The exercise on functional neuroanatomy you are about to complete was devised by five students in the bi-college (Haverford and Bryn Mawr) community: Jessica Kuhn, Jessica Magid, Karen Revere, Elizabeth Caris, and Gray Vargas. The students are enrolled in a Psychology/Biology seminar class called “Topics in Neural and Behavioral Science”. All of the students in the class devised lab exercises, this is the one that was chosen. Several other students from the class have assisted in preparing the lab for you today and will be assisting you as you complete the lab exercises.

The overall objectives of this “Friday in the Lab” session are as follows:

In order to understand the directions that follow in this lab manual, please refer to the following background information:

Definitions:

Objectives:

Introduction:

We will be dissecting a sheep’s brain as an example of a mammalian brain that is relatively similar in organization to the human brain. Since individual neurons are too small to be seen by the naked eye, we will instead be examining various regions of the brain to get an idea of what function they perform.

Procedure: (if you do not want to participate in the actual dissection you can visit http://academic.uofs.edu/department/psych/sheep/framerow.html and complete a virtual dissection)

• Start by removing the meninges, which are the 3 outer layers that protect the brain and spinal cord, most of the outermost layer was probably removed before you received the brain.
• Dura mater – outermost membrane; tough and virtually opaque
• Arachnoid – middle membrane; somewhat transparent
• Pia mater – innermost layer; extremely delicate
• If you look at the cerebrum, you will notice that it is not a flat surface, but it consists of sulci (grooves) and gyri (ridges), which allow for a large amount of material to be condensed into a smaller amount of space. Look for the “T” that is formed between the sulcus running down the center of the cerebrum, and the deep groove (known as the cruciate fissure) that runs across the front third of the brain.
• The gyrus located after the cruciate fissure is known as the primary somatosensory cortex, which is the region where most “touch” (including pressure, pain, temperature, etc.) information is received from the periphery. The organization of this region will be discussed at one of the stations.
• Next, turn the brain over and locate the cranial nerves. These nerves are located on the bottom surface of the brain, and they are arranged in pairs. Each pair of nerves controls a different function associated with the senses and the muscles of the face. For the purpose of this lab you should try to locate the following cranial nerves:
• Olfactory bulbs (part of the Cranial Nerve I system: Olfactory nerve) – two flaps of tissue located near the front of the brain. Smell information is actually sent to the olfactory bulbs through cranial nerve I, but this was removed when the brain was removed from the skull.
• Optic Chiasm (part of the Cranial Nerve II system: Optic nerve) – the structure behind the olfactory bulbs that is shaped like an X. Since information about each side of the visual field is received from both eyes, the optic chiasm is the point where the information from the two eyes is separated to be sent separately to the two halves of the thalamus, and then to the two hemispheres.
• Take one of the 2 brains and cut down the middle between the hemispheres. Now that you are looking at the inside of the midline of the brain try to locate a bundle of white fibers that connects the two hemispheres. This bundle of fibers is known as the corpus callosum, and it transfers information between the cerebral hemispheres.
• Now you have some time to slice the brains in a few different directions and to look at the overall organization. The plates pictured below are photographs of the sheep brain. They are provided below so that you can try to locate the different neural structures, and students and teachers are available to help you identify the structures and to tell you a little bit about their functions.

The following plates were downloaded from the Department of Psychology at the University of Guelph (Canada), where a sheep brain dissection manual is maintained (http://www.psydev.uoguelph.ca/faculty/peters/labmanual/)

Figure 1. the ventral surface of the brain

Figure 2. the midsagittal section

Figure 3. an anterior coronal section

Figure 4. a posterior coronal section

Table 1. Brain structures as labeled in the plates above

Nerve Endings Station

Equipment:

Safety:

Be gentle with pointy things, everyone should use different toothpicks.

Objectives:

Brain contains a kind of map (somatosensory cortex) that reflects the relative number of touch receptors in various parts of the body. The somatosensory cortex contains a “map” of the human body, but since all parts of the body are not equally “sensitive” the areas that contain more sense receptors are represented as larger in the somatosensory cortex. This representation of sensitivity is known as the homunculus.

Figure 5 – the somatosensory cortex

Figure 6 – the homunculus

Human skin contains several different sense receptors that respond to mechanical and thermal stimuli (e.g., touch, pressure, pain, cold, heat).

A sense receptor is a specialized cell that converts a physical or chemical stimulus into action potentials. These action potentials produced by the receptors are conducted to the spinal cord and brain (CNS) for processing and interpretation. The message that is sent to the central nervous system (CNS) is always a train of action potentials, regardless of the kind of stimulus that excites a particular receptor.

Sensory receptors that respond to touch send action potentials through axons that enter the dorsal columns of the spinal cord and ascend to the medulla of the brainstem. These axons then make connections (synapses) with another pathway within the medulla. It is here that the pathway crosses over the brain midline and then continues to the thalamus. The final pathway begins in the thalamus and continues to the specific region of the sensory cortex.

