Neuroanatomic connectivity of the human ascending arousal system critical to consciousness and its disorders

Abstract

The ascending reticular activating system (ARAS) mediates arousal, an essential component of human consciousness. Lesions of the ARAS cause coma, the most severe disorder of consciousness. Because of current methodological limitations, including of postmortem tissue analysis, the neuroanatomic connectivity of the human ARAS is poorly understood. We applied the advanced imaging technique of high angular resolution diffusion imaging (HARDI) to elucidate the structural connectivity of the ARAS in 3 adult human brains, 2 of which were imaged postmortem. High angular resolution diffusion imaging tractography identified the ARAS connectivity previously described in animals and also revealed novel human pathways connecting the brainstem to the thalamus, the hypothalamus, and the basal forebrain. Each pathway contained different distributions of fiber tracts from known neurotransmitter-specific ARAS nuclei in the brainstem. The histologically guided tractography findings reported here provide initial evidence for human-specific pathways of the ARAS. The unique composition of neurotransmitter-specific fiber tracts within each ARAS pathway suggests structural specializations that subserve the different functional characteristics of human arousal. This ARAS connectivity analysis provides proof of principle that HARDI tractography may affect the study of human consciousness and its disorders, including in neuropathologic studies of patients dying in coma and the persistent vegetative state.

Figure 1

Diffusion tractography of the human ascending reticular activating system, as defined by the cuneiform/subcuneiform (CSC) region of interest (ROI). (A, B) Transverse histologic (A) and radiologic (B) sections through the rostral midbrain at the level of superior colliculi (SC) in Case 1. The image in (B) is a non-diffusion-weighted image with b = 0 s/mm²; the CSC ROI is labeled in red. Neuroanatomic landmarks: mamillary bodies (MB), cerebral peduncles (CP), red nuclei (RN), superior colliculi (SC) and periaqueductal grey (PAG). (C, D) Ventral (C) and left lateral (D) views of CSC fiber tracts. Tracts are color-coded according to their direction (red, medial-lateral; green, ventral-dorsal; blue, rostral-caudal). Bundles of CSC tracts are labeled as follows: VTT_R, ventral tegmental tract, rostral; VTT_C, ventral tegmental tract, caudal; DTT_L, dorsal tegmental tract, lateral; DTT_M, dorsal tegmental tract, medial; and MFB, medial forebrain bundle.

Figure 2

Diffusion tractography of the cuneiform/subcuneiform nucleus (CSC) and pontis oralis (PO) pathways. (A, B) Left lateral (A) and zoomed left lateral (B) views of the CSC and PO regions of interest, and the fiber tracts that pass through these regions in Case 1. Fiber tracts are color-coded by the source nucleus of origin (red, CSC; light blue, PO). Tracts in (A) are superimposed on an axial non-diffusion-weighted image with b = 0 s/mm² at the level of the rostral pons. Bundles of CSC and PO fiber tracts are labeled as follows: VTT_R, ventral tegmental tract, rostral; VTT_C, ventral tegmental tract, caudal; DTT_L, dorsal tegmental tract, lateral; DTT_M, dorsal tegmental tract, medial; and MFB, medial forebrain bundle. Of note, red fiber tracts generated by the CSC region of interest extend both rostrally and caudally, but the direction of electrical signaling within each fiber tract (i.e. ascending or descending) cannot be determined.

Figure 3

Neuroanatomic connectivity of brainstem arousal pathways and intrathalamic gating pathways. (A) Dorsal view of fiber tracts originating in the pedunculopontine nucleus (purple) and parabrachial complex (yellow) connecting with the centromedian/parafascicular (CEM/Pf) and central lateral (CL) nuclei via the dorsal tegmental tract, medial (DTT_M) in Case 1. Pedunculopontine and parabrachial fibers also connect with the reticular nuclei (Ret) via the dorsal tegmental tract, lateral (DTT_L), and via an extension of DTT_M (arrowheads). Fiber tracts from the dorsal raphe (turquoise), locus coeruleus (dark blue), pedunculopontine nucleus (purple), and parabrachial complex (yellow) connect with the paraventricular, midline nuclei of the thalamus via the ventral tegmental tract, rostral (VTT_R). (B) Dorsal view of intrathalamic fiber tracts connecting Ret with CL (red) and Ret with CEM/Pf (pink). Fiber tracts in (A) and (B) are superimposed upon an axial non-diffusion-weighted image with b = 0 s/mm² (b0) at the level of the rostral midbrain and a coronal b0 image at the level of the mid-thalamus. Axial and coronal images in (A) are semitransparent, so that the trajectories of VTT_R fiber tracts are visualized coursing rostrally and dorsally to the paraventricular region of the thalamus. Neuroanatomic landmarks: red nuclei (RN), superior colliculi (SC) and peri-aqueductal grey (PAG).

