Clinical anatomy of the cerebral cortex
This is a clinical article highlighting the essential anatomy of the cerebral cortex needed to supplement localising neurological lesions and correlate examination findings.
Cerebral cortex: overview
- The cortex is divided into 2 hemispheres each housing 4 lobes (frontal, parietal, temporal and occipital).
- Lateralisation of function (i.e. which hemisphere the function is located) depends on which hemisphere is said to be 'dominant'.
- In simple terms, this is where language functions reside. In 90% of right handed individuals, this is the left hemisphere. In left-handers, this percentage is less (~70%) and a greater proportion may be right hemisphere dominant.
- This is clinically significant in a stroke for example where speech may or may not be affected depending on which hemisphere is compromised.
1. Somatomotor cortex and premotor area
- An essential concept is somatopic organisation of the motor cortex located in the posterior frontal lobe. This is best represented by the motor homunculus.
- This initially bizarre depiction of the human form roughly represents how we would look proportional to the number of motor units innervating different parts of the body.
- Note how the areas for hands, fingers, lips, tongue and muscles of facial expression are widely represented indicating the heavy motor neurone investment in fine motor control and speech production.
- This has clinical implications in terms of focal lesions compressing areas of the motor strip or when an epileptic focus propagates to adjacent motor units resulting in a distinct pattern of seizure (Jacksonian motor seizures).
NB: The premotor area lies just in front of the motor cortex. It has a complex array of functions and projections to the rest of the cortex. Primarily it is involved in selecting a specific motor sequence from a repertoire of possible motor sequences.
2. Somatosensory cortex
- The same homunculus concept applies to the somatosensory cortex. This time areas with the greatest sensory innervation are represented and mapped onto the postcentral gyrus of the parietal lobe.
- The sensory neurones from the skin eventually synapse with ascending tracts in the spinal cord (dorsal columns) which will eventually cross over and end up in the somatosensory cortex where the information is integrated.
- Thus lesions in the 'sensing' area of the brain result in a loss of the modalities of somatosensation: fine touch, proprioception, vibration sense.
5. Prefrontal cortex
- This area is located in the anterior frontal lobes. It is a significant area in terms of its history and function.
- The case of Phineas Gage, an American construction worker who lived in the 19th Century, provided great insight into functional localisation and a starting point for lesion studies.
- In essence, Mr Gage suffered an iron pole being driven through his frontal lobe. Although he survived, his resultant personality and behaviour were markedly different from his pre-morbid state including a state of disinhibition across many spheres.
- Gage's case led the way for further studies into brain function. We now know that the prefrontal area is responsible for a group of processes grouped under the umbrella term of executive functions.
- This encompasses planning, decision making, risk taking, goal-orientated behaviour and elements of personality and behaviour. In clinical terms, lesions in this area will compromise aspects of these processes.
5. Speech and the temporal lobes
Two essential components of cortical anatomy relating to speech must be understood: a) Wernicke's area b) Broca's area. These are located in the so-called dominant hemisphere, usually the left hemisphere as previously explained.
- External speech is received by our ears and travels to the auditory cortex located in the temporal lobes which passes to Wernicke's area.
- Wernicke's area is located in the temporal lobe, specifically the posterior portion of the superior temporal gyrus. This is the centre of comprehension i.e. where speech is understood.
- This information is passed via the arcuate fasciculus to Broca's area located in the frontal lobe, specifically the posterior portion of the inferior frontal gyrus. This is responsible for the motor aspect of speech and orchestrates signals to brainstem nuclei and the spinal cord. The signals will interact with the cerebellum which co-ordinates the motor actions required to produce the sequence of movements needed for speech.
The above is a highly oversimplified explanation of the speech pathway and speech deficits may be caused by a vast array of lesions in what is a complicated pathway. However it is worth noting three points of clinical significance:
(i) If a patient's speech is affected, it lends strong evidence to the fact that there is a lesion in the dominant hemisphere, more often than not, the left side as previously explained.
(ii) Lesions in or compromising Wernicke's area will result in problems in comprehending speech known as a receptive aphasia. This means the patient will not understand instructions but may well produce a nonsensical largely structured sentence irrelevant to the instructions given.
(iii) Lesions in or compromising Broca's area result in problems articulating speech otherwise known as a motor or expressive aphasia.
6. Visual cortex
- The visual cortex is located in the occipital lobe at the back of the brain. There are actually five visual areas, the most important being the striate cortex named V1 or the primary visual cortex.
- It is a histologically complex and dense area relative to the rest of the cortex. The visual pathway and specific lesions are covered in the article on Neuro-ophthalmology.
- However relevant to the cortical aspect, it is worth noting that this area being at the back of the brain is supplied by the posterior i.e. vertebrobasilar circulation.
- Therefore vertebrobasilar strokes or TIAs are an important cause of visual loss (amongst other symptoms).
- Pure posterior circulation strokes result in macular sparing whereas MCA (middle cerebral artery) strokes do not as the macula is supplied by a branch of the MCA.
Boon et al. Davidson's Principles and Practice of Medicine. 20th Edition. Elsevier. 2006
Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. The Premotor Cortex. Available from: http://www.ncbi.nlm.nih.gov/books/NBK10796/