1. [2 pts] What is the value of studying the behavior of simple organisms? Explain at least 2 reasons.
(See Syllabus pp. 11-14, Young & Griffin articles).
Mostly because of evolution... All organisms are related to one another, some more closely than others. Because more complex organisms are based on the template of a simpler organism’s system, there will be commonalities between the systems and mechanisms.
The genetic code is highly conserved from bacteria to humans, therefore most of the principles that underlie neural mechanisms and behavior are going to be conserved in all organisms.
For example: the action potential is nearly identical in every organism found to possess one. The mechanism of synaptic transmission is the same in all organisms.
From Griffin, "…no uniquely human neuronal or synaptic mechanisms have been discovered."
The brains and behaviors of higher organisms are so complex that it is very difficult to sort out which mechanisms are involved precisely in which behaviors and vice versa. Simpler organisms provide simpler systems that make linking specific neurons are neural pathways to behaviors much easier. By studying the behaviors and underlying neural mechanisms of simpler organisms we can elucidate the building blocks of the human nervous system. By understanding as many connections between behavior and neural pathways as we can in simpler organisms we can construct the human nervous system in the same way that it evolved over millions of years.
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2. [1 pt] Cuvier came up with a theory of how bats navigate & catch insects without using vision. Even though this theory was not correct (he unbelievably thought they used the sense of touch to navigate), why was his theory still important scientifically?
| (p. 20-21 SE) Cuvier’s theory that bats navigated with their sense of touch was important scientifically because it was testable under the scientific method. The scientific method is based on falsification of hypotheses and theories. If a theory or hypothesis is not falsifiable, it is therefore not testable. Cuvier’s theory was put to the test when another experimentalist, Hahn, did an experiment in which he covered bats’ wings with Vaseline thereby making them touch insensitive. The bats could still navigate without any problems. This test proved that the touch theory was not correct—it falsified the touch theory. |
3. [2 pts]
(a)
was the scientist that did the preliminary field studies of moth behavior in response to bat-like supersonic sounds.
What two behaviors in these night-flying moths were discovered from these field studies (describe briefly)?
Tight turns and/or drop straight into bushes (when moth perceived that bat was very near).
Turn in the direction that is directly opposite from where it perceived the source of the bat’s cry and fly straight away.
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(b) What was the next step that this scientist took in elucidating the moth’s behavior? Briefly describe the results and the study performed.
(SE p. 18-30 & syll. p. 16-19)
He then did lab experiments with the moth’s ear. He found that there were primarily 2 cells in the ear that produced these two behaviors in response to bat’s cries.
One cell is more sensitive than the other. This cell mediates the behavior that is induced when the bat’s cry is perceived as being far away from the moth. The bat’s cry is fainter when the bat is further from the moth, so only the more sensitive ear cell fires and elicits the turn directly away from the source and fly straight.
The other ear cell is less sensitive and thus only fires when the bat’s cries are nearer and thus, louder. When the bat is near, both ear cells fire. This triggers the tight turns and/or drop into the brush below behavior that the moth does when it thinks the bat is very close.
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4. [1 pt] Here are two pairs of recordings from 4 different cells: one synapse occurs between each pair of cells. There is one microelectrode that records from the presynaptic cell, and another that records from the postsynaptic cell. One pair of recordings comes from a chemical synapse and the other from an electrical synapse. Which is which AND state two distinguishing features that allowed you to make your determination.
Synapse type 1 = chemical synapse
Synapse type 2 = electrical synapse
Electrical synapses have no or very little delay between the signal that occurs in the presynaptic cell and the one that occurs in the postsynaptic cell (because the cells are directly connected to one another).
Electrical synapses are said to be stereotyped, the signal that occurs in the presyn. cell will produce a signal in the postsyn. cell that looks exactly the same (i.e., the signal reaches the same amplitude and takes the same amount of time). Also, electrical synapses are always excitatory. There is no possibility that an excitatory signal in the presyn. cell can cause an inhibitory signal in the postsyn. cell as can occur in a chemical synapse.
