Socializing the Cell. nerve cells and brains
Genes, then, function as part of a complex cell with its own important history. Cells, in turn, operate as large, intimately connected groups that form coherent organs within a complex, functionally integrated body. It is at this level, when we look at cells and organs within the body, that we can begin to glimpse how events outside the body become incorporated into our very flesh.
Just after the turn of the twentieth century in the Bengal Province of India, the Reverend J. A. Singh ‘‘rescued’’ two children (whom he named Amala and
Kamala), girls succored since infancy by a pack of wolves.19 The two girls could run faster on all four limbs than other humans could on two. They were profoundly nocturnal, craved raw meat and carrion, and could communicate so well with growling dogs at feeding time that the dogs allowed the girls to eat from the same bowls. Clearly these children’s bodies—from their skeletal structure to their nervous systems—had been profoundly changed by growing up with nonhuman animals.
Observations of wild children dramatize what has become increasingly clear to neuroscientists, especially in the past twenty years: brains and nervous systems are plastic. Overall anatomy—as well as the less visible physical connections among nerve cells, target organs, and the brain—change not only just after birth but even into the adult years. Recently, even the dogma that no new cells appear in the adult brain has gone the way of the dodo.20 Anatomical change often results when the body’s nervous system responds to, and incorporates, external messages and experiences.
Examples abound in which a social interaction causes a physical change in the nervous system.21 Two types of studies seem especially relevant to a framework for understanding human sexuality. One concerns the development and plasticity of nerve cells and their interconnections in the central and peripheral nervous systems.22 The other addresses changes in nerve cell receptors that potentially can bind transmitters such as serotonin or steroid hormones such as estrogens and androgens, which can in turn activate the protein synthetic machinery of a particular set of cells.23 These examples show how nervous systems and behaviors develop as part of social systems.
Scientists sometimes disrupt such systems by interfering with the genetic function of one or another component. Analytically, this is akin to removing a spark plug to see whether and how it interferes with the running of an internal combustion engine. For example, scientists have created mice that lack the gene for serotonin receptors and have observed their distorted behaviors.24 But although such experiments provide important information about how cells function and communicate, they cannot explain how mice develop particular behaviors in particular social settings.25
How might social experience affect the neurophysiology of gender? The comparative neurobiologists G. Ehret and colleagues offer an example in their study of paternal behavior in male mice. Males that never have contact with young pups will not retrieve them in the spirit of good fathering (when they inch too far from the nest), but even a few hours or a day spent in the company of baby rats will evoke ongoing paternal pup retrieval. Ehret and colleagues found that early exposure to pups correlated with increased estrogen receptor binding in a number of areas of the brain and decreased binding in one area.26
In other words, parenting experience may have changed the hormonal physiology of the father’s brain as well as the mouse’s ability to care for his pups.
The fact that human brains are also plastic, a concept that recently has begun to make it into the mass media,27 makes it possible to imagine mechanisms by which gendered experience could become gendered soma. Environmental signals stimulate the growth of new brain cells or cause old ones to make new connections.28 At birth the human brain is quite incomplete. Many of the connections between nerve cells and other parts of the body are tentative, requiring at least a little external stimulation to become permanent. In some brain regions, unused neural connections disintegrate throughout the first twelve years of life.29 Thus, early physical and cognitive experience shape the brain’s structure.30 Even muscular movements before birth play a role in brain development.
One way the brain ‘‘hardens’’ a neural connection is by producing a fatty sheath, called myelin, around the individual nerve fibers. At birth the human brain is incompletely myelinated. Although major myelination continues through the first decade of life, the brain is not completely fixed even then. There is an additional twofold increase in myelinization between the first and second decades of life, and an additional 60 percent between the fourth and sixth decades,31 making plausible the idea that the body can incorporate gender-related experiences throughout life.
Finally (for this discussion at least),32 large groups of cells can change their patterns of connectivity—or architecture, as brain scientists call it. For years neuroanatomists have performed experiments to find out what segment of the brain responds when they stimulate an exterior part of the body. Touching the face provokes certain cortical nerves to fire, touching the hand and individual fing ers affects different nerves, the feet still other nerve cells. Textbooks often summarize such experiments with a cartoon of a misshapen body (called a homunculus) superimposed on the brain cortex. Scientists used to think that after early childhood, the shape of the homunculus did not change. But following a series of experiments with other primates, this viewpoint has changed dramatically.33
One recent study compared the representation on the cerebral cortex of the fingers of the left hand of stringed instrument players to age – and gender – matched controls who had no experience with stringed instruments. String players constantly move the second through fifth digits of the left hand. The left hand homunculus was visibly larger for digits two through five compared to both non-string players and to the musicians’ own right hands.34 Or consider people who, blind from a young age, have become accomplished Braille readers.35 Not surprisingly, they have enlarged the hand representation for their Braille-reading fingers. But their brains have made an even more amazing readjustment. They have recruited a region of the cortex that sighted people use to process visual information (the so-called visual cortex) and instead use it to process tactile sensations.36
For both musicians and those blind from birth, cortical reorganization probably takes place during childhood, a fact that confirms something we already know: children have enormous learning capacities. Such studies extend our ideas about learning, however, by showing that the material anatomic connections in the brain respond to external influences. Such knowledge wreaks havoc with both attempts to maintain a distinction between mind and body and attempts to offer up the body as a precursor to behavior. Instead they back up an insistence that the environment and the body co-produce behavior and that it is inappropriate to try to make one component prior to the other.37
The studies on Braille users and musicians show brain plasticity in the young, but can adult brain anatomy change as well? The answer comes from the study of a phenomenon that has long fascinated students of the human brain, from neurosurgeons to phenomenologists: the mystery of the phantom limb. Amputees often feel that the missing part is still present. At first the phantom seems to the patient to be shaped like the missing part. With time, however, the perceived shape changes; in contrast to a real limb, a phantom part feels lighter and hollow. Like a ghost, the phantom limb seems able to penetrate a solid object.38
Someone who has lost a hand may ‘‘feel’’ the missing hand following light stimulation of the lips; a light touch to the face may make someone who has lost an arm ‘‘feel’’ the missing limb, a phenomenon called referred sensation. A series of recent studies tries to explain such sensations with the finding that nerves in the region of the homunculus previously devoted to the now-missing limb are ‘‘taken over’’ by adjacent areas—in the example given, the cortical field connecting exterior stimuli to the face. The size of the homunculus for the intact hand also increases, presumably in response to increased use demanded by the loss of one hand.39 Although remapping of the brain’s cortex probably doesn’t explain all phantom limb phenomena,40 it does provide a dramatic instance of how adult brain anatomy responds to new circum-
How might all this apply to the development of sexual difference and human sexual expression? Answers developed to date have been impossibly vague, in part because we have been thinking too much about individual components and not enough about developmental systems. Paul Arnstein, a practicing nurse concerned with understanding physiological links between learning and chronic pain, writes that: ‘‘The true nature of the central nervous system has eluded investigators because of its fully integrated, constantly changing structure and a symphony of chemical mediators. Each sensation, thought, feeling, movement and social interaction changes the structure and function of the brain. The mere presence of another living organism can have profound effects on the mind and body.’’42 We will begin to understand how gender and sexuality enter the body only when we learn how to study the symphony and its audience together.