Intro to Brain Lecture Notes PDF

Introduction to the Brain – Lecture notes Prof. Steve Edgley, Physiology Development and Neuroscience Michaelmas 2023 E-mail: [email protected] Objectives:  To introduce you to the course and its objectives  To introduce you to basic brain structure, terminology and basic function...

Introduction to the Brain – Lecture notes Prof. Steve Edgley, Physiology Development and Neuroscience Michaelmas 2023 E-mail: [email protected] Objectives:  To introduce you to the course and its objectives  To introduce you to basic brain structure, terminology and basic function As the information-processing organ (literally the nerve-centre) of the body, the brain is incredibly complex. It controls the body systems, receives sensations and turns them into perception, decides on and generates coordinated movement to generate behaviour that is essential for survival and reproduction. To achieve this it learns, in multiple ways. But beyond this, the brain is the seat of our consciousness: it creates our personalities, our moods and emotions, our hopes and fears, holds our wisdom and makes our decisions. Trying to understand how the brain achieves this is a major scientific enterprise. Not surprisingly, our current understanding is limited. Neurobiology as a field is very broad, encompassing many levels from the molecular (the molecules and genes important for neural function), cellular (how do neurons function and signal through synapses), systems (how networks and circuits of neurons process information to generate function), to the function of the organ as a whole (how decisions that generate behaviour, and generate understanding are a made). Clinically relevant disorders can arise from malfunction at any of these levels – from channelopathies (global changes in ion channel function) to psychiatric disorders following focal brain damage. The NAB and NHB courses aim to provide a fundamental overview of these processes required for clinical practice, and therefore focus principally on the systems and whole organ view. BASIC BRAIN STRUCTURE: Almost all of your nerve cells are with you at birth, and most will remain with you until your death. They are the most highly specialised of cells. To consider the vertebrate central nervous system in overview, it is convenient to look at the brain from a developmental point view, where basic layout and origin of the major elements is more easily seen. Vertebrate nervous systems develop from the embryonic ectoderm overlying the notochord, which induces cells in this region to differentiate into neurons. This region folds to form a neural tube (the future spinal cord), with swellings at the future rostral end (brain vesicles) that will form the brain. These are the primordia of the hind-, mid- and forebrain, major divisions which can be identified in the brains of all vertebrates. (Note that there are alternative names for these structures: hindbrain = rhombencephalon made of the Medulla and pons, the midbrain = mesencephalon, the forebrain at this stage may be called the prosencephalon – later it becomes subdivided, but this is complex enough. It’s much easier to use hind mid and forebrain: but you may encounter these alternative more complex names) In mammals the hindbrain further divides into 2 distinct parts: the caudal medulla oblongata (or just medulla) and a more rostral pons. A major structure the cerebellum (little brain) develops on its dorsal surface with which the pons is associated. The forebrain also divides further into a caudal diencephalon (mainly the thalamus and hypothalamus) and a rostral telencephalon (a telencephalic or cerebral vesicle on either side): these give rise to the large cerebral hemispheres, one on each side (or cerebral cortex) which overlay most of the brain. schematic representation of the embryonic brain with the major divisions Unfortunately the situation is not as simple as the diagram suggests: different parts grow disproportionately during development (e.g. the cerebral hemispheres of mammals become huge, especially in man) and the brain folds and bends dramatically. Having grown from the walls of a tube, 2 of the obvious structures (cerebral hemispheres and cerebellum) greatly expand into huge 2-dimensional sheets of tissue with a layered structure (like a stack of carpets), and in many mammals these have buckled and folded to fit into the limited intracranial space. Diagrammatic lateral view of the developing brain showing the major structures As the brain develops, the internal cavity of the neural tube persists to form the fluid-filled ventricular system of the adult brain. The 2 largest are the lateral ventricles within the cerebral hemispheres. These connect with a 3rd ventricle in the midline diencephalon (between thalamus and hypothalamus on either side). These connect through a slender canal (the aqueduct) with the 4th ventricle between the dorsal medulla and the cerebellum. Ventricles shown schematically BASIC FUNCTIONS OF THE MAJOR BRAIN REGIONS: The SPINAL CORD contains a core of neurons, surrounded by fibres running up to and down from the brain. It houses major output systems that control body movement - the somatic motoneurons that make muscles twitch, and the sympathetic preganglionic neurons. It also receives somatosensory afferent information from receptors in the body and skin, as well as some visceral afferent signals: the information it receives is sent through ascending pathways to the brain (some after local spinal processing) and also generates reflexes. The spinal cord acts autonomously in some functions (basic reflexes and locomotion in fish, some reptiles and amphibia), but in mammals, and especially in man, is mainly subordinate to the brain. The hindbrain (medulla & pons) and midbrain together form the BRAINSTEM, from which all of the cranial nerves except 1 and 2 (olfactory and optic) emerge. The brainstem manages many basic autonomic functions as well as managing the overall activity (arousal state) of the forebrain. The medulla oblongata (named from the quadrilateral appearance of the 4th ventricle when seen from above) and pons house major regulatory systems for the body, and can maintain basic functions (e.g. respiration, cardiovascular control) in the absence of a forebrain. Irreversible loss of brainstem function is the criterion for diagnosing death clinically. The pons is expanded ventrally by bundles of fibres that bridge the