SENSORY BIOLOGY

Sensory Biology

Sensory Biology

Introduction

The rotation of the earth around its axis causes 24-hour rhythms in many aspects of the physical environment, while the Earth's revolution around the sun causes seasonal changes. As an adaptation to daily environmental changes, virtually all living systems have developed endogenous circadian clocks with periods of about 24 hours to anticipate changes in the solar day. Anticipation of, rather than passive responsiveness to, the daily cycle is deeply embedded in biology and perhaps first evolved in cyanobacteria 3-4 billion years ago. The photic environment was likely the selective agent for optimization of photosynthesis as well as escape from the mutagenic effects of ultraviolet irradiation.

Discussion

For proper functioning, circadian rhythms have to be synchronized, or entrained, to the day-night cycle. The major synchronizing stimulus in the environment is light. Light, rather than temperature variation or other environmental features, is a reliable signal to indicate whether it is day or night, while changes in day length are a reliable signal to indicate the time of the year. Not surprisingly, circadian clocks are responsive to light to subserve entrainment to the daily and annual cycle. In mammals, the circadian clock is located in the suprachiasmatic nuclei (SCN) at the base of the anterior hypothalamus. Light information reaches the SCN via specialized neuronal projections, of which the retinohypothalamic tract appears sufficient for photic entrainment.

Despite the well-established role for extraretinal photoreception in many vertebrate species, mammals have lost the ability to perceive light extraretinally and photoentrainment is mediated exclusively via the retina (Groos, 2002). Action spectra have indicated a maximal sensitivity of the circadian system in its phase-shifting response to wavelengths of about 500nm and a secondary peak in the near-ultraviolet range of the spectrum. The maximal sensitivity at about 500nm may suggest a role for opsin-based photopigments.

However, mice that are deficient in both rods and cones show normal phaseshifting responses to 509nm monochromatic light pulses (Freedman et al., 1999) and normal suppression of pineal melatonin in response to light (Lucas et al., 1999), indicating that classical photoreceptors do not fully account for light effects on the circadian pacemaker. Recent evidence suggests that a small subset of retinal ganglion cells (about 1% of the total population) contains the photopigment melanopsin (Hattar et al., 2002). Melanopsin is present in the axons, cell bodies, and proximal dendrites of the ganglion cells. Melanopsin-containing ganglion cells innervate the SCN bilaterally and also project to the intergeniculate leaflet (IGL), the pretectum, and, to a lesser extent, the ventral lateral geniculate nucleus. It is suggested, therefore, that melanopsin plays a role in transducing light information to non-image-forming visual brain areas (Hatter et al., 2002).

Melanopsin-containing ganglion cells show direct responsiveness to light (Berson et al., 2002). The ganglion cells also depolarize in response to light, when synaptic input from rods and cones is blocked or when they are surgically detached from the retina. This demonstrates that these ganglion cells are intrinsically photosensitive. The spectral sensitivity function of these ganglion cells shows a peak at ...