Activity 3-5: Brain Pathways—Introduction and Learning Pathway
Lesson 3 Module View
It might be tempting to think about particular brain areas, for instance, the hypothalamus, and imagine that homeostasis
relies only on that region. However, brain regions are connected to each other and typically an entire network of
interconnected regions is responsible for the function under investigation.
Consider the pathway of neurons necessary for us to see. A sensory neuron
in the eye synapses on another outside the retina which will then take
information through the optic chiasm to the thalamus. In the thalamus,
another neuron will take the information to the visual cortex. So, there is not
one organ or one region necessary for the function of sight, but many
interconnected regions.
We will return to sensory pathways in a following activity.
For now, consider first how we learn. The “learning pathway” is a connected
network of brain cells that starts with cells in sensory organs (ears and eyes,
for instance) which acquire information. They transmit that information from
sensory organ to thalamus. The thalamus determines where the information
needs to go in order to perceive that information (see below) and also sends
this information to the hippocampus. In the hippocampus, the momentary
awareness of the sensory input is translated into memory. The hippocampus
corresponds about that memory with the cerebral cortex, primarily the frontal
cortex. Thus learning involves a circuit that connects sensory organ ->
thalamus -> hippocampus -> frontal cortex.
Visit the University of Washington's Digital Anatomist
at http://www9.biostr.washington.edu:80/cgibin/DA/PageMaster?atlas:Neuroanatomy+ffpathIndex/Brain^Dissections/Dorsal^4^(Hippocampal^Form.)+2 (Links to an
external site.) . You will find a number of human brain dissection views here and may explore as you like. But for this
lesson, focus on the hippocampal formation (dorsal 4) view.
Click on various regions of the brain and the digital anatomist will tell you the portion you are looking at it (upper left) and
outline the area represented by that part in yellow. Make sure you find the hippocampus. Once you've gotten familiar with
this region, try the "quiz" and see what you've remembered! It's more challenging than necessary, so don't let the quiz
frustrate you.
There is amazing new work on the impact of our behaviors on the development of our hippocampus. German
researchers studied 40 "identical twin" mice in very rich environments where there were lots of options for things to
do. Depending upon the choices made by these mice, their adult behavior differed. AND neurogenesis (birth of neurons)
in the hippocampus also varied depending upon their activity choices. The authors of this research study say that
although " the animals shared the same life space, they increasingly differed in their activity levels.
These differences were associated with differences in the
generation of new neurons in the hippocampus, a region of the
brain that supports learning and memory," says Kempermann.
"Animals that explored the environment to a greater degree also
grew more new neurons than animals that were more passive."1
Recall patient H.M.? (I hope you remember him —if not, consider
what brain area might be functioning poorly for you to have
forgotten Activity 3-1!) The brain area his neurosurgeon damaged
in an attempt to cure his epilepsy was the hippocampus.The
neurosurgeon also removed his amygdala. The amygdala is the
"fear" center of the brain. It is involved in learning by giving input to this circuitry in cases where we are learning to avoid
something fearful (harm avoidance) or in the case of addiction-related learning, we are losing this harm avoidance.
1: Emergence of Individuality in Genetically Identical Mice", Julia Freund, Andreas M. Brandmaier, Lars Lewejohann, Imke
Kirste, Mareike Kritzler, Antonio Krüger, Norbert Sachser, Ulman Lindenberger, Gerd Kempermann, Science, doi:
http://www.sciencemag.org/lookup/doi/10.1126/science.1235294 (Links to an external site.)
Images:
Brain showing hippocampus: georg@ucsb.edu
Visual Pathway diagram Source: http://commons.wikimedia.org/wiki/File:Optic_nerve_pair_%26_two_brain_hemispheres.jpg (Links to an external site.)
Mouse photo: http://commons.wikimedia.org/wiki/File:Lab_mouse_mg_3294.jpg
Activity 3-6: Brain Pathways—Sensing Pathway
Lesson 3 Module View
In order to know about the world around us, our sensory organs (for instance, eyes and ears) are not enough.
Source: http://upload.wikimedia.org/wikipedia/commons/8/8a/10.1371_journal.pbio.0030137.g001-L-B.jpg
There are many blind people who have eyes that work just fine. But information from the eyes needs to arrive at and be
interpreted by the brain. Information flow in the sensory pathway depends upon sensory organ.
The following pathway corresponds to all senses with the exception of the nose. As before, -> implies a neuron or set of
neurons that are conveying information from one brain area to the next. For the sensory pathway, sensory organ ->
thalamus -> appropriate portion of the cerebral cortex. What is the appropriate portion of the cerebral cortex? Recall that
the thalamus’ job is to relay information depending upon origin. Visual information arrives in the cerebrum in an area just
above the cerebellum. Auditory information completes its journey in the Wernicke’s area.
