Neuroscience: Overview of the Brain PDF

Neuroscience: Overview of the Brain Page 1 of 12 Dr. Paul Walker Session Objectives By the end of this session, students should be able to accurately: 1. Discuss the general organization of the brain and spinal cord using correct anatomical...

Neuroscience: Overview of the Brain Page 1 of 12 Dr. Paul Walker Session Objectives By the end of this session, students should be able to accurately: 1. Discuss the general organization of the brain and spinal cord using correct anatomical terminology. 2. Name the main lobes of the cerebrum and parts of the brainstem. 3. Summarize the cellular basis of neuronal signaling. 4. Distinguish gray vs. white matter both anatomically and functionally. 5. Compare and contrast long ascending and descending pathways in the brain and spinal cord. 6. Relate different brain systems to their general functions. Session Outline I. Anatomical Organization of the Brain & Spinal Cord II. Lobes of the Cerebrum and Parts of the Brainstem III. Cellular Basis of Neuronal Signaling IV. Functional Anatomy of Gray & White Matter V. Long Ascending & Descending Pathways of the Brain & Spinal Cord VI. Brain Systems and their Functional Relevance Supplemental Reading Fundamental Neuroscience for Basic & Clinical Applications, 5th Ed (2018) Haines & Mihailoff, Chapter 1. Elsevier. https://www-clinicalkey- com.proxy.lib.wayne.edu/#!/content/book/3-s2.0- B9780323396325000268?indexOverride=GLOBAL Nolte’s The Human Brain, 8th Ed (2021), TW Vanderah & DJ Gould, Chapters 1 and 3. Elsevier. https://www.clinicalkey.com/#!/content/book/3-s2.0- B9780323653985000199?indexOverride=GLOBAL Nolte’s The Human Brain in Photographs and Diagrams, 5th Ed (2020) TW Vanderah. https://www-clinicalkey-com.proxy.lib.wayne.edu/#!/browse/book/3-s2.0- C20170008030?indexOverride=GLOBAL Neuroscience: Overview of the Brain Page 2 of 12 Dr. Paul Walker I. Anatomical Organization of the Brain & Spinal Cord The central nervous system (CNS) consists of the brain and spinal cord (Fig 1). The brain includes cerebral hemispheres, brainstem, and cerebellum. The spinal cord is connected to the brainstem at the foramen magnum of the skull and is located within the vertebral canal of the bony vertebral column. Divisions of the spinal cord include: Cervical Thoracic Lumbar Sacral Coccygeal The peripheral nervous system (PNS) consists of cranial nerves connected to the brainstem and spinal nerves connected to the spinal cord. Fig 1 Nolte It is important to use correct anatomical terminology when referring to the parts of the brain. The long axis of the cerebrum is in the horizontal plane and is perpendicular to the long axis of the spinal cord. The reason it is like this is because of the folding of the cephalic flexure during embryonic development. Rostral means ‘toward the nose’ or toward the frontal pole of the cerebrum. Caudal means ‘toward the tail’ or toward the bottom of the spinal cord or occipital pole of the cerebrum. Dorsal and ventral refer to the same areas learned in gross anatomy when referencing the body to the anatomical position. Fig 2 Nolte Neuroscience: Overview of the Brain Page 3 of 12 Dr. Paul Walker II. Lobes of the Cerebrum & Parts of the Brainstem The brain is mainly characterized by the cerebrum (cerebral cortex). During development, the cerebral cortex outgrows the cranial cavity. Folds appear on the surface of the cerebral cortex to gain surface area. A single fold is called a gyrus and multiple folds are called gyri. A space between the folds is called a sulcus while multiple spaces are called sulci. Gyri and sulci give the surface of the cerebrum its well- known lumpy appearance. If the surface of the cerebral cortex was stretched out to be flat, your skull would need to be the size of a large pizza box to contain it. Fig 3 Nolte The cerebrum has 2 hemispheres- left and right. Fig 3 shows a lateral view of the left hemisphere. There are many gyri and sulci and they all have names. The central sulcus and lateral sulcus (Sylvian fissure) are 2 important landmarks. There is a also an indent called the preoccipital notch superior to the cerebellum. These 3 landmarks are used to divide the cerebrum into 4 lobes (Fig 4). Frontal Temporal Parietal Occipital Fig 4 Nolte The cerebellum is a separate structure that is not included as part of the cerebrum. The brainstem is also indicated in Fig 4. A good way to study the brainstem is cut the brain in the mid-sagittal plane and look at its medial surface. This cuts the brainstem down the middle. Neuroscience: Overview of the Brain Page 4 of 12 Dr. Paul Walker Fig 5 Nolte Fig 5 shows a medial view of the right hemisphere after mid- sagittal section. The lumpy cerebral cortex is still obvious. An important interhemispheric white matter pathway called the corpus callosum is also visible. The brainstem is also seen anterior to the cerebellum. The thin laminal terminalis is most rostral extent of the brainstem. The brain is protected by a layer of fluid secreted by choroid plexus located within the brain’s ventricular system. You will learn more about this in LEC 19. Fig 6 Nolte Fig 6 includes a color scheme to indicate the different parts of the cerebrum and brainstem. The diencephalon is shaded differently than the rest of the brainstem, but is part of it. The rest of the brainstem caudal to the diencephalon includes the midbrain, pons, and medulla. We’ll learn more about the anatomy of the brainstem later in LEC 28. Neuroscience: Overview of the Brain