Transcript Nervous System

Nervous System
• Central Nervous System (CNS) = brain and
spinal cord (responsible for integration and
memory).
• Peripheral Nervous System (PNS) = cranial
and spinal nerves, autonomic nervous
system, sense organs (both sensory and
motor components).
Central Nervous System
• Brain = same basic plan as in reptiles and
mammals, 3 Divisions:
– Forebrain (Cerebrum) = integration, instinctive
behavior, intelligence
– Midbrain = vision, muscular coordination, physiological
control (homoeostasis)
– Hindbrain (medulla) = links brain with spinal cord and
peripheral nervous system
• Birds and mammals both have enlarged cerebral
and cerebellar hemispheres; the brain in both
Classes makes up 2-9% of total body weight.
Avian Forebrain
• Pallial domains are responsible for learning
and intelligence in vertebrate brains.
• In mammals, cerebral cortex is dominant
portion
• Serves as the seat of higher intelligence.
• Provides a great capacity for learning.
• Bird cortex thin and relatively undeveloped
(thought to be the seat of conditioned
behavior).
Avian Forebrain
• Pallial domains are dominant part of the avian cerebrum
and are cellular homologs in birds and mammals.
• Pallial domains = seat of learning, intelligence, complex
instinctual behaviors.
• In general, pallial domains (= cortex of mammalian brain)
specialized for learning, corpus striatum for stereotypic
behaviors.
• Former model: corpus striatum dominant in birds, so
lower intelligence than mammals.
• Recent evidence: some birds are highly intelligent;
outperform mammals in some advanced learning tasks
(counting, spatial cognition, pattern recognition).
Evolution of Bird and Mammal Brains
• Ancestral Stem-Amniote condition led to
layered cortex in mammals and pallium
differentiated into several regions in birds
• Two Hypotheses: based on neuron connectivity
patterns
• Nuclear-to-Layered Hypothesis = Ancestor with
nuclear pallium (neurons grouped into clumps)
– Evolved into layered arrangement in mammals, but
maintains ancestral connectivity patterns
– Evolved into three semi-layered sets of neurons in birds
Evolution of Bird and Mammal Brains
• Nuclear-to-Claustrum/Amygdala Hypothesis =
Connectivity patterns shared by neurons in layered
mammal cortex and bird pallial divisions evolved
independently
• Pallial divisions in birds (outside of hyperpallium)
represent elaboration of parts of brain homologous
to claustrum and amygdala regions in mammals
– Both with nuclear, rather than layered, arrangement of
neurons
• Not currently known which hypothesis is
correct and both may be partially correct
Avian Midbrain, Spinal Cord, PNS
• Birds with large, well-developed cerebellum
(largest among the vertebrates), associated
with very high degree of muscular
coordination necessary for flight.
• Very large optic lobes are present,
associated with the importance of vision in
birds.
• Spinal Cord - similar in structure to other
tetrapods, cervical and lumbar enlargements
associated with appendages
• PNS similar to that in other vertebrates.
Senses
• Sense of Smell - olfactory lobes are generally
small and birds formerly thought to have a
generally poor sense of smell.
• New evidence suggests that birds have better
developed sense of smell than previously
believed.
– Detect certain odors with similar abilities to mammals
– Number of functional olfactory receptor genes in most
birds is roughly similar to that in humans
– Many birds use smell for daily routines (feeding,
navigation, etc.)
Sense of Smell (cont.)
• Birds with low olfactory receptor gene
numbers and small olfactory bulb region
don’t necessarily have a poor sense of
smell
• Examples:
– Starlings can detect and discriminate volatile
plant compounds in nest materials
– Blue Tits apparently use olfaction to maintain
an aromatic nest environment (for nestlings)
and to detect predators
Senses
• Taste - all birds are capable of tasting
• Birds are equally or less sensitive to certain
ingredients than mammals.
• Birds have fewer taste buds than do
mammals.
Senses (cont.)
