Published today in Current Biology, the study combines extracellular recordings, single-cell labelling, anatomical reconstruction, and immunohistochemistry to characterise individual HD cells within the anterodorsal thalamic nucleus (ADn), a key hub in the brain's spatial navigation system.
HD cells are active when an animal faces a particular direction and are thought to provide a neural representation of heading. While these cells have traditionally been studied as a single functional population, the new work demonstrates substantial diversity within the ADn.
The researchers recorded the activity of individual HD cells in awake mice and examined how their firing was influenced by sensory stimuli and behavioural events. They found that distinct HD cell subpopulations responded differently to light flashes, sounds, and movement, indicating that directional signals are integrated with information related to sensorimotor processing, attention, and arousal.
"Our findings show that head direction cells do much more than simply encode which way an animal is facing," said Associate Professor Tim Viney, senior author of the study. "Different HD cell subpopulations respond to salient sensory events in different ways, suggesting that the brain's navigation system continuously integrates information about the external environment."
The team also identified a previously unrecognised population of calretinin-expressing HD cells (shown in cyan in the image above). These neurons exhibited distinctive firing properties and connectivity patterns compared with neighbouring HD cells (shown in magenta), supporting the idea that molecularly defined cell types make unique contributions to navigation circuits.
By reconstructing the dendrites and projection axons of individual neurons, the researchers further identified an unusual calretinin-positive HD cell type characterised by highly tortuous dendrites and a distinctive descending axon. The authors named this cell the tortuosa head direction cell (example shown below).
Co-first author Dr Sara Hijazi said: "Combining physiology, anatomy and molecular identity at the level of individual neurons allowed us to reveal an unexpected degree of diversity within this circuit. These findings provide a new framework for understanding how thalamic networks process directional and sensory information."
The work adds to a growing body of evidence that functionally defined neural systems are often composed of multiple specialised cell types. Understanding this diversity is important because the ADn selectively expresses several genes associated with neurodevelopmental disorders, while selective degeneration of the same region has been linked to spatial disorientation in early dementia.
The study provides one of the most detailed characterisations to date of neuronal diversity within the mammalian head direction system and establishes a foundation for future investigations into how specific HD cell populations contribute to navigation and cognition.
Publication:
Hijazi, S, Jiang, S, Wülfing, MS, Quach, J, Lachance, PA, Hasselmo, ME, Viney, TJ, 2026. Diversity and sensorimotor specialization of head direction cells in the mouse thalamus. Current Biology
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To read the paper, click here: https://authors.elsevier.com/sd/article/S0960-9822(26)00631-7