Brain and Cranial Nerves 1 PDF

The Brain 14.1 Expected Learning Outcomes: Describe the major subdivisions Overvie and anatomical landmarks of the brain. Describe the locations of its w of the gray and white matter. Describe...

The Brain 14.1 Expected Learning Outcomes: Describe the major subdivisions Overvie and anatomical landmarks of the brain. Describe the locations of its w of the gray and white matter. Describe the embryonic development of the CNS and relate this to adult brain Brain anatomy. Major Landmarks Directional terms: Rostral—toward the forehead Caudal—toward the spinal cord Important anatomical features: Longitudinal fissure—deep groove that separates cerebral hemispheres Gyri—thick folds on brain surface Sulci—shallow grooves between gyri Corpus callosum—thick nerve bundle at bottom of longitudinal fissure that connects hemispheres Surface Anatomy of the Brain Superior View Figure 14.1a  Major Landmarks Three major portions of the brain: Cerebrum (cerebral hemispheres) 83% of brain volume Important features and landmarks: gyri and sulci, longitudinal cerebral fissure, corpus callosum Cerebellum Second largest brain region Contains 50% of the neurons; about 10% of brain volume Located in posterior cranial fossa Separated from cerebrum by transverse cerebral fissure Brainstem Includes diencephalon, midbrain, pons, medulla oblongata Surface Anatomy of the Brain Lateral View Figure 14.1b Medial Aspect of the Brain Figure 14.2a Medial Aspect of the Brain Figure 14.2b Gray and White Matter Gray matter Contains cell bodies, dendrites, and synapses Darker color due to very little myelin present in tissue Forms surface layer (cortex) over cerebrum and cerebellum Forms nuclei deep within brain White matter Bundles of axons White color from lots of myelin Called “tracts” in the CNS, “nerves” in the PNS Deep to cortical gray matter in brain Superficial to gray matter in spinal cord Embryonic Development Nervous system develops from ectoderm, the outermost tissue layer of the embryo Early in third week of development Neural plate sinks and its edges thicken the dorsal midline of embryo thickens the neural groove forms which has folds on to form neural plate each side of it. These folds fuse together to form a tube. The lumen of this tube becomes the ventricles of the brain.  Embryonic Development Over the tube a crest forms to either side. This gives rise to: The inner meninges Peripheral nervous system Other skeletal, integumentary and endocrine structures.  Primary and Secondary Vesicles of the Embryonic Brain By Week 4: Forebrain (prosencephalon) Midbrain (mesencephalon) Hindbrain (rhombencephalon) By Week 5: Forebrain divides into two vesicles Telencephalon Diencephalon Midbrain remains undivided Mesencephalon Hindbrain divides into two vesicles Metencephalon Myelencephalon Figure 14.4 14.2 Meninges, Expected Learning Outcomes: Describe the meninges of the brain. Ventricles, Describe the fluid-filled chambers within the brain. Cerebrospi Discuss the production, circulation, and function of the cerebrospinal fluid nal Fluid, that fills these chambers. Explain the significance of the brain and Blood barrier system. Supply  Meninges Meninges—three membranes surrounding brain and spinal cord Lie between the nervous tissue and bone Protect the brain and provide structural framework for its arteries and veins From outermost to innermost: Dura mater Arachnoid mater Pia mater  The cranial dura mater is comprised of two layers: Outer periosteal layer Meninges Equivalent to periosteum of cranial bones Inner meningeal layer Continues into vertebral canal and forms dural sheath around spinal cord The two layers are separated by dural sinuses, which collect blood circulating through brain  Dura mater presses closely against cranial bones No epidural space (unlike spinal cord) Not directly attached to bone except around foramen magnum, sella turcica, crista galli, and sutures of the Meninges skull Folds of dura mater extend inward, separate some brain regions Falx cerebri separates the two cerebral hemispheres Tentorium cerebelli separates cerebrum from cerebellum Falx cerebelli separates right and left halves of cerebellum  Arachnoid mater Transparent membrane over brain surface Subarachnoid space separates it from pia mater below Pia mater Very thin membrane, not usually visible without microscope Follows all contours of brain Follows arteries as they penetrate into cerebrum Meninges The Meninges of the Brain Figure 14.5 Meningitis