The Brain and Nervous System PDF

4 The Brain and Nervous System Robert Biswas-Diener The brain is the most complex part of the human body. It is the center of consciousness and also controls all voluntary and involuntary movement and bodily functions. It communicates with each part of...

4 The Brain and Nervous System Robert Biswas-Diener The brain is the most complex part of the human body. It is the center of consciousness and also controls all voluntary and involuntary movement and bodily functions. It communicates with each part of the body through the nervous system, a network of channels that carry electrochemical signals. Learning Objectives Name the various parts of the nervous system and their respective functions Explain how neurons communicate with each other Identify the location and function of the limbic system Articulate how the primary motor cortex is an example of brain region specialization Name at least three neuroimaging techniques and describe how they work In the 1800s a German scientist by the name of Ernst Weber conducted several experiments meant to investigate how people perceive the world via their own bodies. It is obvious that we use our sensory organs—our eyes, and ears, and nose—to take in and understand the world around us. Weber was particularly interested in the sense of touch. Using a drafting compass he placed the two points far apart and set them on the skin of a volunteer. When the points were far apart the research participants could easily distinguish between them. As Weber repeated the process with ever closer points, however, most people lost the ability to tell the difference between them. Weber discovered that the ability to recognize these “just noticeable differences” depended on where on the body the compass was positioned. Your The Brain and Nervous System 53 back, for example, is far less sensitive to touch than is the skin on your face. Similarly, the tip of your tongue is extremely sensitive! In this way, Weber began to shed light on the way that nerves, the nervous system, and the brain form the biological foundation of psychological processes. In this module we will explore the biological side of psychology by paying particular attention to the brain and to the nervous system. Understanding the nervous system is vital to understanding psychology in general. It is through the nervous system that we experience pleasure and pain, feel emotions, learn and use language, and plan goals, just to name a few examples. In the pages that follow we will begin by examining how the human nervous system develops and then we will learn about the parts of the brain Measuring “just noticeable differences.” and how they function. We will conclude with a section on how modern psychologists study the brain. It is worth mentioning here, at the start, that an introduction to the biological aspects of psychology can be both the most interesting and most frustrating of all topics for new students of psychology. This is, in large part, due to the fact that there is so much new information to learn and new vocabulary associated with all the various parts of the brain and nervous system. In fact, there are 30 key vocabulary words presented in this module! We encourage you not to get bogged down in difficult words. Instead, pay attention to the broader concepts, perhaps even skipping over the vocabulary on your first reading. It is helpful to pass back through with a second reading, once you are already familiar with the topic, with attention to learning the vocabulary. Nervous System development across the human lifespan As a species, humans have evolved a complex nervous system and brain over millions of years. Comparisons of our nervous systems with those of other animals, such as chimpanzees, show some similarities. Researchers can also use fossils to study the relationship between brain The Brain and Nervous System 54 volume and human behavior over the course of evolutionary history. Homo habilis, for instance, a human ancestor living about 2 million years ago shows a larger brain volume than its own ancestors but far less than modern homo sapiens. The main difference between humans and other animals-- in terms of brain development-- is that humans have a much more developed frontal cortex (the front part of the brain associated with planning). Interestingly, a person’s unique nervous system develops over the course of their lifespan in a way that resembles the evolution of nervous systems in animals across vast stretches of time. For example, the human nervous system begins developing even before a person is born. It begins as a simple bundle of tissue that forms into a tube and extends along the head- to-tail plane becoming the spinal cord and brain. By day 40 of gestation (40 days after fertilization of the egg) the spinal cord, hindbrain, midbrain and forebrain are all visibly distinct. What, exactly, is this nervous system that is developing and what does it do? The nervous system can be thought of as the body’s communication network that consists of all nerve cells. There are many ways in which we can divide the nervous system to understand it more clearly. One common way to do so is by parsing it into the central nervous system and the peripheral nervous system. Each of these can be sub-divided, in turn. Let’s take a closer, more in-depth look at each. And, don’t worry, the nervous system is complicated with many parts and many new vocabulary words. It might seem overwhelming at