Tracing the visual pathway from the receptor organ (eye) through the afferent ... in
CNS), through a synapse in the thalamus (LGN), to the primary visual cortex.
CHAPTER 2 APPROACHES TO STUDYING THE STRUCTURE AND FUNCTION OF SENSORY SYSTEMS 2.1. THE BASIC BUILDING BLOCKS OF SENSORY SYSTEMS. Although each sensory system is highly specialized for a particular purpose, all sensory systems share some basic organizational principles, and include the same basic building blocks. These basic components include: A receptor organ (the eye or ear, for example) is specialized to collect a particular form of energy from the environment and convert the pattern of physical energy present in the environment into a pattern of electrochemical energy that can be utilized by the nervous system to obtain information. A nerve (the optic nerve or auditory nerve, for example) transmits patterns of electrochemical energy, in the form of nerve impulses, from the receptor organ to the brain where these patterns are further processed. The brain is the central processing unit for information from the environment. It performs many different operations that include filtering, comparison, computation, and synthesis of new patterns of information. The motor system (the motor pathways of the central nervous system, the peripheral nerves, and the muscles) is the output through which a sensory stimulus produces a behavioral response. Of course, not all sensory input results in a response! Part of the task of sensory processing is to "decide" whether or not a response is required.
Figure 2-1. A schematic showing the basic building blocks of a sensory system. _____________________________________________________________________________
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Thought question: We often think of behavior in terms of "stimulus-response", specifically a sensory stimulus evoking a motor response. When you are exposed to sensory stimuli, what alternative outcomes are there besides a motor response? Do you think it would be beneficial for every sensory stimulus to always evoke a motor response? Why or why not? _____________________________________________________________________________ 2.2. THE RELATIONSHIP BETWEEN STRUCTURE AND FUNCTION. By looking at the physical structures that make up sensory systems, we can gain some general idea of how they work. In terms of their structure, we can examine sensory systems at several different levels including gross anatomical features, microscopic anatomy, and the molecular makeup of cells in the system. 2.2.1. Gross anatomical structure. The gross anatomical structure of sensory receptor organs, nerves, and the central nervous system provides clues about the mechanisms that optimize the collection of stimulus energy and that direct it to the cells that perform the transduction process. For example, we know that light enters the eye, the lens focuses the light on the back inner surface of the eye, and a large nerve connects the back of the eye to the brain. Simply by observing the structure of the eye, we can conclude that it contains the following: Specialized parts for optimizing energy transfer. These include the transparent cornea that covers the front outer surface of the eyeball, the muscular iris that acts like the diaphragm of a camera to regulate the amount of light entering, the fibrous lens that focuses the light that enters, and the retina that acts as a projection screen for the image. In addition, there are small muscles inside the eye (an important task of these muscles is to regulate the shape of the lens so that the eye can focus on objects that are nearby or far away) as well as muscles attached to the outside of the eyeball (these move the whole eye).
Figure 2-2. The main structures of the eye.
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Supporting tissues. All of the essential parts of the eye are held together by various supporting structures. In the eye, for example, most of the eyeball is covered by a white connective tissue sheath called the sclera. Nerves. Examination of any sensory receptor organ will reveal that it is connected to the central nervous system by one or more nerves. The eye, for example, sends information to the brain via the optic nerve, a large diameter "cable" that exits the back of the eye and connects the eye with the lower surface of the brain. Central Nervous System. Examination of the brain and spinal cord at a fairly superficial level reveals that there are a large number of nerves entering and exiting at various points. It is also clear that different parts of the brain look different, so presumably serve different functions. 2.2.2. Microscopic anatomy. By looking at the fine structure of the tissues and cells that make up sensory systems, we can learn much more about how each part of the system is specialized, and we can make very good guesses about the specific pathways through which information is transmitted from one level of the central nervous system to the next.
Figure 2-3. The retina is made up of several distinct layers, each of which contains a particular type of cell. Light passes through several layers of cells before hitting the photoreceptors, which are the cells that convert light energy to electrochemical energy. Information from the photoreceptors is transmitted to the bipolar cells (middle layer), and from there to the ganglion cells, where patterns of nerve impulses are generated and transmitted to the brain through the fibers of the optic nerve.
