As someone intrigued by both the fields of psychology and biology, I can state without a doubt that the brain is one of the most fascinatingly complex things to ever exist. The brain has the impossible task of processing an insurmountable amount of sensory information it receives every second. This job partially falls to the somatosensory cortex. But what exactly is the function of the somatosensory cortex?
The somatosensory cortex is responsible for receiving and processing sensory information collected across the body, which is sent through the somatosensory tracts. Going beyond processing the sensory information, the somatosensory cortex is also responsible for orchestrating the reactions to the stimuli.
Even if we had a holistic view of every process the somatosensory cortex is involved in – which is not yet possible as we are still trying to grasp the cortex’s full responsibilities – I believe it would be difficult to fully comprehend the extent to which we rely on the somatosensory cortex. Like the rest of the brain, it is crucial to how we live and function. From cortical processing of sensory information to mediating necessary reactions, the importance of this cortex isn't a question.
Somatosensory Cortex Function
The somatosensory cortex has multiple functions revolving around retrieving and processing sensory information.
The somatosensory cortex receives tactile information (touch, pressure, temperature, etc.) from the somatosensory tract, which runs through the body to the spinal cord, brainstem, thalamus, and cerebellum. The somatosensory cortex, in turn, has numerous connections with other brain areas to process the given sensory information.
Where is the Somatosensory Cortex?
The somatosensory cortex is found in the parietal lobe, specifically in the postcentral gyrus. It is situated just behind the central sulcus (a prominent fissure that runs down the side of the cerebral cortex). Meaning it lies behind the primary motor cortex of the frontal lobe, explaining why it is where most sensory and tactile information is processed.
The Regions of the Somatosensory Cortex
The somatosensory cortex is divided into multiple areas based on the allocations of the German neuroscientist Korbinian Brodmann. Brodmann identified 52 distinct brain regions according to differences in cellular composition.
The somatosensory cortex is mainly comprised of four distinct regions, Brodmann’s areas 3a, 3b, 1, and 2. All four regions are involved in processing tactile information but have separate focuses.
Area 3 receives the bulk of somatosensory input. Area 3b mainly processes touch sensation, while area 3a processes the information from proprioceptors (a sensory receptor responding to position, movement, action, and location).
Area 3b then sends touch information to areas 1 and 2 for further processing. Area 1 is used to sense the texture of an object, while area 2 is responsible for perceiving the size and shape of the object. Additionally, area 2 is responsible for proprioception.
The Somatosensory Cortex
The somatosensory cortex is made of many parts that work together to create an effective processing machine. From gathering the sensory information from stimulations inside and outside the entire body to ensuring all this information is processed correctly in the suited areas, this sliver of the brain carries a lot of responsibility.
The Somatosensory Tracts
The somatosensory tracts – also known as the somatosensory system or pathways – process information about somatic sensations such as pain, temperature, touch, position, and vibration. These somatic sensations originate when sensory receptors receive stimuli. Sensory receptors are found throughout the body, mainly located in the skin, muscles, joints, and tendons.
The somatosensory tracts are comprised of three neurons: primary neurons, secondary neurons, and tertiary neurons.
The primary neurons are sensory receptors within the periphery of the somatosensory cortex and can detect various stimuli like touch and temperature. The secondary neurons are found within the spinal cord and brainstem and act as a relay station. The tertiary neurons are located within the thalamus and cerebellum and project to the somatosensory cortex.
All these neurons work together in afferent pathways (which carry signals to the central nervous system (CNS)) in the spinal cord and brainstem. They work by passing information from the periphery and the rest of the body to the brain. These will then terminate in either the thalamus or cerebellum, where the tertiary neurons will project to the somatosensory cortex. This will aid in forming a sensory homunculus (a representational map of the body).
The Sensory Homunculus Map
Within the somatosensory cortex, specified body parts are represented on a sensory homunculus map. Meaning that areas of the somatosensory cortex are connected to specific parts of the body and will only receive the sensory information correlating to its specific part of the body.
Due to this unique set-up, the surface area of the cortex dedicated to a part of the body directly correlates to the amount of sensory information it receives from that area. Thus, areas of the body which are more sensitive than others are represented in the homunculus map by a distortion – showing that those parts of the body take up a disproportionate amount of space.
For example, hands are very sensitive to sensations, so a large area of the somatosensory cortex is solely dedicated to processing the sensory information from those areas. Contrastingly, something like the back is generally less sensitive to sensation, so it will have a much smaller correlating area in the cortex.
Usually, the medial portion of the homunculus tends to represent body parts less sensitive to sensation – such as the hips and below. However, the lateral sides have a larger surface area to accommodate the more sensitive body parts – such as the fingers, lips, eyes, and face.
The Primary Somatosensory Cortex
In the somatosensory cortex, the regions known as Brodmann's areas 3a, 3b, 1, and 2 are generally known as the primary somatosensory cortex, or S1. The S1 receives and processes sensory information from the somatic senses, proprioceptive senses, and some visceral senses.
The S1 is responsible for the first stage of cortical processing, which is handling the somatic sensations. These sensations are collected by receptors positioned throughout the body, which are responsible for detecting touch, proprioception (the position of the body), nociception (noxious/painful stimuli), and temperature.
When a receptor detects one of the sensations mentioned above, the information is then sent to the thalamus, which passes it along to the S1. From there, the information is processed and, if necessary, actioned through a cortical neuron which may respond to the relevant stimulation.