Page 5 of 12 Dr. Paul Walker III. Cellular Basis of Neuronal Signaling Brain function depends on the connective (network) organization of neurons and their mechanism of communication called synaptic transmission. This week you’ll learn some molecular details of synaptic transmission. You’ll then apply this information to neural pathways that regulate sensory, motor, and cognitive functions discussed throughout the course. Fig 7 Nolte Fig 7 illustrates the fundamental principle of synaptic communication between neurons. The neuron to the left contains a cell body. Radiating from the cell body are neuronal processes called dendrites, which are the receiving parts of a neuron and serve as a site for synaptic transmission. The neuron also sends a single process called an axon to other neurons. The axon is covered by myelin which increases the speed of axonal transmission. The axon may branch at its distal region to communicate with multiple neurons via synaptic terminals (boutons). Post-synaptic neurons send their own signals via axons to other targets. IV. Functional Anatomy of Gray & White Matter In this course, we’ll discuss functional areas of the brain that connect to one another via neural pathways. We’ll refer to gray matter & white matter areas. Gray matter refers to collections of neuronal cell bodies. Sometimes we call these areas brain nuclei (plural) or a brain nucleus (singular). The surface of the cerebral cortex is gray matter that contains collections of neuronal cell bodies. White matter refers to regions that contains myelinated axon tracts. Myelin appears white (or lighter) in both fresh and preserved brain. Areas that have a lot of axonal tracts are typical white matter. In the cerebral cortex, the white matter lies deep to the gray matter. Fig 8 Neuroscience: Overview of the Brain Page 6 of 12 Dr. Paul Walker Fig 9 Nolte Neuronal cell bodies are numerous within gray matter of the cerebral cortex and are arranged in columns. Fig 9 shows a ‘Brainbow’ transgenic mouse engineered to express different fluorescent proteins in different cell types to create a colorful image (left). Note the column arrangement of the neuronal processes (apical dendrites) of these cortical neurons. Similarly, cresyl violet (Nissl) stain in the human superior temporal gyrus shows column arrangement of neurons in the gray matter (right). https://doi.org/10.1073/pnas.97.10.5019 Fig 10 In other areas such as the brainstem and spinal cord, neuronal cell bodies are arranged in a cluster called a nucleus. The above cells are stained with markers for the neurotransmitter acetylcholine. The cholinergic cell cluster (nucleus) is located in the dorsal medulla in an area called the Dorsal Motor Nucleus of X (Vagus). They are the VE preganglionic PANS neurons that send axons into the vagus nerve. We have discussed these neurons and their axons several times throughout the year. Now you can understand their nuclear origin in the brainstem. Neuroscience: Overview of the Brain Page 7 of 12 Dr. Paul Walker Fig 11 Myelinated axonal tracts (white matter) are revealed by the Weigert staining method. This method involves the treatment of nervous tissue with potassium dichromate to preserve myelin lipids followed by staining with hematoxylin. Myelinated axonal pathways appear dark due to the staining of myelin sheaths. Gray matter areas (nuclei) devoid of myelinated axons do not stain well and appear light (tan). Below Figs 12-13 are a Weigert stained sections through the spinal cord and medulla cut in the horizontal (axial) plane of the body but at a right angle to the longitudinal axis of the spinal cord. We call these transverse sections of the spinal cord and medulla. In the spinal cord, the white matter (myelinated axons) is on the outside and the gray matter (nuclei) is on the inside. In comparison, the section of medulla looks quite different Fig 12 Each Weigert-stained section of the spinal cord or brainstem has different characteristics that enable you to tell them apart. The medulla section in Fig 13 has a leafy shaped nucleus called the inferior olivary complex. When you see this nucleus, you know you are in the medulla. But exactly where in medulla? The upper (rostral) medulla? The caudal (lower) medulla? We’ll teach you certain landmarks that will allow you to know and you’ll become skilled at recognizing the different levels of each brainstem region. Incidently, the inferior olivary complex is visible in the unstained section shown in Fig 10. Can you see it? Fig 13 Neuroscience: Overview of the Brain Page 8 of 12 Dr. Paul Walker V. Long Ascending & Descending Pathways of the Brain & Spinal Cord Fig 14 The spinal cord and brainstem contain different neural pathways (tracts) that transmit sensory information to higher centers or motor commands from higher centers down to the brainstem and spinal cord. In a Weigert-stained section, these fibers are difficult to distinguish but you’ll learn their general regions. When the brainstem is cut transversely, the axonal tracts are cut perpendicular to the long axis of the pathway. In below Fig 15, the yellow dashed circle marks a descending motor pathway that you’ll learn about in this course. The blue dashed circle marks an ascending sensory pathway. Damage to either pathway along its length can interrupt sensory perception and motor control. Some pathways cross the midline and carry information that affects the opposite side of the body. For this reason, the patient will have a lesion on one side of