• Mechanoreception
– Touch - possess typical touch, pressure,
temperature, and pain receptors.
– Birds are also sensitive to barometric
pressure.
• Many birds can sense oncoming storms and
modify foraging behavior accordingly.
• Pigeons and thrushes can select proper
altitude for migratory flights, presumably
this is true for other birds as well.
Magnetism Detection
• Birds can use information from the earth's
magnetic field for navigation.
• Magnetite Crystals are present near
olfactory nerves (between eyes) of pigeon,
and these may serve as the basis for the
magnetism-detection system.
Hearing
• Ear is divided into same 3 regions as in mammals:
External, Middle, and Internal.
– Middle Ear is only one bone (columella) = transmits
sound vibrations from tympanum to inner ear.
– Inner Ear serves both hearing and equilibrium
functions.
• Optimal Hearing Range = 1 - 5 KHz, Limit = 10
KHz; Owls to lower frequencies and up to 12 KHz.
Overall, the range of optimal hearing in birds is
narrower than that in mammals
P. 193, Gill
Hearing in Owls
• Owls with specializations allowing them to detect
and capture prey by hearing alone.
– Detect low frequency sounds effectively
– Have high numbers of auditory neurons
– Facial discs act as sound collectors and aid in focusing
sound to ear (see p. 194, Gill)
– Asymmetry of external ears - allows binaural
comparison of intensity and frequency, which enables
precise vertical distinction in addition to horizontal
distinction similar to ours.
– Because of this they can capture prey by sound alone.
Echolocation in Birds
• A few birds are capable of echolocation for
navigation (Cave Swiftlet, Oilbird).
• Use low frequency clicks.
• This differs from the high frequency
ultrasound used by bats and is not nearly
as effective.
Avian Vision
• Vision is the most important sensory input for
birds, as they are visual animals.
• Birds have large eyes relative to other vertebrates
(starling eye makes up 15% of head mass,
humans = 1%)
• Shapes of avian eyes vary.
– Globular = diurnal birds with high resolution over great
distances (hawks, etc.)
– Flattened = most birds
– Tubular = nocturnal birds, allows increased
accommodation (focusing) and light-gathering
Flattened
Globular
Tubular
The three general categories of avian eyes
Avian Vision
• Birds with higher visual acuity (resolving
power) than mammals because of higher
numbers of photoreceptors and a slight
magnifying effect of the fovea.
– Raptors and passerines = 2 - 3 times human
abilities
• Generally, birds have higher powers of
accommodation as both the cornea and the
lens change curvature while focusing. Only
the lens changes curvature in mammals.
The avian retina is relatively thick compared with that
in other vertebrates. The increased thickness results in
increased refraction and increased magnification.
(P. 187, Gill)
Avian Vision
• Color Vision - birds have very high numbers of
cones (diurnal birds), which suggests welldeveloped color vision.
• Birds are sensitive to UV light.
– UV light probably more important to short-range visual
communication (such as mate choice) than long-range
communication, because UV wavelengths are more
highly scattered than longer wavelengths in air.
– UV reflectance of plumage increases female
preference for males in some spp., but not in others.
– In Blue Tits, females increase the number of male
offspring in clutch when mated to males with high UV
reflectance in their crest.
Avian Vision
• Diurnal birds have colored oil droplets in the
eyes, probably functions to enhance contrast by
filtering out background "noise."
• Birds do not have stereoscopic vision as do
mammals.
• Optic nerve tracts project only to the opposite
brain hemisphere.
• Most birds see laterally better than forward due
to the lateral position of eyes on the head and
little overlap in fields of view.
Avian Vision
• Pecten = structure composed of blood vessels
and supporting stromal cells; present at exit of
optic nerve, projects into vitreous chamber of
eye.
• May serve a nutritive role for the avascular retina
since it is highly vascularized, but the exact
function is unknown.
• Other proposed functions include:
–
–
–
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reduce glare
regulate pressure or temperature within the eye
perception of movement
light absorption
Pecten