Human Anatomy & Physiology I

Embryonic Development of the Brain3rd week ectoderm thickens to form neural plate, which is later flanked by neural foldsThis neural groove deepens, forming a neural tube by 4th weekdifferentiates into the CNS = brain development begins

Embryonic Development of the BrainBetween ectoderm and neural tube a neural crest forms, and the 3 primary brain vesicles appear:1. FOREBRAIN, or prosencephalon [pros-en-sef-uh-lon]2. MIDBRAIN, or mesencephalon [mes-en-sef-uh-lon]3. HINDBRAIN, or rhombencephalon [rom-ben-sef-uh-lon]The rest of the neural tube becomes the spinal cord

Embryonic Development of the Brainfigure 12.3Week 5 Secondary brain vesicles ariseForebrain divides: telencephalon [tel-en-sef-uh-lon, -luhn] (endbrain) and diencephalon [dahy-en-sef-uh-lon] (interbrain)Hindbrain constricts: metencephalon [met-en-sef-uh-lon] (afterbrain) and myelencephalon [mahy-uh-len-sef-uh-lon] (spinal brain)Eventually the endbrain sprouts two lateral swellings like Mickeys earsThis eventually becomes the cerebrumThe other brain structures form the midbrain, pons, cerebellum and the medulla oblongata [ob-lawng-gah-tuh]All but the cerebellum form the brain stem

Brain continues to grow rapidly; positions changeMidbrain & cervical flexures developSurfaces crease & fold = convolutionsIncrease surface area = more neuronsBrain--major partsBrain stem- continuous with spinal cordMedulla oblongata, pons, midbrainDiencephalon [dahy-en-sef-uh-lon] - above brain stemThalamus, hypothalamus & pineal glandCerebrum- at top and largest partSurface covered with gray matter- cortexBeneath is cerebral white matterCerebellum- back of brain stemMeans little brainCranial meninges [mi-nin-jeez]- dura mater, arachnoid [uh-rak-noid] mater & pia materFigure 10.6b

Ventricles of the brain-filled with cerebrospinal fluidLateral ventricles, one deep within each hemisphere are large C-shaped chambersThey are separated by a thin membrane, the septum pellucidumInterventricular foramen allows comminucation between the lateral ventricle and the narrow third ventricleThe third and fourth ventricles are connected by the canal-like cerebral aqueduct

The Cerebral Hemisphere3 basic regions:Cerebral cortexInternal white materBasal nuclei (gray matter within white matter)Surface folds = gyriShallow grooves = sulciDeeper grooves that separate brain= fissuresLongitudinal Fissure- divides it into left & right hemispheresConnected by corpus collosumTransverse fissure- separates cerebral hemispheres from cerebellumCerebrum- Structure (cont)Each hemisphere has 4 lobesFrontal, parietal, temporal, occipitalCentral sulcus separates frontal & parietalPrecentral gyrus anterior to sulcus= primary motor areaPostcentral gyrus = primary sensory areaEach hemisphere is concerned with the sensory and motor functions of the opposite side fo the body.The two hemispheres are not equal in function.Figure 10.11a

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Function areas of CortexSpecialized areas anatomically locatedSensory areas receive input and responsible for perceptionMotor areas- initiate movementsAssociative areas- complex integration: e.g. memory, emotion, reasoning, etc.Motor AreasMainly from anterior part of hemispherePrimary motor area- precentral gyrusBrocas speech area- interacts with premotor area & primary motor area to regulate breathing and speech musclesSensory AreasPrimary somatosensory area- postcentral gyrus.input includes: touch, proprioception, pain, itching, tickle, temperature Primary visual area- occipital lobePrimary auditory area- temporal lobePrimary gustatory area base of postcentral gyrusPrimary olfactory area- medial aspect of temporal lobeSensory PathwaysRelay information from periphery to cerebral cortex3 neurons in each pathway.Posterior column- medial lemniscus [lem-nis-kuhs] pathwayFine touch- body location, texture, sizeProprioception- [proh-pree-uh-sep-shuhn] position & motion of body partsVibratory sensations- fluctuating touch stimuliAssociation AreasAdjacent to sensory & motor areasconnected with tracts- interpret informationE.g. somatosensory [suh-mat-uh-sen-suh-ree] association areaPosterior to primary somatosensory areaIntegrates sensation- exact shape & texture of object compares with stored memoriesWernikes area- left temporal & parietal [puh-rahy-i-tl] lobesInterprets meaning of speechRight hemisphere adds emotional contentLateralizationLeft gets input from & sends output to right side of body and vice versaLeft important for spoken & written language, numerical & scientific skills & reasoningRight more involved with spatial and pattern recognition and emotional contentBasal NucleiDeep gray = basal nuclei (basal ganglia)Globus palladus, putamen [pyoo-tey-min], caudate nucleus

