Sensory Systems

Question 1

What is the purpose of a sensory system?

Answer 1

Senses the nature of both the internal and external environment, allowing us to make appropriate responses to both.

Question 2

What is the main sensor of the internal environment?

Answer 2

The viscera which are mainly innervated by the vagus nerve (wandering nerve as it goes all over the intestines)

Question 3

What is the sensor of the external environment?

Answer 3

Sensed through somatosensation, through skin receptors, and proprioception which is the sense of where your limbs are in space and how they’re moving

Question 4

What is a sensory receptor?

Answer 4

A specialised neuronal structure sensitive to a particular form of energy (a modality)

Question 5

What is the law of specific nerve energies?

Answer 5

Receptors are (usually) specific to a particular modality, except for nociceptors which are often polymodal

Question 6

What is an adequate stimulus?

Answer 6

The modality to which a receptor responds best

Question 7

What is a receptor potential?

Answer 7

These are graded and so can be weak or strong. Sensory units (a receptor and afferent neuron) give rise to both generator and action potentials

Question 8

What is tonotopic mapping?

Answer 8

Occurs in the auditory cortex. Lower pitches are detected at the anterior and higher pitches at the posterior. Cochlear implants involve microphones being inserted which is able to extract different tones and direct them to the correct pitch destination

Question 9

What is a photoreceptor and photopigment?

Answer 9

A photoreceptor is a cell specialised for light detection. A photopigment is the protein and light absorbing cofactor

Question 10

Describe a “simple eye”

Answer 10

They have pigmented pits that limit the range of directions from which light can reach the photoreceptor through shielding. Acts as a form of phototaxis

Question 11

Describe the “insect compound eye”

Answer 11

Has a photoreceptor in each ommatidia. They have a crystalline structure lens and a cornea that regulates life. Each ommatidia responds to an individual area in space. Gives spatial awareness. Acuity decreases with distance

Question 12

What is acuity?

Answer 12

The capacity for seeing distinctly the details of an object. High acuity requires an array of pixels, each receiving light from a restricted range in the visual space. In the compound eye, one ommatidium is one pixel.

Question 13

Describe a mirror eye

Answer 13

Occurs in the scallop. Contains 60-100 small 1mm eyes. Forms a concave mirror focussing the image onto an array of photoreceptors. Acuity is limited as when the objects are close they cannot be seen as they just get reflected back

Question 14

Describe a lens eye

Answer 14

very high acuity. Each photoreceptor recieves light from a different point in the visual space making it its own pixel. Uses refraction, not reflection to focus the light onto the retina

Question 15

What is refraction?

Answer 15

Focusing the divergent rays from a point in space into a single point on the retinal surface. Degree of refraction depends on the distance form the object. Adjustment is originated in the lens

Question 16

Describe the lens’ response to distant objects

Answer 16

Light rays are near parallel so require little refraction to focus. Lens remains thin. Cilary muscles are relaxed and suspensory ligaments are taught, pulling the lens thin

Question 17

Describe the lens’ response to close objects

Answer 17

Light rays that are diverging require refraction to focus. Think lenses have a higher refractive power. Cilary muscles are contracted, suspensory ligaments are loose and the lens is small and thick

Question 18

Describe accommadation

Answer 18

Changing from distant to close vision. Cilary muscles contract, lens thickens and the refractive power increases. Pupils constrict

Question 19

Describe myopia

Answer 19

Short sightedness so distant objects are blurred. Eyes are too deep and the refractive power is too great. Requires a diverging (concave) lens

Question 20

Describe hyperopia

Answer 20

Long sightedness as the eyes are too shallow. Refractive power of the cornea is too small. Need a converging (convex) lends to compensate.

Question 21

Describe vertebrate photoreceptors

Answer 21

Have lots of plasma membranes in their outer segments as photopigments are membrane associated. The photopigment protein is opsin.

Question 22

Describe how retinaldehyde alters in the presence of light

Answer 22

In opsin photopigments, light absorption relies on an organic cofactor retinaldeyhyde. B carotene is converted to all-cis retinol which is converted to 11 cis retinaldehyde. When light is absorbed this is converted to all trans retinaldehyde

Question 23

What is an opsin protein?

Answer 23

7 TMD GPCRs that bind to retinaldehyde. They translate isomerisation of retinal into a biological signal. They also determine the wavelengths the retinaldehyde absorb.

