The Somatic Sensory Cortex
Somatosensory Cortex: Have you ever wondered if you feel things the same way other people do? How do you know ‘red’ is really the same red to everyone? Maybe the person next to you sees green as red… These thought-provoking questions can’t be answered precisely with science, but we can learn more about how external stimuli, like colors, are processed in the brain. This is where the somatosensory cortex comes in. This part of the brain processes sensations, or external stimuli, from our environment. Before we learn more about the somatosensory cortex, we need to learn a little bit about brain anatomy and where the somatosensory cortex is located.
The brain is the control center of the whole body. It is made up of a right and left side, or lobes, which are connected in the middle by the corpus callosum. Each lobe is devoted to a different function. The outer layer of the brain is called the cerebral cortex. Think of it like the skin on fruit, the skin is the cerebral cortex, and the fruit is the white insides of the apple. The cerebral cortex helps with processing and higher-order thinking skills, like reasoning, language, and interpreting the environment. This image shows a cross-section of the brain, with the cerebral cortex shown as the dark outline.
Primary Somatosensory Cortex
The somatosensory cortex is a part of the cerebral cortex and is located in the middle of the brain. This image shows the somatosensory cortex, highlighted in red in the brain. The somatosensory cortex receives all sensory input from the body. Cells that are part of the brain or nerves that extend into the body are called neurons. Neurons that sense feelings in our skin, pain, visual, or auditory stimuli, all send their information to the somatosensory cortex for processing. The following diagram shows how sensations in the skin are sent through neurons to the brain for processing.
Some neurons are very important and a big chunk of the somatosensory cortex is devoted to understanding their information. The senior scientist sends the most important information to our analyst, and he spends a lot of time understanding it. However, our junior scientists or volunteers gather less important information, so our analyst, or somatosensory cortex, spend less time on that data. Each neuron takes its information to a specific place in the somatosensory cortex. Next, that part of the somatosensory cortex gets to work on figuring out what the information means. Think of it like scientists sending data to a data analyst. Each scientist, like the neuron, gathers information and sends it to a master analyzer or the somatosensory cortex.
Somatosensory Cortex Function
The primary somatosensory cortex is located in the postcentral gyrus and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield, and parallel surface potential studies of Bard, Woolsey, and Marshall. Although initially defined to be roughly the same as Brodmann areas 3, 1 and 2, more recent work by Kaas has suggested that for homogeny with other sensory fields only area 3 should be referred to as “primary somatosensory cortex”, as it receives the bulk of the thalamocortical projections from the sensory input fields.
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Brodmann areas 3, 1, and 2 make up the primary somatosensory cortex of the human brain (or S1). Because Brodmann sliced the brain somewhat obliquely, he encountered area 1 first; however, from anterior to posterior, the Brodmann designations are 3, 1, and 2, respectively.
Brodmann area (BA) 3 is subdivided into areas 3a and 3b. Where BA 1 occupies the apex of the postcentral gyrus, the rostral border of BA 3a is in the nadir of the Central sulcus and is caudally followed by BA 3b, then BA 1, with BA 2 following and ending in the nadir of the postcentral sulcus. BA 3b is now conceived as the primary somatosensory cortex because 1) it receives dense inputs from the NP nucleus of the thalamus; 2) its neurons are highly responsive to somatosensory stimuli, but not other stimuli; 3) lesions here impair somatic sensation; and 4) electrical stimulation evokes the somatic sensory experience. BA 3a also receives dense input from the thalamus; however, this area is concerned with proprioception.
Areas 1 and 2 receive dense inputs from BA 3b. The projection from 3b to 1 primarily relays texture information; the projection to area 2 emphasizes size and shape. Lesions confined to these areas produce predictable dysfunction in texture, size, and shape discrimination.
The somatosensory cortex, like another neocortex, is layered. Like other sensory cortex (i.e., visual and auditory) the thalamic inputs project into layer IV, which in turn projects into other layers. As in other sensory cortices, S1 neurons are grouped together with similar inputs and responses into vertical columns that extend across cortical layers (e.g., As shown by Vernon Mountcastle, into alternating layers of slowly adapting and rapidly adapting neurons; or spatial segmentation of the vibrissae on mouse/rat cerebral cortex).
This area of cortex, as shown by Wilder Penfield and others, is organized somatotopically, having the pattern of a homunculus. That is, the legs and trunk fold over the midline; the arms and hands are along the middle of the area shown here, and the face is near the bottom of the figure. While it is not well-shown here, the lips and hands are enlarged on a proper homunculus, since a larger number of neurons in the cerebral cortex are devoted to processing information from these areas.
Primary Somatosensory Cortex Function
The primary somatosensory cortex is located in a ridge of cortex called the postcentral gyrus, which is found in the parietal lobe. It is situated just posterior to the central sulcus, a prominent fissure that runs down the side of the cerebral cortex. The primary somatosensory cortex consists of Brodmann’s areas 3a, 3b, 1, and 2.
At the primary somatosensory cortex, tactile representation is orderly arranged (in an inverted fashion) from the toe (at the top of the cerebral hemisphere) to mouth (at the bottom). However, somebody parts may be controlled by partially overlapping regions of cortex. Each cerebral hemisphere of the primary somatosensory cortex only contains a tactile representation of the opposite (contralateral) side of the body. The amount of primary somatosensory cortex devoted to a body part is not proportional to the absolute size of the body surface, but, instead, to the relative density of cutaneous tactile receptors on that body part. The density of cutaneous tactile receptors on a body part is generally indicative of the degree of sensitivity of tactile stimulation experienced at a said body part. For this reason, the human lips and hands have a larger representation than other body parts.
