Extended Data Fig. 2: Distribution of PT axon collaterals.
From: Distinct descending motor cortex pathways and their roles in movement
a, Axonal lengths of single-neuron reconstructions within targets in thalamus and medulla (thalamus-targeting PT neurons, n = 8, green; medulla-targeting PT neurons, n = 4, magenta). b, Axonal lengths within other selected PT targets. c, Axonal termini in single-neuron reconstructions within thalamic and medullary targets. d, Axonal termini within other selected PT targets. e, Bulk projections of PTupper and PTlower populations. Groups of cortical neurons were labelled from the thalamus (PTupper; mouse #1), the medulla (PTlower; mouse #2), or both targets (mouse #3) using rAAV2-retro expressing spectrally distinct fluorescent proteins. Top, schematics of the labelling procedures. Left, rostro-caudal level (relative to Bregma). Right, annotated coronal sections taken from the Allen Mouse Brain Atlas54 with imaged area indicated. Both cell types extended axon collaterals to motor-related superior colliculus, but to different parts. Axons from PTupper cells were apparent throughout all superior colliculus layers, with a dense projection to the ventrolateral aspect; PTlower neurons were restricted to the ventral superior colliculus and were concentrated more caudally. Both groups project to the pons, particularly the pontine grey, but with terminations in largely non-overlapping zones. PTupper cells projected to the globus pallidus external segment and broadly targeted the dorsal, lateral and ventral striatum. PTlower cells projected sparsely to the lateral striatum. PTlower neurons projected to the central amygdala and parasubthalamic nucleus. PTlower neurons also made up most of the projection to the red nucleus, parabrachial nucleus, substantia nigra pars compacta, motor and sensory trigeminal nuclei in the hindbrain, and through the medullary pyramids. Both cell types extended axon collaterals locally within the same sublamina as their somata, the subthalamic nucleus, zona incerta, and the midbrain reticular nucleus. PTupper cells project more broadly to layer 1 in motor cortex. Mice 1 and 2 were used for electrophysiological recordings. For this reason, projections were labelled with ChR2(H134R)–YFP.
