A neuronal network generating flexible locomotor behaviour in a simple vertebrate: studies on function and embryonic self-assembly
Lead Research Organisation:
University of Bristol
Department Name: Biological Sciences
Abstract
How do nervous systems allow animals to behave? The challenge here is immediately clear from the vast numbers of neurons (over 2 billion) in a human brain. Less obvious is the minute scale of nervous systems construction with many neurons only 0.01 mm in diameter. Problems of size and complexity have led to the study of simpler animals like snails and squid which have complex behaviour but many fewer, often larger, neurons. A further remarkable feature of nervous systems is that they must self-assemble rapidly during embryonic development to allow early responses that aid survival. How do they do this? Recent genetic work on development has emphasised fundamental features, common to animals as diverse as nematode worms, fruit flies and vertebrates like us. Detailed research on embryos has shown that vertebrate nervous systems share a common plan particularly in core parts like the spinal cord. We can therefore investigate how nervous systems develop and function in the simplest vertebrates. We study just-hatched, 2 day old frog tadpoles. While only 5 mm long and with less than 2000 neurons, they will swim when touched, struggle when grasped by a predator, and stop and attach when they bump into things: behaviour that aids survival. Exploiting the simplicity of the early tadpole nervous system and new methods that we devised, we have recorded activity from most types of neuron controlling movement and now have a uniquely detailed picture of the neuronal circuits for swimming and struggling. In collaboration with computer scientists and mathematicians, we built simplified models of these circuits and uncovered key principals of operation. This theoretical work emphasised commonality in neuronal circuits controlling movement, from snails to mammals, but revealed gaps in our knowledge. As well as asking how early neuronal circuits work we also want to know how they develop. We found that connections between tadpole neurons are not very specific. Broadly, their nerve fibres simply contact the neurons they encounter as they grow. Since neurons responding to sensory stimuli lie at the top and neurons controlling muscles lie at the bottom, those growing near the top will connect to different neurons to those growing near the bottom. Our mathematical models showed that very simple rules could direct nerve fibre growth to form neuronal circuits able to generate swimming activity when stimulated. In this study we will ask whether the ordered structure of the early tadpole nervous system allows functional neuronal circuits to self-assemble in response to 3 chemical gradients known to control growth of nerve fibres along and around the nervous system. To answer this question we need many more electrical recordings to establish exactly how swimming and struggling are initiated by different skin stimuli. We also need more detailed information on morphology for each neuron type. Our study should reveal how nervous systems 'decide' to initiate movement. Using the morphological information we will build a mathematical model of neuron growth to generate the synaptic connections that different types of neuron make with each other to self-assemble neuronal circuits. Using the physiological information we will build models where the different neurons are connected into functional circuits which we can stimulate to find how they generate the neuronal activity that produces swimming and struggling. Our ultimate aim is to see if simple growth rules can allow the self-assembly of neuronal networks which can 'decide' when and how to respond to sensory stimuli, behaving like a tadpole. By making a 'virtual tadpole' whose movements are controlled by our networks we can actually watch them producing behaviour. If successful, our study will lay a foundation for understanding the way more mature, complex nervous systems control movements and how they develop.
Technical Summary
Functional neuronal networks can form rapidly as vertebrates first develop. Molecular biology suggests that hindbrain and spinal cord networks are built on a common plan from organised rows of precursor cells. Detailed studies also show diverse mechanisms that control axon growth and ensure that correct synaptic connections are made. How do these early circuits work and can they form using simple growth rules? In the hatchling Xenopus tadpole our recent work has defined in detail the anatomy, properties and functions of the neurons of the locomotor network in the hindbrain and spinal cord that lets animals swim when touched or struggle when grasped. We will combine our detailed knowledge of circuit structure and function with computer modelling of neuron growth to build a self-assembling locomotor network model, finally visualised as a 'virtual' tadpole, that responds flexibly to stimuli like the real animal. Uniquely in vertebrates, we can critically evaluate our model's success as we know the final motor output and the activity patterns of most network neurons. We will first resolve questions about neuron anatomy and initiation pathway physiology, using modelling to analyse experimental results and generate further questions. We will then test the hypothesis that simple molecular gradients operating during development can direct self-assembly of a minimal functioning neuronal circuit that generates alternating swimming to brief stimuli. Once this proof of principle is established for swimming, we will extend experiments and modelling to test whether (1) continuous stimulation can reconfigure the locomotor network to produce struggling, and (2) longitudinal gradients in synaptic drive can organise the longitudinal spread of activity in swimming and struggling. Our results will show how sensory stimuli initiate distinct locomotor rhythms and provide a platform on which to test hypotheses about how other early vertebrate neuronal networks develop and function.