Chemical synapses are much slower. There is a delay of 1-5 ms between the appearance of a signal in the presyn. cell and the appearance of a signal in the postsyn. cell. This is because the synapse is actually a separation between the two cells. Neurotransmitters are used to bridge this gap between the two cells. The triggering of the release of neurotransmitters from the presyn. cell and the travelling of the neurotransmitters across the gap takes time, this causes the delay.
Also, the signal that occurs in the presyn. cell almost never causes an identical "looking" signal in the postsyn. cell. One AP in the presyn. cell usually only results in the release of enough neurotransmitter to cause a tiny epsp or ipsp in the postsyn cell.
Depending on the type of neurotransmitter released by the presyn. cell, the postsyn. cell may be inhibited or hyperpolarized by the arrival of an AP in the presyn. cell.
NOTE: neurons only release one type of neurotransmitter for their entire "life". So, if you discover an inhibitory neuron that releases GABA, every time its axon terminal is stimulated by an AP it will release GABA onto whatever neurons are in the immediate vicinity. The postsyn. neurons that have GABA receptors will bind GABA and thus be inhibited. A neuron cannot change which neurotransmitter it’s going to release or release a mixture of neurotransmitters. Every neuron is specialized to release one specific type of neurotransmitter.
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5. [1 pt] (a) List two advantages to having electrical synapses in neural circuits?
Electrical synapses are very fast, therefore they are useful in neural pathways that require the fastest transmission of signals (such as those involved in startle reactions).
They are stereotyped: an AP in the presyn. will always trigger an AP in the postsyn. cell (no threshold barriers). This provides for synchronization of reactions. Electrical synapses are often observed in regions where there needs to be a high degree of synchrony in the reaction, such as muscle contractions (in some muscles, the muscle fibers will all be interconnected by electrical synapses to ensure simultaneous contraction of all the muscle fibers within one muscle).
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(b) List two advantages to having chemical synapses in neural circuits?
Chemical synapses create a threshold for signaling. The postsyn. cell will not fire an AP unless several presyn. APs cause several epsps in the postsyn cell such that they sum to surpass the threshold of the postsyn. cell. The creation of a threshold allows a neural pathway to "filter" incoming signals. Only signals of a certain strength and/or duration will cause the transmission of the impulse across a given synapse.
Because a chemical synapse can be either excitatory or inhibitory, the nervous system can create more different types of neural pathways using chemical synapses. Inhibition does not exist in neural pathways that have only electrical synapses. A nervous system can therefore better control neural pathways by using inhibitory input.
Chemical synapses are plastic. They change relative to the frequency with which they are used. If a chem. synapse is used in high repetition for a given duration, that synapse will physically change so that it is easier to stimulate with each arriving new stimulus. These changes create neural freeways for certain types of information. These types of changes are thought to underlie learning and memory.
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6. [1 pt] Does every neuron use an AP to transmit a signal from its input region to its output region? Explain why or why not.
(See p.32 in NC). NO.
An AP is NOT necessary for synaptic transmission. The only requirement for synaptic transmission to occur is that the axon terminal or more generically, the synaptic region of a neuron, be depolarized sufficiently to open voltage-gated Ca+2 channels. Ca+2 comes into the presyn. neuron and triggers the release of neurotransmitters.
APs are just an evolutionary answer to the problem of the degradation of passive signals that occurs over long distances. If a neuron has no long processes, as is the case in many sensory receptor neurons (photoreceptors, hair cells, etc.), then there is no need for an AP. The epsp that is triggered by the arrival of a stimulus is sufficient to trigger neurotransmitter release onto the postsyn. cell and thereby the signal has been communicated across a synapse without the involvement of an AP.
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7. [1 pt] (a) What causes the depolarization phase of the AP, explain briefly.
(b) What causes the repolarization phase of the AP, explain briefly.
| Just as in the depolarization phase, there is a channel and ion flux associated with the repol. phase of the AP. Slow voltage-gated K+ channels open and allow K+ to rush out of the cell down its electrochemical gradient. K+ is a positive ion that is leaving the cell, this causes the lessening of the positive charge inside the neuron. The neuron membrane potential begins to fall or become more negative. |
8. [2 pts] The passive properties of the membrane (aka cable properties) are what determine how far electrical signals can travel inside the neuron. The mathematical term to describe how far a signal can travel before significant degradation occurs is the space constant.
