Laboratory 9 : Brain Anatomy

 This lab will center on a sheep brain dissection and comparison of brain structure across various vertebrate groups. This will illustrate principles of both general neural anatomy and some specializations of the mammalian brain.

Factoid: Did you know that your brain accounts for roughly 20% of your oxygen consumption when you are at rest, but is only about 2% of your body weight? (maybe a little more during tests and a little less on weekends)

Brain Anatomy in a Representative Mammal

The second part of this lab will be an examination of the sheep brain and comparison of gross features of this structure to that of other vertebrates. We are examining vertebrate brains in this lab, but it should be noted that much of what we know about how the nervous system functions we know from squid, flies, and other invertebrate animals. For example, we know that adults of the commonly used model organism Caenorhabditis elegans (a small nematode worm) have 302 neurons and we are beginning to get a handle on what each one does. This might seem distant from a human brain with its 100 billion of neurons, but many of basic principles are the same.

The plan in approaching the problem of dissecting the sheep's brain will be the following: First, an examination of the external features of the specimen will be undertaken. Second, the brain will be inspected in specimens bisected along the longitudinal midline (a sagittal section), and the several features of the medial face of the hemisphere considered. Third and finally, you can section the brain in a coronal manner (a cross section), and examine some of the deep structures of the brain studied from that point of view. In addition to the diagrams available, there will also be a Powerpoint presentation in the computer at your lab station. This will allow you to look at the structures we are covering in full color and make it easier to identify them.
 
 

Subdivisions of the Vertebrate Brain

The vertebrate brain is basically an anterior enlargement of the nervous system. It is subdivided into three basic regions containing five major divisions. These divisions are essentially remnants of our basically segmental organization (think about our vertebral column or abdominal muscles for other examples).

• Forebrain (ancestrally associated with olfaction)
• Telencephalon
• Diencephalon
• Midbrain (ancestrally associated with vision)
• Mesencephalon
• Hindbrain (ancestrally associated with balance and vibration information)
• Metencephalon
• Myelencephalon
Forebrain

This region contains the telencephalon and diencephalon. Ancestrally, this region of the brain functioned primarily in olfaction. A variety of functions are centered here in mammals. Among these are conscious thought, emotion, control of many basic body functions, and control of the endocrine system.

1. Telencephalon: This is the forward-most region of the vertebrate brain. There has been a trend of increasing telencephalic size over the course of vertebrate evolution. In most mammals, the telencephalon is very large and has come to dominate the outer aspect of the brain. Examine the cerebral hemispheres of the telencephalon on your sheep brain and compare their relative size to those of the telencephalon in models of other vertebrate brains (colored light yellow on the models).


Give a rough ranking of telencephalon size in the various vertebrate groups here:

_________ > ____________ > _____________ > ____________ > _____________

1. The mammalian brain shows a characteristic feature. This is a phenomenon called corticalization and it is extreme in many mammals relative to the brains of other vertebrates. What you are looking at on the outside of the sheep brain is referred to as the cerebral cortex. This term refers to the fact that the cell bodies (the Îgray matterâ) are on the outside of the brain while their myelinated axons (the Îwhite matterâ) are on the inside of the brain. Look at this corticalization on both your sheep brain and the pictures on your monitor.


Why is the cortex so wrinkled? The cell bodies are on the outside here, forming the surface of the cortex. So: if you want to add more cells, what do you need? More surface area. This wrinkling of the cortex greatly increases the surface area of the cortex and therefore the number of cells it can contain.

