Brain Plasticity


Introduction


Brain plasticity, a.k.a Neuroplasticity, refers to the brains' ability to "remap" and change according to personal experiences. The brain is composed of multiple cortices and lobes (Figure 1) that mature at different times in a human life span, for example, the frontal lobe is the last to mature and does not mature until the early mid twenties and the amygdala is most active and developing during adolescence.
Figure 1: http://dericbownds.net/uploaded_images/cortex.jpg
Figure 1: http://dericbownds.net/uploaded_images/cortex.jpg


Brain plasticity does not mean that one would find the occipital lobe where the frontal lobe would be normally found during an accident nor does it mean that one lobe would overtake another. This is because once the brain develops, they are generally located where figure 1 illustrates. What brain plasticity refers the strength of the connections between the neuron and the glial cells within the brain.

Glial cells make about 50% of the brains' neurons and they function as the supporting cell for the other neurons. During learning it has been observed that there is an increase of strength between the connections as well as the formation of new cells (Wikipedia).

It has been thought that once passed a critical period the brain cannot change. This idea is true to some extent and it can be observed in other animals, for example, when certain bird species hatches, the first animal it sees it will follow even if the animal is not a human. This can be said about humans as well. There have been feral children found with no explanations that have seemed to have survived in the wilderness with the aid of other animals. Some kids were taught to behave as humans again but some human behaviors, like language, could not be learned. It is known that learning a language is easier when one is younger but if the critical period for language acquisition is missed, which is in the first couple months of infant development, language cannot be learned.Though the critical periods are crucial in development of the brain, there are high adaptable areas in the brain that can adjust to by external changes of training.

Somatosensory Cortex


Figure 2: http://www.karger.com/gazette/67/field/images/field_1.jpg
Figure 2: http://www.karger.com/gazette/67/field/images/field_1.jpg

The Somatosensory cortex is found in the postcentral gyrus and recieves input from touch as well as proprioception. It is also contains the homunculus (figure 2) which means little man. It has been coined this because the somatosensory cortex is somatotopically organized meaning, different sensory cells in the body innervates certain parts of the cortex. Certain parts of the body have greater representation within the cortex and this is illustrated with the size of the picture (Figure 2), for example, the lips have the greatest representation. This was found with the use of microelectrodes. This locates individual cortical neurons and reveals that there are actually four fairly complete maps in the primary somatosensory cortex each with its own areas: Brodmann areas 3a, 3b, 1 and 2. Brodmann area 3a receives the most information about proprioception, while Brodmann area 3b receives information from the skin about touch and is further processed in area 1. Area 2 primarily receives information from the muscles and joints. When there is a slight lesion in area 1, there is an impairment of discriminatory tactile sense and when there is a lesion in area 2, the ability to recognize size and shape of an object is grasped is lost (Kendal).

Somatosensory System

The somatosensory cortex receives information from certain touch receptors, proprioceptors, and pain receptors found within the body and these compose the somatosensory system. The touch receptors are found in different layers within the skin and these include Meissner's corpuscles, Merkel cells, Pacinian corpuscles, and Ruffini endings. Proprioceptors are found in the muscle spindles and the Golgi tendon organs. Pain receptors are wide spread and can be found everywhere in the body tissue.

Ascending Pathway
Figure 3b: http://grants.hhp.coe.uh.edu/clayne/6397/Unit5_files/Unit5.4.jpg
Figure 3b: http://grants.hhp.coe.uh.edu/clayne/6397/Unit5_files/Unit5.4.jpg
Figure 3a: http://grants.hhp.coe.uh.edu/clayne/6397/Unit5_files/Unit5.3.jpg
Figure 3a: http://grants.hhp.coe.uh.edu/clayne/6397/Unit5_files/Unit5.3.jpg
The information that the receptors receive travels up two ascending pathways; dorsal column medial lemniscal pathway (DCML) and the anterolateral pathway. The DCML first order neurons are located in the dorsal root ganglion cells and enters the spinal cord into the dorsal column. It travels up the spinal cord and when it hits the medullary area, the first order axons synapses with the second order neruons in the gracile and cuneate nucleus. It then decussates and travels up through the medial lemniscus to the ventral posterior lateral portion of the thalamus. There, the second order axons synapses with the third order neurons where it innervates the somatosensory cortex. This is illustrated in figure 3a. The anterolateral pathways' first order neurons are also located in the dorsal root ganglion but once the axon enters the spinal cord, it synapses with the second order neuron. The second order neurons immediately decussates and ascends the spinal cord. The third order neurons for the anterolateral track are also found in the ventral posterior lateral portion of the thalamus. This is illustrated in figure 3b. From the anterolateral tract, two tracts branch; the spinoreticular tract and the spinomesencephalic tract. The spinoreticular tract branches in the medullary region and the spinomesencephalic tract branches in the midbrain region.

