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Brain Dissection Guide

Systematic approach to brain dissection covering surface anatomy, lobes, ventricles, deep structures, and brainstem. Essential for understanding neurological examination findings and correlating structure with function.

Learning Objectives

  • ✓Identify cerebral lobes, gyri, and sulci
  • ✓Trace the ventricular system
  • ✓Examine deep gray matter structures (basal ganglia, thalamus)
  • ✓Identify brainstem structures and cranial nerve attachments
  • ✓Understand the blood supply and watershed zones

Prerequisites

  • • Understanding of skull and meningeal anatomy
  • • Knowledge of cranial nerve functions
  • • Review of major motor and sensory pathways
  • • Familiarity with brain imaging (CT, MRI)

Equipment Needed

  • • Brain knife (long, thin blade)
  • • Scalpel
  • • Dissecting scissors
  • • Probe
  • • Brain dissection board
  • • Brain model for reference

Dissection Steps

Step 1:External Surface Examination

Examine the brain externally. Identify the cerebral hemispheres, cerebellum, and brainstem. Note the longitudinal fissure separating the hemispheres and the dura-covered falx cerebri (if present).

Key Structures

Cerebral hemispheresCerebellumBrainstemLongitudinal fissureOlfactory bulbs and tracts
Tip:Handle the brain gently - fixed brain tissue is fragile. Support the brain from below when lifting.

Step 2:Identify Cerebral Lobes

On the lateral surface, identify the four lobes: frontal, parietal, temporal, and occipital. Locate the central sulcus (between frontal and parietal), lateral sulcus (Sylvian fissure), and parieto-occipital sulcus.

Key Structures

Frontal lobeParietal lobeTemporal lobeOccipital lobeCentral sulcusLateral sulcusParieto-occipital sulcus
Tip:The precentral gyrus (motor cortex) is anterior to the central sulcus; the postcentral gyrus (sensory cortex) is posterior.

Step 3:Identify Major Gyri

On the frontal lobe, identify the precentral gyrus, superior/middle/inferior frontal gyri. On the parietal lobe, find the postcentral gyrus and superior/inferior parietal lobules. Identify the superior, middle, and inferior temporal gyri.

Key Structures

Precentral gyrusPostcentral gyrusSuperior frontal gyrusInferior frontal gyrus (Broca's area)Superior temporal gyrus (Wernicke's area)Angular gyrus
Tip:Broca's area is in the inferior frontal gyrus (left hemisphere, speech production). Wernicke's area is in the superior temporal gyrus (left hemisphere, speech comprehension).

Step 4:Examine the Inferior Surface

Turn the brain to view the inferior surface. Identify the olfactory bulbs and tracts, optic nerves and chiasm, infundibulum (pituitary stalk), mammillary bodies, and cerebral peduncles.

Key Structures

Olfactory bulb and tractOptic nerveOptic chiasmInfundibulumMammillary bodiesInterpeduncular fossaUncusParahippocampal gyrus
Tip:The uncus is the medial part of the temporal lobe that herniates in uncal herniation, compressing CN III.

Step 5:Identify Cranial Nerves

Locate the attachment sites of all twelve cranial nerves. CN I and II attach to the forebrain. CN III-XII attach to the brainstem in numerical order from superior to inferior.

Key Structures

CN I (olfactory)CN II (optic)CN III (oculomotor)CN IV (trochlear)CN V (trigeminal)CN VI (abducens)CN VII (facial)CN VIII (vestibulocochlear)CN IX-XI at jugular foramen regionCN XII (hypoglossal)
Tip:CN IV is the only nerve to exit dorsally from the brainstem. Look for it on the posterior surface of the midbrain.

Step 6:Coronal Section

Make a coronal cut through the brain at the level of the mammillary bodies. Identify the cortex, white matter, corpus callosum, lateral ventricles, third ventricle, basal ganglia (caudate, putamen, globus pallidus), internal capsule, and thalamus.