DiencephalonThalamus- critical relay for sensory inputTransmits motor information from cerebellum & basal nuclei to cerebrumHypothalamus- important for homeostasisControl of ANS-regulation of many activitiesControl of pituitary and hormone productionRegulation of emotional & behavior patternsRegulation of eating & drinkingControl of body temperatureRegulation of circadian rhythms & states of consciousnessPineal gland- secretes melatoninFigure 10.6a

Brain Stem- MidbrainConnects pons to DiencephalonLarge tracts = cerebral peduncles (motor)Nuclei = substantia nigra, red nuclei, cranial nerves III & IVSuperior colliculi nuclei involved in tracking visual stimuliInferior colliculi auditory input & startle reflexBrain Stem- PonsPons (bridge)- nuclei & tractsConnect left & right of cerebellumAscending & descending tractsNuclei motor relays from cerebrum to cerebellum , respiration & cranial nerves V, VI, VII, VIIIBrain Stem- MedullaMedulla Oblongata- inferior part of brainstemwhite matter extending between spinal cord & other parts of brainseveral nuclei: cardiovascular center (heart rate)Medullary rhythmicity area (respiratory rhythm)Other sensory & reflex motor areasSome related to cranial nervesCerebellumTwo cerebellar hemispheresPosterior to medulla and pons, below cerebrumCerebellar cortex gray matterTree like white matter & nucleiAttached to brain stem via cerebellar pedunclesCerebellar functionGets wide range of sensory inputCompares with programmed motor activity from cerebral cortexSmoothes & coordinates complex activitiesRegulates posture & balanceRequired for skilled motor activitiesLimbic SystemRing of structures on inner border of cerebrum and floor of diencephalonemotional brain pain , pleasure, anger, affection, docilityInvoluntary activity related to survivalImportant in memory development

Reticular formationNetlike arrangement of gray and white materAscending part = Reticular Activating System (RAS)Projects to cerebral cortex & helps maintain consciousnessInactivation => sleepCerebrospinal Fluid (CSF)Circulates through ventricles of brain and the subarachnoid space.4 ventricles: 2 lateral, third & fourthFormed in choroid plexuses= Specialized capillary networks in wall of ventricles covered by ependymal cellsFlows through ventricles then from 4th to central canal of spinal cord & subarachnoid cellsReabsorbed through arachnoid villi into superior saggital sinusBrain blood supplyRequires ~20% bodys oxygen supply4 min lack => permanent damageRequires continuous glucose supplyProtected by Blood-brain barrierAllows lipid soluble materials: O2, CO2, alcohol, anesthetic agents but controls entry of other materialsCreated by tight capillaries and glial cellsFigure 10.7

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Figure 10.11b

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Figure 10.14a

Sensory Pathways (cont)Spinothalamic pathways-anterior & lateral spinothalamic tractsRelay impulses for pain, tickle, itch & thermal sensations.Somatic Motor PathwaysSignals converge on lower motor neuronsLower motor neurons stimulate muscles directlyInput comes from:Local interneurons- e.g. reflexesUpper motor neurons- corticospinal tractsBasal ganglia- help with muscle toneCerebellum- coordinationFigure 10.15

