AEB Notes - Phase I

Phase I: AEB Scenario Plenary Space Occupying Lesions - Definition: Any lesion that occupies space in the cranial cavity o Progression: Expands cranial volume compresses/distorts/displaces tissue raises intracranial pressure herniation into spaces with lower...

Phase I: AEB Scenario Plenary Space Occupying Lesions - Definition: Any lesion that occupies space in the cranial cavity o Progression: Expands cranial volume compresses/distorts/displaces tissue raises intracranial pressure herniation into spaces with lower pressure - Common Causes o Infections: Brain abscess o Vascular: Intracranial haemorrhage o Neoplastic: Malignant and non-malignant (primary/secondary) o Developmental: Cyst - Investigations: Physical examination (localising signs), neuroradiology/neuroimaging, serology, cultures, lumbar puncture CSF sampling - Diagnosis: Intraoperative smear, biopsy o Nature (tumour?) and type of lesion o Prognosis (prediction of future behaviour) o Has it returned? How has treatment changed it? - Intraoperative Assessment o Smears ▪ Advantages: Rapid, small amount of tissue required (1mm), great cytology ▪ Disadvantages: Limited architectural information o Frozen Sections ▪ Advantages: Rapid, preserves architectural information, similar to paraffin sections (H&E stain) ▪ Disadvantages: Cytology (seen in smears) may not be preserved, needs special equipment, introduction of ARTEFACTS (ice crystals from freezing) - Thorough Examination o FFPE (Formalin-fixed, Paraffin embedded tissue): Tissue is fixed in wax over ~24h and examined using H&E stain o GFAP (immunohistochemistry): Antibody staining and identifying mutations - Management o Biopsy (for pathology) o Neurosurgery (intraoperative tissue fluorescence, allowing tumour visualisation) o Surgical resection  o Radiation oncology o Medical oncology Stroke - Ischaemia in the CNS o Cerebral neurons rely on aerobic metabolism o Normal Blood Flow to CNS: 50mL/100g/min – 15% of total CO ▪ Autoregulation: Cerebral vessels maintain blood flow across a wide range of blood pressure o Stroke Occurs: >90% occlusion o Infarction ▪ Blood flow falls to zero for 4-10min ▪ Blood flow falls below 16mL/100g/min for >1h ▪ Every minute a major cerebral artery is occluded, 1.9 million neurons, 14 billion synapses, and 12km of myelinated fibres are lost - Epidemiology o Prevalence: 6% of all deaths in Australia in 2020 (rank 3) o Incidence: 127 per 100,000 population in 2018 (reduction of 25% since 2001) ▪ ~28,000 first-time strokes in Australia in 2020 = 1 per 19 minutes o Global Lifetime Risk: 1 in 4 o Mortality: 15% in the first 30 days (75% decrease over past 40 years) o Morbidity: By the end of the first year, the majority of stroke survivors remain disabled ▪ Recurrence: 20% in the first 5 years - Costs: Approximately 2% of total cost of disease in Australia (~$6.2 billion/year) o Comprises: Health system costs, lost productivity, costs of care + home modifications, etc. o Additional: $26 billion due to disability and death - Acute Stroke Management o Pathway: Identification urgent transfer to hospital, stroke team mobilised o History: Onset, severity of stroke and deficits, comorbidities, medications ▪ Exclusion of stroke mimics Anatomy Cranial Cavity and Brain Bones of the Skull Lateral, Posterior, and Superior Views NB: A neonate9s skull has an anterior/posterior fontanelle where the bregma and lambda emerge. - Temporal Bone  o Mastoid Process: Contains air chambers (mastoid air cells) o External Acoustic Meatus: Opening of the ear o Styloid Process: Bone projection where multiple facial muscles attach o Zygomatic Process: Extension of the temporal bone that connects to the zygoma (connects to the temporal process of the zygoma to form the zygomatic arch) - Pterion: Meeting of various sutures in the anterior lateral part of the skull o Clinical Significance: Branches of middle meningeal artery passes below this, which may cause a haematoma if ruptured - Frontal Bone: Develops as two bones with the metopic suture between them, which fuse Cranial Cavity (Internal Surface) - Anterior Cranial Fossa o Frontal Bone: Squamous part makes up the outside, orbital part makes up the floor of the anterior cranial fossa o Cribriform Plate of Ethmoid Bone: Many small holes for olfactory sensory nerves to pass from the nasal cavity to the brain o Sphenoid Bone: Lesser wing - Middle Cranial Fossa o Temporal Bone: Squamous and petrous parts o Sphenoid Bone: Greater wing o Foramina: Mainly for cranial nerves - Sphenoid Bone o Boundaries: Anterior/posterior clinoid processes, which are projections from the greater wing o Lesser Wing: Separates the anterior cranial fossa from the middle cranial fossa  o Sella Turcica: Saddle-shaped body, contains the pituitary fossa which contains the pituitary gland, joins the two greater wings of the sphenoid bone - Posterior Cranial Fossa o Occipital Bone: Squamous part forms most of the posterior cranial fossa o Temporal Bone: Partially forms the P.C.F. o Clivus Brain Development Time Notes Cephalic end: 3 dilations (occur due to varying rates of growth), 2 flexures - Primary Brain Vesicles o Prosencephalon (forebrain) o Mesencephalon (midbrain) Week 3 o Rhombencephalon (hindbrain) - Flexures o Cervical Flexure: at junction of hindbrain and the spinal cord o Cephalic Flexure: in the midbrain region Caudal End: remains narrow 5 secondary vesicles develop from primary vesicles (at different rateS) - Prosencephalon telencephalon/diencephalon - Mesencephalon remains - Rhombencephalon metencephalon/myelencephalon 4 New Features - Rhombencephalic isthmus (separates mesencephalon from metencephalon) Week 5 - Pontine flexure: between metencephalon and myelencephalon - Ventricular System in secondary vesicles o Fluid-filled spaces continuous with central canal of spinal cord o In adulthood, CSF is produced by choroid plexus, which is present in each of the four brain ventricles - Central Canal in spinal cord  Adult Structures Primary Vesicle Relevant Secondary Vesicle (Neural Canal/Brain) Lateral ventricles Cerebrum: cerebral Telencephalon hemispheres (from alar plates) - Cortex - White matter - Basal nuclei Prosencephalon 3rd ventricle Thalamus Hypothalamus Diencephalon Epithalamus Pituitary Epiphysis Retina (optic cup + stalk) Cerebral aqueduct Mesencephalon Mesencephalon similar to spinal cord Brain stem: midbrain 4th ventricle Metencephalon Brain stem: pons Cerebellum Rhombencephalon 4th ventricle basal plates medially Brain stem: medulla oblongata alar plates laterally Myelencephalon - Caudal portion: closed off part of medulla - Cranial portion: open part of medulla Lobes of the Cerebral Cortex - Septum Pellucidum: Partition between left and right hemispheres o Lateral Ventricle: Lateral to the septum pellucidum - Thalamus: One L/R, has interthalamic adhesions to connect L/R thalami - Cerebral Aqueduct: Separates two parts of the midbrain o Tegmentum: Larger part (anterior) o Tectum: Smaller part (posterior), has 2 colliculi on either side (2 top, 2 bottom) - Central Sulcus: Divides frontal and parietal lobes - Lateral Sulcus: Divides temporal and parietal lobes - Parieto-occipital Sulcus: Divides occipital and parietal lobes - Insula: Cortex insulated by temporal/frontal lobes, revealed by pulling apart lateral sulcus o Seen well in a coronal section - Limbic Cortex: Collection of many different parts of the brain that are connected o Cingulate Gyrus: Highly prominent - Uncus: Most medial part of the temporal lobe, relevant for olfaction o Olfactory Tract/Valve: Arise from this area passes through the cribriform plate Meninges - Periosteum of Skull - Dura Mater: Can be separated when meningeal layers pass into deep fissures dural venous sinuses (forming partition that separates lobes of the brain) o Periosteal Layer: Thick, fibrous, leathery – closer to the skull o Meningeal Layer – closer to the rest of the meninges o Falx Cerebri: Partition, attaches to the crista galli longitudinal suture tentorium and internal occipital protuberance o Tentorium Cerebelli: Partition (tent-shaped) that passes between cerebellum and cerebral hemispheres o Falx Cerebelli: Small partition between two cerebellar hemispheres - Arachnoid Mater: Thin and web/lace-like o Subarachnoid Space: Between arachnoid and pia mater, contains CSF (from ventricles) and blood vessels - Pia Mater: Thin, adherent to cortex  Dura Mater Arachnoid Mater - Extradural Haematoma: Arterial blood from Middle Meningeal As o Between skull/dura o Applies pressure on brain (elevates intracranial pressure) - Subdural (Dural Border) Haematoma o Venous blood from superior cerebral veins o Between dura and arachnoid mater o Direct damage to brain - Subarachnoid Haematoma o Extravasation of arterial blood o Between arachnoid and pia mater o Usually results from aneurysms Ventricular System - Cerebrospinal Fluid (CSF): Produced within the ventricles escapes into subarachnoid space via foramina in the roof and sides of the ventricle Forebrain Organisation and Cortical Function Cerebral Cortex Structure - Definition: Particular areas of cortex play a key role in performing a certain function, with many other areas involved - Cerebral Cortex: Thin 8rind9 of cell bodies (grey matter) covering a mass of axons + tracts o 2-4mm thick, contains 10-20 billion neurons - Divisions of the Cerebral Cortex o Allocortex: 3-5 layers (10% of total cortex) ▪ Archicortex: Most primitive, represented by the hippocampal formation ▪ Paleocortex: Includes olfactory areas such as the uncus, which is the primary olfactory region o Neocortex: 6 layers which vary in thickness depending on functional localisation (90% of total cortex) - Functional Types (4): Motor areas, sensory areas, association areas, unimodal/multimodal - Lateralisation of Cortex: L/R hemisphere may be