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Introduction to neuroscience > Reflexes and the somatosensory system > Flashcards

Reflexes and the somatosensory system Flashcards

1
Q

What does monosynaptic mean?

A

Only one synapse is involved

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2
Q

What do extensor and flexor muscles do?

A

Extensor- straighten/extend limbs

Flexor- bend/reduce angle at joints of limbs

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3
Q

What is the myotactic/stretch reflex?

A

Tapping on the knee stretches the thigh extensor muscle and associated tendon
Sets in motion process to correct the stretching
Important in maintaining posture and is part of the proprioceptive system

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4
Q

Steps in the myotactic reflex

A

Sensory fibres sense muscle stretch and send signals to SC
Direct monosynaptic connection to motor neuron fires AP which contracts bicep muscle
Simultaneously an inhibitory interneuron ihibits firing of motor neurons connected to the antagonistic muscle to relax it

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5
Q

What is reciprocal inhibition?

A

The relaxing of the antagonistic muscle to the one that is being contracted to accommodate the contraction

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6
Q

What are muscle spindles?

A

Special sensory receptor that detects muscle stretch
One of the body’s many proprioceptors
Found in striated muscle, most abundant in areas of fine motor control

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7
Q

What are muscle spindles innervated by? What do they provide feedback to?

A

Ia sensory fibres

Alpha motor neurons (MNs innervating the muscles around the spindle)

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8
Q

What are gamma motor neurons?

A

Stimulate intrafusal muscle fibres to adjust tension in the muscle spindle as the extrafusal muscle fibres of surrounding muscle contract so that the spindle is never slack

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9
Q

What is a golgi tendon organ (GTO)?

A

Another kind propioceptor that detects muscle tension due to muscle contraction, not muscle stretch (which is detected by muscle spindles)

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10
Q

What happens when GTO sensory afferents are activated?

A

Activation of inhibitory interneurons which inhibit motor neurons that innervate the same muscle
So GTO reflex is a negative feedback circuit that regulates muscle tension to protect it from damage when large force is applied

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11
Q

What is the flexor or withdrawal reflex?

A

Quick contraction of flexor muscles to withdraw a limb from an injurious stimulus (e.g. heat or cut)
Arises from stimulation of nociceptors
Polysynaptic despite speed of response- parallel after discharge circuit

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12
Q

What is a parallel after discharge circuit? Why is this useful?

A

When a sensory neuron outputs to several other neurons that converge back onto a singular neuron
Initial signal is sustained over an extended period

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13
Q

What is the crossed extensor reflex?

A

Provides postural support during withdrawal reflex as rapid withdrawal of a limb would cause imbalance

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14
Q

What happens during the crossed extensor reflex?

A

Stimulus > sensory neuron activates inhibitory and excitatory neurons
Ipsilateral neurons to flexor are excited and cause ipsilateral flexor to contract
Contralateral neurons to extensor are excited and cause contralateral extensor to contract

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15
Q

What is the difference between cranial and spinal reflexes?

A

Cranial reflexes are processed/controlled by the brain, happen in the head region and use cranial nerves
Spinal reflexes are processed/controlled by the spinal cord, happen in the rest of the body and use spinal nerves

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16
Q

Example of a spinal monosynaptic reflex and a spinal polysynaptic reflex

A

Mono- Myotactic/stretch reflex

Poly- Flexor/withdrawl reflex

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17
Q

What are central pattern generators?

A

Local circuits that can generate the pattern of alternating flexion and extension that we see in locomotion
Rhythmic neuron firing

Severing spinal cord connections to the brain doesn’t stop alternation

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18
Q

Where are messiner/tactile corpuscles located? What modality do they detect?

A

Dermal papillae of skin (palms, eyelids, lips, tongue)

Light touch and texture

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19
Q

Where are pacinian/lamellated corpuscles located? What modality do they detect?

A

Dermis, joint capsules and viscera

Deep pressure, stretch, tickle, vibration

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20
Q

Where are ruffini corpuscles located? What modality do they detect?

A

Dermis, subcutaneous tissue, joint capsules

Heavy touch, pressure, skin stretch, joint movements

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21
Q

What sensory receptors are encapsulated nerve endings?

A

Messiner/tactile corpuscles
Pacinian/lamellated corpuscles
Ruffini corpuscles

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22
Q

Where are merkel/tactile discs located? What modality do they detect?

