Neuro-anatomy: Surfaces of the Brain, Functional Areas, and Internal Structures

Embryological Correlation of Brain Structures

• Primary Vesicles ($4^{th}$ week):

  • Forebrain (Prosencephalon): Segregates into the Telencephalon and Diencephalon.
  • Midbrain (Mesencephalon): Remains the Mesencephalon.
  • Hindbrain (Rhombencephalon): Segregates into the Metencephalon and Myelencephalon.

• Secondary Vesicles ($5^{th}$ week) and Adult Derivatives:

  • Telencephalon:
    • Adult Derivative (Walls): Cerebral hemispheres.
    • Cavity: Lateral ventricles.
  • Diencephalon:
    • Adult Derivative (Walls): Thalami, etc.
    • Cavity: Third ventricle.
  • Mesencephalon:
    • Adult Derivative (Walls): Midbrain.
    • Cavity: Cerebral aqueduct.
  • Metencephalon:
    • Adult Derivative (Walls): Pons and cerebellum.
    • Cavity: Upper part of the fourth ventricle.
  • Myelencephalon:
    • Adult Derivative (Walls): Medulla oblongata.
    • Cavity: Lower part of the fourth ventricle.
  • Spinal Cord:
    • Cavity: Central canal.

Anatomy of Brain Surfaces and Lobes

• Brain Surfaces:

  • Superolateral Surface: Contains the central sulcus, lateral fissure, and limen insulae.
  • Medial Surface: Features include the cingulate sulcus, parieto-occipital sulcus, and calcarine sulcus.
  • Basal Surface: Inferior portion containing orbital and tentorial surfaces.

• Lobes and Lobules:

  • Frontal Lobe: Separated from the parietal lobe by the central sulcus.
  • Parietal Lobe: Located between the frontal and occipital lobes.
  • Occipital Lobe: Bounded by the parieto-occipital sulcus (medial) and pre-occipital notch (lateral).
  • Temporal Lobe: Separated from the parietal/frontal lobes by the lateral fissure.
  • Insula: Deep within the lateral fissure.
  • Limbic Lobe: Located on the medial surface surrounding the corpus callosum.
  • Lobules on Superolateral Surface:
    • Superior Parietal Lobule: Separated from the inferior by the intraparietal sulcus.
    • Inferior Parietal Lobule: Includes the supramarginal and angular gyri.
  • Lobules on Medial Surface:
    • Paracentral Lobule: Surrounds the indentation of the central sulcus.
    • Precuneus: Anterior to the cuneus.
    • Cuneus: Between the parieto-occipital and calcarine sulci.

Sulci and Gyri

• Superolateral Sulci:

  • Lateral Fissure: Includes the anterior ramus, ascending ramus, and posterior ramus.
  • Central Sulcus: Separates the primary motor area from the primary sensory area.
  • Frontal Sulci: Superior and inferior frontal sulci.
  • Parietal Sulci: Precentral, postcentral, and intraparietal sulci.
  • Temporal Sulci: Superior and inferior temporal sulci.
  • Occipital sulci: Lunate sulcus; pre-occipital notch.
  • Insular Sulcus: Circular sulcus of the insula.

• Superolateral Gyri:

  • Superior, Middle, and Inferior Frontal Gyri: The inferior frontal gyrus is divided into Pars Opercularis, Pars Triangularis, and Pars Orbitalis.
  • Precentral Gyrus: Home to the primary motor area.
  • Postcentral Gyrus: Home to the primary somatosensory area.
  • Supramarginal Gyrus: Caps the posterior ramus of the lateral fissure.
  • Angular Gyrus: Caps the posterior end of the superior temporal sulcus.
  • Temporal Gyri: Superior, middle, and inferior temporal gyri.

• Medial Sulci and Gyri:

  • Sulci: Cingulate, parieto-occipital, calcarine, marginal ramus, and paracentral ramus.
  • Gyri: Cingulate gyrus (connected to the istmus), medial frontal gyrus, lingual gyrus, and parahippocampal gyrus (basal surface extension).