more dominant in controlling function - Evidence for Functional Localisation o Clinical Studies: Correlate signs and symptoms with locations of injury ▪ Limits of Neuropsychology Tests: Only as useful as the model design, i.e., measures only things deigned as important o Physiological Studies: EEG, direct electrical stimulation ▪ Limits of EEG, MEG: Electrical activity correlates with neuronal activity; large scale; no information about chemical interactions o Modern Imaging Techniques: fMRI (blood flow), PET (radiolabelled glucose) scan ▪ Limits of CT & MRI: Structure only, with no functional information ▪ Limits of fMRI, PET, SPECT: Metabolic usage of tissue (assumes that active tissue increases blood flow o Neurosurgical Studies (e.g., split brain) - Brodmann9s Areas o Sectors of the brain defined solely on their neuronal organisation, which have since been correlated to cortical functions - Thalamus: Large nucleus in the brain (8relay station9) – NOT PART OF THE CORTEX o Function: Sensory information is sent to the thalamus and terminates there, which processes it and drives/activates relevant cortical regions o Association with Cortex: Cortex projects back to modulate thalamic activity Cerebral Neocortex Functional Localisation - Divisions of the Cerebral Neocortex o Motor Areas: From which descending motor tracts arise to result in movement o Sensory Areas: Receive discrete inputs from sensory pathways via relays in the thalamus (e.g., spinothalamic tract) - Primary Cortex: Cortical regions with 8simpler9 functions o Examples: Areas receiving direct sensory input (somatosensory, vision, auditory) or directly involved in movement production (motor cortex) - Association Areas: Concerned with more 8complex9 functions o Function: Analysing and correlating the information from the primary cortex with other information o General Locations: Usually adjacent to the primary cortices o Examples: Anterior-most part of the frontal lobes (prefrontal cortex); posterior part of the parietal lobe (posterior association areas) o Unimodal Association Areas: Adjacent to corresponding primary cortical area; receives pre-sorted information from the adjacent primary cortex used in higher level processing ▪ Function: Enables recognition of qualities of stimuli (e.g., texture, colour, shape, weight) ▪ Lesions in Sensory Association Areas inability to recognise stimuli based on sensory cues (e.g., agnosia) o Multimodal Association Areas: Parieto-temporo-occipital association area integrates information from multiple cortical areas; concerned with high-level cognitive functions ▪ Function: Mediates spatial orientation, attention (particularly on right side – right sided lesions can cause 8neglect9), language processing (particularly on left side) ▪ Examples: Prefrontal, parieto-temporal, and limbic areas Primary Areas of Function - Frontal Lobe: Personality, consciousness; emotional control; motor control o Key Structures: Motor areas, frontal eye field, prefrontal cortex, motor speech areas o Primary Motor Cortex: Precentral gyrus; motor to contralateral body; corticospinal tract (where most fibres of the voluntary motor pathway arise) ▪ Input: Strong input from cerebellum via the thalamus ▪ Hand Hook: Deals with hand movements ▪ Motor Homunculus: Represents muscles of contralateral half of the body in somatotopic pattern, with lower limb most medial (along longitudinal fissure) and head most lateral (towards lateral sulcus); regions with fine motor control have larger areas of cortex devoted to them (head/face) ▪ Lesions: Result in contralateral flaccid paralysis that becomes spastic after several days (due to lack of inhibition) o Motor Association Area: Supplementary motor cortex and premotor cortex o Supplementary Motor Cortex (SMA) (association area): Motor control for posture ▪ Relations: Anterior to precentral gyrus, more superiorly than inferiorly, and on the medial surface ▪ Importance: Movements retrieved from memory – active when mentally rehearsing movements o Premotor Cortex (PMA) (association area): Higher order processing, integrating motor information which is initiated in the precentral gyrus ▪ Relation: Inferior to the supplementary motor association cortex ▪ Importance: Movements involving sensory cues ▪ Lesions: Result in a form of apraxia – inability to perform complex movements guided by sensory cues (e.g., dressing oneself), producing less severe weakness but greater spasticity than patients with isolated precentral gyrus lesions o Frontal Eye Fields: Controls eye movements o Frontal Gyri ▪ Superior, middle, and inferior portions o Prefrontal Cortex/Prefrontal Association Area: Responsible for executive functions (intellect, predictive and planned behaviour) ▪ Most of frontal lobe anterior to the motor areas ▪ Responsible for working memory, learning, problem-solving, judgement (dorsolateral part), emotional/social behaviour (orbitofrontal part), motor planning, and mood – particularly depression and mania (anterior part of cingulate gyrus, with some evidence of laterality) ▪ Lesions: Result in inappropriate social behaviour, emotional lability, poor concentration, motivation, and abstract reasoning - Parietal Lobe: Spatial coordination, integrating sensory information into a single precept (cognition) o Key Structures: Primary somatosensory cortex, integration areas (interfaces between sensory, visual, and auditory areas), association area for contralateral consciousness, and language function (dominant hemisphere) o Primary Somatosensory Area (S1): Postcentral gyrus ▪ Inputs: Sensory input like touch, proprioception, pain from contralateral side of the body via thalamus ▪ Somatosensory Homunculus: Somatotropic – lower limbs on 8banks9 of longitudinal fissure, with areas having richer sensory innervation occupying larger areas of cortex ▪ Lesions: Loss of ability to perceive and localise a sensory stimulus (e.g., pinprick) – parietal neglect results from lesion of parietal lobe o Posterior Association Area: one of the largest multimodal association areas ▪ Function: Integration of somatosensory information with other cortical areas ▪ Superior Parietal Lobule (unimodal): Attention and visuospatial perception, including manipulation of objects; interpretation of general sensory information, consciousness of contralateral body ▪ Inferior Parietal Lobule: Interpretation of emotions from facial stimuli; interpretation of general sensory information; language, maths, body image ▪ Lesions: Cause loss of object recognition based on touch (e.g., astereognosis) - Temporal Lobe: Smell, hearing, emotions, language, memory o Key Structures: Primary auditory cortex and auditory association area, language (Wernicke9s area, dominant hemisphere), primary olfactory cortex and olfactory association area, emotional processing o Temporal Gyri ▪ Superior temporal gyrus (auditory/language) ▪ Middle/Inferior temporal gyri (memory, language, multimodal integration) ▪ Transverse temporal gyrus (primary auditory area, conscious awareness of sound – frequency, pitch, and location) o Primary Auditory Area: Upper part of superior temporal gyrus ▪ Input: Both ears – one-sided lesion does NOT cause significant hearing loss ▪ Organisation: Tonotopic – low frequency sounds are anterolateral, high frequency sounds are most posteromedial o Auditory Association Area (unimodal) ▪ Overlaps with Wernicke9s area ▪ Function: Interpretation of sounds – on dominant side, important for understanding language; on non-dominant side, responsible for understanding language prosody (tone of voice) ▪ Lesion of Wernicke9s Area (dominant hemisphere): Inability to understand language, both spoken and written - Occipital Lobe: Vision o Key Structures: Primary visual cortex (surrounds calcarine sulcus), visual association cortex, centre for visual perception o Primary Visual Area: Surrounds calcarine sulcus ▪ Input: Contralateral half of the visual field of both eyes, via thalamus ▪ Organisation: Retinotopic organisation – map of the contralateral half of the visual field ▪ Lesions: Result in cortical blindness  o Visual Association Area (unimodal): Surrounds primary visual area in occipital lobe, extends into inferolateral part of temporal lobe ▪ Input: Lateral geniculate body ▪ Function: Recognition of form, colour, movement, and attentional aspects of vision ▪ Lesions: Result in various types of visual agnosia (inability to perceive colour, movement, or recognise faces – prosopagnosia) - Insula: Gustation, visceral sensations, vestibular cortex o Relations: Deep to lateral sulcus - Limbic System: Control of innate behaviours associated with survival and, in particular, memory - Uncus: Primary olfactory cortex o Relations: Anterior extremity of the parahippocampal gyrus, continuous with the parahippocampal gyrus Speech and Language - Broca9s Area (multimodal): Motor speech – articulation of words o Relations: Posterior part of inferior frontal gyrus o Broca9s Aphasia (8expressive9): Speech is slow and slurred, but comprehension is normal – sentences are not fluent - Wernicke9s Area (multimodal): Receptive speech o Relations: Inferior parietal lobule + part of left superior temporal gyrus (planum temporale) o Wernicke9s Aphasia (8receptive9): Lack of understanding of speech, both spoken and written fluent but meaningless speech and/or incorrect grammar - Arcuate Fasciculus: A short subcortical tract that links Broca9s area and Wernicke9s area o Lesion: Results in conduction aphasia – patient knows what they want to say, but words produced are nonsensical spoken sense cannot be repeated - Global Aphasia: Loss of ability to comprehend or produce speech - Parieto-temporal Association Cortex o Function: Relays information to premotor area and prefrontal areas, integrating somatosensory, visual, and auditory