LESSON 1.2 WORKBOOK How does brain structure impact its function?

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1 DEFINITIONS OF TERMS Central nervous system (CNS) contains the brain and spinal cord. Peripheral nervous system (PNS) includes all the nerves outside the brain and spinal cord. For a complete list of defined terms, see the Glossary. LESSON 1.2 WORKBOOK How does brain structure impact its function? In this lesson, you ll be dissecting a sheep s brain. During the dissection you ll localize and identify major brain structures. By understanding where these structures are localized you ll begin to appreciate how the brain is organized spatially. Once you understand spatial organization we can begin to investigate how the different parts connect to control behavior. If you have an iphone or an ipad you can download a great free app that will allow you to look at the structures of the brain in 3D. These pictures are worth a thousand words as we examine more closely how the brain is organized. The app is available FREE from the itunes store. Just search 3D brain. How can we study our brains? Before we get too much further in our discussion of how the brain is organized, let s take a short tour of the nervous system as a whole to orient you on all the different parts, how they re classified and what their functions are. First of all we need to remember that your nervous system has basically three functions it receives information via our various sensory systems; it makes sense of these sensations and decides what an appropriate response should be; and it executes that response. To complete these three functions, our nervous system uses its two main branches - the central nervous system (CNS) and the peripheral nervous system (PNS). Sensations come in from the environment via the PNS. The PNS delivers this information to the CNS which then evaluates the information and decides how to respond. Finally, the CNS sends a signal the PNS in order to be able to execute the response. Your central nervous system (CNS) includes your brain and spinal cord while your peripheral nervous system includes all the nerves in your head, body and limbs that lie outside the brain and spinal cord (Figure 4). Let s start by briefly talking about the What are the three basic functions of your nervous system? What is the CNS? What is the PNS? peripheral nervous system and spinal cord, then we can concentrate on the brain for the remainder of Lesson 1.2 this lesson. 8

2 DEFINITIONS OF TERMS Peripheral nervous system (PNS) includes all the nerves outside the brain and spinal cord. Somatic nervous system - part of the PNS that controls voluntary movement. Autonomic nervous system part of the PNS that controls the function of organs and glands. For a complete list of defined terms, see the Glossary. LESSON READING Your Peripheral Nervous System (PNS) The PNS can be further divided into the somatic nervous system, which controls voluntary muscles, and the autonomic nervous system, which controls the function of organs and glands. The autonomic nervous system has two divisions: Sympathetic nervous system is nicknamed the fight-or-flight system because it prepares our body when energy expenditure is necessary, such as during times of stress or excitement. This system increases heart rate and blood pressure, stimulates secretion of adrenaline, and increases blood flow to the skeletal muscles. Parasympathetic nervous system helps our body conserve and store energy for later use. This system increases salivation, digestion, and storage of glucose and other nutrients, as well as slowing the heart and decreasing respiration. Overall, the peripheral nervous system connects with non-neuronal cells at one end and the central nervous system at the other. The neurons of the PNS can be divided into two classes: Sensory neurons bring sensations such as smell, touch, hearing, taste and pain to the CNS where they are evaluated to determine what response is needed. Motor neurons execute those responses. Motor neurons of the somatic nervous system control voluntary responses such as muscle contractions, whereas motor neurons of the autonomic nervous system control involuntary responses such as changes in heart rate. Peripheral nerves are protected by the organs they travel through, and in cases of injury or disease peripheral nerves are able to regenerate. Figure 4: Peripheral and central nervous systems. The CNS is in pink, and contains all neurons in the brain and spinal cord. The PNS is in blue, Are you aware of your somatic nervous system? What about your autonomic nervous system? Which part of your autonomic nervous system is working as you are reading this page? What about if you heard a fire alarm? and contains all neurons not in the Lesson 1.2 brain or spinal cord. 9

