Lecture 1 Brain Organization PDF

Organization of the nervous system Prepared by: Mohammad Abuawad. MD.PhD An-najah National University Faculty of Medecin and Health sciences  Division of the Nervous System The SNS is responsible for the innervation of skeletal muscles. Embryology of...

Organization of the nervous system Prepared by: Mohammad Abuawad. MD.PhD An-najah National University Faculty of Medecin and Health sciences  Division of the Nervous System The SNS is responsible for the innervation of skeletal muscles. Embryology of the brain - The CNS begins as a neural tube - The lumen of the tube (neurocoel) is filled with fluid Embryology of the brain - In the fourth week of development, the cephalic area of the neural tube enlarges to form: - Prosencephalon - Mesencephalon - Rhombencephalon Embryology of the brain Prosencephalon eventually develops to form: -Telencephalon: forms the cerebrum -Diencephalon: forms the epithalamus, thalamus, and hypothalamus Mesencephalon Does not subdivide Becomes the midbrain Rhombencephalon eventually develops to form: - Metencephalon: forms the pons and cerebellum - Myelencephalon: forms the medulla oblongata Major Regions and Landmarks of the Brain -Medulla oblongata - Pons - Cerebellum - Mesencephalon (midbrain) - Diencephalon - Cerebrum (telencephalon)  Gross cerebral structure Gyri (s., Gyrus) Gyri are the upward folds, on the surface of the cerebral hemispheres. Sulci (s.,Sulcus) Sulci are the downward folds on the surface of the cerebral hemispheres, between the gyri. Convolutions Convolutions are the collective name for the gyri and sulci. They are the raised and depressed surfaces of the brain. Because brain growth is confined by the skull, the brain folds in on itself as it grows. Theorists surmise that the more gyri and sulci one has, the larger one’s brain surface is and the more brain capacity one has for brain functions. Fissure A fissure is a deep groove in the surface of the brain. Gross cerebral structure Medial Longitudinal Fissure The medial longitudinal fissure separates the right and left cerebral hemispheres. Central Sulcus (also called the Sulcus of Rolando) The central sulcus separates the frontal and parietal lobes. It also separates the primary motor cortex from the primary somatosensory cortex. Lateral Fissure (also called Fissure of Sylvius) This sulcus separates the temporal lobe from the frontal lobe. Parieto-occipital sulcus This sulcus separates the parietal lobe from the occipital lobe. Gross cerebral structure Precentral Gyrus The precentral gyrus is the primary motor cortex. This area handles voluntary motor movement. It is located just anterior to the central sulcus. Postcentral Gyrus The postcentral gyrus is the primary somatosensory cortex. It is located just posterior to the central sulcus. This is the part of the brain that mediates the detection of physical sensation.  Cerebral lobes Each hemisphere has 4 separate lobes. Frontal Lobes - The borders of the frontal lobes are the lateral fissure and the central sulcus. - The frontal lobes mediate cognition (intelligence, problem solving, and short-term memory), expressive language, motor planning, mathematical calculations, - The prefrontal lobe mediates executive functions (organization, planning, sequencing, and motivation), self-insight, and regulation of emotions and working memory. - The frontal lobes develop mostly after birth. Development is not thought to be complete until late adolescence or early adulthood. Cerebral lobes Cerebral lobes Parietal Lobes - The parietal lobes sit just posterior to the frontal lobes. - The inferior border is the temporal lobe and the lateral fissure. - Their functions are sensory detection, perception, and interpretation. Temporal Lobes - The temporal lobes are the most inferior or caudal lobes. - They have a poorly defined posterior border, the anterior occipital lobe. - Their functions are audition (hearing), comprehension of language, and long- term memory. Occipital Lobes - The occipital lobes are the most posterior lobes. - They are responsible for the interpretation of visual stimuli from the optic pathways. Cerebral lobes Insula -The insula is a portion of the cerebral cortex that lies deep in the lateral fissure. - It is covered from view by the frontal, parietal, and temporal lobes. - The insula has a role related to basic survival mechanisms such as taste (gustation), sensation of the viscera, and autonomic and homeostasis functions. - The insula also functions in emotional processing, including empathy, self awareness, and emotional regulation.  