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Chapter 08 CNS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I. Structural Organization of the Brain Central Nervous System Composed of the brain and spinal cord a. Receives input from sensory neurons and directs activity of motor neurons that innervate muscles and glands b. Association neurons integrate sensory information and help direct the appropriate response to maintain homeostasis and respond to the environment. Central Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gyrus Sulcus Corpus callosum Cerebrum Meninges Spinal cord Central canal Tentorium cerebelli Cerebellum Embryonic Development 1. From the ectoderm comes a groove that will become the neural tube around 20 days after conception. This will eventually become the CNS. 2. Between the neural tube and the developing epidermis, a neural crest forms. This will become PNS ganglia. Embryonic Development of the CNS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neural crest Neural groove Neural crest Cranial neuropore Neural canal Neural tube Caudal neuropore Neural crest Neural groove Neural groove Neural crest Waldrop Wall of yolk sac Embryonic Development 3. By week 4 after conception, three distinct swellings are seen on the neural tube: a. Prosencephalon (forebrain) b. Mesencephalon (midbrain) c. Rhombencephalon (hindbrain) Embryonic Development 4. By week 5, these regions differentiate into five regions. a. The prosencephalon divides into the telencephalon and diencephalon. b. The mesencephalon remains the mesencephalon c. The rhombencephalon divides into the metencephalon and myelencephalon. Developmental Sequence of the Brain Weeks 4 and 5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Three primary vesicles Wall Five secondary vesicles Adult derivatives of Walls Cavities Cavity Telencephalon Prosencephalon (forebrain) Mesencephalon (midbrain) Diencephalon Mesencephalon Cerebral hemisphere Thalamus Hypothalamus Lateral ventricles Midbrain Aqueduct Pons Rhombencephalon Metencephalon (hindbrain) Third ventricle Upper portion Cerebellum Myelencephalon Medulla oblongata Spinal cord of fourth ventricle Lower portion Later development a. Telencephalon two cerebral hemispheres and the two lateral ventricles (remnant of the tube) b. Diencephalon the thalamus, hypothalamus, and the third ventricle c. Mesencephalon the midbrain and cerebral aqueduct d. Metencephalon the pons, cerebellum, and upper fourth ventricle e. Myelencephalon the medulla oblongata and lower fourth ventricle f. The posterior neural tube becomes the spinal cord Choroid plexuses and cerebrospinal fluid Consists of simple cuboidal to columnar epithelium (ependymal cells) in close association with blood capillaries Project into the roofs of the ventricles Secrete cerebrospinal fluid (CSF) into the ventricles and central canal of the cord. CSF is an ultrafiltrate of blood and is returned to blood Ventricle of the Brain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lateral ventricle Interventricular foramen Lateral ventricle Third ventricle Third ventricle Interventricular foramen Mesencephalic aqueduct Mesencephalic aqueduct Fourth ventricle Fourth ventricle (a) To central canal of spinal cord (b) To central canal of spinal cord Brain statistics 1. Gray matter forms the cortex and deep nuclei; white matter is deep forming tracts 2. The adult brain has 100 billion neurons. 3. It weighs about 1.5 kg (3−3.5 pounds). 4. It receives 15% of the total blood flow to the body per minute. 5. Scientists have demonstrated neurogenesis (the formation of new brain cells from neural stem cells) in adult brains within the subgranular zone of the hippocampus and subventricular zone of the lateral ventricles II. The Cerebrum Introduction 1. 2. 3. 4. Derived from the telencephalon Largest portion of the brain - 80% of the mass Responsible for higher mental functions Consists of a right and left cerebral hemisphere connected internally by the corpus callosum Cerebral Cortex 1. The outer region of the cerebrum composed of 2−4 mm gray matter with underlying white matter. 