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Dinesh

Dinesh club

Posted: 19 Oct 2019

Taken: 19 Oct 2019

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Excerpt •
The Myth of An Afterlife •
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Michael Martin •
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Keith Augustine •
Second excerpt •
MAKING SPACE •
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Jennifer Groh •

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Figure 6.2 ~ Penfield homunculus

Figure 6.2 ~ Penfield homunculus
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 Dinesh
Dinesh club
The visual cortex’s astounding sensitivity in mapping the external world onto the brain, as dramatic as it may seem, is not entirely unexpected: the scientific understanding of the structural organization of the brain dates back to the early days of the last century. In 1937, Wilder Penfield and Edwin Boldrey described an astoundingly precise structured mapping of the human body onto the sensory and motor cortices.

Control of movement is carried out by many regions of the brain, including some of the most evolutionarily primitive, such as the cerebellum and spinal cord, as well as some of the most evolutionarily recent. Such as the frontal cortex. Most sensorimotor processing (i.e., that which involves both sensation and movement) passes through an area of the neocortex (the evolutionarily recent top six neural layers of the brain). This area modulates perception and the processing of sensory stimuli (in the sensory cortex) and movement (in the motor cortex), and is collectively known as the sensorimortor cortex (Figure 6.2) Simulation of one area of the sensory cortex led a subject to experience a sensation in their pinkie finger, stimulation of an immediately adjacent area led to sensation in their ring finger and so on. These findings led to a representation of the human body’s topography with brain known as the “Penfield homunculus” -- a rather horrifyingly distorted cartoon with immense hands. The distortion arises because larger areas of the brain are dedicated to receiving inputs from the sending output to, more sensitive areas of the body (e.g., the fingers, tongue, genitalia, etc.).

The cortical homunculus is now known to be significantly more complex than Penfield originally envisioned, but it remains a cornerstone of both motor and sensory neuroscience (Schott, 1993). From the motor perspective, a revealing symptom of some epileptic seizures is the so called Jacksonian march, in which tremors move through the limbs in the precise order in which those limbs are encoded in the motor cortex; these areas are sequentially activated as the seizure activity moves through the brain. Neurosurgeons use a related concept during intraoperative testing, delivering weak electrical stimuli to areas of the motor cortex in order to precisely localize specific functions prior to surgery. ` Page 140/142

THE MYTH OF AN AFTERLIFE
4 years ago. Edited 15 months ago.
 Dinesh
Dinesh club
This drawing shows the layout of the body maps in the somatosensory cortex. See how big the hands and lips are compared to other areas? The somatosensory cortex is found in the middle of your brain (front and back), extending from the top of your head around to the sides near your ear. It is adjacent to the motor cortex, which is responsible for guiding movements. The motor cortex has a similar map of the body. These maps were characterized by the pioneering epilepsy surgeon and scientist, Wilder Penfield.

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The visual system is not the only place where the brain employs maps to organize spatial information. Much the same happens for your sense of touch. . . . . Each touch receptor monitors a particular region of the body surface, which is its receptive field. The signals from these touch receptors are sent along even longer axons, extending all the way from the receptor in the skin to the spinal cord. So individual neurons can be several feet long, from, say, the tip of your toe all the way to your spinal cord in your back! The axons from the same location travel together and from synapses of neighboring neurons in the spinal cord, which in turn send axons to neighboring regions in the body region of the thamalus and from thence to the cortex. All along the way, the pattern of input continues to match the pattern of locations of the body surface, so the neurons that receive input originating in the toe, elbow, or nose are clustered together in distinct zones. The body surface maps of your skin is thus duplicated at each stage along the neural road into the brain. In the ‘somatosensory’ cortex, the cortex responsible for body-related information, . . . . Page 87

Penfield stimulated all over the brain in his patients, and he asked them to describe what they noticed. Patients reported a variety of sensations -- sights, sounds, touch -- depending on where the electrode was placed. When Penfield stimulated the somatosensory cortex, the sensation would felt at a particular location of the body surface. Moving the electrode to a slightly different location would move the sensation to an adjacent location on the body. Penfield thus built up detailed maps of how the body surface was represented to the somatosensory and nearby motor cortex that are still in use today. ~ Page 104

MAKING SPACE
15 months ago. Edited 15 months ago.

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