Chapter 6 Part 2 Flashcards
How does light reach the retina ?
It passes through the cornea, the aqueous chamber, the lens and the vitreous chamber to reach the retina.
What is the function of the cornea and the lens ?
They help to focus the image on the retina.
Describe the retina
Human retina is like a deep cup. It is 3/4rths of a sphere with a dia of 22mms. There are 7 million cone cells and 7 00 million rod cells in the retina.
What is the macula
The center of the retina is called the macula.
What is the function of the rod cells and the cone cells
Cone cells - High acuity vision and colour
Rod cells - Sensitive to very dim light and quick moving objects
What is the blind spot ?
Just medial to the macula, there is an area where the axons of the retinal ganglion leaves the eyeball. This is the optic nerve. Since there are no photoreceptors in this area, it is called the blindspot. It is 1.5mm to 2 mm.
Does all mammals have macula
There is no distinct macula in all mammals except simian primates, instead many mammals have a horizontal concentration of retinal cells called the visual streak. This is useful for scanning horizon to detect the predators.
Describe the colour channels in different animals.
Humans and related primates are trichromats - meaning they respond to 3 colurs RBG. Dogs and mice are dichromats meaning they are sensitive to two colours and can see a range of colours limited by the pigments.
Lampreys need to detect changes in water composition in order to breed therefore has 5 colour pigments . This means they need a 5 colour element television to look convincing.
Describe the retinal cell layers
3 main cell layers of retina
Photoreceptor layer - Deepest layer. Furthest from the entry part of the light rays. Contains rods and cones. The light sensitive processes of the rods are partly embedded in the retinal pigmental epithelial layer. The exchange of retinoids between the RPE and photoreceptor is necessary for transduction.
Bipolar cell layer - Second layer. Contains bipolar cells, amacrine and horizontal cells. The role of bipolar cells is to receive information from the photoreceptors and send signals to the ganglion cells. Role of the amacrine and horizontal cells is to modulate the signals.
Ganglion cell layer - contain large ganglion cells that transmit information from the bipolar cells to the lateral geniculate nucleus of the thalamus , the suprachiasmatic nucleus of the hypothalamus and the superior colliculus of the midbrain via the optic tract.
What is the difference between signal processing in the macula and the edges of retina ?
In the edges of the retina the image is distorted due to the bipolar cells receiving signals from hundred or more rod cells. This is to increase sensitivity from dim light The macula, however, is specialised to receive signal from two or three cones, increasing our capacity to resolve fine detail.
What are opsins?
Opsins are photopigment molecules which captures the light striking the rods and the cones and converts them to synaptic signals. This process is called transduction
How is transduction different to other sensory systems?
Transduction causes closure of sodium channels in the photoreceptors causing hyperpolarisation of the membrane and reducing the amount of neurotransmitter released, whereas in other sensory systems, the neurons depolarised and increase the amount of neurotransmitter released.
How does the bipolar cells respond to the neurotransmitter interupptions?
Half of the bipolar cell hyperpolarize decreasing the release of glutamate and the other half, invert the signal to depolarise and increase transmitter release. This is referred as on and off responses. These signals are further received by ganglion cells generating AP, which travels along the optic nerve, sending the information to the brain.
How is the information processed in the temporal(lateral and nasal(medial) retinal?
Axons from the ganglion cells in the nasal retina crossover in the optic chiasm, sending information to the opposite side of the brain. the axons from the temporal retina do not crossover. In humans, 50% of the retina is temporal, while in rodents only 10% of the retina is temporal. Therefore, 10% of the fibres remain uncrossed in most mammals.
What is cataract?
It is an expanding area of opacity that blocks useful vision in that eye. It is an entirely curable of blindness, which can be treated by replacing the affected lens.
What happens when the optic chiasm is damged?
Most commonly damaged by pituitary tumours. When completely damaged, the person has vision immediately in the front and lateral visual fields are lost.
Which part of the brain the visual field processed?
It is ipsilateral processing. Therefore, the right visual field is processed in the right hemisphere and the left visual field is processed in the left hemisphere.
What is binocular vision?
Humans have eyes in front of the head, and having overlapping visual fields. This binocular vision enables high degree of stereoscopic vision and is very precise.
Do humans have more than one cortical area that analyses vision?
They have primary visual cortex that analyse visual data. There are surrounding cortical areas that make meaning of the data for different functions such as face recognition and the circadian rhythm. Birds have detailed subsystems for detecting prey, looking at the food in the beak and for collision avoidance.
Where is facial recognition area located?
It is located , along the fusiform gyrus on the medial side of the temporal lobe. It is close to area 17, the primary visual cortex.
What is the pinna and why is it important?
It the folds of cartilage surrounding the ear canal. It serves to reflect and attenuate sound. It also modifies sound when we turn our heads in a was the brain can determine the direction it came from.
Why is the auditory canal important?
It helps to amplify the sound.
How is the sound transmitted to the inner ear?
The tympanic membrane vibrates and causes the tiny bones of the malleus, incus and stapes to vibrate and convert into high pressure sound vibrations in the fluid of the inner ear.
Describe the cochlea and its functions.
The cochlea is the auditory receptor organ. It has three fluid filled chambers: the Scala media, Scala vestibuli and Scala tympani. The latter two contain perilymph(similar to extra cellular fluid) and Scala media contains endolymph(similar to intracellular fluid). These scalae are twisted into three spiral staircases.
A strip of sensory epithelium in the floor of Scala media is the organ of corti. It has hair cells which transform fluid waves into electrical nerve impulses. The hairs are columnar and covered in cilia. The tectorial membrane lies on top of the cilia and moves forth and back each cycle of sound changing the membrane potential each time.
