What is a Homunculus?
The term homunculus is Latin and is referred to as ‘little man’. In neuroanatomy, the cortical homunculus is either the motor or sensory distribution along the cerebral cortex of the brain. It was discovered by the great scientist ‘Wilder Penfield and Edwin Boldrey in the year 1937. It is a map that corresponds body part to touch sensitivity. The proportion of the sensory cortex to the size of the body region is irregular. For example, a small area is devoted to sensations that are arising from the trunk, and a large cortical area is devoted to the face and lips.
There are three homunculi:
- Motor homunculus present on the primary motor cortex of the precentral gyrus.
- Somatosensory homunculus on the primary sensory cortex of the postcentral gyrus.
- Motor homunculus present in the internal capsule.
How did Penfield Construct the Map of the Cortex?
Penfield first mapped the cortex in humans while operating the brain of the patients who had epilepsy and other disorders. The patient was awake during the surgery and was able to stimulate different parts of the brain, able to tell his experience. Penfield noticed that certain parts of the brain activated sensations on corresponding parts of the brain which were always on the opposite side. Based on this experiment, he was able to construct the map of the cortex which was published in the year 1950. He also found something funny about the limbs of the map. On the motor cortex map, he found that the foot look freakishly small compared to the hand, and the arm is just a tiny spindle. Similarly, on the somatosensory map, the fingers appeared enormous when compared with the tiny hip. The body parts look out of proportion as it is represented in terms of motion and feeling and not based on the physical size of the body part.
Brain Maps Include: Sensory Homunculus and Motor Homunculus
The brain is like maps that form a pattern on the surface of the brain that can be seen at various levels of the organization. The areas that control motor functions map to the front-most areas of the cerebral cortex and the areas that receive and process the sensory information are more towards the back of the brain. These maps of sensory and motor areas can be discriminated into regions with more specific functions. Consider an example, the part of the cerebral cortex that receives visual input from the retina is in the very back of the brain which is the occipital lobe. Similarly, the auditory information that comes from the ears is from the side of the brain which is the temporal lobe and sensory information that is received from the skin is conveyed to the top of the brain at the parietal lobe.
Both sensations and movements of different parts of the body are controlled by different areas of the brain. For instance, when you move your thumb finger, a particular area on the top of the brain triggers a signal that informs the muscles on your thumb finger to move. Furthermore, on touching the thumb finger, the neighboring area of the brain receives a signal about the touch sensation. When someone looks at his hand or arm, it is the arm that is considered to be bigger. Does it mean that the arm has larger areas of the brain focussed on its control? But that is not true because sensation and movements of our hand are controlled by more of our brain than sensation and movements of our arm. This larger area allows finer movements and has a better sense of touch. The area of the brain is focused on each body part, and it is represented by a homunculus.
Homunculus: Motor Cortex
- The motor cortex comprises three different regions in the frontal lobe which include: primary motor cortex, premotor cortex, and the supplementary motor area.
- The motor cortex is anterior to the central sulcus and extends down to the Sylvian fissure. This area is histologically known as Brodmann’s area 4 (primary motor cortex).
- The primary motor cortex (M1) lies on the precentral gyrus and the anterior paracentral lobule present on the medial surface of the brain. Of the three motor cortices, the primary motor cortex requires less amount of electric current to bring out a movement.
- Individual muscle movements are associated with the activity from widespread parts of the primary motor cortex. Furthermore, the stimulation of small regions of the primary motor cortex brings about movements that require the activity of numerous muscles. Thus, the primary motor cortex homunculus does not represent the activity of individual muscles but includes the movements of individual body parts, which often require the coordinated activity of large groups of muscles throughout the body.
- The topological representation of the homunculi is arranged in an anatomical fashion that represents the contralateral side. It indicates that the primary cortex in the right cerebral hemisphere represents the motor activity present on the left side of the body and vice-versa.
- It is crucial to recognize that the density of receptors for various parts of the body is not the same. That is why the homunculus represents variation in sizes as it extends over the cortex.
Motor Homunculus: Topological Representations
- The face and mouth region are near the Sylvian fissure.
- The arm and the hand area are at the midportion of the primary motor cortex.
- The trunk is near the apex of the brain.
- The leg and the foot areas are in the part of the primary cortex that dips into the longitudinal fissure.
Difference between Motor and Sensory Homunculus
- It is the map that displays the sensory processing of the neurological connections.
- The type of processing is sensory.
- The signal received is transmitted through the thalamus.
- Brodmann’s area is 1, 2, and 3.
- It is the map that displays the motor processing of the neurological connections.
- The type of processing is motor.
- The signal received is transmitted through the frontal lobes.
- Brodmann’s area is 4.
Context and Applications
This topic is significant in the professional exams for both undergraduate and graduate courses, especially for
- Bachelor of Science in Zoology
- Bachelor of Science in General Physiology
- Master of Science in Human Physiology
- Master of Science in Anatomy and Physiology
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