first but through the figures and a little study you can get it. The Central Nervous System (CNS): The Neurons inside the Brain The Central Nervous System, or CNS for short, is made up of the brain and spinal cord (see Figure 1). The CNS is the portion of the nervous system that is encased in bone (the brain is protected by the skull and the spinal cord is protected by the spinal column). It is referred to as “central” because it is the brain and spinal cord that are primarily responsible for processing sensory information —touching a hot stove or seeing a rainbow, for example —and sending signals to the peripheral nervous system for action. It communicates largely by sending electrical signals through individual nerve cells that make up the fundamental building blocks of the nervous system, called neurons. There are approximately 100 billion Figure 1: The central nervous system The Brain and Nervous System 55 neurons in the human brain and each has many contacts with other neurons, called synapses. If we were able to magnify a view of individual neurons we would see that they are cells made from distinct parts (see Figure 2). The three main components of a neuron are the dendrites, the soma, and the axon. Neurons communicate with one another by receiving information through the dendrites, which act as an antenna. When the dendrites channel this information to the soma, or cell body, it builds up as an electro-chemical signal. This electrical part of the signal, called an action potential shoots down the axon, a long tail that leads away from the soma and toward the next neuron. When people talk about “nerves” in the nervous system, it typically refers to bundles of axons that form long neural wires along which electrical signals can travel. Cell-to-cell communication is helped by the fact that the axon is covered by a myelin sheath—a layer of fatty cells that allow the signal to travel very rapidly from neuron to neuron. Figure 2: The parts of a neuron If we were to zoom in still further we could take a closer look at the synapse, the space between neurons (see Figure 3). Here, we would see that there is a space between neurons, called the synaptic gap. To give you a sense of scale we can compare the synaptic gap to the thickness of a dime, the thinnest of all American coins (about 1.35 mm). You could stack approximately 70,000 synaptic gaps in the thickness of a single coin! As the action potential, the electrical signal reaches the end of the axon, tiny packets of chemicals, called neurotransmitters, are released. This is the chemical part of the electro- chemical signal. These neurotransmitters are the chemical signals that travel from one neuron to another, enabling them to communicate with one another. There are many different types of neurotransmitters and each has a specialized function. For example, serotonin affects sleep, The Brain and Nervous System 56 hunger and mood. Dopamine is associated with attention, learning and pleasure. It is amazing to realize that when you think— when you reach out to grab a glass of water, when you realize that your best friend is happy, when you try to remember the name of the parts of a neuron—what you are experiencing is actually electro-chemical impulses shooting between nerves! The Central Nervous System: Looking at the Brain as a Whole Figure 3: A view of the synapse between neurons If we were to zoom back out and look at the central nervous system again we would see that the brain is the largest single part of the central nervous system. The brain is the headquarters of the entire nervous system and it is here that most of your sensing, perception, thinking, awareness, emotions, and planning take place. For many people the brain is so important that there is a sense that it is there—inside the brain—that a person’s sense of self is located (as opposed to being primarily in your toes, by contrast). The brain is so important, in fact, that it consumes 20% of the total oxygen and calories we consume even though it is only, on average, about 2% of our overall weight. It is helpful to examine the various parts of the brain and to understand their unique functions to get a better sense of the role the brain plays. We will start by looking at very general areas of the brain and then we will zoom in and look at more specific parts. Anatomists and neuroscientists often divide the brain into portions based on the location and function of various brain parts. Among the simplest ways to organize the brain is to describe it as having three basic portions: the hindbrain, midbrain and forebrain. Another way to look at the brain is to consider the brain stem, the Cerebellum, and the Cerebrum. There is another part, called the Limbic System that is less well defined. It is made up of a number of structures that are “sub-cortical” (existing in the hindbrain) as well as cortical regions of the brain (see Figure 4). The brain stem is the most basic structure of the brain and is located at the top of the spine and bottom of the brain. It is sometimes considered the “oldest” part of the brain because we can see similar structures in other, less evolved animals such as crocodiles. It is in charge of a wide range of very basic “life support” functions for the human body including breathing, digestion, and the beating of the heart. Amazingly, the brain stem sends the signals to keep these processes running smoothly without any conscious effort on our behalf. The Brain and Nervous System 57 The limbic