2. The two cerebral hemispheres seem very separate, but are in fact connected by a sort of neural "information superhighway". This is the corpus callosum and it is large a bundle of axons which carry information between the two sides of the cerebral cortex (this structure consists of ~250 million axons in humans).
1. To see the corpus callosum, spread the cerebral hemispheres apart and look for a bundle going from one side to the other.
3. Flip your sheep brain over now and examine the ventral surface. Olfactory information is received from the nose and the olfactory epithelium first at the olfactory bulbs. These are the bulbous structures at the front of the brain on the underside. Leading from the olfactory bulbs are the olfactory tracts. These are large bundles of axons from cells in the olfactory bulb that carry olfactory information into the brain. The olfactory nerves entering the olfactory bulbs are the first cranial nerves.
4. Looking medially (towards the center) on the ventral surface, you can also see the paths visual information takes into the brain. The optic nerves are large whitish structures that cross here before entering the underside of the brain. These nerves are not part of and do not enter the telencephalon, illustrate the second pair of cranial nerves (the optic nerves).
2. Diencephalon: The diencephalon lies almost completely hidden under telencephalon in your sheep brain. It is more obvious in other vertebrates with smaller telencephalons (the diencephalon is indicated in dark yellow on the models). The diencephalon contains several important structures including the thalamus, hypothalamus, pineal gland, and posterior lobe of the pituitary (an extension of the hypothalamus). The thalamus and hypothalamus will be more obvious when you examine the brain in cross section.
1. For now, locate the pineal gland on the sagittally sectioned brain. The pineal is the main source of the hormone melatonin, which regulates both behavioral and physiological rhythms like sleep/wake cycles.
2. Find the pituitary gland. This lies near the crossing of the optic nerves. However, it sometimes becomes detached, so try to find someone with an intact pituitary gland on his or her sheep brain if there is not one on yours.
3. On the sagittal sectioned brain, locate the hypothalamus and the thalamus. The hypothalamus is important in both endocrine function and motivated behavior (fight, flight, feeding, and sexual behavior). The thalamus is important in relaying sensory information to higher brain centers in the cortex.
Midbrain

The only brain division in this region is the mesencephalon (colored blue on the models). The midbrain is the region ancestrally associated with processing visual information in vertebrates and this is still true of non-mammals. This information is processed in the visual cortex of mammals, but the midbrain is still an important Îwaystationâ for this information as it is passed along to these higher brain centers. This region is also important for processing auditory information.

1. Locate the superior colliculi (singular: colliculus) on your sheepâs brain and on the monitor. It may be necessary to Îpeekâ between the cerebellum and the cortex on a whole brain or look at a sagittally sectioned brain at this point. This area is important for controlling eye movements.
2. Locate the inferior colliculi. You really will have to be looking at a sagittally sectioned brain for this as these lie under the superior colliculi. These structures are important for processing auditory information.
Hindbrain

The hindbrain includes two major divisions and is important in several functions. The metencephalon is colored pink on the brain models while the myelencephalon is colored ivory. Ancestrally, this area is important in vertebrates for processing balance and vibrational information. A lot of processing still goes on here even in mammals. The cerebellum is a critical center in movement control. The medulla serves as both a relay station for balance and auditory information and has areas that regulate heartbeat, blood pressure, and breathing.

1. Metencephalon
1. Cerebellum: The cerebellum is the prominent ribbed structure just behind the cerebral cortex. On the models, it forms an upward extension of the metencephalon. From an inspection of the surface, you can identify the distinct hemispheres making up the cerebellar cortex.


The cerebellum makes up 10% of the volume of the human brain, but contains 50% of it's the neurons. Although the cerebellum does not directly affect perception or initiate movement, it indirectly regulates motor control and posture by comparing intention with performance and making necessary changes to the nervous signals that go to the muscles.

2. Pons: The pons is the large bulge beneath the anterior part of the cerebellum. It is continuous with the medulla immediately behind it. The pons is another important way station for information: fibers from the cortex pass through the pons on their way to the cerebellum.
2. Myelencephalon
1. Medulla: The medulla lies between the pons and the spinal cord. The medulla serves as both a relay station for balance and auditory information and has areas which regulate heartbeat, blood pressure, and breathing. Remember the areas sensitive to blood pH that regulate breathing rate that we talked about? They are in the medulla.
Sheep Brain ÷ Lateral View




Sheep Brain ÷ Lateral view, sagittal section