Posts Tagged ‘ brain

Videos of the Brain

Some great videos illustrating brain anatomy. First, anatomy and function using color coded 3D animation.

Next, a more in depth dissection with Ami Cohen, a graduate student at USCB.

The Inside of the Brain

Now let’s work our way inside the brain.

At the base of the brain the spinal cord becomes enlarged, and is known as the brain stem. The lowest level of the brain stem is the medulla, rising up from the spinal cord. The medulla controls the most basic of operations including cardiac, respiratory, vomiting and vasomotor centers, as well as regulating autonomic functions, such as breathing, heart rate and blood pressure.

As we move further up the brain stem we find the pons, involved in general arousal of the nervous system, assisting in controlling autonomic functions, sleep, and relaying sensory information between the cerebrum and cerebellum. This relaying is necessary as the cerebellum is seemingly detached from the rest of the brain, lying off to the back of the brain stem. It is involved in other basic functions, most notably motor control. It does not initiate movement, but acts as a grand director – receiving input from sensory systems and from other parts of the brain and spinal cord, and integrating these inputs to fine tune gross motor activity. The cerebellum is also involved in motor learning, such as a baby deer tottering around and correlating sensory input to motor output in order to use its legs.

The midbrain controls the visual and auditory systems as well as eye movement. Portions of the midbrain called the red nucleus and the substantia nigra are involved in the control of body movement. The darkly pigmented substantia nigra contains a large number of dopamine-producing neurons, degeneration of which is associated with Parkinson’s disease. Barry Kidston, a 23-year-old chemistry graduate student in Maryland, synthesized MPPP (a synthetic opioid painkiller) in 1976 after learning about it from an obscure 1947 paper, but screwed up a few tiny technical details leaving traces of MPTP behind. It took only three days of self-injection of the tainted MPPP for the trace MPTP to irreversibly damage the substantia nigra and for Barry to start exhibiting symptoms of Parkinson’s disease.

The bulbous thalmus extends out of the top of the brain stem, and it situated between the cerebral cortex and the midbrain, both in terms of location and function. Its functions include relaying sensory input and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep and alertness. It is surrounded by basal ganglia which are primarily involved in action selection, or the decision of which of several possible behaviors to execute at a given time. Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The basal ganglia can therefore be thought of as a “behaviour switch” influenced by signals from many parts of the brain, including the “executive” prefrontal cortex.

Centered deep below the thalamus lies the hypothalamus, the connection between the nervous system and the endocrine (hormonal) system. The hypothalamus controls body temperature, hunger, thirst, fatigue, and circadian cycles. Sweeping up from the hypothalamus on both sides are several other structures collectively known as the limbic system, or “paleomammalian brain”. These ancient structures infuse emotional overtones in our experience and into our affective responses, affect long term memory, behaviour, and the sense of smell. One limbic component, the hippocampus, helps us process data and relay them to be stored in various memory circuits.

The limbic system was originally proposed as the emotional center of the brain, with the neocortex taking the role of “hard computation”. This has been cast into controversy with the discovery that the hippocampus is involved in memory retention, and the boundaries of the limbic system have been redrawn again and again since the concept was originally proposed. Some scientists propose scrapping the concept of a seperate limbic system entirely.

The Outside of the Brain

The brain is divided into two hemispheres, one on the left and one on the right, connected by a thick band of tissue called the corpus callosum. Oddly enough, the right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body. Activity in the left hemisphere is correlated with speech, writing, language, and calculation. Activity in the right hemisphere is correlated with spatial abilities, face recognition, and some musical abilities.

The outside of the brain is a grey layer, folded into strange ridges, and called the cerebral cortex, or the “gray matter”. The gray color comes from billions of nerve cells, and connects to bundles of of white, insulated fibers – “white matter”. You can think of it like an outer “rind” – in humans, it makes up 80% of the brain. In monkeys, only about 66%. The amount of “gray matter” appears to contribute significantly to how advanced a species is, and humans have the most of it.

The brain itself can be divided roughly into four lobes.

The frontal lobes help make us human. The association cortex in this region helps us generate goals that are personally desirable, to determine how socially appropriate they are, and then to decide which behaviors will result in the best future outcome. Many higher order executive functions arise in the frontal lobes, including those that direct speech and other precisely managed movements. The frontal lobe contains most of the dopamine-sensitive neurons in the cerebral cortex.
Behind lie the parietal lobes. These lobes not only receive sensation but allow fine discrimination between them. When we fumble in our pocket, which coin is a nickel and which coin is a quarter? Our fingers tells us “this one”, as the parietal lobes make abstract representations of the sensations from our fingers, relating these representations to the type of coin.
Below each parietal lobe, deep to the temple on either side, are the temporal lobes. They decode and interpret what we hear and see, and process other more elaborate, patterned sensory messages. For instance, the left temporal lobe is important in understanding language-related concepts.
The occipital lobes at the back of the brain register impulses concerned with vision. They then pattern them into streams of visual messages that are relayed forward to both the temporal and parietal lobes. Note that they do not relay patterns of photons hitting your eye, but more abstract and compressed representations. These mental images are generated and regenerated, and could be thought of as our “mind’s eye”, the mental templates that allow us to recognize and insert meaning into what we see. Strong increases in occipital activity is noted under the influence of certain psychedelic drugs.