Two Nerve Cells In Direct Contact

[Guest guest]

Two Nerve Cells In Direct Contact

http://www.sciencedaily.com/releases/2006/11/061108103929.htm

For the first time, scientists at the Max Planck Institute for

Neurobiology in sried near Munich have been able to show how

two nerve cells communicate with each other from different

hemispheres in the visual centre. This astoundingly simple circuit

diagram could at a later date provide a model for algorithms to be

deployed in technical systems (Nature Neuroscience, October 10, 2006)

A fly, flying along a corridor, produces through its movement a

constant shift of the pictures of the environment on its eyes

(illustrated with arrows). This " vector field " must be analysed on a

higher level of the visual centre, called the Lobula plate, so as to

control and correct the flight course. Turns are controlled by the

direct connection of two nerve, the HSE cell (right) and the H2-

Zelle (left). (Image: MPI for Neurobiology - Schorner)Ads by

Google Advertise on this site

Movements in space create in humans and animals so-called optical

flow fields which are characteristic for the movement in question.

In a forward movement, the objects flow by laterally, objects at the

front increase in size and objects further away hardly change at

all. At a higher level in the visual centre in the brain, there must

be a computation of the visual information, so that animals can

differentiate between their own movement and movement of their

environment and are able to correct their course if necessary. It is

important for the analysis of flow fields that the movement

information from both eyes is merged so that the whole flow field

can be assessed. In their current study, Karl Farrow, Jürgen Haag

and Borst have for the first time proved the direct link

between two nerve cells, one in each half of the brain, combining

the movement signals from both the facetted eyes of a fly.

In the blow fly, the nerve cells that analyse optical flow fields,

called tangential cells, are located in the lobula plate. There are

only 60 of these tangential cells for each half of the brain and

each of these 60 cells can be identified individually. The

scientists in sried have looked closely at one cell, the H2

cell. This cell exhibits a strong preference for rotational flow

fields such as that which arises when the fly turns around its

body's vertical axis. Interestingly, this cell seems initially to

react only to the movements in front of its own eye (ipsilateral),

but remain blind to movements in front of the other eye

(contralateral). However, if the ipsilateral movement stimuli are

combined with the contralateral, it is seen that the latter do

indeed modulate the reactions to ipsilateral movement stimuli. " The

preference of the H2 cell for rotational stimuli is due to a non-

linear coordination of the movement stimuli from both eyes, and it

was this non-linearity that we wanted to investigate further, " said

Borst.

The next step was to analyse the circuit diagram of the tangential

cells of the lobula plate in detail. This was based on a multitude

of experiments in which the connections between the cells within one

lobular plate and those between the two hemispheres were examined.

In the end it turned out that there were two ways in which movement

information from one half of the brain could reach the H2 cell in

the other: firstly, directly from the so-called HSE cell, which is

electrically linked to the H2 cell in the opposite hemisphere and

secondly, indirectly via the CH cell, which receives information via

several intermediate stops from the other half of the brain and

which inhibits the H2 cell on the same side with chemical synapses.

Both connections were in principle suitable for achieving the effect

described; however the question remained: which of the two is the

crucial one?

The Max Planck scientists therefore blocked the two possible routes

selectively with laser ablation (the cell is filled with fluorescent

dye which has a toxic effect when strongly stimulated) and tested

sensitivity to rotation in the H2 cell. A long series of these

technically very difficult experiments provided unambiguous proof:

if the ipsilateral CH cell was destroyed, no effect was seen on the

rotation sensitivity of the H2 cell. However, if the contralateral

HSE cell was removed from the circuit, the rotation sensitivity of

the H2 cell disappeared. It was blind to the movement stimuli in

front of the other eye, irrespective of whether it was combined with

the ipsilateral movement stimuli.

Borst enthused about the discovery: " The genius of this

circuitry is in its simplicity: with a single electrical link

between two cells from the halves of the brain, one cell is

selective for rotation flow fields. " Whether nature has constructed

similarly simple mechanisms in mammals is still unclear - the

circuitry of the nerve cells in the relevant areas in the cerebral

cortex has not yet been sufficiently explained to allow experiments

of this nature to be carried out. And it is rather doubtful whether

removing a single cell from the many billions of cells in the

cerebral cortex would have an effect.

Clearly, however, this does not mean that the findings made by the

fly researchers in sried will be without consequence for other

areas of science. For example, engineers developing navigating

robots and driving assistance systems rely on simple and robust

algorithms such as those realized by nature in insects. The

mechanisms of optical flow field analysis are supremely suitable for

technical implementation. As part of two projects supported by the

Federal Ministry for Education and Research (the Bernstein Center in

Munich and " Cognition in Technical Systems " (CoTeSys)) the

neurobiologists in sried, in collaboration with their

colleagues from the Technical University in Munich, will be working

more intensively on this area over the next few years. The Neuronal

Information Processing department under the leadership of

Borst has recently participated in the graduate training

program " School of Systemic Neurosciences " at the Ludwig Maximilian

University in Munich.