information – mediates spatial orientation and attention  o Inferior Temporal Area: Related to visual memory – left (usually symbols) and right (complex shapes and scenes) NB: Multimodal association areas include language areas and the parieto-temporal association cortex - Lateralisation of Function o Dominant hemisphere (usually left) ▪ Language ▪ Mathematical/computational skills ▪ Intellectual functions involving rational and symbolic thought processes o Non-dominant hemisphere ▪ Limited or nil language capacity ▪ Recognition of complex 3D structures and patterns, faces and images ▪ Creative artistic abilities, including music ▪ Intellectual functions involving more non-verbal, intuitive processes Summary Diagrams Forebrain: Internal Capsule and Subcortical Tracts Ventricular System - Function: Production and circulation of CSF throughout the brain and spinal cord - Lateral Ventricles: Derived from the telencephalon o Relations: Has anterior (frontal), posterior (occipital), and inferior (temporal) horns, with main body in parietal lobe o Separated By: Septum pellucidum (thin partition of white fibres) - 3rd Ventricle: Derived from the diencephalon - Cerebral Aqueduct: Derived from the midbrain (mesencephalon) - 4th Ventricle: Derived from the hindbrain o Leads To: Central canal of the spinal cord; subarachnoid space o Apertures: 2 lateral apertures, 1 medial aperture subarachnoid space - Choroid Plexus: Produces CSF Deep Nuclei/Basal Ganglia - Deep Nuclei Definition: Grey matter (neuron cell bodies) encapsulated by white matter - Lentiform Nucleus o Relations ▪ Wedge-shaped nucleus embedded within the cerebral white matter ▪ Bounded laterally by the external capsule (thin sheet of white matter) ▪ Bounded medially by a (much thicker) internal capsule ▪ Lateral to the thalamus o Globus Pallidus: Lighter staining, medial portion (has an external/internal portion) o Putamen: Extends beyond the GP anteriorly, superiorly, and posteriorly - Caudate Nucleus o Relations: Occupies lateral wall of the lateral ventricle (LV) o Head: Bulges into anterior horn of LV, continuous inferiorly with the putamen of the lentiform nucleus o Body: Within body of LV o Tail: Curves around the thalamus into the inferior horn of LV - Claustrum o Relations: Thin layer of grey matter that covers the external capsule, lateral to the lentiform nucleus o Unclear function - Amygdala (Amygdaloid Body) o Relations ▪ Located anteriorly in the inferior horn (LV) ▪ Deep to the uncus (primary olfactory area) in the temporal lobe o Function: Part of limbic system, involved in emotional control - Hippocampus o Relations: Region of cortex located along the medial edge of the temporal lobe, forming the medial wall of the LV9s inferior horn - Corpus Striatum o Components: Caudate nucleus + lentiform nucleus o Forms: Telencephalic parts of the basal nuclei Cerebral White Matter - Projections: Cortical areas to subcortical structures (e.g., thalamus, basal nuclei, parts of the brainstem, and spinal cord) o Function: Connecting cortex with the spinal cord (e.g., corticospinal fibres) o Relations: From the cerebral cortex downwards, the tracts form the corona radiata, capsules (internal, external, extreme), and the tracts of the brainstem and spinal cord o Pathway: Corona radiata internal capsule crus cerebri pons pyramids of medulla o Internal Capsule: Between the lentiform nucleus, laterally, and the caudate nucleus and thalamus medially (alternatively, between thalamus and basal nuclei) ▪ Anterior Limb: Between head of caudate nucleus (medially) and lentiform nucleus (laterally); contains the anterior thalamic radiation, connecting thalamus to prefrontal and cingulate cortices; contains frontopontine fibres ▪ Posterior Limb: Between thalamus (medially) and lentiform nucleus (laterally); contains the middle (superior) thalamic radiation (passing between thalamus and motor + somatosensory areas of cortex); contains corticospinal, corticobulbar, and corticopontine tracts ▪ Genu: Junction of anterior and posterior limb ▪ Retrolentiform Part: Portion behind the lentiform nucleus; contain optic radiation and corticopontine fibres ▪ Sublentiform Part: Fibres passing under the lentiform nucleus; contains auditory radiation (passes to auditory cortex in temporal lobe); contains some fibres of the optic radiation (Meyer9s loop fibres) ▪ Clinical Significance: Most common site of cerebral haemorrhage o External Capsule: Thin layer of white matter on outer surface of putamen, formed mainly by fibres from the cortex to the putamen (corticostriate fibres) o Extreme Capsule: Thin layer of white matter lying between claustrum and insula - Forebrain Commissures o Function: Connects corresponding contralateral parts of the brain (e.g., corpus callosum) o Corpus Callosum (200-300 million fibres, larger in females than males) ▪ Divisions: Rostrum, genu (interconnecting prefrontal areas), body, and splenium (interconnecting occipital lobes) ▪ Forceps Minor: From genu to frontal lobe ▪ Forceps Major: From splenium to occipital lobe  ▪ 8Split Brain9 Surgery: Transects the corpus callosum o Anterior Commissure (~1 million fibres) ▪ Relations: Inferior to the rostrum of corpus callosum, located in anterior wall of LV ▪ Function: Connects inferior parts of the temporal lobes, including the olfactory areas o Fornix: Fibre bundle formed by axons from the hippocampus, with its body containing the commissure that connects the hippocampi ▪ Relations: Passes from posterior end of the hippocampus, up over the thalamus, with the body passing anteriorly until it reaches the front of the thalamus diverges from midline as it turns ventrally ▪ Boundaries: Forms anterior boundary of the interventricular foramen before it enters the hypothalamus, where it terminates ▪ Septum Pellucidum: Attached inferiorly to the fornix, which forms part of the medial boundary of the collateral trigone and inferior horn of the LV o Hippocampal Commissure ▪ Relations: Formed by some fibres crossing the midline to enter the opposite fornix, beneath the splenium o Hippocampus ▪ Relations: Medial edge of temporal lobe ▪ Function: Memory and learning, spatial navigation, emotions ▪ Shape: S-shaped structure - Association Fibres: Connecting cortical areas of the same hemisphere o Adjacent Gyri: Short association fibres o Lobes: Long association fibres which form distinct bundles o Arcuate Fasciculus: Connects auditory/receptive speech area (Wernicke9s) and motor speech areas (Broca9s) o Superior Longitudinal Fasciculus: Frontal, parietal, occipital, temporal o Inferior Longitudinal Fasciculus: Occipital to temporal o Cingulum: Part of limbic system, frontal + temporal to parahippocampal  o Uncinate Fasciculus: Expressive speech, Broca9s area (frontal) to temporal Cerebral Blood Supply Internal Carotid Circulation (Anterior) - Common Carotid Artery: Bifurcates at C4, forming the internal/external carotid As o Carotid Body: Chemoreceptors o Carotid Bulb: Baroreceptors - Cerebral Arterial Circle: Closed by 1 anterior communicating artery and 2 posterior communicating arteries - Internal Carotid Artery: Supplies anterior circulation o Origin: CCA at carotid bifurcation o Course: Enters the cranium through the carotid canal, in the lateral edge of the foramen lacerum (sealed by a cartilage plate) emerges in the anterior part of the petrous temporal bone carotid groove winds around the anterior clinoid process in the cavernous sinus starts to branch, lateral to the optic nerve o Classification ▪ Cerebral (Supraclinoid) – part after piercing dura (C5+C6+C7) ▪ Cavernous – within cavernous sinus ▪ Petrous – within petrous temporal bone, C2+C3  ▪ Cervical – neck o Bends: Carotid canal, cavernous sinus o Cortical Branches: Ophthalmic A, anterior choroidal A, anterior cerebral A anterior communicating A, middle cerebral A ▪ Ophthalmic A: Arises as ICA exits cavernous sinus, lying close to the anterior clinoid process passes through optic canal into orbit ▪ Anterior Choroidal A (AChA): Passes around cerebral peduncles to reach lateral geniculate bodies (associated with vision) by travelling atop the optic tract passes above uncus, enters choroidal fissure, then ventricle supplies choroid plexus, optic tract, and internal capsule ▪ Anterior Cerebral Artery (ACA): Crosses olfactory nerve, passes superior to optic nerve enters longitudinal fissure, passes over the corpus callosum Supplies (Functional Localisation): Paracentral lobules, sensory areas for pelvis + lower limb + genitalia, motor areas for pelvis and lower limb Cortical Branches: Medial surface of frontal and parietal lobes, and a finger9s breadth lateral to the longitudinal fissure (gives off pericallosal and callosomarginal branches) ▪ Middle Cerebral Artery (MCA): Runs in lateral sulcus, supplying the frontal, temporal, and parietal lobes Supplies (Functional Localisation): Primary somatosensory + motor areas for contralateral head + upper limb + trunk, language areas (dominant hemisphere), auditory areas, input association areas Cortical Branches: Lateral surface (i.e., all motor areas except that of the lower limb); basal ganglia through lenticulostriate branches Strokes: Involved in 2/3 of all ischaemic strokes o Perforating Branches/Ganglionic As: Medial striate arteries (branches of ACA), lateral striate artery (branches of MCA) ▪ Medial Striate Arteries: Supply caudate head and the anterior limb of internal capsule ▪ Lenticulostriate Arteries: Supplies striatum and posterior limb of the internal capsule Vertebrobasilar Circulation (Posterior) - Vertebral Artery: Supplies posterior circulation, passing through the transverse foramina from C6 to C1 foramen magnum cranial cavity o Origin: Posterior, superior part of the subclavian artery - Basilar Artery: Lies on pons, with numerous pontine branches o Origin: Vertebral