A3 Perception of Stimuli Flashcards
What do sensory receptors do?
Receptors detect changes in the environment. The environment, particularly its changes, stimulate the nervous system via sensory receptors. The nerve endings of sensory neurones act as receptors for example, touch receptors. In other cases there are specialised receptor cells that pass impulses to sensory neurones, as with the light-sensitive rod and cone cells of the eye.
What types of specialised receptors do humans have?
- Mechanoreceptors respond to mechanical forces and movements.
- Chemoreceptors respond to chemical substances.
- Thermoreceptors respond to heat.
- Photoreceptors respond to light.
What are olfactory receptors?
Olfaction is the sense of smell. Olfactory receptor cells are located in the epithelium inside the upper part of the nose. These cells have cilia which project into the air in the nose. Their membrane contains odourant receptor molecules, proteins which detect chemicals in the air. Only volatile chemicals can be smelled in the air within the nose. Odourants from food in the mouth can pass through mouth and nasal cavities to reach the nasal epithelium.
There are many different odourant receptor proteins, each encoded by a different gene. In some mammals such as mice there are over a thousand different odour receptors, each which detects a different chemical or group of chemicals. Each olfactory receptor cell has just one type of odourant receptor in its membrane, but there are many receptor cells with each type of odourant receptor, distributed through the nasal epithelium. Using these receptor cells most animals, including mammals, can distinguish a large number of chemicals in the air or in water in the case of aquatic animals. In many cases the chemical can be detected in extremely low concentrations but the human sense of smell is very insensitive and imprecise compared to that of other animals.
Label a structure of the eye?
THIS TILL NEEDS TO BE DONE! USE PAGE 527
What are rods and cones?
Rods and cones are photoreceptors located in the retina.
How does an image form in the eye?
Light entering the eye is focused by the cornea and the lens onto the retina, the thin layer of light-sensitive tissue at the back of the eye. Two main types of photoreceptor are present in the human retina, rods and cones. Many nocturnal mammals have only rods and cannot distinguish colours. Rods and cones are stimulated by light and so together detect the image focused on the retina and convert it into neural signals.
What is the difference between rod cells and cone cells?
They are both photoreceptors on the retina that help us convert light into neural signals. However they differ in their sensitivities to light intensities and wavelengths.
Rods are very sensitive to light, so work well in dim light. In very bright light the pigment in them is temporarily bleached so for a few seconds they do not work. Rod cells absorb a wide range of visible wavelengths of light but cannot respond selectively to different colours, so they give us black and white vision.
There are three types of cone, which absorb different ranges of wavelengths of light. They are named according to the colour that they absorb most strongly; red, blue or green. When light reaches the retina, the red, green and blue cones are selectively stimulated. By analysing the relative stimulation of each of the three cone types, the colour of light can be very precisely determined, though experiments show that people’s perception of colour differs quite a lot. Cones are only stimulated in bright light and therefore colour vision fades in dim light.
If it is very bright which photoreceptors are you using?
Cone cells work very well in bright light, the red, green and blue photoreceptor cone cells are stimulated in bright light, they are selectively stimulated and by analysing the relation stimulation of each one the colours around you can be very precisely determined.
Rod cells tend, in bright light, to have their pigments temporarily bleached and do not work for a few seconds. They work in dim lights and give us black and white vision.
Why are some people colour blind?
Red-green colour-blindness is a common inherited condition in humans and some other mammals. It is due to the absence of, or defect in, the 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. The normal alleles of both genes are dominant and the alleles that cause the red-green colour-blindness are recessive. Red-green colour blindness is therefore much more common in males who only have one X chromosome, that females, and they inherit the condition from their mother.
Label the structure of the retina?
THIS STILL NEEDS TO BE DONE PAGE 529
What are bipolar cells?
Bipolar cells send the impulses from rods and cones to ganglion cells.
Rod and cone cells synapse with neurons called bipolar cells in the retina. If rod or cone cells are not stimulated by light they depolarise and release an inhibitory neurotransmitter onto a bipolar cell, causing it to become hyperpolarised and not transmit impulses to its associated retinal ganglion cell. When light is absorbed by a rod cell or a cone cell it becomes hyperpolarised and stops sending inhibitory neurotransmitter to the bipolar cell and so the bipolar cell can depolarise and send an impulse to the ganglion cells.
Groups of rod cells send signals to the brain via a single bipolar cell, so the brain cannot distinguish which rod absorbed the light. The images transmitted to the brain by rods alone are lower resolution, like a grainy photograph, whereas those based on the cones are sharper because each cone cell sends signals to the brain via its own bipolar cell.
Which are sharper cone cells or rod cells and why?
Cone cells produce a much sharper image because each cone cell sends signals to the brain via one bipolar cell. Each cone cell has one bipolar cell and so the brain can tell exactly which cone cell detected the light. Rod cells however are grouped and a whole group uses the same bipolar cell and therefore the brain cannot tell which rod cell detected the light, this produces a much low resolution image.
How do rod and cone cells get their impulses to the brain?
They are synapses to bipolar cells which carry the impulses to the brain.
When no light is absorbed by a cone or rod cell they become depolarised and send an inhibitory neurotransmitter to the bipolar cells causing the bipolar cells to hyperpolarise meaning that it does not reach the threshold potential to release an impulse to the ganglion cells.
When light is absorbed the rod and cone cells themselves become hyperpolarised and so do not reach the threshold potential to send the inhibitory neurotransmitter to the bipolar cell. The bipolar cell can then depolarise and send impulses to the ganglion cells.
This is perhaps the opposite than you might think, the automatic is to send inhibitory, and then when light is absorbed it hyperpolarises the cell so it cannot send an inhibitory neurotransmitter to the bipolar cell to hyperpolarise it. So it then is depolarised and does reach the threshold potential, activating a ganglion cell.
What do the ganglion cells do?
The 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 are therefore able to receive impulses from the rod and cone cells.
Ganglion cells also have long axons along which impulses pass to the brain. Impulses are passed at a low frequency when the ganglion cell is not being stimulated and at an increased rate in response to stimuli from bipolar cells.
The axons of ganglion cells pass across the front of the retina to form a central bundle at the ‘blind spot’, so called because their presence makes a gap in the layer of rods and cones. The axons of the ganglion cells pass via the optic nerve to the optic chiasma in the brain.
How do impulses get from the rod cells to the brain?
The impulses go from the rod cells to the bipolar cells to the ganglion cells which the axons of which pass via the optic nerve into the optic chiasma in the brain?
What is the blind spot?
The blind spot is where the axons of the ganglion cells pass across the front of the retina and form a central bundle. It the part of the retina where they pass through to the optic nerve which leads into the optic chiasma in the brain.
