Cecil College Neurons in Vertebrates Discussion Minimum of 375 words, max about 450. 375 words is about a full page (without your name on top and whatever

Cecil College Neurons in Vertebrates Discussion Minimum of 375 words, max about 450. 375 words is about a full page (without your name on top and whatever else you write on top) 1.5-spacedNeuroscienceThese should be submitted through safe assign, which is NOT set up yet, but will be. Safe assign is through blackboard and what it does is it checks for plagiarism and close paraphrasing between anything anywhere and your own write-up. If you have copy/paste issues and close paraphrases, you will not qualify for extra credit. Please, do your own work- it is merely a page to a page and a half (a walk in the park!!) Removing a single neuron in a vertebrate brain forever
abolishes an essential behavior
Alexander Heckera, Wolfram Schulzea?, Jakob Ostera, David O. Richtera, and Stefan Schustera,1
a
Department of Animal Physiology, University of Bayreuth, 95440 Bayreuth, Germany
Edited by L. B. Vosshall, The Rockefeller University, New York, NY, and approved January 3, 2020 (received for review October 23, 2019)
The giant Mauthner (M) cell is the largest neuron known in the
vertebrate brain. It has enabled major breakthroughs in neuroscience but its ultimate function remains surprisingly unclear: An
actual survival value of M cell-mediated escapes has never been
supported experimentally and ablating the cell repeatedly failed
to eliminate all rapid escapes, suggesting that escapes can equally
well be driven by smaller neurons. Here we applied techniques to
simultaneously measure escape performance and the state of the
giant M axon over an extended period following ablation of its
soma. We discovered that the axon survives remarkably long and
remains still fully capable of driving rapid escape behavior. By
unilaterally removing one of the two M axons and comparing
escapes in the same individual that could or could not recruit an M
axon, we show that the giant M axon is essential for rapid escapes
and that its loss means that rapid escapes are also lost forever.
This allowed us to directly test the survival value of the M cellmediated escapes and to show that the absence of this giant
neuron directly affects survival in encounters with a natural
predator. These findings not only offer a surprising solution to
an old puzzle but demonstrate that even complex brains can trust
vital functions to individual neurons. Our findings suggest that
mechanisms must have evolved in parallel with the unique significance of these neurons to keep their axons alive and connected.
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ablation phenotype startle response
predator–prey axon initial segment
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T
he giant neurons found in the escape circuitry (1–5) and sensory
centers (1, 6, 7) of many animals have always been instrumental
in major breakthroughs in neuroscience—from the classic experiments in the squid axon to the discovery of axonal protein
synthesis in the Mauthner (M) cell of teleost fish (8, 9). Despite
their outstanding role in the study of proximate mechanisms, the
ultimate reason for having and maintaining such gigantic neurons
has received far less interest and poses major puzzles that have
never been addressed. Specifically, in the M cell of teleost fish,
experiments over the past decades have repeatedly shown that
rapid escapes are still possible after removal of the giant M cell (8–
20), suggesting that these starts could equally well be driven by
smaller neurons, such as the smaller serial homologs MiD2cm and
MiD3cm (8, 9, 12–14, 16, 20–22). Even the role of the rapid escapes for survival is not as clear as it seemed: The few studies
available fail to detect a survival function of rapid escapes (23, 24),
a survival value has been questioned on theoretical grounds (11–
13, 25), and attempts to analyze the survival value of fast, shortlatency responses show that responding quickly can even be a
disadvantage: Survival depends, among many other aspects, on
speed and maneuverability of the predator and its prey as well as
the mechanism used for prey capture (26–28).
So, when the role of the M cell in escape behavior can be taken
over by smaller neurons and when M cell-driven rapid escapes are
not of clear survival function, what could then be the reasons for
having the giant M cell? Is it a “leftover” that is tied to the basic
plan of a fish? The M cell forms remarkably early, in the late
gastrula (29), and its major function could be to guide neuronal
growth. However, even then it would not be clear why such
guidance could not be achieved by smaller versions of the M cells.
3254–3260 | PNAS | February 11, 2020 | vol. 117 | no. 6
Here we present a series of experiments that were specifically
aimed at solving the puzzle of what is the ultimate role of the M
cell. Using larval and adult zebrafish, we discovered that ablation
experiments require the utmost care to ensure that not only the
soma but also the axon has degenerated. If one waits until the
axon is lost, then the ability to escape rapidly is also lost completely. Furthermore, experiments with an ecologically relevant
voracious predator of zebrafish larvae in the wild show that this
loss translates directly into a loss of fitness. Moreover, loss of the
cell could never be compensated throughout the lifetime of
the fish. Our findings demonstrate a clear survival function of the
giant M cell. They thus solve a major evolutionary puzzle and
end the controversy of whether M cells are truly needed to initiate the fastest escapes. Furthermore, we show that an individual giant neuron in the vertebrate CNS can play a crucial role for
survival—at the full risk that losing that neuron does affect
survival.
Results
Simultaneously Monitoring Behavior and Decay of the Cell. Most
earlier ablation work controlled for the absence of the M cell
soma but not for the absence of the axon (10–18, 20). Most
importantly, it was previously unknown if and how long the axon
would survive after the soma was gone. To examine these important issues, we used two-photon laser irradiation in transgenic
zebrafish larvae. This allowed us to specifically irradiate the M
cells and to induce Wallerian-like degeneration (30, 31) in either
one or both M axons of a larva while demonstrably leaving associated structures intact (SI Appendix, Fig. S1 and Movies S1 and
Significance
The Mauthner cell is the largest neuron known in the vertebrate brain and, in fish, mediates rapid escape behavior. Ablating this neuron has repeatedly failed to eliminate rapid
escapes, a survival role of these escapes has not been supported experimentally, and it is unknown which advantage the
enormous size and complexity of the cell conveys. By taking
care to ensure ablations remove not only the soma but also the
giant axon, we find that rapid escapes are lost forever and that
this loss directly affects survival in predator–prey assays. The
Mauthner cell thus is an example in which a survival-critical
function depends on an individual neuron whose axon appears to have unusual capacities to remain functional after
severe injury.