Publications
Berkowitz A
(2010)
Roles for multifunctional and specialized spinal interneurons during motor pattern generation in tadpoles, zebrafish larvae, and turtles.
in Frontiers in behavioral neuroscience
Borisyuk R
(2011)
Modeling the connectome of a simple spinal cord.
in Frontiers in neuroinformatics
Buhl E
(2015)
Sensory initiation of a co-ordinated motor response: synaptic excitation underlying simple decision-making.
in The Journal of physiology
Buhl E
(2012)
The role of a trigeminal sensory nucleus in the initiation of locomotion.
in The Journal of physiology
Conte D
(2021)
A simple method defines 3D morphology and axon projections of filled neurons in a small CNS volume: Steps toward understanding functional network circuitry.
in Journal of neuroscience methods
Hull MJ
(2016)
Modelling Feedback Excitation, Pacemaker Properties and Sensory Switching of Electrically Coupled Brainstem Neurons Controlling Rhythmic Activity.
in PLoS computational biology
Li WC
(2010)
Specific brainstem neurons switch each other into pacemaker mode to drive movement by activating NMDA receptors.
in The Journal of neuroscience : the official journal of the Society for Neuroscience
Roberts A
(2014)
Can simple rules control development of a pioneer vertebrate neuronal network generating behavior?
in The Journal of neuroscience : the official journal of the Society for Neuroscience
Roberts A
(2013)
Encyclopedia of Computational Neuroscience
Roberts A
(2010)
How neurons generate behavior in a hatchling amphibian tadpole: an outline.
in Frontiers in behavioral neuroscience
Roberts A
(2012)
A functional scaffold of CNS neurons for the vertebrates: the developing Xenopus laevis spinal cord.
in Developmental neurobiology
Winlove CI
(2012)
The firing patterns of spinal neurons: in situ patch-clamp recordings reveal a key role for potassium currents.
in The European journal of neuroscience
Winlove CI
(2011)
Pharmacology of currents underlying the different firing patterns of spinal sensory neurons and interneurons identified in vivo using multivariate analysis.
in Journal of neurophysiology
Description | Through our research using a simple, vertebrate, model organism (hatchling frog tadpole), we have provided the first neuron-by-neuron identification of a brain pathway by which trigeminal sensory information initiates motor activity. This included the identification of a new sensory nucleus in the hindbrain. We developed a new measuring microscope system for reconstruction of individual neurons. This involved collaboration with Scientifica to use one of their Patchstar micromanipulators to move a microscope slide in three dimensions with <0.1 um precision while observing dye filled neurons. In conjunction with software purchased from Scientifica, and additional processing code written by our Plymouth collaborators, this allowed us to define the three dimensional coordinates of nervous tissue and the neurons lying within it. Problems with suitable landmarks on the biological material have, so far, limited the exploitation of this technique and prevented publication of the methodology. However, the new system has contributed significantly to providing the detailed anatomical data on which our computer modelling of neuron growth and network formation is based. We set up an effective collaboration with the School of Computing and Mathematics at University of Plymouth (Prof Roman Borisyuk). This collaboration has played a key role in the development of computer models of neuronal network growth and activity. The strength and future potential of this collaboration lies in the combination of computer modelling and detailed biological experiments. As a result of the collaboration with Plymouth University, we developed a computer model for axon growth in the spinal cord and validated it using detailed biological measurements. This model has then been used to generate networks of spinal cord neurons with biologically realistic connectivity. |
Exploitation Route | In terms of academic routes to impact, our new methodologies and findings will inform other in the field and also provide a basis for our own future research into the neuronal basis for initiation and operation of motor activity, particularly locomotion. In terms of non-academic routes, the information will be used though public engagement and our web site to inform the public about this area of research. |
Sectors | Education |
URL | http://www.bristol.ac.uk/biology/research/behaviour/xenopus/circuit.html |
