Layer 5 Corticofugal Projections from Diverse Cortical Areas: Variations on a Pattern of Thalamic and Extrathalamic Targets

Corticofugal L5 targets of V1. A, V1 injection site where EYFP label is confined to L5. Selected thalamic targets are the (B) lateral dorsal, central lateral, and pulvinar nuclei and (C) intergeniculate leaftlet, and extrathalamic targets (D) caudoputamen, (E) anterior pretectal nucleus, and (F) superior colliculus. Conventions follow Figure 1. Low-power images correspond to Allen Mouse Brain Reference Atlas panels as follows: (A) 84, (Bi) 63, and (Ci) 77, where arrowhead indicates terminals along the border of pulvinar and dorsal lateral geniculate nucleus (Di) 57, (Ei) 81, and (Fi) 87. APN, Anterior prectectal nucleus; CC, corpus callosum; CL, central lateral nucleus; CP, caudoputamen; IGL, intergeniculate leaflet; LD, lateral dorsal nucleus; LGd, dorsal lateral geniculate nucleus; LGv, ventral lateral geniculate nucleus; LH, lateral habenula; MD, mediodorsal nucleus; NOT, nucleus of the optic tract; OP, olivary pretectal nucleus; POm, posterior medial nucleus; PPT, pedunculopontine nucleus; Pul, pulvinar; SC_ig, intermediate gray of the superior colliculus; SC_iw, intermediate white of the superior colliculus; SC_op, optic layer of the superior colliculus; SC_sg, superficial gray of the superior colliculus; thal, thalamus; TRN, thalamic reticular nucleus; VAL, ventral anterior-lateral nucleus; Vl, lateral visual cortex; VPl, ventral posterolateral nucleus; VPm, ventral posteromedial nucleus; Vpm, posteromedial visual cortex.

Corticofugal L5 targets of M1. A, M1 injection site where EYFP label is confined to L5, selected thalamic targets are the (B) posterior medial, central lateral nucleus, mediodorsal, ventral medial, rhomboid and (C) parafascicular nuclei, and extrathalamic targets, (D) caudoputamen, (E) anterior pretectal nucleus, and (F) superior colliculus. Conventions follow Figure 1. Low-power images correspond to Allen Mouse Brain Reference Atlas panels as follows: (A) 46, (Bi) 73, (Ci) 75 (Di) 45, (Ei) 85, and (Fi) 87. ACA, Anterior cingulate cortex; ACB, nucleus accumbens; APN, anterior prectectal nucleus; CC, corpus callosum; CM, central medial nucleus; CL, central lateral nucleus; CP, caudoputamen; fr, fasciculus retroflexus; LH, lateral habenula; M2, secondary motor cortex; MD, mediodorsal nucleus; MRN, midbrain reticular nucleus; PAG, periaqueductal gray; PCN, paracentral nucleus; PF, parafascicular nucleus; POm, posterior medial nucleus; Pul, pulvinar; Re, nucleus reuniens; RH, rhomboid nucleus; RN, red nucleus; SC_dg, deep gray of the superior colliculus; SC_dw, deep white of the superior colliculus; SC_ig, intermediate gray of the superior colliculus; SC_iw, intermediate white of the superior colliculus; SC_op, optic layer of the superior colliculus; SC_sg, superficial gray of the superior colliculus; SC_zo, zonal layer of the superior colliculus; SMT, submedial nucleus; thal, thalamus; VAL, ventral anterior-lateral nucleus; VM, ventral medial; VPl, ventral posterolateral nucleus; VPm, ventral posteromedial nucleus.