What is the primary somatosensory cortex and what does it do?
The primary somatosensory cortex is responsible for processing somatic sensations. These sensations arise from receptors positioned throughout the body that are responsible for detecting touch, proprioception (i.e. the position of the body in space), nociception (i.e. pain), and temperature. When such receptors detect one of these sensations, the information is sent to the thalamus and then to the primary somatosensory cortex.
The primary somatosensory cortex is divided into multiple areas based on the delineations of the German neuroscientist Korbinian Brodmann. Brodmann identified 52 distinct regions of the brain according to differences in cellular composition; these divisions are still widely used today and the regions they form are referred to as Brodmann’s areas. Brodmann divided the primary somatosensory cortex into areas 3 (which is subdivided into 3a and 3b), 1, and 2.
The Somatosensory Cortex Is Responsible For Processing
The axons arising from neurons in the ventral posterior complex of the thalamus project to cortical neurons located primarily in layer IV of the somatic sensory cortex (see Figure 9.7; also see Box A in Chapter 26 for a more detailed description of cortical lamination). The somatic sensory cortex in humans, which is located in the parietal lobe, comprises four distinct regions, or fields, known as Brodmann’s areas 3a, 3b, 1, and 2. Although area 3b is generally known as the primary somatic sensory cortex (also called SI), all four areas are involved in processing tactile information. Experiments carried out in nonhuman primates indicate that neurons in areas 3b and 1 respond primarily to cutaneous stimuli, whereas neurons in 3a respond mainly to stimulation of proprioceptors; area 2 neurons process both tactile and proprioceptive stimuli. Mapping studies in humans and other primates show further that each of these four cortical areas contains a separate and complete representation of the body. In these somatotopic maps, the foot, leg, trunk, forelimbs, and face are represented in a medial to the lateral arrangement.
Although the topographic organization of the several somatic sensory areas is similar, the functional properties of the neurons in each region and their organization are distinct (Box D). For instance, the neuronal receptive fields are relatively simple in area 3b; the responses elicited in this region are generally to stimulation of a single finger. In areas 1 and 2, however, the majority of the receptive fields respond to the stimulation of multiple fingers. Furthermore, neurons in area 1 respond preferentially to particular directions of skin stimulation, whereas many area 2 neurons require complex stimuli to activate them (such as a particular shape). Lesions restricted to area 3b produce a severe deficit in both texture and shape discrimination. In contrast, damage confined to area 1 affects the ability of monkeys to perform accurate texture discrimination. Area 2 lesions tend to produce deficits in finger coordination, and in shape and size discrimination.
A salient feature of cortical maps, recognized soon after their discovery, is their failure to represent the body in actual proportion. When neurosurgeons determined the representation of the human body in the primary sensory (and motor) cortex, the homunculus (literally, “little man”) defined by such mapping procedures had a grossly enlarged face and hands compared to the torso and proximal limbs (Figure 9.8C). These anomalies arise because manipulation, facial expression, and speaking are extraordinarily important for humans, requiring more central (and peripheral) circuitry to govern them. Thus, in humans, the cervical spinal cord is enlarged to accommodate the extra circuitry related to the hand and upper limb, and as stated earlier, the density of receptors is greater in regions such as the hands and lips. Such distortions are also apparent when topographical maps are compared across species. In the rat brain, for example, an inordinate amount of the somatic sensory cortex is devoted to representing the large facial whiskers that provide a key component of the somatic sensory input for rats and mice (see Boxes B and D), while raccoons overrepresent their paws and the platypus its bill. In short, the sensory input (or motor output) that is particularly significant to a given species gets relatively more cortical representation.
Somatosensory Cortex Definition
Previous chapters described the ways in which the different somatosensory receptors respond to specific types of somatosensory stimuli and that the receptors, by virtue of their selective sensitivities, extract specific information about the somatosensory stimulus. The specificity of the receptors forms the basis for a parsing (i.e., a sorting) of somatosensory experience into separate “information channels” or pathways. For example, sharp-pricking pain is mediated in the neospinothalamic (information channel) pathway, whereas proprioception is mediated in the medial lemniscus pathway. Recall that the receptor’s extraction of somatosensory information is very specific (e.g., during limb movement, muscle spindles respond to muscle stretch, whereas Golgi tendon organs respond to muscle contraction) and the processing of this extracted information is kept separate along most of the ascending pathway. In addition to this parsing of stimulus information, the somatosensory system is also organized to provide a somatotopic representation of the body surface and parts. The resulting spatial maps provide the anatomical basis for our ability to localize somatosensory stimuli and for our sense of a ‘body image”.
As described above, the nervous system reduces somatosensory experience into parallel streams of neural activity – a decomposition of the experience into stimulus fragments spread over body pieces. So how does one have a sense of “oneness” of the body and how does one identify an object by handling it? One can do so because somatosensory information converges in the parietal lobe of the cerebral cortex to provide a cohesive perception of the body and of somatosensory stimuli.
The first part of this chapter will present additional details about the general organization of the somatosensory system and how somatosensory information is represented and processed in the parietal cortex. This understanding of the general organization of the somatosensory pathways will be used in the clinical assessments of somatosensory function.
What is the role of the somatosensory cortex?
What is the function of the somatosensory system?
The somatosensory system is the part of the sensory system concerned with the conscious perception of touch, pressure, pain, temperature, position, movement, and vibration, which arise from the muscles, joints, skin, and fascia.