Visit the University of Washington's Digital Anatomist's page on pathways
http://www9.biostr.washington.edu:80/cgibin/DA/PageMaster?atlas:Neuroanatomy+ffpathIndex/SUBJECTS/3D^Pathways+2 (Links to an external site.) and select
the somatosensory radiation. Somatosensory refers to our sense of touch. Clicking on regions of this diagram will give
you labels and playing the movie will rotate the image for you. Next, select the auditory radiation and then the optic
radiation to see how information that arrived at the same brain structure (thalamus) ended up in distinct areas of the brain
(auditory cortex and visual cortex respectively).
Perhaps the most striking example of brain organization comes from the somatosensory cortex. Our sense of touch is
perceived by a brain region located above the ears called the somatosensory cortex.
In the image on the left, a portion of the somatosensory cortex in human and sheep is color coded to show the portions of
the brain that are responsible for detecting touch by each of the five digits on the hand.
This color code corresponds to the colors of the digits which are transmitting sensory information.
Activity 3-8: Brain Pathways—Reticular Pathway
Lesson 3 Module View
The reticular pathway allows us to feel sleepy or wakeful. Reticular pathway information comes from our senses through
the brain stem and arrives at the thalamus (as with sensory information). But upon arrival, in addition to determining how
we perceive these incoming stimuli, the thalamus-to-cerebrum connection also determines how we respond to them. If
we’re sound asleep, some sounds will rouse us and others will not.
The reticular pathway serves the purpose of enabling sleep when we are in need of sleep. It helps us feel sleepy and
helps us keep our sleep from being interrupted if we have not satisfied our sleep quotient. The reticular system uses
electrical inputs from the thalamus and chemical inputs in the form of adenosine (a brain chemical whose concentration is
directly correlated with the need for sleep). If we have a lot of adenosine, not only will we feel sleepy, but inputs from the
thalamus will be easier to ignore.
Activity 3-9: Brain Pathways—Reward Pathway
Lesson 3 Module View
Our final pathway is the one we visited earlier in the video clip pertaining to brain scans. Recall that the methamphetamine
user showed changes in a brain region when using methamphetamine. These changes were present in a region of the
midbrain near to the limbic pathway.
Our brains have prepared us for survival both for ourselves and for our species. We therefore have a pleasure center that
is stimulated when we engage in activities that promote our survival, such as eating, and promote the survival of humans
generally, as in mating. This pleasure center can also be stimulated by activities that do not relate to survival, such as
gambling and video games. It is likely that these activities are addictive for some in part because they stimulate our
pleasure center, our reward pathway.
We understood that this region of the brain was involved in pleasure as far back as 1950. Watch the following
video. The audio on this video is a little challenging. Focus on the depressed patient who has been hooked up to an
electrode that allows her to stimulate her reward pathway. What are the signs (including descriptions she uses) that
indicate she is stimulating a pleasure center?
http://www.youtube.com/watch?v=de_b7k9kQp0&NR=1 (Links to an external site.)
Similarly, rats will go to great lengths to stimulate their reward pathway. By training a rat that the press of a lever will result
in stimulation of the reward pathway, it was observed that rat would press the lever to obsession. Some have been
observed to press the lever without bothering to stop for food or water. Move the needle that administers stimulation just a
bit to one side and the mouse will stop pressing the lever. In the previous video, how do we know the animal will go to
great lengths to stimulate the reward pathway?
The site of stimulation that caused people and rodents to self-stimulate repeatedly is called the nucleus accumbens (NA).
The reward pathway centers on the NA. Inputs regulate the release of a neurochemical called dopamine in the NA.
Release of dopamine here sends a message to the frontal cortex that you have engaged in something highly pleasurable
(and important for your survival). Drugs that cause an increase in dopamine release here also reward you with pleasure
and stimulate your desire to repeat that experience. These drugs may work directly on the NA, or on a brain region nearby
that regulates the NA. This area, the ventral tegmental area (VTA), communicates to the NA, which communicates to the
frontal cortex. But the frontal cortex also feeds back, reinforcing drug seeking behaviors.
In adolescents, the prefrontal cortex develops more slowly than the nucleus accumbens. Think of the frontal cortex as the
"brakes"—it helps to reign you in, to inform you about unwise choices. Think about the nucleus accumbens as the "gas
pedal"—it helps you go for it, to be heedless to inhibition. Now, what does that mean about adolescents' risk of addiction?