3 DEFINITIONS OF TERMS Cerebrospinal fluid (CSF) - the fluid that bathes the brain and spinal cord. Meninges protective membranes that cover the brain and spinal cord Ventricles - the spaces inside the hollow brain and spinal cord that are gilled with cerebro spinal fluid. For a complete list of defined terms, see the Glossary. LESSON READING Your Central Nervous System (CNS) The central nervous system (CNS) is also divided into different parts - the spinal cord and the brain. The sensations that are received in the periphery via sensory neurons first enter the spinal cord and then pass into the brain. Then once the brain has decided on a response, output from the brain passes into the spinal cord before it exits to the somatic or autonomic peripheral motor neurons in the periphery. The central nervous system is protected from damage by the bony skull and vertebrae. Both the brain and spinal cord are cushioned by sheets of protective membranes called meninges (Figure 5). The brain also contains a series of hollow, interconnected chambers called ventricles which are filled with cerebrospinal fluid (CSF). The largest of these chambers are the lateral ventricles which are located in the center of the brain (Figure 6). The CSF serves two main functions - it provides the brain with nutrients and it cushions the meninges to protect the brain. Fourth ventricle Lateral ventricles Third ventricle Figure 5: Meninges. The brain is protected in part by the meninges which are fluid filled membranes covering the brain. (A) The meninges have three layers: the pia mater, the arachnoid, and the dura mater. (B) Meningitis results from inflammation of the meninges. Despite these multiple levels of protection, in cases of injury or disease, the CNS is unable to regenerate. What is the role of the meninges? What are two functions of the cerebrospinal fluid? Figure 6: Ventricles. The ventricles are interconnected chambers that are filled with cerebro- Lesson 1.2 spinal fluid (CSF). 10

4 DEFINITIONS OF TERMS Myelin fatty substance that insluates most nerves. White matter portions of the nervous system that appear white in color because they are composed of myelinated axons. Grey matter portions of the nervous system that appear grey in color because they are composed of neuron cell bodies and unmyelinated axons. For a complete list of defined terms, see the Glossary. LESSON READING Your Spinal Cord (CNS) The spinal cord is a long, conical structure, approximately as thick as your little finger. Its main function is to act as a two-way track that collects the sensory information from the periphery to pass it onto the brain, and then to collects the motor responses from the brain to pass onto the somatic and autonomic nervous systems. We can divide the spinal cord into four regions, each controlling a specific region of the body. Starting from the top (Figure 7): The cervical region serves the neck and arms. The thoracic region serves the trunk. The lumbar region serves the legs. The sacral region serves the bowels and bladder. The spinal cord is arranged so the neurons traveling up into the brain and down out of the brain are arranged on the outside. These neurons are coated with a layer of fatty insulation that appears white, called myelin. As we will see later, myelin makes the signals that are transmitted along neurons move more efficiently. Because of this white appearance, this area of the spinal cord is referred to as white matter. The area where connections between the peripheral and central nervous system neurons are made is in the middle of the spinal cord, and lacks myelin. Because of this it appears grey in comparison to the white matter. So, this area is referred to as grey matter. Crossing over B. A. One interesting thing to note about the neurons traveling up and down the spinal cord is that they cross over from one side to another. Because of this cross, each side of the brain receives sensory information from the opposite side of the body. Similarly, the spinal cord output neurons also cross from one side of the body to the other so that each side of the brain also controls the responses of the opposite side of the body. What is the function of the spinal cord? What side of the brain controls the left side of the body? What side of the brain controls the right side of the body? Lesson Brainstem Spinal cord Vertebra Cervical Thoracic Lumbar Sacral Figure 7: The spinal cord. The spinal cord is segmentally arranged. The segments are grouped into 4 major divisions: cervical, thoracic, lumbar, and sacral. (A) The spinal cord is encased in vertebral bone. (B) The spinal cord has pathways along which sensory information can be conveyed to the brain (indicated in red), and motor information can be transmitted from the brain to the body (indicated in blue).

