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Chapter 12 The Central Nervous System Part C Shilla Chakrabarty, Ph.D. Copyright © 2010 Pearson Education, Inc. Electroencephalogram (EEG) • Records electrical activity that accompanies brain function • Measures electrical potential differences between various cortical areas (a) Scalp electrodes are used to record brain wave activity (EEG). Copyright © 2010 Pearson Education, Inc. Brain Waves • Patterns of neuronal electrical activity • Generated by synaptic activity in the cortex • Each person’s brain waves are unique • Can be grouped into four classes based on frequency measured as Hertz (Hz) Alpha waves (8–13 Hz)—regular and rhythmic, low-amplitude, synchronous waves indicating an “idling” brain Beta waves (14–30 Hz)—rhythmic, less regular waves occurring when mentally alert Theta waves (4–7 Hz)—more irregular; common in children and uncommon in adults Delta waves (4 Hz or less)—high-amplitude waves seen in deep sleep and when reticular activating system is damped, or during anesthesia; may indicate brain damage Copyright © 2010 Pearson Education, Inc. Brain Waves Indicate State Of The Brain 1-second interval Alpha waves—awake but relaxed Beta waves—awake, alert Theta waves—common in children Delta waves—deep sleep (b) Brain waves shown in EEGs fall into four general classes. • Brain waves change with age, sensory stimuli, brain disease, and chemical state of the body • EEGs are used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic lesions • A flat EEG (no electrical activity) is clinical evidence of death Copyright © 2010 Pearson Education, Inc. Epilepsy • Epilepsy occurs in 1% of the population • A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking • Epilepsy is not associated with intellectual impairments • Epileptic seizures are categorized as: Absence seizures, or petit mal Mild seizures seen in young children where the expression goes blank Tonic-clonic (grand mal) seizures Victim loses consciousness, bones are often broken due to intense contractions, may experience loss of bowel and bladder control, and severe biting of the tongue Control of Epilepsy • Anticonvulsive drugs • Vagus nerve stimulators implanted under the skin of the chest can keep electrical activity of the brain from becoming chaotic Copyright © 2010 Pearson Education, Inc. Consciousness • Conscious perception of sensation • Voluntary initiation and control of movement • Capabilities associated with higher mental processing (memory, logic, judgment, etc.) • Clinically defined on a continuum that grades behavior in response to stimuli Alertness Drowsiness (lethargy) Stupor Coma • Loss of consciousness (e.g., fainting or syncopy) is a signal that brain function is impaired Copyright © 2010 Pearson Education, Inc. Sleep • State of partial unconsciousness from which a person can be aroused by stimulation • Two major types of sleep (defined by EEG patterns) Nonrapid eye movement (NREM) Rapid eye movement (REM) • First two stages of NREM occur during the first 30–45 minutes of sleep • Fourth stage is achieved in about 90 minutes, and then REM sleep begins abruptly Copyright © 2010 Pearson Education, Inc. Awake REM: Skeletal muscles (except ocular muscles and diaphragm) are actively inhibited; most dreaming occurs. NREM stage 1: Relaxation begins; EEG shows alpha waves, arousal is easy. NREM stage 2: Irregular EEG with sleep spindles (short high- amplitude bursts); arousal is more difficult. NREM stage 3: Sleep deepens; theta and delta waves appear; vital signs decline. (a) Typical EEG patterns Copyright © 2010 Pearson Education, Inc. NREM stage 4: EEG is dominated by delta waves; arousal is difficult; bed-wetting, night terrors, and sleepwalking may occur. Figure 12.21a Sleep Patterns • Alternating cycles of sleep and wakefulness reflect a natural circadian (24-hour) rhythm • RAS activity is inhibited during, but RAS also mediates, dreaming sleep • The suprachiasmatic and preoptic nuclei of the hypothalamus time the sleep cycle • A typical sleep pattern alternates between REM and NREM sleep Awake REM Stage 1 Stage 2 Non REM Stage 3 Stage 4 Time (hrs) (b) Typical progression of an adult through one night’s sleep stages Copyright © 2010 Pearson Education, Inc. Importance of Sleep • Slow-wave sleep (NREM stages 3 and 4) is presumed to be the restorative stage • People