David Shenk explains multiple times that nothing “...from eye color to intelligence...” (198) is fixed and all of our traits and actions are influenced by the GxE paradigm. When referring to the brain’s ability to “...adapt and organize [itself] in response to specific experience” (35), this is known as our brain’s “plasticity”: the brain’s “...capacity to become, over time, what we demand of it” (36). However, Shenk contradicts himself by stating that “there are, of course, strict limits to plasticity. Every functioning human brain has an intricate and unchanging design, billions of years in the making” (201). If our brains have supposedly intricate, “unchanging” designs, then how does Shenk propose that this is not true of other organs and traits, such as intelligence, weight, hair color, etc? On the other hand, exactly how has our brain evolved over time to account for the GxE paradigm? Use evidence from the book as well as Chapter 49 of the Campbell textbook (“Nervous Systems”) and relate this to the biological themes of evolution and/or structure and function.
Ethan Homedi (ethanhomedi@yahoo.com)
First of all, I don’t believe David Shenk meant to contradict himself at all. The limitations he meant by “intricate and unchanging design” refer to the specialized functions of the human brain. For example, the temporal lobe of the brain is associated with hearing, the occipital lobe with vision, and so on (Campbell 1075). David Shenk himself explains that, “various lobes and neural pathways are dedicated to specific functions…”(201). An analogy of this idea would be to say that the plasticity of AP Biology knowledge would be limited for London cabbie drivers simply because it isn’t related to their job, which is analogous to a specialized function of a lobe. However, visual recognition would be subject to plasticity because it’s relevant to a cabbie driver’s job. Thus, plasticity is limited if structure and function do not match. Otherwise, “this evolved design also includes an enormous capacity to learn and adapt…”(201).
ReplyDeleteAs for how traits are evolved, it relates to David Shenk’s thesis of GxE interaction. Genes are either turned on or off due to the influence of the environment. David Shenk quotes Anders Ericsson: “‘When individuals deliberately push themselves beyond…relative comfort and engage in sustained strenuous physical activity, they [induce] an abnormal state for cells…[which] will trigger the activation [of] dormant genes within the cells’ DNA,… caus[ing] bodily reorganization and adaptive change’” (69). For example, when athletes train in high altitude locations, the hypoxic conditions activate the hypoxia—inducible factor 1 transcriptional activator complex (HIF-1) which turns on erythropoietin-coding genes. Erythropoietin will then promote increased production of red blood cells (http://www.jbc.org/content/272/36/22642.short).Thus, environment influences gene expression and in turn can enhance a certain trait. The brain, of course, is anything but exempt from this idea—the brain is impacted by GxE interaction as well.
Although the basic architecture of the central nervous system is, as David Shenk says, “intricate and unchanging”, the brain has a capacity called neural plasticity to be remodeled. Neurons have two extensions: dendrites that receive signals and axons that send signals. The axon connects the signal-sending presynaptic cell with the signal-receiving postsynaptic cell. The axon divides into several branches as it reaches the postsynaptic cell. Synapses are the junctions where the branched end of an axon transmits information to the postsynaptic cell (Campbell 1048). Synapse activity is why reshaping of nervous system occurs. When activity at one synapse correlates with other synapses, the connection is reinforced. The opposite is also possible. The net result is this: increased signaling between pairs of neurons or decreased signaling between pairs of neurons (Campbell 1079). By strengthening synaptic connections of neurons, memory is improved (http://www.medicalnewstoday.com/releases/60455.php). By improving memory, many traits or skills at an activity are indirectly improved. This concept fits right in the paradigm of GxE. In 2000, Genomics Institute of the Novartis Research Foundation and Princeton University found that mice raised in the enriched environment showed more gene expression involved in formation of new synapses and strengthening of existing synapses (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC18858/).
Lastly, the theme of structure and function can be applied. For example, axons can be extremely long—up to over a meter in giraffes! This structure is critical for those axons that connect neurons from the spinal cord to muscle cells of the feet (Campbell 1048). Also, dendrites have a large surface area to volume ratio to receive signals (http://www.wikilectures.eu/index.php/Structure_and_Function_of_Axons_and_Dendrites). Finally, just fact that humans exhibit cephalization at all reflects structure and function to accommodate our advanced thinking and functions (such as AP Biology blogging)—as opposed to cnidarians’ simple nerve net (Campbell 1065).
