Brain Structure-Function, Blood Supply, Formation

Brain Structure-Function, Blood Supply, Formation

1. Functional divisions of the brain - CNS

   • Brainstem
     • Medulla
     • Pons
     • Midbrain
     • Superior (visual) & inferior (auditory) COLLICULI
     • PINEAL BODY: regulates circadian rhythms - synthesise melatonin. Pineal tumours can compress midbrain and block cerebral aqueduct - vCSF from 3rd to 4th ventricle
     • SENSORY and MOTOR functions via CN's and BODY VIA SPINAL CORD
   • Cerebellum
     • Maintaining POSTURE. Coordinate HEAD & EYE mvmts. FINE-TUNING mvmts. MOTOR LEARNING.
   • Diencephalon
     • Thalamus
       • Essential for transferring ALL SENSORY INFO (minus olfaction). Relay sensory info to CORTEX.
         • GATES & MODULATES sensory info
         • Integrates MOTOR control
         • Influences ATTENTION & CONSCIOUSNESS
     • Hypothalamus
       • Associated with 3rd ventricle & cerebral aqueduct
       • Regulates HOMEOSTASIS & SEXUAL REPRODUCTION BEHAVIOURS
         • eg. drinking, growth, eating, maternal behaviour, circadian rhythms
         • Connected to Pituitary (HPA Axis) - hormonal secretions
       • Extensive connections to rest of CNS
   • Cerebrum
     • Cortex
       • 'HIGHER FUNCTIONS': PERCEPTION. MOTOR PLANNING. COGNITION. EMOTION. MEMORY
         • SENSORY CORTEX
         • MOTOR CORTEX
           • SMA + PMA provide info INTO Primary motor cortex (M1). First step for voluntary mvmt. Then from M1, motor neurones in cerebral cortex > spinal cord > skeletal muscle
             • > PNS
           • Brodmann's Areas
             • Anatomical differences/organisation across brain mean cortex areas have different function. Structure of axons relates to function.
       • Discovered via Temporal Lobectomy 1953 - Hippocampus removed > cured epilepsy but induced anterograde amnesia
     • Amygdala
       • Social BEHAVIOUR & EMOTION
     • Hippocampus
       • MEMORY
     • Basal nuclei/ganglia
       • MVMTS
     • White matter
       • Axons connecting different parts of brain - to&from cortex / between strutures (hidden beneath cerebral cortex)
         • Tracts - 3 types:
           • PROJECTION: Internal Capsule - sensory fibres thalamus>cortex
           • ASSOCIATION: Short fibres, Sup & Inf Longitudinal Fasc, Cingulum, Uncinate Fasc, Perpeindicular Fasc
           • COMMISURAL: Corpus Callosum, Anterior & Posterior & Hippocampal commissures

2. Blood Supply

   • Circle of Willis
     • NB. Anterior spinal artery supplies surface of spinal cord
     • Occlusion > loss of pain and temp sensation. Dorsal sensory functions are relatively preserved
   • Spinal blood supply
     • Aorta > L/R Post intercostal artery > Post branch > Segmental spinal artery > Segmental medullary artery OR Ant/Post Radicular artery
   • Cerebral blood supply
     • Cerebral arteries supply:
       • Watershed Infarcts - if perfusion pressure is low in terminal branches (ie. where territories overlap)
       • Anterior - motor & sensory cortex of LOWER LIMB
       • Middle - motor & sensory cortex of UPPER LIMB & FACE, also AUDITORY CORTEX
         • Most common site for ISCHAEMIC STROKE > Contralateral hemiparesis and hemisensory loss - affects FACE and UPPER + LOWER EXTREMITIES
           • Think F.A.S.T. (Face, Arms, Speech, Time)
       • Posterior - whole of VISUAL CORTEX
   • Cerebral blood flow
     • REGULATION
       • WHY NEEDED?
         • TOO LOW > ischaemic damage
         • TOO HIGH > ^intracranial pressure > oedema & crushing of brain tissue, shifting of structures, restricts blood flow, herniation
       • Flow can be adjusted very RAPIDLY at microvascular level (as soon as 1sec after neuronal excitation)
       • NEURAL - symp & parasymp innervation
       • METABOLIC
         • Local ^metabolism > vO2 ^CO2
           • vpH of ECF > VASODILATION of vascular SMOOTH MUSCLE
           • Hypoxia > ^K+ > vasodilation.
           • Low O2 > ADENOSINE released > vasodilation
             • *[Adenosine] changes can be used as rapid detector for stroke
       • LOCAL CONTROL
         • GLUTAMATE release (main neurotransmitter for blood flow reg) > acts on astrocytes (wrap around vessels) > ^Ca2+ > activate NO-synthase > NO release > Vessels DILATE
     • ~15% of Cardiac output. Should be 50ml/min per 100g of brain weight.
       • Functional MRI (fMRI)
         • Detect rapid changes. Used with vascular markers/contrasting agents eg. GADOLINUM
         • Neuronal signals cause changes in blood flow > can see this via OXYHb & DEOXYHb.

