In one breath
The somatosensory cortex is where the third-order neurons of the ascending sensory pathways end, chiefly in SI (areas 3, 1 and 2) in the postcentral gyrus and SII in the wall of the Sylvian fissure. SI carries the sensory homunculus, a body map in which the face (lips) and the hand (fingers) take up the most room, and it is built of columns that each handle one modality. A cortical lesion hits proprioception and fine touch hardest, barely touches pain and temperature, and all but abolishes the cortical sensations: localization, discrimination and stereognosis.
Builds on: Thalamus: nuclei, functions and thalamic syndrome · Ascending tracts: dorsal column and anterolateral system · Pathways for specific sensations · Leads to: Sensory lesions and sensory function tests · Cortical integration and motor cortex
Sensory cortical areas
At least four separate areas of cortex receive somatosensory input.
| Area | Where it lies |
|---|---|
| SI: areas 3 (3a, 3b), 1, 2 | Postcentral gyrus |
| SII | Upper bank of the lateral (Sylvian) sulcus |
| Area 5, the association cortex | Posterior parietal lobe |
| Supplementary sensory area | Medial wall of the parietal lobe |
- Your book calls SI and SII the primary somatosensory areas, and the other areas secondary.
- Area 5 is also called the higher somatosensory cortex, because most of its input comes from SI and SII. Damage here leaves subtler sensory deficits.
- The supplementary sensory area is part of the posterior parietal association area.
- Other areas that respond to body stimuli:
- the precentral gyrus (area 4): the primary motor cortex, but it receives somatosensory signals too;
- area 7b, the cortex around SII.
Primary or secondary: SII
Your book counts SII as a primary area alongside SI, and both take their input straight from the thalamus. Worth knowing, though not in your pages: most other books call SII the second, or secondary, somatosensory area. In your exam, give the book’s answer: SI and SII are primary, and area 5 and the supplementary sensory area are among the secondary areas.
Connections
- SI and the thalamus: every sub-area of SI is linked both ways, point to point, with VPL. The main projection back to the thalamus comes from area 3a.
- SII takes input from the ventrobasal nucleus of the thalamus.
- Serial flow: information passes step by step from area 3b to area 5, and area 3a is linked both ways with areas 1 and 2.
- SI and SII have extensive two-way connections.
- Area 5: its only thalamic input is the lateral posterior nucleus, which itself gets no direct somatosensory input.
- Across the midline: SI and SII are joined to the matching areas of the other hemisphere through the corpus callosum.
- To the motor cortex: SI, SII, area 5 and the supplementary sensory area all project to the precentral gyrus (areas 4 and 6), which regulates movement. This is one basis of sensorimotor coordination.
- Monoamine input: SI and SII also take monoaminergic fibres from brainstem nuclei, above all the locus ceruleus and the raphe nuclei.
- On top of all this, the sensory areas are richly connected with one another.
Somatosensory area I (SI)
SI, the primary somatosensory area, lies in the parietal lobe, on the back wall and floor of the central sulcus.
Input and output
- Input: the VPL nucleus of the thalamus, SII, and the brainstem (locus ceruleus and raphe nuclei).
- Output: the VPL nucleus, the supplementary sensory area, SII, area 5, and the motor cortex (areas 4 and 6).
Sub-regions by modality
SI is divided into areas 3 (with 3a and 3b), 1 and 2. The split rests on modality: whether the cells answer touch on the skin (cutaneous) or input from muscles and joints (deep).
| Area | Responds mainly to |
|---|---|
| 3a | Deep: muscles and joints |
| 3b | Cutaneous: skin |
| 1 | Both cutaneous and deep |
| 2 | Deep: muscles and joints |
- Areas 3a and 2 respond poorly to skin stimuli.
- Worth knowing, though not in your pages: from front to back, area 3a lies in the floor of the central sulcus, 3b on its back wall, area 1 on the crown of the postcentral gyrus and area 2 on its back slope.
The sensory homunculus
- SI holds two complete maps of the body, one in area 3b and one in area 1. This map is the sensory homunculus.
- Each hemisphere maps the opposite half of the body, which is why a one-sided lesion shows on the other side.
| Body part | Place on the cortex |
|---|---|
| Face | Lateral part of the postcentral gyrus |
| Hand and upper limb | Dorsolateral part |
| Lower limb | Medial surface of the hemisphere |
- Area follows use, not body size. The largest areas belong to the face, especially the lips (used in speech), and to the hand, especially the fingers (used in skilled work).
- The trunk and back get only small areas.
