Mi and Tm medulla neurons
systemVisionspeciesD. melanogaster3D view49 of 626 cellssources6
Mi (medulla intrinsic) and Tm (transmedullary) neurons are columnar neurons of the fruit fly’s medulla. Mi1, Tm3, Mi4 and Mi9 are the major inputs to the T4 motion detectors, and Tm1, Tm2, Tm4 and Tm9 the major inputs to T5.
| Also known as | Mi1 · Mi4 · Mi9 · Tm1 · Tm2 · Tm3 · Tm4 · Tm9 · Medulla intrinsic neurons · Transmedullary neurons |
|---|---|
| Region | Medulla (optic lobe); Tm axons reach the lobula |
| System | Vision: eye and optic lobe |
| In the display bundle | 49 of the 626 reconstructed neurons in the site’s MaleCNS display bundle, which shows 626 of the model’s 166,700 neurons: Mi1 24, Mi4 2, Mi9 4, Tm1 6, Tm2 7, Tm3 2, Tm4 3, Tm9 1 |
What they are
The names come from the classic Golgi survey of the optic lobe (Fischbach and Dittrich, 1989): Mi neurons stay within the medulla, while Tm neurons send an axon from the medulla to the lobula. Both are columnar, repeating across the medulla’s retinotopic columns.
What they do
An electron-microscopy reconstruction of the medulla identified a motion detection circuit and found that the connections onto individual motion-sensitive neurons were consistent with their direction selectivity (Takemura et al., 2013).
Patch-clamp recordings showed that Mi1 and Tm3 respond to brightness increments, with Mi1 delayed relative to Tm3, and that Tm1 and Tm2 respond to brightness decrements, with Tm1 delayed relative to Tm2. These are the delayed and non-delayed signals that the classic correlator model of motion detection requires (Behnia et al., 2014).
None of the main inputs to T4 is itself direction-selective; selectivity arises in the T4 dendrites (Strother et al., 2017). On the OFF side, T5 receives its major input from Tm1, Tm2, Tm4 and Tm9, which act as an array of spatiotemporal filters; silencing them in various combinations shows that all four contribute to OFF motion detection to different degrees (Serbe et al., 2016). Connectome comparisons of the two pathways are given in (Shinomiya et al., 2019).
Behavior
Silencing each T4 input type identified which ones are necessary for T4 direction selectivity and for behavioral responses to ON motion (Strother et al., 2017).