Sensory Systems

Front 1

Which sensory neurons of a moth are responsible for detecting bat calls?

Back 1

Auditory neurons housed in the ears of the moth

Front 2

Where on the moth's body are the ears located?

Back 2

In the abdomen - each ear opens to the external surface under the wing.

Front 3

Moth ear when bat produces call

Back 3

Vibration of tympanic membrane, stretching of dendrites of auditory membranes.

Front 4

Mechanoreceptors

Back 4

= sensitive to pressure and movement.
• Involved in senses of touch, hearing, and balance.
• Detect important body signals such as blood pressure.
• Sense of body position = proprioception – depends on mechanoreceptors.
• E.g., mechanoreceptors used for vertical determination in honey bees.

Front 5

Chemoreceptors

Back 5

= sensory receptors that detect chemical signals.
• Sense blood oxygen levels and pH.
• We use them to detect taste and smells.

Front 6

Thermoreceptors

Back 6

= detect heat and cold.
• E.g. pit organ of snakes.

Front 7

Electroreceptors and magnetoreceptors

Back 7

= sense electric and magnetic fields.
• Electroreceptors - Help shark locate their prey.

Front 8

Photoreceptors

Back 8

= detect light.
• Vision relies on this.

Front 9

Sensory transduction

Back 9

• Sensory receptors have voltage difference across their cell membranes.
• Called resting membrane potential.
• Inside of cell membrane = typically negative with respect to outside.
• Sensory receptors can alter their membrane potential in response to incoming signals.
• =Important for signal transduction and conveying info from sensory receptors in the periphery to integrating centres, such as the brain.

Front 10

Changes in membrane potential

Back 10

• Membranes of sensory receptor cells contain receptor proteins that are specialised to detect incoming sensory signals.
• Incoming stimuli causes a change in shape of the receptor proteins.
• Triggers a chain of events within the cell that leads to a change in membrane potential.
• Transduction pathways vary depending on sensory receptor type.

Front 11

Importance of change in membrane potential

Back 11

• Changes in membrane potential caused by the detection of an incoming stimulus generate signals that can be sent to integrating centres of the nervous system, such as the brain.
• Here the signals are interpreted so that the animal can respond to its environment appropriately.
• Changes in membrane potential belong to two main type:
1) Graded potentials that vary in amplitude depending on the size of the stimulus
• local event
• if large enough can give rise to action potentials
2) ‘Action potentials’ (also called ‘nerve impulses’) which travel along nerves and transfer info from one part of nervous system to another.

Front 12

Afferent neurons

Back 12

nerve cells that conducts sensory info from the periphery into the central nervous system.

• Within the central nervous system (including the brain), sensory info arriving from the periphery is integrated and interpreted, so that signals can be sent to muscles (or to glands of the body) to bring about appropriate movements (or physiological responses)

Front 13

Efferent neurons

Back 13

nerve cells that send signals from the central nervous system to muscles and glands.

Front 14

Sensory receptor cells

Back 14

• Some sensory receptors are modified nerve cells; these are called sensory neurons.
• Others are epithelia-derived cells.
• Each epithelia-derived cell is closely associated with an afferent neuron that conveys info towards the central nervous system.
• Sensory neurons, incoming stimuli activate receptor proteins that usually lead to depolarisation of the receptor cell membrane (the inside of the cell membrane becomes less negative with respect to the outside).
• This is called generator potential.
• If this local, graded potential is large enough, it will trigger action potentials in the sensory neuron and these will be conducted along the cell to other regions of the nervous system, such as the brain.
• Similar mechanism operates in epithelia-derived sensory cells except that such cells do not generate action potentials.
• Incoming stimuli activate receptor proteins in the surface of the receptor cell causing a change in membrane potential, referred to as receptor potential.
• Response to this local, graded change in membrane potential, calcium channels open and Ca2+ ions flow into the cell causing neurotransmitter to be release onto the adjacent primary afferent neuron
• The neurotransmitter binds to receptors in the membrane of the adjacent afferent neuron.
• If sufficient depolarisation of the afferent neuron occurs, action potentials will be generated.

