Chapter 14 The Brain and Cranial Nerves

• Largest organ in the body at almost 3 lb.

• Brain functions in sensations, memory, emotions, decision making, behavior

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Principal Parts of the Brain

• Cerebrum • Diencephalon – thalamus & hypothalamus • Cerebellum • Brainstem – medulla, pons & midbrain

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Protective Coverings of the Brain

• Bone, meninges & fluid • Meninges same as around the spinal cord – dura mater – arachnoid mater – pia mater • Dura mater extensions – falx cerebri – tentorium cerebelli – falx cerebelli

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Blood Supply to Brain

• Arterial blood supply is branches from circle of Willis on base of brain (page 699) • Vessels on surface of brain----penetrate tissue • Uses 20% of our bodies oxygen & glucose needs – blood flow to an area increases with activity in that area – deprivation of O2 for 4 min does permanent injury • at that time, lysosome release enzymes • Blood-brain barrier (BBB) – protects cells from some toxins and pathogens • proteins & antibiotics can not pass but alcohol & anesthetics do – tight junctions seal together epithelial cells, continuous basement membrane, astrocyte processes covering capillaries

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Cerebrospinal Fluid (CSF)

• 80-150 ml (3-5oz) • Clear liquid containing glucose, proteins, & ions • Functions – mechanical protection • floats brain & softens impact with bony walls – chemical protection • optimal ionic concentrations for action potentials – circulation • nutrients and waste products to and from bloodstream

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Origin of CSF

• Choroid plexus = capillaries covered by ependymal cells – 2 lateral ventricles, one within each cerebral hemisphere – roof of 3rd ventricle – fourth ventricle

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Drainage of CSF from Ventricles

• One median aperture & two lateral apertures allow CSF to exit from the interior of the brain

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Flow of Cerebrospinal Fluid

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Reabsorption of CSF

• Reabsorbed through arachnoid villi – grapelike clusters of arachnoid penetrate dural venous sinus • 20 ml/hour reabsorption rate = same as production rate

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Hydrocephalus

• Blockage of drainage of CSF (tumor, inflammation, developmental malformation, meningitis, hemorrhage or injury • Continued production cause an increase in pressure --- hydrocephalus • In newborn or fetus, the fontanels allow this internal pressure to cause expansion of the skull and damage to the brain tissue • Neurosurgeon implants a drain shunting the CSF to the veins of the neck or the abdomen

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Medulla Oblongata

• Continuation of spinal cord • Ascending sensory tracts • Descending motor tracts • Nuclei of 5 cranial nerves • Cardiovascular center – force & rate of heart beat – diameter of blood vessels • Respiratory center – medullary rhythmicity area sets basic rhythm of breathing • Information in & out of cerebellum • Reflex centers for coughing, sneezing, swallowing etc

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Ventral Surface of Medulla Oblongata

• Ventral surface bulge – pyramids – large motor tract – decussation of most fibers • left cortex controls right muscles • Olive = olivary nucleus – neurons send input to cerebellum – proprioceptive signals – gives precision to movements

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Dorsal Surface of Medulla Oblongata

• Nucleus gracilis & nucleus cuneatus = sensory neurons – relay information to thalamus on opposite side of brain • 5 cranial nerves arise from medulla -- 8 thru 12

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XII = Hypoglossal Nerve

• Controls muscles of tongue during speech and swallowing • Injury deviates tongue to injured side when protruded • Mixed, primarily motor

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XI = Spinal Accessory Nerve

• Cranial portion – arises medulla – skeletal mm of throat & soft palate • Spinal portion – arises cervical spinal cord – sternocleidomastoid and trapezius mm.

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X = Vagus Nerve

• Receives sensations from viscera • Controls cardiac muscle and smooth muscle of the viscera • Controls secretion of digestive fluids

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IX = Glossopharyngeal Nerve

• Stylopharyngeus m. (lifts throat during swallowing) • Secretions of parotid gland • Somatic sensations & taste on posterior 1/3 of tongue

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VIII = Vestibulocochlear Nerve

• Cochlear branch begins in medulla – receptors in cochlea – hearing – if damaged deafness or tinnitus (ringing) is produced • Vestibular branch begins in pons – receptors in vestibular apparatus – sense of balance – vertigo (feeling of rotation) – ataxia (lack of coordination)

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Injury to the Medulla

• Hard blow to the back of the head may be fatal • Cranial nerve malfunctions on same side as injury; loss of sensation or paralysis of throat or tongue; irregularities in breathing and heart rhythm

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Pons

• One inch long • White fiber tracts ascend and descend • Pneumotaxic & apneustic areas help control breathing • Middle cerebellar peduncles carry sensory info to the cerebellum • Cranial nerves 5 thru 7

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VII = Facial Nerve

• Motor portion – facial muscles – salivary & nasal and oral mucous glands & tears • Sensory portion – taste buds on anterior 2/3’s of tongue

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VI = Abducens Nerve

• Lateral rectus eye muscle

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V = Trigeminal Nerve

• Motor portion – muscles of mastication • Sensory portion – touch, pain, & temperature receptors of the face • ophthalmic branch • maxillary branch • mandibular branch

