week 8 Flashcards
4 somatic senses
touch, temperature, proprioception, nociception
5 special senses
vision, hearing, equilibrium, taste and smell
receptor potential
a receptor cell converts stimulus energy into a graded change in membrane potential. – receptor may then release neurotransmitter to affect a neuron, if the receptor is itself a neuron, it may fire action potentials.
transductions
receptors convert stimuli into electrical signals
receptor cells
in some sensory systems (vision), receptor cells are neurons; in others (hearing) they are non neuronal epithelial cells
adeuqate stimulus
form of energy to which a receptor cell is most responsive, but many receptors also respond to other forms of energy
perceptual threshold
weakest stimulus that will cause a conscious perception in an organism
receptor threshold
weakest stimulus that will cause a response in the receptor
what allows secondary and higher neurons to combine data from many receptors
convergence
stimulus modality
what stimulus is it? light sound, etc
population coding of intensity
Stronger stimuli may activate more neurons.
frequency coding
Stronger stimuli may make individual neurons fire at a faster rate
what sensory systems indicate modality by
labeled lines
labeled lines
modality is revealed by which axons carry a signal
phasic cells
respond briefly to any cells and then cease firing
phasic-tonic cells
react to change but dont return all th way to sero firing when the stimulus is constant, so they also carry information about its steady level
tonic cells
maintain their activity when the stimulus is not changing, signalling its present level
temporal changes
changes through time
what cells are phasic
many retinal cells are phasic
contrast
location where there is strong contrast are called edges
where do most sensory pathways run
via thalamus to cortex
exception: olfactory pathways don’t project via the thalamus
equilibrium pathways project mainly to cerebellum
spatial changes
differences between neighbouring regions in space; also called contrast
why is sensory processing inference?
sense data are incomplete and ambiguous so the brain has to infer
lateral inhibition
cells inhibit their neighbours or they inhibit the cells their neighbours excite
how do sensory system accentuate edges
lateral inhibition
what suspends the lens
ligaments called zonules
lens
trasnparent disc that focuses light
2 chambers in eye
anterior chamber (front of lens) and vitreous chamber (behind lens)
vitreous chamber
filled with vitreous body (clear jelly) that helps maintain the eyeballs shape
anterior chamber
filled with aqueous humour (plasma like fluid)
what does light enter the eye through
cornea
cornea
transparent bulge at the front of the eye, continuous with the white of the eye (sclera)
where do cornea and lens focus light
retina
retina
inner lining of the eye that contains photoreceptors
when and how does the pupil constrict
in bright light, parasympathetic signals from the brain contract the ring-shaped pupillary constrictor muscles, shrinking the pupils
how does light pass from cornea to lens
through a hole in the iris called a pupil
when and how does the pupil dilate
In the dark, sympathetic signals contract the radial pupillary dilator muscle of the iris, dilating the pupil
how does the pupil control the depth of field
when the pupil is tightly constricted we have full depth of field, when pupil is dilated, we have a shallow depth of field
what is light refracted by
cornea (2/3rd) and lens (1/3rd)
what is lens made of
it is a mesh of long cell without nuclei, packed with clear proteins called ccrystallins and ‘zippered’ together in concentric layers for flexibility. it has no blood supply but abrsorbs nutrients from aqueous humor.
shape of lens
convex - thick in the middle and thinner at edges
what does the convex shape of lens do
makes light rays converge to a focal point
concave lenses
thinner in the middle and fatter at the edges
what does refraction depend on
angle of incidence
when does a light ray not bend at all
when it strikes the lens at right angles
what does a rounder lens do
bends light more, so has a closer focal point
for clear vision where must the focal point fall
retina
what happens if an object draws closer but lens stays flat
focus falls behind the retina
how do we make a lens rounder
parasympathetic nerve signals contract the ring-shapes, smoothe ciliary muscle, reducing tension in the zonules, making the lens rounder, so light rays bend more and the focal point moves forward.
how do we make a lens flatter
sympathetic signals relac the ciliary muscle, making the lens flatter for far vision
accomodation
rounding the lens for near vision
near point of accomodation
closest point a person can focus
myopia; how is it solved?
(near-sightedness) the focal point falls in front of the retina; solved by concave lens in front of the eye
hyperopia; how it solved?