Interesting facts:

• When an area of the body is missing the somatosensory cortex can reconfigure itself.
• Homunculus means “little person”
• Homunculus is different in different animals

Something to think about – We will return to this at the vision and olfaction stations, but it is interesting to think about the fact that we are normally not aware of our clothes touching us, or of the seat we are sitting on. This is because sensory receptors stop responding after an extended period of constant stimulation, but once we move even slightly we notice the change.

Procedure:

• What parts of the body seem more sensitive to you? Where do you think you will have the most touch receptors?
• Each group should choose a subject, a measurer, and a recorder.
• Use two toothpicks, start far apart, go closer and closer (making sure to touch the toothpicks at the same time), asking the subject if they feel two points or one (they’re blindfolded/closing their eyes), continue until they don’t feel two. (Touch randomly with one to keep them on their toes) Record when they can’t feel two points on each of the different body regions in the Table 1 (next page).
• Calculate the reciprocals (1/measurement)—the bigger the reciprocal, the more touch receptors and the larger the representation on the somatosensory cortex map.
• Make a graph of the reciprocals (or the distances).

Discussion Questions:

How does this map affect our life?

How is it evidenced in every day life?

Why is this arrangement beneficial for us?

Visual System Station

Objectives:

Background:

The eye is derived from neural tissue, and actually processes information (as opposed to just transmitting it). Light enters the eye, hits the retina, activates a series of rods and cones, which become excited and cause retinal ganglion cells to fire. Rods and cones differ in the wavelength of light they are most sensitive to, and how strongly they respond to light energy.

Rods are smaller than cones and concentrated in the periphery of the retina.

Cones are concentrated in the fovea (center of the retina), and contain 3 different pigments.

Figure 7 – the eye

Part 1: Visual system anatomy

Equipment:

• Forceps
• Dissected sheep brain
• Gloves

Procedure:

• Knowing that the brainstem (colliculus, pulvinar nucleus) is a more evolutionarily primitive structure compared to the cortex, what kinds of visual information do you think each of these pathways processes?
• Why do you think the visual cortex is broken down into so many different areas? (Figure 9)

Figure 9 – the visual cortex

Part 2: The blind spot

Introduction:

Whether you know it or not, you have a blind spot on the retina of each eye where there no photoreceptors.

Equipment:

Procedure:

• Make a tester by marking + on the far right side of a piece of notebook paper.
• Stand with your back to a wall, with your head touching the wall.
• Hold the tester 500 mm (0.5 m or 50 cm) in front of your eye. (It may help to have someone help you.)
• Close your right eye and look at the + with your left eye.
• Place a pencil eraser on the far left side of the tester, and slowly move the pencil eraser to the right.
• When the eraser disappears, mark this location on the tester. Call this point "A."
• Continue moving the eraser to the right until it reappears. Mark this location on the tester. Call this point "B."
• Repeat the measurements until you are confident that they are accurate.
• Measure the distance between the spots where the eraser disappeared and reappeared.
• To calculate the width of your blind spot on your retina, let's assume that 1) the back of your eye is flat and 2) the distance from the lens of your eye to the retina is 17 mm. We will ignore the distance from the cornea to the lens.
• With the simple geometry of similar triangles, we can calculate the size of the blind spot because triangle ABC is similar to triangle CDE. So, the proportions of the lines will be similar.

Figure 10 – the blind spot

Discussion Questions:

Why do you think you have a blind spot?

What are some ways your brain and visual system compensates for this blind spot?

What would a larger or smaller blind spot mean for your vision?

Part 3: Color Vision

Introduction:

As mentioned before, there are two types of photo receptors located in the eye, rods and cones. They respond to different kinds of light. Each rod or cone responds to two wavelengths of light (for example, rods respond to either red or green light). The purpose of this next exercise is to investigate the characteristics of rods and cones with respect to color.

Procedure:

• Load the “Lilac Chaser” animation on the computer (http://www.michaelbach.de/ot/col_lilacChaser/index.html)
• Follow the directions on screen; play with different colors and saturation levels.

Discussion Questions:

Why do you think you see colors that aren’t actually there? Using what you know about how the nervous system works, how can you explain this fatigue? Do you think the fatigue observed visually can be applied to other parts of the nervous system, not just sensory systems?

