Sensory Receptors Flashcards
What are sensory receptors
Tranducers: convert different stimuli into frequency of action potentials - inform brain about internal and external environment
3 types of sensory receptors
explain each
- Mechanoreceptors: stimulated by mechanical stimuli (pressure/stretch/deformation) - gives skin sensations of touch and pressure
- Proprioceptors: mechanoreceptors in joints/muscles that signal info about body or limb position
- Nociceptors: respond to painful stimuli (heat/tissue damage)
sensory modality
stimulus type that activates a particular receptor
What is an adequate stimulus
the form of energy to which a receptor normally responds
- energy required to activate a sensory receptor
What are sensory recepots usually highly sensitive to
One specific energy form: but can be activated by other intense stimuli
2 types of proprioceptro and what each measures
- Muscle spindle - length of muscle
- Golgi tendon - tension of muscle
What are cutaneous receptors, and what 3 types do they include?
A cutaneous receptor is the type of sensory receptor found in the skin ( the dermis or epidermis). They are a part of the somatosensory system. Cutaneous receptors include mechanoreceptors (pressure or distortion), nociceptors (pain), and thermoreceptors (temperature).
Where are cutaneous receptors found, and what is their function
Cutaneous receptors are found at the distal ends of the primary sensory axon; they act as dendrites, in which threshold stimuli lead to the firing of an action potential at the initial segment of the primary sensory axon.
What are cuteneous receptors a part of
the somatosensory system
What is a receptor/generator potential
Skin mechanoreceptors/proprioceptors:
sensorty receptor transduction inveolves ion channels opening/closing to change memb potential of nerve cells. Adequate stimulus causes a graded membreane potential change - aka receptor/generator potential
What is the adequate stimulus in cutaneous mechanoreceptors and proprioceptors called and what does it do
membrane deformation - this activates stretch-sensitive ion channels (ions flow across membrane and change it’s potential locally)
When will an action potential fire in a receptor potential
once stimulus intensity has reached a threshold where it causes depolarisation to open voltage-gated ion channels - AP fires
In a sensory nerve, what does a larger stimulus cause and what is this called
A larger receptor potential and a higher frequency of AP - called frequency coding of stimulus intensity
What can happen in mechanoreceptors if a stimulis persists (e.g. socks on)
AP persist, or drop off in AP occus, allowing brain to process new or changing events - adaptation
Explain adaptation of mechanoreceptors in detail
Stimulus causes above threshold generator potential that triggers APs. Generator potential declines rapidly below threshold and AP stop - mechanoreceptor only signals the onset of a stimulus and responds only to a change or novel event
Explain and give examples of:
1. Rapidly/modertely-adapting receptors
2. Slowly-adapting receptors
- Rapidly/modertely-adapting receptors: only generate signal when stimulus turned on or off (pacinian corpuscles and meissner’s corpuscles)
- Slowly-adapting receptor: generate signal throughout stimulus (Merkel’s discs and ruffini endings)
Nociceptors in terms of adaptation
Do NOT adapt: important not to ignore painful stimulus
Explain the pacinian corpuscle (what is it/structure)?
- Best understood mechanoreceptor
- Comprises a myelinated nerve with a neked nerve ending enclose by a CT capsule of layered membrane lamellae. Each layer is separated by fluid. - like onion
How does the pacinian corpuscle respond to a stimulus
- mechanical stimulus deforems the casule and the nerve ending
- Stretches nerve ending and opens ion channels
- Na+ influx causes local depolarisation - receptor/generator potential
- AP are generated and fire where myelination begins (because regenerative Na+ channels cluster at the nodes of Ranvier)
How does the pacinian corpuscle show rapid adaption
- AP fire and stimulus on (see other flashcards for process)
- Fluid rapidly redistributes within capsule lamellae, this spreads the stimulus impact out laterally - minimizing downward deformation
- downward force causing mechanical stretch to nerve endings stops - so AP stop firing
- As stimulus is withdrawn capsile lamellae sping back and AP fire again
- Can detect ON/OFF phases of mechanical stimulus
What do capsules in pacinian/sensory receptors do
Enchance sensory functio: without them (if lamellae are removed) than a continous receptor/generator and thus, AP would be produced. Instead of responding to ON/OFF stimuli.
how is strength of stimulus encoded in nerve action potentials
Stimulus intensity is encoded in two ways: 1) frequency coding, where the firing rate of sensory neurons increases with increased intensity and 2) population coding, where the** number of primary afferents responding increases** (also called RECRUITMENT)
Receptive field
Specific area where stimulus occured
Give the simplist case of sensory receptors having a receptive field
1 receptive field associated with 1 sensory neurons (primary sensory neurone) which synapses 1 CNS neurone (secondary neruone in spinal cord)
What does our ability to tell 2 points apart on the skin depend on
- Receptive field size
- Neuronal convergence -do pathways combine?
Explian neuronal convergence
- Can occur in sensory neurons with neighbouring receptive fields
- Allows simultaneous sub-threshold stimuli from multiple primary receptive fields to summate at a secondary neurone, forming a larger secondary receptive firld and initiating an AP
What would lots of convergens and a large secondary receptive field indicate
a relatively insensitive area
Acunity
Ability to locate a stimulus on skin and differentiate it from another closeby
What can low acuity be caused by
high convergence
Explain lateral inhibition
- receptors on edge of stimulus more strongly inhibited than receptors near the cenetre
- Enhances the contrast between relevant and irrelevant information - sharpens the sensory code
Where does all sensory information from the body go?
