The postcentral gyrus is on the lateral surface of the parietal lobes between the central sulcus and postcentral sulcus. The postcentral gyrus contains the primary somatosensory cortex, a significant brain region responsible for proprioception. This region perceives various somatic sensations from the body, including touch, pressure, temperature, and pain. After stimulation, these peripheral somatosensory receptors relay through the dorsal spinal cord and terminate in the postcentral gyrus where the stimuli are perceived.
The postcentral gyrus is found on the lateral surface of the anterior parietal lobe, caudal to the central sulcus, and corresponds to Brodmann areas 3b, 1, and 2. The primary somatosensory cortex perceives sensations on the contralateral side. The topographic organization of this region is known as the sensory homunculus, or “little man.” This organization of the somatosensory map is such that the medial aspect is responsible for lower extremity sensation, the dorsolateral aspect is responsible for the upper extremity, and the most lateral aspect is responsible for the face, lips, and tongue. However, regions of the homunculus that require high sensory acuity and resolution take up a larger area on the somatosensory map. For example, the hands, face, and lips necessitate fine somatosensory perception relative to other regions, such as the leg or torso. The postcentral gyrus also houses the secondary somatosensory cortex, which appears to play a role in the integration of somatosensory stimuli and memory formation.
The early central nervous system first appears as the neural tube. Over time, the anterior portion of the neural tube develops into the forebrain, midbrain, and hindbrain, and the dorsal neural tube differentiates to become the somatosensory pathway in the brain and spinal cord. In the forebrain, a variety of factor gradients, including SHH, WNT, and BMP, gives rise to the telencephalon. By the end of the fifth month of development, the central sulcus has formed separating the precentral and postcentral gyri. Between the late third trimester and birth, the primary somatosensory cortex spatially differentiates forming the topographic sensory map.
The anterior and middle cerebral arteries provide the postcentral gyrus blood supply. The anterior cerebral artery is responsible for perfusing the medial third of the postcentral gyrus, while the middle cerebral artery perfuses the lateral two-thirds of the postcentral gyrus. Venous blood drains through the superior sagittal sinus for the superior two-thirds of the postcentral gyrus and the superficial Sylvian veins to the transverse sinus for the inferior third of the postcentral gyrus.
Dorsal Column-Medial Lemniscus Pathway
The dorsal column-medial lemniscus pathway is the primary somatosensory pathway for fine touch, vibration, two-point discrimination, and proprioception. This pathway is comprised of three neurons that connect the mechanoreceptor to its specific region within the primary somatosensory cortex. Meissner's corpuscles, Merkel's disks, and Ruffini's corpuscles are stimulated by touch, vibration, and skin tension, respectively leading to the production of an action potential in the first-order neuron. These first-order neurons are pseudounipolar with their cell bodies found in the dorsal root ganglion. Axons below the T6 level travel up the medial dorsal column as the fasciculus gracilis, while axons above the T6 level travel up the lateral dorsal column as the fasciculus cuneatus. These columns synapse with the second-order neurons in the nucleus gracilis and nucleus cuneatus in the medulla. The second-order neurons crossover to the contralateral side and ascend, forming the medial lemniscus. Like the dorsal columns, the medial lemniscus is also spatially organized. However, rather than being structured in a medial to the lateral direction, the lower extremity axons are found more ventrally, and the upper extremity axons more dorsal. The medial lemniscus continues into the midbrain and synapses in the ventral posterior nucleus of the thalamus. Again, these synapses topographically organize with the ventral posterolateral nucleus responsible for somatosensation of the body and the ventral posteromedial nucleus responsible for somatosensation of the head. Finally, the third-order neurons ascend through the posterior internal capsule to the specific region within the primary somatosensory cortex.
The spinothalamic tract (also known as the ventrolateral system) is the somatosensory pathway for crude touch, pressure, nociception, and temperature. The spinothalamic tract divides into two systems; the anterior spinothalamic system is responsible for crude touch and pressure, and the lateral spinothalamic system is responsible for pain and temperature sensation.
In the anterior spinothalamic system, A-beta fibers carrying crude touch and pressure stimuli travel through the dorsal root ganglion and synapse in the ipsilateral dorsal horn. A-beta fibers are myelinated and have a large diameter compared to other fiber types in the spinothalamic tract. The second-order neuron passes through the anterior white commissure of the spinal cord and ascends as the anterior spinothalamic tract on the contralateral side. Axons from the body synapse in the ventral posterolateral thalamus and third-order neurons travel through the posterior limb of the internal capsule and terminate in their appropriate region within the postcentral gyrus.
