1. [1 pt] Why does the crayfish have a neuron specialized to inhibit or "turn off" its stretch receptor neurons that sense the stretch level of its abdominal segment muscles?
See Syllabus p. 34.
"Crustaceans have stretch receptors extending between abdominal segments. These are mechanoreceptors responding to movement of the body segments. If stretched too much they shut down the movement in progress to protect the musculature. PROBLEM – the alarm response can easily make this happen, BUT it doesn’t, because the alarm response inhibit the mechanoreceptors." taken from the Syllabus 2000.
So the crayfish has inhibitory control of its stretch receptors to allow a massive muscular contraction that is necessary when an alarm response has been evoked (e.g., a tail flick). |
2. [2 pts] List the following steps in the order they occur during excitation-contraction coupling in vertebrate muscles.
- (1) End-plate potential travels outside of end-plate region
(2) Ca+2 comes out of the sarcoplasmic reticulum (aka lateral cisterna)
(3) Neuronal AP
(4) Depolarization of end-plate region
(5) Voltage-gated channels open
(6) AP travels down t-tubules into muscle fiber
(7) Ligand-gated channels open
(8) Voltage-gated channels on sarcoplasmic reticulum open
(9) muscle AP
(10) Actin & myosin slide along each other to produce a contraction
(11) ACh release
3. [2 pts] Arthropods use very few neurons to run their muscles. What organization makes this possible AND distinguishes them from the organization of vertebrate muscles and their innervating motor axons? List two reasons.
| NOTE: ONE NERVE IS NOT SYNONOMOUS WITH ONE NEURON. ONE NERVE = MANY NEURONS.
one motor axon/neuron innervates more than one anatomical muscle.
one motor axon/neuron innervates all the muscle fibers that make up an anatomical muscle.
Verts never have the same motor axon serving more than one anatomical muscle and usually have hundreds of motor axons just to serve one anatomical muscle, that is, there can be as many as one motor axon for each muscle fiber within an anatomical muscle. |
4. [2 pts] Contrast how a vertebrate achieves graded whole muscle contractions with how an arthropod achieves the same task.
Vertebrates achieve graded contractions by what's called motor-unit recruitment. A motor unit is the motor axon and all of the muscle fibers that it controls within a larger muscle. So each muscle can be composed of hundreds of motor units in Vertebrates. Since there are many motor axons that control one muscle in a vertebrate, to get the entire muslce to do a very forceful contraction, it is necessary to evoke all of the motor axons that control each muscle fiber within that muscle. A smaller contraction can be achieved simply by only evoking a few of the motor axons that control a few of the muscle fibers in one particular muscle. Notice it is the # of motor units involved in the overall muscle contraction that determine its strength NOT the size of the motor unit. (Size of the motor unit indicates how finely controlled that muscle is.)
Also, vertebrate muscle fibers contract in an all-or-none fashion by way of a muscle fiber AP. So at the level of each muscle fiber there can be gradation of contraction.
Arthropods usually have only one motor axon per all the muscle fibers within a muscle. This means that the entire muscle is one motor unit. Therefore, there cannot be any motor-unit recruitment in arthropods. The way graded contractions are achieved is instead by changing frequency of APs within the single innervating motor axon. High frequency APs = lots of neurotransmitter (glutamate) released onto the muscle fibers = high degree of contraction. Low freq of APs = tiny amounts of neurotransmitter released onto muscle fibers = small contractions.
Arthropod muscle fibers do not have the ability to conduct APs, therefore each muscle fiber CAN contract in a graded fashion and this is exactly what happens.
Arthropods can also control the strength of their muscle's contractions by the additional inhibitory and excitatory neurons that synapse onto the muscle fiber. |
5. [1 pt] What is the term to describe when similar organs or mechanisms evolve independently of each other in very distantly related organisms due to the same selection pressure?
How did Darwin use this as an explanation for the evolution of the various groups of electrical fish?
See SE p. 205-207
Because the same ability (to emit weak electric discharges) was found in a wide variety of freshwater fishes that were not related to each other, Darwin concluded that it must be evidence that the same selection pressure acted on more than one species of fish to produce the same adaptation, albeit by different evolutionary origins. The common element was the environment that these fish lived in: murky rivers where vision could not play a role in prey capture, navigation, and mate finding, all of which are crucial to the survival of an individual and a species.
NOTICE: Darwin DID hypothesize a convergent evolution mechanism, although he did not call it this. Also, Darwin was speculating on the origin of only the weakly electric freshwater fish (there are many different species of these types of fish), he was not speculating about the relatedness of weakly electric freshwater fish to the strongly electric fish. |
6. [2 pts] What is the difference between the purpose of the strong pulses that the strongly electric fish give off vs. the weak pulses that the weakly electric fish give off?
See SE p. 210-212
Strong electric fish only emit the electrical discharge when they detect the presence of a prey item or when they are being "harrassed" such that they need to defend themselves. It does not use its electroreceptive abilities to navigate.
Weak electric fish emit electrical discharges continuously regardless of sensory input. These fish use their discharges to navigate their environment, communicate with other electric fish (antagonism & mate finding), and in prey finding. |
7. [1 pt] Which of these sensory receptors are involved in passive electroreception? (See SE p. 225)
mechanoreceptors
marginal bodies
electrocytes
ampullary receptors
tuberous receptors
8. [2 pts] How do several small, modified muscle cells produce electrical discharges of up to 500 V in strongly electric fish?