their brain that affects functions on the opposite side of their body. An example is ischemia (stroke) damage to the left hemisphere of the brain affecting speech centers while also affecting movements of the right side of the body. Fig 15 Neuroscience: Overview of the Brain Page 9 of 12 Dr. Paul Walker Fig 16 Nolte Fig 16 provides an example of an ascending sensory pathway from the periphery that transmits tactile (touch) information to the brainstem and cerebral cortex where conscious perception of touch occurs. The sensory pathway detects peripheral touch on one side of the body, but the chain of synaptic connections relays the information across the midline to the opposite side of the brain. If the pathway is severed prior to crossing the midline, the patient loses sensation on the same side of the body (ipsilateral) as the lesion location. If the pathway is severed after it has crossed the midline, the patient loses sensation on the opposite side of the body (contralateral) as the lesion location. Fig 17 is an example of a descending motor pathway from cerebral cortex to spinal cord that transmits motor commands to the spinal cord. The motor pathway starts on one side of the body but crosses the midline in the caudal medulla and affects spinal motor neurons on the opposite side. If the pathway is severed prior to crossing the midline, the patient loses motor control on the opposite side of the body (contralateral) as the lesion location. If the pathway is severed after it has crossed the midline, the patient loses motor control on the same side of the body (ipsilateral) as the lesion location. There are places where both pathways can be lesioned together resulting in combined sensory and motor deficits. We will practice predicting patient signs and symptoms associated with lesions affecting the different pathways of the brain. It gets complicated, but if you know the basic principles of the pathways, you can figure out the deficits. Fig 17 Nolte Neuroscience: Overview of the Brain Page 10 of 12 Dr. Paul Walker VI. Brain Systems and their Clinical Relevance Fig 18 Nolte You’ll find that we often discuss brain systems that control bodily functions and human behaviors. For example, we’ve recently reviewed micturition and defecation reflexes at sacral spinal levels and mentioned brainstem (pons) and higher centers (cerebral cortex) that provide conscious override of these reflexes. There are many brain systems that we’ll cover in this course that are neural networks connecting different brain areas together. For now, let’s learn a few brain regions and try to link together some functions that will be later emphasized in this course. Cerebral Cortex (Fig 18) Frontal Lobe: Planning/Scheming/Judgment Commands for movement Motor speech Parietal Lobe: Sensory Perception Higher cognitive functions (language perception, math, symbols) Temporal Lobe Memory, Emotions, Perception, Cognition Occipital Lobe: Vision & visual perception (e.g. color, shapes, etc) Diencephalon (Fig 19) Fig 19 Nolte Thalamus Sensory relay center Motor relay center Limbic functions (memory, emotions, cognition) Hypothalamus Master endocrine regulator Body functions (thirst, feeding) Energy metabolism Temperature regulation Sleep/Wake/Alertness ANS control Motivational behaviors Reproduction & nurturing behaviors Neuroscience: Overview of the Brain Page 11 of 12 Dr. Paul Walker Fig 20 Nolte Cerebellum (Fig 20) Interacts with: Vestibular system (balance, equilibrium) Midbrain (red nucleus) Pons (pontine nuclei) Medulla (inferior olivary nucleus) Spinal cord (unconscious proprioception) Functions: Body Movements Motor Coordination Cognitive Functions Fig 21 Nolte Basal Ganglia (Fig 21) Parts Caudate Nucleus Putamen Globus pallidus Subthalamic nucleus Substantia nigra Functions: Body Movements Motivation/Reward Cognitive Functions Fig 22 Nolte Limbic System (Fig 22) Parts Cingulate cortex Orbitofrontal cortex Medial temporal lobe Hippocampus Amygdala Basal forebrain Functions: Memory Emotions Mood Personality Cognition Neuroscience: Overview of the Brain Page 12 of 12 Dr. Paul Walker Fig 23 Nolte Brainstem (Fig 23) Midbrain CN III, IV Pons CN V, VI, VII, VIII Medulla CN VIII, IX, X, XII Functions: Visual Reflexes Brainstem motor pathways ANS Reflexes Cranial nerve circuits Conduit for ascending and descending pathways Reticular formation LEC 20 & LEC 28 will provide additional information on brainstem structure and function and we’ll review this content in Neuro Labs 2-3. Some Advice on Mastering Neuroscience Content As with other disciplines, practice & repetition is your KEY to success. Start by downloading notes & slides and viewing the lectures. When available, view the recommended short videos (vignettes) that accompany certain lectures & neuro labs. For each structure, ask: 1. What is it? 2. What does it connect to? 3. What information does it transmit? 4. What happens if it is lesioned? The Neuro Labs promote active learning. The neuro lab sessions are designed to foster practice and repetition. You and your peers will complete worksheets together during lab that reinforce content presented in lecture. Use additional resources such as recommended texts or websites. Look for opportunities to learn neuroanatomy from unexpected sources. For example, the Acland videos viewed to prepare for the GA interactive sessions contain some beautiful examples of cranial nerve relationships within the brainstem and cranial cavity. Integrate this information where possible.