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Midbrain

• One inch in length • Extends from pons to diencephalon • Cerebral aqueduct connects 3rd ventricle above to 4th ventricle below

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Midbrain in Section

• Cerebral peduncles---clusters of motor & sensory fibers • Substantia nigra---helps controls subconscious muscle activity • Red nucleus-- rich blood supply & iron-containing pigment – cortex & cerebellum coordinate muscular movements by sending information here from the cortex and cerebellum

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Dorsal Surface of Midbrain

• Corpora quadrigemina = superior & inferior colliculi – coordinate eye movements with visual stimuli – coordinate head movements with auditory stimuli

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IV = Trochlear Nerve

• Superior oblique eye muscle

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III = Oculomotor Nerve

• Levator palpebrae raises eyelid (ptosis) • 4 extrinsic eye muscles • 2 intrinsic eye muscles – accomodation for near vision (changing shape of lens during reading) – constriction of pupil

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Reticular Formation

• Scattered nuclei in medulla, pons & midbrain • Reticular activating system – alerts cerebral cortex to sensory signals (sound of alarm, flash light, smoke or intruder) to awaken from sleep – maintains consciousness & helps keep you awake with stimuli from ears, eyes, skin and muscles • Motor function is involvement with maintaining muscle tone

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Cerebellum

• 2 cerebellar hemispheres and vermis (central area) • Function – correct voluntary muscle contraction and posture based on sensory data from body about actual movements – sense of equilibrium

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Cerebellum

• Transverse fissure between cerebellum & cerebrum • Cerebellar cortex (folia) & central nuclei are grey matter • Arbor vitae = tree of life = white matter

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Cerebellar Peduncles

• Superior, middle & inferior peduncles attach to brainstem – inferior carries sensory information from spinal cord – middle carries sensory fibers from cerebral cortex & basal ganglia – superior carries motor fibers that extend to motor control areas

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Diencephalon Surrounds 3rd Ventricle

• Surrounds 3rd ventricle • Superior part of walls is thalamus • Inferior part of walls & floor is hypothalamus

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Thalamus

• 1 inch long mass of gray mater in each half of brain (connected across the 3rd ventricle by intermediate mass) • Relay station for sensory information on way to cortex • Crude perception of some sensations

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Thalamic Nuclei

• Nuclei have different roles – relays auditory and visual impulses, taste and somatic sensations – receives impulses from cerebellum or basal ganglia – anterior nucleus concerned with emotions, memory and acquisition of knowledge (cognition)

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Hypothalamus

• Dozen or so nuclei in 4 major regions – mammillary bodies are relay station for olfactory reflexes; infundibulum suspends the pituitary gland • Major regulator of homeostasis – receives somatic and visceral input, taste, smell & hearing information; monitors osmotic pressure, temperature of blood

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Functions of Hypothalamus

• Controls and integrates activities of the ANS which regulates smooth, cardiac muscle and glands • Synthesizes regulatory hormones that control the anterior pituitary • Contains cell bodies of axons that end in posterior pituitary where they secrete hormones • Regulates rage, aggression, pain, pleasure & arousal • Feeding, thirst & satiety centers • Controls body temperature • Regulates daily patterns of sleep

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Epithalamus

• Pineal gland – endocrine gland the size of small pea – secretes melatonin during darkness – promotes sleepiness & sets biological clock • Habenular nuclei – emotional responses to odors

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Subthalamus & CVO

• Subthalamus – small area just inferior to thalamus – work with basal ganglia, cerebrum & cerebellum to control body movements • Circumventricular organs – in walls of 3rd & 4th ventricles – monitor changes in blood chemistry because lack blood brain barrier (parts of hypothalamus, pineal & pituitary gland) – sites of entry of HIV virus into brain (dementia)

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Cerebrum (Cerebral Hemispheres)

• Cerebral cortex is gray matter overlying white matter – 2-4 mm thick containing billions of cells – grew so quickly formed folds (gyri) and grooves (sulci or fissures) • Longitudinal fissure separates left & right cerebral hemispheres • Corpus callosum is band of white matter connecting left and right cerebral hemispheres • Each hemisphere is subdivided into 4 lobes

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Lobes and Fissures

• Longitudinal fissure (green) • Frontal lobe • Central sulcus (yellow) – precentral & postcentral gyrus • Parietal lobe • Parieto-occipital sulcus • Occipital lobe • Lateral sulcus (blue) • Temporal lobe • Insula

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Insula within Lateral Fissure

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Cerebral White Matter

• Association fibers between gyri in same hemisphere • Commissural fibers from one hemisphere to other • Projection fibers form descending & ascending tracts

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Basal Ganglia

• Connections to red nucleus, substantia nigra & subthalamus • Input & output with cerebral cortex, thalamus & hypothalamus • Control large automatic movements of skeletal muscles

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Limbic System

• Parahippocampal & cingulate gyri & hippocampus • Emotional brain--intense pleasure & intense pain • Strong emotions increase efficiency of memory