- Worth knowing, though not in your pages: the leg area, on the medial surface, is supplied by the anterior cerebral artery. The face and arm areas, on the lateral surface, are supplied by the middle cerebral artery.
Draw it: the sensory homunculus
Draw a coronal section of one hemisphere through the postcentral gyrus, with the medial surface on one side and the lateral sulcus low on the other. Lay the body along the cortex, starting deep on the medial surface, running over the top and down the lateral side. The order is: genitals, foot and leg (on the medial surface), hip (at the top), trunk, neck, shoulder, arm, a hand with very large fingers and thumb, then the face with huge lips, and the tongue and throat nearest the lateral sulcus. Label “leg medial, hand dorsolateral, face lateral” and “lips and fingers largest, trunk small”.
Columnar organization
- In SI above all, neurons are stacked in columns that run at right angles to the cortical surface.
- Neighbouring columns handle different modalities. For example, one column in area 3b answers skin stimuli from rapidly adapting mechanoreceptors. A column in area 3a answers deep stimuli, mainly from slowly adapting mechanoreceptors.
- Worth knowing, though not in your pages: Mountcastle described these columns. The cells in one column share one modality and one small patch of body surface.
Functions
- SI does the first stage of processing of somatosensory input.
- It also does some higher-order processing, such as sensing the direction of an applied stimulus.
- Most of what we know about SI comes from studying lesions.
Effects of lesions
A cortical lesion never wipes out body sensation completely.
| Sensation | After a cortical lesion |
|---|---|
| Proprioception, fine touch | Most affected |
| Pain, temperature | Least affected |
| Localization, discrimination, stereognosis | Almost abolished |
The deficit depends on which part is damaged:
- SI of one side: every kind of body sensation (somesthesis) is clearly impaired on the opposite side.
- Area 3 alone: discrimination tasks cannot be learnt, even with repeated trials.
- Area 1: telling hard from soft and smooth from rough is badly impaired. Other sensory learning is spared.
- Postcentral gyrus: kinesthesia is impaired, so the patient cannot judge the position or passive movement of his body parts.
- Area 5: stereognosis alone suffers; other touch sensations stay intact.
- Pain and temperature suffer least of all.
Pain is not lost in a cortical lesion
Your book’s thalamus chapter explains why: the Thalamus: nuclei, functions and thalamic syndrome itself perceives pain, temperature and crude touch to some degree, so these survive damage to the cortex. What the cortex adds is locating, discriminating and recognizing objects by touch, and those are what a cortical lesion takes away.
Somatosensory area II (SII)
- Site: the upper wall of the Sylvian fissure, the cleft that separates the temporal lobe from the frontal and parietal lobes.
- Map: the feet lie deepest, at the floor of the Sylvian fissure, and the head lies at the lower tip of the postcentral gyrus.
- Cells: most answer light touch of hair or skin (rapidly adapting mechanoreceptors); they answer deep stimuli poorly.
- Lesion: learning that depends on tactile discrimination suffers. SII does not seem to shape the processing done in SI to any real extent.
Association cortex
- Site: the parietal lobe, hence the name parietal association cortex.
- Input: mainly from SI, and also from SII and the visual cortex.
- Main job: relating the body to the space around it (extrapersonal space), as in hand–eye coordination.
- By hemisphere: in the non-dominant hemisphere it deals with spatial relations; in the dominant hemisphere, with language.
Applied: non-dominant parietal lesion
The patient can no longer relate his body to the space around him. Asked to copy a geometric figure, he draws it distorted: constructional apraxia. He may also develop hemineglect, disowning the opposite half of his body. Worth knowing, though not in your pages: in most people the non-dominant hemisphere is the right one, so the neglected side is usually the left; such a patient may shave or dress only the right half of his body.
Exam-answer skeleton: "Somatosensory cortex: its areas, connections and the sensory homunculus" (short note)
- Definition: where the third-order sensory neurons end. List SI, SII, area 5, the supplementary sensory area and the other responsive areas.
- SI: its site (postcentral gyrus, areas 3a, 3b, 1 and 2), input and output.
- Connections: VPL, SII, area 5, the corpus callosum and the motor cortex.
- Sub-regions by modality: 3a and 2 deep, 3b cutaneous, 1 both.
- The sensory homunculus: draw it; face and hand largest, trunk small.
- Columnar organization.
- Effects of lesions: what is most, least and almost completely lost, then the area-by-area deficits.
- SII and the parietal association cortex in a line or two each, with hemineglect.
Asked in exams
- Somatosensory cortex and the sensory homunculus: Dec 2016, 4 marks · Aug 2024, MCQ