• Basal Sulci and Gyri:

  • Sulci: Olfactory sulcus, orbital sulci, collateral sulcus, rhinal sulcus, and parahippocampal sulcus.
  • Gyri: Straight gyrus (gyrus rectus), orbital gyri, occipitotemporal gyri (medial and lateral), uncus, and lingual gyrus.

Functional Areas of the Superolateral Surface

• Broca’s Motor Speech Area:

  • Location: Opercular and triangular parts of the inferior frontal gyrus.
  • Function: Production of meaningful language.
  • Dominance: Left hemisphere dominant in right-handed individuals; no dominance in left-handed individuals.
  • Lesions: Motor aphasia (non-fluent speech, visible effort to produce sound).

• Wernicke’s Auditory Association Area:

  • Location: Supramarginal gyrus and angular gyrus.
  • Function: Comprehension of spoken and written language.
  • Lesions: Receptive aphasia (producing meaningless words/phrases, words not in sequence).

• Prefrontal Cortex:

  • Location: Entire frontal lobe except motor areas.
  • Function: Judgment, foresight, and perception. Works with the limbic system.
  • Case Study: Phineas Gage.

• Frontal Eye Field:

  • Location: Posterior end of the middle frontal gyrus.
  • Function: Voluntary eye movement (horizontal plane), allowing eyes to follow objects. The left field forces eyes toward the right.
  • Lesions: Eyes deviate to the damaged side as the undamaged field overrides.

• Premotor Area:

  • Location: Anterior to the precentral gyrus, medial surface of paracentral lobule, and posterior ends of superior, middle, and upper inferior frontal gyri.
  • Function: Gross coordination of voluntary motor activity via basal nuclei and spinal tracts.
  • Lesions: Uncoordinated gross voluntary movement.

• Primary Somatomotor Area:

  • Location: Precentral gyrus.
  • Function: Segmental upside-down presentation of the body; voluntary motor activity of the contralateral side.
  • Homunculus (Motor):
    • Inferior: Head and Neck.
    • Middle: Upper Limb ($UL$).
    • Superior: Trunk.
    • Medial: Lower Limb ($LL$).

• Primary Somatosensory Area:

  • Location: Postcentral gyrus.
  • Function: Perception of pain, temperature, crude touch, vibration, proprioception, and fine touch from head, neck, trunk, and limbs.
  • Homunculus (Sensory): Mirroring the motor layout (Inferior: Head/Neck; Medial: Lower Limbs).

• Sensory Association Area:

  • Location: Superior parietal lobule.
  • Function: Integrates somatosensory and visual input; perception of size, shape, textures; goal-directed movements.
  • Lesions: Neglect of the contralateral half of the body and visual area (e.g., shaving only half the face).

• Visual Areas:

  • Primary Visual Area: Located on the medial surface of the occipital lobe next to the calcarine sulcus. Receives input from the lateral geniculate body ($LGB$).
  • Visual Association Area: Located in the occipital part of the parietal lobe and inferolateral temporal lobe. Analyzes form, color, movement, and depth. Outputs to frontal eye fields and superior colliculus.

• Auditory Areas:

  • Primary Auditory Area: Located in the short anterior transverse temporal gyrus (within lateral fissure). Involved in sound reflex movements.
  • Auditory Association Area: Located in the long posterior transverse gyri (planum temporale). Function is comprehension of sound.

• Other Specialized Areas:

  • Taste Area: Located in the parietal operculum; connected to pharynx and tongue.
  • Olfactory Area: Located in the anterior pole of the temporal lobe (piriform cortex and periamygdaloid area). Lesions cause olfactory hallucinations.
  • Limbic Lobe: Located in the cingulate gyrus and medial temporal edge. Involved in emotion, memory, learning, and fight-or-flight reactions.