Key Structures

Cerebral cortexWhite matterCorpus callosumLateral ventriclesCaudate nucleusPutamenGlobus pallidusInternal capsuleThalamusThird ventricle
Tip:The internal capsule carries motor fibers from the cortex. Strokes here cause contralateral hemiplegia.

Step 7:Examine the Ventricular System

Trace the ventricular system: lateral ventricles (with frontal, occipital, and temporal horns), foramina of Monro to third ventricle, cerebral aqueduct through midbrain, to fourth ventricle.

Key Structures

Lateral ventriclesForamen of MonroThird ventricleCerebral aqueduct (of Sylvius)Fourth ventricleChoroid plexusSeptum pellucidum
Tip:CSF is produced by choroid plexus in all ventricles (most in lateral). The aqueduct is the narrowest point and common site of obstruction.

Step 8:Midsagittal Section

Make a midsagittal cut through the brain to separate the hemispheres. Identify the corpus callosum (genu, body, splenium), septum pellucidum, fornix, pineal gland, and medial surface structures.

Key Structures

Corpus callosumSeptum pellucidumFornixCingulate gyrusPineal glandSuperior and inferior colliculiCalcarine sulcusCuneus
Tip:The calcarine sulcus in the occipital lobe contains the primary visual cortex. The area above is the cuneus.

Step 9:Examine the Brainstem

Separate the brainstem and cerebellum. Identify midbrain (cerebral peduncles, superior and inferior colliculi), pons (basilar part, middle cerebellar peduncles), and medulla (pyramids, olives, gracile and cuneate tubercles).

Key Structures

Cerebral pedunclesSuperior colliculiInferior colliculiPonsMiddle cerebellar pedunclePyramidsOlivesGracile tubercleCuneate tubercle
Tip:The pyramidal decussation is at the junction of medulla and spinal cord - explains contralateral motor control.

Step 10:Examine the Cerebellum

Cut the cerebellum in the midsagittal plane. Identify the vermis, cerebellar hemispheres, arbor vitae (white matter pattern), dentate nucleus, and cerebellar peduncles.

Key Structures

VermisCerebellar hemisphereArbor vitaeDentate nucleusSuperior cerebellar peduncleMiddle cerebellar peduncleInferior cerebellar peduncleFourth ventricle
Tip:The dentate nucleus is the largest cerebellar nucleus and sends output via the superior cerebellar peduncle.

Clinical Correlations

  • •Stroke: MCA territory (lateral cortex) causes contralateral hemiparesis (face and arm > leg), aphasia if dominant hemisphere
  • •Uncal herniation: temporal lobe herniates through tentorial notch, compressing CN III (dilated pupil) and cerebral peduncle
  • •Hydrocephalus: obstruction of CSF flow (often at aqueduct) causes ventricular enlargement
  • •Parkinson's disease: loss of dopaminergic neurons in substantia nigra affects basal ganglia circuitry

Common Mistakes to Avoid

  • ✗Confusing caudate nucleus with thalamus (caudate has a C-shape and follows the lateral ventricle)
  • ✗Difficulty distinguishing the internal capsule from surrounding structures
  • ✗Losing orientation when making cuts through the brain
  • ✗Not correlating surface gyri with underlying structures

Study Questions

  1. What structures would be affected by an MCA stroke?
  2. Trace the path of CSF from production to absorption
  3. What is the significance of the blood-brain barrier?
  4. How does damage to the internal capsule differ from cortical damage?
  5. Why does uncal herniation cause CN III palsy?

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FAQs

Common questions about this dissection

Fresh brain tissue is very soft and difficult to handle. Formalin fixation cross-links proteins, making the tissue firmer and easier to dissect while preserving anatomical relationships.

Motor/sensory deficits point to contralateral cortex or descending/ascending tracts. Language deficits suggest dominant (usually left) hemisphere. Coordination deficits suggest ipsilateral cerebellum. Cranial nerve deficits localize to brainstem level.

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