The Brain: What’s Going On in There?

The brain as explained by Pinky and the Brain:


The Structure of a Sheep’s Brain



external image sheep-brain-sagittal.jpg

In general, the sheep brain is easy to work with because of it's size, availability, and relevance in comparative dissection (easily compared to the human brain dissected).

Sheep Brain vs. Human Brain: Structure and Functions

The sheep's brain is significantly smaller than a human brain. As evidenced by the pictures below, the sheep's frontal lobe is also significantly smaller than a human's. The brain stem and cerebellum are in different places due to the different physical structure of sheeps and humans (sheeps walk on all fours and humans walk upright).

The cerebrum is a region of both sheep's and humans' brains. Located in the cerebrum are the frontal, parietal, occipital, and the temporal lobes of the human brain. The cerebrum is split into two hemispheres (the right hemisphere and the left hemisphere) connected by the corpus callosum. The cerebrum is sometimes called the "new brain" and is responsible for higher level thinking. Presumably, the "new brain" was the last part of the brain to evolve.


Each of these sections of the cerebrum have different functions:

The Temporal lobe- controls auditory functions, memory, emotion, smell, music, fear, sense of identity, and language. There are two sulci the temporal lobe has that separate the temporal lobe into three gyri. Those gyri are the Superior Temporal Gyrus, the Middle Temporal Gyrus, and the Inferiro Temporal Gyrus. Sound recognition is mostly found in the Superior Temporl Gyrus.

The Frontal lobe- controls higher mental functions such as: decision making, problem solving, and planning, behavior, abstract thought processes, attention, creative thought some emotion, intellect, reflection, judgment, initiative, inhibition, coordination of movements (with the cerebellum), generalized and mass movements, some eye movements, sense of smell, muscle movements, skilled movements, some motor skills, physical reaction, future consequences, override/suppress unacceptable social decisions, recognizing similarities and differences of things, and libido (sexual urges).

Parietal lobe- has somatic sensory cortex to help feel pain, is the reception site and processing center of sensory information and comprehension, some language and reading functions, and some visual functions.

Occipital- controls visual and reading functions. This lobe helps us to identify color, and interpret each color is different. It is located behind the parietal lobe, and aids us in understanding and distinguish between different shapes.

The cerebellum is another region of the brain that sheep and humans share. Its functions in both organisms are: provides timing for muscle activity & provides a sences of balance, initiates corrective impulses, works with the brain stem.
It provides feedback and controls motor output and the sense of proprioception, which gives us an intuitive map of the location of our bodies.

Sheeps and humans also share the brain stem, which regulates autonomic functions, such as breathing and heartbeat. Located here are the pons, medulla oblongata and the spinal cord.

A final major region is the Mid Brain. This contains the hypothalamus, the thalamus, the limbic system, the hippocampus and many other structures in both sheep and humans. The Mid Brain is responsible for a variety of functions, such as memory (hippocampus) and emotion (hypothalamus). The reward pathway is also located here. The Mid Brain and cerebellum are called the "old brain" because they are responsible for essential instinctual functions and presumably evolved first.


Human Brain (Left) Sheep Brain (Right)

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Differences
1. The human brain is heavier and longer than a sheep’s brain.
2. The sheep’s brain has a more developed olfactory bulb when compared to the human brain, giving them a sharper sense of smell.
3. The human brain is rounded, whereas the sheep’s brain is elongated in shape.
4. The human brain has a larger frontal lobe than the sheep’s brain. This allows humans to engage in higher level thinking, such as problem solving.
5. The major difference between the human brain and the sheep brain is that the human brain can produce thoughts, words (orally and verbally), and ideas, whereas the sheep brain can not.
6. The sheep's pineal gland measures day length and prepares the sheep for the reproductive season through secretion of melatonin once a year. The pineal gland in humans is more active because they have a more frequent reproductive cycle.