There are 3 properties of the membrane that determine this. (1) membrane resistance, which is how easily the charge or current (i.e. the signal) can leave the neuron across the membrane, which would result in signal degradation. This first property was most affected by the evolution of myelin.
(2) intracellular, internal, or cytoplasmic resistance, which is how easily the charge or current can flow within the neuron or axon without being impeded by intracellular structures. This property is most directly affected by the diameter of the neuron or axon.
(3), capacitance, which is the ability of the membrane to store or bind up some of the charge or current.
9. [1 pt] Why is it that dendrites are typically described as being chemically excitable, but not electrically excitable, but the axon is electrically excitable?
The type of channels that are present in a region determine the type of excitation that triggers this area to initiate a signal. Dendrites typically have only ligand-gated channels or channels that open when a certain chemical has bound to the channel. Therefore, regions of a cell that have only these types of channels are said to be chemically excitable. Whenever a certain chemical or neurotransmitter is released, it binds to these special channels and ions flow through them, thus initiating a signal.
Areas of a cell that are electrically excitable are so because they contain voltage or electrically-gated channels. These channels open in response to a membrane potential change. The axon has populations of these types of channels. Axons are typically described as being electrically excitable.
Dendrites do not have voltage-gated channels, so they cannot be described as being electrically excitable.
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10. [1 pt]
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Which presynaptic neuron (choose between #1, 2 or 3) has the largest effect on changing the Vm (membrane potential) of the postsynaptic cell? (Assume all inputs are excitatory.)
Explain why.
| This question was not graded, everyone was just given a freebie 1 point because I realized upon grading that it was a poorly worded question. Essentially, I was trying to get you to realize that presyn. neurons that synapse onto a neuron closer to the axon hillock (where all inputs are summed and if threshold is reached, an AP is initiated), have more of an effect on the signal generation than neurons that synapse further from the axon hillock. So in this case #1 is closest to the axon hillock. Why does it have a greater effect even if the signal from neurons 1, 2 and 3 are all the exact same strength? Because it has less distance to travel and therefore it will degrade less and is more likely to help the axon hillock region reach threshold and trigger an AP. |
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11. [1 pt] In vertebrates, (a) myelin functions to insulate an axon to maximize the distance currents (movement of ions) can flow inside the axon. The small bare parts of the axon in between these regions are called (b). It is in these areas (b) the Nodes of Ranvier that currents flow across the axon membrane to regenerate the action potential. How is this region (b) of the axon membrane specialized to this function?
| It is only in this region of the axon that the voltage-gated channels exist. Current = the movement of charged particles, such as ions. So in biology, typically currents are carried by the flow of ions across the membrane. If currents flow at the Nodes, then that must mean there are channels that allow the ions the cross the membrane at these regions. This is the case. The myelin sheaths that are in between the Nodes do not have channels, so there is no way that ions can flow across the membrane except at the Nodes. At the Nodes, the large depolarization from the AP at the previous Node opens the voltage-gated Na+ channels at the current Node. Na+ flows into the cell down its electrochemical gradient and thus, regenerates the AP. |
12. [1 pt] Using the lobster stretch receptor, Kuffler was able to demonstrate the mechanism of the inhibitory neurotransmitter, GABA. This neurotransmitter causes Cl- channels to open on the postsynaptic cell, which results in Cl- rushing into the cell. The membrane potential of the postsynaptic cell is then said to be hyperpolarized. Whenever a neuron is hyperpolarized, it is much more difficult to stimulate that neuron to reach threshold and trigger an AP.
13. [2 pts] There were two examples described in lecture of very primitive organisms that demonstrate that whereas they may not have a true nervous system, they have many of the components of nervous systems.
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Organism
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Sensory Input
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What is Effector & How is it modified by the sensory input?
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What is the associated change in membrane potential?