2. Characterized by raised folds called gyri separated by depressed grooves called sulci; together called convolutions 3. Each hemisphere is divided by deep sulci or fissures into 5 lobes - Frontal , Parietal, Temporal, Occipital, Insula Lobes of the Cerebrum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Precentral gyrus Superior frontal gyrus Superior frontal sulcus Frontal poles Central sulcus Superior frontal gyrus Postcentral gyrus Parietal lobe Central sulcus Frontal lobe Lateral sulcus Superior frontal sulcus Longitudinal fissure Occipital lobe Temporal lobe Parietal lobe Cerebellar hemisphere Occipital poles (a) (b) Frontal and Parietal Lobes Separated by the central sulcus The precentral gyrus (primary motor cortex) is located in the frontal lobe and is responsible for motor control; neurons called upper motor neurons The postcentral gyrus (primary somatosensory cortex) is in the parietal lobe and is responsible for somatosensory sensations (coming from receptors in the skin, muscles, tendons, and joints) Maps of the Precentral and Postcentral Gyri Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. “Homunculus” Central sulcus Somatosensory cortex Motor cortex Thumb, fingers, and hand Lower arm Facial expression Upper arm Upper leg Trunk Lower leg Pelvis Pelvis Trunk Neck Upper arm Lower arm Hand, fingers, and thumb Upper leg Salivation Vocalization Mastication Lower leg Foot and toes Foot and toes Genitals Upper face Lips Teeth and gums Swallowing Tongue and pharynx Longitudinal fissure Insula Insula Parietal lobes Central sulcus Motor cortex Somatosensory cortex Frontal lobes (a) (b) Temporal, Occipital, and Insula Lobes Temporal lobe: auditory centers Occipital lobe: vision and coordination of eye movements Insula: encoding of memory and integration of sensory information with visceral responses; receives olfactory, gustatory, auditory, and pain information Functional Regions of the Cerebrum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central sulcus Primary motor cortex involved with the control of voluntary muscles Somatosensory cortex for cutaneous and proprioceptive senses Frontal lobe Parietal lobe Motor speech area (Broca’s area) General interpretive area Auditory area Lateral sulcus Occipital lobe Interpretation of sensory experiences, memory of visual and auditory patterns Combining visual images, visual recognition of objects Cerebellum Temporal lobe Brain stem Functions of the Cerebral Lobes Mirror Neurons Found in frontal and parietal lobes to integrate sensory and motor neural activity Becomes active during goal-directed actions or through observation of someone else performing such actions Connected through the insula and cingulate gyrus to emotion centers in the brain May be involved in the ability to learn social skills and language Have been implicated in autism (autism spectrum disorder) Visualizing the Brain X-ray computed tomography (CT): looks at soft tissue absorption of X-rays Positron emission tomography (PET): radioactively labeled fluoro-deoxyglucose injected into the blood; emits gamma rays in active tissues 1) Used to monitor cancer 2) Used to study brain metabolism, drug distribution in the brain, and changes in blood flow following activity Visualizing the Brain Magnetic resonance imaging (MRI): Protons in tissues are aligned by powerful magnets. The chemical composition of different tissues results in differences in proton alignment. 1) Can be amplified using MRI contrast agents injected before imaging 2) Shows clear definition between gray matter, white matter, and cerebrospinal fluid Visualizing the Brain: MRI Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lateral ventricle Third ventricle White matter of cerebrum Gray matter of cerebrum Courtesy of Dr. Llinas, New York University Medical Center Visualizing the Brain Functional magnetic resonance imaging (fMRI): visualizes increased neuronal activity in different brain regions indirectly by looking at blood flow 1) Release of the neurotransmitter glutamate increases vasodilation of blood vessels in the area. 2) Active brain regions receive more oxyhemoglobin; called the BOLD response for blood oxygenation level dependent contrast Visualizing the brain Magnetoencephalogram (MEG) 1) Based on magnetic fields produced by postsynaptic currents 2) Sensors are SQUIDS – superconducting quantum interference devices 3) More accurate than EEGs Visualizing the Brain Electroencephalogram (EEG): Electrodes on the scalp detect synaptic potentials produced by cell bodies and dendrites in the cerebral cortex. Visualizing the Brain Four patterns are usually seen: a) Alpha waves (10-12 cycles/sec): active, relaxed brain. Best recorded from parietal lobes and occipital lobe. b) Beta waves (13-25 cycles/sec): produced with visual stimulation and mental activity. Best recorded from frontal lobe. c) Theta waves (5-8 cycles/sec): seen in sleep or stress; occipital and temporal lobes d) Delta waves (1-5 cycles/sec): seen in sleep of adults and in awake infants EEG Wave Patterns Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Alpha Beta Theta Delta 1 sec Techniques for Visualizing Brain Function Sleep May be genetically controlled, although sleep is affected by environmental factors Neurotransmitters involved Histamine – wakefulness Adenosine & GABA – sleep Serotonin – reduces REM sleep and stimulates non-REM sleep Sleep Two recognized categories: 1) REM (rapid eye movement): state when dreams occur. Activities similar to Beta waves are seen here. 2) Non-REM (resting sleep): divided into four stages, determined by EEG waves seen. Stages 3 and 4 are often called slow-wave sleep, characterized by delta waves. Sleep pattern 1) When people first fall asleep, they enter non-REM sleep and progress through the four stages. 