THE CENTRAL NERVOUS SYSTEM

1. CHAPTER # 12(c) THE CENTRAL NERVOUS SYSTEM

2. Functional Brain Systems • Networks of neurons that work together and span wide areas of the brain • Limbic system • Reticular formation

3. Limbic System • Structures on the medial aspects of cerebral hemispheres and diencephalon • Includes parts of the diencephalon and some cerebral structures that encircle the brain stem

4. Fiber tracts connecting limbic system structures Septum pellucidum Diencephalic structures of the limbic system Corpus callosum •Fornix •Anterior thalamic nuclei (flanking 3rd ventricle) •Anterior commissure Cerebral struc- tures of the limbic system •Hypothalamus •Mammillary body •Cingulate gyrus •Septal nuclei •Amygdala •Hippocampus •Dentate gyrus •Parahippocampal gyrus Olfactory bulb Figure 12.18

5. Limbic System • Emotional or affective brain • Amygdala—recognizes angry or fearful facial expressions, assesses danger, and elicits the fear response • Cingulate gyrus—plays a role in expressing emotions via gestures, and resolves mental conflict • Puts emotional responses to odors • Example: skunks smell bad

6. Limbic System: Emotion and Cognition • The limbic system interacts with the prefrontal lobes, therefore: • We can react emotionally to things we consciously understand to be happening • We are consciously aware of emotional richness in our lives • Hippocampus and amygdala—play a role in memory

7. Reticular Formation • Three broad columns along the length of the brain stem • Raphe nuclei • Medial (large cell) group of nuclei • Lateral (small cell) group of nuclei • Has far-flung axonal connections with hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord

8. Reticular Formation: RAS and Motor Function • RAS (reticular activating system) • Sends impulses to the cerebral cortex to keep it conscious and alert • Filters out repetitive and weak stimuli (~99% of all stimuli!) • Severe injury results in permanent unconsciousness (coma)

9. Reticular Formation: RAS and Motor Function • Motor function • Helps control coarse limb movements • Reticular autonomic centers regulate visceral motor functions • Vasomotor • Cardiac • Respiratory centers

10. Radiations to cerebral cortex Visual impulses Auditory impulses Reticular formation Descending motor projections to spinal cord Ascending general sensory tracts (touch, pain, temperature) Figure 12.19

11. Electroencephalogram (EEG) • Records electrical activity that accompanies brain function • Measures electrical potential differences between various cortical areas

12. (a) Scalp electrodes are used to record brain waveactivity (EEG). Figure 12.20a

13. 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)

14. Types of Brain Waves • 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

15. 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 intofour general classes. Figure 12.20b

16. Brain Waves: State of the Brain • Change with age, sensory stimuli, brain disease, and the chemical state of the body • EEGs 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

17. Epilepsy • A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking • Epilepsy is not associated with intellectual impairments • Epilepsy occurs in 1% of the population

18. Epileptic Seizures • 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

19. 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

20. Consciousness • Conscious perception of sensation • Voluntary initiation and control of movement • Capabilities associated with higher mental processing (memory, logic, judgment, etc.) • Loss of consciousness (e.g., fainting or syncopy) is a signal that brain function is impaired

21. Consciousness • Clinically defined on a continuum that grades behavior in response to stimuli • Alertness • Drowsiness (lethargy) • Stupor • Coma

22. 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)

23. Sleep • 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

24. 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. NREM stage 4: EEG is dominated by delta waves; arousal is difficult; bed-wetting, night terrors, and sleepwalking may occur. (a) Typical EEG patterns Figure 12.21a

25. 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

26. Awake REM Stage 1 Stage 2 Non Stage 3 REM Stage 4 Time (hrs) (b) Typical progression of an adult through onenight’s sleep stages Figure 12.21b

27. 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

28. 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

29. Language • Language implementation system • Basal nuclei • Broca’s area and Wernicke’s area (in the association cortex on the left side) • Analyzes incoming word sounds • Produces outgoing word sounds and grammatical structures • Corresponding areas on the right side are involved with nonverbal language components

30. 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

31. Outside stimuli General and special sensory receptors Afferent inputs Temporary storage (buffer) in cerebral cortex Data permanently lost Data selected for transfer Automatic memory Forget Short-term memory (STM) Forget Data transfer influenced by: Excitement Rehearsal Association of old and new data Retrieval Long-term memory (LTM) Data unretrievable Figure 12.22

32. Transfer from STM to LTM • 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

33. Categories of Memory • 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

34. Categories of Memory • 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

35. Protection of the Brain • Bone (skull) • Membranes (meninges) • Watery cushion (cerebrospinal fluid) • Blood-brain barrier

36. Meninges • Cover and protect the CNS • Protect blood vessels and enclose venous sinuses • Contain cerebrospinal fluid (CSF) • Form partitions in the skull

37. Meninges • Three layers • Dura mater • Arachnoid mater • Pia mater

38. Skin of scalp Periosteum Bone of skull Dura mater Periosteal Meningeal Superior sagittal sinus Arachnoid mater Pia mater Arachnoid villus Subdural space Blood vessel Falx cerebri (in longitudinal fissure only) Subarachnoid space Figure 12.24

39. Dura Mater • Strongest meninx • Two layers of fibrous connective tissue (around the brain) separate to form dural sinuses

40. 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

41. Superior sagittal sinus Falx cerebri Straight sinus Crista galli of the ethmoid bone Tentorium cerebelli Falx cerebelli Pituitary gland (a) Dural septa Figure 12.25a

42. 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

43. Skin of scalp Periosteum Bone of skull Dura mater Periosteal Meningeal Superior sagittal sinus Arachnoid mater Pia mater Arachnoid villus Subdural space Blood vessel Falx cerebri (in longitudinal fissure only) Subarachnoid space Figure 12.24

44. Pia Mater • Layer of delicate vascularized connective tissue that clings tightly to the brain

45. Cerebrospinal Fluid (CSF) • Composition • Watery solution • Less protein and different ion concentrations than plasma • Constant volume

46. Cerebrospinal Fluid (CSF) • Functions • Gives buoyancy to the CNS organs • Protects the CNS from blows and other trauma • Nourishes the brain and carries chemical signals

47. 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 choroid plexus of each ventricle. Cerebral aqueduct Lateral aperture 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. Fourth ventricle Median aperture 2 Central canal of spinal cord 3 CSF flows through the subarachnoid space. (a) CSF circulation 4 CSF is absorbed into the dural venous sinuses via the arachnoid villi. Figure 12.26a

48. 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

49. 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.) Cavity of ventricle (b) CSF formation by choroid plexuses Figure 12.26b

50. Blood-Brain Barrier • Helps maintain a stable environment for the brain • Separates neurons from some bloodborne substances