Basics of the Central Nervous System UNIT 3 THE CEREBRUM AND THE CEREBRAL HEMISPHERES AND THEIR FUNCTIONS Structure 3.0 Introduction 3.1 Objectives 3.2 The Cerebrum and the Cerebellum 3.3 The Brain Stem 3.4 The Diencephalon 3.5 The Cerebrum 3.5.1 The Cerebral Cortex 3.5.2 Functional Areas of the Cerebral Cortex 3.6 The Cerebellum 3.6.1 Difference between Cerebrum and Cerebellum 3.7 Study of the Brain 3.8 Cerebral Hemisphreres 3.8.1 Left Brain and Right Brain 3.8.2 The Hands and the Two Hemispheres 3.8.3 Cerebral Dominance 3.8.4 Functions of the Left Hemisphere 3.8.5 Functions of the Right Hemisphere 3.8.6 Hemispheric Lobe Functions 3.8.7 Lateralisation or Plasticity of Hemispheric Function 3.9 The Limbic System 3.10 The Forebrain 3.11 Lobes of the Brain 3.11.1 Frontal Lobe 3.11.2 Parietal Lobe 3.11.3 Temporal Lobe 3.11.4 Occipital Lobe 3.12 Let Us Sum Up 3.13 Unit End Questions 3.14 Suggested Readings 3.0 INTRODUCTION This unit discusses the two main aspects of the brain namely the cerebrum and the cerebral hemispheres and their functions. The first section starts with the cerebrum and the cerebellum followed by a discussion on the brain stem and its various functions. This is followed by a description of the diencephalon and its functions. The next topic to be taken up is the cerebrum within which we discuss the cerebral cortex and the functional areas of the cerebral cortex. We then take up the description of the cerebellum and bring out the differences between the 42 cerebrum and the cerebellum. How to learn about the brain and what are the The Cerebrum and the Cerebral Hemispheres and various methods available to us to learn about the brain is discussed next which their Functions includes the MRI, PET and other such equipments which help to understand what goes on within the brain. Then the two hemispheres are discussed of the brain and we take up the description of the left and the right brain followed by the how the two hands are controlled and managed by the two hemispheres. Then we discuss about the cerebral dominance and the functions of the right and left hemispheres. Then we discuss the limbic system, the forebrain and the four lobes of the brain. 3.1 OBJECTIVES After completing this unit, you will be able to: • Define and describe cerebrum and the cerebellum; • Differentiate between the cerebrum and the cerebellum; • Describe the diencephalon, and the brain stem; • Explain the cerebral cortex and its functions; • Describe the two hemispheres of the brain; • Explain how the right and left hands are controlled by the two sides of the brain; • Define the limbic system and its functions; • Eklucidate the role of the forebrain; and • Analyse the functions of the four lobes of the brain. 3.2 THE CEREBRUM AND THE CEREBELLUM The brain, with the spinal cord and network of nerves, controls information flow throughout the body, voluntary actions, such as walking, reading, and talking, and involuntary reactions, such as breathing and heartbeat. The human brain is a soft, shiny, grayish white, mushroom shaped structure. Encased within the skull, the brain of an average adult weighs about 3 lb (1.4 kg). At birth, the average human infant’s brain weighs 13.7 oz (390 g); by age 15, the brain has nearly reached full adult size. The brain is protected by the skull and by a three layer membrane called the meninges. Many bright red arteries and bluish veins on the surface of the brain penetrate inward. Glucose, oxygen, and certain ions pass easily from the blood into the brain, whereas other substances, such as antibiotics, do not. The four principal sections of the human brain are: • the brain stem, • the diencephalon, • the cerebrum, and • the cerebellum. 43 Basics of the Central Nervous System 3.3 THE BRAIN STEM Underneath the limbic system is the brain stem. This structure is responsible for basic vital life functions such as breathing, heartbeat, and blood pressure. Scientists say that this is the “simplest” part of human brains because animals’ entire brains, such as reptiles (who appear early on the evolutionary scale) resemble our brain stem. The brain stem connects the brain with the spinal cord. All the messages that are transmitted between the brain and spinal cord pass through the medulla, which is a part of the brain stem. This it does through fibers. The fibers on the right side of the medulla cross to the left and those on the left cross to the right. As a result, each side of the brain controls the opposite side of the body. The medulla also controls the heartbeat, the rate of breathing, and the diameter of the blood vessels and helps to coordinate swallowing, vomiting, hiccupping, coughing, and sneezing. Another component of