system is a collection of highly specialized neural structures that sit at the top of the brain stem, which are involved in regulating our emotions. Collectively, the limbic system is a term that doesn’t have clearly defined areas as it includes forebrain regions as well as hindbrain regions. These include the amygdala, the thalamus, the hippocampus, the insula cortex, the anterior cingulate cortex, and the prefrontal cortex. These structures influence hunger, the sleep-wake cycle, sexual desire, fear and aggression, and even memory. The cerebellum is a structure at the very back of the brain. Aristotle referred to it as the “small brain” based on its appearance and it is principally involved with movement and posture although it is also associated with a variety of other thinking processes. The cerebellum, like the brain stem, coordinates actions without the need for any conscious awareness. The cerebrum (also called the “cerebral cortex”) is the “newest,” most advanced portion of the brain. The cerebral hemispheres (the left and right hemispheres that make up each side of the top of the brain) are in charge of the types of processes that are associated with more awareness and voluntary control such as speaking and planning as well as contain our primary sensory areas (such as seeing, hearing, feeling, Figure 4: General areas of the brain (Image: biology corner) and moving). These two hemispheres are connected to one another by a thick bundle of axons called the corpus callosum. There are instances in which people—either because of a genetic abnormality or as the result of surgery—have had their corpus callosum severed so that the two halves of the brain cannot easily communicate with one another. The rare split-brain patients offer helpful insights into how the brain works. For example, we now understand that the brain is contralateral, or opposite-sided. This means that the left side of the brain is responsible for controlling a number of sensory and motor functions of the right side of the body, and vice versa. Consider this striking example: A split brain patient is seated at a table and an object such as a car key can be placed where a split-brain patient can only see it through the right visual field. Right visual field images will be processed on the left side of the brain and left visual field images will be processed on the right side of the brain. Because language is largely associated with the left side of the brain the patient who sees car key in the right visual field when asked The Brain and Nervous System 58 “What do you see?” would answer, “I see a car key.” In contrast, a split-brain patient who only saw the car key in the left visual field, thus the information went to the non-language right side of the brain, might have a difficult time speaking the word “car key.” In fact in this case, the patient is likely to respond “I didn’t see anything at all.” However, if asked to draw the item with their left hand—a process associated with the right side of the brain—the patient will be able to do so! See the outside resources below for a video demonstration of this striking phenomenon. Besides looking at the brain as an organ that is made up of two halves we can also examine it by looking at its four various lobes of the cerebral cortex, the outer part of the brain (see Figure 5). Each of these is associated with a specific function. The occipital lobe, located at the back of the cerebral cortex, is the house of the visual area of the brain. You can see the road in front of you when you are driving, track the motion of a ball in the air thanks to the occipital lobe. The temporal lobe, located on the underside of the cerebral cortex, is where sounds and smells are processed. The parietal lobe, at the upper back of the cerebral cortex, is where touch and taste are processed. Finally, the frontal lobe, located at the forward part of the cerebral cortex is where behavioral motor plans are processed as well as a number of highly complicated processes occur including speech and language use, creative problem solving, and planning and organization. One particularly fascinating area in the frontal lobe is called the “primary motor cortex”. This strip running along the side of the brain is in charge of voluntary movements like waving goodbye, wiggling your eyebrows, and kissing. It is an excellent example of the way that the various regions of the brain are highly specialized. Interestingly, each of our various body parts has a unique portion of the primary motor cortex devoted to Figure 5: The 4 lobes of the cerebral cortex it (see Figure 6). Each individual finger has about as much dedicated brain space as your entire leg. Your lips, in turn, require about as much dedicated brain processing as all of your fingers and your hand combined! Because the cerebral cortex in general, and the frontal lobe in particular, are associated with such sophisticated functions as planning and being self-aware they are often thought of as a higher, less primal portion of the brain. Indeed, other animals such as rats and kangaroos The Brain and Nervous System 59 Figure 6: Specific body parts like the tongue or fingers are mapped onto certain areas of the brain including the primary motor cortex. while they do have frontal regions of their brain do not have the same level of development in the cerebral cortices. The closer an animal is to humans on the evolutionary tree—think chimpanzees and gorillas, the more developed is this portion