The Secondary Somatosensory Cortex
Structurally, the secondary somatosensory cortex – known as the S2 – consists of Brodmann's areas 40 and 43. This area is located posterior to the primary somatosensory cortex, in the parietal operculum region in the upper part of the lateral sulcus.
In comparison to the S1, there is little information available on the exact structural organization and function of the S2. But we have been able to discern some of its responsibilities, as well as theorize about possible processes it is involved in.
The S2 is responsible for spatial and tactile memory associated with sensory experiences. It responds to both somatosensory and visual stimuli. Thus, it is believed to be responsible for tactile object recognition, episodic memory, visuospatial processing, reflections upon self, and aspects of consciousness.
Not only is the S2 connected to the S1, but it is also believed to receive direct projections from the thalamus, in addition to having connections to the hippocampus and amygdala – making it responsible for part of our decision-making process.
This area has been shown to contain many complex somatotopic representations of the body, suggesting there are multiple subregions of this area. The S2 is also thought to represent the sensory discriminative aspects of pain.
Furthermore, the S2 is also believed to perform higher-order functions like sensorimotor integration, information integration from the two body halves, attention, learning, and more.
Somatosensory Cortex Dysfunction
Like all areas of the brain, any dysfunction or damage in the somatosensory cortex is noticeable in how a person functions or if they can function at all. Whether the root of the problem is a traumatic brain injury, or something an individual was born with, damage to this area of the brain can manifest in numerous ways.
Damage to the Somatosensory Cortex
Damage to the somatosensory cortex can be caused by; lesions to one or more areas, a brain injury, cerebral palsy, or the result of a stroke. The symptoms of damage are dependent on which specific area was affected.
Symptoms of damage to the somatosensory cortex:
– One may experience numbness or paraesthesia (a tingling sensation in certain body parts). This is caused by damage to the cortex, which then affects the receptors in the body in specific areas. The larger areas on the homunculus map are the most susceptible to numbness.
– This numbness can also result in difficulty detecting temperature, which raises a safety issue when an individual cannot recognize if a surface temperature is causing tissue damage.
- Inability to Localise Sensations
–One may be unable to pinpoint where a sensation has taken place on their body, though they may be able to narrow it down to a general region or limb. This general identification is possible due to undamaged receptors or brain regions still being able to localize.
– Similarly, one with damage to this area can experience tactile agnosia. This includes having difficulty recognizing things being traced on their skin and not being able to identify objects by touch alone.
- Inability to Judge Weight and Pressure
– Another possible symptom is the inability to judge the weight of objects. An individual may be unable to identify if an object was light or heavy after carrying it. Likewise, people with this damage may find it difficult to judge physical pressure; though they may know pressure has been applied to their body, they won't be able to tell the degree or severity to which it was applied.
- Phantom Limb Syndrome
– A common experience in people who had limbs amputated is to experience sensations, or even pain, in the amputated limb. Studies suggest that this pain correlates to changes in the S1 which no longer receives the expected sensory input from the amputated limb.
Several neurological disorders may occur due to abnormal processing of somatosensory information by the S1 or damage in the somatosensory cortex itself.
Examples of neurological disorders related to dysfunction or damage in the somatosensory cortex:
- Multiple Sclerosis (MS)
– In MS, the body's own immune system mistakenly attacks and damages the myelin (fatty material) around nerves. The myelin sheath is important for protecting and insulating nerves so messages can be sent between the brain and the body.
– MS causes a type of lesion which results in the loss of proprioception (the body’s ability to sense movement, action, and location) or exteroception (sensitivity to stimuli outside of the body.)
- Parkinson’s Disease
– Parkinson’s is a brain disorder that causes unintended or uncontrollable movements, such as shaking, stiffness, and difficulty with balance and coordination.
– One argument is that abnormal spatial and temporal processing of sensory information produces incorrect signals for the execution of voluntary movement. Meaning there is an issue with the processing within the somatosensory cortex and with the response specificity, which results in the transmission of less-differentiated information to cortical regions.
– Dystonia is a disorder characterized by involuntary muscle contractions that cause slow, repetitive movements or abnormal postures. The movement can be painful, and some individuals may have a tremor or other neurological symptoms.
– Evidence suggests that both motor and somatosensory functions may be defective in dystonia. Abnormal processing of the somatosensory input in the central nervous system (CNS) can lead to inefficient sensorimotor integration, contributing to the generation of dystonic movements.
– It is highly unlikely that abnormal somatosensory input is the only driver of dystonia, especially since the most dramatic symptoms seem to be motor in nature.
– Ataxia describes a group of disorders characterized by poor muscle control that affects coordination, balance, and speech. Any area of the body can fall victim, but people with ataxia most commonly have difficulties with balance, walking, hand coordination, speaking, and swallowing.
– Most forms of ataxia result from damage to areas of the brain that control muscle coordination (the cerebellum) and other areas it is connected to (e.g., the somatosensory cortex).
The somatosensory cortex has an incredibly important job, as proven by the fact that without it, we wouldn't be able to process any sensory stimuli or regulate our movements – and that’s only the beginning.
Predictably, the somatosensory cortex is an incredibly complex region of the brain that we don’t fully understand yet. Further research is necessary not only so we can grasp the entire role of the S2, but also so we can understand how dysfunction or damage to this region of the brain can manifest in disorders; as well as how we can use this knowledge to help treat or cure these disorders and diseases.