Meningitis—inflammation of the meninges Serious disease of infancy and childhood Especially between 3 months and 2 years of age Bacterial or viral invasion of CNS through nose and throat Pia mater and arachnoid are most often affected Can cause swelling of the brain, enlargment of the ventricles, and hemorrhage Signs include high fever, stiff neck, drowsiness, intense headache May progress to coma, then death within hours of onset Diagnosed by examining CSF obtained by lumbar puncture (spinal tap)  Ventricles and Cerebrospinal Fluid Ventricles—four fluid-filled chambers within brain Two lateral ventricles One in each cerebral hemisphere Third ventricle Narrow medial space beneath corpus callosum Fourth ventricle Small triangular chamber between pons and cerebellum  Ventricles and Cerebrospinal Fluid Ventricles are connected Interventricular foramen— pore that connects lateral ventricles to third ventricle Cerebral aqueduct—tube running through midbrain that connects third ventricle to fourth ventricle Central canal—tube that connects to fourth ventricle and runs through center of spinal cord Ventricles and Cerebrospinal Fluid 3 Figure 14.6a,b I always think of the Star Trek Enterprise when I look at the ventricles. Ventricles and Cerebrospi nal Fluid Figure 14.6c  Ventricles and Cerebrospinal Fluid Cerebrospinal fluid (CSF) Clear, colorless liquid Fills the ventricles and canals of CNS Surrounds brain and bathes its external surface Production of CSF begins with filtration of blood plasma through capillaries of the brain Choroid plexus—spongy mass of blood capillaries on the floor of each ventricle Ependyma—type of neuroglia that lines ventricles and covers choroid plexus Ependymal cells modify the filtrate. Compared to plasma, CSF has more sodium and chloride, less potassium, calcium, glucose, and very little protein Ventricles and Cerebrospinal Fluid CSF continuously flows through the CNS Driven by its own pressure, beating of ependymal cilia, and pulsations of the brain produced by each heartbeat Path through ventricles: Secreted in lateral ventricles Through intervertebral foramina into third ventricle Down the cerebral aqueduct into the fourth ventricle Third and fourth ventricles add more CSF along the way Ventricles and Cerebrospinal Fluid All CSF ultimately escapes through three pores that lead into subarachnoid space of brain and spinal cord surface Median aperture Two lateral apertures CSF is reabsorbed by arachnoid granulations Cauliflower-shaped extensions of the arachnoid meninx Protrude through dura mater into superior sagittal sinus CSF penetrates the walls of the villi and mixes with the blood in the sinus  Figure 14.7  Brain only contributes 2% of adult body weight but receives 15% of the blood Blood 750 mL/min Neurons have high demand for ATP Supply and (therefore oxygen and glucose) so constant supply of blood critical the Brain 10-second interruption—loss of Barrier consciousness 1- to 2-minute interruption— System significant impairment of neural function 4-minute interruption—irreversible brain damage  The brain barrier system regulates what substances can get from bloodstream into tissue fluid of the brain Although blood is crucial, it can also contain harmful Blood agents The brain barrier system is selectively Supply and permeable the Brain Highly permeable to water, glucose, and lipid-soluble substances such as oxygen, carbon dioxide, alcohol, caffeine, nicotine, anesthetics Barrier Slightly permeable to sodium, potassium, chloride, waste products urea and creatinine System Two points of entry must be guarded: Blood capillaries throughout the brain tissue Capillaries of the choroid plexus  At the blood capillaries, the brain is Blood protected by the blood–brain barrier Consists of tight junctions between endothelial Supply and cells that form the capillary walls Anything leaving the blood must pass through the the Brain cells and not the gaps between them Endothelial cells can be selective (unlike simple Barrier gaps) Exclude harmful substances, allow System necessary ones to pass through to brain tissue  Blood At the choroid plexus, brain is protected Supply and by the blood–CSF barrier Forms tight junctions between the the Brain ependymal cells Tight junctions are absent from ependymal Barrier cells elsewhere Important to allow exchange between System brain tissue and CSF The brain barrier system (BBS) can be an obstacle for delivering medications such as antibiotics and cancer drugs Trauma and inflammation can damage BBS