For example, starting at the receptor organ, we can identify the cells that are involved in transduction and information processing. If we cut a thin section through the eye and look at it under a microscope, we will see that the back of the eye (the retina) contains several layers of cells. Cells in each layer look different, suggesting that they perform different functions. If we follow the nerve fibers from the eye, we will see that they originate from a certain type of cell in
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a particular layer of the retina and that these cells are contacted by other cells that, in turn, are contacted by the photoreceptors. Once the optic nerve enters the brain, we can trace its course by microscopic examination of sections through the different levels of the brain. We can follow the fibers of the optic tract as they proceed through the brain, and we can see that they terminate on several different groups of cells within the brain, located in areas widely separated from one another. The fact that the cell groups that receive input from the optic nerve are widely separated and very different in structure suggests that information is processed in very different ways in each of these cell groups.
Figure 2-4. Tracing the visual pathway from the receptor organ (eye) through the afferent nerve (optic nerve, becoming optic tract in CNS), through a synapse in the thalamus (LGN), to the primary visual cortex.
2.2.3. Molecular structure and cell biology. By examining the molecules that make up the cell membranes and internal structures of the cells that are present in sensory systems, we can learn how energy from the environment (e.g., mechanical energy) is converted to electrical energy. We can learn how cells communicate with one another, and how they integrate information from different sources to make “decisions”. In the eye, for example, the receptor cells contain molecules that are sensitive to light; these molecules are called photopigments. When light hits the photopigment, it causes a chemical reaction that leads to a change in the internal state of the cell. This change in internal state causes the photoreceptor cell to release a molecule that affects the membrane of a nearby cell, but how that cell is affected depends on its own molecular makeup and state.
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2.3. DIRECT APPROACHES TO STUDYING FUNCTION. We can study the function of sensory systems directly by using various methods to observe the normal properties and activity of the organs, tissues, and cells that make up sensory systems, or by perturbing the system in some way and observing the result.
2.3.1. Lesion studies. The earliest and most basic studies of the function of the brain and sensory systems involved removal of specific parts of the system, either through injury or disease in human patients, or through experimentally produced lesions in laboratory animals. Lesion studies have provided fundamental information about which parts of the brain are involved in various sensory and perceptual functions.
Fgure 2-5. Lesions at various points along the visual pathway predictably result in different deficits. For example, cutting the right optic nerve severs all of the fibers from the right eye and results in complete blindness in the right eye. Because of the way fibers cross from one side to the other at the optic chiasm, cutting the chiasm destroys the crossed fibers from the nasal (innermost) part of the retina of both eyes, resulting in blindness in the outside half of the visual field in both eyes. Cutting the right optic tract destroys the crossed fibers from the nasal half of the left retina and the uncrossed fibers from the temporal (outer) half of the right retina, resulting in blindness in the outer half of the left eye's visual field and the inner half of the right eye's visual field.
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Formerly, it was only possible to determine where a neurological patient's lesion was located by performing an autopsy after their death. Now it is often possible to localize lesions using non-invasive brain scan methods such as CAT scans, PET scans, and MRI. _____________________________________________________________________________ Thought question: What sort of information would you need to have in order to prove that a lesion in a particular part of the brain caused a specific behavioral or perceptual deficit? _____________________________________________________________________________ 2.3.2. Recording activity of receptors and nerve cells. Recording the electrical activity of nerve cells has provided detailed information about how individual cells at different levels of a sensory system react when specific stimuli are present in the environment, and how they communicate with one another. Such electrophysiological recording of neural activity can be combined with permanent or reversible lesions, application of drugs that alter specific processes within the cell, or injection of neural tracers that are transported to the input cells or target cells of the structure from which the recording was obtained.
Figure 2-6. Recordings of the electrical activity of nerve cells can be made by placing a fine wire electrode or glass pipette near the outside of the cell (left) or by inserting a very fine glass pipette inside the cell (right). The activity recorded will look different depending on whether the electrode is outside or inside the cell.
Figure 2-7. It is possible to use a hollow glass electrode to inject substances that are taken up by neurons and transported to their targets (anterograde transport); similarly, substances that are taken up by nerve terminals in the area of the injection are transported back to the cell bodies that send projections to the area of the injection (retrograde transport).
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