5 LESSON READING Your Brain The brain is also organized into areas of white matter where neurons travel and gray matter where connections between different neurons are made. In addition it can also be divided into distinct areas, each of which perform a specific function. Starting from the region where the spinal cord connects to the brain, these areas are called the brainstem, diencephalon, cerebellum, and cerebrum (Figure 8). We will take a look at each of these areas in turn. What is the function of the following brain structures? What symptoms would you see if they were damaged? Would the patient survive? Medulla The Brainstem Figure 8: Main brain areas. The brain can be subdivided into the brainstem, diencephalon, cerebellum, and cerebrum (or cerebral hemisphere). The brainstem is an evolutionarily old area of the brain where the spinal cord and the brain connect. Part of the brainstem consists of sensory neurons that are traveling into the brain, and motor neurons that are traveling out of the brain. But the brainstem also has its own functions, that divide it into 3 parts, from the bottom, closest to the spinal cord, to the top, closest to the brain itself. The medulla controls breathing, heart rate and digestion. As you can imagine these are critical functions, and it is difficult to survive when the medulla is damaged. The pons (from the Latin that means bridge) is a part of the brainstem that acts like a bus station connecting upper levels of the brain (the cortex) with the spinal cord and a part of the brain called the cerebellum. These connections allow the brain not only to give instructions about which movements to make, but also to monitor those movements as they are happening. Pons Midbrain The midbrain is also involved with coordinating movements. In this case it coordinates eye movement responses to visual and auditory stimulation. Lesson

6 LESSON READING The Diencephalon Moving on upwards, the diencephalon is located at the upper end of the brain stem. It has two parts that perform functions that are critical for life: What is the function of the following brain structures? What symptoms would you see if they were damaged? Would the patient survive? Thalamus The thalamus acts as a relay station (like a post office) where all the major ascending sensory pathways from spinal cord and brainstem connect to neurons destined for the upper parts of the brain in the cortex. There are also reciprocal connections from the cortex to the thalamus. The thalamus is thought to be the first area in the brain where consciousness can be experienced. We ll talk more about the thalamus and how important these connections are when we talk about epilepsy and seizures. The hypothalamus is tiny! Only 1 oz. in adult humans, yet it is the master regulator of homeostasis controlling heart rate, blood pressure, blood composition, eating behaviors, and body temperature to name but a few of its functions. It also links body responses to emotions. We ll talk more about the hypothalamus when we talk about sleep. The Cerebellum The cerebellum lies behind and on top of the pons (Figure 9). It communicates with both the spinal cord and the cortex. The cerebellum monitors how the intention to perform a motor movement compares with how well the movement is actually being executed. It can then adjust the response to make sure the intention is being executed accurately. Amazingly, you are completely unaware of the cerebellum as it works it functions below the level of consciousness. Figure 9:The cerebellum. The cerebellum lies just behind the pons Lesson 1.2 and is critical for controlling motor 13 Cerebellum movements. Hypothalamus Cerebellum