of their brain. The Peripheral Nervous System In addition to the central nervous system (the brain and spinal cord) there is also a complex network of nerves that travel to every part of the body. This is called the peripheral nervous system (PNS) and it carries the signals necessary for the body to survive (see Figure 7). Some Figure 7: The peripheral nervous system The Brain and Nervous System 60 of the signals carried by the PNS are related to voluntary actions. If you want to type a message to a friend, for instance, you make conscious choices about which letters go in what order and your brain sends the appropriate signals to your fingers to do the work. Other processes, by contrast, are not voluntary. Without your awareness your brain is also sending signals to your organs, your digestive system, and the muscles that are holding you up right now with instructions about what they should be doing. All of this occurs through the pathways of your peripheral nervous system. How we study the brain The brain is difficult to study because it is housed inside the thick bone of the skull. What’s more, it is difficult to access the brain without hurting or killing the owner of the brain. As a result, many of the earliest studies of the brain (and indeed this is still true today) focused on unfortunate people who happened to have damage to some particular area of their brain. For instance, in the 1860s a surgeon named Paul Broca conducted an autopsy on a former patient who had lost his powers of speech. Examining his patient’s brain, Broca identified a damaged area—now called the “Broca’s Area”—on the left side of the brain (see Figure 8). Over the years a number of researchers have been able to gain insights into the function of specific regions of the brain from these types of patients. An alternative to examining the brains or behaviors of humans with brain damage or surgical lesions can be found in the instance of animals. Some researchers examine the brains of other animals such as rats, dogs and monkeys. Although animals brains differ from human brains in both size and structure there are many similarities as well. The use of animals for study can yield important insights into human brain function. Figure 8: Broca's Area (Image: Charlyzon) In modern times, however, we do not have to exclusively rely on the study of people with brain lesions. Advances in technology have led to ever more sophisticated imaging techniques. Just as X-ray technology allows us to peer inside the body, neuroimaging techniques allow us glimpses of the working brain. Each type of imaging uses a different technique and each has its own advantages and disadvantages. The Brain and Nervous System 61 Positron Emission Tomography (PET) records metabolic activity in the brain by detecting the amount of radioactive substances, which are injected into a person’s bloodstream, the brain is consuming. This technique allows us to see how much an individual uses a particular part of the brain while at rest, or not performing a task. Another technique, known as Functional Magnetic Resonance Imaging (fMRI) relies on blood flow. This method measures changes in the levels of naturally occurring oxygen in the blood. As a brain region becomes active, it requires more oxygen. This technique measures brain activity based on this increase oxygen level. This means fMRI does not require a foreign substance to be injected into the body. Both PET and fMRI scans have poor temporal resolution , meaning that they cannot tell us exactly when brain activity occurred. This is because it takes several seconds for blood to arrive at a portion of the brain working on a task. One imaging technique that has better temporal resolution is Electroencephalography (EEG), which Above: A PET scan - Below: An fMRI scan (Image: measures electrical brain activity instead of blood flow. Erik1980) Electrodes are place on the scalp of participants and they are nearly instantaneous in picking up electrical activity. Because this activity could be coming from any portion of the brain, however, EEG is known to have poor spatial resolution, meaning that it is not accurate with regards to specific location. Another technique, known as Diffuse Optical Imaging (DOI) can offer high temporal and spatial resolution. DOI works by shining infrared light into the brain. It might seem strange that light can pass through the head and brain. Light properties change as they pass through oxygenated blood and through active neurons. As a result, researchers An EEG cap (Image: Chris Hope) can make inferences regarding where The Brain and Nervous System 62 and when brain activity is happening. Conclusion It has often been said that the brain studies itself. This means that humans are uniquely capable of using our most sophisticated organ to understand our most sophisticated organ. Breakthroughs in the study of the brain and nervous system are among the most exciting discoveries in all of psychology. In the future, research linking neural activity to complex, real world attitudes and behavior will help us to understand human psychology and better intervene in it to help people. The Brain and Nervous System 63 Outside Resources Video: Animation of Neurons http://www.youtube.com/watch?v=-SHBnExxub8 Video: Split Brain Patient http://www.youtube.com/watch?v=ZMLzP1VCANo Web: Animation of the Magnetic Resonance Imaging (MRI) http://sites.sinauer.com/neuroscience5e/animations01.01.html Web: Animation of the Positron Emission Tomography (PET) http://sites.sinauer.com/neuroscience5e/animations01.02.html Web: Teaching resources and videos for teaching about the brain, from Colorado State University: http://www.learner.org/resources/series142.html Web: The Brain Museum http://brainmuseum.org/ Discussion Questions 1. In your opinion is learning about the functions of various parts of the brain by studying the abilities of brain damaged patients ethical. What, in your opinion, are the potential benefits and considerations? 