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Chapter 28: 11-21

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The Human Brain

WebLecture Topics

Animal Nervous Systems

Not all animals have nervous systems. Sponges lack nerve cells of any kind, so that response to stimulus occurs at the individual cell level, based on the interaction of the cell membrane with chemicals or conditions in its environment.

The simplest new systems are neural nets, found in Cnidarian species such as Hydra and sea anemone. While there is no brain or concentration of neurons in such a system, the neurons are interconnected and capable of exchanging information in a way that controls coordinated movement from different parts of the organism. Echinoderms like starfish also have nerve nets and a central nerve ring with no identifiable "brain" organ.

Structures: brain, spinal chord, nerves, cerebrospinal fluid

The concentration of nerve tissue at one end of a bilaterally symmetric animal is called encephalization. The simplest animals to show this are planaria, which have the beginnings of a primitive nerve cluster in the "head" end. Animals with bilateral symmetry have more developed nerve systems and localized clusters of nerves (ganglia) that can coordinate "left" and "right" body part motions to work together. In invertebrates, the nervous system, like the circulatory system is mostly unprotected by any special bone structure or covering. Only in insects, where the chitin exoskeleton covers the head, thorax, and abdomen, are gangla protected.

Vertebrates have well-developed nervous systems with clearly defined brain, spinal chords, and nerve extensions. The brain and spinal chord are surrounding by tissues flooded with cerebrospinal fluid, mostly water but containing small amounts of plasma proteins. It is produced at the rate of 500mL per day by ependymal cells in different parts of the brain (specifically the choroid plexus at the top of the brain stem, and the surface tissues of the subarachnoid space — aren't you glad you asked?). Because these tissues are not isolated from the rest of the body, the cerebrospinal fluid leaches into surrounding tissues and is absorbed and processed by the lymphatic system. The brain to some extent "floats" in this fluid, which (like amniotic fluid for a developing fetus) supports, protects, and provides a balanced chemical environment for the brain and spinal column nerve tissues.

The brains of more advanced species usually contain three primary areas: forebrain, midbrain, and hindbrain, but the sections differ in function and size, corresponding to the animal's need for perception and control in its habitat. The human brain is atypical in the size of the cerebrum section, which in humans is devoted to analytic thinking.

CNS: Somatic and Autonomic Systems

The central nervous system (CNS) contains the brain and spinal cord, that is, those parts of the nervous system that analyze signals, identify perceptions from memory, and order a response.

PNS: Parasympathetic, sympathetic, and Enteric divisions

The peripheral nervous system (PNS) consists of all the rest of the nerves that extend from the CNS to every other part of the body.

You may find this diagram useful in keeping the different parts straight:

Wikipedia CNS diagram

The Human Brain

Wikipedia Brain Diagram

In the detailed diagram above, see if you can identify the cerebral cortex, the corpus callosum, the thalamus, the hypothalamus, the cerebellum, the brainstem, the pons, and the medulla oblongata.

Each part of the brain controls different functions. Until the last three decades, most of our knowledge about which part of the brain controlled a given function came from analyzing abilities affected or lost by different instances of brain injuries or disease. Modern technologies such as MRI scanning now allows us to determine which parts of the brain are active during different activities, from sleeping to reading or solving mathematics problems to running or playing piano. This human brain map shows the areas of the cerebellum that process information to determine sequences, help you spell written words, identify spoken words, respond emotionally to a situation, or visualize the alphabet shapes. Each of these activities engages a different part of the brain, and studying the parts and their development help us understand how infants and children learn, and how development (or lack of development) of a seemingly unconnected skill such as crawling on all fours may affect the rate at which you learn to read.

The brain is a very adaptable ("plastic" or trainable) organ, and in the event of injury or experience, some functions can be transferred to another part of the brain.

Regulating Sleep

While we often think of sleeping as a period where the body and brain shut down, many important biological "housekeeping" functions occur during this period of reduced sensory input, so one of the more important functions of the brain is regulating hormones that control sleep patterns. While the cerebrospinal fluid mentioned above has almost no flow when humans are awake, the flow from the brain into the body increases dramatical during sleep, as though the body were washing out wastes and detritus from the nervous system.

At the same time that this physical cleanup occurs, the brain moves through different internal processing steps, including deep sleep and "REM" sleep (marked by rapid eye movements under the lids).

In deep sleep, body cells increase their production of proteins to repair damage or replace used cells. Emotional responses and decision-making processes shut down, allowing other parts of the brain the process short term memories and fix them as long term information to work more efficiently. The immune system may actually release hormones like cytokinins induce sleep to provide more resources for higher levels of infection-fighting than are possible during waking periods.

During REM sleep we dream, possibly trying to create stories and explanations out of random signals still being transmitted as the memory areas process recent events. Such processing appears to be critical to the learning process: without REM sleep, we don't remember what we've studied or learned recently. For our safety, the pons shuts down motor signals to the spinal cord, so that if you dream about catching a baseball or running from bears, you don't actually try to stretch out your arms or race down the hall in your sleep. Preventing or disturbing REM sleep patterns can lead to seizures, disorientation, and hallucinations.

The Limbic System

The limbic system appears to be the brain center for emotion, behavior, and long term memory, and (perhaps not all that surprising if you think about how important smell might be to survival), the processing location for the olfactory nerve signals. The main components are


Study of the limbic system is relatively new, so there are many questions and much controversy over exactly how these anatomical divisions work together, and even whether they should be considered all parts of the same system. Memory formation is one of the great unanswered questions of modern neuroscience. One of the important areas of brain research focusses on "neuroplasticity", or the brain's ability to learn and adapt to new situations, including new kinds of stimuli, by changing the connections between existing neurons, or by neurogenesis, that is, the development of new neurons in the brain as a response to injury. Until recently, neuroscientists believed that the brain's physical composition was fixed by late childhood, and that damage to existing neurons could not be repaired or the neurons replaced, but research in the last two decades supports that the brain can and does change, and not only in response to injury. Disciplined meditation (deliberately relaxing and focussing on specific imagery, especially if it is inspirational for the practitioner), appears to change the brain's physical behavior, enabling it to deal more effectively with attention disorders, anxiety, fear, or the body's response to injury.

Levels and conditions of exercise appear to affect brain functions as well. Neuroscientists studied two groups of mice to see how well they could learn to swim a water maze and how quickly they would learn to avoid an unpleasant stimulus by moving away from it. They then gave one set of mice the opportunity to exercise on a rodent wheel whenever they wanted, but the second group had to run a treadmill at speeds and durations set by the scientists. The two groups of mice both improved in performance in the water maze, but only the second group got better at the avoidance test. Examinations of brain chemistry showed that the first group had actual molecular changes in only one part of their brains, while the forced-exercise mice changed in several areas. It wasn't clear whether this was because the mice had to learn to deal with forced vs. voluntary exercise, or whether the different form of exercise (rodent wheel vs. treadmill) played a part. But the studies do imply that different experiences actually shape the chemical function and eventually the structure and neural responses of our brains, and that self-discipline or medical treatment may be effective in increasing learning ability or dealing with diseases like epilepsy.

Healthy Brains

The Franklin Institute, a museum in Philadelphia originally founded by Benjamin Franklin, has a set of brain-related activities. You can check out how information travels your neural network, follow a visual pathway, make predictions based on previous knowledge, focus your perception, and explore how music changes the way we feel.