3. Formation

   • Development of SPINAL CORD
     • Development of BRAIN REGIONS
     • Forms ~WEEK 3 from ECTODERMAL LAYER
       • Neural groove > neural tube over several days
         • Anterior neuropore defect > ANENCEPHALY Posterior neuropore defect > SPINA BIFIDA
           • Give mothers FOLIC ACID supplements
         • Neuroepithelial cells > neuroblasts. Form MANTLE layer (around neuroepithelial layer) > GREY MATTER OF SPINAL CORD.
           • MARGINAL layer (outermost layer) contains nerve fibres emerging from neuroblasts in mantle layer. These are MYELINATED > white appearance > WHITE MATTER OF SPINAL CORD.
     • Vertebral column & dura lengthen faster than neural tube > Spinal cord terminates L2/L3, then CAUDA EQUINA. Dural sac & subarachnoid space terminate S2. Below L2/L3, FILUM TERMINALE (threadlike extension of pia mater) attaches to coccyx > regression of spinal cord.
       • No spinal cord below L3 allows EPIDURAL ANAESTHESIA & LUMBAR PUNCTURE
   • Development of VENTRICULAR SYSTEM
     • allows fluid to flow between lateral & 3rd ventricles
     • Canal runs through brain and continues down spinal cord
   • Development of CORTEX
     • BIRTH = all of the neurons you will ever have.
       • SYNAPSES form at a FASTEST rate in first 3 years. At 2/3yo, the brain has DOUBLE as many synapses as in adulthood. Surplus connections 'PRUNED' throughout childhood and adolescence.
         • Grey matter: neurones with different structures, all interconnected
       • NO NEW neurones formed throughout life, apart from in: SUBVENTRICULAR ZONE and DENTATE GYRUS of HIPPOCAMPUS
     • Brain size doubles in first year. By 3yo it has reached 80% of its adult volume
     • More complex dendritic tree > more synapses can form > more complex brain
   • MENINGES
     • CSF
       • Disorders!
         • HYDROCEPHALUS - CSF continuously produced - BLOCKAGES in circulation/drainage/production > ^intracranial pressure
           • Blockage most common @ interventricular foramen & cerebral aqueduct
           • in newborn > VENTRICULAR & SKULL DILATION
           • in adult (cranial cavity closed) > HEADACHE/VOMITING/NAUSEA/BRADYCARDIA/HYPERTENSION/LOSS OF CONSCIOUSNESS/BRAIN STEM DYSFUNCTION
           • Ttment: shunt to remove fluid. If caused by tumour > remove/shrink
       • 150ml total, 25ml in ventricles. 500ml produced per day - secreted by CHOROID PLEXUS
         • CHOROID PLEXUS = capillary network separated from ventricles by choroid epithelial cells.
           • Blood-Brain Barrier BBB
             • = Endothelial cells + Tight junctions
           • Fluid filters into ventricles - other substances may be transported - ie. CSF is an ultrafiltrate of blood
           • NB. CP in lateral ventricles continuous with CP in 3rd ventricle
       • 1) Removes WASTE products. 2) TRANSPORTS SIGNALLING molecules. 3) Makes brain BUOYANT. 4) SUPPORTS/CUSHIONS/DISTRIBUTES PRESSURE on brain.
     • DURA mater
       • Two layers - close together apart from along sagittal midline
         • MENINGEAL / PROPER / DEEP LAYER Continuous with dura of spinal cord
           • Falx Cerebri & Tentorium Cerebelli
             • Tentorium roofs over posterior cranial fossa. An opening, the 'tentorial notch', allows midbrain to pass through
         • ENDOSTEAL / PERIOSTEAL / SUPERFICIAL LAYER Continuous with periosteum on outside of skull at foramina not cont with dura of spinal cord
       • Provides protection - provides space for superior sagittal sinus > CSF venous drainage
       • Disorders!
         • Lesions (eg. heamatoma/tumors) cause brain to PRESS against dura (tough immovable membrane), or HERNIATE through tentorial notch.
     • ARACHNOID mater
       • Separated from dura by subdural space (filled with film of fluid) Separated from pia by subarachnoid space (filled with CSF, BLOOD VESSELS & NERVES)
         • bridges over sulci of brain
           • ARACHNOID VILLI - arachnoid projects through dura into venous sinuses > form arachnoid granulations along sinuses
             • 1-way valves – allow CSF to drain into sinuses > venous system REABSORPTION: greater hydrostatic pressure in subarachnoid space; greater osmotic pressure in sinus
     • PIA mater
       • Extend into brain sulci. Covers cerebral arteries
     • Disorders!
       • HEADACHE - brain has no pain receptors so caused by stretch/irritation of meninges/vessels
       • BACTERIAL MENINGITIS - inflammation due to Streptococcus Pneumoniae & Neisseria Meningitides. Immune response > cerebral oedema > ^internal pressure > herniation & poor blood supply
         • Diagnosis: CSF examination via lumbar puncture - ^White cell count ^Protein vGlucose

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