Lecture 10: Special Senses I

Question 1

What is sensory transduction?

Answer 1

conversion of an external or internal stimulus to electrical signals in neurons

Question 2

What does sensory transduction require?

Answer 2

a change in the membrane potential of a sensory neuron terminal

Question 3

In mammals, what must the stimulus produce?

Answer 3

action potentials in neurons

Question 4

When does transduction occur?

Answer 4

occurs when the environment interacts with a structure that can detect and convert that interaction into a change in properties of a sensory neuron to alter the probability of that neuron firing action potentials

Question 5

Which sensory stimuli are chemical?

Answer 5

taste, olfaction, nutrients (in gastrointestinal tract, not sensed directly but change behaviour), inflammation and pain

Question 6

Which sensory stimuli are electromagnetic?

Answer 6

vision and electroreceptors (not seen in most mammals)

Question 7

Which sensory stimuli are detected by mechanoreceptors?

Answer 7

touch, stretch, pain (pinch etc.), proprioception (muscle stretch and length, joint angle, position of limb in space), audition

Question 8

What does sensory modality refer to?

Answer 8

the actual stimulus detected by the peripheral sensory structure

Question 9

What is sensation often a construct from?

Answer 9

several sensory modalities or neurons that supply different parts of the body

Question 10

How does the brain interpret firing in a sensory axon? Give an example.

Answer 10

interprets firing in a sensory axon according to where the peripheral terminals of that axon are located e.g. an axon supplying the ear cannot signal the presence of light, both because the ear lacks photoreceptors and because the brain will interpret the signal as sound

Question 11

What do sensory transduction mechanisms determine? Give an example.

Answer 11

what elements of the environment can activate a specific sensory axon – these form the receptive field of that neuron e.g. auditory axons are activated by a range of sound frequencies with a characteristic maximum effect to produce a “tuning curve”

Question 12

What is the full receptive field for an auditory axon?

Answer 12

its tuning curve and the ear it supplies

Question 13

What do receptive fields in higher centres depend on?

Answer 13

integration and summation of the defined receptive fields of the primary sensory neurons that provide the original information

Question 14

What are examples of the receptive fields of higher order neurons being very abstracted?

Answer 14

position of a sound source in relation to the head and body, a familiar face, association of fear with a particular scent

Question 15

Which neurons are rapidly adapting?

Answer 15

neurons whose firing is transient despite maintained stimulus

Question 16

Which neurons are slowly adapting?

Answer 16

neurons that fire throughout a maintained stimulus

Question 17

What is glabrous skin?

Answer 17

hairless skin

Question 18

What are Pacinian corpuscles?

Answer 18

vibration detectors

Question 19

What are Ruffini endings and Merkel cells complexes?

Answer 19

slowly adapting receptors

Question 20

What are Meissner corpuscles?

Answer 20

rapidly adapting receptors

Question 21

What are free nerve endings?

Answer 21

pain, cool and warm receptors

Question 22

How does simple mechanotransduction occur?

Answer 22

axon terminal is inside a specialised sensory structure - in this case a Pacinian corpuscle
deforming the terminal membrane opens stretch- sensitive cation channels (Piezo2) to depolarise terminal membrane – generator (receptor) potential

Question 23

What does mechanical distortion result in?

Answer 23

depolarises sensory nerve terminals to trigger action potentials

Question 24

What does the response to mechanical stimulus depend upon?

Answer 24

coupling between mechanical stimulus and stretch sensitive ion channels in terminal membrane

Question 25

What is coupling between mechanical stimulus and stretch sensitive ion channels in the terminal membrane determined by?

Answer 25

physical properties of the sensory structure (corpuscle, free ending, etc)

Question 26

What do Merkel cells express and where do they express it?

Answer 26

piezo2 in membrane structures on apical surface

Question 27

What happens in Merkel cells during deformation?

Answer 27

deformation opens Piezo2 which depolarises the cell and triggers Ca2+
action potential leading to release of transmitter (serotonin, 5-HT) that excites nerve terminal

Question 28

What does transduction depend on?

Answer 28

the environment of the transducing structure

Question 29

Where can detection mechanisms be located?

Answer 29

in axon terminals or specialised receptor cells that excite, or inhibit, axon terminals

Question 30

What must specialised receptor cells communicate with and change?

Answer 30

the membrane potential of sensory axons to produce signals that will ultimately affect behaviour

Question 31

What is the role of muscle spindles?

Answer 31

encode length using generator potentials like those of cutaneous afferents (in parallel sensory system)

Question 32

What is the role of Golgi tendon organs?

Answer 32

encode tension via generator potentials like those of cutaneous afferents (in series sensory system)

Question 33

What can muscle spindles be tuned by?

Answer 33

activity in γ motor neurons

change rate of adaptation

Question 34

What are muscle spindles?

Answer 34

specialised groups of muscle fibres within an embedded capsule

Question 35

What are the two types of muscle fibres possessed by muscle spindles?

Answer 35

nuclear bag fibres with all nuclei in a central “bag” and nuclear chain fibres whose nuclei are distributed along the length

Question 36

Which primary afferent terminals wrap around the two types of muscle fibres in spindles?

Answer 36

group Ia primary afferent terminals wrap around the nuclear region of bag fibres and group II primary afferents run along the chain fibres

Question 37

What happens in muscle spindles when a muscle is stretched?

Answer 37

bag intrafusal and chain intrafusal muscle fibres are stretched, which deforms their respective nerve terminals -> opens Piezo2 channels initiating APs -> when the extrafusal muscle contracts the tension in the intrafusal muscle fibres is reduced thus closing Piezo2 channels -> thus, groups Ia and II afferents signal the length of the muscle

Question 38

What is the implication of nuclear bag fibres having no contractile proteins where the Ia endings form spirals?

Answer 38

transmission of force to the ending differs from what is seen with the linear group II endings

Question 39

What are the two different responses of Ia afferents to stretch?

Answer 39

initial high frequency firing is called the dynamic response and is proportional to the rate of stretch
lower frequency firing during maintained stretch is static response and encodes magnitude of stretch

Question 40

What type of responses do group II axons show?

Answer 40

only show static responses

Question 41

What is the role of ɣ-motoneurons?

Answer 41

tune the static and dynamic responses of Ia primary afferents
specialised motor innervation of intrafusal muscle fibres

Question 42

What do Golgi tendon organs signal and how?

Answer 42

signal muscle tension
when muscle is stretched Piezo2 in terminals is deformed and there is depolarisation in Ib afferent
when muscle contracts tendon organ is stretched so Ib afferent is depolarised

Question 43

Where are Golgi tendon organs found?

Answer 43

where muscle fibres meet tendons