See SE pp. 232-233
the alignment of the cells that produce these electrical discharges is critical. They are all aligned in parallel stacks such that each tiny discharge from each cell sums to produce one large discharge.
there is also quite a lot of nervous system control to ensure that all the electrocytes (discharge-producing cells) fire their discharge simultaneously (synchrony of discharge is critical to the summation). |
See Module 3.
An AP is not really necessary for synaptic transmission. All that matters is that somehow the pre-synaptic region of the neuron is depolarized. If it is sufficiently depolarized, voltage-gated Ca+2 channels that sit in the pre-synaptic membrane are stimulated to open. Ca+2 floods into the pre-synaptic cell down its electrochemical gradient. Influx of Ca+2 triggers the synaptic vesicles containing neurotransmitter to exocytose their contents into the synaptic cleft. If the neurotransmitter binds to receptors on the post-synaptic cell then synaptic transmission has occurred.
Notice how the influx of Ca+2 and the release of neurotransmitter is graded!! |
10. [1 pt] Which of the following characterize(s) inner hair cells (more than one may be correct)?
Function to dampen sensitivity of the ear
Tansduce sound waves
Possess stereocilia
Possess stretch-sensitive channels
Function to contract the tectorial membrane
11. [1 pt] Hair cells have no axons, but can still release neurotransmitter onto other neurons.
Not all neurons have axons, but still release neurotransmitter as a mode of communicating with other cells. So, does this cell have an AP? Is there any need for one?
True
False
12. [2 pts] Name three sensory systems that we’ve learned about in this course that permit "visualization" of the environment without eyes, and for each one, give an example of an organism that evolved the sensory ability.
| In pit vipers (e.g., rattlesnakes), they have two pit organs on either side of their head that are very sensitive to infrared radiation (heat given off by the warm body temp of mammals). This sensory system is so complex that the snake can actually form somewhat of an image in its brain of the scene only using the input from the IR sensitive pits, no vision.
Echolocation in bats. Using super-sonic frequencies emitted in pulses and listening for the returns, a bat can successfully navigate a complex terrain environment and catch small prey items without the use of its visual system at all.
Electroreception, particularly in the sharks and weak electric fish. Sharks just sense the electric fields that their prey give off to find their prey buried in the sand (they can't see the prey, but they can still find it). Weak electric fish create an electrical halo around their bodies such that they can detect nearby objects (objects distort the electrical halo), this allows them to navigate their murky environments where vision is of little use. |
13. [2 pts] A compound eye is composed of many or discrete individual "mini-eyes". Each one has it’s own or focusing element. One unit of the compound eye is composed of many cells. The cells that form the light-sensitive region of the eye or the rhabdom are called . There are typically 8-12 of these cells per unit. is the visual pigment in compound eyes. Compound eyes are found in which group of organisms (not a subgroup)? .
14. [3 pts] Limulus visual system is an excellent example of input processing or filtering that occurs before the signal reaches the central nervous system. Lateral inhibition in the Limulus eye is a classic example of this pre-CNS filtering. What is lateral inhibition and what is its purpose?
The purpose of lateral inhibition is to enhance edge contrast in an image. Nearly every organism with compound or complex eyes has this ability.
Lateral inhibition refers to the ability of signals to be transmitted laterally in the retina from one "photoreceptor cell" to another. The signals are inhibitory. So when light strikes a given ommatidia (or photoreceptor) this cell/unit of cells affects the signals sent to the brain that tell the brain that light is striking the retina, but these photoreceptors/ommatidia also send inhibitory signals to the neighboring photoreceptor cells/ommatidia. Inhibition is stronger on the nearest neighbors and falls of in strength in concentric circles away from the "light-excited" ommatidia/photoreceptor. "This results in enhanced contrast (increased difference in response) between neighboring units subjected to different intensities of light. Contrast enhancement is greatest for units immediately opposite each other across the bright-dim border, since the lateral effects diminish with distance." Taken from Eckert, 1989. |
15. [2 pts] How does the compound eye of Limulus differ from a compound eye in a true fly?
Limulus compound eyes are composed of hundreds of ommatidia, just like in the compound eye of the fly. The main difference is in the cellular structure within each ommatidia. In Limulus, each ommatidia has 12 retinula cells that surround one eccentric cell like the wedges of an orange. The eccentric cell has one very long dendrite that extends up into the cluster of retinula cells. This dendrite is depolarized by the depolarization of the retinula cells that occurs whenever light strikes the rhabdomeres because the eccentric cell is connected to each retinula cell by gap junctions. The axon of the eccentric cell forms the optic nerve, which transmits the signals from the retinula cells to the brain. The ommatidia of the fly compound eye does not have an eccentric cell and usually, there are only 7-8 retinula cells per ommatidia. The second cell in line to receive the output from the retinula cell is the optic cartridge.
Another difference between Limulus and fly eyes is the structure of the rhabdome. The light-sensitive portion of each retinula cell is called the rhabdomere. In Limulus, the rhabdomeres of each of the 12 retinula cells fuses into one structure. Because of this fusion all the retinula cells of an ommatidia look at the same part of the visual field. In flies, the rhabdomeres of each retinula cells are separate and are organized in a such a way that each retinula cell within a single ommatidia can view different parts of the visual field. (This is a much more complex organization). |