regions of the hypothalamus involved in the regulation of a variety of autonomic functions, food and water intake, the initiation and coordination of reproductive functions and the regulation of hormonal output. Locate these structures and surrounding landmarks on your preserved brain specimens, and on the Mid-Sagittal Brain Image (Figure 1.7C). Discuss with your lab instructors how the retina develops from the diencephalon and locate the optic nerves and their partial crossing at the optic chiasm. Caudal or central to the optic chiasm the optic nerves become the optic tracts. 

5. Telencephalon 

Examine the cerebral hemispheres, noting differences in the shape of the rostral and caudal poles and the elaborate sulci and gyri. Viewing the superior surface of the cerebral hemispheres identify the interhemispheric or longitudinal fissure (Figure 1.7A) separating the two hemispheres. In a specimen of the whole brain, gently spread apart the 

hemispheres to view the large corpus callosum, consisting of over 100 million nerve fibers connecting the two hemispheres. Note the C-shaped configuration of the corpus callosum by examining a preserved brain specimens, the Sagittal MRI image and on the Mid-Sagittal Brain Image (Figure 1.6A-B). The anterior bend of the callosum is the genu and the posterior bend is the splenium. On lateral surface of the cerebral hemisphere find the lateral sulcus, sometimes called the Sylvian fissure (Figure 1.7B & Figure 1.8); this separates the temporal lobe from the rest of the hemisphere. Within the lateral sulcus lies the primary auditory cortex of the temporal lobe. Gently lift the two borders of the lateral sulcus and feel the buried insula. How is the insula formed during development? It should be possible to locate the central sulcus , which extends from the interhemispheric fissure over the lateral surface, almost to the lateral sulcus. Rostral or anterior to the central sulcus is the precentral gyrus that contains the primary motor area of the cortex. The gyrus caudal to the central sulcus is the postcentral gyrus and is the primary somato-sensory area of the cortex. All sulci and gyri rostral to the central sulcus make up the frontal lobe. The parietal lobe extends caudally from the central sulcus back to the parieto-occipital sulcus (better seen on sagittal-cut brains). The remaining cortical area caudal to the parieto-occipital sulcus is the occipital lobe. Within the occipital lobe lies the primary visual cortex and several higher order visual areas.

Blood Supply of the Brain

Review what you learned in Gross Anatomy about the blood supply to the brain, including the distribution of the internal carotid and vertebral arteries, the cerebral arterial circle of Willis, the areas supplied by the anterior, middle and posterior cerebral arteries (Figure 1.9A). Inspect whole and sagittal brain sections to identify the locations of these primary 

blood vessels supplying the surfaces of the brain (Figure 1.9B). During future laboratory sessions, the blood supply to specific regions (e.g. spinal cord, brainstem, thalamus, cerebral hemispheres) will be carefully studied. An excellent youtube video reviewing the arteries of the circle of willis and their distribution is Intracranial Arterial Anatomy – Circle of Willis.

Medial Surface and the Ventricular System

The cut mid-sagittal surface of the half-brain reveals several structures and features that are not visible on the whole brain (Figure 1.10). As you study it, be sure you can determine the borders of the five brain divisions: myelencephalon (medulla), (metencephalon (pons and cerebellum), mesencephalon (midbrain), diencephalon (thalamus, hypothalamus, subthalamus and epithalamus) and telencephalon (cerebral cortex, basal ganglia, amygdala, olfactory bulb and hippocampus)and can identify all of the structures previously described. Fill in the diagram of this surface (Interactive 1.2). 

On the sagittal-brain specimen, examine the shape of the ventricular system: trace the merging of the central canal of the spinal cord with the fourth ventricle, and trace the shape of the fourth ventricle. The lateral recess should be seen better than before. Note that the CSF producing choroid plexus is only located caudal to the lateral recess (Figure 1.11B) that is, only in the medullary half of the IVth ventricle. Choroid plexus cannot form in the metencephalon (rostral IVth ventricle), since the cerebellum developed over the roof plate. The fourth ventricle narrows into the cerebral aqueduct of the midbrain, lying beneath the colliculi. Now several structures of the diencephalon are visible. Trace its anterior, posterior, inferior and superior borders and note that the medial surface of this area forms the lateral wall of the narrow third ventricle.

Now exam the shape of the ventricular system using the AR Brain app ventricular viewer. 