Hemispheric Specialization Left hemisphere -Speech center, writing, language, mathematics - This hemisphere controls movement on the right side of the body. - It receives sensory information from the right side of the body. Right hemisphere - Analysis by touch, spatial visualization - The right hemisphere controls movement on the left side of the body. - It receives sensory information from the left side of the body.  Split-brain humans Corpus callosum disconnection. Roger Sperry’s Split Brain Experiments (1959–1968) Sperry tested the idea in humans with their corpus callosum severed as treatment for epilepsy, a seizure disorder. The left hemisphere interpreted language but not the right. each hemisphere of the brain is responsible for movement and vision on the opposite side of the body Sperry shared the Nobel Prize in Physiology or Medicine in 1981for his split-brain research. Demonstrating language comprehension in the right hemisphere If a split-brain person sees a word in the left visual field, he will says he sees nothing. Why?  The Wada procedure Developed by Juhn Wada Simple technique for studying the function of single cerebral hemisphere A fasting acting barbiturate such as sodium amytal is injected into the carotid artery on one side of the neck The drug is carried to the ipsilateral hemisphere where it acts as anesthetic for about 10 minutes The limb on the side of the body contralateral become paralyzed and somatic sensation is lost. By asking the patient to answer Qs: - if the injected hemisphere is dominant for speech, the patient will be completely unable to talk until the anesthesia wears off. - If the injected hemisphere is not dominant, will continue to speek throughout the procedure. The Wada procedure  Gray matter VS white matter The cerebral hemispheres consist of gray and white matter. Gray Matter - Areas where gray matter covers part of the central nervous system (CNS) are called the cortex. - Gray matter sits on the surface of the cerebrum and the cerebellum. - Gray matter extends deep within the CNS and forms nuclei body such as the basal ganglia. - It has a grayish appearance because it consists of nerve cell bodies (nuclei). - The gray matter is non-myelinated brain matter. Ganglia - Ganglia are collections of neural cell bodies (or nuclei) usually located outside of the CNS, or in the peripheral nervous system (PNS). Example: Dorsal root ganglia Gray matter VS white matter White Matter - White matter is located beneath the gray matter, in the internal regions of the cerebrum and cerebellum. - White matter consists of myelinated fiber tracts or neuronal axons. The Central White Matter Consists of bundles called: Association fibers: tracts that interconnect areas of neural cortex within a hemisphere Commissural fibers: tracts that connect the two hemispheres (corpus callosum) Projection fibers: tracts that link the cerebrum with other regions of the brain and spinal cord Sagittal section through the brain  Diencephalon The diencephalon consists of 4 structures 1. Thalamus 2. Hypothalamus 3. Epithalamus 4. Subthalamus Thalamus - Thalamus means “egg-shaped.” - There are 2 thalamic lobes, 1 in each hemisphere. - A flattened gray band called the interthalamic adhesion connects the thalamic lobes. - The thalamus contains 26 pairs of nuclei. Functions of the thalamus 1. All sensory information, except olfaction, travels through the thalamus before it reaches the cortex and is consciously interpreted. 2. However, the thalamus does not passively relay sensory information. It is a dynamic structure that works collaboratively with the cortex in feedforward, feedback, and loop circuits. 3. The thalamus receives motor information from the cerebral hemispheres and relays it to the motor receptors, 4. The thalamus has a role in sleep-wake cycles and consciousness/alertness. The thalamus acts as a screen for information traveling to the cortex, inhibiting less important information that can be handled at a subcortical level and alerting the cortex to important information that must be dealt with at a conscious level. Thalamus Important thalamic nuclei include the following: 1. Lateral geniculate nucleus: responsible for visual processing 2. Medial geniculate nucleus: responsible for auditory processing 3. Ventrolateral nucleus: responsible for the organization of motor responses 4. Ventral posterolateral nucleus: responsible for