arteries join at the pontomedullary junction  o Terminates as posterior cerebral arteries - Posterior Cerebral Artery: Curves around the midbrain passes between temporal lobe and cerebellum occipital lobe o Supplies (Functional Localisation): Visual cortex o Cortical Branches: Supply inferior part of temporal lobe and occipital lobe - Anterior Spinal A: Runs on anterior spinal cord - Posterior Spinal A: Runs on posterior spinal cord - Cerebellar Blood Supply o Superior Cerebellar Artery: Winds around cerebral peduncle supplies superior cerebellum o Anterior Inferior Cerebellar A: Passes posterolaterally supplies anterior and inferior cerebellum o Posterior Inferior Cerebellar A: Largest branch of vertebral A, passes between medulla and cerebellum supplies cerebellum (inferior vermis, central nuclei, medulla) - Pontine Blood Supply: Pontine branches of the basilar artery o Radially distributed with peripheral and deep branches - Medullary Blood Supply: Radially distributed with peripheral and deep branches o Posterior inferior cerebellar o Anterior spinal artery o Posterior spinal artery o Direct branches Cerebral Arterial Circle - Perforating Arteries = Ganglionic Arteries o Anteromedial Group: Anterior hypothalamus, septal area, o Medial Striate As: Caudate head, anterior limb of internal capsule o Lenticulostriate Group: Striatum, posterior limb of internal capsule o Posteromedial Group: Medial midbrain, anterior thalamus, hypothalamus, GP o Posterolateral Group: Posterolateral midbrain, posterior thalamus - Internal Capsule Blood Supply o Anterior Limb ▪ Upper: Lenticulostriate branches of MCA ▪ Lower: Recurrent branch / Medial Striate branches of ACA o Genu ▪ Upper: Lenticulostriate branches of MCA ▪ Lower: Direct branches of ICA + recurrent branch / medial striate branches of ACA o Posterior Limb ▪ Upper: Lenticulostriate branches of MCA ▪ Lower: AChA (branch of ICA) o Retrolentiform / Sublentiform Parts ▪ AChA and branches of PCA Brainstem - Overview: Formed by the medulla, pons, and midbrain - Function: Similar to the spinal cord, for motor and sensory regions of the head and neck o Contents: Vital centres for control of cardiorespiratory function, posture and movement, wakefulness/sleep o Conduit for Tracts: Ascending sensory tracts from spinal cord and descending motor tracts from forebrain Ventral/Anterior Surface - Medulla: Extension of the spinal cord o Pyramids (two white columns): Medially, form the ventral surface of the medulla ▪ Formed by the corticospinal tract (descending motor tract), which transmits motor information from the cerebral cortex to the spinal cord o Pyramidal Decussation: Junction of medulla and spinal cord, 90% of fibres from each pyramid cross the midline to enter the contralateral side of the spinal cord o Olives: Visible, lateral to the pyramids in the upper (rostral) part of the medulla o Cranial Nerves ▪ CN 6-8 arise from the pontomedullary junction ▪ CN 9-11 arise in a row down the lateral side of the olive ▪ CN 12 arises as rootlets medial to the olive - Pons o Basis Pontis: The base forms the pons9 ventral surface, with a basilar groove in the midline occupied by the basilar artery o Middle Cerebellar Peduncle: Extensions of the basis pontis on each side which enter the cerebellum o CN 5 (Trigeminal Nerve): Arises from the middle cerebellar peduncle - Midbrain o Cerebral Peduncles (Crus Cerebri): Two massive white structures which form the ventral surface of the midbrain, separated by the interpeduncular fossa o CN 3: Arises from the interpeduncular fossa Dorsal/Posterior Surface - Medulla o Gracile Fasciculus: Medially, forms dorsal surface of the medulla carries somatosensory information (discriminative touch, proprioception, sense of vibration) from the lower limbs and trunk ▪ Gracile Tubercle/Nucleus: Enlarged upper end of the gracile fasciculus o Cuneate Fasciculus: Laterally, forms the dorsal surface of the medulla carries somatosensory information (discriminative touch, proprioception, sense of vibration) from the upper limbs and trunk ▪ Cuneate Tubercle/Nucleus: Enlarged upper end of the cuneate fasciculus o 4 Ventricle: Gracile and cuneate th fasciculi disappear as the central canal opens to form the 4th ventricle, lying between the cerebellum (behind) and the medulla/pons (in front) ▪ Obex: Caudal limit of the 4th ventricle ▪ Floor: Rhomboid fossa – median fissure in midline, and sulcus limitans further laterally, a fetal remnant separating basal plate (motor) from alar plate (sensory) derivatives ▪ Superior/Inferior Medullary Vela: Roof and floor of the 4th ventricle ▪ Lateral Boundaries: Cuneate tubercle, inferior cerebellar peduncle, superior cerebellar peduncle - Isthmus: Junction between pons and midbrain - Midbrain o Superior/Inferior Colliculi: Paired, form the dorsal surface of the midbrain collectively referred to as the tectum o CN 4 (Trochlear Nerve): Emerges just caudal to the inferior colliculus winds around the MCP to reach the ventral surface Major Tracts - Tegmentum: Central core of the brainstem, located between the ventricles and bordered by the cerebral aqueduct, 4th ventricle, and central canal o Reticular Formation: Mixture of grey and white matter, involved in regulation of some autonomic functions, pain transmission, regulation of sleep/wake cycles, and some motor control o Spinal Nucleus of Trigeminal Nerve: Replaces dorsal horn of the spinal cord and extends throughout the medulla - Corticospinal Tract o Transmits: Fine, voluntary motor control o Cells of Origin: Primary motor cortex (pre-central gyrus) o Pathway: Base of cerebral peduncle base of pons pyramid of medulla o Decussation: ~85% of fibres cross in pyramidal decussation, at the lower end of the medulla - Medial Lemniscus: Second part of ascending somatosensory pathway to the primary somatosensory cortex, terminating in the thalamus o Transmits: Discriminative touch, proprioception, sense of vibration o Cells of Origin: Contralateral gracile and cuneate nuclei o Decussation: Caudal medulla - Spinothalamic Tract: Second part of the ascending somatosensory pathway to the primary somatosensory cortex o Transmits: Pain, temperature, light touch o Cells of Origin: Contralateral dorsal horn o Decussation: Within 2 segments of level of origin o Termination: Thalamus and brainstem (reticular formation) Internal Anatomy - Gracile/Cuneate Nuclei: Caudal half of the medulla, receiving somatosensory information from the gracile and cuneate fasciculi, which have axons that cross the midline and form the medial lemniscus - Inferior Olivary Nuclei: Large, prominent in rostral medulla; associated with cerebellum - Pontine Nuclei: Scattered masses of grey matter throughout the base of the pons, which have axons that cross the midline to form the middle cerebellar peduncles as they enter the cerebellum - Superior/Inferior Colliculi: Found in the midbrain, involved in visual (superior) and auditory (inferior) reflex activity - Periaqueductal Grey: Surrounds cerebral aqueduct (midbrain), regulates pain transmission, also involved in autonomic control and defensive behaviour - Red Nucleus: Rostral part of midbrain, involved in motor control - Substantia Nigra: Throughout midbrain, motor control Structure and Ultrastructure of Neurons Neuron Types - Parts of the Neuron o Cell body – biosynthetic centre and receptive region o Dendrites – receptive regions o Nucleus and nucleolus (stains heavily) – DNA storage o Nissl bodies – rough ER o Axon hillock + axon – place where the axon potential starts and travels o Myelin sheath and Nodes of Ranvier o Terminal branches (telodendria) o Axon terminals – secretory component - Neuron Classification (by dendritic branches) o Multipolar o Bipolar o Unipolar (pseudounipolar) o Anaxonic Staining - Myelin Stain: Fibres with myelin - Nissl Stain: Neurons – cationic stain that stains nucleic acids (DNA/RNA) - Golgi Stain: Randomly stains ~1% of cells in their entirety o Demonstrates dendritic morphology - Immunohistochemical: Uses antibodies (e.g., NeuN) to stain specific enzymes or proteins - Fluorescent Proteins: Inserted in genome of transgenic animals (e.g., Brainbow staining) Ultrastructure of Neurons - Neuron Membrane o Receptors (transmembrane proteins) to receive neurotransmitters – may be metabotropic (trigger secondary messengers) or ionotropic (coupled ion channels) o Ion Channels: Ligand-gated or voltage gated - Nucleus: Centrally placed o Contains prominent nucleolus which can be revealed with Nissl stain - Cytoplasm o Prominent rough ER (Nissl substance protein synthesis) in soma and dendrite, but not the axon o Golgi apparatus (packaging of proteins) in soma only o Many mitochondria (energy production) in soma, dendrites, and axon - Cytoskeleton o Microtubules/Neurotubules: Axoplasmic transport (20nm diameter, hollow) ▪ Axoplasmic Transport: Microtubules form 8railway tracks9 (up to 400mm/day) – anterograde (away from soma; neurotransmitter proteins) or retrograde (towards soma; metabolic products) o Neurofilaments (10nm d.) and Microfilaments (5nm d.): Maintain neuron shape and anchor membrane molecules ▪ Elongation of axon and dendrites in growing neurons - Myelin Sheath: Not present on all axons o Schwann Cells (PNS) / Oligodendrocytes (CNS) wrap themselves around axons to form a tight, lipid-rich myelin sheath ▪ Schwann cells wrap around one cell only o Segmentation: Internodes are separated by Nodes of Ranvier – contain high concentration of voltage-gated ion channels o Conduction Velocity: Myelin sheath increases conduction velocity from 0.1m/s to 80m/s by forcing current to jump from node to node - Synapses: 10,000 per neuron (avg.), may be up to 250,000 o Presynaptic Compartment: Contains neurotransmitter vesicles o Postsynaptic Compartment: Contains neurotransmitter receptors o Electrical