Plasticity of the Somatosensory Cortex


The homunculus represents the general layout of the body but the somatotopic mapping of the somatosensory cortex has a surprisingly high level of plasticity. It seems that the more use an area that is represented of the homunculus has, the more space it takes in the somatosensory cortex. The opposite can be said as well; the less use a certain limb of the body has the less room it takes up in the somatosensory cortex. In the case of amputees, some may experience pain or sensation where there is no limb. This is the phenomena called phantom limbs. Eventually as time passes, amputees learn to go about everyday life without a limb and the other areas that are being used to compensate for the loss of the limb will begin to take over the empty space. For example, if the ring finger was cut off, the amputee would start to use their little finger or middle finger more and these connections within the brain would increase. Of course this is just a hypothetical situation and is difficult to research because of ethical reasons.

Proprioceptive plasticity
Rat_brain_wo_whisker.JPG
figure 4b shows the mapping of the somatosensory cortext without the whiskers

There has been a study conducted with rats that
Rat_brain_whisker.JPG
Figure 4a shows the somatopic mapping of the somatosensory cortex in the rat with the intact whiskers: Kendal pg 762
shows the brain's ability to adapt to external changes. This study cut the whiskers off a rat to see the changes within the somatosensory cortex. Figure 4a shows the somatotopic mapping of the rat's brain with their whiskers and figure 4b shows the somatotopic mapping of the somatosensory cortex after the whiskers were cute. It is clear that the forelimb and periocular areas of the brain increased to compensate for the lack of whiskers. Not only has the forelimb and periocular areas they taken over the space where the whiskers had but they took over more of the cortex as well. This is because the whiskers are crucial in that they let the rats know how far they are away from a certain object. The periocular and the forelimbs need to have more sensitivity to allow the rat to function normally as if the whiskers were still intact.


Though conducting studies with human subjects can be ethically difficult, there has been a study (Mogilner et al) done that shows the somatosensory change after surgical correcton of syndactyly. Generally, the digits have a specific area in the brain where they occupy (Figure 5a) and this can be said about syndactyly subjects as well (figure 5b), but what is interesting to note is not only does use affect somatotopic mapping, but their proprioceptive properties as well can change the somatotopic mapping. The information from the muscle spindles and the golgi tendon relays the changes that the corrective surgery produce and the somatotopic mapping changes. Also, because the digits now have more mobility, the cortical representation of these digits expand as well. This is shown in figure 5c.
Sintyly3.JPG
Figure 5c: Shows syndactyly subject after corrective surgery and the change in the somatotopic mapping (Mogilner)

Sintyly1.JPG
Figure 5a shows normal somatotopic mapping of digits (Mogilner)

Sintyly2.JPG
Figure 5b: Shows somatotopic mapping of a subject with syndactyly (Mogilner)


Trained plasticity
Training of certain limbs can also increase the space it takes up in the somatosensory cortex as well. It has been shown in monkeys that when the somatosensory system of digits 2, 3, and 4 were trained the cortical representation regions in Brodmann area 3b increases drastically. This is shown in figure 6. This idea of training comes in very handy when there is damage to the brain, for example a stroke. A major loss of function that is usually tied with a stroke victim is loss of speech. This is because when there is not enough blood to the brain, that specific part that is not receiving blood begins to die. To use the loss of speech as an example, that area would be the motor cortex. The motor cortex, like the somatosensory cortex is also highly plastic and is somatotopically organized as well. This allows there to be a retraining of the mouth to allow the stroke victim to speak again. Even though training can bring back speech, there is only so much the brain can be retrained. The skill level of the ability to speak would not return to the skill level of speech before the stroke occurred.
Training.JPG
Figure 6 Kendal pg 390


