Neural system adaptations Sensory adaptations Bio 325 Lecture 21 March 29, 2011

1. Neural system adaptationsSensory adaptationsBio 325 Lecture 21 March 29, 2011 Neurons Giant neurons Cricket localization mechanism Vertebrate retinas: accomodation Owl eyes and eyespots Owl hearing

2. Interneurons sometimes show their function in their shape • Parts of a neuron: cell body (soma), dendriticarborization, axon. An example is the omega interneuron of the insect CNS, the name suggested by the shape. • Omega neuron, here filled with green dye via the electrode that monitored its firing activity; omega neurons occur in the prothoracic ganglion of a cricket as an overlain mirror-image pair. Gerry Pollack, Montreal

3. (From J. Insect Physiology, George Boyan, 1984)Interneurons that receive auditory inputnote topographical similarity of omegas of cricket Gryllus and katydid TettigoniaNeurons can be identified across taxa based upon their morphology, e.g., making binaural comparisons or as in the case of AN1 conveying activity from the prothoracic ganglion to the brain.

4. Interneurons and brain neurons that discriminate song pattern

5. Nervous systems Giant axons are widespread in animals, e.g., Annelids, Arthropods, Molluscs • One of the earliest neurons studied was the giant axon of squids. • In 1936 J.Z. Young discovered certain long structures present in squids (previously thought to be blood vessels) were actually nerves. They were axons of unusually large diameter: up to 1 mm in diameter. • A typical axon is about 40 microns in diameter: these giants are about 700 microns. • These cells came to be used by physiologists trying to understand depolarization of the nerve cell membrane and to be called GIANT AXONS.

6. Stellate ganglion in squid mantle. The stellate nerve contains the giant axon. This motor nerve is very large in diameter and conducts very rapidly, ensuring nearly synchronous activation of mantle muscles in a jet-propelled escape. Time of arrival differences of the motor commands are made almost simultaneous over the mantle Pictures taken from a website illustrating the dissecting out of a nervous preparation of the squid giant synapse (USCRC IBRO)

7. Sound localization in crickets •             Human ears are separated by about 21 cm. And this distance is about the wavelength of a 800 Hz sound. For this wavelength the sound ‘shadow’  (diffraction) cast by the human head gives an intensity difference of about 8 dB. For higher frequencies, e.g., a tone of 10 kHz, this sound ‘shadow’ becomes more pronounced, e.g., a 20 dB drop in intensity at the farther ear for 10 kHz. Humans make use of such intensity differences, turning their head to equalize sound levels and so to face in the direction of the source of a sound.

8. From a Scientific American article by Franz Huber (1985) Blue illustrates the cricket’s acoustic tracheal system This is a pressure difference system in which sound is conveyed to the front and to the rear of both tympana; there are four points of sound access

9. Sound localization in crickets • Female crickets must also use ears to localize sounds, most importantly the position of calling males in the darkness. Cricket ears are quite directional and a female can orient to a  singing male quite effectively. But this directionality cannot be based upon body diffraction (body ‘sound shadow’) because crickets are too small in relation to the wavelength of the sound they use. The call of a cricket commonly has a carrier in the range of 4.5 kHz. The wavelength of this sound is between 8.6 cm (4000 Hz) and 6.9 cm (5000 Hz); in other words about 7 cm. The cricket’s body width is only about 6 mm and the distance between her ears with the legs in walking position is no more than 1 cm. Just as a water wave in the ocean with a very long wavelength relative to a piling sweeps on by the piling without much reflection effect, so the sound of the male cricket’s call sweeps by the body of the female with little or no diffraction. There is not a significant drop in intensity on her lee side. So it matters not how a female cricket changes the direction in which she faces, she cannot localize a song source by diffraction.

10. So how does she work this miracle? The directional mechanism used involves cross-body transfer of sound and differing path lengths that affect the phase of the cricket’s calling song. The cricket has a pressure difference ear – one which conducts sound to the rear of the eardrum as well as receiving it on the front. So the activity of the eardrum is a function of the pressure changes both outside and inside. Sound from a male’s call travels to the front of the near eardrum. It also reaches the inner surface of this same eardrum via three other different body routes: two spiracles on the thorax and the other eardrum. These are connected internally by an h-shaped tracheal system. The path lengths of the three routes do change as the female turns. Thus the sound pressures on the back of the eardrum will change in phase relative to those on the outside.  So right and left eardrums do show different activity as a function of direction. Sound reaching the back of the eardrum later than sound reaching the front is shifted in time, i.e., its phase has changed. Since time of arrival between front and back is so important, it has apparently also been important for crickets to make just a single very pure-tone frequency – to make the phase effects clearer.

