The Brain: Structure, Function, and Clinical Correlates

Cerebral Cortex Histology

The cerebral cortex is the outermost layer of the brain, responsible for higher-order brain functions. It is composed of six distinct layers, each with specialized roles in processing neural information.

• Layers II-IV: Primarily afferent (receiving) layers, integrating sensory input from other brain regions.

• Layers V and VI: Mainly efferent (sending) layers, projecting signals to subcortical structures and the spinal cord.

Functional Areas of the Cerebral Cortex

The cortex is divided into lobes, each with specialized functional areas:

Frontal Lobe

• Motor Cortex (Area 4) & Premotor Cortex (Area 6): Control voluntary movement. Lesion: Contralateral spastic paresis (muscle weakness and increased tone on the opposite side).

• Frontal Eye Field (Area 8): Controls voluntary eye movements. Lesion: Deviation of eyes toward the side of the lesion (ipsilateral).

• Broca’s Speech Area (Areas 44 & 45): Responsible for speech production. Lesion: Expressive, nonfluent aphasia (Broca’s aphasia): comprehension intact, but speech and writing are impaired. Mnemonic: "Broca’s is for Broken speech."

• Prefrontal Cortex (Areas 9–12): Involved in concentration, orientation, abstract thinking, judgment, and problem-solving. Lesion: Loss of initiative, inappropriate behavior, gait apraxia, sphincteric incontinence, and inappropriate social behavior (e.g., use of obscene language, urinating in public).

Parietal Lobe

• Sensory Cortex (Areas 3, 1, 2): Processes somatic sensations. Lesion: Contralateral hemi-hypesthesia (reduced sensation) and astereognosis (inability to recognize objects by touch).

Temporal Lobe

• Primary Auditory Cortex (Areas 41 & 42): Processes auditory information. Unilateral lesion: Slight hearing loss. Bilateral lesion: Cortical deafness.

• Wernicke’s Speech Area (Area 22): Responsible for language comprehension. Lesion: Receptive, fluent aphasia (Wernicke’s aphasia): speech is fluent but nonsensical; comprehension is impaired. Mnemonic: "Wernicke is Wordy but makes little sense."

• Olfactory Bulb, Tract, and Primary Cortex (Area 34): Processes smell. Lesion: Ipsilateral anosmia (loss of smell).

Occipital Lobe

• Primary Visual Cortex (Area 17): Processes visual information. Unilateral lesion: Contralateral homonymous hemianopia and lower quadrantanopia. Bilateral lesion: Cortical blindness.

• Visual Association Cortex (Areas 18 & 19): Integrates visual information for interpretation.

Blood Supply of the Brain

The brain receives blood from the internal carotid and vertebrobasilar systems. Disruption of blood flow can cause specific neurological deficits.

Internal Carotid Artery (ICA) and Branches

• Ophthalmic Artery → Central Artery of the Retina: Occlusion: Blindness.

• Anterior Cerebral Artery: Supplies medial cortex from frontal pole to parieto-occipital sulcus. Occlusion: Motor and sensory deficits in the leg and foot.

• Anterior Communicating Artery: Connects anterior cerebral arteries; common site for aneurysm (circle of Willis).

• Middle Cerebral Artery: Supplies lateral hemisphere, including Broca’s and Wernicke’s areas and frontal eye field. Occlusion: Motor and sensory deficits in face and arm, aphasia, and eye deviation.

• Other ICA Branches: Posterior communicating artery, anterior choroidal artery, lateral striate arteries ("arteries of stroke").

Vertebrobasilar System

• Subclavian Artery → Vertebral Artery: Gives rise to anterior spinal artery and posterior inferior cerebellar artery (PICA), supplying medulla and cerebellum.

• Basilar Artery: Formed by vertebral arteries; branches include pontine arteries, labyrinthine artery, anterior inferior cerebellar artery (AICA), and superior cerebellar artery.

• Posterior Cerebral Artery: Branch of basilar artery; supplies midbrain, occipital lobe, visual cortex, and inferior temporal lobe. Occlusion: Contralateral hemianopia with macular sparing.