Corticofugal L5 targets of PFC. A, PFC injection site where EYFP label is confined to L5. Selected targets are (B) thalamic submedial nucleus and extrathalamic targets (C) caudoputamen and (D) periaqueductal gray. Conventions follow Figure 1. Low-power images correspond to Allen Mouse Brain Reference Atlas panels as follows: (A) 32, (Bi) 65, (Ci) 46, and (Di) 104. ACB, Nucleus accumbens; AM, anteromedial nucleus; CC, corpus callosum; CP, caudoputamen; DTN, dorsal tegmental nucleus; IAM, interanteromedial nucleus; ILA, infralimbic cortex; LDT, laterodorsal tegmental nucleus; ORBl, lateral orbital cortex; ORBm, medial orbital cortex; ORBvl, ventrolateral orbital cortex; PAG, periaqueductal gray; PCG, pontine central gray; PL, prelimbic cortex; SMT, submedial nucleus; Re, nucleus reuniens; RH, rhomboid nucleus; VAL, ventral anterior-lateral nucleus; VM, ventral medial nucleus.

Many cortical terminal fields are unconstrained by cytoarchitectural boundaries. Backset, Schematic section adapted from Allen Mouse Brain Reference Atlas (panel 74; http://mouse.brain-map.org/static/atlas). Red overlay represents region shown at high power. Frontset, A field of labeled S1 terminals spanning the parafascicular, posterior medial nucleus, and pulvinar. Bottom right, Scale bar, 50 µm. Insets, Terminals are evident across this swath, not just in the most densely labeled regions. Morphology varies, with smaller terminals in the parafascicular (left) compared with the posterior medial nucleus (right). Inset, Scale bar, 5 µm.

En passant varicosities in the thalamic reticular nucleus: (A) from S1, (B) from V1, (C) from M1, and (D) from PFC, labeled fibers passing through the thalamic reticular nucleus (red outline) from the internal capsule to the dorsal thalamus. For different injection sites, this passage occurs at different positions along the rostrocaudal axis, corresponding to Allen Mouse Brain Reference Atlas (http://mouse.brain-map.org/static/atlas) (planes 67, 65, 61 and 61, respectively). Scale bars: A-Di (low power), 500 µm; (A-Dii) (high power), 50 µm. Arrowheads indicate en passant swellings. int, Internal capsule; thal, dorsal thalamus; TRN, thalamic reticular nucleus; ZI, zona incerta.

L5 terminal size is pathway-specific. A, For a given pathway, bars represent mean terminal size and error bars indicate SEM among animals (n = 2-4 per group). For most pathways, average terminal size falls between the small and large terminals established in mouse somatosensory thalamus. The average terminal size in the ventral posteromedial nucleus of Rbp4-Cre negative mice and in the posterior medial nucleus of Cre⁺ mice is shown to the left and extended across the graph for easy comparison with these previously established small and large populations. *p < 0.001, significant difference from terminals in the ventral posteromedial nucleus (ANOVA with animal and pathway as between-subjects variables, Tukey HSD post hoc test). Star represents significant difference (p < 0.001) from terminals in the posterior medial nucleus (ANOVA with animal and pathway as between-subjects variables, Tukey HSD post hoc test). B, Population distributions of terminal size for pathways of interest. Bi, S1 terminals in the posterior medial nucleus exhibit fewer small and more large terminals than in the posterior medial nucleus. Bii, PFC terminals in submedial nucleus and V1 terminals in the pulvinar exhibit a terminal size distribution similar to S1 terminals in posterior medial nucleus. Biii, S1 and M1 terminals in the parafascicular nucleus exhibit a distribution of small terminals similar to the posterior medial nucleus but, similar to the ventral posteromedial nucleus, lack large terminals. Biv, Terminals in example extrathalamic pathways exhibit more terminals of intermediate size (arrowhead). APN, Anterior prectectal nucleus; CL, central lateral nucleus; CP, caudoputamen; IGL, intergeniculate leaflet; LD, lateral dorsal nucleus; MD, mediodorsal nucleus; PAG, periaqueductal gray; PF, parafascicular nucleus; POm, posterior medial nucleus; Pul, pulvinar; RH, rhomboid nucleus; SC, superior colliculus; SMT, submedial nucleus; VM, ventral medial nucleus; VPm, ventral posteromedial nucleus.