Description | The findings have been used to inform the wider public about elements of neuroscience through public talks (though Bristol Neuroscience) |
First Year Of Impact | 2013 |
Sector | Education,Other |
Impact Types | Cultural,Societal |
Description | Plymouth Computing and Mathematics (Borisyuk) |
Organisation | University of Plymouth |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | Supply of biological data: electrophysiological and neuroanatomical measurements |
Collaborator Contribution | Computer modelling and data analysis |
Impact | Software, published scientific papers and material for talks to academic and non-academic audiences |
Start Year | 2009 |
Title | Measuring microscope |
Description | Microscope with its stage controlled by a modified, computer-driven manipulator plus additional software to allow three dimensional reconstruction of dye-filled neurons |
Type Of Technology | New/Improved Technique/Technology |
Year Produced | 2011 |
Impact | Initial interest but too soon to judge impact |
Title | SC2D axon growth model |
Description | Matlab software for generating sets of axon trajectories of specified type, using parameter values based on biological data. |
Type Of Technology | Software |
Year Produced | 2012 |
Impact | Initial interest; too soon to judge impact |
Description | 'Bristol Neuroscience' talk (AR) 10 year celebration |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | Yes |
Geographic Reach | Local |
Primary Audience | Schools |
Results and Impact | Provoked questions and discussions afterwards As above |
Year(s) Of Engagement Activity | 2013 |
Description | @ Bristol Brain awareness week 2012 |
Form Of Engagement Activity | Participation in an activity, workshop or similar |
Part Of Official Scheme? | Yes |
Geographic Reach | Local |
Primary Audience | Public/other audiences |
Results and Impact | Interest expressed by public participants See above |
Year(s) Of Engagement Activity | 2012 |
Description | Bristol Neuroscience (SRS), 10 year celebration |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | Yes |
Geographic Reach | Local |
Primary Audience | Public/other audiences |
Results and Impact | Provoked questions afterwards As above |
Year(s) Of Engagement Activity | 2013 |
Description | Huber Lecture |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | Yes |
Geographic Reach | International |
Primary Audience | Participants in your research and patient groups |
Results and Impact | Talk generated questions and discussion afterwards Too soon to judge |
Year(s) Of Engagement Activity | 2014 |
Description | Lecture for invited Japanese public audience, Sapporo at Hiroshima University |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | National |
Primary Audience | Public/other audiences |
Results and Impact | Elicited discussion after the talk Raised some public awareness of the particular scientific issues |
Year(s) Of Engagement Activity | 2014 |
Description | Neuroregeneration Centre Edinburgh: invited research seminar (Feb 2016) |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Regional |
Primary Audience | Professional Practitioners |
Results and Impact | Research seminar outlining progress in understanding a simple vertebrate model system to researchers investigating neural regeneration |
Year(s) Of Engagement Activity | 2016 |
Description | Science Club, Clifton College |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Local |
Primary Audience | Schools |
Results and Impact | Provoked questions and discussion As above |
Year(s) Of Engagement Activity | 2013 |
Description | Tadpole Website (tadpoles.org.uk) |
Form Of Engagement Activity | Engagement focused website, blog or social media channel |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Public/other audiences |
Results and Impact | Website containing multi-level information on tadpoles that uses these familiar animals to provide entry into study of brains and behaviour |
Year(s) Of Engagement Activity | 2016,2017 |
URL | http://tadpoles.org.uk |
Description | The Boggling Brain Show |
Form Of Engagement Activity | Participation in an activity, workshop or similar |
Part Of Official Scheme? | No |
Geographic Reach | Local |
Primary Audience | Public/other audiences |
Results and Impact | Good feedback from participants (adults and children) Interest in subject matter expressed by participants |
Year(s) Of Engagement Activity | 2011 |