2) Next, a person ascends back up the stages of nonREM sleep to REM sleep. 3) This cycle repeats every 90 minutes, and most people go through five per night. 4) If allowed to awaken naturally, people usually do so during REM sleep. 5) Slow-wave is prominent in the first part of sleep, while REM is prominent in the second half Sleep pattern REM Sleep Brain waves during REM similar to beta waves. The limbic system (involved in emotion) is very active during REM sleep. Breathing and heart rate may be very irregular. Benefits consolidation of nondeclarative memories Non-REM Sleep As you fall asleep, neurons decrease their firing rates, decreasing blood flow and energy metabolism. Breathing and heart rate are very regular. Non-REM sleep may allow repair of metabolic damage done to cells by free radicals and allows time for the neuroplasticity mechanisms needed to store memories. Benefits consolidation of spatial and declarative memories Basal Nuclei Masses of gray matter located deep in the white matter of the cerebrum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Motor cerebral cortex Thalamus Claustrum Putamen Basal nuclei Lentiform nucleus Globus pallidus Corpus striatum Caudate nucleus Cerebellum Spinal cord Basal Nuclei Most prominent is the corpus striatum; composed of: a. Caudate nucleus b. Lentiform nucleus; made up of the putamen and the globus pallidus Also includes subthalamic nucleus of the diencephalon and substantia nigra of the midbrain Degeneration of dopaminergic neurons from the substantia nigra to the corpus striatum causes Parkinson’s disease Motor circuit a. Neurons from motor cortex sends axons to the striatum (caudate and putamen) b. Striatum sends axons to the globus pallidus c. Globus pallidus sends axons to the thalamus d. Thalamus sends axons to the motor cortex This completes a motor circuit. This circuit stimulates appropriate movements and inhibits unwanted movement. Neurons in the nigrostiatal tract (substantia nigra striatum) secretes dopamine, which excites the direct pathway and inhibits the indirect pathway Motor circuit – Direct Pathway Motor Cortex (+) (+++) Muscle Glutamate Glutamate Thalamus (+) ↓ GABA (+) Striatum (caudate and putamen) (+) GABA (-) Globus pallidus (interna) Dopamine Substantia nigra Direct pathway “turns up” motor activity Motor circuit – Indirect Pathway (- - -) Motor Cortex (-) Muscle Glutamate Thalamus Glutamate (-) GABA Indirect pathway “turns down” motor activity Globus pallidus (+) (interna) Striatum (+) (caudate and putamen) Glutamate (-) Dopamine Substantia nigra GABA (-) Subthalamic nuclei (+) ↓ GABA Globus pallidus (externa) The Motor Circuit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glutamate neurotransmitter (excitatory) Caudate Dopamine neurotransmitter (excitatory) Putamen GABA neurotransmitter (inhibitory) Thalamus Globus pallidus Subthalamic nucleus Substantia nigra Cerebral Lateralization Each side of the precentral gyrus controls movements on the contralateral (opposite) side of the body due to decussation of fibers. Somatesthetic sensation from each side of the body projects to contralateral sides of the postcentral gyrus. Communication between the sides occurs through the corpus callosum; this is severed in severe forms of epilepsy. Cerebral Lateralization (Dominance) Some tasks seem to be performed better by one side of the brain than the other. a. Right hemisphere: visuospatial tasks, recognizing faces, composing music, arranging blocks, reading maps b. Left hemisphere: Language, speech, writing, calculations, understand music Cerebral Lateralization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Olfaction Olfaction Speech, writing Left ear Right ear Simple language comprehension Main language center Spatial concepts Calculation Left visual half field Right visual half field Split brain Language 1. Most of the knowledge of how the brain controls language has come from studying people with speech problems called aphasias. 