Life in the Fast Lane

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A number of peculiar things that you may not know about the human brain may astound you. Size doesn’t matter — at least when referring to brains, the misnomer that we only use 10% of our brains is a myth, and yes, there are differences between the sexes as to how we process information, and truth behind the adage that men are more rational and women are more emotional.

Visualization of a diffusion tensor imaging (DTI) measurement of a human brain. Depicted are reconstructed axon tracts that run through the mid-sagittal plane. Especially prominent are the U-shaped fibers that connect the two hemispheres through the corpus callosum (the fibers come out of the image plane and consequently bend towards the top) and the fiber tracts that descend toward the spine (blue, within the image plane). Image Thomas Schultz

Evolution of the brain — from the earliest shrewlike mammals through primates to hominids — is marked by a steady increase in encephalization, or the ratio of brain to body size. Neanderthals had larger brains at adulthood than present-day humans.

Via:Nursing Programs

The human brain has been estimated to contain 50 to 100 billion neurons, of which about 10 billion are cortical pyramidal cells. These cells pass signals to each other by scores of synaptic connections — as many as 1,000 trillion.

Drawing of the human brain, showing several important structures. Image US National Institute for Aging

The adult human brain weighs on average about 3 pounds (1.5 kilos) with a size of about 1,130 cubic centimeters in women and 1,260 in men, although there is substantial individual variation.

There are differences in size between the sexes — men with the same body height and body surface area as women have an average of 3.5 ounce (100 gram) heavier brains.

The brain is very soft, with a consistency similar to soft gelatin or firm tofu. Despite being referred to as ‘grey matter,’ the live cortex is a pinkish-beige color and slightly off-white in the interior.

Bisection of the head of an adult man, showing the cerebral cortex and underlying white matter. Photo US National Library of Medicine

4 Lobes
From the outside, the cerebral cortex is nearly symmetrical, with left and right hemispheres. Anatomists conventionally divide each hemisphere into 4 ‘lobes’ — the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.

The lobes are named after the bones of the skull that overlie the structure of the cortex with one exception — the border between the frontal and parietal lobes is shifted backward to the central sulcus, a deep fold that marks the line where the primary somatosensory cortex and primary motor cortex come together.

The four lobes of the cerebral cortex. Photo Mysic

3 Functional Divisions
Researchers who study the functions of the cortex divide it into 3 functional categories of regions, or areas.

One consists of the primary sensory areas, which receive signals from the sensory nerves and tracts by way of relay nuclei in the thalamus. Primary sensory areas include the visual area of the occipital lobe, the auditory area in parts of the temporal lobe and insular cortex, and the somatosensory area in the parietal lobe.

A second category is the primary motor area, which sends axons down to motor neurons in the brainstem and spinal cord. This area occupies the rear portion of the frontal lobe, directly in front of the somatosensory area.

The third category consists of the remaining parts of the cortex, which are called the association areas. These areas receive input from the sensory areas and lower parts of the brain and are involved in the complex process of perception, thought, and decision making.

Brain Imaging
There are several methods for detecting brain activity changes by 3-dimensional imaging of local changes in blood flow. The older methods are SPECT and PET, which depend on injection of radioactive tracers into the bloodstream. The current method — functional magnetic resonance imaging (fMRI) — has considerably better spatial resolution and involves no radioactivity.

A scan of the brain using fMRI. Photo Washington Irving

Using the most powerful magnets currently available, fMRI can localize brain activity changes to regions as small as one cubic millimeter. The downside is that the temporal resolution is poor — when brain activity increases, the blood flow response is delayed by 1 to 5 seconds and lasts for at least 10 seconds. fMRI is a very useful tool for learning which brain regions are involved in a given behavior, but gives little information about the temporal dynamics of their responses.

Language
In human beings, the left hemisphere usually contains the specialized language areas. This may be true for 97% of right-handed people, but about 19% of left-handed people have their language areas in the right hemisphere, and as many as 68% of them have some language abilities in both the left and the right hemisphere.

The 2 hemispheres are thought to contribute to the processing and understanding of language — the left hemisphere processes the linguistic meaning of prosody (the rhythm, stress, and intonation of connected speech), while the right hemisphere processes the emotions conveyed by prosody.

Studies of children have shown that if a child has damage to the left hemisphere, the child may develop language in the right hemisphere instead. The younger the child, the better the recovery. So, although the ‘natural’ tendency is for language to develop on the left, human brains are capable of adapting to difficult circumstances, if the damage occurs at a young enough age.

Location of two brain areas that play a critical role in language, Broca’s area and Wernicke’s area. Image US NIH publication 97-4257.

The first language area within the left hemisphere to be discovered is Broca’s area, named after Paul Broca, who discovered the area while studying patients with aphasia, a language disorder. Broca’s area doesn’t just handle getting language out in a motor sense — it seems to be more generally involved in the ability to process grammar itself, at least the more complex aspects of grammar. For instance, it handles distinguishing a sentence in passive form from a simpler subject-verb-object sentence — the difference between ‘The boy was hit by the girl’ and ‘The girl hit the boy.’

The second language area to be discovered is called Wernicke’s area, after Carl Wernicke, a German neurologist who discovered it while studying patients who had similar symptoms to Broca’s area patients but damage to a different part of their brain. Wernicke’s aphasia is the term for the disorder occurring upon damage to a patient’s Wernicke’s area.

But Wernicke’s aphasia doesn’t only affect speech comprehension. People with Wernicke’s aphasia also have difficulty recalling the names of objects, often responding with words that sound similar, or the names of related things, as if they are having a hard time recalling word associations

Sources: Online MBA and Wikipedia

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