E1 Stimulus and response

E.1.1 Define the terms stimulus, response and reflex in the context of animal behavior

A stimulus is a change in the environment (internal or external) that is detected by a receptor and elicits a response. A reflex is a rapid, unconscious response

E.1.2 Explain the role of receptors, sensory neurons, relay neurons, motor neurons, synapses and effectors in the response of animals to stimuli.

see notes from 6.5 and from textbook

Aim 7: Data logging using an EKG sensor to analyze neuromuscular reflexes could be used

E.1.3 Draw and label a diagram of a reflex arc for a pain withdrawal reflex, including the spinal cord and its spinal nerves, the receptor cell, sensory neuron, relay neuron, motor neuron and effector. Include white and grey matter, and ventral and dorsal roots.

Use the diagram above and the textbook to compete a diagram with all of the structures.

E.1.4 Explain how animal responses can be affected by natural selection, using two examples

Two examples.

The bird Sylvia   atricapilla (blackcap) breeds during the summer in Germany and, until recently, migrated to Spain or other Mediterranean areas for winter. However, studies show that 10% of blackcaps now migrate to the UK instead. To test whether this change is genetically determined or not (and, therefore, whether it could have developed by natural selection or not), eggs were collected from parents who had migrated to the UK in the previous winter and from parents who had migrated to Spain. The young were reared and the direction in which they set off, when the time for migration came was recorded. Birds whose parents had migrated to the UK tended to fly west, wherever they had been reared, and birds whose parents had migrated to Spain tended to fly south-west. Despite not being able to follow their parents at the time of migration, all the birds tended to fly in the direction that would take them on the same migration route as their parents. This and other evidence suggests that blackcaps are genetically programmed to respond to stimuli when they migrate so that they fly in a particular direction. The increase in the numbers of blackcaps migrating to the UK for the winter may be due to warmer winters and greater survival rates in the UK.TOK: There are many poor examples of supposed links between animal responses and natural selection. It is easy for us to guess how the behavior of an animal might influence its chance of survival and reproduction, but experimental evidence from carefully controlled trials is always needed to back up our intuitions

E2 Perception of stimuli

E.2.1 Outline the diversity of stimuli that can be detected by human sensory receptors, including mechanoreceptors, chemoreceptors, thermoreceptors and photoreceptors. Details of how each receptor functions are not requiredTOK: Other organisms can detect stimuli that humans cannot. For example, some pollinators can detect electromagnetic radiation in the non-visible range. As a consequence, they might perceive a flower as patterned when we perceive it as plain. To what extent, therefore, is what we perceive merely a construction of reality? To what extent are we dependent upon technology to “know” the biological world? Sensory receptors are classified as followsChemoreceptors Electroreceptors MechanoreceptorsPhotoreceptors Thermoreceptors

E.2.3 Describe what is meant by each of the term in E.2.2 with reference to one named example of each type of receptor.

ChemoreceptorsChemoreceptors have special proteins in their membranes that bind to a particular substance, causing a depolarization of the axon, and sending an action potential. Chemoreceptors are involved with sense of smell, taste, and blood pH. ElectroreceptorsMuscle contractions generate electrical fields that are conducted through water. Electroreceptors can sense these fields. Sharks have electroreceptors that can detect muscles contractions. Air is a poor conductor, so terrestrial organism do not use this kind of sensorMechanoreceptorsMechanoreceptors are sensitive to movement. The lateral line in fish detects vibrations in the environment. Our inner ear detects our body’s position and movement with three semicircular canals in the inner ear. These are connected to a system of hair cells. A change in the speed or direction of the body will move fluids in one of the canals bending the hairs and causing action potential to be sent to the brainPhotoreceptorsRods and cone cells in our retina contain photo pigments. Rhodopsin, is the photopigment found in rods; and iodopsins (there are 3 types) are found in cones. These pigments are broken down when exposed to light, causing an action potential to be sent to the brain.Thermoreceptors Thermoreceptors are found in the skin. Cool receptors send an action potential when the temperature drops. Warm receptors located deeper in the skin; send action potential when the temperature increases. The temperature center in the hypothalamus also contains thermoreceptors to regulate blood temperatureFill in the chart

Sensory receptor Where it is found; examples What is the function

E.2.2 Draw and label a diagram of the structure of the human eyeThe diagram should include the sclera, cornea, conjunctiva, eyelid, choroid, aqueous humor, pupil, lens, iris, vitreous humor, retina, fovea, optic nerve and blind spot

E.2.3 Annotate (i.e. explain what is happening) a diagram of the retina to show the cell types and the direction in which light movesInclude names of rod and cone cells, bipolar neurons and ganglion cells.

E.2.4 Compare rod and cone cellsInclude: 1) use in dim light versus bright light, 2) one type sensitive to all visible wavelengths versus three types sensitive to red, blue and green light 3) passage of impulses from a group of rod cells to a single nerve fiber in the optic nerve versus passage from a

rods conesdim light versus bright light

sensitivity to visible wavelengths

number of cells per neuron

single cone cell to a single nerve fiber.

E.2.5 Explain the processing of visual stimuli, including edge enhancement and contralateral processingEdge enhancement occurs within the retina and can be demonstrated with the Hermann grid illusion.Contralateral processing is due to the optic chiasma, where the right brain processes information from the left visual field and vice versa. This can be illustrated by the abnormal perceptions of patients with brain lesions

Hermann grid illusion

E.2.6 Make and label a diagram of the earInclude pinna, eardrum, bones of the middle ear, oval window, round window, semicircular canals, auditory

nerve and cochlea

E.2.7 Explain how sound is perceived by the ear, including the roles of the eardrum, bones of the middle ear, oval and round windows, and the hair cells of the cochleaThe roles of the other parts of the ear are not expected

E.2.2 Label a diagram of the structure of the human eye. ( Repeated )

E.2.3 Annotate a diagram of the retina to show the cell types and the direction in which light moves.

