Lecture 6- Mechanosensory Transduction Flashcards
What needs to happen for an external or internal stimulus to be detected by the brain?
-To be detected by the brain, any external or internal stimulus must be converted to electrical signals in neurons
– Sensory transduction
– In mammals, signals must produce action potentials in neurons
– Without action potentials signal is unlikely to ultimately reach the brain
What are the types of sensory stimuli?
- chemical
- electromagnetic
- mechanoreceptors
What are the chemical types of sensory stimuli?
– Taste
– Olfaction
– Nutrients, in gastrointestinal tract
– Inflammation
– Pain
What are the electromagnetic types of sensory stimuli?
– Vision
– Electroreceptors
-in humans have a very limited range of what we perceive, the visible light and the heat via warmth receptors
What types of mechanoreceptor stimuli are there?
– Touch
– Pain – pinch, etc
– Proprioception, joint angle, position of limb in space
– audition
How does mechanotransduction work in touch?
- Cutaneous primary afferent – sensory neuron
- Has terminal inside a specialised sensory structure – Pacinian corpuscle
- Deformation of terminal membrane opens stretchsensitive cation channels and causes depolarization of terminal membrane – generator (receptor) potential
- cutaneous primary afferent= terminal within the skin, deformation of this sensory neurons changes the ion flow= depolarisation then fires an AP and coducts back into the spinal cord, activates the pathway
- the ion channels in the membrane are mechanosensitive= stretch= open, release= close, simple arrangement
What are the two types of skin?
- glabrous (soles of your feet and hands)
- hairy (the rest)
What are the types of nerve endings supplying the skin?
- Free nerve endings, Meissner corpuscle, Merkel cell neurite complex, Ruffini ending and Pacinian corpuscle (according to depth)
- in hairy skin there is an extra one supplying the hair
- at 44C warm detectors, at 55 you will activate cutaneous nociceptros= pain Meisner= right underneath epidemris, in the dermis, consist of one axon with lot of branches inside a sdack of connecting tissue filled with liquid Ruffini= nerve endings, in a sack -deepest= most sensitive= pacinian corspuscle
- pacinian normally detect vibration
- merkel and ruffini= respond throughout the stimulus, force is modified with the depth of the nerve
- meissner= sack around the nerve ending= buffers the force but they are at teh top, they also fire at the beginning and end = rapidly adapting
- all convey info to the spinal cord quickly (myelinated)
What do free nerve endings do?
pain, cool and warm receptors
What do Pacinian corpuscles detect?
-vibrations
Which of the skin mechanoreceptors are rapidly and slow adapting?
- Ruffini endings and Merkel cell complexes are slowly adapting receptors
- Meissner corpuscles are rapidly adapting, as are the Pacinian corpuscles
What is the role of the sensory structure in the mechanoreceptors (what the ending looks like)?
-Mechanical distortion depolarizes sensory nerve terminals to trigger action potentials
- Response shape depends on coupling between mechanical stimulus and stretch sensitive ion channels in terminal membrane
- Coupling determined by physical properties of the sensory structure (corpuscle, free ending, etc
What was the experiment with Pacinian corpuscles to show how important the ending is?
- Pacinian corpuscle= big enough to strip it and can compare the response the naked nerve ending
- when nerve terminal in pacinian sack thing= as you push it deformes the nerve ending= causes depolarisation if big enough= then AP, the force generated is dissipated into the pacinian sack, so the depol lasts very short time= only one AP! fires at the start and at the end of the deformation
- receptive potential= what is generated in the nerve look up
- when only a nerve ending= deform the terminal and the receptor potential look different, get potential, declines a bit and when the eliciting stimulus is removed no AP, as no pillow!
How is information coded in different types of cutaneous sensory neurons?
- Only the slowly adapting Merkel’s discs encode the Braille pattern
- Other types of afferent encode different aspects of the stimulus
- merkel give the pattern that is present
- merkel disks are incredibly important for this sort of info, the others will tell you about physical force and vibration
What is another form of mechanosensory behaviour?
- Three types of mechanoreceptor axon in skeletal muscle
- Muscle spindles (A) encode length using generator potentials similar to those of cutaneous afferents
- In parallel sensory system
- Golgi tendon organs (B) encode tension, again via generator potentials similar to those of cutaneous afferents
- In series sensory system • Muscle spindles can be tuned by activity in γ motor neurons
– Change rate of adaptation
- detect force and length of muscle
- muscle spindle afferents= supply modiffied muscle fibres, surrounded by a connective tissue structures, receive two classes of primary afferent endings= 1A primary afferent (high speed) those send nerve termineal into the muscle spindle central, when spindle is stretched then the afferent fires
- then group 2 /slower like cutaenous) parallel to the muscle fibres, so when the spindle is stretched= firing follows the stretch of the spindle itself -when muscle contract the spindle is unloaded, shortening and the 1A wouldn’t deform,
- muscle spindle= length detector
- second class of primary afferent also group 1= more rapid than 1A, these are 1B= supply Golgi tendon organs, thes elie within the tendon of every muscle
- when muscle lengthenes and tension goes then will stretch, if contracts still will stretch= so whatever happens the golgi will fire even during a contraction
- these let the brain know what happens in the muscle,important for proprioception
How are APs generated in the ear?
- when vibration moves the cilia in that direction (right) it pulls the smaller cilia= open K+ channels
- the region where th ecells are then high K+ conc, and low Na+, so when K+ channel opens= it flows into the cell and causes depolarisation
- when the system goes in the other direction= the potassium channels close then K+ pumped and the cell back to rest slightly hyperpolarisation = here it is different with potassium as high conc of K+ in the region out the inner hair cells
- the tip links are critical= break them and will never work again, that means that any very large vibration is likely to damage the hair cells= tear the tip links= so then deaf -from the inner hair cell to thenerve terminal= epithelial cells relase neurotransmitter= to a synapse
- calcium increase upon the depol. and cause vesicles to release
- need mechanosensitive channels in the tips of teh cillia and chemical transmission at the base of the hair cell at the terminal
- have to have both
