Brain Anatomy

Overview: Major Divisions

1. Cerebral Hemispheres (Telencephalon)

Frontal Lobe

• Bounded posteriorly by the central sulcus (of Rolando) and inferolaterally by the Sylvian (lateral) fissure
• Contains the precentral gyrus - the primary motor cortex (voluntary movement)
• The inferior frontal gyrus includes Broca's area (speech production, dominant hemisphere)
• Prefrontal cortex: executive function, personality, decision-making
• Key gyri: superior, middle, inferior frontal gyri

Parietal Lobe

• Bounded anteriorly by the central sulcus
• Postcentral gyrus = primary somatosensory cortex
• The intraparietal sulcus separates the superior parietal lobule from the inferior parietal lobule
• The inferior parietal lobule contains the supramarginal gyrus and angular gyrus (language processing)
• No sharp lateral demarcation from the occipital or temporal lobes

Temporal Lobe

• Lies below the Sylvian fissure
• Contains the primary auditory cortex (Heschl's gyri on the superior temporal plane)
• Wernicke's area (speech comprehension) in the posterior superior temporal gyrus of the dominant hemisphere
• Medial temporal lobe: hippocampus and amygdala (memory and emotion)

Occipital Lobe

• Most posterior lobe, separated from parietal lobe medially by the parieto-occipital sulcus
• Contains the primary visual cortex (V1) along the calcarine fissure
• The cuneus (above calcarine fissure) and lingula (below) are key medial landmarks

Insula (5th lobe)

• Buried within the depths of the Sylvian fissure, covered by the frontal, parietal, and temporal opercula
• Involved in interoception, taste, pain, and autonomic regulation

Key White Matter Structures

• Corpus callosum: the large C-shaped commissure connecting homologous areas of both hemispheres; divided into rostrum, genu, body, and splenium
• Internal capsule: V-shaped white matter tract containing corticospinal and corticobulbar fibers; divided into anterior limb, genu, and posterior limb
• Corona radiata: the fan-shaped expansion of the internal capsule radiating to the cortex

2. Diencephalon

Thalamus

• Paired egg-shaped gray matter masses flanking the third ventricle
• Acts as the relay station for almost all sensory input to the cortex (except olfaction)
• Key nuclei: VPL (somatosensory relay), LGN (visual), MGN (auditory), VL/VA (motor)

Hypothalamus

• Below the thalamus, around the third ventricle
• Controls the autonomic nervous system, pituitary gland, hunger, thirst, temperature regulation, circadian rhythms, and reproductive behavior
• Connected to the pituitary via the infundibular stalk

Other Diencephalon structures

• Subthalamus: contains the subthalamic nucleus (STN), part of the basal ganglia circuit
• Epithalamus: contains the pineal gland (melatonin synthesis) and habenular nuclei

3. Basal Ganglia

• Caudate nucleus and putamen together = striatum (main input structure)
• Globus pallidus (internal and external segments)
• Substantia nigra (in the midbrain) and subthalamic nucleus are functionally part of this circuit
• Role: regulation of voluntary movement, procedural learning, habit formation

4. Brainstem

Midbrain (Mesencephalon)

• Most rostral brainstem segment, connects to the diencephalon
• Tectum: dorsal plate containing the superior colliculi (visual reflexes) and inferior colliculi (auditory reflexes)
• Tegmentum: contains cranial nerve nuclei (CN III, IV), red nucleus, substantia nigra, periaqueductal gray (pain modulation)
• Cerebral peduncles: large ventral columns carrying corticospinal fibers
• Cranial nerves: CN III (oculomotor) exits ventrally; CN IV (trochlear) exits dorsally - the only CN to exit dorsally

Pons

• Bulging ventral structure; connects to the cerebellum via the middle cerebellar peduncles
• Contains nuclei for CN V (trigeminal), VI (abducens), VII (facial), VIII (vestibulocochlear)
• The pontine reticular formation plays a key role in arousal and consciousness
• The locus coeruleus (norepinephrine) and raphe nuclei (serotonin) are located here

Medulla Oblongata

• Most caudal brainstem segment, continuous with the spinal cord at the foramen magnum
• Houses cardiovascular, respiratory, and vomiting centers - vital for survival
• Contains CN IX (glossopharyngeal), X (vagus), XI (spinal accessory), XII (hypoglossal)
• Key surface landmarks: pyramids (corticospinal tracts), inferior olives (cerebellar coordination), pyramidal decussation (where corticospinal fibers cross)
• The reticular formation extends throughout the brainstem, regulating consciousness, pain, and autonomic function

5. Cerebellum

• Located in the posterior cranial fossa, behind the pons and medulla
• Connected to the brainstem by three pairs of cerebellar peduncles (superior, middle, inferior)
• Divided into two hemispheres and a midline vermis
• Cortex has a distinctive folded appearance (folia)
• Function: coordination of movement, balance, fine motor control, motor learning
• Does NOT initiate movement but refines and coordinates it

6. Ventricular System and CSF

• Choroid plexus (modified ependymal cells) produces ~0.5 L of CSF per day
• CSF circulates around the brain and spinal cord in the subarachnoid space, then is reabsorbed into the venous system (mainly via arachnoid granulations)

7. Meninges (Protective Coverings)

1. Pia mater - delicate, adheres directly to brain surface
2. Arachnoid mater - spiderweb-like; CSF flows in the subarachnoid space between pia and arachnoid
3. Dura mater - tough outer layer; forms folds like the falx cerebri and tentorium cerebelli

8. Cortical Functional Areas (Brodmann Map Summary)