2. Two areas are identified as important : a. Broca’s area b. Wernicke’s area Broca’s Area Located in left inferior frontal gyrus Controls motor aspects of speech Broca’s aphasia involves slow, poorly articulated speech. There is NO impairment in understanding. Interestingly, other actions of the tongue, lips, and larynx are not affected; only the production of speech is affected. Wernicke’s Area Located in left superior temporal gyrus, left anterior occipital lobe and left inferior parietal lobe Controls understanding of words. Information about written words is sent by the occipital lobe (visual cortex). Wernicke’s aphasia involves production of rapid speech with no meaning, called “word salad.” Language (spoken and written) comprehension is destroyed. Speech To speak, word comprehension originates in Wernicke’s area and is sent to Broca’s area along the arcuate fasciculus (association fibers). Broca’s area sends information to the motor cortex to direct movement of appropriate muscles. Speech Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Motor cortex (precentral gyrus) Motor speech area (Broca’s area) Wernicke’s area The Limbic System Group of brain regions responsible for emotional drives Areas of the cerebrum included: cingulate gyrus, amygdala, hippocampus, septal nuclei, anterior insula The hypothalamus and thalamus (in the diencephalon) are also part of this system The Limbic System Papez circuit a. The fornix connects the hippocampus to the mammillary bodies of the hypothalamus, which sends neurons to the thalamus. b. The thalamus sends neurons to the cingulate gyrus, which sends neurons to the hippocampus, completing the circuit. The Limbic System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Corpus callosum Thalamus Cingulate gyrus Fornix Mammillary body Septal nucleus Amygdala Preoptic nucleus Hippocampus Olfactory bulb Olfactory tract Cortex of right hemisphere Hypothalamus Limbic System Once called the rhinencephalon, or “smell brain,” because it has direct connection with olfaction. There are few synaptic connections between the limbic system and the cerebral cortex, which is why it is hard to control your emotions. Limbic System Emotions controlled by the limbic system: Aggression: areas in the amygdala and hypothalamus Fear: amygdala and hypothalamus Hunger/satiety: hypothalamus Sex drive: the whole system Goal-directed behaviors: hypothalamus and other regions Memory Brain areas involved: a. Studies of people with amnesia reveal that areas of the temporal lobe, hippocampus, caudate nucleus, and dorsomedial thalamus are involved in memory. b. The amygdala is important in learning fear responses. c. The prefrontal cortex may be involved in complex problem solving and working memory– very short-term memory. Brain areas d. Left inferior frontal lobe – mathematical calculations e. Hippocampus is the critical component 1) Acquire new information 2) Consolidation of short-term memory to longterm memory f. Inferior temporal lobe – storage of long-term visual memories Types of Memory Short-term memory: recent events; transferred to long-term memory through process of memory consolidation 1) Memory consolidation occurs in the medial temporal lobe, hippocampus, and amygdala. 2) Sleep is needed for optimum memory consolidation. Long-term memory Requires actual structural change - Activation of genes, synthesis of mRNA, production of proteins, and formation of new synapses Long-term memory can be classified into: a) Nondeclarative (implicit): memory of simple skills, how to do things b) Declarative (explicit): memory of things that can be verbalized. People with amnesia have impaired declarative memory; further broken into: 1) Semantic: facts 2) Episodic: events Categories of Memory Synaptic Changes in Memory Short-term memory involves a recurrent circuit (reverberating circuit) where neurons synapse on each other in a circle. Interruption of the circuit destroys the memory because there was no structural change. Long-term memory requires a relatively permanent change in neuron chemical structure and synapses. Synaptic Changes in Memory – LTP Long-term potentiation (LTP) in the hippocampus is a good example. 1) Synapses that are stimulated at a high frequency exhibit increased excitability. 2) In these synapses, glutamate is secreted by the presynaptic neuron. 3) The postsynaptic neuron has both AMPA and NMDA receptors for glutamate. 4) Glutamate binds to AMPA receptor, allowing Na+ in. Synaptic Changes in Memory – LTP 5) This depolarizes the cell and activates NMDA receptor channels (which were inactive due to a Mg2+ blocking the pore). 6) NMDA allows Ca2+ and Na+ in. 7) The Ca2+ binds to a protein called calmodulin, which in turn activates an enzyme called CaMKII. 