Synapses: Exist but are rare in mammals (e.g., gap junctions) o Types: Axodendritic, axosomatic, axoaxonic - Dendritic Spines: Dynamic synaptic sites o Believed to underlie mechanisms of learning and memory - Glial Cells: Supporting cells of the nervous system that are as numerous as neurons o Astrocytes: Nourish (metabolic support), repair (structural support), and electrolyte balance; contribute to BBB; guide neuronal growth and aid synapse formation ▪ Larger, more oval/granular, somewhat lighter o Microglia: Removal and invading organism or dead/damaged tissue via phagocytosis ▪ Nuclei look like commas o Oligodendrocytes and Schwann Cells: Myelin sheath ▪ Oligodendrocytes: Smaller, rounder, darker Cranial Nerves: Organisation and Distribution Overview - Classification: 12 pairs of nerves arising from the brain and passing through foramina in the skull which are part of the peripheral nervous system o Key Difference from Spinal Nerves: No regular intervals, and can have sensory/motor/mixed functions o Supply of Structures: Mostly in head and neck ▪ Special Sensory: 1, 2, 8 ▪ Motor: 3, 4, 6, 11, 12 ▪ Mixed/Both: 5, 7, 9, 10 ▪ Parasympathetic: 3, 7, 9, 10 ▪ Eye Movements and Pupil Constriction: 3, 4, 6 - Brainstem: CN 2 – 11 o Usually carrying ipsilateral sensory information or motor output o CN 2 arises from the diencephalon o CN 3 and CN 5 carry some contralateral motor output Organisation - Cranial Nerve Nuclei o Relations to Sulcus Limitans ▪ Sensory: Lateral ▪ Branchial Motor: Medial ▪ Visceral Motor: In between o Foramina of the Skull ▪ CN I: Olfactory foramina ▪ CN II: Optic canal ▪ CN III, IV, V1, VI: Superior orbital fissure ▪ CN V2: Foramen rotundum ▪ CN V3: Foramen ovale ▪ CN VII, VIII: Internal acoustic meatus  ▪ CN IX, X, XI (descending): Jugular foramen ▪ CN XI (ascending): Foramen magnum ▪ CN XII: Hypoglossal canal o Corticobulbar Tracts: All except VII and XII Eye Movements and Pupil Constriction (3, 4, 6) - Somatic Motor Nerves: 3, 4, 6, (12 – not in control of eye movements/pupil constriction) o III: Oculomotor, some parasympathetic fibres ▪ Origin: Midbrain, oculomotor nucleus – anterior to the cerebral aqueduct  ▪ Function: Eye movements, opening of eyelid, constriction of pupil, focusing, proprioception ▪ Clinical Test for Injury: Differences in pupil size, altered pupillary response to light, eye tracking alteration Diplopia: Strabismus downwards and to affected side due to paralysis of MR and IO, impaired medial and vertical movements Ptosis: Paralysis of levator palp. sup Mydriasis: Dilated pupils Difficulty Focusing: Unreactive to light, no accommodation ▪ Motor Neuron Course: Passes forwards between posterior cerebral A and superior cerebellar A the cavernous sinus9 lateral wall superior orbital fissure branches into superior/inferior division Superior Division: Levator palpebrae superioris, superior rectus Inferior Division: Inferior oblique, medial rectus, inferior rectus ▪ Parasympathetic Neuron Course: Edinger-Westphal nucleus travels with the motor neurons to the inferior division ciliary ganglion ▪ Motor Function: All extraocular muscles except SO and LR, as well as the levator palpebrae superioris (upper eyelid) ▪ Parasympathetic Function: Sphincter pupillae (bright light + accommodation), ciliary muscle (accommodation, makes lens fatter) o IV: Trochlear ▪ Origin: Posterior surface of midbrain, just below the inferior colliculus – the only nerve to arise from the dorsal surface of the brainstem ▪ Supplies: Superior oblique muscle ▪ Course: Trochlear nucleus in midbrain fibres decussate lateral wall of cavernous sinus SOF crosses optic nerve superior oblique ▪ Clinical Test for Injury: Inability to rotate eye inferomedially, reading from top to bottom Diplopia: Strabismus upwards in contralateral eye, patient may perform a compensatory head tilt to the affected side o VI: Abducens ▪ Origin: Anteriorly, between pons and medulla ▪ Supplies: Lateral rectus ▪ Course: Pons (nucleus on the floor of the 4th ventricle, forming 8facial colliculus9), exiting at pontomedullary junction of brainstem long course in  subarachnoid space cavernous sinus, close to ICA SOF lateral rectus (ipsilateral) Some fibres pass upwards in medial longitudinal fasciculus to contralateral medial rectus, coordinating movement between eyes ▪ Clinical Test for Injury: Ability to rotate eye inferolaterally, reading from top to bottom Diplopia: Strabismus, affected eye unable to move laterally Esotropia: Deviation medially due to unopposed MR - Extraocular Muscles o Superior/Medial/Lateral/Inferior Rectus: Pull from insertion on eyeball origin ▪ Lateral Rectus: Abducts eye by action (aligns with axis of orbit) ▪ Medial Rectus: Adducts eye by action o Superior/Inferior Oblique: Passes forwards through a trochlea, inserts onto the back of the eyeball contraction allows sight in opposite direction Trigeminal Nerve (5) - Three Parts: Ophthalmic (V1), Maxillary (V2), Mandibular (V3) o Ophthalmic (sensory): SOF o Maxillary (sensory): Foramen rotundum exits through the infraorbital foramen o Mandibular (sensory and motor): Foramen ovale infratemporal fossa o Contains no parasympathetic fibres, but branches suspend the PS ganglia - Sensory Component o Main somatosensory nerve for head o General pain, temperature, discriminative touch, and proprioception o Cell bodies in trigeminal ganglion - Motor Component o Muscles of Mastication: Masseter, mylohyoid, digastric o Tensor Tympani: Dampens sound o Tensor Veli Palatini: Prevents food from entering nasopharynx, opens auditory tube - Course: Leaves anterior aspect of pons sensory ganglion/nuclei in the middle cranial fossa; V1 + V2 + V3 CN V ganglion enters as CN V splits into 3 nuclei o Mesencephalic Nucleus (sensory): Proprioception – jaws + teeth o Main Sensory Nucleus (sensory): Touch + proprioception o Spinal Sensory Nucleus (sensory): Pain + temperature o Trigeminal Motor Nucleus (motor, runs underneath CN V ganglion and passes with V3/mandibular): Muscles of mastication  - Normal Function: Corneal reflex, sense of touch, pain, and temperature, clenching teeth, moving mandible side to side o Effect of Damage ▪ Decreased sensation on face ▪ Increased pain ▪ Impaired chewing ▪ Trigeminal neuralgia – needle-like sensation o Clinical Test for Injury ▪ Sensory: Touch, corneal reflex ▪ Motor: Clench jaw and palpate muscles of mastication (masseter), open mouth against resistance Facial Nerve (7) - Motor Component o Muscles of facial expression o Stapedius m. (in middle ear) o Posterior belly of digastric and stylohyloid muscles o Facial motor nucleus (pons) loops around abducens - Parasympathetic Component o Head glands (not parotid): lacrimal, submandibular, sublingual, mucous membranes - Special Sensory: Taste from anterior 2/3 of the tongue - Sensory: Small area of skin of external ear - Nuclei o Facial Nerve Nucleus (motor): Fibres from this pass around nucleus of CN VI o Superior Salivatory Nucleus (glands), fibres pass through parotid gland o Solitary Tract: Taste o Spinal Trigeminal: Sensory ear - Course: 4 paths, intracranial + intratemporal o Brainstem enters internal acoustic meatus through facial canal in petrous temporal bone exits skull via stylomastoid foramen  Highlighted parts 8hitchhike9 on branches of V1/V3 to get to the lacrimal gland/tongue - Normal Function: Facial expression, taste o Effects of Damage ▪ Inability to control facial muscles (droopy mouth – Bell9s palsy), ipsilateral inability to close eye ▪ Distorted taste o Clinical Test for Injury  ▪ Motor: Close eyes, smile, whistle, frown, raise eyebrows ▪ Sensory: Taste - Facial Nerve Pathway Principles o Nuclei are divided into upper/lower for upper/lower face control o Does not follow the corticobulbar pathway o All parts of the face except the ipsilateral lower quadrant can be made to contract with one cerebral hemisphere o Unilateral Corticonuclear Damage: Weakness of only the contralateral lower quadrant o Unilateral Facial Nerve/Nucleus Damage: Weakness of the entire ipsilateral half of the face o Corticonuclear Fibres to Facial Motoneurons ▪ Upper: Originate in motor cortex upper half of nucleus some cross, most stay on ipsilateral side, distributed bilaterally to facial motoneurons exits as facial nerve ▪ Lower: Originates in motor cortex lower half of nucleus all fibres cross over leaves as facial nerve ▪ Upper half of the face is controlled by both hemispheres while lower quarters have contralateral control Vestibulocochlear Nerve (8) - Special Sensory: Hearing and balance o Sensory information from hearing and balance receptors in the ear - Effects of Damage: Hearing loss, balance, dizziness Glossopharyngeal Nerve (9) - Functions o Special Sensory: Taste from posterior 1/3 of tongue o Somatosensory: Pharynx and posterior 1/3 of tongue, upper pharynx, outer ear and eardrum, carotid body + carotid sinus o Motor: Stylopharyngeus (swallowing and speech) o Parasympathetic: Parotid gland (salivary) - Course: From upper part of medulla oblongata, between olive and inferior cerebellar peduncle passes laterally in posterior cranial fossa exits via jugular foramen - Nuclei o Nucleus Ambiguus: Stylopharyngeus o Inferior Salivatory: Parotid gland o Solitary Tract: Taste o Spinal Trigeminal: Touch and pain - Effects of Damage o Loss of taste from posterior tongue o Difficulty swallowing and dry mouth o Decreased sensations at the back of the tongue, soft palate, pharynx, and ear o Diminished gag reflex – uvula is deviated to the contralateral side - Clinical Test for Injury o Gag reflex o Swallowing o Coughing Vagus Nerve (10) - Function: Swallowing, taste, speech, respiratory, CVS, GI regulation, sensations of hunger/fullness, intestinal discomfort o Special Sensory: Taste (a few taste buds on epiglottis) o Motor: Muscles of pharynx and larynx, important