Blood and allow pathogens to enter brain tissue Circumventricular organs (CVOs)— Supply and places in the third and fourth ventricles where the barrier is absent the Brain Allows blood direct access to the brain Enables the brain to monitor and Barrier respond to fluctuations in blood System glucose, pH, osmolarity, and other variables CVOs afford a route for invasion by the human immunodeficiency virus (HIV)  Stroke (cerebral vascular Stroke accident—CVA) Interruption of blood supply causes death of brain tissue Can occur at any age, but two-thirds are over 65 years old Second greatest cause of death (after coronary artery disease) About 50% of stroke patients live less than one year Two kinds of stroke distinguished longer by cause: 1. Hemorrhagic stroke Rupture of a cerebral or subarachnoid blood vessel Ischemic stroke 2. Obstruction of a blood vessel Blood clot (thrombosis) Lipid deposit (atherosclerosis) 14.3 The Expected Learning Outcomes: Hindbrain List the components of the hindbrain and midbrain and their functions. and Describe the location and functions of the reticular formation. Midbrain The Medulla Oblongata Medulla oblongata Begins at foramen magnum of skull Extends about 3 cm rostrally and ends at a groove just below pons Slightly wider than spinal cord The Medulla Oblongata Anatomical features of the medulla oblongata Pyramids Ridges on anterior surface, resemble side-by-side baseball bats Separated by anterior median fissure Cranial nerves Four pairs begin or end in medulla—VIII (in part), IX, X, XII Olives Prominent bulges lateral to each pyramid Gracile and cuneate fasciculi of spinal cord Continue as two pairs of ridges on posterior medulla Contain sensory fibers; synapse in gracile and cuneate nuclei The Brainstem Figure 14.8a The Brainstem 2 Figure 14.8b The Medulla Oblongata All ascending and descending fibers connecting brain and spinal cord pass through medulla Medial lemniscus Axons of gracile and cuneate nuclei decussate and form ascending (sensory) tract to thalamus Corticospinal tracts Descending motor tracts in pyramids; carry signals down to skeletal muscles Inferior olivary nucleus Relay center for signals to cerebellum Reticular formation Network of nuclei; extends through medulla, pons, midbrain Contains cardiac, vasomotor, and respiratory centers Cross Sections of the Brainstem (Medulla Oblongata) Figure 14.9c The Pons Pons—anterior bulge in brainstem, rostral to medulla Develops from metencephalon Cerebellar peduncles—thick stalks on posterior pons that connect it (and the midbrain) to the cerebellum Ascending sensory tracts Descending motor tracts Pathways in and out of cerebellum Cranial nerves V, VI, VII, and VIII Sensory roles: hearing, equilibrium, taste, facial sensations Motor roles: eye movement, facial expressions, chewing, swallowing, urination, and secretion of saliva and tears Reticular formation in pons contains additional nuclei concerned with sleep, respiration, posture The Pons 2 Figure 14.2a Cross Sections of the Brainstem (Pons) Figure 14.9b  The Midbrain Mesencephalon becomes one brain structure, the midbrain Short segment of brainstem that connects hindbrain to forebrain Contains cerebral aqueduct Surrounded by central gray substance involved in controlling pain Contains continuations of medial lemniscus and reticular formation Contains motor nuclei of two cranial nerves that control eye movements: CN III (oculomotor) and CN IV (trochlear)  Mesencephalon Tectum: roof-like part of the midbrain posterior to cerebral aqueduct The Four prominent bulges: Superior colliculi—visual attention, tracking moving objects, and some reflexes Midbrain Inferior colliculi—relays signals from inner ear to thalamus and other parts of the brain Cerebral peduncles: two anterior midbrain stalks that anchor the cerebrum to the brainstem Each peduncle has three parts: tegmentum, substantia nigra, and cerebral crus The Midbrain Figure 14.2a  Tegmentum Dominated by red nucleus Pink color due to high density of blood vessels Cerebral Connections go to and from cerebellum for motor control Substantia nigra Peduncles of Black nucleus pigmented with melanin Motor center that relays inhibitory signals to thalamus and basal nuclei suppressing the Midbrain unwanted body movement Degeneration of neurons leads to tremors of Parkinson’s disease Cerebral crus Bundle of nerve fibers that connect cerebrum to pons Carries corticospinal tracts Cross Sections of the Brainstem (Midbrain) Figure 14.9a  The Reticular Formation Loose