Author contributions: A.H. and S.S. designed research; A.H., W.S., and J.O. performed
research; A.H., W.S., and D.O.R. contributed new reagents/analytic tools; A.H., W.S.,
and S.S. analyzed data; and A.H. and S.S. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This open access article is distributed under Creative Commons Attribution-NonCommercialNoDerivatives License 4.0 (CC BY-NC-ND).
1
To whom correspondence may be addressed. Email: stefan.schuster@uni-bayreuth.de.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/
doi:10.1073/pnas.1918578117/-/DCSupplemental.
First published January 30, 2020.
www.pnas.org/cgi/doi/10.1073/pnas.1918578117
A
intact
R
L
R
ipsi
contra
L
ablated
M cell
B
***
Latency (ms)
100
n.s.
***
recruit the remaining intact M axon, these were not statistically
different—both in response time (latency) and release probability—from escapes made by completely untreated siblings (latency: P = 1 for intact vs. ipsilateral; Fig. 1B; one-way ANOVA;
n = 862 and 150 escapes from n = 18 and 5 larvae; response
probability: P = 0.2 for intact vs. ipsilateral; Fig. 1C; one-way
ANOVA; n = 1,572 and 440 stimulations from n = 18 and 5
larvae). This important control shows directly that the effect was
not due to unspecific side effects of the ablation or of the general
treatment. Our findings thus suggest that it may have been the
absence of the particular individual axon that caused the massive
drop in performance.
An Intact Axon Initial Segment Is Necessary for Rapid Escape Behavior.
10
We next discovered that two specific aspects of escape behavior,
its threshold and its performance characteristics, were mediated by
two different compartments of the M cell (Fig. 2A). When only the
50
30
10
ipsi
n.s.
100
A
intact
+ AIS
– AIS
– axon
*
***
B
n.s.
100
20
E
n.s.
10
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S2). Furthermore, we developed techniques that allowed us to
monitor—in the same larva—both the state of its degenerating M
axon and the larva’s escape performance over the long time it
took until the complete decay of the axon (SI Appendix, Fig. S1
and Movie S1). To probe escape performance at various degeneration stages of the axon, we selected a stimulus that demonstrably also activated the homologous neurons that are
thought to compensate for the loss of the M cell (SI Appendix,
Fig. S2) (8, 9, 14, 16, 20–22). Surprisingly, this approach revealed
that a unilateral complete loss of one of the axons of the two M
neurons specifically abolishes those rapid escapes (contralateral
to where the soma of the missing M neuron was) that would have
recruited the missing axon (9, 14) (Fig. 1A). Exploiting the
symmetry of the system for an intrinsic control, we found that
when the very same individual larva produced escapes that could
Hecker et al.
D
**
0
n.s.
*
10
30
Latency (ms)
50
1.0
15000
CDF
Fig. 1. The giant axon of the M neuron is essential for escape performance.
(A) Outline of the idea to use unilateral ablations: An action potential in one
of the two M cells elicits powerful contraction of the contralateral body side,
causing it to bend (Left). After unilateral ablation of the soma and axon (red),
only bending commanded by the ablated contralateral cell body should be
affected, but bending to the other side should not. Unspecific side effects of
ablation could thus be tested by comparing escapes made toward that side in
the unilaterally ablated larvae with escapes made by untreated siblings L, left;
R, right. (B and C) Escape latency (B) and release probability (C) in untreated
control larvae (n = 862 escapes from 18 larvae) and in ablated larvae for escapes that either could use the remaining cell (ipsi; n = 138 escapes from 5
larvae) or not (contra; n = 43 escapes from 5 larvae). Significance is as indicated
(one-way ANOVA; latency: ***P 0.12, logrank test). (E and F) Analysis of the survivors at the end of the experiment (i.e., after 7 h) using their fluorescence signals confirms higher capture rates of M
neuron-ablated fish (E; **P = 0.008, Mann–Whitney U test) but no higher capture rate of sham-ablated fish (F; P = 0.64, Mann–Whitney U test). Boxplots show
capture rates as determined in the individual groups (medians are indicated by thick black horizontal lines).
3256 | www.pnas.org/cgi/doi/10.1073/pnas.1918578117
Hecker et al.
Once the Cell Is Lost, a Behavioral Function Is Also Lost Forever. We
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next examined whether the drastic consequences of losing an M
axon early during ontogeny might be compensated during maturation of the larval nervous system or later during the months of life as
an adult fish. To examine this possibility, we established procedures
that allowed us to rear unilaterally ablated larvae together with their
untreated siblings (Fig. 4A). This way, we could reexamine the
early effects that the ablations had in the larvae and compare
them with the effects found 5 mo later in the adults. Remarkably,
the absence of a single M cell still caused the specific effects on
response probability and on latency as it did—about half a year
before—in the larva. Starts that could not recruit an M axon
were far less likely than were starts shown by the same fish that
could recruit the remaining M axon (Fig. 4B; Student’s t test, P