the brain stem is the pons (meaning bridge). The pons conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. They convey impulses between the cerebral cortex. The pons, and the spinal cord is a section of the brain stem known as the midbrain, which also contains visual and audio reflex centers involving the movement of the eyeballs and head. The brain stem is made of the midbrain, pons, and medulla. Let us see what these structures do: i) Midbrain: The midbrain is the smallest region of the brain that acts as a sort of relay station for auditory and visual information. The midbrain controls many important functions such as the visual and auditory systems as well as eye movement. Portions of the midbrain called the red nucleus and the substantia nigra are involved in the control of body movement. The darkly pigmented substantia nigra contains a large number of dopamine-producing neurons are located. The degeneration of neurons in the substantia nigra is associated with Parkinson’s disease. ii) Pons: In Latin, the word pons literally means bridge. The pons is a portion of the hindbrain that connects the cerebral cortex with the medulla oblongata. It also serves as a communications and coordination center between the two hemispheres of the brain. As a part of the brainstem, the pons helps in the transferring of messages between various parts of the brain and the spinal cord. iii) Medulla: This structure is the caudal most part of the brain stem, between the pons and spinal cord. It is responsible for maintaining vital body functions, such as breathing and heartrate iv) Cranial nerves: Twelve pairs of cranial nerves originate in the underside of the brain, mostly from the brain stem. They leave the skull through openings and extend as peripheral nerves to their destinations. Among these cranial nerves are the olfactory nerves that bring messages about smell and the optic nerves that conduct visual information. 44 The Cerebrum and the 3.4 THE DIENCEPHALON Cerebral Hemispheres and their Functions The diencephalon lies above the brain stem and embodies the thalamus and hypothalamus. The thalamus is an important relay station for sensory information, interpreting sensations of sound, smell, taste, pain, pressure, temperature, and touch. The thalamus also regulates some emotions and memory. The hypothalamus controls a number of body functions, such as heartbeat rate and digestion, and helps regulate the endocrine system and normal body temperature. The hypothalamus interprets hunger and thirst, and it helps regulate sleep, anger, and aggression. 3.5 THE CEREBRUM The cerebrum constitutes nearly 90% of the brain’s weight. Specific areas of the cerebrum interpret sensory impulses. For example, spoken and written language are transmitted to a part of the cerebrum called Wernicke’s area. Wernicke’s area is the region of the brain that is important in language development. The Wernicke’s Area is located on the temporal lobe on the left side of the brain and is responsible for the comprehension of speech (Broca’s area is related to the production of speech). Language development or usage can be seriously impaired by damage to the Wernicke’s Area. Broca studied patients with language deficits. Later after their death when he autopsied, he found a sizable lesion in the left inferior frontal cortex. Subsequently, Broca studied eight other patients, all of whom had similar language deficits along with lesions in their left frontal hemisphere. This led him to make his famous statement that “we speak with the left hemisphere” and to identify, for the first time, the existence of a “language centre” in the posterior portion of the frontal lobe of this hemisphere. Now known as Broca’s area, this was in fact the first area of the brain to be associated with a specific function that is in this case language. Ten years later, Carl Wernicke, a German neurologist, discovered another part of the brain, this one involved in understanding language, in the posterior portion 45 Basics of the Central of the left temporal lobe. People who had a lesion at this location, could speak, Nervous System but their speech was often incoherent and made no sense. Wernicke’s observations have been confirmed many times since. Neuroscientists now agree that there is a sort of neural loop that is involved both in understanding and in producing spoken language. At the frontal end of this loop lies Broca’s area, which is usually associated with the production of language, or language outputs . At the other end (more specifically, in the superior posterior temporal lobe), lies Wernicke’s area, which is associated with the processing of words that we hear being spoken, or language inputs.