Front 15

How is stimulus encoded

Back 15

• As stimulus intensity increases, size of the change in membrane potential (i.e. the amplitude of the graded potentials generated) will also increase and so will the frequency of action potentials generated.
• Stimulus intensity is encoded in action potential frequency.
• Some sensory receptors consist of a single sensory neuron.
• However, complex sensory organs, like the human eye, consist of large numbers of sensory receptor cells, together with accessory structures that help gather or amplify the sensory signals.

Front 16

Survey of transduction

Back 16

• Protein embedded in membrane, fixed by links with extracellular and cytoskeleton.
• When membrane is stretched TRP channel opens.
• Allows cations i.e. positively charged ion moves through pore.
• Inside of cell always negative with respect to outside (Resting membrane potential)
• Inside of cell becomes less negative = depolarisation. Membrane usually polarised (negative).
• How much you stretch membrane = size of the change in potential (depends on size of the stimulus_ = a graded potential.
• Inside becomes briefly positive = action potential. (Nerve impulses to central nervous system).
• Current always moves from positive to negative.
• Action potentials begin at the “trigger zone” – voltage-gated Na+ channels.
• Channels along axon, proteins sitting in membrane.
• Not effected by stretch, sensitive to change in voltage across the protein.
• If change potential across membrane, they don’t like that, they suddenly open up if membrane is depolarised.
• Na+ channel opened up and more Na+ comes into the membrane.
• Rapid influx of Na+
• =Briefly becomes positive, then channel closes off again.
• Each channel sets off down the axon.

Front 17

Key points of transduction

Back 17

• Stretching opens TRP channel proteins.
• The sensory cell is depolarised.
• If this graded potential reaches threshold…
• Action potentials carry the info to other areas of the nervous system.
• The more intense the stimulus, the higher the frequency of action potentials.

Front 18

Round dance

Back 18

o Provides no info about the direction of the food source.
o Provides info about odours associated with the food source and quality of nectar.

Front 19

Waggle dance

Back 19

o Directional info.
o Direction indicated by the angle of the straight run of the dance relative to vertical.
o Same as the angle between the direction of the food and the sun, as measured from the hive.
o Use sun as compass.
o Relies of bees being able to determine very accurately the angle of their body with respect to vertical (i.e. the position of their body in relation to the forces of gravity).
o Sensory receptors enable bees to detect body position (proprioception) are called graviceptors.
o These are mechanoreceptors, sensitive to stretching of the sensory- receptor cell membrane.

Front 20

Gravity receptors in bees

Back 20

Key gravity receptor organs are located at the joint between the head and the thorax (neck organ) and between the thorax and the abdomen (petioles organ).
• Neck organ is particularly important for informing the bee whether it is facing upwards or downwards on the vertical axis of the comb.
• Head of the honey bee sits on two chitinous projections from the thorax.
• Surrounding these projections are rows of sensory hairs.
• The hairs fit exactly into the flat surface of the head capsule, so that if the head is positioned normally (i.e. standing of horizontal surface), each hair has the same amount of contact with the head.
• On vertical surface weight of abdomen pulls abdomen down (gravity).
• Bristle hairs bend on side, mechanical pressure.
• Hairs = sensory receptors.
• Sensory hairs stimulated by movement of the bee.
• The centre of gravity of a bee’s head is below the neck joint, so the head acts like a pendulum.
• If bee starts walking up vertical surface the head will move with respect to gravity because pins do not sit in centre of head, so centre of gravity is off centre, therefore weight of back of the head is less than the front.
• If bee is standing on horizontal surface then there is very little friction between cuticle of head and sensory hairs, not much bending, pushing on hairs.
• Vertical surface:
o Upwards: head tilts forwards towards ventral surface.
o Downwards: head falls away from dorsal surface.
• Head can’t go sideways, animal can’t tell if it’s going sideways, therefore other sensory hairs of abdomen between thorax and abdomen.

Front 21

Bee directions

Back 21

Vertical
Up: forward, ventral
Down: falling away, dorsal

Front 22

Experiment for bees

Back 22

• Can use an innate response in the honey bee.
• Show an innate response if bee is placed on vertical surface – it will run upwards.
• Big piece of board, mark into sections, place honey bee in centre, see where it goes.
• Bees run upwards when placed on a vertical surface like you find with many insects e.g. flies.
• = Innate response.

