The brain is the most complex organ in the body, containing approximately 86 billion neurons. It is divided into the cerebrum, cerebellum, and brainstem, each with specialized functions.
Gross Divisions of the Brain
| Division | Embryologic Origin | Components |
|---|---|---|
| Forebrain (prosencephalon) | Telencephalon | Cerebral hemispheres, basal ganglia |
| Diencephalon | Thalamus, hypothalamus, epithalamus | |
| Midbrain (mesencephalon) | Mesencephalon | Tectum, tegmentum, cerebral peduncles |
| Hindbrain (rhombencephalon) | Metencephalon | Pons, cerebellum |
| Myelencephalon | Medulla oblongata |
Cerebrum
The cerebrum is the largest part of the brain, accounting for approximately 85% of brain weight. It is divided into two hemispheres by the longitudinal fissure.
Cerebral Hemispheres
| Feature | Description |
|---|---|
| Weight | ~1150 g (80-85% of total brain weight) |
| Surface area | ~2500 cm² (folded) |
| Cortical thickness | 1.5-4.5 mm |
| Neuron count | ~16 billion in cerebral cortex |
Hemispheric connections:
- Corpus callosum: Largest commissure (~200 million axons)
- Anterior commissure
- Hippocampal commissure
Gray matter: Cerebral cortex (outer layer) White matter: Internal capsule, corona radiata, commissural fibers, association fibers, projection fibers
Lobes of the Cerebrum
| Lobe | Location | Primary Functions |
|---|---|---|
| Frontal | Anterior to central sulcus | Motor, executive function, language (Broca), personality |
| Parietal | Posterior to central sulcus, above lateral sulcus | Somatosensory, spatial orientation, attention |
| Temporal | Inferior to lateral sulcus | Auditory, language (Wernicke), memory, emotion |
| Occipital | Posterior | Vision |
| Insula | Deep within lateral sulcus | Taste, interoception, emotion |
Functional Areas of the Cortex
Motor areas:
- Primary motor cortex (precentral gyrus, Brodmann area 4)
- Premotor cortex (area 6)
- Supplementary motor area
- Frontal eye fields (area 8)
- Broca area (areas 44, 45) - dominant hemisphere
Sensory areas:
- Primary somatosensory cortex (postcentral gyrus, areas 1, 2, 3)
- Primary visual cortex (area 17, calcarine sulcus)
- Primary auditory cortex (areas 41, 42, transverse temporal gyri)
- Primary gustatory cortex (insula, frontal operculum)
- Vestibular cortex (parietal insular)
Association areas:
- Prefrontal cortex (executive functions)
- Posterior parietal cortex (spatial attention)
- Wernicke area (area 22, language comprehension)
Basal Ganglia
The basal ganglia are a group of subcortical nuclei involved in motor control, cognition, and emotion.
| Nucleus | Location | Function |
|---|---|---|
| Caudate nucleus | Lateral to lateral ventricle | Motor learning, cognition |
| Putamen | Deep to insula | Motor control |
| Globus pallidus (internal, external) | Medial to putamen | Output (internal), modulation (external) |
| Subthalamic nucleus | Diencephalon | Modulation of globus pallidus |
| Substantia nigra (pars compacta, pars reticulata) | Midbrain | Dopamine (compacta), output (reticulata) |
Circuit: Cortex → Striatum → GPi/SNr → Thalamus → Cortex
Limbic System
The limbic system is involved in emotion, memory, and motivation.
| Structure | Location | Function |
|---|---|---|
| Hippocampus | Medial temporal lobe | Memory formation, spatial navigation |
| Amygdala | Anterior temporal lobe | Fear, emotion, social behavior |
| Cingulate gyrus | Medial surface above corpus callosum | Emotion, pain, attention |
| Fornix | Connects hippocampus to mammillary bodies | Memory circuit |
| Mammillary bodies | Posterior hypothalamus | Memory (part of Papez circuit) |
| Septal nuclei | Below corpus callosum | Pleasure, reward |
| Olfactory bulb | Below frontal lobe | Smell |
Thalamus
The thalamus is the major relay station for sensory and motor information.