Above was posted by Linda Xu (lindaxu22@hotmail.com)
ReplyDeleteI agree with Linda in that Shenk doesn’t contradict himself; he is merely saying that the human brain can grow via plasticity, but there are “strict” limits. He is saying that our brains have designs where some aspects are unchangeable and other aspects that are changeable. I would have to disagree with Shenk in that there are “strict” limits to brain plasticity because of the integral role that brain plasticity has played in evolution. Brain plasticity is the “capacity to become, over time, what we demand of it” (Shenk 36) and allows the brain to adapt to the environment. Many studies have proven that experience can sculpt the features of brain structure. Shenk uses the example of how “experienced taxi drivers had a greatly enlarged posterior hippocampus” (Shenk 35). Shenk quotes Harvard psychiatrist Leon Eisenberg about how “the cortex has a remarkable capacity for remodeling after environmental change” (Shenk 35). There have been people who have had to get an either an entire hemisphere removed or their corpus callosum removed due to damage or to help reduce the frequency of seizures in those with epilepsy. Even after these people have had chunks of their brain removed, they are able to live and function normally due to brain plasticity. If the surgery occurs at a young age, human brains can adapt by creating new synaptic connections that allow the remaining brain to “take over” the functions of the section of brain that has been removed. In this sense, the brain can recover functions that may have been lost due to damage. According to a study done by Michael Johnston, plasticity is “influenced by genetic factors such as mutations in brain-derived neuronal growth” (http://onlinelibrary.wiley.com/doi/10.1002/ddrr.64/abstract).
ReplyDeleteThe limits that Shenk refers to aren’t exactly “strict”, but some limits include damage to the broca that would inhibit a person from talking and damage to the Wernicke’s area that would inhibit a person from understanding language (Campbell 1076). But from many studies, the brain seems to prevail in remodeling itself when the environment or physical structure of the brain changes. Over time, the brain has accommodated to the GxE equation because the “turning on” and “turning off” of certain genes due to the environment stimuli that can influence the reorganization of “neuronal networks in response to environmental stimulation” (http://onlinelibrary.wiley.com/doi/10.1002/ddrr.64/abstract). I agree with Shenk in that the brain does seem to have an unchanging design when humans are born and a normal brain is compartmentalized and has the same basic structure, but the how the brain reacts to environmental stimuli makes each brain unique. The brain is divided up into different lobes that are specialized to carry out specific functions (Campbell 1075).
- Akshay Ramachandran (ramachandran.akshay11@gmail.com)
Part 2
ReplyDeleteThe theme of structure and function in the brain is crucial to plasticity and brain development. Shenk provides the example of the London cabbies to prove that intensive practice actually does physically change us because of plasticity. Shenk cites Anders Ericsson, who said “when individuals deliberately push themselves beyond the zone of relative comfort and engage in sustained strenuous physical activity…they induce an abnormal state for cells in some physiological systems…these biochemical states will trigger the activation of dormant genes within the cells’ DNA” (69). The stretching of the brain allows the brain to become “engraved” with new configurations that allow a buildup of talent. The structure of the brain allows for plasticity as different types of brain cells, such as neurons, glia, and vascular cells , all of which rely on “a highly specialized cellular organization” to transfer information throughout the body (Campbell 1048). These factors are involved in neuroplasticity (the lifelong ability of the brain to reorganize neural pathways based on new experiences). In order to learn or memorize a fact or skill, there must be persistent functional changes in the brain that represent the new knowledge. Neuroplasticity does not consist of a single type of morphological change, but includes several different processes that occur throughout an individual’s lifetime (http://faculty.washington.edu/chudler/plast.html). Without neuroplasticity, human evolution would slow down exponentially or wouldn’t occur.
- Akshay Ramachandran (ramachandran.akshay11@gmail.com)