Anatomy of the Brainstem and Cerebellum

• Medulla Oblongata:

  • External Surface: Anterior median fissure, Pyramids (corticospinal and corticonuclear fibers), Pyramidal decussation, Olives (inferior olivary nucleus), Ventrolateral (pre-olivary) sulci where $CN\,XII$ emerges, and Posterolateral (post-olivary) sulci where $CN\,IX, X, XI$ exit.
  • Dorsal Surface: Posterior median sulcus, Obex, Gracile tubercle (sensations from below $T7$), Cuneate tubercle (sensations from $C1$ to $T7$), and Inferior cerebellar peduncle.

• Pons:

  • Anterior View: Basilar groove for the basilar artery. Cranial nerves $VI, VII, VIII$ emerge at the junction.
  • Lateral Surface: Middle cerebellar peduncle and $CN\,V$ exit.
  • Internal Structure: Tegmentum (divided into ventral and dorsal by $CN\,V$).

• Cerebellum:

  • Structure: Two hemispheres joined by the vermis. Cerebellar cortex features folds called folia.
  • Nuclei (Medial to Lateral): Fastigial, Globose, Emboliform, Dentate.
  • Vermis Lobules:
    • Superior Vermis: Lingula, Central, Culmen, Declive, Folium.
    • Inferior Vermis: Tuber, Pyramid, Uvula, Nodule.
  • Lobes: Anterior, Posterior, Flocculonodular.
  • Fissures: Primary (separates anterior/posterior lobes) and Horizontal (divides posterior lobe).
  • Function: Maintenance of balance, muscle tone, and coordination of skilled voluntary movement.

• Midbrain (Mesencephalon):

  • Posterior Surface: Corpora quadrigemina (superior and inferior colliculi). $CN\,IV$ emerges below inferior colliculi.
  • Lateral Surface: Brachia of colliculi. Superior brachium connects to $LGB$ (visual reflex). Inferior brachium connects to $MGB$ (auditory reflex).
  • Anterior Surface: Cerebral peduncles (basis pedunculi), Interpeduncular fossa, and Posterior perforated substance.
  • Internal Structures: Tectum (roof), Tegmentum, Substantia nigra, Red nucleus, and Periaqueductal grey matter.

The Diencephalon

• Components: Thalamus ($80\,\%$ of diencephalon), Epithalamus, Metathalamus, Hypothalamus, and Subthalamus.

• Thalamus:

  • Basic Roles: Relay station for all sensory tracts (except olfactory), motor connections to basal nuclei/cerebellum, processing EEG rhythms/sleep-wake cycles, and limbic connections for emotion.
  • Pathology: Thalamic syndrome (noxious pain, emotional instability).

• Epithalamus: Consists of the Pineal gland (secretes melatonin), Habenular nucleus, and Medullary stria of the thalamus.

• Metathalamus:

  • Lateral Geniculate Nucleus ($LGN$): Visual relay between optic tract and occipital lobe.
  • Medial Geniculate Nucleus ($MGN$): Auditory relay for tone discrimination.

• Hypothalamus:

  • Weight: Approximately $4\,g$.
  • Functional Areas: Preoptic, Supraoptic (dorsal to optic chiasm), Tuberal, and Mamillary.
  • Activities: Regulates body temperature, hunger, thirst, sexual activity, and autonomic/endocrine rhythms.
  • Autonomic Control: Anterior/Medial areas control parasympathetic responses; Lateral/Posterior areas control sympathetic responses.

Medullary Centres (White Matter)

• Association Fibres: Connect different parts of the same hemisphere.

  • Short: Arcuate fibres.
  • Long: Cingulum, Uncinate fasciculus, Superior longitudinal fasciculus, Fronto-occipital fasciculus, Inferior longitudinal fasciculus, and Arcuate fasciculus.

• Commissural Fibres: Connect corresponding areas of two hemispheres.

  • Corpus Callosum: Includes the Rostrum, Genu, Body (corpus), and Splenium. Extensions include Forceps minor, Forceps major, and Tapetum.
  • Other Commissures: Anterior (smell), Posterior (consensual light reflex), Habenular, and Hippocampal.