Nuclear groups:
| Group | Nuclei | Function |
|---|---|---|
| Anterior | Anterior nucleus | Limbic circuit (memory) |
| Medial | Dorsomedial nucleus | Prefrontal cortex connections |
| Lateral | Lateral dorsal, lateral posterior, pulvinar | Association cortex |
| Ventral | VA, VL, VPL, VPM | Motor (VA/VL), sensory (VPL/VPM) |
| Posterior | LGN, MGN | Vision (LGN), hearing (MGN) |
| Intralaminar | Centromedian, parafascicular | Arousal, pain |
Hypothalamus
The hypothalamus regulates homeostasis, endocrine function, and autonomic activity.
| Region | Nuclei | Function |
|---|---|---|
| Preoptic | Medial preoptic | Thermoregulation, sexual behavior |
| Supraoptic | Supraoptic, paraventricular | ADH, oxytocin production |
| Tuberal | Arcuate, ventromedial, dorsomedial | Appetite, satiety, growth hormone regulation |
| Mammillary | Mammillary bodies | Memory (Papez circuit) |
| Posterior | Posterior nucleus | Heat production, arousal |
Brainstem
The brainstem connects the cerebrum to the spinal cord and contains nuclei for essential life functions.
Midbrain
| Structure | Function |
|---|---|
| Cerebral peduncles | Corticospinal, corticobulbar tracts |
| Substantia nigra | Dopamine production, motor control |
| Red nucleus | Motor coordination |
| Periaqueductal gray | Pain modulation |
| Superior colliculus | Visual reflexes |
| Inferior colliculus | Auditory reflexes |
| Oculomotor (III) and trochlear (IV) nuclei | Eye movement |
Pons
| Structure | Function |
|---|---|
| Pontine nuclei | Corticopontocerebellar relay |
| Middle cerebellar peduncle | Cerebellar input |
| Trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII) nuclei | Cranial nerve functions |
| Pontine reticular formation | Arousal, respiration |
Medulla Oblongata
| Structure | Function |
|---|---|
| Pyramids | Corticospinal tracts (decussation here) |
| Olivary nuclei | Motor coordination, cerebellar relay |
| Hypoglossal (XII), glossopharyngeal (IX), vagus (X), accessory (XI) nuclei | Cranial nerve functions |
| Nucleus tractus solitarius | Visceral sensory relay |
| Medullary reticular formation | Respiration, cardiovascular control |
Cerebellum
The cerebellum coordinates movement, balance, and motor learning.
| Lobe | location | Function |
|---|---|---|
| Flocculonodular lobe | Inferior | Vestibular, balance |
| Anterior lobe | Superior, anterior | Spinocerebellar, limb coordination |
| Posterior lobe | Superior, posterior | Cerebrocerebellar, motor planning |
Cerebellar peduncles:
- Superior: Output (to thalamus)
- Middle: Input (from cortex via pons)
- Inferior: Input (from spinal cord, brainstem)
Blood Supply of the Brain
Arterial Supply
Internal carotid arteries (anterior circulation):
| Branch | Territory |
|---|---|
| Anterior cerebral artery | Medial frontal/parietal lobes |
| Middle cerebral artery | Lateral frontal/parietal/temporal lobes, basal ganglia |
| Anterior choroidal | Internal capsule, hippocampus |
Vertebral arteries (posterior circulation):
| Branch | Territory |
|---|---|
| Posterior inferior cerebellar (PICA) | Lateral medulla, inferior cerebellum |
| Basilar artery | Brainstem, cerebellum |
| Anterior inferior cerebellar (AICA) | Lateral pons, cerebellum |
| Superior cerebellar (SCA) | Superior cerebellum |
| Posterior cerebral artery | Occipital lobe, medial temporal lobe |
Circle of Willis:
- Anterior communicating artery
- Anterior cerebral arteries (A1 segments)
- Internal carotid arteries
- Posterior communicating arteries
- Posterior cerebral arteries (P1 segments)
Venous Drainage
Dural venous sinuses:
- Superior sagittal sinus
- Inferior sagittal sinus
- Transverse sinus
- Sigmoid sinus
- Cavernous sinus
- Straight sinus
- Confluence of sinuses
Veins:
- Superficial cerebral veins (drain to sagittal sinus)
- Deep cerebral veins (internal cerebral, basal vein of Rosenthal → great vein of Galen)
- Cerebellar veins
- Brainstem veins