Front 23

Bees on horizontal surface

Back 23

• Bee is using sensory system in neck to determine which way is up.
• Hairs stimulated by movement of bees head.
• Not stimulated on horizontal surface.
• Change tilting of head, put block of lead on back of the head, changes the way head moves, head will go backwards when running up not forward, innate response bees run down.

Front 24

Snakes detect prey in the dark

Back 24

• Highly specialised sensory organs that enable them to detect heat (infrared radiation) radiating from objects at a distance,
• Organs called pit organs or labial pits.
• House extremely sensitive thermoreceptors that enable some snakes to detect mammalian prey.
• Thought a while ago that they had good vision, but they blindfolded the snake and had no problem finding prey. What was the cue? Warmth generated by warm blooded animals.
• Experiment: light bulb wrapped in sac. Light bulb off (cold) = doesn’t attack. Light bulb on (warm) = attacks.
• Just warmth? Experiment = dead mouse (cold), doesn’t find mouse easily but if in front can smell, taste it therefore will eat it.

Front 25

Anatomy of pit organs

Back 25

• Scales along the upper and lower jaws have behind them an elaborate network of nerves that lead into the two branches of the trigeminal nerve that convey info detected by pit organs to the brain.

• When the pit organs detect a rise in temp resulting from a brief pulse of infrared radiation, the trigeminal nerve carries a signal to the brain.
• Response can be recorded in the brain within 35 ms.


Not part of olfaction systems
Distinct openings
Not nostrils
Sensory receptors inside pit organ

Front 26

Important structural feature of pit organs

Back 26

1) Membrane where sensory receptor neurons located sits suspended between two air filled chambers
• Keeps receptors separate from snakes own body
• If membrane attached to snakes body would be affected by own body temp
2) Membrane in detail, not just one sensory neuron, there are lots
• Very important for function of this organ
• Distributed right across membrane, each receptor cell has ending in diff part of membrane
• This is crucial
• Collectively gather and form trigeminal nerve
• Which sends info to brain

Front 27

Multiple sensory neurons in pit organs

Back 27

• Snake can form picture of what it’s looking at
• Membrane in pit organ acting like a retina in an eye
• Forms image
• Point to point mapping on membrane from heat coming from prey

Front 28

Where does info go in snake

Back 28

• Goes into area of brain
• Optic tectum
• Same info from eye goes
• Point to point mapping in brain
• Same as humans with eyes

Front 29

Image of mouse from snakes view

Back 29

• Movement of snakes: Side by side, lines prey up.
• Comparing info from left pit organ and right pit organ.
• Snake knows when head is in a position when image coming into two pit organs is similar, in that instance prey has to be right in front of them.

• Closer snake gets to mouse the more diff the image will be in pit organs.

• Like human left and right eye with finger bringing it closer.
• Stereoscopic perception
• Compare image for distance
• Brain blends them together

Front 30

Electrosense

Back 30

• Electroreception = ability to sense electric fields or weak electrical discharges.
• Common in aquatic organisms.
• Sharks have very keen electric sense.
• E.g. hammer head can detect stingrays buried in the sand by sweeping its head (which contains electroreceptors) over the bottom of the ocean like a metal detector.

Front 31

Feeding responses of shark

Back 31

1) Hungry shark passes in the vicinity of a flounder completely buried in the sand, it will detect using its electrosense and attack it.
2) To exclude the possibility of olfaction: covered flounder with an agar chamber perfuse with water that exited some distance from the flounder. Shark still detected correct location.
3) When pieces of fish were placed in the agar chamber the shark searched for the food where the perfusing water exited. Shark attacked odour, so sharks detect odour but not the only cue they use i.e. backup cue.
4) If agar chamber was covered in nonconducting plastic film, electrical signal was attenuated and shark passed by.
5) Artificial electric field of the same magnitude that is generated by the breathing movements of a flounder excites the shark, and immediately attacked.

Front 32

Sharks electrosensory organs

Back 32

• Sharks have elaborate electrosensory organs that are located in a series of pores distributed across the head. Look like tiny black dots.

