Gross Anatomy of the Brain & Cranial Nerves

Central vs. Peripheral Nervous System

• Principal divisions
  • Central Nervous System (CNS)
  • Components: \text{brain} and \text{spinal\ cord}.
  • Core roles: interpretation of incoming sensory input; issuing motor instructions based on current input and past experience.
  • Peripheral Nervous System (PNS)
  • Components: \text{cranial\ nerves}, \text{spinal\ nerves}, \text{ganglia}, sensory receptors.
  • Two broad subdivisions:
    1. Sensory (afferent) portion: conducts impulses to the CNS.
    2. Motor (efferent) portion: conducts impulses from the CNS.
    • Somatic (voluntary) division → controls skeletal muscle.
    • Autonomic Nervous System (ANS) → controls smooth muscle, cardiac muscle, glands.
• Functional dependence: voluntary, learned behaviors (e.g., speech, reasoning) disappear if sensory history is erased—illustrates CNS supremacy in defining individuality.

Embryologic Brain Development

• Initial structure: hollow \text{neural\ tube} along dorsal midline.
• Week 4: rostral end expands → primary brain vesicles
  1. Prosencephalon (forebrain)
  2. Mesencephalon (midbrain)
  3. Rhombencephalon (hindbrain)
• Week 5: formation of secondary brain vesicles
  • Prosencephalon → Telencephalon (cerebrum), Diencephalon.
  • Mesencephalon remains undivided.
  • Rhombencephalon → Metencephalon (pons + cerebellum), Myelencephalon (medulla).
• Adult derivatives & neural-canal remnants
  • Telencephalon → cerebral hemispheres; cavity = lateral ventricles.
  • Diencephalon → thalamus, hypothalamus, epithalamus, retina; cavity = third ventricle.
  • Mesencephalon → midbrain; cavity = cerebral aqueduct.
  • Metencephalon/Myelencephalon → brain-stem regions + cerebellum; cavity = fourth ventricle → central canal.

Brain Regions & Landmarks

Cerebral Hemispheres

• Largest, most superior mass; surface folded into:
  • Gyri = elevated ridges.
  • Sulci = shallow grooves.
  • Fissures = deeper grooves.
• Major fissures/sulci
  • Longitudinal fissure → separates hemispheres.
  • Transverse cerebral fissure → cerebrum vs. cerebellum.
  • Central sulcus → divides frontal from parietal lobes.
  • Lateral sulcus → temporal from parietal/frontal.
  • Parieto-occipital sulcus (medial) → parietal vs. occipital.
• Lobes (named for overlying cranial bones) + insula (buried within lateral sulcus).
• Gray vs. White matter
  • Cortex (gray): neuron cell bodies; site of conscious mind.
  • White matter: myelinated tracts.
  • Association fibers (within one hemisphere).
  • Commissural fibers (between hemispheres); largest = corpus callosum.
  • Projection fibers (cortex ↔ lower CNS); fan-shaped superiorly = corona radiata; condenses to internal capsule.
• Basal nuclei (basal ganglia): caudate nucleus, putamen, globus pallidus, amygdaloid body.
  • Functions: initiation & regulation of voluntary motor activity.
  • Putamen + caudate = striatum (striped appearance via internal capsule).
• Functional cortical areas (examples):
  • Primary motor cortex: conscious skeletal control (precentral gyrus).
  • Broca’s area: speech muscles & planning.
  • Primary somatosensory cortex: skin/proprioceptive input (postcentral gyrus).
  • Visual, auditory, olfactory cortices + related association areas.

Diencephalon

• Thalamus
  • Paired egg-shaped masses; enclose third ventricle; joined by interthalamic adhesion.
  • Relay & integration center for sensory input headed to cortex.
• Hypothalamus
  • Forms ventral walls/floor of third ventricle.
  • Autonomic control: body temp, water balance, metabolism, drives (hunger, thirst, sex).
  • Key external landmarks: optic chiasma (visual pathway), infundibulum/pituitary stalk, mammillary bodies (olfactory relay).
• Epithalamus
  • Roof of third ventricle.
  • Contains pineal gland (melatonin) & choroid plexus (CSF production).

Brain Stem

• Midbrain (mesencephalon)
  • Anterior: cerebral peduncles (motor tracts).
  • Posterior: corpora quadrigemina →
  • Superior colliculi (visual reflex).
  • Inferior colliculi (auditory reflex).
  • Cerebral aqueduct traverses midbrain (third → fourth ventricle).
• Pons (metencephalon)
  • Bridge of conduction tracts connecting cerebrum ↔ cerebellum ↔ medulla.
• Medulla oblongata (myelencephalon)
  • Continuous with spinal cord; site of pyramidal tract decussation.
  • Houses vital autonomic centers: cardiac, respiratory, vasomotor; plus reflex centers for vomiting, swallowing, etc.

Cerebellum

• Two hemispheres + central vermis; lobes = anterior, posterior, flocculonodular.
• Gray cortex surrounding white arbor vitae.
• Functions: subconscious coordination of skeletal movement, balance, equilibrium.
• Connected to brain stem by superior, middle, inferior cerebellar peduncles.