deprived of REM sleep become moody and depressed • REM sleep may be a reverse learning process where superfluous information is purged from the brain • Daily sleep requirements decline with age • Stage 4 sleep declines steadily and may disappear after age 60 Sleep Disorders • Narcolepsy : Lapsing abruptly into sleep from the awake state • Insomnia: Chronic inability to obtain the amount or quality of sleep needed • Sleep apnea: Temporary cessation of breathing during sleep Copyright © 2010 Pearson Education, Inc. Language • Language implementation system o Basal nuclei o Broca’s area and Wernicke’s area (in the association cortex on the left side) o Analyzes incoming word sounds o Produces outgoing word sounds and grammatical structures • Corresponding areas on the right side are involved with nonverbal language components Copyright © 2010 Pearson Education, Inc. Memory • Storage and retrieval of information • Two stages of storage • Short-term memory (STM, or working memory)—temporary holding of information; limited to seven or eight pieces of information • Long-term memory (LTM) has limitless capacity • Factors that affect transfer from STM to LTM Emotional state—best if alert, motivated, surprised, and aroused Rehearsal—repetition and practice Association—tying new information with old memories Automatic memory—subconscious information stored in LTM Copyright © 2010 Pearson Education, Inc. Memory: STM and LTM Outside stimuli General and special sensory receptors Afferent inputs Temporary storage (buffer) in cerebral cortex Automatic memory Data permanently lost Data selected for transfer Short-term memory (STM) Forget Forget Data transfer influenced by: Excitement Retrieval Rehearsal Association of old and new data Long-term memory Data unretrievable (LTM) Copyright © 2010 Pearson Education, Inc. Categories of Memory 1. Declarative memory (factual knowledge) • Explicit information • Related to our conscious thoughts and our language ability • Stored in LTM with context in which it was learned 2. Nondeclarative memory • Less conscious or unconscious • Acquired through experience and repetition • Best remembered by doing; hard to unlearn • Includes procedural (skills) memory, motor memory, and emotional memory Copyright © 2010 Pearson Education, Inc. Brain Structures Involved in Declarative Memory • Hippocampus and surrounding temporal lobes function in consolidation and access to memory • ACh from basal forebrain is necessary for memory formation and retrieval Thalamus Basal forebrain Touch Prefrontal cortex Hearing Vision Taste Smell Hippocampus (a) Declarative memory circuits Sensory input Association cortex Thalamus Medial temporal lobe (hippocampus, etc.) Prefrontal cortex ACh ACh Basal forebrain Copyright © 2010 Pearson Education, Inc. Brain Structures Involved in Nondeclarative Memory • Procedural memory • Basal nuclei relay sensory and motor inputs to the thalamus and premotor cortex • Dopamine from substantia nigra is necessary • Motor memory—cerebellum • Emotional memory—amygdala Copyright © 2010 Pearson Education, Inc. Sensory and motor inputs Association cortex Basal nuclei Thalamus Dopamine Premotor cortex Premotor cortex Substantia nigra Thalamus Basal nuclei Substantia nigra (b) Procedural (skills) memory circuits Copyright © 2010 Pearson Education, Inc. Figure 12.23b Protection of the Brain • Bone (skull) • Membranes (meninges) • Watery cushion (cerebrospinal fluid) • Blood-brain barrier Copyright © 2010 Pearson Education, Inc. Superior sagittal sinus Subdural space Subarachnoid space Copyright © 2010 Pearson Education, Inc. Skin of scalp Periosteum Bone of skull Periosteal Dura Meningeal mater Arachnoid mater Pia mater Arachnoid villus Blood vessel Falx cerebri (in longitudinal fissure only) Figure 12.24 Dura Mater • Strongest meninx • Two layers of fibrous connective tissue (around the brain) separate to form dural sinuses Copyright © 2010 Pearson Education, Inc. Dura Mater • Dural septa limit excessive movement of the brain • Falx cerebri—in the longitudinal fissure; attached to crista galli • Falx cerebelli—along the vermis of the cerebellum • Tentorium cerebelli—horizontal dural fold over cerebellum and in the transverse fissure Copyright © 2010 Pearson Education, Inc. Superior sagittal sinus Straight sinus Crista galli of the ethmoid bone Pituitary gland Falx cerebri Tentorium cerebelli