Unlike the anterior spinothalamic system, the lateral spinothalamic system is made up of multiple nerve fiber types. A-delta fibers are myelinated nerves with a smaller diameter than A-beta fibers. Type I A-delta fibers respond to mechanical and chemical stimuli but have a high threshold of activation for heat stimuli, whereas type II A-delta fibers are sensitive to heat and have a high mechanical threshold. Together, A-delta fiber afferents signal for the "first" pain response, a rapid nociceptive stimulation that triggers a reflex arc to remove the body from the painful stimulus. C-fibers are unmyelinated, small diameter nociceptive nerves that can respond to mechanical and temperature stimuli. C-fibers are slowly conducting neurons responsible for the "second" pain, the burning, or aching pain associated with an injury. Like the anterior spinothalamic system, the lateral spinothalamic system synapses in the dorsal horn and crosses to the contralateral side at the same level of the spinal cord; however, these fibers ascend as the lateral spinothalamic tract. Again, these fibers will synapse in the ventral posterolateral nucleus of the thalamus, travel through the corona radiata, and synapse in the correct topographic region within the primary somatosensory cortex.
Although there are interhemispheric and interindividual variations in the gross appearance of postcentral gyri, the overall topographic sensory distribution remains the same. However, postcentral gyrus grey matter volume has shown to correlate positively with improved somatosensory processing.
Common surgical considerations regarding the postcentral gyrus or structures near the postcentral gyrus, such as the precentral gyrus, posterior parietal lobe, or insula, include tumor resection surgery, brain mapping for treatment of seizures, and treatment of neurodegenerative diseases. Cancers that can manifest near the postcentral gyrus include gliomas, astrocytomas, oligodendrogliomas, and meningiomas. Patients with lesions in the primary somatosensory cortex experience somatosensory deficits, especially in the hands and face. Uncontrolled growth of these tumors near or within the postcentral gyrus makes surgical resection difficult to perform without any post-operative somatosensory loss. However, new developments in cerebral tumor resection techniques have shown improvements in glioma and meningioma microsurgery resections near the postcentral gyrus. Another improvement in tumor resection surgeries has been preoperative transcortical magnetic stimulation in conjunction with fMRI. This method has shown to improve resection margins in precentral glioma resections and preserve motor functions. Repetitive transcranial magnetic stimulations have shown to improve tactile discrimination and reorganize the somatosensory map.
Other surgical considerations involving the postcentral gyrus are the use of deep brain stimulation for the treatment of Parkinson disease. Patients with Parkinson disease undergo deep brain stimulation and dopaminergic therapy to correct motor deficits. The subthalamic nucleus and globus pallidus internus are two common targets for deep brain stimulation to treat motor symptoms seen in Parkinson disease. Unfortunately, these therapies can negatively impact somatosensation and the primary somatosensory cortex. Positron emission tomography and magnetoencephalography reveal deep brain stimulation can result in deleterious effects on somatosensation while effectively treating Parkinson disease-related motor symptoms. It is important to monitor patients undergoing deep brain stimulation to ensure they are not experiencing any lasting sensory deficits.
The postcentral gyrus is at risk of damage due to strokes. The two major arteries that supply the postcentral gyrus are the anterior and middle cerebral arteries. Sensory deficits can often help to determine which artery is affected and the location of the infarct. For example, ischemic stroke in the anterior cerebral artery will affect the medial postcentral gyrus and may present with sensory deficits in the contralateral leg. A stroke in the middle cerebral artery may show a contralateral sensory loss in the face or upper extremity, depending on the location of the infarct. Cerebral infarctions to these arteries will often have accompanying motor deficits, aphasia, and visual deficits depending on the location of the occlusion.
Nociception pathways can be suppressed by ascending and descending modulating pathways. The pain perception circuit in the brain includes the primary somatosensory cortex, insula, anterior cingulate gyrus, prefrontal cortex, and thalamus. These regions are important for the perception of the painful stimulus and play a role in learning and memory to prevent the painful stimulus from occurring in the future. Although this system is essential for responding to acute pain, dysregulation of the nociceptive pathway can lead to chronic pathologic pain. Descending pain modulating pathways function to prevent chronic pain, which may manifest as overactive and hypersensitive nociception.
Descending pain modulating pathways appear to originate in the periaqueductal grey and rostroventral medial medulla. Activation of the descending pain modulating pathway leads to the activation of inhibitory interneurons in the spinal cord and subsequent release of endogenous opioids and acetylcholine. Opioid ligands bind to the presynaptic receptors hyperpolarizing the neuron and preventing the release of substance P, thus stopping the transmission of the painful stimulus while leaving non-nociceptive sensation intact. This pathway is also the target for exogenous opioid pharmacotherapy for the treatment of pain. However, presynaptic opioid receptors become tolerant with continuous drug therapy; prolonged opioid administration leads to the downregulation of the opioid receptors at the presynaptic neuronal surface. Therefore, patients should not receive treatment with opiates for chronic pain.
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