Perception of stimuli Flashcards
Sensory receptors
Receptors detect changes in the environment. The nerve endings of sensory neurons act as receptors, ex: touch receptors.
There are also specialized receptor cells that pass impulses to sensory neurons, ex: the light-sensitive rod and cone cells of the eye.
Photoreceptors
Rods and cones are photoreceptors located in the retina.
Light entering the eye is focused by the cornea and the lens onto the retina.
Many nocturnal mammals have only rods and cannot distinguish colors.
Rods and cones are stimulated by light and so together detect the image focused on the retina and convert it into neural signals.
Rods
Rods are very sensitive to light, so work well in dim light. In very bright light the pigment in them is temporarily bleached so they don’t work for a few seconds. Rod cells absorb a wide range of visible wavelengths of light but cannot respond selectively to different colors so they give us black and white vision.
Cones
Three types of cone, named according to the color they best absorb: red, blue, green.
When light reaches the retina, the red, blue and green cones are selectively stimulated. Cones are only stimulated by bright light and therefore colour vision fades in dim light.
Red-green color-blindness
Common inhertied condition in humans + some mammals. It is a result of a defect / asence of a gene for photoreceptor pigments essential to either red or green cone cells.
Both genes are located on the human X chromosome, so it is a sex linked condition.
Normal alleles of both genes are dominant and the alleles that cause red-green color blindness are recessive.
Red-green color blindness is more common among males (only X chromosome) than females.
Males inherit the allele that causes the condition from their mother.
Bipolar cells
Bipolar cells send the impulses from rods and cones to ganglion cells.
When light is absorbed by a rode or cone cdell it becomes hyperpolarized and stopis sending inhibitory neurotransmitter to the bipolar cell. The bipolar cell can therefore depolarize, activiating the adjacent ganglion cell.
Images transmitted by only rodes = lower resolution; images transmitted based on cones = sharper b.c each cone cell sends signals to the brain via its own bipolar cell.
Ganglion cells
Ganglion cells send messages to the brain via the optic nerve.
Retinal ganglion cells have cell bodies in the retina with dendrites that form synapses with bipolar cells. They also have long axons by which impulses travel to the brain.
Impulses are passed at a low frequency when the ganglion cell is not being stimulated and at an increased frequency in response to stimuli from bipolar cells.
The axons of the ganglion cells pass across the front of the retina to form a central bundle and the “blind spot”.
The azons of the ganglion cells pass via the optic nerve to the optic chiasma in the brain.
Vision from the right and left fields
The information from the right field of vision from both eyes is sent to the left part of the visual cortex and vice versa.
The crossing over of axons between the left and right sides occurs in the optic chiasma.
The middle ear
Structures in the middle ear transmit and amplify sound.
Air filled chamber between the outer ear and inner ear.
A thin sheet of flexible tissue (eardrum) separates the iddle ear from the outer ear.
Two other thin sheets of tissue (oval and round windows) separate the middle ear from the inner ear.
3 tiny bones are in the middle ear (mallus, incus, stapes) which articulate w/ eachother to form a connection between the eardrum and the oval window.
These bones (ossicles) transmit vibrations from the eardrum to the oval window amplifying sound x20.
During very loud sounds the delicate sound-reception components of ear are protected by contraction of the muscles attached to the bones in the middle ear, weakening the connections between the ossicles and dampening the vibrations.
The cochlea
Sensory hairs of the cochlea detect sounds of specific wavelengths.
Part of the innear ear where vibrations are transduced into neural signals.
It is tubular, coiled, fluid-filled.
Within it are layers of tissue (membranes) to which sensory cells are attached.
Each of these cells has a byndle of hairs, stretching from one membrane to another.
When vibrations are transmitted from the oval window into the cochlea, they resonate with the hair bundles of partoicular hair cells, stimulating them.
We can detect pitch through selective activation of different hair cells.
The auditory nerve
Impulses caused by sound perception are transmitted to the brain via the auditory nerve.
When a hair cell in the cochlea is depolarized by the vibrations that consititue sound, it realeases neurtransmitter across a synapse stimulating an adjacent neuron.
This triggers an action potential in the sensory neuron which propagates to the brain along the auditory nerve.
Cochlear implants
If the hair cell in the cochlea is defecttive, hearing aids are not efficient for deafness. If the auditory nerve functions properly, a cochlear implant is needed.
Don’t fully restore normal hearing - improve it and usually allow speech recognition.
EXTERNAL parts are:
- a microphone to detect sounds,
- a speech processor that selects the frequencies used in speech and filters out other frequencies,
- a transmitter that send the processed sounds to the internal parts.
INTERNAL parts are implanted in the mastoid bone behind the ear. They consist of:
- a recceiver that picks up sound signals from the transmitter,
- a stimulator that converts these signals into electrical impulses
- an array of electrodes that carry these impulses to the cochlea.
- The electrodes stimulate the auditory nerve directly and bypass the non-functional hair cells.
Detecting head movements
Hair cells in the semicircular canals detect movment of the head.
There are three fluid-filled semicircular canals in the inner ear.
Each has a swelling at one end in which there is a group of sensory hair cells, with their hairs embedded in gel to form a structure called the cupula.
When the head moves in the plane of one of the semicircular canals, the still wall of the canal moves with the head.
Due to inertia the fluid inside the canal lags behind. There is therefore a flow of fluid past the cupula.
This is detected by the hair cells, which send impulses to the brain.
The 3 semicircular canals are at right angles to eachother, so each is a different plane. Therefore they can detect mvoement by the relative amount of stimulation of the hair cells in each of them.