Question 24

Describe the photoreceptors’ cellular response

Answer 24

When in the dark, sodium enters the cones but when in the light sodium channels close allowing hyperpolarisation. Photoreceptors respond to light with graded hyperpolarisation. This results in a reduction in glutamate release in their synaptic terminals

Question 25

What is a retinal ganglion cell?

Answer 25

They form a single layer and recieve input from multiple bipolar cells. They don’t contribute to the visual image but detect light intensity. They generate action potentials to be sent to the optic nerve. Each RGC has its own receptive field.

Question 26

Describe a cone receptor response

Answer 26

These hyperpolarise to light, inhibiting glutamte release. Glutamate is always released, until there is light. When the release is stopped the bipolar cell depolarises and the signal is sent

Question 27

Describe the purpose of a horizontal cell

Answer 27

Involved in centre-surround inhibition. When hyperpolarised they inhibit the signal transfer between cones and bipolar cells. The organisation is important as they allow the boring parts of the visual scene to be ignored. Horizontal cells provide an antagonistic surround

Question 28

Describe off cells

Answer 28

These depolarise in the presence of glutamate and hyperpolarise to light. Presence of glutamate opens cation channels. They are excited when light intensity in the centre of the RF is lower than the surround

Question 29

Describe on cells

Answer 29

These hyperpolarise in the presence of glutamate and depolarise to light. They respond to glutamate via a metabotropic GPCR where the presence of glutamate causes the closure of the cation channels. They are excited when light intensity in the centre of the RF is higher than the surround

Question 30

How does the firing pattern of ganglion cells encode visual information?

Answer 30

Spatial contrast in grey scales due to centre:surround info. Movement in a particular direction (which is supported by amacrine cells). Looming and brightness information

Question 31

What are the different colour cones and what wavelengths do they maximally respond to?

Answer 31

Blue - short - 433 nm
Green - middle - 533 nm
Red - long - 564 nm

Question 32

What is the role of opsin proteins?

Answer 32

They determine spectral sensitivity due to light dependent coupling to transducin allowing isomerism of retinaldehyde.
The amino acids surrounding the opsin proteins determine the shift from UV wavelengths. This means your genome determines the code for where the peak of your sensitivity

Question 33

Why are men more likely to suffer from colour blindness?

Answer 33

LWS and MWS opsin proteins are x linked

Question 34

What is the average number of cone opsin genes in mammals? What is different for mice? How many do chickens have?

Answer 34

Most mammals have 2. Mice have one green and one UV. Chickens have 4, with one UV cone.

Question 35

Describe differences in the number of cone opsin genes in fish?

Answer 35

Fish at the top of the surface have all 4 as they are exposed to all wavelengths of the sun. Fish at the bottom have 2 which are both sensitive to bluish pigment

Question 36

Describe the evolutionary history of cone opsin genes

Answer 36

Outside mammals all branches of the evolutionary tree had 4. Our common ancestor had 2 (nocturnal) and we then re-evolved a 3rd. In new world monkeys the 3rd opsin is on the x chromosome and became polymorphic, some therefore have both versions. All males are colourblind.

Question 37

How does information of the visual field get passed to the thalamus?

Answer 37

The superior part corresponds directly whereas the bottom part cross over at the optic chiasm. Visual information is passed form the optic nerve to the optic chiasm, then to the LGN (in the thalamus) and then to the primary visual cortex.

Question 38

What is the thalamus?

Answer 38

Part of the diencephalon. Essential link in the transfer of sense info to the rest of the cortex (apart from in smell). It integrates info from the cerebellum and the basal ganglia. Is involved in sleep/wake attention. Has 50 sub nuclei.

Question 39

What are the major groups of specific/relay nuclei in the thalamus?

Answer 39

Anterior, medial and ventral - involved in somatosensory and motor systems
Posterior - contains the medial (auditory) and lateral (vision) geniculate nuclei and the PULVINAR which is most heavily developed in humans.

Question 40

What is the thalamic reticular nucleus?

Answer 40

Surrounds the thalamus. Creates thin sheets of cells above the thalamus. Covers the whole of the thalamus.

Question 41

What is the perigeniculate nucleus?

Answer 41

Talks to the LGN. All cell are GABAergic