Veins of the Brain

• Superior Cerebral Veins (Bridging Veins): Drain into superior sagittal sinus. Laceration: Subdural hematoma.

• Great Vein of Galen: Drains deep cerebral veins into straight sinus.

• Middle Meningeal Artery: Branch of maxillary artery; supplies dura. Laceration: Epidural hematoma.

The Limbic System

The limbic system regulates emotions, motivation, and certain survival behaviors.

• Responsible for: Feeding, Fighting, Feeling, Flight, and Sex (Mnemonic: The 5 Fs).

Thalamus

The thalamus is the largest division of the diencephalon and acts as a relay center for sensory and motor signals.

• Pulvinar: Largest nucleus; lesion causes sensory dysphasia (speech impairment).

• Anterior Nucleus: Part of the Papez circuit (emotion, limbic system).

• Mediodorsal Nucleus: Memory processing.

• Lateral Geniculate Nucleus: Visual relay (Mnemonic: Lateral to Look).

• Medial Geniculate Nucleus: Auditory relay (Mnemonic: Medial for Music).

• Ventral Lateral Posterior Nucleus: Proprioception, pressure, pain, touch, vibration (spinothalamic tract).

• Ventral Medial Posterior Nucleus: Facial sensation, including pain (trigeminothalamic tract).

• Ventral Anterior/Lateral Nuclei: Motor function.

Hypothalamus

The hypothalamus is a diencephalic structure involved in autonomic, endocrine, and limbic functions, maintaining homeostasis.

Functions of Hypothalamic Nuclei

• Paraventricular & Supraoptic Nuclei: Thirst, water balance; produce ADH and oxytocin. Lesion: Diabetes insipidus.

• Lateral Nucleus: Hunger center. Lesion: Starvation and emaciation. Mnemonic: Little (Lateral) food makes you hungry.

• Ventromedial Nucleus: Satiety center. Lesion: Hyperphagia, obesity, and aggressive behavior. Mnemonic: Zap ventromedial nucleus, you grow ventrally and medially.

• Suprachiasmatic Nucleus: Controls circadian rhythms.

• Septate Nucleus: Sexual urges and emotions.

• Anterior Nucleus: Temperature regulation (cooling, parasympathetic). Lesion: Hyperthermia.

• Posterior Hypothalamic Nucleus: Temperature regulation (heat conservation, sympathetic).

Summary Table: Hypothalamic Nuclei Functions

General Clinical Definitions

• Chorea: Sudden, jerky, purposeless movements (basal ganglia lesion, e.g., Huntington’s disease). Mnemonic: Choral dancing.

• Athetosis: Slow, writhing movements, especially of fingers (basal ganglia lesion).

• Hemiballismus: Sudden, wild flailing of one arm (subthalamic nucleus lesion). Mnemonic: "Half ballistic" like throwing a baseball.

• Tremor:

  • Cerebellar tremor: Intentional tremor (during movement).

  • Basal ganglion tremor: Resting tremor (at rest, e.g., Parkinson’s disease).

Cerebrospinal Fluid (CSF)

Cerebrospinal fluid (CSF) cushions the brain and spinal cord, provides nutrients, and removes waste. Its formation, flow, and composition are clinically significant.

Formation of CSF

• Produced mainly by ependymal cells of the choroid plexus in the brain ventricles.

• Additional CSF formed by diffusion and filtration from blood vessels along ventricular walls.

• CSF composition is similar to extracellular fluid (ECF).

• CSF is renewed about four times per day.

• CSF formation is independent of intraventricular pressure; absorption depends on pressure.

Flow of CSF

• Produced in lateral ventricles

• → Third ventricle via Foramen of Monroe

• → Aqueduct of Sylvius

• → Fourth ventricle

• → Foramina of Luschka and Magendie

• → Subarachnoid space

• → Absorbed through arachnoid villi into venous sinuses

CSF Analysis: Differential Diagnosis Table

Example: A patient with turbid, yellow CSF, high protein, low glucose, and many PMNs likely has bacterial meningitis.

Additional info: The above notes integrate clinical correlations and mnemonics to aid memory and understanding of brain structure and function. The tables summarize hypothalamic nuclei functions and CSF findings for differential diagnosis.