Brain Neuroanatomy — Cortical Maps, Basal Ganglia, Thalamus, and Brainstem Nuclei (Part 1) (Part 1 of 2)

Section 1: Cerebral Cortex — Lobes, Brodmann Areas, and Language Regions

BASIC-01 · ~02:00

Bottom line: the cerebral cortex is divided into four lobes by three landmark sulci. The central sulcus separates frontal from parietal; the lateral (Sylvian) sulcus separates temporal from frontal and parietal above; the parieto-occipital sulcus separates parietal from occipital. The high-yield Brodmann areas distill to seven: area 4 is primary motor cortex (M1) on the precentral gyrus; areas 1, 2, 3 are primary somatosensory cortex (S1) on the postcentral gyrus; area 17 is primary visual cortex (V1) on the calcarine sulcus; areas 41, 42 are primary auditory cortex (A1) on Heschl gyrus; areas 44, 45 are Broca area in the dominant inferior frontal gyrus; area 22 is Wernicke area in the dominant superior temporal gyrus. The motor and sensory homunculi run with face and hand laterally on the cortex and the leg draped over the medial surface into the interhemispheric fissure — the single rule that drives every cortical stroke vignette.

The cerebral cortex is the convoluted outer mantle of gray matter that covers the cerebral hemispheres. Its surface folds into gyri, the ridges, and sulci, the grooves, an arrangement that triples cortical surface area without enlarging the cranial vault. Three landmark sulci divide each hemisphere into four lobes. The central sulcus separates the frontal lobe from the parietal lobe. The lateral or Sylvian sulcus separates the temporal lobe from the frontal and parietal lobes above. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.

The frontal lobe occupies everything anterior to the central sulcus and superior to the lateral sulcus. It contains the primary motor cortex, often called M1, which is Brodmann area 4, located on the precentral gyrus immediately anterior to the central sulcus. Anterior to M1 sit the supplementary motor area and the premotor cortex, which handle movement planning and sequencing. The frontal lobe also contains the frontal eye fields at Brodmann area 8, which control contralateral horizontal saccades; the prefrontal cortex for executive function; and Broca area at Brodmann areas 44 and 45 in the inferior frontal gyrus of the dominant hemisphere. The dominant hemisphere is the left side in approximately 95 percent of right-handed people.

The parietal lobe sits between the central sulcus and the parieto-occipital sulcus. It contains the primary somatosensory cortex, S1, at Brodmann areas 1, 2, and 3, located on the postcentral gyrus. The parietal lobe also contains the somatosensory association cortex posteriorly and the parietal association areas for spatial attention. Lesions of the non-dominant parietal lobe, typically the right side, produce hemispatial neglect of the contralateral world. Dominant parietal lesions can produce Gerstmann syndrome, which is acalculia, agraphia, finger agnosia, and left-right confusion.

The temporal lobe lies inferior to the lateral sulcus. It contains the primary auditory cortex at Brodmann areas 41 and 42 on Heschl transverse gyrus deep within the lateral sulcus; Wernicke area at Brodmann area 22 in the superior temporal gyrus of the dominant hemisphere; and the medial temporal structures, including the hippocampus and amygdala, for declarative memory.

The occipital lobe is dedicated to vision. The primary visual cortex, V1, at Brodmann area 17, lines the calcarine sulcus on the medial occipital surface. Visual association cortex at Brodmann areas 18 and 19 wraps around V1 and processes form, motion, and color.

The motor and sensory homunculi run along M1 and S1 respectively, and they share the same somatotopy: face and hand laterally, trunk and arm in the middle, leg and foot draped over the medial surface and into the interhemispheric fissure. This medial-leg arrangement is the single most useful bedside vascular localization in stroke neurology. The leg representation is supplied by the anterior cerebral artery, which runs along the medial hemisphere. The arm and face representations are supplied by the middle cerebral artery, which runs along the lateral hemisphere. As a result, anterior cerebral artery strokes produce contralateral leg weakness greater than arm weakness, while middle cerebral artery strokes produce contralateral arm and face weakness greater than leg weakness. Internalize that one rule and dozens of stroke vignettes pivot from confusing to obvious.

Broca aphasia results from injury to Brodmann areas 44 and 45 in the dominant inferior frontal gyrus, typically from a left middle cerebral artery superior division stroke. Speech is non-fluent and effortful, with telegraphic short utterances, agrammatism, impaired repetition, and relatively preserved comprehension. Patients are usually aware of and frustrated by their deficit. Because Broca area sits adjacent to the lateral motor cortex serving the face and hand, Broca aphasia is commonly accompanied by right facial weakness and right arm hemiparesis.

Wernicke aphasia results from injury to Brodmann area 22 in the dominant superior temporal gyrus, typically from a left middle cerebral artery inferior division stroke. Speech is fluent but empty, with normal prosody and rate but paraphasic errors and made-up words. Comprehension is impaired, repetition is impaired, and patients are typically unaware of their deficit. Because Wernicke area sits below the lateral sulcus and away from the motor strip, Wernicke aphasia is often unaccompanied by hemiparesis. Both Broca and Wernicke aphasia lose repetition because the arcuate fasciculus that connects them is involved in either lesion.