The word "homunculus" means little man in Latin. But in neuroanatomy, the cortical homunculus represents either the motor or the sensory distribution along the cerebral cortex of the brain. The motor homunculus is a topographic representation of the body parts and its correspondents along the precentral gyrus of the frontal lobe. While the sensory homunculus is a topographic representation of the body parts along the postcentral gyrus of the parietal lobe.

The sensory homunculus is a topographic representation of the sensory distribution of the body found in the cerebral cortex. This topograph usually has body parts illustrated along the surface of the postcentral gyrus of the parietal lobe. The topography of the sensory homunculus corresponds to the contralateral side of the body. Starting from the medial aspect of the postcentral gyrus, this area corresponds to the sensory territory of the genitals and lower extremities. Moving more laterally along the postcentral gyrus, the superior to superolateral regions of the gyrus corresponds to the neck, shoulder, and torso. While along the lateral side of the postcentral gyrus, this region corresponds to the upper extremity and hand. The inferolateral region of the postcentral gyrus corresponds to the facial and bulbar structures. Interestingly, the face is the structure that takes up the most area of the sensory homunculus.[1][2][3][4][5]

The sensory homunculus is a map along the cerebral cortex of where each part of the body is processed. The sensations occur all along the body. The impulses from the body will be sent into the spinal cord and eventually back to the brain to be processed. While the trigeminal nerve carries the face's sensations. The thalamus processes all these sensory impulses, and then the thalamus will send it to the cerebral cortex. The final destination of the sensory impulses is along the postcentral gyrus. This sensory map is what the sensory homunculus illustrates.

During embryology, the nervous system is developed from the ectodermal germ layer. In the beginning, the notochord is beneath the ectoderm and induces the ectoderm to differentiate into the neuroectoderm and the neural plate. It is the neural plate that will further differentiate into the neural tube and the neural crest cells. The neural tube is responsible for developing the central nervous system. While the neural crest is responsible for the development of the peripheral nervous system.

The neural tube will further differentiate into the prosencephalon, mesencephalon, and the rhombencephalon. First, the prosencephalon is further divided into the telencephalon and the diencephalon. The telencephalon will develop into the cerebral hemispheres, basal ganglia, and the lateral ventricles. This makes the telencephalon the origin of the postcentral gyrus. While the diencephalon will develop into the thalamus, hypothalamus, and the third ventricle. Secondly, the mesencephalon will develop into the midbrain and the cerebral aqueduct. Lastly, the rhombencephalon will further differentiate into the metencephalon and the myelencephalon. The metencephalin later develops into the pons, cerebellum, and the upper part of the fourth ventricle. Finally, the myelencephalon will develop into the medulla and the lower part of the fourth ventricle.

Arterial

The perfusion of the postcentral gyrus is predominantly from the middle and anterior cerebral artery. The middle and anterior cerebral artery arises from the internal carotid artery. Once the internal carotid artery reaches the brain, it splits into the middle cerebral artery and the anterior cerebral artery. The middle cerebral artery ascends the brain between the temporal and the frontal lobe. This path corresponds to the perfusion territory of the middle cerebral artery. The middle cerebral artery will perfuse the inferior and lateral region of the postcentral gyrus, which corresponds to the face and hand part of the homunculus. As for the anterior cerebral artery, it will ascend between the two frontal lobes. The anterior cerebral artery will go on and perfuse the medial and superior region of the postcentral gyrus. The medial and superior part of the postcentral gyrus corresponds to the legs and the genitals respectively on the homunculus. The region of the postcentral gyrus that corresponds to the neck, shoulders, and torso is considered the watershed area. The watershed region received perfusion at the point where the distal ends of the anterior and middle cerebral arteries meet.

Venous

The venous blood and cerebral spinal fluid in the brain drain into the dural venous sinuses. The dural sinuses derive from the folds of the meninges. The dura sinuses are unique structures that drain venous blood similar to veins, but dura sinuses lack valves. The venous blood from the dura sinuses drains towards the internal jugular veins. Then the internal jugular veins merge with the subclavian vein to form the brachiocephalic vein. Both the left and right brachiocephalic veins join to form the superior vena cava. The venous blood from the superior vena cava empties into the right atrium of the heart.

Lymphatics

The lymphatic system of the brain was once believed to be nonexistent. But in recent findings, researchers found that the lymphatic system of the brain is embedded in the dura mater layer of the meninges. The lymph drains from the dura mater towards the cervical lymph nodes. The lymph from the cervical lymph nodes will either drain into the right lymphatic duct if it's on the right side of the head or the thoracic duct if it's on the left side of the head. Both the right lymphatic duct and the thoracic duct will drain back into the central circulation.[6][7][8]

The sensory homunculus does not contain nerves. But the sensory impulses from the body's sensory receptors such as the free nerve endings, Meissner corpuscles, Pacinian corpuscles, Merkel disc, and Ruffini corpuscles are processed in the thalamus and sent to the postcentral gyrus of the parietal lobe. In contrast, the sensory of the face follows the distribution of the trigeminal nerve (CN V).

Sensory Receptors and Senses

• Free nerve endings - pain and temperature
• Meissner corpuscles - fine/light touch and position sense
• Pacinian corpuscles - vibration and pressure
• Merkel discs - pressure, deep static touch, and position sense
• Ruffini corpuscles - pressure, slippage of objects along the surface of the skin, and joint angle change

Spinal Cord Tract

The free nerve endings are responsible for sensing pain and temperature. The sensation of pain and temperature gets transmitted to the brain via the spinothalamic tract in the spinal cord, while transmissions from the Meissner corpuscles, Pacinian corpuscles, Merkel disc, and Ruffini corpuscles sensations travel to the brain via the dorsal column in the spinal cord. Once these tracts arrive in the brain, they synapse in the ventral posterolateral nucleus of the thalamus. Finally, the thalamus will send the impulse to the postcentral gyrus of the parietal lobe (sensory cortex).[9][10][11]

The sensory receptors on the face will send impulses toward the brain following the distribution of the trigeminal nerve. The impulses from the face will synapse in the ventral posteromedial nucleus of the thalamus. Finally, these impulses will get transmitted to the sensory cortex in the brain.