7 LESSON READING The Cerebrum The cerebrum forms the bulk of the CNS (Figure 10). The cerebrum consists the three deep-lying structures surrounded by the cerebral cortex. These three structures also have distinct functions. The basal ganglia are involved in the intention to move (like when you re lying in bed and then suddenly you re up, but you haven t consciously jumped out of bed and put your feet on the floor). The hippocampus is involved with making memories, as we saw with H.M. The amygdala is involved in creating emotional states. It then works with the hippocampus to coordinate the emotional states with the correct hormonal responses (think fight or flight). Cingulate cortex Hypothalamus Corpus callosum Amygdala Hippocampus Basal ganglia Thalamus Cerebral cortex Figure 10: The Cerebrum. The cerebrum consists of the cerebral cortex and three deep-laying structures: basal ganglia, hippocampus, and amygdala. The two hemispheres of the cerebral cortex are connected via the corpus callosum. (The thalamus and hypothalamus, which together compose the diencephalon, are also shown for spatial reference.) The outer layer of the cerebrum is called the cerebral cortex. The cortex contains at least 30 billion individual cells. Approximately half are the neurons that transmit information around the nervous system. Just like in the spinal cord the neurons are arranged in layers of white matter where neurons are traveling, and grey matter where they are connecting. In the cortex these layers are alternating. The cerebral cortex is divided into two hemispheres, one on the left and one on the right. Although they superficially look the same they are neither structurally nor functionally symmetrical. Each hemisphere receives sensory information from, and sends motor instructions to, the opposite side of the body. Even though the two cerebral hemispheres perform somewhat different functions, our perceptions and our memories are unified. This unity is accomplished by the corpus callosum, a large band of neurons that travels between corresponding parts of the left and right hemispheres connecting them and providing both sides of the cortex with the same information one world through two eyes. What is the function of the following brain structures? What symptoms would you see if they were damaged? Would the patient survive? Basal ganglia Hippocampus Amygdala Corpus callosum Lesson

8 LESSON READING Each hemisphere of the cortex can be divided into 4 lobes, each of which has a different function (Figure 11): The frontal lobe is concerned with planning for future action and with control of movement. The parietal lobe is concerned with receiving sensory information and with body image. The occipital lobe is concerned with vision. The temporal lobe is concerned with hearing, learning and memory and emotion. What is the function of the following brain structures? What symptoms would you see if they were damaged? Would the patient survive? Frontal lobe Frontal lobe Planning, motor control Temporal lobe Hearing, memory, learning, emotion Brainstem Parietal lobe Sensation, body image Occipital lobe Vision Figure 11: The four lobes of the cerebral cortex. The cerebral cortex is divided into four lobes: frontal, temporal, parietal and occipital. Each lobe has many characteristic folds and grooves. The folds are called gyri (singular gyrus), and the grooves are called sulci (singular sulcus). Together the gyri and sulci increase the area of the cortex considerably increasing the amount of information it can handle. The two most prominent sulci are: The longitudinal sulcus (or fissure) which separates the left and right hemispheres The central sulcus which separates the frontal lobe from the parietal lobe Notably, mammals lower in the evolutionary scale than humans have many fewer sulci and gyri than humans. Parietal lobe Occipital lobe Temporal lobe Lesson Spinal cord Cerebellum

9 LESSON READING Getting information into and out of the brain As we saw before, the brain communicates with the rest of the body via the cranial nerves (that supply the head) and the spinal nerves (that supply the body). These nerves are part of the PNS. Since we ve not dealt with them before, let s end by taking a look at the cranial nerves. There are twelve pairs of cranial nerves that attach to the bottom surface of the brain before they enter it via the brainstem (Figure 12). Many of them deal with sensory and motor functions in the head and neck region in the same way that spinal neurons do. Others convey what we call the special senses (vision, smell and taste and hearing) to the brain. For example, the olfactory sensory neurons transmit olfactory information from receptors in the nose to the brain, while the optic nerve transmits visual signals from the eye to the brain. The optic nerves partially cross before entering the brain at the optic chiasm. Figure 12:The cranial nerves. Twelve pairs of cranial nerves attach to the bottom surface of the brain and innervate the head and neck region. The three we focus on are: olfactory (CNI), optic (CNII), and vagus (CNX). Finally, the vagus nerve is an important autonomic cranial nerve that regulates the functions of organs of the chest and abdomen such as the heart, lungs and digestive system. What are two functions of cranial nerves? If the vagus nerve was damaged what symptoms would you see? Lesson

10 STUDENT RESPONSES What differences are there between your central and peripheral nervous systems? (Be sure to address their overall functions, and ability to regenerate). Remember to identify your sources Given what you know about how your brain controls the function of your body, if you met a stroke patient who had difficulty moving his left leg, what half of his brain was affected by the stroke? Lesson