Visual Plasticity


An interesting phase that was worth researching is the belief that people who have a sense deficiency have their other senses heightened. If the somatosensory system can change with use it makes sense to assume that blind people have better sense of smell, touch, or hearing. Though these beliefs are plausible, there are some set back. A study (Rosenbluth et al) showed that blind children sense of smell is not better than their seeing peers. Another study (Ferdenzi et al) showed that adults that have been visually impaired also do not have a better sense of smell then seeing adults. How does this compare with the belief that blind people have a heightened sense of touch? A 1994 study by Morrongiello showed that blind people can use their touch sense to analyze an object faster but could not correctly guess what the object was compared to people who were blindfolded. Lastly, another thought that hold some truth is the idea that blind people can hear better. If a person was blind at a very young age or born with this disability, their ability to distinguish different pitches is heightened. This hold true only with those individuals who were blind from a very young age. Those individuals who lost their sight gradually have the same pitch recognition as a sighted person.

There is some interesting technological research being done that can still stimulate the visual cortex for people with blindness. The first major improvement was the walking stick but now there other more modern advances that allow for blind people to walk without the fear of bumping into anything. For example, there are special sonar headphones to help the blind walk. Also, another technological invention is when an electrode is put on the tongue to provide vibration. This allows the blind person to know where they are in space by be trained to understand what each specific vibration is (Renier). There are many other different "seeing" aids and most use sensory substitution.

Glossary of Terms

Brain Plasticity

- How brain structures can change itself from experience

Glial cells

- cells that maintain homeostasis and provides support and protection for the brain's neurons

Critical period

- Time in early stages of an organism's life that is sensitive to environmental stimuli

Feral children

- human child that has been isolated from human contact and is unfamiliar with human care, love, social behavior, and language

Somatosensory cortex

- Area in the brain that processes information from various systems involving touch and proprioception

Homunculus

- term used to refer to the representation of the human being

Epidermis

- outer most layer of skin

Dermis

- middle layer of skin that is in between the epidermis and the subcutaneous tissue

Meissner's corpuscles

- Found in the skin and is ensitive to light touch

Merkel cells

- Found in the skin and is sensitive light touch and can discriminate shapes and textures

Pacinian Corpuscles

- Found in the skin and is sensitive to pain and pressure

Ruffini endings

- Found in the subcutaneous tissue and is sensitive to stretch and contributes to control of finger position and movement

Phantom limbs

- Sensation that an amputated or missing limb is still connected to the body

Proprioception

- Sense of where the limbs are in space

Syndactyly

- Condition when 2 or more digits are fused together

Sensory substitution

- The use of one sense to aid in the awareness of another sense.


Quiz


Multiple guess

1) Which cortex is the last to mature when observing brain development
a) Amygdala
b) Motor cortex
c) Somatosensory cortex
d) Frontal lobe

2) Glial cells make up approximately what percent of the brain
a) 20%
b) 30%
c) 50%
d) 60%

3) Where are the first order neurons in the ascending pathways
a) dorsal horn
b) dorsal root ganglion
c) ventral horn
d) ventral root

4) Which of these is NOT part of the somatosensory system
a) muscle spindles
b) joint receptors
c) touch receptors
d) none of the above

5) What are the two main ascending pathways
a) Dorsal Column medial lemniscus; spinoreticular tract
b) Dorsal Column medial lemniscus; anterolateral tract
c) anterolateral tract; spinomesencephalic tract
d) spinomesencephalic tract; spinoreticular tract

True or False

1) Brain plasticity is the the brain's ability to overgrow and migrate to other areas in the brain
a) True
b) False

2) Brain plasticity only occurs in subjects with injury
a) True
b) False

3) The somatosensory cortex is the primary cortex where brain plasticity occurs
a) True
b) False