tactile-sensory processing Hypothalamus - The hypothalamus is located just anterior and inferior to the thalamus. - There are 2 hypothalamic lobes, 1 in each hemisphere. Functions include the following 1. Regulates the autonomic nervous system (ANS) 2. Releases hormones from the pituitary gland, adrenal glands, Secretion of antidiuretic hormone and oxytocin 3. Regulates temperature 4. Regulates hunger and thirst 5. Regulates sleep-wake cycles (circadian rhythms) and fatigue 6. Works collaboratively with the limbic system in the expression of emotions The epithalamus - Contains the pineal gland Produces the hormone melatonin Regulates our day/night cycles Subthalamus - The subthalamus is a deep structure and is considered a thalamic nuclei group located caudal to the thalamus. - The function of this structure is not well understood, but some researchers believe it to be a component of the basal ganglia that may function in action selection. - There is some evidence that the subthalamic nucleus has a role in impulse control.  Other structures of the diencephalon Pituitary Gland - There is only one pituitary gland in the body. - The pituitary gland is an endocrine gland that secretes hormones that regulate growth, reproductive activities, and metabolic processes. - These hormones from anterior pituitary gland include the following: 1. Growth hormone (GH) 2. Prolactin (PRL), for lactation 3. Luteinizing hormone (LH), for reproduction 4. Follicle stimulating hormone (FSH), for reproduction 5. Thyroid stimulating hormone (TSH), for metabolism 6. Adrenocorticotrophic hormone (ACTH), for the regulation of stress The pituitary works collaboratively with the hypothalamus. The synthesis and secretion of these hormones are controlled by neuropeptides released from the hypothalamus. Pituitary Gland - Hormones from posterior pituitary gland include the following: 1. Antidiaruticc hormone (ADH) 2. Oxcytocine Infundibulum The infundibulum is the stalk that extends from the hypothalamus andcholds the pituitary gland. PosteriorCommissure - The posterior commissure connects the right and left halves of the diencephalon. - This structure allows communication between the hemispheres if the corpus callosum is lesioned or removed because of pathology. AnteriorCommissure - The anterior commissure connects the olfactory bulb to the amygdala and may have a role in olfaction. - This structure allows communication between the hemispheres if the corpus callosum is lesioned.  Structures located near diencephalon (but not part of the diencephalon) Corpus Callosum - The corpus callosum is the largest commissure in the brain. - It allows the right and left cerebral hemispheres to communicate with each other. Optic Chiasm - A chiasm is a crossing-over point. - The optic chiasm is a cross-shaped connection located between the optic nerves. - It is a midline structure specifically located at the base of the brain just superior to the pituitary gland. Mammillary Bodies - The mammillary bodies are nuclei groups that form attachments with the hypothalamus and fornix and may play a role in the processing of memory. Internal Capsule - The internal capsule is a large fiber bundle that connects the cerebral cortex with the diencephalon. - All descending motor messages from the motor cortex travel through the internal capsule to the thalamus, brain-stem, spinal cord, and the skeletal muscles. - Sensory information from the sensory receptors ascends in the spinal cord, through the brainstem, to the thalamus, through the internal capsule, to primary sensory cortex.  Basal ganglia - Control the cycles of arm and leg movements when walking. The basal ganglia consist of 3 primary structures 1. Caudate nucleus 2. Putamen 3. Globus pallidus - Threre are other structures that are considered to be part of the basal nuclei: subthalamic nucleus of the diencephalon, the substantia nigra of the midbrain Corpus Striatum Corpus striatum is a collective name for the caudate, putamen. Lenticular Nucleus Lenticular nucleus is a collective name for the globus pallidus and putamen. Claustrum The claustrum is not considered to be part of the basal ganglia, although it is a group of nuclei located just lateral to the external capsule and just medial to the insula. Subthalamic Nucleus - The subthalamic nucleus is a thalamic nuclei group located caudal to the thalamus. - It contains glutaminergic neurons with projections to the internal globus pallidus. - In addition to motor control, the subthalamic nucleus is believed to play roles in the timing of cognitive decision making and the processing of emotional information. Substantia Nigra - The substantia nigra is located in the midbrain. - the pars compacta (SNpc) containing dopaminergic neurons: produces dopamine, a neurotransmitter that functions in movement and mood regulation - the pars reticulata (SNpr) with inhibitor gamma-aminobutyric acid-containing (or GABAergic) neurons.  