for swallowing and speech o Sensory ▪ Pain, temperature, touch from lower pharynx and larynx (involved in cough reflex), as well as oesophagus ▪ Wall of external acoustic meatus and tympanic membrane, neck, and trunk o Parasympathetic: Viscera of thorax and abdomen, glands associated with GIT - Course o Dorsal nucleus of vagus leaves medulla oblongata between olive and inferior cerebellar peduncle leaves skull through jugular foramen joined by cranial accessory N gives off laryngeal nerves passes down into thorax and abdomen - Nuclei o Nucleus ambiguous o Dorsal nucleus of vagus nerve o Solitary tract o Spinal trigeminal - Effects of Damage o Hoarseness or loss of voice o Impaired swallowing and GI motility o Diminished gag reflex – uvula deviated to contralateral side o Abnormal heart rate - Clinical Test for Injury: Test with CN IX Accessory Nerve (11) - Motor: Supplies sternocleidomastoid and trapezius muscles - Course: Arises from both medulla and cervical spinal cord spinal fibres enter cranial cavity via foramen magnum exits cranial cavity via jugular foramen o True Accessory N: From spinal roots of accessory N (C1-5) Hypoglossal Nerve (12) - Function: Tongue movements of speech, food manipulation, and swallowing - Motor: All muscles of the tongue except two, entering from beneath - Course: Emerges from medulla, between pyramid and olive leaves skull through hypoglossal canal travels between internal jugular V and internal carotid arteries, joins cervical plexus to supply some muscles in head/neck o Fibres arise in motor cortex pass down and synapse in nucleus of opposite side leaves through nerve of opposite side - Effects of Damage: Difficulty in speech and swallowing, atrophy of tongue, inability to protrude tongue - Clinical Test for Injury: Tongue function (deviation to side of lesion) Cerebellum External Features - Relations: Located in posterior cranial fossa, inferior to tentorium cerebelli o Dorsal extension of the pons that forms the roof + lateral sides of the 4th ventricle o Attached to the brainstem via 3 pairs of cerebellar peduncles o Contains more than half of the neurons of the brain - Function: Input from the vestibular system, spinal cord, and cerebral cortex o Works like a supercomputer o Involved in maintenance of balance, posture, and coordination of movement (but also has other, non-motor functions) synchronisation of muscle activity ▪ Appropriate muscles, appropriate time + sequence, correct amount of force o Damage: Disturbances in balance and equilibrium, as well as lack of motor coordination without paralysis  - Components: Two hemispheres, joined in the midline by the vermis o Outer Covering: Cerebral cortex, highly folded forms transversely oriented folia, separated by sulci or fissures o Centre: Medullary white matter o Nuclei: 4 pairs of deep cerebellar nuclei - Fissures (2): Posterolateral, and primary fissures - Lobes (3): Flocculonodular, anterior, and posterior lobes o Anterior Lobe: Anterior 2/3 of superior surface, separated from posterior lobe by the primary fissure o Posterior Lobe: Posterior 1/3 of superior surface and most of the inferior surface ▪ Cerebellar Tonsils (2): Swellings present just lateral to the inferior vermis o Flocculonodular Lobe: Nodule and paired floccule, forming the rostral end of the inferior vermis ▪ Separated from posterior lobe and inferior vermis by posterolateral fissure ▪ Covered on ventricular surface by inferior medullary velum ▪ Relations: Adjacent to middle cerebellar peduncle, rostral to the inferior part of the posterior lobe - Zones (3): Vermis, paravermal zone, and lateral hemisphere Internal Features - Peduncles (3) o Inferior Cerebellar Peduncle (ICP): Input from ipsilateral spinal cord (proprioception, touch) + input/output for medulla o Middle Cerebellar Peduncle (MCP): Input from contralateral pontine nuclei (ultimately from cortex) o Superior Cerebellar Peduncle (SCP): Output to midbrain + thalamus (then cortex) - Deep Cerebellar Nuclei (paired on both sides, from medial to lateral) o Fastigial Nucleus: Input from vermis and flocculonodular lobe, dorsal to the apex o Interposed Nucleus: Input from paravermal zone o Dentate Nucleus: Input from lateral hemisphere (largest and most lateral) - Cortex: Hyperfolded, with each fold corresponding to a folium o Layers (3): Molecular layer, Purkinje cell layer, granular layer (+ white matter) - Purkinje Cells: Output cells of the cerebellar cortex, with synaptic inputs from a variety of sources (about 200,000 synapses per cell) o Axons: Project mainly to deep cerebellar nuclei o Inferior Olive: Direct input from it via climbing fibres o All Other Sources: Input from them via mossy fibres which synapse on granule cells (whose axons form parallel fibres each synapses on up to 500 Purkinje cells) - Basic Circuitry o Function: Deep cerebellar nuclei output is the sum of mossy fibres 8sculpted9 by Purkinje cell output, while Purkinje cell activity is 8programmed9 by climbing fibres o Inputs arrive as mossy fibres after sending a collateral branch in a deep cerebellar nucleus terminate in granular layer on granule cells (clumps called glomeruli) o Granule cells send axons into molecular layer, which split and form parallel fibres along folia long axes synapse on Purkinje cell dendrites (one plane which is perpendicular to folia long axis) axons project to deep cerebellar nucleus (inhibitory) deep cerebellar nuclei send cerebellar output to brainstem + forebrain o Each Purkinje cell receives one climbing fibre originating from the contralateral inferior olive  ▪ All synapses are excitatory, except the Purkinje cell synapse (which is inhibitory) Functional Subdivisions - Vestibulocerebellum (archicerebellum, flocculonodular lobe) o Function: Maintains balance, coordinates eye movements with head movements o Inputs: Vestibular receptors (via CN VIII) and vestibular nuclei ▪ Vestibular input reaches nodule and flocculus directly and indirectly (via vestibular nucleus) using mossy fibres, 1 of which goes straight to the fastigial nucleus cerebellar cortex output (via Purkinje cells) projects to vestibular nucleus and fastigial nucleus fastigial nucleus projects to vestibular nuclei and reticular formation reticulospinal and vestibulospinal tracts act on spinal motoneurons of proximal musculature rectify posture, maintain balance o Control: From vestibular nuclei, MLF axons influence motor neurons in the spinal cord (balance) and extraocular nuclei for CN III, IV, and VI (eye movements) ▪ Adjusts coordination of eye-head movement, via vestibular nucleus o Lesions ▪ Inability to maintain balance (falling to one side, wide-based gait), especially when eyes are closed ▪ Abnormal eye movements (nystagmus) and difficulties maintaining gaze - Spinocerebellum (paleocerebellum, vermis and paravermal zone) o Function: Postural adjustment during movement execution o Peripheral Input: Proprioceptive and tactile inputs (somatosensory system) via relays in the spinal cord (for trunk and limbs) or trigeminal nuclei (for the head) ▪ Motor Cortex: Instructs spinocerebellum about intended movement via corticopontine projections to pontine nuclei ▪ Upper Limb: Cuneocerebellar tract ▪ Lower Limb: Spinocerebellar tracts ▪ Course: Reach vermal/paravermal zones while sending collaterals to fastigial and interposed nuclei form somatotopic maps with proximal muscles in vermal zone and distal muscles in paravermal zone ▪ Axons from head and trunk terminate in a topographic manner in the vermis o Outputs (from vermis): Project to the fastigial nucleus sends projections to the reticular formation influences LMNs in the spinal cord ▪ Purkinje Cells: Detect error between intended and actional movement compute adjustment project output to fastigial and interposed nucleus ▪ Cerebellar Output: Depends on deep cerebellar nucleus ▪ Vermal Zone: Fastigial nucleus projects to vestibular nuclei and reticular formation reticulospinal and vestibulospinal tracts act on spinal motoneurons of proximal musculature for adjusting posture ▪ Paravermal Zone: Interposed nucleus projects to red nucleus and thalamus motor cortex corticospinal tract acts on spinal motoneurons of distal musculature for adjusting movement of limbs, hands, and feet o Inferior Olive: Can modulate cerebellar circuitry during motor learning  o Lesions: Isolated lesions are rare and usually occur in conjunction with lesions of the neocerebellum - Cerebrocerebellum (neocerebellum, cerebellar hemispheres/lateral zone) o Function: Plans movement before execution o Input: Loop that allows premotor cortex to use the computing power of the cerebellum to plan ▪ Premotor Cortex: Projects to cortex of lateral hemisphere via corticopontine and pontocerebellar tracts, while sending collaterals to dentate nucleus ▪ Purkinje Cells: Project to dentate nucleus dentate nucleus projects to red nucleus and thalamus motor/premotor cortex ▪ Inferior Olive: May modulate cerebellar circuitry during motor learning Cerebellar Syndromes - Cerebellar Ataxia: Lack of coordination - Flocculonodular Syndrome (medial) o Problem with balance o Staggering, wide-based gait o Abnormal eye movements (nystagmus) - Anterior Lobe Syndrome (~vermis) o Broad-based staggering gait, jerky limb movements, slurred speech, NO nystagmus - Neocerebellar Syndrome: Damage to lateral hemisphere, effects on ipsilateral side o Hypotonia, hyporeflexia o Intention tremor and dysmetria misjudging distance, defective FTN test o Dysdiadochokinesia (problem with rapid alternating movement) o Decomposition of movement (puppet-like) o Scanning speech - Chronic Alcoholism: May cause cerebellar degeneration and symptoms of both types of cerebellar syndrome o Symptoms: Broad-based staggering gait, slurred speech, jerky limb movements - Finger-To-Nose Test (FTN): Test of