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principles of normal function > week 4 sensory receptors and reflexes > Flashcards

week 4 sensory receptors and reflexes Flashcards

1
Q

what are three types of sensory receptors

A
  • mechanoreceptors (mechanical stimuli)
  • proprioceptors (mechanoreceptors in joints and muscles)
  • nociceptors (painful stimuli)
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2
Q

what do mechanoreceptors (type of sensory receptors) respond to

A
  • stimulated by mechanical stimuli (pressure, stretch, deformation)
  • gives us skin sensations of touch and pressure (e.g. don’t squeeze an egg)
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3
Q

what do proprioreceptors do (type of sensory receptors)

A
  • they are mechanoreceptors in the joints and muscles

- they signal information about body or limb position (e.g. touching nose)

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4
Q

what do nocicoptors do (type of sensory receptor)

A
  • respond to painful stimuli

heat and tissue damage

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5
Q

what is sensory modality

A
  • the stimulus type that activates a particular receptor
  • i.e. touch, pressure, joint angle, pain
  • sensory receptors are highly sensitive to one specific energy form but they can be activated by other intense stimuli
    (e. g. poke in eye with stick, mechanical poke may cause visual stimulation by ‘seeing start’)
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6
Q

how does sensory transduction in the skin work

A
  • adequate stimulus in skin mechanoreceptors and proprioceptors is caused by membrane deformation (something impacts your skin)
  • this activates stretch-sensitive ion channels
  • ion channels open
  • adequate stimulus causes generator/receptor potential
  • when depolarisation reaches threshold action potentials fire
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7
Q

what is the frequency coding of stimulus intensity in sensory nerves

A
  • a larger stimulus causes a larger receptor potential and a higher frequency of action potentials
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8
Q

what is a pacinian corpuscle

A
  • a mechanoreceptor

- senses vibration

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9
Q

what do Merkel’s receptors respond to

A
  • sense steady pressure and texture
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10
Q

what do meissner’s corpuscle respond to

A
  • responds to flutter and stroking movements
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11
Q

what does Ruffini corpuscle respond to

A
  • skin stretch
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12
Q

what are rapidly/moderately adapting receptors

A
  • if stimulus persists and doesn’t stop then firing of action potentials will stop until stimulus is removed then we will get an action potential again
  • e.g. pacinian and meissner’s corpuscles
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13
Q

what are slowly adapting receptors

A
  • if stimulus persists and doesn’t stop then action potentials won’t stop either, action potentials will stop once stimulus is removed
  • e.g. markers discs and Ruffini endings
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14
Q

why do nociceptors not adapt to persistent stimuli

A
  • nociceptors do not adapt because it is important not to ignore painful stimuli
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15
Q

what is the structure of a pacinian corpuscle

A
  • it is a mechanoreceptor that responds to vibration
  • made up of a militated nerve with a naked nerve ending
  • enclosed by a connective tissue capsule of layered membrane lamellae, each layer is separated by fluid
  • this gives it it’s rapidly adapting qualities
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16
Q

how does the pacinian corpuscle respond

A
  1. a mechanical stimulus deforms the capsule and nerve ending
  2. this stretches the nerve ending and opens the ion channels
  3. sodium influx causes local depolarisation and we get a receptor/generator potential
  4. action potentials are generated and fire where myelination begins
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17
Q

what is a receptive field in sensory receptors

A
  • sensory neuron is activated by stimuli coming from a specific area called the receptive field
  • this field can be large (so not as sensitive or specific) or small
  • size of this receptive field increases when there is neuronal convergence (which is when there are more than one paths the signal can take to the brain, many neurones synapsed onto just one neurone)
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18
Q

what does convergence of sensory neurones allow you to do

A
  • convergence of three primary sensory neurones in the skin, convergence allows simultaneous sub-threshold stimuli from the 3 neurones to sum at the secondary neurone
  • this forms a larger secondary receptive field and initiates action potentials
  • so lots of convergence and a large secondary receptive field indicates a relatively insensitive area
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19
Q

what is acuity

A
  • that ability to locate a stimulus on the skin and differentiate it from another close by
  • low actuity is caused by high convergence
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20
Q