Starting just rostral to the superior colliculi, locate the pineal body of the epithalamus. Near the dorsal (superior) border of the diencephalon the choroid plexus of the third ventricle is found. Although the body of the egg-shaped thalamus lies buried deep in the hemispheres, most of the medial surface of the thalamus (Figure 1.12) is visible. There may be a small cut surface representing the intrathalamic adhesion where the thalami of the two hemispheres are often connected (this varies in each brain). Rostrally, the interventricular foramen (foramen of Monro) connects the third ventricle with the lateral ventricle. Note that the choroid plexus is continuous through this “hole” with the choroid of the lateral ventricle. The slightly depressed area below the thalamus is the medial surface of the hypothalamus; here the third ventricle is slightly wider. Note the cut surfaces of the optic chiasm, the infundibulum and of the mammillary bodies. Use Figure 1.11A-C, the ventricular cast supplied by your instructor and the preserved serial horizontal brain slices to review the structure and continuity of the ventricular system and the flow of CSF through the ventricular system, subarachnoid spaces and its absorption into the blood stream via the arachnoid granulations. The total adult volume of CSF is approximately 150cc. The choroid plexus is the principal source of CSF, producing approximately 20cc per hour. Disturbances of the flow of CSF or the rate of production can lead to a condition known as hydrocephalus. Watch the video Circulation in Ventricles and Dural Sinuses for a review of the ventricular system and the flow of CSF. 

Stretching between the fornix and the corpus callosum and forming the medial wall of the anterior horn and body of the lateral ventricle is the septum pellucidum. The septum pellucidum is often missing from the sagittal brain allowing you to look inside to see the cavity of the lateral ventricle. Examine its continuity with the foramen of Monro and with the third ventricle. If your septum pellucidum is intact do not damage it, but use another group’s specimen to examine the ventricle. The ventrolateral surface of the lateral ventricle is formed by part of the corpus striatum, and can be felt rostrally as a distinct bulge (later this will be identified as the head of the caudate). Finally, trace the transverse fissure: this is formed by the backward growth of the cerebral hemispheres over the brain stem. It begins at the space between the cerebral hemisphere and the cerebellum, and continues rostrally under the corpus callosum but over the thalamus (e.g., over the roof of the third ventricle). This space is filled with a double layer of pia (one layer from the dorsal surface of the thalamus and one from the ventral surface of the hemisphere); “ the double layer of pia is called the velum interpositum.

Special Dissections

During this lab session, your instructors will demonstrate a brain sliced horizontally, exposing internal structures not visible in other preparations. Try to identify the major brain divisions and the structures you have learned earlier. Pay particular attention to the internal mediolateral and dorsoventral extent of the structures, the relationship of the brain stem to the telencephalon, as well as the three-dimensional shape of the ventricular system and structures such as the cerebral cortex, fornix, corpus callosum and lateral ventricles which form C-shaped structures of the brain running from inferior to superior, then anterior to posterior and finally moving from superior to inferior (Figure 1.13). Have your instructor demonstrate how the caudate nucleus of the striatum also shares this shape and trajectory using the horizontal sections. Go to the horizontal section outlines in Interactive 1.3 and draw in and label as many of the italicized structures as you can. 

Interactive 1.3 Label Structures on horizontal brain sections 

Try to identify these structures and the relationships that could not be seen in the whole brains and in sagittal cut brains: 

1. The exposed white matter of the cortex consists of (a) long association fibers, connecting distant regions of cortex; (b) short association fibers, or “U”-fibers, connecting adjacent gyri (you can see these along the broken edges of the dissected cortex; and (c) interhemispheric fibers, the most massive being the corpus callosum. 
2. The frontal, parietal and temporal lobes meet at the lateral sulcus to form an operculum (or covering) over the buried insular cortex. Caudal to this, on the posterior surface of the lateral sulcus you will see exposed the gyri of the auditory cortex. 
3. Deep to the insular cortex lies the corpus striatum; these are the internal nuclei of the telencephalon. Note also the relationship of the corpus striatum to the internal capsule and to the thalamus. The internal capsule is composed of axons running between the cerebral cortex and the rest of the brain. 
4. The lateral ventricle is seen as it extends into the frontal, occipital and temporal lobes; identify the anterior, posterior and inferior horns of the lateral ventricles. Look inside the lateral ventricle and follow the choroid plexus anteriorly to the interventricular foramen of Monro and then around into the inferior horn; note that the choroid plexus does not extend into the posterior horn. Lateral to the choroid plexus is the corpus striatum, medial is the thalamus. 
5. The hippocampus is visible in the inferior horn of the lateral ventricle. Identify the fornix as a large bundle of fibers that leaves the hippocampus and follows the lateral ventricle as it travels toward the mammillary bodies and the septal region. 

Coronal Sections Through Squirrel Monkey Brain 

Your instructor will provide you with serial coronal sections through a monkey brain. Use these sections to test your ability to determine the relative rostral – caudal positions of different sections. Use the organization of the grey vs. white matter and the appearance of the ventricles and other structures to position these sections relative to each other.