appendicular ataxia o Requirements: Coordination of agonist/antagonist muscles acting on multiple joints (shoulder, elbow, wrist)  o Association: Intention tremor, which increases when approaching target Basal Nuclei (Ganglia) Connections of the Basal Ganglia - Motor Control: Initiated in the motor cortex, but is regulated by loops through the basal ganglia and thalamus o Glutamate: Excitatory o GABA: Inhibitory - Feedback Loops (2): Act in parallel but have opposite effects o Direct: Facilitates movement o Indirect: Suppresses unnecessary movement o GPi and GPe: Active at rest and exerts inhibitory effects on thalamus and subthalamic nuclei  - Direct Loop: Provides positive feedback to the cortex increases motor actions o Disinhibits thalamic neurons and excites connections to the cortex - Indirect Loop: Negative feedback to cortex reduces unwanted motor actions o Excitatory thalamic neurons that project back to cortex are inhibited o Net negative effect for the cortex o Activation of the indirect loop through the putamen suppresses movement - Substantia Nigra: Extra adjustments via the nigrostriate pathway, DA from pars compacta o Releases: Dopamine acts on both loops in the striatum, on D1/D2 receptors o Function: Increases movement ▪ Stimulates direct loop (D1 receptors, excitatory) ▪ Inhibits indirect loop (D2 receptors, inhibitory) - Parallel Loops (through the basal ganglia) o Motor Circuit: via putamen, globus pallidus, VA/VL thalamus o Cognitive Circuit: via caudate, globus pallidus, DM thalamus o Limbic Circuit: NAcc, ventral pallidum, anterior thalamus Dyskinesia (Motor Disorders of Basal Ganglia) - Hypokinetic Disorder (e.g., Parkinson9s disease, difficulty initiating movement) o Stop-start: Degeneration of dopamine neurons in substantia nigra compacta - Hyperkinetic Disorder (unwanted and involuntary movements, decreased activity of STN) o Hemiballismus: Damage to subthalamic nucleus breaks the indirect loop uncontrollable ballistic movements of the contralateral arm/leg o Huntington9s Chorea: Degeneration of indirect striatal GABAergic cells controlling the indirect loop uncontrollable movement in the limbs, face, and tongue - Parkinson9s Disease o Symptoms ▪ Bradykinesia (reduced movements) ▪ Expressionless face (mask) ▪ Rigidity (flexors and extensors) ▪ Resting tremor (rolling pill) ▪ Abnormal posture (stooped) o Pathophysiology ▪ Dopamine neurons of SNc modulate the striatal neurons of the direct and indirect loop in opposite directions ▪ Excitatory effect via D1 receptors on GABAergic neurons of direct loop ▪ Inhibitory effect via D2 receptors on GABAergic neurons of indirect loop o Degeneration of Dopamine Neurons impaired direct pathway and facilitated indirect pathway ▪ No facilitation of direct loop via D1 and no inhibition of indirect loop via D2 ▪ Balance between direct and indirect loop shifts towards indirect loop ▪ More GPi inhibition of thalamus ▪ Less movement and difficulty initiating movement Body Control Motor Control: Reflexes Overview of Motor System - Skeletal Muscle: Performs voluntary contractions - Activation Pathway: Upper motor neuron in brain (from precentral gyrus) through the brainstem Lower motor neuron (a-motoneuron) in segments of the spinal cord o Purpose: Controlling movement and regulating force o Issues: Maximum force can be too strong for tendons/bones fractures (e.g., in epileptic seizures) - Neuromuscular Coupling: Presynaptic axon terminal releases ACh onto postsynaptic membrane in neuromuscular junction action potential Ca2+ influx contraction o A single muscle fibre contraction (8twitch9) produces the same force for a constant muscle fibre length o Higher Firing Rate of A-Motoneuron: Muscle fibre twitches fuse (intracellular Ca2+ accumulation), larger force production - Motor Unit: One a-MN innervates multiple muscle fibres o Small Motor Unit: One a-MN controls only a few muscle fibres for fine motor skills (e.g., fingers, eyes) o Large Motor Unit: One a-MN controls many muscle fibres (e.g., legs) - Reflex: A relatively unpredictable, involuntary, and stereotyped response to an eliciting stimulus, defined by its reflex arc o Spinal Reflexes: Integration centre = spinal cord, with reflexes able to be mapped to spinal segments (input through dorsal root, output through ventral route) ▪ Monosynaptic Reflexes: Occur within one segment ▪ Polysynaptic Reflexes: Span more than one segment o Brain: Descending tracts modulate spinal reflexes integration of reflexes into voluntary movement programs - A-MN Activity o Alpha and gamma MNs receive complex, pressed input from numerous descending tracts voluntary movement patterns interact with regulating input (representing visual, vestibular, and processed somatosensory information from muscle spindles and other peripheral sensors) Myotatic Reflex (Stretch) - Type of Reflex: Monosynaptic o Stimulus: Muscle stretch (increase in length) o Sensor: Muscle spindle, 8intrafusal9 fibres o Afferent: Type Ia sensory fibres (very fast) o Integration: Monosynaptic excitatory connection to the a-MN in the ventral horn o Efferent: a-MN, motor axon o Effector: Muscle (extrafusal fibres – contractile 8working9 fibres, A-alpha input) o Response: Muscle lengthening is met by muscle contraction (homeostasis, example of negative feedback) - Muscle Spindle: Specialised, small skeletal muscle fibres to sense length o Afferents: Group Ia fibres o Efferent Input: Gamma fibres connect to the spindle9s contractile ends and regulate the gain + sensitivity of the stretch reflex o Alignment: In parallel with the extrafusal fibres o Physiological Contraction: Occurs in normal body, seen in Case 2 ▪ Co-activation allows for steady spindle response during contraction and information to be continuously delivered to the CNS - Polysynaptic Side-Effects: Monosynaptic reflex sends collateral impulses to neighbouring spinal cord segments, through an inhibitory interneuron (for the antagonist) o Effect: Antagonist inhibition, to avoid agonist/antagonist co-activation o Examples ▪ Elbow: Flexor (m. biceps brachii, C5,6) ´ extensor (m. triceps brachii, C7,8) ▪ Knee: Flexor (m. biceps femoris, L5, S1) ´ Extensor (m. quadriceps, L4) - Clinical Application o Normal Function: Load change, muscle stretch ▪ How much the involuntary reflex interacts with the voluntary motor program (8hold the glass9) is regulated by descending pathways from the brain ▪ Higher motor centres modulate the gain of the reflex o Aim: Screening spinal cord function (segment-wise) functional localisation ▪ Upper Motor Neuron Lesion: Myotatic reflex enhanced (disinhibition), hyperreflexia due to lack of inhibition from higher motor centres (e.g., brain, brainstem, cerebellum) ▪ Lower Motor Neuron Lesion: Myotatic reflex attenuated/absent ▪ Lesions of the Pyramidal Tract: Increased gain of the myotatic reflex hyperreflexia, clonus (present in MS, stroke, ALS) ▪ Asymmetry: Never normal o Process: Striking tendon with a small hammer rapid stretch of the muscle spindle, leading to muscle contraction ▪ Biceps Reflex: C5, C6 ▪ Triceps Reflex: C6, C7 ▪ Knee Extensor: M. quadriceps femoris, patellar reflex L4 ▪ Foot Flexor: M. triceps surae, Achilles tendon reflex S1 ▪ Finger Flexors: Hoffman reflex C7, C8, C9 ▪ Discus Prolapse (L5/S1) reflex loss objectively proves nerve damage Polysynaptic Reflexes - Golgi Tendon Reflex: Regulates maximum muscle tension o Function: Protective – excess force/load may rupture muscles o Stimulus: Increase in muscle tension (force/area) o Sensor: Golgi tendon organ, braided collagen fibres + nerve fibres o Afferents: Type Ib sensory fibres (not as fast as Ia) o Integration: Activation of inhibitory interneuron, inhibiting a-MN o Efferent: Aa fibre (reduced firing) o Effector: Muscle (extrafusal fibres) o Response: Muscle relaxation – sudden stop of contraction when tension becomes too large, as fibres that exert maximum effort must recover through motor unit cycling o Alignment with Muscle Fibres: In series - Flexor/Withdrawal Reflex o Function: Protective, much faster than cortical processing o Sensor: Skin sensors (e.g., pain, heat – nociceptors) o Afferent: Slow fibres (type III, IV) o Integration: Spinal cord o Efferent: Motor axons, Aa o Effect: Arm retraction (flexor activation retraction, assisted by antagonist relaxation and inhibition) about 0.2-0.5s after trigger, with many synapses involved o In Lower Extremities: As the triggered leg is elevated, the other leg extends to maintain stability and push body away from the trigger ▪ Requirements: Multisegmented, polysynaptic activation of both sides of the spinal cord (the below is for painful stimulus on right foot) ▪ Right knee flexor (S1) + / Right knee extensor (L3/4) – ▪ Left knee flexor (S1) – / Left knee extensor (L3/4) + Motor Control: Spinal Pathways and Motor Cortex Spinocerebellar Tract - Spindle Information: From myotatic reflex, also goes to cerebellum via the spinocerebellar tracts (though monosynaptic processing of the information occurs in the spinal cord) - Description: Thick myelination, very fast - Target: Spinocerebellum (vermis) – muscle tone, anti-gravity, unconscious processing control of the ipsilateral half of the body - Symptoms of Lesions: Gait ataxia (drunken walk), ataxia of stance (Romberg test), muscle hypotonia, hyporeflexia (both more typical of neocerebellar lesions) Pyramidal System and Tracts - Corticospinal Tract: Connects cortical upper motoneurons to lower motoneurons in the spinal cord (e.g., for arms and legs) – two on each side o Lateral Corticospinal Tract: Carries 90% of the axons limb muscles (decussates) o Anterior/Ventral/Medial Corticospinal Tract: Smaller diameter, ends in the thoracic spine segments axial/trunk muscles (does not decussate) - Corticobulbar Tract: Connects cortical upper motoneurons with lower