trivia: Can you see what I see? I'm trying to see how respiratory cycles, circadian rhythms, accomodation, dark adaptation, pupillary light reflex, photic sneezing and trigeminal nerve and oculomotor nerve work together in humans in air and water, especially with reference to humans apnea diving to dark depths and focusing on food gathering, surfacing and speedy exhalation, and alternatively backfloating while resting in the mid-day sunlight. Some of the info below may not be perfectly accurate, still ongoing investigative biomedical research.
"They also observed that the phase of the spontaneous circadian rhythm did not lock. This is because, circadian rhythm is very flexible. In humans, the cycle repeats about every 24.2 hours. The circadian clock is reset by light and our circadian apparatus is exquisitively sensitive to lights falling on the retina. The retina sends this light (for synchronization) to the SCN via the retino-hypothalamic tract. This synchronization or entrainment can now 'phase lock' the circadian rhythm.
Clinical implication of circadian (circa=about; dian=day) rhythm is enormous. Our sleep-wake cycle, growth hormone and cortisol secretion are only a few example. A person in whom the circadian period is short will rise early (early bird?) and a 'night owl' will have his/her circadian period short. Curiously, our sleepiness, tendency to sleep and occurrence of REM sleep peaks (resulting from endogenous circadian rhythm) when we are about to rise; and our endogenous clock reaches its peak about 1-3 hrs before our habitual bedtime.
Humans are the only species with exposed white sclerae.
20% of European/Asian humans sneeze at bright sunlight after having been dark-adapted. Humans can see clearly underwater (conclusively tested in Thai Moken children seafood forage divers and Scandinavian children) by employing 'visual accomodation', which is a form of dark-adaptation of the oculomotor nerve (CN#3) in association with the trigeminal nerve, the pupils constrict voluntarily, the complementary opposite of the pupillary light reflex and photic sneeze reflex, since this is not found in our mammalian kin, it must be assumed to have occurred in our seashore diving ancestors as selection for improved seafood foraging efficiency.
The Edinger-Westphal nucleus supplies preganglionic parasympathetic fibers to the eye, constricting the pupil and accommodating the lens. Edinger-Westphal nucleus (also known as the accessory oculomotor nucleus) is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve (cranial nerve III), supplying the constricting muscles of the iris. http://en.wikipedia.org/wiki/Edinger-Westphal_nucleus
The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa), comprising coordinated changes in vergence, lens shape and pupil size. It is dependent on cranial nerve II (afferent limb of reflex), higher centres and cranial nerve III.
When bright light is shone on the eye, it will automatically constrict. This is the pupillary reflex, which is an important test of brainstem function. Furthermore, the pupil will dilate if a person sees an object of interest.
The oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil. The pupil gets wider in the dark but narrower in light. When narrow, the diameter is three to four millimeter. In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 4 to 5 mm. The constriction of the pupil and near vision are closely tied. In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity; in the dark this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye. The pupil dilates in extreme psychical situations (e.g., fear) or contact of a sensory nerve, such as pain.