Figure 4

Ascending reticular activating system (ARAS) connectivity with the hypothalamus. (A, B) Ventral (A) and rostral (B) views of ARAS connectivity with the hypothalamus in Case 1. (C) Ventral view from (A) with display of tract end-points. All fiber tracts and end points are color-coded according to their source nucleus of origin: turquoise, dorsal raphe; green, median raphe; dark blue, locus coeruleus; purple, pedunculopontine nucleus; yellow, parabrachial complex; and pink, ventral tegmental area. End-points for all source-nucleus tracts terminate in the hypothalamus but only dorsal raphe, locus coeruleus, pedunculopontine, and parabrachial complex pathways connect with the anterior region of the hypothalamus (arrowheads, arrows, and asterisks in A–D. (D) High-zoom view of tract end-points in the left anterior hypothalamus demonstrates termination sites of locus coeruleus, pedunculopontine, parabrachial complex, and dorsal raphe pathways, some of which overlap within the same voxel (dimensions 562 × 609 × 641 µm). All fiber tracts and end-points are superimposed upon an axial non-diffusion-weighted image with b = 0 s/mm² (b0) at the level of the red nuclei and a coronal b0 image at the level of the mid-thalamus (inset). The axial and coronal b0 images in (A) are semi-transparent so that the trajectories of fiber tracts are seen as they cross each respective plane. Bundles of fiber tracts are labeled as follows: VTT_R, ventral tegmental tract, rostral; VTT_C, ventral tegmental tract, caudal. Neuroanatomic landmarks: 3V, third ventricle; CP, cerebral peduncles; RN, red nuclei.

Figure 5

Connectivity of the cuneiform/subcuneiform nucleus (CSC) and pontis oralis (PO) in Cases 1, 2, and 3. (A–C) A dorsal view of left-sided CSC fiber tracts (red) and PO fiber tracts (blue) is shown for all 3 high angular resolution diffusion imaging (HARDI) scans (A, Case 1; B, Case 2; C, Case 3). For each scan, a summary of key HARDI sequence parameters is provided below the fiber tracts; the 3 planes (sagittal, coronal, and axial) onto which the fiber tracts are superimposed are shown as non-diffusion-weighted images with b = 0 s/mm². The 2 divergent branches of the dorsal tegmental tract (DTT) – DTT lateral (DTT_L) and DTT medial (DTT_M) – are seen in each case. In addition, each HARDI scan demonstrates that DTT_M connects with and passes through the central lateral nucleus (red, CL) and the centromedian/parafascicular complex (pink, CEM/Pf) of the thalamus, and a bundle of DTT_M fiber tracts projects rostrally and laterally to the reticular nucleus of the thalamus (purple, Ret) (white arrows, A, B). Of note, the Ret, CL and CEM/Pf thalamic nuclei are shown in only 1 coronal slice in (B, C) so that the fiber tracts can be optimally visualized, whereas these nuclei are shown as 3-dimensional structures in (A). Neuroanatomic landmarks: CC, splenium of the corpus callosum; IC, inferior colliculus; P, pineal gland; Thal, thalamus.

Figure 6

Connectivity of intrathalamic gating pathways and brainstem arousal pathways projecting to the thalamus, hypothalamus, Forel’s fields (FF), and zona incerta (ZI). (A) Dorsal view of intrathalamic fiber tracts that connect the reticular nucleus (Ret) with centromedian/parafascicular complex (CEM/Pf; pink fiber tracts) and that connect Ret with the central lateral nucleus (CL, red fiber tracts) in Case 1. Many red Ret-CL fiber tracts extend caudally into CEM/Pf, FF, and ZI, and pink Ret-CEM/Pf tracts also extend into FF and ZI. Brainstem arousal pathways are displayed as fiber tract end-points, color-coded by the source nucleus from which the tracts are generated: pedunculopontine nucleus (purple); parabrachial complex (yellow); dorsal raphe (turquoise); locus coeruleus (dark blue); median raphe (green); and ventral tegmental area (pink). (B) High-zoomed dorsal view from within the left CEM/Pf, which is semitransparent to allow for visualization of intrathalamic fiber tracts and pedunculopontine tract end-points (arrow) within this nuclear complex. (C) Zoomed dorsal view of right side of image in (A) shows that pedunculopontine fiber tracts terminate in FF and ZI (purple end points) in close proximity to fiber tracts that connect these regions to Ret via pathways that pass through CEM/Pf and CL (arrowheads, A, C). Fiber tracts in (A–C) are superimposed on an axial non-diffusion-weighted image with b = 0 s/mm² (b0) at the level of the rostral midbrain and a coronal b0 image at the mid-thalamus. Neuroanatomic landmarks: 3V, third ventricle, red nuclei (RN), superior colliculi (SC), peri-aqueductal grey (PAG).

Figure 7

The neuroanatomic connectivity of the serotonergic-related dorsal raphe (DR). (A) Left lateral oblique view of DR fiber tracts (turquoise) in Case 1 superimposed on an axial non-diffusion-weighted image with b = 0 s/mm² (b0) at the level of the mid-pons and a coronal b0 image at the level of the mid-thalamus. The images are semi-transparent so that the trajectories of fiber tracts are seen as they cross the coronal plane. (B) Left lateral view superimposed on axial b0 image at the level of the mid pons and a sagittal b0 image at the midline of the specimen. (C) Dorsal view superimposed on an axial b0 image at the level of the inferior colliculi (caudal midbrain) and a coronal b0 image at the dorsal thalamus. Fiber pathways from DR are seen connecting with the inferior colliculi bilaterally (arrows). Bundles of fiber tracts are labeled as follows: VTT_R, ventral tegmental tract, rostral; VTT_C, ventral tegmental tract, caudal. Neuroanatomic landmarks: red nuclei (RN), superior colliculi (SC) thalamus (Thal), pineal gland (P).