Chapter 14 The Brain and Cranial Nerves The Brain • The Brain is part of Central Nervous System (CNS) • It is divided into 6 major parts: – – – – – – The cerebrum The diencephalon The mesencephalon The cerebellum The pons The medulla oblongata Major divisions of the Brain The real thing Fig.14.01b The Human Brain • Ranges from 750 cc to 2100 cc • Contains almost 98% of the body’s neural tissue • Average weight about 1.4 kg (3 lb) Protection of the Brain • The brain is protected by bone, meninges, and cerebrospinal fluid • Harmful substances are shielded from the brain by the blood-brain barrier Cerebrospinal fluid • Protects by absorbing shock (it “floats” the brain. • Maintains the electrochemical environment (opitmal pH & osmolality). • Circulates nutrients and wastes. The Ventricles Formation of CSF Choroid Plexuses Figure 12.25a Fig. 14.04c Cirulaation of CSF Meninges • Three connective tissue membranes lie external to the CNS – dura mater, arachnoid mater, and pia mater • Functions of the meninges – Cover and protect the CNS – Protect blood vessels and enclose venous sinuses – Contain cerebrospinal fluid (CSF) – Form partitions within the skull The Meninges of the Brain Dura Mater • Three dural septa extend inward and limit excessive movement of the brain – Falx cerebri – fold that dips into the longitudinal fissure – Falx cerebelli – runs along the vermis of the cerebellum – Tentorium cerebelli – horizontal dural fold extends into the transverse fissure • Falx cerebri, tentorium cerebelli, and falx cerebelli Dural Folds Figure 14–3b Arachnoid Mater • The middle meninx, which forms a loose brain covering • It is separated from the dura mater by the subdural space • Beneath the arachnoid is a wide subarachnoid space filled with CSF and large blood vessels • Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood Blood flow, O2 and the BBB • Though only 2% of body mass the brain consumes 20% of the oxygen and glucoseof the resting body. • No O2 for a minute = dizziness – 4 minutes = brain damage – 6 minutes = death • The brain is protected by the blood-brain barrier. Blood-Brain Barrier: Functions • Selective barrier that allows nutrients to pass freely • Is ineffective against substances that can diffuse through plasma membranes • Absent in some areas (vomiting center and the hypothalamus), allowing these areas to monitor the chemical composition of the blood • Stress increases the ability of chemicals to pass through the blood-brain barrier The Bloodbrain barrier The Brain Stem • The brain stem is comprised of 3 main subdivisions: – The Medulla Oblongata – The Pons – The Mesencephalon The diencephalon & brainstem Medulla Oblongata: 3 Groups of Nuclei 1. Autonomic nuclei: – control visceral activities 2. Sensory and motor nuclei: – of cranial nerves 3. Relay stations: – along sensory and motor pathways Nuclei of the medulla oblongata Summary: The Medulla Oblongata Table 14-2 • Links cerebellum with mesencephalon, diencephalon, cerebrum, and spinal cord The Pons Figure 14–6c The Pons: 4 Groups 1. Sensory and motor nuclei of cranial nerves V, VI, VII, VIII 2. Nuclei involved with respiration: – – apneustic center and pneumotaxic center modify respiratory rhythmicity center activity The Pons: 4 Groups 3. Nuclei and tracts: – process and relay information to and from cerebellum 4. Ascending, descending, and transverse tracts: – transverse fibers (axons) • link nuclei of pons with opposite cerebellar hemisphere The Cerebellum • An autonomic & motor processing center Figure 14–7a Functions of the Cerebellum 1. Adjusts postural muscles 2. Fine-tunes conscious and subconscious movements Structures of the Cerebellum Figure 14–7b Structures of the Cerebellum (1 of 2) • Folia: – surface of cerebellum – highly folded neural cortex • Anterior and posterior lobes: – separated by primary fissure Structures of the Cerebellum (2 of 2) • Cerebellar hemispheres: – separated at midline by vermis • Vermis: – narrow band of cortex • Flocculonodular lobe: – below fourth ventricle Summary: The Cerebellum Table 14-3 The Mesencephalon Figure 14–8a Midbrain nuclei • Substantia nigra: dopamine release, control of subconscious muscle movents. • Red nuclei: synapses between neurons of cerebellum and cerebrum. Also origin of oculomotor nerve (CN III) and trochlear nerve (CN IV). Some major functions mesencephalon • Cerebral peduncles: contain motor axons connecting cerebrum to brain stem and sensory axons that connect medulla to thalamus • Tectum: – Superior colliculi: visual reflexes, including pupillary reflexes. – Inferior colliculi: auditory path from ear to thalamus. – Startle reflex Summary: The Mesencephalon Table 14-4 The Diencephalon • The Thalamus - switch board for the cerebrum. Includes the: – Geniculate nuclei (part of the ventral group): • Medial processes auditory information. • Lateral processes visual input. – Other ventral nuclei connect motor & sensory areas of cortex to cerebellum & spinal cord – Anterior & medial nuclei • Anterior connects to limbic system & hypothalamus • Medial connects with cortex & limbic system The Diencephalon The Thalamus: Filters ascending sensory information for primary sensory cortex. Relays information between basal nuclei and cerebral cortex Figure 14–9 Those pesky thalamic nuclei Fig. 14.09a-d Fig. 14.09e Oblique section of brain showing the Thalamus and basal nuclei Summary: Thalamic Nuclei Table 14-5 The Hypothalamus Functions of the hypothalamus • ANS control – , heart rate, respiration, sweating, digestion, urination, stress