web of gray matter that runs vertically through all levels of the brainstem and into the upper spinal cord Occupies space between white fiber tracts and brainstem nuclei Has connections with many areas of cerebrum More than 100 small neural networks without distinct boundaries Figure 14.10  Somatic motor control Adjust muscle tension to maintain tone, balance, and posture, especially during body movements The Reticular Relay signals from eyes and ears to cerebellum Formation- Integrate visual, auditory, balance and motion stimuli into motor coordination Function Gaze centers—allow eyes to track and fixate on objects Central pattern generators—neural pools that produce rhythmic signals to the muscles of breathing and swallowing  Cardiovascular control Cardiac and vasomotor centers of medulla oblongata The Reticular Pain modulation Some pain signals ascend through the Formation- reticular formation Some descending analgesic pathways Function begin in the reticular formation and end in the spinal cord where they block pain signals.  Sleep and consciousness Reticular formation plays a central role The Reticular in consciousness, alertness, and sleep Injury here can result in irreversible Formation- coma Function Habituation Reticular activating system modulates activity in cerebral cortex so that it ignores repetitive, inconsequential stimuli  The Cerebellum Cerebellum Largest part of hindbrain Second largest part of the brain as a whole Contains more than half of all brain neurons—about 100 billion Many small granule cells Large Purkinje cells have axons that synapse on deep nuclei General structure Right and left cerebellar hemispheres connected by vermis Superficial cortex of gray matter with folds (folia), branching white matter (arbor vitae), and deep nuclei The Cerebellum Cerebellar peduncles— three pairs of stalks that connect brainstem and cerebellum (their fibers carry signals to and from cerebellum) 1. Inferior peduncles: connected to medulla oblongata Most spinal input enters the cerebellum through inferior peduncle 2. Middle peduncles: connected to pons Most input from rest of the brain enters through middle peduncle 3. Superior peduncles: connected to the midbrain Carries cerebellar output  The cerebellum has long been known to be important for motor coordination and locomotor ability Recent studies have also revealed several The sensory, linguistic, emotional, and other nonmotor functions including: Cerebellum Comparing textures of objects Perceiving space (as tested by pegboard puzzles) - Function Recognizing objects from different views Keeping judge of elapsed time and maintaining tapping rhythm Directing eye movements to compensate for head movement Judging pitch of tones; distinguishing between spoken words Helping in verbal association tasks Planning, scheduling, and emotion control Many hyperactive children have small cerebellums The Cerebellum Figure 14.11  Expected Learning Outcomes: Name the three major components of the diencephalon and describe their locations and functions. Identify the five lobes of the 14.4 The cerebrum and their functions. Describe the three types of tracts in Forebrain the cerebral white matter. Describe the distinctive cell types and histological arrangement of the cerebral cortex. Describe the location and functions of the basal nuclei and limbic system. The Forebrain Forebrain consists of two parts: Diencephalon Encloses third ventricle Most rostral part of the brainstem Telencephalon Develops chiefly into the cerebrum The Forebrain 2 Figure 14.4c The Diencephalon: Forms part of walls and floor Hypothalamus of third ventricle Extends anteriorly to optic chiasm Extends posteriorly to mammillary bodies Each mammillary body contains three or four mammillary nuclei that relay signals from limbic system to thalamus The hypothalamus attaches to the pituitary through a stalk-like structure called the infundibulum The Diencephalon: Hypothalamus Essential role in homeostatic regulation of all body systems Functions of hypothalamic nuclei Hormone secretion Controls anterior pituitary, thereby regulating growth, metabolism, reproduction, and stress responses Produces posterior pituitary hormones for labor contractions, lactation, and water conservation Autonomic effects Major integrating center for autonomic nervous system Influences heart rate, blood pressure, gastrointestinal secretions, motility, etc. The Diencephalon: Hypothalamus Functions (continued): Memory Mammillary nuclei receive signals from hippocampus Emotional behavior and sexual response Anger, aggression, fear, pleasure, contentment, sexual drive The Diencephalon: Epithalamus Epithalamus Very small mass of tissue Composed of: Pineal gland: endocrine gland Habenula: relay from the limbic system to the midbrain Thin roof over the third ventricle The Epithalam us Figure 14.2a The Cerebrum The largest, most conspicuous part of human brain Seat of sensory perception, memory, thought, judgment, and voluntary motor actions Two hemispheres divided by longitudinal fissure Connected by white fibrous tract, the corpus callosum Gyri and sulci: increase amount of cortex in the cranial cavity, allowing for more information-processing capability Each hemisphere has five lobes named for the cranial bones overlying them The Cerebrum 2 Figure 14.1a,b The Cerebrum Frontal lobe Rostral to central sulcus Voluntary motor functions, motivation, foresight, planning, memory, mood, emotion, social judgment, and aggression Parietal lobe Between central sulcus and parieto- occipital sulcus Integrates general senses, taste, and some visual information Occipital lobe Caudal to parieto-occipital sulcus Primary visual center of brain The Cerebrum Temporal lobe Lateral and horizontal; below lateral sulcus Functions in hearing, smell, learning, memory, and some aspects of vision and emotion Insula (hidden by other regions) Deep to lateral sulcus Helps in understanding spoken language, taste and integrating information from visceral receptors The Five Lobes of the Cerebrum and Some of Their Key Functions Figure 14.13  Most of the volume of cerebrum is white matter The Glia and myelinated nerve fibers that Cerebral transmit signals White Tracts are bundles of nerve fibers in the Matter central nervous system Three types of tracts: Projection tracts Commissural tracts Association tracts  The Cerebral White Matter Projection tracts Extend vertically between higher and lower brain and spinal cord centers Example: corticospinal tracts Commissural tracts Cross from one cerebral hemisphere to the other allowing communication between two sides of cerebrum Largest example: corpus callusum Other crossing tracts: anterior and posterior commissures Association tracts Connect different regions within the same cerebral hemisphere Long fibers connect different lobes; short fibers connect gyri within a lobe Tracts of Cerebral White Matter Figure 14.14 The Cerebral Cortex Neural integration is carried out in the gray matter of the cerebrum Cerebral gray matter found in three places: Cerebral cortex Basal nuclei Limbic system Cerebral cortex—covers surface of the hemispheres Only 2 to 3 mm thick Cortex constitutes about 40% of brain mass Contains 14 to 16 billion neurons 90% of human cerebral cortex is neocortex—six- layered tissue that has relatively recent evolutionary origin Histology of the Neocortex Figure 14.15 The Cerebral Cortex Contains two principal types of neurons: Stellate cells Spheroid somas with dendrites projecting in all directions Receive sensory input, process information locally Pyramidal cells Tall and conical, with apex toward the brain surface A thick dendrite with many branches with small, knobby dendritic spines Include the output neurons of the cerebrum Output neurons—axons leave the cortex and connect with other parts of the CNS The Limbic System Limbic system—important center of emotion and learning Prominent components: Cingulate gyrus: arches over corpus callosum in frontal and parietal lobes Hippocampus: in medial temporal lobe (memory functions) Amygdala: immediately rostral to hippocampus (emotion functions) There is a limbic system in each cerebral hemisphere Limbic system components are connected through a loop of fiber tracts allowing for somewhat circular patterns of feedback Limbic system structures have centers for gratification and aversion Gratification: sensations of pleasure or reward Aversion: sensations of fear or sorrow  The Basal Nuclei Basal nuclei—masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus Receive input from the substantia nigra of the midbrain and the motor areas of the cortex Send signals back to both of these locations Involved in motor control The Limbic System 2 Figure 14.16  The Basal Nuclei At least three brain centers form the basal nuclei and are collectively called the corpus striatum Caudate nucleus Putamen Globus pallidus Lentiform nucleus—putamen and globus pallidus together The Basal Nuclei Figure 14.17a, b Photo: Biophoto Associates/Science Source