How We See Whats Going On In There: Brain Imaging Techniques

external image mri_brain.jpgMRI (Magnetic Resonance Imaging)MRIs use the detection of radio frequency signals produced by shifted radio waves in a magnetic field to produce detailed, three-dimensional images and videos of brain. Because physicians are able to see various angles with the images and videos, as well as magnify the images they are able to better evaluate certain areas of the brain which may not be noticed with other imaging methods.
3D MRI of a Human Brain


PET Sca
external image copet.gifn (Positron Emission Tomography Scan) A scanner is able to detect a radioactive glucose which is injected into the body 40 minutes before the PET scan in order to produce an image of the brain. When the radioactive material gets into the bloodstream, it goes to areas of the brain that use it. So, oxygen and glucose accumulate in brain areas that are metabolically active. When the radioactive material breaks down, it gives off a neutron and a positron. When a positron hits an electron, both are destroyed and two gamma rays are released. Gamma ray detectors record the brain area where the gamma rays are emitted. Since PET scans provide a functional view of the brain it is often used in conjunction with MRIs or CT Scans to see both the structural and functional views of the brain.

See full size image
See full size image

EEG (Electroencephalography) - Records the electrical activity (impulses) in the brain by the firing of neurons. This method of imaging may be used to diagnose strokes, tumors, and epilepsy. With an EEG you are able to see “real time” activity within the brain. When getting an EEG, a person is asked to wear a bonnet that contains electrodes. These electrodes pick up impulses from the brain.

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CT Scan (Computed Tomography Scan)CT scans use a series of X-ray beams which pass through the head. CT scans take several consecutive X-rays to create "slices" of the brain. The images are then developed on sensitive film. This method creates cross-sectional images of the brain and shows the structure of the brain.
(Class Notes, Mayo Clinic Website)




The Reward System of the Brain


external image slide-9.gifThe Reward System: a collection of structures located in the limbic system in the brain that regulates behavior by inducing sensations of pleasure.
- Drives us to eat, drink, sleep and reproduce, all instincts controlled by the old brain (i.e. limbic system)
- The
major neuropathways: mesolimbic and the mesocortical pathways.
- The mesolimbic pathway connects to the
ventral tegmental area (VTA) using the medial forebrain bundle and then to the nucleus acumbens. This is where dopamine is released and creates a euphoric feeling that rewards our bodies for completing an instinctual drive or an action necessary for survival. This area of the brain plays a large role in fear, laughter, avoidance of pain, seeking of pleasure, and many strong emotions. The ventral tegmental area plays an especially large role in addiction.
*Essentially: Life sustaining activities activate a series of specialized nerve cells that are devoted to producing and regulating pleasure.
The reward system rewards us for fulfilling desires of the old brain. This system, while normally encourages us to do things necessary to our survival, can be "hijacked" by drugs of abuse.

Video on the Reward System and Drug Abuse

How Normal Behaviors Activate the Reward System vs. Drugs of Abuse

Normal Behavior activate the Reward System..external image ss-2.gif Life sustaining activities activate a series of specialized nerve cells (VTA, NA) that are devoted to producing and regulating pleasure through the release of dopamine. The reward system evolved to reward and encourage behaviors which help ensure survival.
Ex - seeking food, reproduction, shelter, drink, etc

Feelings of pleasure motivate us to repeat those behaviors. Each time the reward system is activated the brain remembers activity in the prefrontal cortex. The more we engage in a pleasurable act, the more we learn to repeat it until we do it without even thinking about it.


Drug's Affect on the Reward System...
    • Blocking reuptake channels
    • Stimulating neurotransmitters to be released without an electrical impulse
    • Releasing more neurotransmitters from the vesicles in response to an impulse

Drugs of abuse "hi-jack" the reward system and replace longings for survival instincts with cravings for the drug.

The Contrasting Roles of the ‘Old Brain’ and ‘New Brain’

The "Old Brain"

- The part of the brain that humans share with other animals
- Composed of three parts: the brain stem, the limbic system (thalamus, hypothalamus, basal ganglia, amygdala, and the hippocampus), and the cerebellum.

The brain stem functions: reproduction, self-preservation, circulation of blood, breathing, sleeping, and the contractions of muscles in response to external stimuli.
The limbic system functions: generate motivations and emotions, specifically fear and aggression. The limbic system uses memories and sensory input to generate emotional responses to different situations. It also provides "reward" for survival behavior such as, eating, sleeping, and drinking (by realeasing dopamine).
The cerebellum function: moderate some of the instinctual reactions of the old brain.


The "New Brain"
- the more highly evolved part of our brain
- composed of the cerebrum
- there are four lobes in the cerebrum: temporal, occipital, frontal, parietal
- controls higher thinking
- ex. problem solving, analyzing, making connections
- can also override instinctual drives coming from "Old Brain"