4) Trained and proprioceptive plasticity are closely related
a) True
b) False

5) The phrase "blind people have heightened senses" has some truth to it
a) True
b) False

Short answer
1) The somatosensory cortex is highly mapped out but even within the somatosensory cortex there are specific areas where information is sent. Explain each area and describe what information they receieve

2) Describe two ways which the somatasensory cortex can change according to the environment

Answers

Multiple guess
1) d
2) c
3) d
4) d
5) b
True or False
1) b
2) b
3) b
4) a
5) a
Short Answer
1) Theses areas within the somatosensory cortex that are also highly somatotopically organized are the Brodmann areas 3a, 3b, 1, and 2. Brodmann area 3a receives information about proprioception so their information comes via the joint receptors and the muscle spindles. Brodmann area 3b information from the touch receptors located in the layers of the epidermis. These include Meissner's corpuscles, Merkel cells, Pacinian corpuscles, and Ruffini endings. Brodmann area 1 also receives information about touch and further processes the information that Brodmann area 3b receives. Brodmann area 2 receives information from the muscles and joints.

2) The two ways that the somatosensory cortex changes is through proprioceptive changes and training. Studies have shown that when limbs have been physically removed or changed, the somatosensory cortex changes accordingly; the study on subjects with syndactyly. In the case of removal, the area in the somatosensory cortex also vanishes because it is being unused, and in its place, the representative areas in the cortex that receive information on the limbs that are being used more often to compensate for the missing limb begin to grow due to the glial cells creating more connections. In the case of training, if a simulus is repeatedly applied to one of the fingers, there are more connections in the representative areas in the cortex which makes those areas grow. It also makes the brain more aware of stimulus in those trained areas.

Suggested links


http://listverse.com/2008/03/07/10-modern-cases-of-feral-children/
List of 10 modern cases of feral children. Interesting to see what animals seemed to have adopted them.

http://www.ted.com/talks/lang/eng/michael_merzenich_on_the_elastic_brain.html
Talk on how to use brain plasticity to increase our best ability

http://www.positscience.com/human-brain/brain-plasticity/about-brain-plasticity
Answers basic questions about brain plasticity and outlines cognitive growth and decline

http://www.sensorysubstitution.co.uk/
A website that explores the future of sensory substitution


References


Kandell ER, Schwartz JH, Jessell TM. Principles of Neural Science. 4th ed. New York City: McGraw-Hill Companies, Inc., 2000.

Mogilner, A., Grossman, J., Ribary, U., Joliot, M., Volkmann, J., Rapaport, D., & ... Llinás, R. (1993). Somatosensory cortical plasticity in adult humans revealed by magnetoencephalography. Proceedings Of The National Academy Of Sciences Of The United States Of America, 90(8), 3593-3597. Retrieved from EBSCOhost.

Ferdenzi, Camille; Coureaud, Gerard; Camos, Valerie; Schaal, Benoist. (2010, January 1). Attitudes toward everyday odors for children with visual impairments: a pilot study The Free Library. (2010). Retrieved December 19, 2010 from http://www.thefreelibrary.com/Attitudes toward everyday odors for children with visual impairments:...-a0216896243

Rosenbluth, Ruth, Ephraim S. Grossman, and Marsha Kaitz. "Perception Abstract." Perception Homepage. 20 July 1999. Web. 19 Dec. 2010. <http://www.perceptionweb.com/abstract.cgi?id=p3001>

Montreal Neurological Institute / McGill University (2004, July 23). The Blind Really Do Hear Better. ScienceDaily. Retrieved December 19, 2010, from http://www.sciencedaily.com­ /releases/2004/07/040723093712.htm

RENIER, L., & DE VOLDER, A. G. (2005). COGNITIVE AND BRAIN MECHANISMS IN SENSORY SUBSTITUTION OF VISION:: A CONTRIBUTION TO THE STUDY OF HUMAN PERCEPTION. Journal of Integrative Neuroscience, 4(4), 489-503. Retrieved from EBSCOhost.