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Glass Sponge (Hexactinellid)
Syll p. 53
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Noxious chemicals in water (polluted water) OR mechanical stimulus |
The effector is the choanocyte or special cell that creates water currents in the sponge. The choanocytes stop moving their flagella, which ceases the inflow of water in the sponge when one of the two adequate stimuli are received by sensory cells. |
There is a large depolarization or AP that is conducted by voltage-gated channels within the trabecular syncitium. |
Protist: Paramecium
Syll. p. 31-32
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Mechanical Stimulus to posterior or anterior of cell. |
Effectors are the cilia that provide the locomotory ability of the cell. The cilia’s beating direction and frequency are changed by the mechanical stimulus. (The cell backs away if the stimulus is on the anterior region, the cell speeds up its forward movement if stim. is on posterior). |
There are two different changes in the Vm depending on where the stim. occurred. If the stim. was anterior, the Vm is depol (like an AP). If the post. is stim, the Vm is hyperpolarized. |
14. [1 pt] Polychaetes are a group of segmented worms within the phylum Annelida , that have a great variety of nervous system complexity. We can understand this difference in nervous system complexity by examining this group of worms’ behavior and anatomy. Here are two polychaetes that exhibit this range of complexity within this group.
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Myxicola sp. A tube-dwelling polychaete. Never comes out of its tube, only fan-like feeding apparatus sticks out.
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Nereis sp. An errant polychaete. Actively crawls and swims around searching for food--a predator.
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Which worm do you think has a more complex nervous system and WHY?
See Mod. 1
Observations about the polychaetes:
Both worms have bilateral symmetry.
Nereis: an active predator, exhibits unidirectional locomotion. Notice the anterior region: sensory structures are concentrated here (= cephalization), you would expect the nervous system to also be concentrated in the anterior region as well. Because it is an active predator, it needs advanced locomotion control (muscle control) and tracking abilities.
Myxicola: a sessile tube-dwelling polychaete. Does not exhibit unidirectional locomotion. Doesn’t really exhibit any sort of active locomotion. What about it’s anterior region, notice much in the way of sensory structures--not really, just the feeding appendage. It doesn’t really need to have any well-developed sensory structures because it doesn’t hunt for its food. Therefore it does not display the same level of cephalization that Nereis does.
Conclusion: Nereis has a more advanced, well-developed nervous system than Myxicola.
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See Syllabus p. 55-58 and supplement to Mod 2: the Cnidarian Nervous System.
Below is all the ways that Dr. Case described the Cnidarian nervous systems as being unique.
Neuroid or Epithelial conduction - use non-nerve cells, like epithelial cells (~skin cells) to conduct APs so that it can quickly conduct an AP from one site to another. These epithelial cells are connected to another by gap junctions or electrical synapses for extremely fast propagation of signals.
3 separate conduction pathways: one is fast, the other two are slower. This allows the organism to have the info that is most important for survival conducted along the fastest pathway while the not so important stuff is conducted along the slower pathways.
Pseudo-Giant Axons: Electrically couple several small diameter axons that are carrying info from the same location to the same location so that they effectively function as one giant axon. Note: Jellyfish have no true giant axons.
General Note: if a membrane is electrically excitable it does not mean it has electrical synapses, it just means that the membrane must be able to conduct a potential or current, which usually means that the membrane has voltage-gated channels in it.
Electrical synapses are also called gap junctions. The jellyfish have two types of electrical synapses (see drawing below); Both help the jellyfish increase the propagation speed of nerve impulses. Notice how the electrical synapses in the top drawing lie in the direction of the AP, whereas the electrical synapses in the 2nd drawing lie perpendicular to the direction of the AP.
En-Passant synapses - whenever a neuron crosses another neuron, it communicates by passing the electrochemical signal onto that other neuron. This is different from other organisms where you only have communication occurring at the axon terminals (the end of axons).
Bipolar neurons/Bi-directional Synapses - the neurons in the jellyfish have the ability to propagate electrochemical signals in either direction away from the site of stimulation. This is because of the distribution of channels in the membranes and the fact that they can release neurotransmitters at sites other than the axon terminal.
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Sensory
Ganglions
Interneruons
Motor
Afferent
Efferent
transduction whereby stimuli is taken in from a different environment and made "readable" to the nervous system.
mechano-receptor that specifically are specialized to detect gravity. Organisms with statocysts generally use them to orient their body relative to Earth's gravitational force.
19. [1 pt] The following recordings from 2 different types of sensory neurons show the response of those sensory neurons’ axons to the appropriate suprathreshold stimulus of a certain duration and intensity.
(See p. 80-81 in NC)