what helps us locate a stimulus so precisely in sensory receptors

A
  • lateral inhibition

- the neurones near the stimulus are inhibited by the neurone being stimulated

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21
Q

what are three types of proprioceptors

A
  • muscle spindles (monitor muscle length, control reflexes and voluntary movements)
  • golgi tendon organs (monitors tension on tendons)
  • joint receptors (monitor joint angle and tension on joint)
22
Q

what is are muscle spindles in proprioceptors

A
  • type of proprioceptor
  • monitors muscle length
  • controls reflexes and voluntary movements
23
Q

what are golgi tendon organs

A
  • type of proprioceptor
  • monitors tension on tendons
  • tension produced by muscle contraction so monitoring muscle tension
24
Q

what are joint receptors

A
  • type of proprioceptor

- monitor joint angle and tension on joint

25
Q

what do proprioceptors do

A
  • send sensory info to the spinal cord and onto the brain and so control our voluntary movements
  • muscle spindles and golgi tendon organs provide the sensory info to spinal cord neurones which can generate spinal reflex movements
  • they provide sensory information to perceive limb and body position and movement in space = kinaesthesia
26
Q

what are intrafusal muscle fibres

A
  • muscle spindles (type of proprioceptor)
  • they are contained within capsule
  • these lie in parallel with normal (extrafusal) muscle fibres
27
Q

what are the two types of intrafusal fibre

A

nuclear bag fibres = nuclei grouped together in middle

nuclear chain fibres = nuclei lined up in chain in the middle

28
Q

what is wrapped around the centre of intrafusal fibres

A
  • Ia afferent nerves wrapped around centre

- this forms annulospiral ends

29
Q

which bit of the intrafusal fibres contain sarcomeres

A
  • the ends, so the ends can contract

- the centre of the intrafusal fibres cannot contract and just get stretched when the ends contract

30
Q

what innervates the contraction of the ends of the intrafusal muscle fibres

A
  • gamma motorneurones cause contraction of the ends of the intrafusal fibres
31
Q

what happens to centre of intrafusal spindle when the ends contract

A
  • ends contract via gamma motorneurones
  • this stretches the centre of the muscle spindles (stretch in annulospiral endings)
  • this opens stretch sensitive ion channels
  • this creates local changes in the generator potential, when muscle stretches there is an increase in action potentials (the quicker the muscle stretches the more action potentials)
  • regenerative action potentials then run in Ia afferent nerves back to spinal cord
32
Q

what does resting membrane potential of intrafusal fibres (muscle spindle) depend on

A
  • depends on length of muscle

- the faster the muscle contracts the more action potentials there are

33
Q

what happens when an agonist stretches

A
  • the antagonist relaxes
34
Q

are tendons elastic or inelastic

A

inelastic

35
Q

how do muscles develop tension

A
  • contracting to stretch the tendons creates tension
    (relaxed muscle = little to no tension)
    (passive contraction = bit of tension)
    (deliberate contraction = lots of tension)
36
Q

what happens to the golgi tendon organs during contraction

A
  • increased tension in tendons

- stretches nerve endings (1b afferent) in golgi tendon organs and initiates action potentials in the 1b afferent fibres

37
Q

what is the arrangement of muscle spindles and golgi tendon organs in relation to muscle fibres

A
  • muscle spindle lies in parallel with muscle fibres

- golgi tendon organs lie in series with muscle

38
Q

with nerves do muscle spindle and golgi tendon organs have

A
  • muscle spindle = 1a
  • golgi tendon organs = 1b
  • 1a faster than 1b
39
Q

what happens to golgi tendon organs and muscle spindle during isometric contraction

A

(e. g. holding weight)
- golgi tendon organ 1b afferents fire action potentials
- muscle spindle 1a afferents do not fire action potentials

