Transcript Neural system adaptations Sensory adaptations

Neural system adaptations
Sensory adaptations
Bio 325 Lecture 21 March 29, 2011
Neurons
Giant neurons
Cricket localization mechanism
Vertebrate retinas: accomodation
Owl eyes and eyespots
Owl hearing
Interneurons sometimes show their function in
their shape
•
•
Gerry Pollack, Montreal
Parts of a neuron: cell body
(soma), dendritic arborization,
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.
(From J. Insect Physiology,
George Boyan, 1984)
Interneurons that receive
auditory input
note topographical similarity of
omegas of cricket Gryllus and
katydid Tettigonia
Neurons 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.
Interneurons and brain neurons
that discriminate song pattern
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.
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 jetpropelled 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)
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.
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
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.
•
So how does she work this miracle? The directional mechanism used involves crossbody 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.
•
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.
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.
•
Vertebrate eyes: frog motion
detectors
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.
Reptile eye
scleral ossicles
accomodation (focussing)
Bird eye: like reptile but with an additional muscle functioning in accomodation
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
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
Barn Owl
O.W.L Center
Quiet flight; depth perception;
•
•
•
•
•
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.”
Bilateral asymmetry in parabolic auditory sensitivity fields is an adaptation to
capture prey using the prey’s incidental-movement sounds
Eyespots as a startle defense
P.J. Pointing