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Brain Injuries

• Causes of damage – displacement or distortion of tissue at impact – increased intracranial pressure – infections – free radical damage after ischemia • Concussion---temporary loss of consciousness – headache, drowsiness, confusion, lack of concentration • Contusion--bruising of brain (less than 5 min unconsciousness but blood in CSF) • Laceration--tearing of brain (fracture or bullet) – increased intracranial pressure from hematoma

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Sensory Areas of Cerebral Cortex

Receive sensory information from the thalamus Primary somatosensory area = postcentral gyrus = 1,2,3 Primary visual area = 17 Primary auditory area = 41 & 42 Primary gustatory area = 43

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Motor Areas of Cerebral Cortex

• Voluntary motor initiation – Primary motor area = 4 = precentral gyrus • controls voluntary contractions of skeletal muscles on other side – Motor speech area = 44 = Broca’s area • production of speech -- control of tongue & airway

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Association Areas of Cerebral Cortex

• Somatosensory area = 5 & 7 (integrate & interpret) • Visual association area = 18 & 19 (recognize & evaluate) • Auditory association area(Wernicke’s) = 22(words become speech) • Gnostic area = 5,7,39 & 40 (integrate all senses & respond) • Premotor area = 6 (learned skilled movements such as typing) • Frontal eye field =8 (scanning eye movements such as phone book)

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Aphasia

• Language areas are located in the left cerebral hemisphere of most people • Inability to use or comprehend words = aphasia – nonfluent aphasia = inability to properly form words • know what want to say but can not speak • damage to Broca’s speech area – fluent aphasia = faulty understanding of spoken or written words • faulty understanding of spoken or written words – word deafness = an inability to understand spoken words – word blindness = an inability to understand written words • damage to common integrative area or auditory association area

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Hemispheric Lateralization

• Functional specialization of each hemisphere more pronounced in men • Females have larger connections between 2 sides • Damage to left side produces aphasia • Damage to same area on right side produces speech with little emotional inflection

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Electroencephalogram (EEG)

• Brain waves are millions of nerve action potentials in cerebral cortex – diagnosis of brain disorders (epilepsy) – brain death (absence of activity in 2 EEGs 24 hours apart) • Alpha -- awake & resting • Beta -- mental activity • Theta -- emotional stress • Delta -- deep sleep

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II -- Optic Nerve

• Connects to retina supplying vision

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I -- Olfactory Nerve

• Extends from olfactory mucosa of nasal cavity to olfactory bulb • Sense of smell

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Developmental Anatomy of the NS

• Begins in 3rd week – ectoderm forms thickening (neural plate) – plate folds inward to form neural groove – edges of folds join to form neural tube • Neural crest tissue forms: – spinal & cranial nerves – dorsal root & cranial nerve ganglia – adrenal gland medulla • Layers of neural tube form: – marginal layer which forms white matter – mantle layer forms gray matter – ependymal layer forms linings of cavities within NS

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Dorsal View of Neural Groove

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Development of Principal Parts

• By end of 4th week, 3 anterior enlargements occur – prosencephalon – mesencephalon – rhombencephalon

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Development of Principal Parts

• By 5th week, 5 enlarged areas exist • Prosencephalon – telencephalon – diencephalon • Mesencephalon • Rhombencephalon – metencephalon – myelencephalon • Neural tube defects – associated with low levels of folic acid (B vitamins) – spina bifida is failure to close of vertebrae – anencephaly is absence of skull & cerebral hemispheres

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Aging & the Nervous System

• Years 1 to 2 – rapid increase in size due to increase in size of neurons, growth of neuroglia, myelination & development of dendritic branches • Early adulthood until death – brain weight declines until only 93% by age 80 – number of synaptic contacts declines – processing of information diminishes – conduction velocity decreases – voluntary motor movements slow down – reflexes slow down

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Cerebrovascular Accident (CVA)

• Third leading cause of death after heart attacks and cancer • 2 types of strokes – ischemic due to decreased blood flow – hemorrhagic due to rupture of blood vessel • Risk factors – high blood pressure, high cholesterol, heart disease, diabetes, smoking, obesity, alcohol • Tissue plasminogen activator (t-PA) used within 3 hours of onset will decrease permanent disability

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Transient Ischemic Attack (TIA)

• Episode of temporary cerebral dysfunction • Cause – impaired blood flow to the brain • Symptoms – dizziness, slurred speech, numbness, paralysis on one side, double vision – reach maximum intensity almost immediately – persists for 5-10 minutes & leaves no deficits • Treatment is aspirin or anticoagulants; artery bypass grafting or carotid endarterectomy

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Alzheimer Disease (AD)

• Dementia = loss of reasoning, ability to read, write, talk, eat & walk • Afflicts 11% of population over 65 • Loss of neurons that release acetylcholine • Plaques of abnormal proteins outside neurons • Tangled protein filaments within neurons • Risk factors -- head injury, heredity • Beneficial effects of estrogen, vitamin E, ibuprofen & ginko biloba

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