• The pores lead into canals that are filled with electrically conductive jelly and lined with modified hair cells (epithelia-derived cells).
• These are the electroreceptor cells that respond to changes in electrical charge within the ampulla.
• Walls of canal are very high resistance.
• Multiple sensory receptor cells and sensory neurons in membrane.
• Receptor neuron constantly firing, spontaneous activity – tonic receptors.
• The change in the membrane potential of the electroreceptor cell triggers the release of neurotransmitter from the electroreceptor cell onto afferent neurons located at the base of the cell.
• This triggers action potentials in the afferent neurons that are then conveyed to the brain.

• Top trace shows that there is spontaneous activity in the afferent neuron.
• This suggests that electroreceptor cells constantly release small amounts of neurotransmitter onto the adjacent sensory afferent neurons.

Front 33

Live flounder buried in sand

Back 33

• Generates small electric signals that can be picked up by the electroreceptor cells of the shark.
• Outside of the shark electroreceptor becomes less positive.
• This depolarises the receptor cell.
• Voltage-gated calcium channels in the receptor cell membrane open
• Ca2+ flows into the receptor cell.
• This lead to mobilisation of vesicles containing transmitter.
• Transmitter released from receptor cell increases.
• This increases the frequency of action potentials in the afferent neuron.

Front 34

Flounder relaxes muscles

Back 34

• Repolarisation of shark electroreceptor cell membrane.
• Close of Ca2+ channels.
• Reduction of transmitter release.
• Action potential frequency in the electrosensory neurons of the shark decreases.

Front 35

Taste

Back 35

Modified epithelial cell
• Vertebrate taste bud illustrates well the diversity of signal transduction mechanisms found in some sensory receptor organs.
• Taste receptors are chemoreceptors.

Front 36

Vertebrate taste bud

Back 36

• Consists of a pore contain sensory receptor cells and support cells.
• Apical surface of each receptor cell is covered with microvilli that project into a pore open to the surface of the body.
• Receptor proteins on the microvilli detect chemicals dissolved in saliva or other fluids.

Front 37

Signal transduction for salty substances

Back 37

• Na+ from salty food enters through a Na+ channel. (Not voltage gated)
• Resulting polarisation opens voltage gated Ca2+ channels.
• Influx of Ca2+ causes neurotransmitter release.

Front 38

Signal transduction for sour substances

Back 38

• H+ ions from sour foods block the K+ channel.
• This blockage prevents K+ from leaving the cell.
• Resulting depolarisation opens voltage gated Ca2+ channels.
• Influx of Ca2+ causes neurotransmitter release.

Front 39

Signal transduction for sweet substances

Back 39

• Sweet substance binds to its receptor, causing a conformational change.
• Activated G protein, activates adenylate cyclase.
• Ac catalyses the conversion of ATP to cAMP.
• The cAMP activates a protein kinase that phosphorylates and closes a K+ channel.
• Resulting depolarisation opens voltage gated Ca2+ channels.
• Influx of Ca2+ causes neurotransmitter release.

Front 40

Signal transduction for bitter substances

Back 40

• Bitter substance binds to its receptor, causing a conformational change.
• The activated G protein activates phospholipase C (PLC).
• PLC catalyses the conversion of POP2 and the second messenger IP3.
• IP3 causes the release of Ca2+ from intracellular stores.
• Influx of Ca2+ causes neurotransmitter release.

Front 41

Food discrimination

Back 41

Discriminating good food from bad.
Large number of natural bitter compounds are known to be toxic.
The ability to detect bitter-tasting, toxic compounds at low thresholds provides an important protective function.

Front 42

Examples of animals with different receptors

Back 42

Chemoreceptor - frog
Mechanoreptor - bee
Electroreceptor - shark
Thermoreceptor – snake

Front 43

Moth auditory

Back 43

• Moth has simple auditory
• Two auditory sensory receptor neurons
• Dendrite input region, embedded in tympanic membrane,
• Axon sends info to nervous system.

Front 44

Moth detect bat calls

Back 44

• High frequency, high energy calls
• Tympanic membrane vibrates
• As air pressure comes in, sound waves come in, vibrates, pulls on dendrites in sensory receptor neuron, then compressing them, going back and forth with stretching and compressing.
• Trp channels open Na+, influx of Na+, depolarise cell, and generate action potentials.
• Negative inside of cell resting membrane potential.
• Info goes to nervous system
• Compress reduce amount of Na+ coming into cell, repolarising cell.