IC2 Sensation, Transduction Flashcards
What is sensory transduction?
Properties of stimulus such as touch, sound, vision, pain, are encoded using action potential signal
External stimulus is is transduced to electrical charge/signal which is typically depolarizing in nature, hence generating action potential
The transformation of energy in external stimulus to an action potential across the membrane is called sensory transduction
This is followed by generation of action potential at the trigger zone of the associated afferent
4 Phases of Sensory Transduction
Phase 1
- Application of stimulus and depolarization
- Application of stimulus to the receptor in the periphery that results in depolarization of the receptor membrane
- Opening of channels embedded in the receptor membrane
- New influx of cations into the receptor, depolarizes the receptor membrane
4 Phases of Sensory Transduction
Phase 2
- Generation of action potential at the threshold potential at the trigger zone
- Receptor potential travels to the trigger zone of the associated primary afferent
- Depolarization reaches the threshold membrane potential at the trigger zone and generates action potential
The trigger zone is located in the axon hillock in the cell body of the neuron, it is highly concentrated with voltage-gated Na+ channels
4 Phases of Sensory Transduction
Phase 3
- Propagation to CNS (along axons)
- Action potential discharge is propagated along the axon to CNS (e.g., sensory cortex)
4 Phases of Sensory Transduction
Phase 4
- Synaptic transmission at axon terminal
- Action potential discharge reaches the axon terminal where it triggers the release of transmitter => synaptic transmission
What are sensory receptors?
What is a receptive field?
Sensory receptors are dendrites of sensory neurons with specialized sensory nerve ending or specialized epithelial cells that recognize a stimulus and initiate sensory transduction by creating action potentials
Receptors in turn send information to the CNS via afferent nerve fibers aka primary afferents
Area on skin where stimulus will excite the receptor is known as the receptive field
What are the stimulus to the following sensory receptors:
- Mechanoreceptor
- Nociceptor
- Chemoreceptor
- Photoreceptor
- Thermoreceptor
- Proprioceptor
- Mechanoreceptor: touch - pressure
- Nociceptor: painful noxious stimuli, temp >43dc, chemicals
- Chemoreceptor: chemicals
- Photoreceptor: light
- Thermoreceptor: temp
- Proprioceptor: position of body in space
What is a unimodal or polymodal receptor?
Mechanoreceptor is unimodal
- 1 adequate stimuli: light pressure
Nociceptor is polymodal
- multiple stimuli: thermal, mechanical, chemical (e.g., capsaicin => Trpv1)
What is a modality?
Modality is a class of stimuli
Modalities include:
- Vision
- Sound
- Somatosensation (which encompass submodalities: non-noxious and noxious stimuli, proprioception, temperature sensation) - somatic sensations at the primary somatosensory cortex
Examples of afferents
Mechanoreceptors: Pacinian Corpuscle, Merkel’s disks, Meissner’s corpuscles, Ruffini ending
Nociceptors: Free nerve ending
What is Pacinian Corpuscle?
Enclosed nerve ending of sensory neurons, covered by connective tissue; embedded in skin of fingertip
It is a physiological receptor that is activated by external stimuli of touch
It converts physical energy of non-painful external stimuli into electrical signal
What are free nerve endings?
Nociceptors are physiological receptors of sensory neurons that have free nerve endings
Activated by noxious (painful, tissue-damaging) stimuli
Embedded in the skin
Free nerve endings can result in sensory transduction in two ways - mechanical and chemical/thermal
*Polymodal receptor
Mechanical:
- Application of stimulus to the receptor in the periphery stretches the membrane of the receptor
- Opening of channels embedded in the membrane
- New influx of cations into the receptor, depolarizes the receptor membrane
Chemical (noxious heat from chemicals):
- Pharmacological receptor, Trpv1 is embedded in the membrane of the free nerve endings; binding of capsaicin to Trpv1 causes it to open, allowing influx of cations (Trpv1 is a calcium-permeable ion channel)
- (TRPV1 was identified as a receptor responsible for mediating the intense burning sensation following exposure to heat greater than approximately 43°C., or capsaicin, the pungent ingredient of hot chilli peppers)
AB-fiber vs C-fiber vs AD- fiber
AB fiber is associated with mechanoreceptors.
AB fiber:
- Myelinated
- Fastest conduction
- Saltatory conduction (nodes of Ranvier): electrical impulse skips from node to node
C-fiber and AD- fiber are axons associated with nociceptors (primary afferents of the nociceptors)
C-fiber:
- Unmyelinated
- Slower conduction
- Action potential generated point to point, multiple Na+ channels in close proximity
AD- fiber
- Thinly myelinated
- Faster conduction
Explain the mechanism of congenital insensitivity to pain with anhidrosis (CIPA)
Absence of NGF-dependent AD- and C- fibers
- Due to loss of function mutations in the NTRK1 gene which is responsible for the production of nerve growth factors
- Result in insensitivity to both superficial and deep painful stimuli (decrease/loss of sensation)
*Note that AB-fibers are still present, and hence sensation of touch/vibration is intact