The retina is the only part of the CNS which is directly observable (ganglion cells). Light is coming through the eye from the right.

There are three layers of neurones shown, photoreceptors, bipolar and ganglion cells which also reflects the order of activity.

The ganglion cells and bipolar cells are transparent and do not significantly reduce the intensity of light passing to the photo receptor.

The photoreceptor absorbs the light which changes the rate of neurotransmitter produced at the first synapse (S1)

The head of the photoreceptor cell contains the light sensitive pigments.

The Bipolar cell (named after its two processes at either side of the cell body) responds by changing rate of neurotransmitter released to the Ganglion cell.

The ganglion cell generates the impulse which will travel along the axon of the ganglion to the brain. Notice that that these axons are group together to form the optic nerve. Also note that the cell body of the ganglion cell is in the in the retina.

(a)Bipolar cell forming synapses with more than one photoreceptor. b) Bipolar cells connecting together rods and cones (c)Ganglion cell collecting input from a group of photoreceptors (receptive field) (d)Axon of the ganglion cell forming the optic nerve at (i) (e)Summation of rod photoreceptors gives low visual acuity (resolution) Non-fovea arrangement (f)Another form of summation (G)The arrangement of cones in the fovea provides a high level of visual acuity (resolution) (h). The impulse traveling along the axon of the ganglion neurone can therefore be mapped to a precise region of the retina (i) Axons of the ganglion converge together to form the nerve fibres of the optic nerve

E.2.4 Compare rod and cone cells.

Consequences of the information:

Rods are used when light levels are low (dim) but they cannot produce colour vision in such conditions. The detail provided by rods is less than the cones because of the ratio of rods to ganglion neurones.

Cones allow colour vision in humans but this is only functional when the light intensity is high.

 

E.2.5 Explain the processing of visual stimuli, including edge enhancement and contralateral processing.

This section of the syllabus relates to how stimuli to the eye are processed.

Edge enhancement is a ‘pre- central nervous system ‘processing of information on the retina itself. This processing is not carried out by part of the brain but by the organisation of the retinal cells.

Contralateral processing is the way in which the brain collects and integrates information from the eyes to create the perception of seeing. Both these processes require a more detailed knowledge of the retina and brain. It should also be noted that this biology is still the subject of much research and the ideas presented are hypothetical.

Edge Enhancement:

Stare at the square; you might notice that there is a kind of ‘white glow’ around the outside.

This glow is called edge enhancement and it results from retinal processing of information.

The purpose is to provide a greater contrast at the edges of objects.

Such ‘edge awareness' provides more detail to the visual system of the environment.

 

Lateral Inhibition and Edge enhancement

A key to understanding the concept of edge enhancement is to consider the idea of lateral inhibition:

When stimulated, rod or cones may pass information to the bipolar cells and then the ganglion. The depolarisation of the ganglion cell generates neural activity in the optic nerve. However if the rod or cone cell stimulates the horizontal cell this can inhibit other photoreceptors and their ganglion cells which are further away.

The photoreceptor (rod or cone is at the top)

The bipolar neurone in black One horizontal cell is shown

One ganglion cell

Notice that there is a one to one mapping of photoreceptor to ganglion, with this photoreceptor comes from the fovea.

   

Above is a schematic diagram of the fovea, each photoreceptor is being stimulated by incoming light (yellow) to pass information to its bipolar neurone and in turn to the ganglion.

However, the horizontal cells will partially inhibit the adjacent photoreceptor.

A is stimulated by bright light as is photoreceptor B, C, D, and all other photoreceptors in the image.

However the stimulated photoreceptor also stimulates the the horizontal cells which inhibit the output of adjacent photoreceptors.

Out put across the retina would be the same.

The next model takes the idea above and applies it to 'edge' when we stare at the edge of the black square on a light back ground.

the illusion of the Hermann grid. Such illusions have provided a great deal of insight into how the eye and brain work together. The main interest of the grid are the light grey square at the intersections. Note that if you look at them directly then they disappear. How are is this illusion explained? 

Lateral inhibition and edge enhancement are thought perhaps to be part of the process that causes this. When we look at one of the squares of the grid the spaces between (white lines) seem very bright. In this area the receptive fields of the fovea with their high visual acuity (resolution) are causing edge enhancement. However just out of the focus of our attention (non fovea) in the peripheral fields we are aware of the ‘grey squares’. The suggestion is that in these regions of peripheral vision:

The fields have lower resolution Stimulation of the ganglion cell is a consequence of

summation.

Contralateral Processing

Contralateral processing is the way in which the brain collects and integrates information from the eyes to create the perception of seeing.

 Stimuli from the left visual field enter both eyes.

The stimulated receptive fields of the left and right eye stimulate the ganglion cells which provide neural activity along the optic nerve.

Look closely, the left visual field information from ganglion in BOTH eyes goes to the right side of the brain.

Equally for the right visual field object, the information ends up in the left side of the brain.

The cross over point for optic nerves is the Optic Chiasm.

The optic nerves for synapses in the lateral geniculate nucleus with neurones from the primary visual cortex.

The brain is able to integrate the 2 D information of the retina  back into a 3 D perception of the ‘real world’.