40
Q

what is alpha gamma co-activation in proprioception

A
  • when muscle spindle 1a afferents report muscle shortening to brain and then gamma motor neurones fire action potentials to cause contraction of ends of intrafusal muscle (muscle spindle)
  • this maintains spindle sensitivity to stretch (prevents them getting floppy)
41
Q

what is the stretch reflex

A
  • you have muscle stretch (maybe due to holding heavy load, arm drops due to weight and muscle stretches)
  • muscle spindles are stimulated (1a afferents fire)
  • this causes a reflex muscle contraction
  • muscle shortens to previous length (so you don’t drop heavy weight)
  • so aim is to maintain length of muscle, stretch causes reflex contraction
42
Q

what are the three types of connections 1a afferents make in response to stretch

A
  • 2 in spine, 1 in brain
    1. afferents directly contact alpha motorneurones in stretched muscle, causes rapid contraction of agonist muscle, this is a monosynaptic reflex (one synapse, no interneurones)
    2. afferents connect indirectly with antagonist muscle, spindle afferents activate inhibitory interneurones, this decreases activation of alpha motorneurons to antagonist muscle, antagonist muscle relaxes (agonist therefore contracts), this is called reciprocal inhibition
    3. spindle afferents travel up dorsal columns to thalamus and somatosensory cortex and tell the brain about the length of muscle
43
Q

what nerves are involved in the inverse stretch reflex which is mediated by the golgi tendon organ

A
  • 1b afferent nerves
44
Q

what nerves are involved in the stretch reflex mediated by the muscle spindle

A
  • 1a afferent nerves
45
Q

what is the inverse stretch relfex

A
  • protects muscle against damage
  • rapid relaxation of agonist and rapid contraction of antagonist
  • avoids muscle contracting so hard that the tendon is ripped off the bone
46
Q

what happens in the inverse stretch reflex which is mediated by the golgi tendon organ

A
  • muscle contracts and shortens
  • this pulls on the tendon 1b afferent nerves in the golgi tendon organ which increases the firing of action potentials in the 1b
  • activation of inhibitory interneurones to agonist muscle (agonist relaxes)
  • activation of excitatory interneurones to antagonist muscle (antagonist contracts)
  • info about muscle tension is sent ascends in dorsal columns to somatosensory cortex
47
Q

what is the flexor/withdrawal reflex

A
  • nociceptors involved (type of proprioceptors which respond to pain)
  • withdraw part of the body away from painful stimulus and in towards body (so flexes affected part)
48
Q

what are the steps in the flexor/withdrawal reflex

A
  1. increased activity in the flexor muscle of the affected part via excitatory neurones
  2. at the same time, antagonistic extensors inhibited via a number of excitatory and inhibitory interneurones
  3. excitatory interneurones cross the spinal cord and excite the contralateral extensors (to balance out movement, one arm flex the other extends)
  4. other interneurones cross the spinal cored and they inhibit the contralateral flexors
  5. sensory info ascends to the brain in the contralateral spinothalamic tract
    (similar movements to walking)
49
Q

how is the golgi tendon organ reflex over-ridden

A
  • consciously by voluntary input from the CNS
  • so if you are holding important load such as baby and think I don’t want to drop this
  • descending voluntary excitation of alpha motorneurones overrides the inhibition from the golgi tendon organ and maintains muscle contraction and so prevents the golgi tendon organ reflex
50
Q

how is the stretch reflex over-ridden

A
  • strong descending inhibition in an anxious patient hyper polarises alpha motor-neurones and the stretch reflex can be hard to evoke
  • so absence of stretch reflex doesn’t always mean nerve damage
51
Q

can the withdrawal reflex be overridden

A
  • it can be … briefly
  • you can mentally stop yourself from dropping say a bowl of hot soup because you know that it is going to burn your legs
  • similarly, anticipating pain can increase the vigour of the withdrawal when a painful stimulus arrives
52
Q

what is facilitation in spinal reflexes

A
  • like how if you burned your finger you would have a small withdrawal reflex but if you put your whole hand on a hot surface you would have a large withdrawal response and jump away
  • so facilitation enhances the effect of sensory inputs
  • so say you have receptors A,B,C, if A stimulated, it also depolarises B slightly and if C is stimulated is also depolarises B slightly, so if A and C are stimulated at the same time C will also reach threshold and therefore we will have a bigger reaction

principles of normal function (15 decks)

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