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Let’s recap some of the information we learned this week and touch upon some questions that arose in the discussions…
We explored the single cell, its structure, and its function. The dendrites bring information towards the cell body (or soma), and the axon carries it away to the terminal buttons.
The is an image of a neuron.
In order to achieve an action potential, the wave of excitation must be strong enough to carry over the axon hillock, at which point the cell fires at its full force. Cells either fire at full force or don’t fire at all (all or none principle). By the way, although not labeled on the figure above, the axon hillock is located just between the soma and the axon. Think of it as a communication gatekeeper between the soma and the axon, allowing certain messages to pass ***
The inside of the neuron is negatively charged compared to its outside environment (approx. -70mV). This is made true by the special distribution of negatively and positively charged ions. An abundance of organic anions within the cell makes the internal cell environment negatively charged compared to the external environment, where positively charged sodium ions predominate. The process of diffusion and electrostatic pressure play a key role here, and you should read up on these properties if you don’t already understand them (see image below).
Image of ion distribution
The contrast in electrical charge between the inside and outside cell environments enables the cell to work like a battery. In fact, without this charge, the cell is unable to communicate with other cells. If an imbalance in ions occurs, the cell may not be able to maintain its membrane potential. The cell may become unresponsive and stop firing, or it may become too sensitive and fire indiscriminately. Cells could even die. These problems can lead to functional disturbances in the individual including motor dysfunction, paralysis, seizures, hallucinations, etc.
Given the importance of maintaining the appropriate electrical charge, it should now make sense to you why cells expend so much energy even when they are at “rest.” The cell wall must ensure that ions don’t roam about freely. How does the cell membrane do this?
The membranes of neurons and glia cells are composed of a phospholipid bilayer, with hydrophobic (water-fearing) tails and hydrophilic (water-loving) heads. Look at the image above – the tails point inward (pink area).
The “water-fearing” (hydrophobic) tails repel the extracellular fluid, prohibiting the fluid from entering the cell. The resting cell membrane, therefore, is impermeable to most ions. An action potential is required to open special channels embedded in the cell membrane, allowing certain ions to pass ***
During an action potential, special channels open and allow sodium (Na+) ions to rush into the cell, causing the internal charge of the cell to become positive in relation to the outside of the cell. The charge, therefore, is reversed, but only for a millisecond. Potassium (K+) pumps open, allowing potassium to leave the cell, and the charge within the cell begins to become negative again. Sodium-potassium pumps now activate and restore the proper balance of the ions, and the cell repolarizes and returns to resting potential (see graph below).
Displaying graph of action potential.
At what point does an action potential occur?
Dr. Sapolsky explains the role of the axon hillock well. Incoming signals can be thought of as waves of energy, and the axon hillock as a wall. If the wave is small, it swells and slowly dies, but it doesn’t get over the wall, and an action potential is not triggered. If the wave is large, it gets over the hillock and triggers the action potential.
How large does the wave have to be to get past the axon hillock? It depends on the cell. What influences the axon hillock’s threshold? All sorts of things, like experience and learning. Cells become more or less responsive, depending on the situation.
Imagine you’re in a field and feeling very hungry. There are several fruits that are unfamiliar to you, so you taste of a few. You learn that the blue ones are delicious and sweet; the red ones are bitter and disgusting. The next time you’re in this field, which fruit are you likely to eat and which are you likely to avoid? What allowed this learning to take place? Experience, of course. But what did the experience do to your nervous system? You guessed it. It caused your nervous system to change and adapt to the new information. Cells in your visual system changed (you can recognize the fruit by sight); cells in your hippocampus changed (you can recall having eaten these fruits before); cells in your limbic system changed (you certainly had an emotional reaction to eating delicious vs. disgusting fruit, and this memory is now recorded and available); cells in your gustatory and olfactory cortexes changed, etc. These cells, therefore, now react differently when you see these fruits.
Without such changes, we’d be incapable of surviving. If you never learned to fear a saber-toothed tiger, then you’d be eaten alive and would be unable to pass ***
This week we move on to chapter 3, which reviews the structure of the nervous system and its various divisions. Given the visual nature of this information, your task this week is to create a Mind Map of the nervous system. A Mind Map can be useful in learning the location and functions of basic anatomical structures. For those of you who are more visual and/or artistic learners, this is your chance to shine.
The following videos explain how to go about creating a Mind Map along with some examples provided below:
How to Make the Perfect Mind Map and Study Effectively (Links to an external site.)
7 Steps to Mind Maps (Links to an external site.)
Body Systems (Links to an external site.)
Health (Links to an external site.)
Meningitis (Links to an external site.)
The Science of Breath (Links to an external site.)
As you plan your Mind Map, consider the content below. You don’t have to include all this information in your work, but you should include a comprehensive array of information that will help you accurately map the nervous system and associated functions.
Development of the human brain from prenatal to postnatal development. Include the 3 layers of meninges.
The divisions of the telencephalon. What primary functions are associated with these divisions? What does it mean when we say that a function is lateralized?
The two major structures of the diencephalon. What important functions do they subserve?
Describe the two major structures of the midbrain and the two major structures of the hindbrain. Explain what important functions they subserve.
Describe the peripheral nervous system. Compare the functions of the sympathetic and parasympathetic divisions of the autonomic nervous system.
NOTE: While you may use the internet for templates and images to help create your Mind Map, you are expected to do a substantial portion of the work by hand!
Please be thorough and present the information in an organized, concise fashion. Take a picture or scan your Mind Map; then upload it directly to Discussion Board. Explain your Mind Map to the class.
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There are also videos for this lecture.
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