(explain)
Brain: relayed via spinal cord to the thalamus and on to the somatosensory cortex with the most sensitive areas innervating the larges cortical space
What are proprioceptors
mechanoreceptors signalling body or limb position
What do proprioceptors include?
3
- Muscle Spindles: monitor muscle length and rat of change of muslce length
- Golgi Tendon Organs: monitor tension on tendons
- Joint Receptors: monitor joint angle, rate of angular movement and tesnion on the joint
What do the 3 proprioceptors do
(3 things)
- Send sensory information to the spinal cord and then on to the brain so that it can control voluntary movements
- Muscle spindles and GTO provide sensory info to spinal cord neurons which can generate spinal reflex movements
- Provide sensory information to perceive limb and body position and movement in space - kinaesthesia
What are a few specialized muscle fibres with their own sensory nd motor innervation called
Intrafusal muscle fibres
Most contractile skeletal muscle fibres are “extrafusal”
Intrafusal muscle fibres
explain + orientation
Contained within a capsule and called muscle spindles: muscle spindles lie in parrallel with muscle fibres
Indepentent of GTO fibres
What do muscle spindles do?
stretch detectors, and sense how much and how fast a muscle is lengthened or shortened
What stimulates:
* Intrafusal fibres
* Extrafusal fibres
- Intrafusal fibres - gamma motorneurones
- Extrafusal fibres - alpha motorneurons
Explain alpha and gamma motorneurones
Motor neurons are divided into two groups. Alpha motor neurons innervate extrafusal fibers, the highly contracting fibers that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibers, which contract only slightly.
How many nuclei do muscle fibres have
Multiple - are multinucleated
What are the 2 typed of intrafusal fibre
- nuclear bag fibres - bag shaped with neclei collected together
- nuclear chain fibred - nucleu lined up in a chain
What is the difference between 1a and 1b afferent?
1a is the largest and thus the most rapidly conducting. 1b, from the Golgi tendon organ, is slower and 2 is the slowest. Afferents from nuclear bag fibers signal velocity. Like the Pacinian corpuscles, bag fibers adapt quickly when stretched.
Explain process of innervation of intrafusal fibres (how do we know the length of out muscles)
Primary endings from 1a afferent nerves wrap around centre of intrafusal fibres: froming annulospiral endings. Ends of intrafusal fibre conntain contractile sarcomeres but the central area has not contractile elements. Gamma motorneurons innervate and cause contraction of these contractile ends. When m-n fires, two ends contract and shorten but central area does not: it gets stretched out.
Stretch sensitive ion channels open, creating a local generator potential, causing regenerative APs in the 1a afferent fibres. Diff between 2 points (strengths of AP) informs about rate of change of length.
During muscle stretch, what is AP frequency proportional to
velocity of the stretch
Summarise how spindle and joint receptor information together inform the brain about joint position
- Muscles work in opposition e.g. when agonist contracts, antagonist relaxes and joint moves
- Stretching the agonist increases and shortening the agonist (contracting it) reduces spindle discharge
- opposite true for antagonist muscle
- overall, this infor gives the brain info on joint position
how do GTO monitor muscle tension
- Nerve endings of GTO mingle with the tendon bundles at ends of muscles. They are stretch receptors and monitor stretch of tendon
- tendons = inelastic, so muscles must develop tension my contracting to stretch the tendons
- Muscle contraction inc tension in tendons
- Stretches the nerve endings of the GTO
- Initiates AP in the group 1b afferent fibre from the GTO
- GTOs lie in series with the muscle fibres
Describe the orginisation of muscle proprioceptors
Muscle spindles/GTO
Muscle spindle in parrallel and GTO in series with extrafusal muscle fibres
Why must we also have gamma motor innervation onto the contractile ends of muscle spindles (in addition to normal alpha motorneurons)?
If had no gamma m-n then when a m-n fire and muscle contracts, muscle spindles would stay same length and not fire AP from 1a afferent annulospiral endings. brain not be informed of change in muscle length. Limits fnctional usefulness of that muscle
What does alpha and gamma motoneurons firing together prevent
A drop off in 1a firing during contraction
What is the solution to ensuring muscle contraction and info sent to brain
- Alpha m-n fire causing extrafusal fibre contraction
- Gamma m-n fire causing intrafusal fibre ends to contract. This stretches the central sensory elements and restores tension in the spindle.
- (and) Resets the sensitivity of the central sensory part of the spindle to match the new muscle length
so spindle 1a afferents report muscle shortening to the brain and gamma m-n fire to maintain the spindle sensitivity to stretch
a-g coactivation
alpha-gamma coactivation
Norm for voluntary movements:
* alpha m-n activated causing contraction
* gamma m-n activated in parallel to maintain spindle sensitivity
Summarise the function of muscle, tendon and joint proprioceptors
Work together to inform brain on movements and position of our body in space. Also act automatically to control movements via spinal cord reflexes.
How are muscle spindles continually reset
by firing a and gamma m-n together to endure the spindles are equally sensitive at all different muscle lengths
Where do gamma motorneurons innervate
contractile ends of the muscle spindle (intradusal fibre)
1a afferent nerve fibres wrap round centre of intrafusal fibre
What does pascinian carpuscle detect
vibration
Name for sense of body position in space
Kinaesthesia
What class of m-n innervates the contractile ends of muscle spindles
Gamma m-n
Alpha innervate extrafusal muscle fibres