MemoryProcess for storing & retrieving informationInvolves structural & functional changesInvolves association areas, parts of limbic system & diencephalonSkill memory also involves cerebellum & basal gangliaCentral Nervous System, Spinal Nerves, And Cranial NervesCranial Nerves (table 9.6)IOlfactory- sensory (smell)IIOptic- sensory (vision)IIIOculomotor- motor (eye)IVTrochlear- motor (eye)VTrigeminal- Mixed Sensory around eyes & upper mouth motor to chewingVIAbducens- motor (eye)VII Facial- mixedSensory to front of tongue & motor to facial expression, lacrimal and some salivary glandsCranial NervesVIII Vestibulocochlear- sensory (ear)IXGlossopharyngeal- mixedSensory for rest of tongue, pharynx & palate, blood pressureMotor to pharyngeal muscles, parotid salivary glandXVagus- mixed (major visceral nerve)Sensory from pharynx, ear, diaphragm, visceral organs in ventral cavityMotor to palatal & pharyngeal muscles & organs in ventral cavityXIAccessory- Motor to voluntary muscles including sternocleidomastoid and trapeziusXII Hypoglossal- motor to tongue

AgingRapid growth during first few yearsSize of neurons & proliferation of neuroglia increasesIncreases development of dendritic branches & synaptic contactsDecline in brain mass from early adulthood onSpinal Cord StructureProtection and CoveringsSpinal cord in vertebral cavity-Surrounded by boneWrapped in meninges- 3 layers of connective tissueSpinal cord meninges are continuous with brain meningesSpinal MeningesEpidural space lined with fatDura mater- tough ,dense connective tissueExtends to 2nd sacral vertebra Well beyond spinal cordArachnoid mater- collagen and elastic fibersSubarachnoid space- cerebral spinal fluid circulates in this spacePia mater- transparent layer adheres to surface of brain & spinal cordContains blood vesselsFigure 10.1

Gross Anatomy Of Spinal CordRuns to 2nd lumbar vertebraRoots of spinal nerves for lumbar, sacral & coccygeal nerves in vertebral cavity before leaving = Cauda EquinaEnlargements: cervical & lumbarInclude nerves for upper & lower limbsEach spinal segment gives rise to a spinal nerve 31 pairs

Figure 10.2

Internal Structure Of Spinal CordTwo grooves- left & right halvesAnterior median fissure & posterior median sulcusGray matter- 3 horns on each sideAnterior, posterior, lateralAnterior- somatic motor neuronsPosterior- sensory neuronsLateral- autonomic motor neuronsInternal Structure Of Spinal Cord (cont)White matter- organized into columnsAnterior, posterior & lateral white columnsEach column contains one or more tracts having a common destinationSensory = ascending tractsCarry information toward brainMotor = descending tractsCarry information down spinal cordFigure 10.3

Spinal NervesServe particular area of bodyContain 2 bundles of axons = rootsDorsal root- only sensory axonsSwelling called dorsal root ganglionContains Cell bodies of sensory neuronsVentral root- axons of somatic & autonomic motor neuronsSpinal Nerves (cont)Named and numbered according to level of vertebra they emerge fromC1-8, T1-12, L1-5, S1-5 & 1 coccygealC1 from above atlasRest through intervertebral foraminaSpinal Nerve CompositionRoots unite to form nerve at foraminaMixed sensory & motor axonsEach axon wrapped in endoneuriumAxons grouped in fascicles wrapped in perineuriumOuter covering = epineuriumFigure 10.4

Distribution Of Spinal NervesAfter leaving vertebra nerves branchSome join with axons from neighboring nerves to form plexusesNames then relate to area they are in or region innervatedSpinal nerves T2-T11 do not form plexuses= intercostal nervesSupply abdominal muscles, skin of chest & back and muscles between robs.

PlexusesCervical plexus- posterior head, neck, shoulders & diaphragmBrachial plexus-upper limbs & some neck & shoulder musclesLumbar plexus- abdominal wall, external genitals & part of lower limbse.g. ilioinguinal, femoral, obdurator nervesSacral plexus- buttocks, perineum & lower limbse.g. Gluteal, sciatic & pudendal nervesFigure 10.2

Spinal Cord FunctionsRoutes signals along pathwaysGray matter integrates signals Reflex = fast involuntary sequence of actions in response to a stimulusInborn reflex e.g. withdrawal reflexCan also have learned reflexes, e.g. driving skillsCan be spinal or cranial integration

Reflex arc (patellar reflex)Sensory receptor- responds to stimulusTap below patellaSensory neuron- to dorsal horn & brainIntegrating center- e.g. single synapseSensory to motor neuronsMotor neuron- from center to effectorVia ventral hornEffector- responder (muscle or gland)Patellar reflex- rectus femoris contractsFigure 10.5