Impulses reach the SCN from retinal receptors which lie particularly in the lower and nasal quadrant of the retina. The receptors are the retinal ganglion cells and their light-sensitive pigment is melanopsin which is particularly sensitive to radiation with a wavelength of around 460nm. These cells are glutamatergic and comprise around 4% of the fibres in the optic nerve in most species.

The retinal ganglion cells are distinct from the rods and cones which lead to the sensation of vision, and whose fibres run in the optic nerves, synapse in the lateral geniculate nuclei and connect to the primary visual sensory cortex in the occipital lobes.

Impulses from retinal ganglion cells travel in the retino-hypothalamic tract, which also runs within the optic nerve, to monosynaptically reach the SCN independent of any visual sensation.

Other impulses from the retinal ganglion cells reach the pretectum, superior colliculus and sub-paraventricular zone. These probably mediate the pupillary light reflex and the effects of light exposure on NREM and REM sleep, but do not lead to any visual sensations.

The cholinergic pedunculopontine and laterodorsal tegmental (LDT/PPT) nuclei and basal forebrain neurones also project to the SCN, as do the ascending reticular activating system and other areas of the brainstem (

Fig. 2.6). These probably mediate the effects of physical exercise on advancing or delaying the sleep phase. This is also influenced by melatonin which inhibits the activity of the SCN and promotes sleep.

References