2. Are research results on the brain more compelling to you than are research results from survey studies on attitudes? Why or why not? How does biological research such as studies of the brain influence public opinion regarding the science of psychology? 3. If humans continue to evolve what changes might you predict in our brains and cognitive abilities? 4. Which brain scanning techniques, or combination of techniques, do you find to be the best? Why? Why do you think scientists may or may not employ exactly your recommended techniques? The Brain and Nervous System 64 Vocabulary Action Potential A transient all-or-nothing electrical current that is conducted down the axon when the membrane potential reaches the threshold of excitation. Axon Part of the neuron that extends off the soma, splitting several times to connect with other neurons; main output of the neuron. Brain Stem The “trunk” of the brain comprised of the medulla, pons, midbrain, and diencephalon. Broca’s Area An area in the frontal lobe of the left hemisphere. Implicated in language production. Central Nervous System The portion of the nervous system that includes the brain and spinal cord. Cerebellum The distinctive structure at the back of the brain, Latin for “small brain.” Cerebrum Usually refers to the cerebral cortex and associated white matter, but in some texts includes the subcortical structures. Contralateral Literally “opposite side”; used to refer to the fact that the two hemispheres of the brain process sensory information and motor commands for the opposite side of the body (e.g., the left hemisphere controls the right side of the body). Corpus Callosum The thick bundle of nerve cells that connect the two hemispheres of the brain and allow them to communicate. Dendrites Part of a neuron that extends away from the cell body and is the main input to the neuron. The Brain and Nervous System 65 Diffuse Optical Imaging​ (DOI) A neuroimaging technique that infers brain activity by measuring changes in light as it is passed through the skull and surface of the brain. Electroencephalography (EEG) A neuroimaging technique that measures electrical brain activity via multiple electrodes on the scalp. Frontal Lobe The front most (anterior) part of the cerebrum; anterior to the central sulcus and responsible for motor output and planning, language, judgment, and decision-making. Functional Magnetic Resonance Imaging (fMRI) Functional magnetic resonance imaging (fMRI): A neuroimaging technique that infers brain activity by measuring changes in oxygen levels in the blood. Limbic System Includes the subcortical structures of the amygdala and hippocampal formation as well as some cortical structures; responsible for aversion and gratification. Myelin Sheath Fatty tissue, that insulates the axons of the neurons; myelin is necessary for normal conduction of electrical impulses among neurons. Nervous System The body’s network for electrochemical communication. This system includes all the nerves cells in the body. Neurons Individual brain cells Neurotransmitters Chemical substance released by the presynaptic terminal button that acts on the postsynaptic cell. Occipital Lobe The back most (posterior) part of the cerebrum; involved in vision. Parietal Lobe The Brain and Nervous System 66 The part of the cerebrum between the frontal and occipital lobes; involved in bodily sensations, visual attention, and integrating the senses. Peripheral Nervous System All of the nerve cells that connect the central nervous system to all the other parts of the body. Positron Emission Tomography (PET) A neuroimaging technique that measures brain activity by detecting the presence of a radioactive substance in the brain that is initially injected into the bloodstream and then pulled in by active brain tissue. Soma Cell body of a neuron that contains the nucleus and genetic information, and directs protein synthesis. Spatial Resolution A term that refers to how small the elements of an image are; high spatial resolution means the device or technique can resolve very small elements; in neuroscience it describes how small of a structure in the brain can be imaged. Split-brain Patient A patient who has had most or all of his or her corpus callosum severed. Synapses Junction between the presynaptic terminal button of one neuron and the dendrite, axon, or soma of another postsynaptic neuron. Synaptic Gap Also known as the synaptic cleft; the small space between the presynaptic terminal button and the postsynaptic dendritic spine, axon, or soma. Temporal Lobe The part of the cerebrum in front of (anterior to) the occipital lobe and below the lateral fissure; involved in vision, auditory processing, memory, and integrating vision and audition. Temporal Resolution A term that refers to how small a unit of time can be measured; high temporal resolution means capable of resolving very small units of time; in neuroscience it describes how precisely in time a process can be measured in the brain.