midline on the ventral surface. These are the axons of cells with inputs from the cerebral cortex, which are destined for the cerebellum (little brain) which lies on the dorsal surface of the hindbrain, and covers the entire dorsal surface of the medulla and pons. The cerebellum is found in all vertebrates and grows dramatically in size with phylogeny: It plays a major role in the control of movement, where it is particularly important for learning how to plan movements, especially complex ones. There is currently heated debate as to whether the cerebellum contributes to non-motor functions (e.g. cognition). The MIDBRAIN is the smallest of the brain divisions in mammals. On the dorsal surface (often called the tectum in non-mammalian vertebrates) of the midbrain are 2 pairs of bulges the superior (rostral) and inferior (caudal) colliculi. These are receiving and processing areas for visual and auditory information, respectively. In reptiles, amphibians and fish these are the principal processing areas for these senses (alternatively called the optic and auditory tectum, respectively). In mammals they are important for initiating rapid movement in response to sensory stimuli and as pre-cerebral sensory processing centres, but visual and auditory processing for perception have been taken over by the cerebral cortex. The 2 anatomical divisions of the FOREBRAIN (diencephalon and telencephalon) are not simple functionally. The diencephalon consists principally of the thalamus (a large structure on either side) and a hypothalamus. The hypothalamus is of fundamental importance as a regulator of homeostasis which you met in HOM; it controls the endocrine system (via the pituitary), the autonomic nervous system (through the brainstem) and drives motivated behaviour through connections with other forebrain structures, including regions of cerebral cortex. The thalamus and cerebral cortex are closely interlinked with specific regions of the cortex being reciprocally interlinked with corresponding parts of the thalamus. The function of the thalamus is integral to sleep and wakefulness, in addition to roles in attention and motivation. The two CEREBRAL HEMISPHERES form by far the largest part of the brain in mammals. These structures make your behaviour intelligent (hopefully) and are essential for your consciousness. They contain circuitry of phenomenal complexity, with connections between the layers of cortex and between different regions. The 2 separate cerebral hemispheres are interconnected by a huge bundle of fibres that cross the midline (the corpus callosum).  A mid-sagittal section through an adult human brain showing the corpus callosum, the tract of fibres that connect the 2 cerebral hemispheres. All of the cerebral cortex has the same basic plan, but it is not uniform and is actually a mosaic of histologically distinct areas in which various parts of the plan are specialised. These were first described by Brodmann. Some areas have relatively simple structure - e.g. the hippocampal cortex has 3 layers and is associated with some forms of memory. Most of the cortex (neocortex) has 6 layers. Specific regions are responsible primarily for processing (discrimination and localisation) each of senses and others for motor output. These are primary cortical areas, specialised for processing information of one modality, and are reciprocally connected to parts of the thalamus related to that modality. Other regions have their principal connections to and from other cortical regions, and in general receive and associate information of many different modalities: these more complex areas are called association cortex. In man a huge proportion of the cerebral hemispheres (particularly the frontal lobes and parts of the parietal and temporal lobes) is association cortex, which is important for our cognitive abilities. These contain areas of cortex with structures that are unique to humans. It is important to note that, although there are cerebral cortical areas with clearly specialised functions, some functions of the brain are not precisely localised but are distributed through the cortex, these include elong-tem memory and consciousness. Other large forebrain structures are buried beneath the cerebral cortex – these are the basal ganglia and amygdala, which have complex functions: the basal ganglia are associated with learned selection and expression of movement, amygdala with learned assessment the emotional significance of an environment. BASIC SYSTEMS After basic lectures on biology of neurones, Michaelmas term lectures are in blocks covering the major sensory systems of the brain (visual, auditory, olfactory & gustatory, somatosensory), and lectures on development and regeneration. Lent term lectures move on to motor systems and motivation and arousal. Note that these divisions are convenient for teaching, but this reductionist approach is oversimplified: e.g. movements depend on sensation and vice versa, so clear division between sensory and motor function is impossible: structures involved in sensorimotor integration belong to several systems. The association areas that make up the majority of the cerebral cortex defy definition into these systems. The artificial division of the brain into systems becomes obvious when it comes to complex brain functions such as emotion, motivation, learning and memory and cognition. Given the large differences between human and animal brains, the course divides at this point. Veterinary students have a course on the neurobiology of animal behaviour. Medical students have a series of lectures on higher functions and human behaviour. The association cortex of the frontal lobes, particularly large in man, are concerned with forward planning and decision making (among various other complex functions), and the temporal-parietal-occipital association areas, associated with perception, language and other complex cognitive functions. Recent developments in functional brain imaging frequently show that large, spatially separate areas of association cortex are involved in such functions. Appreciation of how such complex functions operate is particularly important in man for psychology and psychiatric disorders, but also for understanding changes in animal behaviour. To reiterate – in this lecture I have mentioned the basic structure and function of the brain, but you will see this in more detail in the following lectures. Enjoy the course!