(far-sightedness) the focal point falls behing the retina ; solved by convex lens in front of eye
presbyopia
with advancing age the lens stiffens, hindering accommodation
where are photoreceptors?
retina
phototransduction
conversion of light energy into electrical energy
2 main types of photoreceptors
cones (6 mil) rods (120 mil)
how do rods and cones respond to stimuli?
they are neurons, though they do not fire action potentials, but instead respond to stimuli with graded membrane potentials
basic structure of rods and cones (outer segment)
membrane folds into disc-like layers which contain visual pigments that respond to light
basic structure of rods and cones (inner segment)
there are nucleus and organelles for protein synthesis; and in a basal layer, a synapse that releases glutamate
where do both types of receptors point towards
back of the eye
what happens to photoreceptors in light
when light hits them, pigment molecules change shape, starting a chemical cascade that hyperpolarizes the cell, reducing its release of glutamate
what happens to photoreceptors in dark
get depolarizedd and release more glutamate
type of pigment in rods
rhodopsin
types of pigments in cones
3 types of pigments in 3 types of cones
where are photoreceptors most densely packed
macula, a central disk and especially in its central pit called the fovea
what do we use the fovea for
detailed vision - 5 degreed away from its center acuity is quartered, at 20 degrees it falls below thee standard for legal blindness
where are there no receptors
blind spot, it is the hole where axons carrying visual information exit the eyeball to form the optic nerve
what happens when lights go dim
rods dark adapt
What are rods responsible for
they can detect single photons. but the operate only in low light: in daylight the are bleached out, their rhodopsin is broken down so they cant sense light
what are cones reponsible for
right light
they are less sensitive than rods; responsible for vision in bright light and for distinguishing colours, but they don’t operate in dim conditions
distribution of rods and cones
fovea contains most exclusively cones. more peripheral retina contains mainly rods
where is convergence the greatest
peripheral retina
layers of neurons in retine
photoreceptors synapse onto bipolar cells which synapse onto ganglion cells
where is convergence the least
fovea
receptive field
also called visual field, region of the retina where light affects the cell’s activity
what type of receptive fields do bipolar cells have
center-surround fields
types of center surround receptive field in bipolar cells
on center or off center
on center cells
excited by light in the center of their field and inhibited by light in the surround. so these cells respond most when a light spot fills their center and the surround is dark.
off centre cells
inhibited by light in the centre, and excited by light in the surround. they respond best when a dark spot fills their center and the surround is light.
when do bipolar cells react
contrast
they respond with graded potentials and do not fire action potentials
how do ganglion cells react
they fire action potentials
similarity between ganglion cells and bipolar cells
they have on-center and off-center surround receptive field and detect contrast
where does a ganglion cell farther out get input from
many receptors, mostly rods
very sensitive to light but poor at reporting spatial detail as it blends information from a wide swathe of receptors
where does a ganglion cell near the fovea get iinput from?
few photoreceptrs and mostly cones
less sensitive to light but have =e better spatial resolution
types of ganglion cells
M cells, P cells, melanopsin ganglion cells
M cells
Large, magnocellular ganglion cells; provide inforamtion that is used by the brain to infer the movement of objects
10%
P cells
small, parvocellular gaanglion cells
provide infoamrtion that is used to infer form and fine detail
70%
melanopsin ganglion cells
photoreceptors with own visual pigment (melanopsin)
they project to the suprachiasmatic nucleus, a center for circadian rhythms
how does visual information leave retinas
in optic nerves/cranial nerve II
where do half the optic nerve fibers cross
optic chiasm
which fibers optic nerve cross
fibers from the nasal half of each retina cross; those from the temporal retinas do not
why do optic fibers cross
in the eye, the right side of the scene projects on the left side of the retina; i.e. on the nasal side of the right retina and temporal side of the left retina
how does visual info move
information moved from the chiasm to thalamus and then to cortex
what are nerve bundles emerging from the chiasm called
optic tracts
what do optic tracts end in
2 lateral geniculate nuclei in the thalamus, which project via the optic radiation to primary visual cortex, V1
where is V1 located
occipital lobe
how are many visual area in brain organized
retinotopicallu; neurons close to each other in brain get information from close together parts of the retina
this arrangement is found in lateral geniculate nuclei, V1 and others higher visual processing areas
explain how the retinotopic map does not preserve areas
the fovea, which cover only a small part of the retina projects to large areas in the V1