Olfactory Fatigue Station

Equipment:

2 bottles of different aromatherapy scents

Stopwatch

Ruler

Notebook paper

Objectives:

Explore mechanisms behind olfactory sensation and olfactory fatigue

Background:

Olfaction is the sense of smell. To learn how it works, let’s imagine a certain smell in the air. These chemical molecules enter your nasal passage and then get trapped in the mucus layer in your nose. Olfactory epithelium neurons project cilia (which act as dendrites) into the mucus layer in the nasal passage so passing molecules will bind to the receptors on the ends of the cilia. There are hundreds of different types of receptors which recognize thousands of smells. The chemical molecules of each odor have a receptor that it will fit into, like a lock and key. These neurons have axons that lead to the olfactory bulb which is right under the frontal lobe in the brain. Some neurons of the olfactory bulb lead to the olfactory tubercle where the message is continued to the limbic system, thalamus and cortex. The thalamus and cortex are thought to give us a conscious sense of smell while the limbic system, which includes the amygdala and hippocampus, is thought to be involved in the emotional experience of smell such as inducing memories. Other neurons of the olfactory bulb lead to the olfactory cortex where odors can be identified.

Figure 11 - Olfaction from the nose to the brain

Olfactory fatigue is when, after smelling one specific smell for a large amount of time, the smell is no longer noticeable. This is because the neurons of the olfaction system become accustomed to the continual odor molecules and stop causing action potentials (this concept will also be discussed at the somatosensory and visual stations). This is due to a block of ions flowing in the receiving neuron which stops signaling to the brain. When a new odor enters the mucus membrane, the neurons are reactivated.

Figure 12 - An olfactory neuron

Figure13 - Olfaction in the brain

Procedure:

Why would remembering odors be important for survival?

Why would you want to have olfactory fatigue?

What would your world be like if you lost your sense of smell?

Corpus Callosum and Hemispheric Communication Station

Objectives:

Background:

The left brain is primarily responsible for language production and processing (i.e. reading, writing, speaking), and the right brain is primarily responsible for object/face recognition. In the case of a normal individual with an intact corpus callosum, information about visual stimuli can be shared between the hemispheres through the corpus callosum even though after passing through the optic chiasm each hemisphere is only receiving input about one half of the visual field. With a split-brain patient, information coming into the right hemisphere is confined and cannot be shared with the left, nor can the left hemisphere share information with the right. To demonstrate, take the below image of a male/female face:

The right half of the visual field is a man’s face and the left is a woman’s face. If the split-brain patient focuses on the dot, the man’s face will project to the left hemisphere and the woman’s face will project to the right hemisphere. When shown a picture of a whole man’s face and a whole woman’s face and asked to point to the image just seen, the patient will point to the woman’s face. However, if asked to verbally say which face was just seen, the patient will say a man’s face. This is because the man’s face projected to the language side of the brain (LH) and the woman’s face projected to the object recognition side of the brain (RH).

With normal individuals, the connectedness of the two hemispheres can be highlighted by attempting bimanual tasks. Since information is shared between the hemispheres, it is nearly impossible to complete two tasks simultaneously without there being interference between the hemispheres (see protocol for goal b). This activity not only focuses on the importance of the corpus callosum in allowing information to move freely throughout the brain, but it also speaks to the degree of lateralization within the brain. While lobectomy patients speak to the re-wiring capability of the brain, there are certain basic functions that are specific to each hemisphere of the brain (i.e. language and object recognition).

Procedure:

• Look at the brain section at this station, can you locate the corpus callosum?
• Do you remember its function?
• How does its location makes it optimal for transferring information from one side of the brain to the other?
• Discuss its composition (i.e. white matter, axons with cell bodies in either hemisphere, etc)
• Describe the basic functions of each hemisphere, what kinds of things does the corpus callosum allows us to do?
• Discuss what this “highway” reveals about the specificity of function in each hemisphere

Part I - Bimanual Coordination

Procedure:

• Try to separately draw a star with one hand and a triangle with their other hand. How easy is this task is when you are only concentrating on one image and one hand?
• Shut your eyes (or blind-fold) and try drawing the star and triangle at the same time now
• Most students will make two drawings that look very similar because the motor connections are shared (by way of what structure? Corpus callosum!!) between the hemispheres, making it difficult to draw two different shapes
• If that structure were severed so that it could not connect the hemispheres (as in a split-brain patient), a bimanual drawing task would be easy

Part II - Split Brain Demo on Computer

Procedure:

• Complete computer activity (http://nobelprize.org/medicine/educational/split-brain/index.html) where you can guess what a split-brain patient can or cannot process depending on which hemisphere a series of tasks are flashed
• Discuss your predictions with the group

Discussion Questions:

Why do you think the brain evolved to have hemispheric specification of function?

How would being a split-brain patient affect our daily lives? Would it?

Can you think of any ways that split-brain patients might learn to compensate for their hemispheric disconnectedness?

Can you think of any sicknesses that might require the severing of the corpus callosum?

References:

Purves, D., Augustine, G.J., Fitzpatrick, D., Katz, L.C., LaMantia, A., McNamara, J.O., and Williams, S.M. (2001). Neuroscience 2nd ed. Sunderland, MA: Sinauer Associates, Inc.

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