Ventricular System & CSF

• Cavities: lateral (pair) → interventricular foramina → third ventricle → cerebral aqueduct → fourth ventricle → central canal & subarachnoid space (via 3 apertures: 1 median, 2 lateral).
• CSF production: choroid plexuses (capillary knots with ependyma) in each ventricle.
• Circulation path summarized above; returned via arachnoid granulations to dural venous sinuses (esp. superior sagittal sinus).
• Homeostasis: formation ≈ reabsorption; blockage → hydrocephalus (infant skull expands; adult intracranial pressure rises → neural damage).

Meninges & Cranial Sinuses

• Dura mater (double-layered in skull)
  • Periosteal (outer) adheres to bone; meningeal (inner) continues as spinal dura.
  • Dural folds/septa
  • Falx cerebri: between cerebral hemispheres; attaches to crista galli.
  • Falx cerebelli: between cerebellar hemispheres.
  • Tentorium cerebelli: horizontal; separates cerebellum from occipital lobes.
  • Venous dural sinuses (e.g., superior sagittal sinus) lie between layers.
• Arachnoid mater
  • Web-like; separated from dura by subdural space; bridges to pia via subarachnoid space filled with CSF.
  • Arachnoid granulations pierce dura → CSF drain ports.
• Pia mater
  • Delicate, vascular membrane clinging to brain surface.
• Clinical tie-ins: meningitis (inflammation) may spread to brain → encephalitis; CSF tap (lumbar puncture) assists diagnosis.

Cranial Nerves Overview (PNS)

• Total pairs: 12 (Roman-numeral ordered, I–XII).
  • Mnemonic: “On Occasion Our Trusty Truck Acts Funny—Very Good Vehicle Anyhow.”
• General notes
  • Mostly mixed; exceptions: I (Olfactory) & II (Optic) are purely sensory; VIII predominantly sensory.
  • All except I & II arise from brain stem; only X (Vagus) traverses thoraco-abdominal cavity.
• Snapshot (origin/course & key functions)
  • I Olfactory: olfactory epithelium → cribriform foramina → bulbs; smell.
  • II Optic: retina → optic canal → chiasma → tracts → thalamus → visual cortex; vision.
  • III Oculomotor: ventral midbrain → superior orbital fissure; eye movements, lens & pupil control.
  • IV Trochlear: dorsal midbrain → superior orbital fissure; superior oblique muscle.
  • V Trigeminal (ophthalmic, maxillary, mandibular): face sensation, mastication.
  • VI Abducens: pons → superior orbital fissure; lateral rectus (abduction).
  • VII Facial: pons → internal acoustic meatus → stylomastoid foramen; facial expression, taste (anterior 2/3), glands.
  • VIII Vestibulocochlear: inner ear → internal acoustic meatus; equilibrium, hearing.
  • IX Glossopharyngeal: medulla → jugular foramen; taste (posterior 1/3), carotid receptors, swallowing.
  • X Vagus: medulla → jugular foramen → thorax/abdomen; parasympathetic to viscera, sensory from viscera.
  • XI Accessory: spinal cord (C1–C5) → jugular foramen; sternocleidomastoid & trapezius.
  • XII Hypoglossal: medulla → hypoglossal canal; tongue movement.
• Standard clinical tests:
  • Smell identification, eye chart, pupillary light reflex, facial symmetry tasks, tuning fork, gag & cough reflexes, shoulder shrug, tongue protrusion, etc.

Sheep vs. Human Brain (Key Comparative Anatomy)

• Sheep olfactory bulbs ≫ human → heightened reliance on smell for foraging & danger detection.
• Human cerebrum proportionally larger; fissures deeper & more numerous → advanced cognitive capacity.
• Sheep cerebellum orientation differs; lacks longitudinal division by falx cerebelli present in humans.
• Mammillary body: single rounded in sheep vs. paired in humans.
• Fornix smaller in sheep (reduced limbic connectivity correlates with less complex emotional processing).

Dissection & Lab Activities (Highlights)

• Activity 1: Identify external cerebral structures on models/specimens.
• Activity 2: Internal anatomy via midsagittal/coronal cuts; locate corpus callosum, ventricles, basal nuclei, diencephalon components, brain-stem parts, cerebellar arbor vitae.
• Activity 3: Cranial nerve identification on ventral brain; functional testing list (aromatic oils, penlight, tuning fork, etc.).
• Sheep brain dissection steps
  1. Remove dura; examine arachnoid “cottony” web & pia.
  2. Identify ventral structures: olfactory bulbs, optic chiasma & tracts, infundibulum, mammillary body, cerebral peduncles, cranial nerves III–XII.
  3. Dorsal view: cerebellar peduncles, corpora quadrigemina, pineal gland.
  4. Median section: reveal septum pellucidum, fornix, thalamus, hypothalamus, cerebral aqueduct, fourth ventricle, arbor vitae.
• Safety & cleanup: wear gloves, use dissecting tray, dispose of tissue per protocol; retain spinal cord segment if required.

Ethical & Practical Considerations

• Understanding brain structure–function relationships underpins neurosurgical planning, neurological examination, and treatment of disorders (e.g., hydrocephalus shunting, cranial nerve lesions).
• Comparative dissections respect animal sources; highlight evolutionary adaptations (e.g., olfaction in prey species).