• Projection Fibres: Connect the brain to the spinal cord. Example: Internal capsule (divided into anterior limb, genu, and posterior limb).

Basal Nuclei and Ventricular System

• Basal Nuclei (Extrapyramidal System):

  • Main Nuclei: Caudate nucleus, Lentiform nucleus (Putamen and Globus pallidus), Amygdaloid body, and Claustrum.
  • Associated Structures: Subthalamic nucleus, Substantia nigra, and Red nucleus.
  • Function: Linked processing stations for motor feedback and coordinated instinctive movements.

• Ventricular System and CSF:

  • Spaces: Lateral ventricles, Interventricular foramen of Monro, Third ventricle, Cerebral aqueduct of Sylvius, Fourth ventricle, and Central canal.
  • CSF Flow: Produced in choroid plexuses; flows from lateral to third to fourth ventricles; exits via the median foramen of Magendie and lateral foramina of Luschka into the subarachnoid space.

Meninges and Cisterns

• Meningeal Layers: Scalp, Skull, Epidural space (potential), Dura mater (periosteal and meningeal layers), Subdural space (potential), Arachnoid mater, Subarachnoid space (contains CSF and vessels), and Pia mater.

• Dural Partitions: Falx cerebri, Falx cerebelli, Tentorium cerebelli, and Diaphragma sellae.

• Dural Venous Sinuses: Superior/Inferior sagittal, Straight, Confluence, Transverse, Sigmoid, Cavernous (contains $ICA$ and $CN\,III, IV, VI, V1, V2$).

• Subarachnoid Cisterns:

  • Cisterna Magna (Cerebellomedullary): Between cerebellum and medulla.
  • Superior Cistern: Contains the great cerebral vein of Galen.
  • Interpeduncular Cistern: Contains $CN\,III$ and posterior cerebral arteries ($PCA$).
  • Ambient Cistern: Forms a ring around the midbrain; contains $PCA$, basal vein of Rosenthal, and $CN\,IV$.
  • Pontine Cistern: Contains the basilar artery and $CN\,VI, VII, VIII$.
  • Lumbar Cistern: Extends from $L2$ to $S2$; contains the cauda equina.

Arterial Supply of the Brain

• Carotid System: Originates from the Internal Carotid Artery ($ICA$).

  • Branches: Middle Cerebral Artery ($MCA$), Anterior Cerebral Artery ($ACA$), Ophthalmic artery, Anterior choroidal artery, and Posterior communicating artery.

• Vertebrobasilar System: Originates from the Vertebral Arteries.

  • Branches: Anterior/Posterior spinal arteries, $PICA$ (Wallenberg syndrome), $AICA$, Labyrinthine (Internal auditory) artery, Pontine branches, Superior cerebellar arteries, and $PCA$.

• Circle of Willis: Complete in less than $80\,\%$ of people. Components: $ICA$, $ACA$, Anterior communicating, $MCA$, Posterior communicating, and $PCA$.

• Arterial Territories:

  • ACA: Medial surface (except occipital), orbital surface of frontal lobe.
  • MCA: Superolateral surface (excluding ACA strip and occipital), insula, temporal pole. Branch "Arteries of Stroke": lateral lenticulostriate arteries.
  • PCA: Occipital lobe, inferior temporal lobe, primary visual area.

Cranial Nerves Summary ($I-XII$)

Questions, Discussion, and Homework

• Homework Assignments:

  • Identify every cranial nerve on an image or photograph.
  • Describe the exact position of each cranial nerve on the brainstem.
  • Identify the specific foramen each nerve passes through when exiting the cranial cavity.
  • Memorize the number, name, and classification/function of every cranial nerve.

• Key Origin Points:

  • $CN\,I$: Telencephalon.
  • $CN\,II$: Diencephalon.
  • $CN\,III, IV$: Midbrain.
  • $CN\,V, VI, VII, VIII$: Pons.
  • $CN\,IX, X, XI, XII$: Medulla oblongata.
  • Parasympathetic Nerves: $III, VII, IX, X$.