motoneurons in the brainstem o Corticomesencephalic (CN 3, oculomotor nerve) o Corticopontine (CN 7, facial nerve) - Medulla Oblongata: 80-90% of the axons decussate, resulting in a pyramidal appearance on the macroscopic brainstem surface motor cortex controls the contralateral part of the body - Precentral Gyrus: Primary motor cortex o Neocortex: 6 layers (I – VI), with layer V = internal pyramidal layer o Betz Cells: Very large neurons with a pyramidal shape under the microscope, which have axons that form the pyramidal tract Extrapyramidal System - Overview: Networks that set up a framework of motor neuron excitability, tonus, and sensitivity to reflex loops o Connected in parallel to the pyramidal tract – brain (UMN) to spine (LMN) - Definition: Complex network connecting numerous brain regions (basal nuclei, red nucleus, substantia nigra, reticular formation) and the local spinal cord networks - Function: Integration of voluntary movement with the unconscious (e.g., proprioceptive information from muscle spindles, tendon organs, visual input, vestibular information, etc.) - Brainstem Herniation: Decerebrate rigidity (tonic extension) - Rubrospinal Tract: From red nucleus (midbrain) to spinal cord o Receives collateral from the corticospinal tract o Communicates with the cerebellum (error correction, fine tuning,  8smoothing9 of movements) output through rubrospinal tract to a-MN (LMN) in the spinal cord o Function: Motor learning and direct movement regulation - Tectospinal Tract o Mesencephalic tectum controls eye movements, pathway integrates head-eye-body coordination (e.g., visual pursuit, response to visual/audio stimuli – blinking and turning away from loud noises) - Vestibulospinal Tract o Vestibular system in the inner ear contributes information about position in space, rotation, acceleration, and balance o Function: Postural reflexes, maintain upright posture, translating vestibular information into movement (e.g., sports, jumping, fast rotations, avoiding falls) - Reticulospinal Tract o Reticular Formation: Loose network of neurons extended throughout the whole brainstem, with less defined nuclei ▪ Pontine RF: Activates extensors (anti-gravity) ▪ Medullar RF: Inhibits extensors o Function: Sets overall level of tonus, motor activity, postural reflexes Motor Cortices - Primary Motor Cortex (M1): Encodes activation of single muscles and groups of muscles, encoding force production and adaptions to external influences o The anterior parts of the motor cortex determine more complex movements o Distorted somatotopic map based on neuron density in area o Direction of Movement: Activates muscles moving in desired direction and inhibits muscles moving in the opposite direction o Prolonged Electrical Stimulation: M1 encodes complex, meaningful movements ▪ Stimulation of the same M1 region encodes the same movement from different starting positions - Premotor Area (PM): Extended region frontal to M1, lateral of the SMA, with a direct connection to the spinal cord o Function: Encodes action sequences, connecting with visual and afferent areas ▪ Hand-related activity (grasp, manipulation), coordinated eye-head movements o Lesions: Isolated M1 and PM lesions recover more quickly due to direct connections to the spinal cord (combined lesions recover much less) - Supplementary Motor Area: Extended region frontal to M1, near the midline and medial to the PM o Function: Encodes bilateral movements, multi-muscle + multi-joint movements and sequences, and motor sequence rehearsing Vision: Eye and Optics Optics - Optical Function o Image Formation: The eye uses refraction to focus light emitted/reflected by objects in the environment into an image on the retina o Transduction: Image on retina (and its properties) is converted from EM radiation into membrane depolarisation ▪ Frequency: Colour ▪ Amplitude: Brightness ▪ Location: Near/Far/Direction - Refraction: Bending of light rays when passing through on transparent medium to another o Increases With: Larger difference in refractive index; angle of incidence further from perpendicular o Refractive Index: Ratio of the speed of light in a vacuum to speed of light in the medium (RI of air = 1.00, RI of water = 1.33 at 25°C) - Lenses o Convex: Converge rays emitted from a point back to a focal point real image ▪ Focal Length: Distance between lens centre and focal point ▪ Focal Length of Eye: 0.017m o Concave: Diverge rays, considered to form a virtual image extrapolate back to get a negative focal length o Power: Determined by radius of curvature and RI of the medium compared to air ▪ Reciprocal of focal length (m) in dioptres (D) ▪ The powers of concave lenses are expressed in negative dioptres ▪ Power of Eye = 58D/59D, with two-thirds being the air/cornea interface  - Lens Accommodation: Increases the power of the eye to bring near objects into focus o Near Vision: Parasympathetic stimulation increasing power of lens o Emmetropic (Normal) Eye: Objects at infinity in focus on the retina while the lens is relaxed o Presbyopia: Decrease in accommodation with as, as the lens becomes stiffer and loses elasticity ▪ In young people, accommodation can change the lens power by up to 14D o Hyperopia (long sight): Usually not corrected unless severe, as patient can use accommodation of their lens to compensate ▪ Presbyopia ultimately, reading glasses with a convex lens o Myopia (short sight): Eye is too long, or a cornea with too much power ▪ Corrected using a concave lens Photoreception - Retina: Complex neural structure that transforms light into neural potentials o Photoreceptor Layer: Rods and cones packed very densely at the back of the eye, so light must travel through all the overlying areas (rods larger than cones) ▪ Components: Outer segment, inner segment, cell body, foot ▪ Opsin Molecules: Different between rods and 3 types of cones, altering the photon wavelength they best capture o Optic Nerve: Only ganglion cells and their axons form the optic nerve - Retinal Vessels: Enter through the optic disc and spare the macula (few in central area of retina) o Optic Disc: Produces functional blind spot o Fovea: Site where image is centred - Opsins: Membrane-spanning proteins that bind retinal o Location: Sits in pocket inside opsin molecule o Action: Captures photon changes shape expels opsin molecules - Transduction: Cascade beginning with opsin, which activates second messengers o Location: In stacked membrane discs of the rod or cone, which are packed with opsin and associated machinery o In the Dark ▪ cGMP levels in cytosol are high, Na+ channels (gated by cGMP) are open ▪ Na+ enters cell depolarisation spreads from outer segment to terminal ▪ Ca2+ channels open in response to depolarisation Ca2+ influx ▪ Transmitter exocytosis graded potentials in bipolar cell o In Light: One photon activates one opsin, which can activate many transducins, each activating many phosphodiesterases, each using up many molecules of cGMP ▪ Light is absorbed by photopigment retinal and opsin dissociate ▪ Transducin is activated Phosphodiesterase is activated ▪ cGMP levels in cytosol decrease Na+ channels close hyperpolarisation ▪ Calcium channels close transmitter release is decreased ▪ Reduction in graded potential caused in bipolar cell  - Light Sensitivity o Rods: Monochromatic low-light vision o Cones: High-acuity colour vision - Central Fovea: Elements other than cones are displaced laterally, causing a pit Colour Vision - Photoreceptor Types: Preferentially absorb particular wavelengths of light o S Cone (short wavelength): Blue o M Cone: Green (prefer shades of orange) o L Cone: Red (prefer shades of orange)  o Bell-Shaped Absorbance Curve: Makes colour ambiguous at night, when only rods are used, but ratio of activity in 3 cones determine colour by day - Colour Production o Black: Subtractive colour processes combine o White: Additive colour processes combine - Anomalous Trichromacy o Both red and green cones are present, but peak absorbance of one of the opsins has been shifted closer to the other o Red/Green opsins are evolutionarily recent (in primates but not carnivores), closely related, and both found on the X chromosome ▪ Males more likely to have condition than females because only one copy of each opsin exists o Presentation: Reds and greens seem more similar than to normal viewers, as do purples and greys - Protanopia/Deuteranopia/Tritanopia: Absence of cone type (R, G, B) Vision: Central Pathways/Cortex Visual Fields and Pathways - Visual Field Representation: One in each retina, split up into visual hemifields o Projections: Each visual field projects to the contralateral thalamus (LGN) and visual cortex (V1), with the optic nerve fibres crossing at the optic chiasm o Retinal Pathways: Axons of ganglion cells in the retina form the optic nerve ▪ P-Type Ganglion Cells: Colour-sensitive, slow, small fields ▪ M-Type Ganglion Cells: Colour-insensitive, fast, large fields - Lateral Geniculate Nucleus: Preserves specific inputs in separate layers o Eye Input: Ipsilateral (2, 3, 5) and contralateral (1, 4, 6) o M Cells: 1, 2 o P Cells: 3, 4, 5, 6 - Optic Radiation: Links LGN in thalamus and occipital cortex  Function Result of Lesion in P-Pathway Result of Lesion in M-Pathway Colour vision Deficit Normal Texture perception Deficit Normal Pattern perception Deficit Normal Acuity Deficit Normal Contrast perception Deficit Normal Flicker perception Normal Deficit - Cortical Magnification: Fovea is represented over a relatively larger area of V1 than more peripheral retinal regions less convergence and smaller receptive fields higher acuity - Lesion Effects - Targets for Retinal Axons: Visual information also goes to other targets in the brain o Suprachiasmic Nucleus: Circadian rhythm o Superior Colliculus: Orientation o Pretectum: