Lecture 10: Special Senses I Flashcards
What is sensory transduction?
conversion of an external or internal stimulus to electrical signals in neurons
What does sensory transduction require?
a change in the membrane potential of a sensory neuron terminal
In mammals, what must the stimulus produce?
action potentials in neurons
When does transduction occur?
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
Which sensory stimuli are chemical?
taste, olfaction, nutrients (in gastrointestinal tract, not sensed directly but change behaviour), inflammation and pain
Which sensory stimuli are electromagnetic?
vision and electroreceptors (not seen in most mammals)
Which sensory stimuli are detected by mechanoreceptors?
touch, stretch, pain (pinch etc.), proprioception (muscle stretch and length, joint angle, position of limb in space), audition
What does sensory modality refer to?
the actual stimulus detected by the peripheral sensory structure
What is sensation often a construct from?
several sensory modalities or neurons that supply different parts of the body
How does the brain interpret firing in a sensory axon? Give an example.
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
What do sensory transduction mechanisms determine? Give an example.
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”
What is the full receptive field for an auditory axon?
its tuning curve and the ear it supplies
What do receptive fields in higher centres depend on?
integration and summation of the defined receptive fields of the primary sensory neurons that provide the original information
What are examples of the receptive fields of higher order neurons being very abstracted?
position of a sound source in relation to the head and body, a familiar face, association of fear with a particular scent
Which neurons are rapidly adapting?
neurons whose firing is transient despite maintained stimulus
Which neurons are slowly adapting?
neurons that fire throughout a maintained stimulus
What is glabrous skin?
hairless skin
What are Pacinian corpuscles?
vibration detectors
What are Ruffini endings and Merkel cells complexes?
slowly adapting receptors
What are Meissner corpuscles?
rapidly adapting receptors
What are free nerve endings?
pain, cool and warm receptors
How does simple mechanotransduction occur?
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
What does mechanical distortion result in?
depolarises sensory nerve terminals to trigger action potentials
What does the response to mechanical stimulus depend upon?
coupling between mechanical stimulus and stretch sensitive ion channels in terminal membrane
What is coupling between mechanical stimulus and stretch sensitive ion channels in the terminal membrane determined by?
physical properties of the sensory structure (corpuscle, free ending, etc)
What do Merkel cells express and where do they express it?
piezo2 in membrane structures on apical surface
What happens in Merkel cells during deformation?
deformation opens Piezo2 which depolarises the cell and triggers Ca2+
action potential leading to release of transmitter (serotonin, 5-HT) that excites nerve terminal
What does transduction depend on?
the environment of the transducing structure
Where can detection mechanisms be located?
in axon terminals or specialised receptor cells that excite, or inhibit, axon terminals
What must specialised receptor cells communicate with and change?
the membrane potential of sensory axons to produce signals that will ultimately affect behaviour
What is the role of muscle spindles?
encode length using generator potentials like those of cutaneous afferents (in parallel sensory system)
What is the role of Golgi tendon organs?
encode tension via generator potentials like those of cutaneous afferents (in series sensory system)
What can muscle spindles be tuned by?
activity in γ motor neurons
change rate of adaptation
What are muscle spindles?
specialised groups of muscle fibres within an embedded capsule
What are the two types of muscle fibres possessed by muscle spindles?
nuclear bag fibres with all nuclei in a central “bag” and nuclear chain fibres whose nuclei are distributed along the length
Which primary afferent terminals wrap around the two types of muscle fibres in spindles?
group Ia primary afferent terminals wrap around the nuclear region of bag fibres and group II primary afferents run along the chain fibres
What happens in muscle spindles when a muscle is stretched?
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
What is the implication of nuclear bag fibres having no contractile proteins where the Ia endings form spirals?
transmission of force to the ending differs from what is seen with the linear group II endings
What are the two different responses of Ia afferents to stretch?
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
What type of responses do group II axons show?
only show static responses
What is the role of ɣ-motoneurons?
tune the static and dynamic responses of Ia primary afferents
specialised motor innervation of intrafusal muscle fibres
What do Golgi tendon organs signal and how?
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
Where are Golgi tendon organs found?
where muscle fibres meet tendons