Falx cerebelli (a) Dural septa Copyright © 2010 Pearson Education, Inc. Figure 12.25a Arachnoid Mater • Middle layer with weblike extensions • Separated from the dura mater by the subdural space • Subarachnoid space contains CSF and blood vessels • Arachnoid villi protrude into the superior sagittal sinus and permit CSF reabsorption Pia Mater • Layer of delicate vascularized connective tissue that clings tightly to the brain Copyright © 2010 Pearson Education, Inc. Superior sagittal sinus Subdural space Subarachnoid space Copyright © 2010 Pearson Education, Inc. Skin of scalp Periosteum Bone of skull Periosteal Dura Meningeal mater Arachnoid mater Pia mater Arachnoid villus Blood vessel Falx cerebri (in longitudinal fissure only) Figure 12.24 Cerebrospinal Fluid (CSF) • Composition Watery solution Less protein and different ion concentrations than plasma Constant volume • Functions Gives buoyancy to the CNS organs Protects the CNS from blows and other trauma Nourishes the brain and carries chemical signals Copyright © 2010 Pearson Education, Inc. Superior sagittal sinus 4 Choroid plexus Arachnoid villus Interventricular foramen Subarachnoid space Arachnoid mater Meningeal dura mater Periosteal dura mater 1 Right lateral ventricle (deep to cut) Choroid plexus of fourth ventricle 3 Third ventricle 1 CSF is produced by the Cerebral aqueduct Lateral aperture Fourth ventricle Median aperture Central canal of spinal cord (a) CSF circulation Copyright © 2010 Pearson Education, Inc. 2 choroid plexus of each ventricle. 2 CSF flows through the ventricles and into the subarachnoid space via the median and lateral apertures. Some CSF flows through the central canal of the spinal cord. 3 CSF flows through the subarachnoid space. 4 CSF is absorbed into the dural venous sinuses via the arachnoid villi. Figure 12.26a Choroid Plexuses • Produce CSF at a constant rate • Hang from the roof of each ventricle • Clusters of capillaries enclosed by pia mater and a layer of ependymal cells • Ependymal cells use ion pumps to control the composition of the CSF and help cleanse CSF by removing wastes Ependymal cells Capillary Section of choroid plexus Connective tissue of pia mater Wastes and unnecessary solutes absorbed CSF forms as a filtrate containing glucose, oxygen, vitamins, and ions (Na+, Cl–, Mg2+, etc.) (b) CSF formation by choroid plexuses Cavity of ventricle Figure 12.26b Copyright © 2010 Pearson Education, Inc. Blood-Brain Barrier • Helps maintain a stable environment for the brain • Separates neurons from some blood-borne substances • Composition Continuous endothelium of capillary walls Basal lamina Feet of astrocytes Provide signal to endothelium for the formation of tight junctions • Selective barrier Allows nutrients to move by facilitated diffusion Allows any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics • Absent in some areas, e.g., vomiting center and the hypothalamus, where it is necessary to monitor the chemical composition of the blood Copyright © 2010 Pearson Education, Inc. Capillary Neuron Astrocyte (a) Astrocytes are the most abundant CNS neuroglia. Copyright © 2010 Pearson Education, Inc. Figure 11.3a Homeostatic Imbalances of the Brain Traumatic brain injuries • Concussion—temporary alteration in function • Contusion—permanent damage • Subdural or subarachnoid hemorrhage—may force brain stem through the foramen magnum, resulting in death • Cerebral edema—swelling of the brain associated with traumatic head injury Copyright © 2010 Pearson Education, Inc. Homeostatic Imbalances of the Brain • Cerebrovascular accidents (CVAs)(strokes) Blood circulation is blocked and brain tissue dies, e.g., blockage of a cerebral artery by a blood clot Typically leads to hemiplegia, or sensory and speed deficits Transient ischemic attacks (TIAs)—temporary episodes of reversible cerebral ischemia Tissue plasminogen activator (TPA) is the only approved treatment for stroke Copyright © 2010 Pearson Education, Inc. Homeostatic Imbalances of the Brain • Degenerative brain disorders • Alzheimer’s disease (AD): a progressive degenerative disease of the brain that results in dementia • Parkinson’s disease: degeneration of the dopaminereleasing neurons of the substantia nigra • Huntington’s disease: a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei and cerebral cortex Copyright © 2010 Pearson Education, Inc.