The Limbic System - Located between the cerebrum and the diencephalon - Functions: 1. Establishes emotional states 2. Links the conscious functions with the unconscious autonomic functions 3. Facilitates memory storage and retrieval -The limbic system consists of: 1. Cingulate gyrus 2. Dentate gyrus 3. Parahippocampal gyrus 4. Hippocampus Hippocampus - The hippocampus is located within the parahippocampal gyrus. - In a coronal section, the hippocampus looks like a seahorse (in Latin, hippocampus means “seahorse”). Functions: 1. plays important roles in the consolidation of information from short-term memory to long-term memory particularly for memories that are traumatic or emotionally laden. 2. Research has shown that neurogenesis—or the cell birth and maturation of neurons—occurs in the hippocampus throughout life. The fornix - Tract of white matter connecting the hippocampus with the hypothalamus - Many fornix fibers extend to the mamillary bodies - Mamillary bodies control reflex movement associated with eating  Brainstem The brainstem is composed of 3 basic structures 1. Midbrain 2. Pons 3. Medulla The brainstem controls vegetative functions: 1. Respiration 2. Cough and gag reflex 3. Pupillary response 4. Swallowing reflex Medulla oblongata - Continuous with the spinal cord to the brain stem - All communication between the brain and spinal cord passes through the medulla oblongata - Nuclei in the medulla oblongata are: Relay stations for sensory or motor pathways Associated with cranial nerves connected to the medulla oblongata Associated with the autonomic control of visceral organs Medulla oblongata Nuclei of cranial nerves - Contains sensory and motor nuclei of cranial nerves: VIII, IX, X, XI, and XII - Autonomic nuclei Cardiovascular centers (cardiac and vasomotor) Respiratory centers (rhythmic breathing) The Pons - The pons is a prominent bulge superior to the medulla oblongata and consists of: Sensory and motor nuclei for cranial nerves: V, VI, VII, and VIII midbrain - Mesencephalon (also called the midbrain) - Consists of two pairs of nuclei collectively called corpora quadrigemina Responsible for processing auditory and visual stimuli: 1. Auditory processing occurs in the inferior colliculus 2. Visual processing occurs in the superior colliculus - Consists of nuclei of the reticular formation: Involved in maintaining alertness  Reticular formation -The reticular formation is diffusely located in the brainstem. -plays a role in screening information before it reaches the cortex. -Important information critical to functional survival is amplified, while less important information is habituated or not readily noticed by the cortex. The reticular formation consists of 2 systems: 1. Reticular activating system (RAS) 2. Reticular inhibiting system (RIS) Reticular Activating System - The RAS is the brainstem center that is involved in states of wakefulness and sets the general level of brain activation. - It plays a role in alerting the cortex to attend to important sensory stimuli. - The RAS is located in the rostral midbrain. Reticular Inhibiting System - The RIS is the brainstem center that is involved in states of unconsciousness such as sleep, stupor, or coma. - It plays a role in habituation or screening of unimportant information so that the cortex can concentrate on information critical to survival or goal-directed behavior. - The RIS extends from the caudal midbrain to the caudal medulla. The reticular formation is also divided into 3 columns: 1. Raphe nuclei: involved in the neurochemical synthesis of serotonin and mood regulation 2. gigantocellular reticular nuclei : involved in motor coordination 3. parvocellular reticular nuclei: regulate respiratory function, specifically exhalation  Cerebellum The cerebellum consists of: Two hemispheres Folia (folds similar to gyri of the cerebrum) Anterior and posterior lobes Flocculonodular lobes Vermis (separates the hemispheres) Arbor vitae Cerebellar peduncle Cerebellum  The cerebellum - Cerebellar cortex: subconscious coordination of movements - Arbor vitae: connects cerebellar cortex with cerebellar peduncles - Cerebellar peduncles: 1. Superior: connect cerebellum with mesencephalon,diencephalon, and cerebrum 2. Middle: communicate between cerebellum and pons 3. Inferior: connect cerebellum with the medulla oblongata  Ventricular system The Ventricles of the Brain -The ventricles are the hollow spaces in the brain that contain cerebrospinal fluid (CSF). -There are four ventricles Ventricles 1 and 2 (called lateral ventricles) are in the cerebral hemispheres (separated by a partition called the septum pellucidum) Ventricle 3 is in the diencephalon Ventricle 4 lies between the pons and the cerebellum Lateral ventricles (ventricles 1 and 2) Main portion (body) of the ventricle lies in the parietal lobes Each has an anterior horn extending into the frontal lobe Each has a posterior horn extending into the occipital lobe Each has an inferior horn extending into the temporal lobe Each communicates with the third ventricle through the interventricular foramen (foramen of Monro) Third ventricle - Communicates with the fourth ventricle Through the cerebral aqueduct (aqueduct of sylvius) = it is a common site of blockage. Fourth ventricle - Communicates with the central canal of the spinal cord - The central canal begins in the caudal medulla and descends all the way down the spinal cord.  Cranial Meninges: -The meninges are located between the skull and brain, and they cover the spinal cord. - -They form a seal around the central nervous system (CNS). -There are 3 layers of meninges: dura mater, arachnoid mater, and pia mater. The Dura Mater - The dura mater is the outermost meningeal layer. - It is a very tough and thick membrane that is attached to the inner-surface of the - cranium. - Consists of two layers 1. Endosteal layer This is the layer felt within the anterior fontanel of a baby 2. Meningeal layer Space between the two layers is called the dural sinus Dural Sinuses - The dural sinuses function as large veins and are located above the frontal and parietal lobes. - The sinuses act like a circulatory system, allowing cerebral veins to empty into them. - The cerebral veins also receive cerebrospinal fluid (CSF) from the subarachnoid space via the arachnoid villi. - These fluids are then returned to their general circulatory systems. The meningeal layer forms folds called: falx cerebri tentorium cerebelli falx cerebelli diaphragma sella 1. Falx cerebri Meningeal layer that extends into the medial longitudinal fissure 2. Tentorium cerebelli Separates the cerebellar hemispheres from the cerebral hemispheres Extends across the cranium at right angles to the falx cerebri 3. Falx cerebelli Extends from the tentorium cerebelli to separate the cerebellar hemispheres 4. Diaphragma sellae Lines the sella turcica of the sphenoid bone It anchors the dura mater to the sphenoid bone It encases the pituitary gland  The Arachnoid Mater - The arachnoid mater is the middle meningeal layer and is located just below the subdural space. - Subarachnoid Space: Beneath the arachnoid is the subarachnoid space, which holds the CSF. - The arachnoid looks like a spider web (arachnoid means “spider” in Greek). Consists of projections called arachnoid granulations CSF flows through these to enter into blood circulation Cisterns The cisterns are openings or large spaces in the subarachnoid space. Cisterna Magna (also called the Cerebellar Medullary Cistern) - The cisterna magna is the largest subarachnoid cistern and is located between the cerebellum and the medulla. - It is often used as a shunt placement in hydrocephalus. The Pia Mater -The pia mater is the deepest meningeal layer - Attached to the surface of the brain - Follows the sulci and gyri of the brain - Helps to anchor the larger blood vessels of the cerebrum  Cerebrospinal Fluid (CSF) - CSF is a clear, colorless fluid that bathes and nourishes the brain and spinal cord. - Functions: 1. Provides protection of the brain and spinal cord, the fluid acts as a shock absorber. 2. Provides support 3. Transports nutrients to the CNS tissue and Transports waste away from the CNS 4. CSF helps in diagnosis and is examined for its rate of pressure and fluid composition. 5. CSF also has a role in the transport of some hormones throughout the central nervous system. Formation of CSF - Produced by the ependymal cells of the choroid plexus - They actively transport nutrients, vitamins, and ions into the CSF - They actively remove waste from the CSF Choroid Plexus - The choroid plexus is made up of the vascular structures in the brain that protrude into the ventricles and produce CSF. - All of the ventricles contain choroid plexus, but the lateral ventricles contain the most. Arachnoid Villi -CSF is reabsorbed in the arachnoid villi and returns to blood circulation through the venous sinuses. -The arachnoid villi are projections of the arachnoid mater into the dura mater. CerebrospinalFluid Pressure -CSF maintains a constant circulatory pressure. -The formation of CSF is independent of the pressure. - Even if CSF pressure increases, CSF continues to be produced. NB: -CSF composition and rate of flow are used for diagnostic purposes to identify disease processes. -Example: Spinal tap (or lumbar puncture) is a procedure in which the spinal cavity is punctured with a needle to extract CSF for diagnostic purposes. Circulation of CSF CSF from the choroid plexus of the lateral ventricles Flows through the interventricular foramen Flows into the third ventricle Flows into the cerebral aqueduct Flows into the fourth ventricle Through the lateral aperture and median aperture and central canal of the spinal cord Flows into the subarachnoid space Flows around the brain and spinal cord eventually enters circulation via the arachnoid granulations (villi) Circulation of CSF Foramen of Magendie (also called Median Aperture) - There is only one foramen of Magendie, which is an opening in the fourth ventricle (in the rostral medulla). - The foramen of Magendie opens to the subarachnoid space below the cerebellum. - The subarachnoid space is the space between the arachnoid membrane and the pia mater. This foramen is also a potential site of CSF blockage. Foramina of Luschka (also called Lateral Apertures) -There are 2 foramina of Luschka. -These are openings in the fourth ventricle (in the pons) through which CSF can exit to the subarachnoid-space. -The foramina are potential sites of CSF blockage.  HYDROCEPHALUS - Hydrocephalus is an abnormal accumulation of pressure and fluid that results in compression of neural tissue and enlargement of the ventricles. - This condition causes intracranial pressure and progressive enlargement of the head in childhood if not corrected. - If onset occursin adulthood, the condition can be lethal if not addressed. 1. Noncommunicating Hydrocephalus -Noncommunicating hydrocephalus occurs when blockage in the ventricular system prevents the CSF from reaching the arachnoid villi for reabsorption. -This condition occurs from obstruction of the foramina or cerebral aqueduct. - The most commonly obstructed site is the cerebral aqueduct. 2. Communicating Hydrocephalus - Communicating hydrocephalus results from impaired reabsorption of CSF that does not occur from blockage of the foramina. - In communicating hydrocephalus, the subarachnoid space can become narrowed or blocked as a result of prior bleeding or meningitis. 3. Congenital Hydrocephalusin Infants Etiology 1. Blockage (particularly in the foramina of Luschka and Magendie) 2. Excessive production of CSF for unknown reasons 3. Meningitis, causing adhesions and resultant blockages in the subarachnoid space 4. Tumors of the choroid plexus, causing excessive CSF production 5. Hemorrhage or inflammation; the ependyma (the lining of the ventricles) is especially sensitive to viral infections during embryonic development. Common Sites of Blockage 1. Cerebral aqueduct 2. Foramen of Luschka 3. Foramen of Magendie. Pathological Effects - The infant’s skull expands to accommodate the increased fluid. The cranial sutures separate. - Head expansion and bulging of the fontanels - Compression of neural tissue - Because the skull can expand, increased intracranial pressure is usually not present; intelligence is often spared. Treatment - If hydrocephalus was caused by a blockage, then treatment requires a shunt, or tube, that bypasses the blockage. - If there is excessive production of CSF, treatment requires a shunt usually placed from the fourth ventricle to the abdomen to drain the excess CSF. - If diagnosed, hydrocephalus can be successfully treated in utero.  Blood–Brain Barrier The lining of the blood vessels consists of endothelial cells that are highly interconnected by tight junctions Due to this tight connection, only lipid-soluble material can pass from the blood to the cells of the brain and spinal cord Water-soluble material can only pass via the action of transport mechanisms The transport mechanisms are very specific