Neuro anatomy
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Cerebral Cortex & Lobes - Preclinical Neuroanatomy
Overview
The cerebral cortex is a thin (2-5 mm) layer of neurons covering the surface of all the convolutions of the cerebrum. Despite its thinness, it has a total area of roughly 0.25 m² and contains over 80 billion neurons. The folding pattern of gyri (ridges) and sulci (furrows) dramatically increases surface area within the skull. - Guyton and Hall, p. 716
The Four Lobes
The surface of each hemisphere is divided into lobes by major sulci (fissures):
FIGURE: Lateral (A) and inferomedial (B) views of the cerebral hemispheres - Localization in Clinical Neurology, 8e
1. Frontal Lobe
- Lies anterior to the Rolandic (central) sulcus
- Divided into superior, middle, and inferior frontal gyri by two anteroposterior sulci
- The inferior frontal gyrus has three parts: opercular, triangular, and orbital portions
- The precentral gyrus (just anterior to the central sulcus) = primary motor cortex
- The posterior part of the inferior frontal gyrus on the left = Broca's area (speech production)
- Functions: voluntary movement, executive function, personality, language production
- Localization in Clinical Neurology, p. 1182
2. Parietal Lobe
- Lies posterior to the central sulcus, superior to the Sylvian fissure
- The postcentral gyrus = primary somatosensory cortex
- Contains the supramarginal gyrus and angular gyrus (important for language processing)
- The intraparietal sulcus separates the superior and inferior parietal lobules
- Functions: somatosensory processing, spatial awareness, body image, visuospatial tasks (right hemisphere dominant)
- Localization in Clinical Neurology, p. 1183
3. Temporal Lobe
- Lies inferior to the Sylvian (lateral) fissure
- Divided into superior, middle, and inferior temporal gyri
- The transverse gyrus of Heschl on the superior bank of the Sylvian sulcus = primary auditory cortex
- Wernicke's area (posterior superior temporal gyrus, left hemisphere) = auditory language comprehension
- Functions: auditory processing, language comprehension, memory (hippocampus lies within)
- Localization in Clinical Neurology, p. 1183
4. Occipital Lobe
- Most posterior lobe, separated by the parietooccipital fissure (medial side)
- Contains the calcarine fissure and surrounds it the primary visual cortex (V1)
- Contains cuneus (superior) and lingual gyrus (inferior)
- Functions: visual processing
- The left occipital lobe is often larger than the right - Localization in Clinical Neurology, p. 1185
The Insula (5th lobe)
- Lies buried deep within the Sylvian fissure, hidden beneath the frontal and parietal opercula
- Involved in visceral sensation, taste, and interoception
Cortical Layers (Neocortex)
Most of the cerebral cortex is neocortex - it has 6 layers numbered from surface to depth:
FIGURE: Structure of the cerebral cortex, layers I-VI - Guyton and Hall, p. 716
| Layer | Name | Key Contents | Function |
|---|---|---|---|
| I | Molecular layer | Mainly fibers, few neurons | Horizontal integration |
| II | External granular layer | Small granule (stellate) cells | Intracortical processing |
| III | External pyramidal layer | Medium pyramidal cells | Corticocortical connections |
| IV | Internal granular layer | Dense granule cells + horizontal fibers | Receives thalamic sensory input |
| V | Internal pyramidal (ganglionic) layer | Large pyramidal cells (Betz cells in motor cortex) | Main output to brainstem/spinal cord |
| VI | Multiform (fusiform) layer | Spindle-shaped neurons | Output to thalamus |
Key rules to remember:
- Input arrives at layer IV (from thalamus)
- Output leaves from layers V and VI (to spinal cord, brainstem, thalamus)
- Sensory areas have thick layer IV (koniocortex = "dust cortex" - densely packed granule cells)
- Motor areas have thick layer V with giant Betz cells, thin/absent layer IV (agranular cortex)
- Guyton and Hall, p. 716; Localization in Clinical Neurology, p. 1186
Types of Cortex
| Type | Layers | Location | Example |
|---|---|---|---|
| Neocortex | 6 layers | Most of cerebral cortex | Motor, sensory, association areas |
| Archicortex | 1-3 layers | Ancient cortex | Hippocampus (memory) |
| Paleocortex | 2-3 layers | Old cortex | Olfactory cortex |
- Neuroscience: Exploring the Brain, 5e, p. 637-638
Primary vs. Association Cortex
- Primary cortical areas: first to receive sensory input or directly project to motor neurons
- Primary motor cortex (precentral gyrus)
- Primary somatosensory cortex (postcentral gyrus)
- Primary auditory cortex (Heschl's gyrus)
- Primary visual cortex (around calcarine fissure)
- Association cortex: gives rise mainly to corticocortical connections; integrates and interprets information
- Localization in Clinical Neurology, p. 1183
Neuron Types in the Cortex
Three main types - Guyton and Hall, p. 716:
- Granular (stellate) cells - short axons, function as interneurons; some excitatory (glutamate), some inhibitory (GABA); concentrated in sensory areas
- Pyramidal cells - large, triangular; source of long output fibers to spinal cord and subcortical structures
- Fusiform cells - spindle-shaped; contribute to cortical output
Hemispheric Lateralization
| Left Hemisphere | Right Hemisphere |
|---|---|
| Language production (Broca's) | Spatial and constructional skills |
| Language comprehension (Wernicke's) | Directed attention |
| Analytic and mathematical skills | Body image |
| Temporal sequencing | Emotional comprehension (tone of voice) |
| Dominant in most right-handed people | Melody and tone discrimination |
- Localization in Clinical Neurology, p. 1185
Key Sulci/Landmarks to Know
| Landmark | Significance |
|---|---|
| Central sulcus (Rolandic fissure) | Separates frontal from parietal lobe |
| Lateral fissure (Sylvian) | Separates frontal/parietal from temporal |
| Parietooccipital sulcus | Separates parietal from occipital (medial) |
| Calcarine fissure | Primary visual cortex on both banks |
| Cingulate sulcus | Separates cingulate gyrus from frontal lobe (medial) |
Sources: Guyton and Hall Textbook of Medical Physiology, p. 716 | Localization in Clinical Neurology, 8e, pp. 1182-1187 | Neuroscience: Exploring the Brain, 5e, pp. 636-638
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