8) CaMKII causes more AMPA receptors to fuse to the plasma membrane. This alone strengthens the synapse–it becomes more sensitive to glutamate release (EPSP). Synaptic Changes in Memory – LTP 9) Rise in Ca2+ also causes long-term changes in postsynaptic neurons a) Ca2+ enters the nucleus and binds to calmodulin b) Activates protein kinase that activates a transcription factor called CREB (cyclic AMP response element binding) protein c) Activates genes to produce mRNA and proteins, including dendritic spines with AMPA receptors inserted. Synaptic Changes in Memory – LTP Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) (b) (both): Reprinted from Esther A. Nimchinsky, Bernardo L. Sabatini, and Karel Svoboda, “Structure and Function of Dendritic Spines,” Annual Review of Physiology, Volume 64: 313–353 © 2002 by Annual Reviews, www.annualreviews.org Synaptic Changes in Memory – LTP d. A retrograde messenger (likely NO) is released into the synapse, and cause presynaptic axon to change and release more glutamate, which increase LTP e. Endocannabinoids may lift inhibition from GABAreleasing neurons on the synapse, further strengthening it. Synaptic Changes in Memory Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Presynaptic axon Glutamate 4. Increased release of glutamate from presynaptic axon AMPA receptor Na+ Na+ Ca2+ 3. Increased Na+ diffusion through more AMPA receptors 1. Glutamate binds to AMPA and NMDA receptors NMDA receptor Postsynaptic membrane of dendrite CaMKII Nitric LTP Ca2+ oxide induction as retrograde messenger 2. Ca2+ goes through NMDA receptors into cytoplasm, activates CaMKII Alzheimer’s Disease Most common form of dementia Characteristics 1) Loss of cholinergic fibers in hippocampus and cerebral cortex 2) Accumulation of extracellular proteins called senile plaques 3) Accumulation of intracellular proteins forming neurofibrillary tangles Alzheimer’s Disease Amyloid precursor protein (APP) is broken down into peptides called amyloid beta (Aβ) 1) Aβ forms dimers and oligomers that join to form the fibers in the β-pleated sheet structure that forms the amyloid senile plaques 2) Soluble dimers and oligomers of the 42-amino acid form of Aβ causes Alzheimer’s 3) Forms 1% of early onset Alzheimer’s have a mutation in the APP gene or the presenilin gene; most have “sporadic” form with environmental and genetic interactions Alzheimer’s Disease Tau protein 1) Normal tau proteins bind to and stabilize microtubules in axons 2) In Alzheimer’s, they aggregate and become insoluble forming the neurofibrillary tangles 3) Soluble, intermediate tau proteins are more toxic Alzheimer’s Disease Toxic changes in Alzheimer’s Disease 1) Loss of synapses and dendritic spines 2) Reduced LTP 3) Reduced excitotoxicity leading to neuron apoptosis 4) Mitochondrial release of reactive oxygen species causing oxidative stress and apoptosis People with APOe4 gene have an increased chance of developing Alzheimer’s Current treatments 1) 2) 3) 4) Acetylcholinesterase inhibitors Antagonists of glutamate Drugs for depression Many others are in clinical trials Neural Stem Cells in Learning a. Neural stem cells have been found in the hippocampus, and scientists suspect that neurogenesis is part of learning. b. In mice, physical activity and an enriched environment promote neurogenesis. c. Aging and stress reduce neurogenesis. Emotions and Memory Emotions sometimes strengthen and other times weaken memory formation. a. If the memory has an emotional component, the amygdala is involved in memory formation. b. Stress impairs memory consolidation in the hippocampus and working memory function of the prefrontal cortex. c. Posttraumatic stress disorder may result in hippocampal atrophy. d. Memories are stored but retrieval is hindered Emotions and Memory The amygdala and hippocampus have receptors for stress hormones, such as cortisol. It is thought that cortisol may strengthen emotional memory formation via the amygdala but weaken hippocampal memory formation and memory retrieval. Brain Regions Involved in Emotion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) (b) (both): Reprinted Figure 2 (1st and 3rd panels) with permission from RJ Dolan, “Emotion, Cognition, and Behavior” Science 298: 1191–1194. Copyright 2002 AAAS a. Yellow = prefrontal cortex; mint green = cingulate gyrus b. Purple = insula; mint green = cingulate gyrus; red = amygdala Prefrontal Cortex Orbitofrontal region: ability to consciously experience pleasure and reward; receives input from all the senses and the limbic system Damage here results in severe impulsive behavior. Lateral prefrontal area: motivation, sexual desire, and cognitive functions