11. The mirror-image structure of the overlain omega neurons reflects a function in contralateral inhibition that enhances binaural contrast: input from the left inhibits input from the right and makes right-left localization more dramatic.

12. Vision in animals • A photoreceptor is a neurosensory cell found in animal retinas that converts electromagnetic radiation (in our case in the range we call light) into nerve activity. Proteins in the cell absorb photons to achieve depolarization of the neuron membrane. Examples are rods and cones. The rods are narrower than the cones and distributed differently across the retinas of different species. • There are major functional differences between rods and cones. Rods are extremely sensitive, and can be triggered by a very small number of photons.Cones require much larger numbers of photons in order to produce a signal. • The human retina contains about 120 million rod cells and 5 million cone cells. The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal. Some species of owls have a tremendous number of rods in their retinas — the eyes of the tawny owl are approximately 100 times more sensitive at night than those of humans.

13. Vertebrate eyes: frog motion detectors

14. Frog motion detectors • Different ganglion cells in the retina of a frog respond differently to images formed on the retina. The peripheral sensory system is discriminating danger and food etc.

15. Reptile eyescleralossiclesaccomodation (focussing)

16. Bird eye: like reptile but with an additional muscle functioning in accomodation

17. Nocturnal adaptation in vertebrate eyes • An effective nocturnal eye should collect as much light as possible. A large pupil is needed. And with a large pupil must come a large lens in order to avoid spherical aberration (edge distortion effects: the periphery of a lens is not as effective as its middle). So the cornea and the anterior chamber and the lens must all get larger. A larger lens will have an increased focal length and to successfully focus the image produced by the lens onto the retina the lens will need an increased curvature and/or the eye will need to become longer. • The typical nocturnal eye involves: large cornea, large (nearly spherical) lens, large pupil, large anterior chamber. In nocturnal species with poor vision (e.g., rat) the eyes may not be unusually large and acuity* (resolving ability) may be poor. But species with good nocturnal vision will have eyes as large as can be fitted into the head and often a tubular shape. • Animals with tubular eyes include bush babies and owls. A tubular eye cannot be rotated laterally and so an animal like an owl must compensate and either turn its whole body or its head. The owl has a remarkable capacity for head turning: 270 degrees! This is an adaptation correlated with its tubular eyes. • Nocturnal eyes are dominated by rods: in certain species ONLY rods: bush babies, bats, nocturnal snakes & lizards. Fine art america

18. Tapetum: eyeshine • Visual acuity: resolving power: the ability to distinguish fine detail (not the same as sensitivity: the ability to detect small quantities of light) • Good visual acuity is a property of the cones; high visual sensitivity is a property of the rods • Cones predominate in the retinae of diurnal animals • Rods predominate in the retinae of nocturnal animals • In some lizards and in squirrels, active only in the day, all photoreceptors are cones • Under low light intensities sensitivity becomes more important than acuity: a further adaptation for nocturnal vision is the tapetum • This is a device for increasing sensitivity: it is a reflecting layer within the eye on the inner surface of the choroid

19. Barn Owl O.W.L Center Quiet flight; depth perception;

20. Payne, R.S. 1971. Acoustic location of prey by barn owls (Tyto alba). J. exp. Biol. 54: • 535-573. • External ears of owls (as with birds in general) are hidden by head feathers that make the head look bilaterally symmetrical. But in some owl species, underneath the feathers, the two ear openings are dramatically asymmetrical. “One ear has its opening above the horizontal plane, the other below it” (Payne 1971). • This is an adaptation for hunting prey (rodents) by listening in the dark to their incidental sounds as they scurry through dead leaves on the forest floor. • “Eight parallel rows of feathers…form the heart-shaped periphery of the face. The opening of each ear lies at the focus of one-half of this heart, a curving wall of feathers which is almost, but not exactly, parabolic. The feathers in each curving wall are highly modified, having reduced vanes and rachises which, for their size are unusually thick…. They are also more densely packed than any other feathers on the owl’s body. If they are removed the array of holes formed by their empty sockets shows hexagonal ‘closest packing ‘[e.g., honey comb] indicating that they are as close together as physically possible….These adaptations suggest that there have been strong selective pressures favouring a curving wall that reflects sound – the usual sound-absorbent properties of feathers having been circumvented by emphasis of those surfaces which could act as reflectors, and orientation of them normal to incoming sound waves.”

21. Bilateral asymmetry in parabolic auditory sensitivity fields is an adaptation to capture prey using the prey’s incidental-movement sounds

22. Eyespots as a startle defense P.J. Pointing