response • Hormones - ADH, oxytocin, GnRH et al. • Basic emotions - rage, sex drive, fear, pain pleasure. • Thirst & appetite • Temperature & sleep/wake (circadian) cycles The Hypothalamus • Lies below thalamus Figure 14–10a Summary: The Hypothalamus Table 14-6 The Limbic System • Is a functional grouping that: – establishes emotional states – links conscious functions of cerebral cortex with autonomic functions of brain stem – facilitates memory storage and retrieval The Limbic System Figure 14–11a The Limbic System Figure 14–11b Limbic System • Structures located on the medial aspects of cerebral hemispheres and diencephalon • Includes the rhinencephalon, amygdala, hypothalamus, and anterior nucleus of the thalamus • Parts especially important in emotions: – Amygdala – deals with anger, danger, and fear responses – Cingulate gyrus – plays a role in expressing emotions via gestures, and resolves mental conflict • Puts emotional responses to odors – e.g., skunks smell bad Limbic System: Emotion and Cognition • The limbic system interacts with the prefrontal lobes, therefore: – One can react emotionally to conscious understandings – One is consciously aware of emotion in one’s life • Hippocampal structures – convert new information into long-term memories Summary: The Limbic System Table 14-7 The Cerebrum Major divisions of the Cerebrum • Hemispheres - left and right split down the middle by the longitudinal fissure. • Lobes: – Frontal, parietal, occipital, temporal and insula – Delineated by the sulci (singular sulcus): • Central • Parieto-occipital • Lateral Lobes of the Cerebrum Landmarks of the cerebrum White Matter of the Cerebrum Myelinated fibers (axons) 1. Association fibers 2. Commissural fibers 3. Projection fibers Figure 14–13 Cerebral White Matter The Basal Nuclei Figure 14–14b, c Functions of Basal Nuclei • Are involved with: – the subconscious control of skeletal muscle tone – the coordination of learned movement patterns (walking, lifting) Functional map of the cerebrum Fig. 14.15 Motor and Sensory Areas of the Cortex Figure 14–15a Interpretive Areas of Cortex Figure 14–15b Hemispheric Lateralization Figure 14–16 The Left Hemisphere • In most people, left brain (dominant hemisphere) controls: – reading, writing, and math – decision-making – speech and language The Right Hemisphere • Right cerebral hemisphere relates to: – senses (touch, smell, sight, taste, feel) – recognition (faces, voice inflections) The Electroencephalogram Figure 14–17 Types of Brain Waves • Alpha waves – 8 - 13 Hz, regular and rhythmic, lowamplitude, slow, synchronous waves indicating an “idling” brain • Beta waves – 14 - 30 Hz, rhythmic, more irregular waves occurring during the awake and mentally alert state • Theta waves – 4 - 7 Hz, more irregular than alpha waves; common in children but abnormal in adults • Delta waves – 1 - 5 Hz, high-amplitude waves seen in deep sleep and when reticular activating system is damped Brain Waves: State of the Brain • Brain waves change with age, sensory stimuli, brain disease, and the chemical state of the body • EEGs can be 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 Cranial Nerves Figure 14–18 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. On Old Olympus’ Towering Top A Frenchman Viewed Greek Villagers At Home The 12 cranial nerves The Olfactory Nerve (I) Figure 14–19 The Optic Nerves (II) Figure 14–20 The Oculomotor Nerves (III) Figure 14–21 The Trochlear Nerves (IV) Figure 14–21 The Abducens Nerves (VI) Figure 14–21 The Trigeminal Nerves (V) Figure 14–22 The Facial Nerves (VII) Figure 14–23 The Vestibulocochlear Nerves (VIII) Figure 14–24 The Glossopharyngeal Nerves (IX) Figure 14–25 The Vagus Nerves (X) Figure 14–26 The Accessory Nerves (XI) Figure 14–27 The Hypoglossal Nerves (XII) Figure 14–27 Summary: Cranial Nerves Table 14-9 (1 of 2) Summary: Cranial Nerves Table 14-9 (2 of 2) Examples of Cranial Reflexes Table 14-10 Clinical Stuff • Epilepsy • CVA & TIA • Degenerative Diseases – Alzheimers – Huntington’s – Parkinbson’s Epilepsy • A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking, characteristic of epileptic seizure • Epilepsy is not associated with, nor does it cause, intellectual impairments • Epilepsy occurs in 1% of the population Epileptic Seizures • Absence seizures, or petit mal – mild seizures seen in young children where the expression goes blank • Grand mal seizures – victim loses consciousness, bones are often broken due to intense convulsions, loss of bowel and bladder control, and severe biting of the tongue Control of Epilepsy • Epilepsy can usually be controlled with anticonvulsive drugs • Valproic acid, a nonsedating drug, enhances GABA and is a drug of choice • Vagus nerve stimulators can be implanted under the skin of the chest and can keep electrical activity of the brain from becoming chaotic Cerebrovascular Accidents (Strokes) • Caused when blood circulation to the brain is blocked and brain tissue dies • Most commonly caused by blockage of a cerebral artery • Other causes include compression of the brain by hemorrhage or edema, and atherosclerosis • Transient ischemic attacks (TIAs) – temporary episodes of reversible cerebral ischemia • Tissue plasminogen activator (TPA) is the only approved treatment for stroke Degenerative Brain Disorders • Alzheimer’s disease – a progressive degenerative disease of the brain that results in dementia • Parkinson’s disease – degeneration of the dopamine-releasing 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