A 
Superficial skin (epidermis)
Light touch. textures, edges, shapes
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23
Q

Where are free nerve endings located? What modality do they detect?

A

Widespread in epithelia and connective tissues

Pain, heat, cold

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24
Q

Where are hair follicles located? What modality do they detect?

A

Widespread in epithelia

Varied according to type

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25
Q

What sensory receptors are unencapsulated nerve endings?

A

Merkel/tactile discs and free nerve endings

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26
Q

Why are some sensory receptors encapsulated?

A

Capsules are involved in filtering to give specific frequencies

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27
Q

What sensory receptors are phasic?

A

Messiner corpuscles
Pacinian corpuscles
Some hair folicle subtypes

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28
Q

What sensory receptors are tonic?

A

Ruffini corpuscles
Merkel discs
Some subtypes of hair follicles

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29
Q

What are the sub types of primary afferent axons from the skin in order from fastest to slowest?
What are they called from the muscles?

A

A (split into alpha, beta, gamma), B, C

I, II, III, IV

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30
Q

How is sensory information organised in the spinal cord?

A

Cell bodies of different sensory neurons are grouped in the dorsal root ganglion
Their projections are organised into different layers of the dorsal horn
Spatially organised

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31
Q

What are the medial lemniscal tracts?

A

Carry mechanoreceptive and proprioceptive signals to the hypothalamus

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32
Q

What is the spinothalamic tract?

A

Carries pain and temperature signals to the thalamus

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33
Q

What does it mean when a neuron is commissural?

A

Crosses the midline (2nd order)

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34
Q

What do first order neurons do?

A

Detect the stimulus and transmit it to the spinal cord

35
Q

What do second order neurons do?

A

Relay the signal to the thalamus as it is the gateway to the cortex
In both medial lemniscal and spinothalamic tracts they cross the midline

36
Q

What do third order neurons do?

A

Carry the signal from the thalamus to the cortex

37
Q

What is topological organisation?

A

Spatial arrangement of the objects relative to one another

38
Q

In the ML system, what pathway do 1st order neurons from the upper body follow? Where do they synapse onto 2nd order neurons?

A

Lateral pathway

Cuneate nucleus

39
Q

In the ML system, what pathway do 1st order neurons from the lower body (below T6) follow? Where do they synapse onto 2nd order neurons?

A

Medial pathway

Gracile nucleus

40
Q

What are the dorsal column nuclei?

A

Cuneate nucleus and gracile nucleus

41
Q

In the ML system where do 2nd order neurons cross the mid line? What does this mean in terms of their topology?

A

Medial lemniscus

Reversed- upper body neurons are medial and lower body neurons are lateral upon entering the thalamus

42
Q

In the ML system, where do 3rd order neurons from the upper body synapse onto?

A

Lateral part of somatosensory cortex

Topology reversed again

43
Q

In the ML system, where do 3rd order neurons from the lower body synapse onto?

A

Medial cortical neurons

Topology reversed again

44
Q

Why are projections topologically organised like this?

A

Forms a map of the body in the somatosensory cortex due to the DRG being organised into dermatomes

45
Q

What is a dermatome? How do these arise?

A

Specific region of skin innervated by a sensory ganglion

The dermis of each region is derived from its somite

46
Q

What is a somite? How do these develop into dermatomes?

A

Iterated embryonic structures that give rise to the underlying musculature and skeleton
In the embryo, each sensory ganglion (DRG) is associated with a specific somite and subsequently innervates the tissues arising from that somite

47
Q

How many pairs of dorsal roots are there?

A

31

48
Q

How can you measure the size of a receptive field?

A

By assessing the ability to discriminate two sharp points set apart at different distances

49
Q

Why is more cortex dedicated to areas where receptive fields are smaller?

A

Where receptive fields are large, discrimination is low (legs and arms), whereas where receptive fields are small, discrimination is high (fingers)
The number of sensory neurons innervating an area depend on the behavioral importance of that area

50
Q

How are sensory modalities organised in the postcentral gyrus?

A

Different sensory modalities are localised along the sagittal axis

51
Q

What is the specificity theory?

A

Pain is a distinct sensation, detected and transmitted by specific receptors and pathways to specific pain regions of the brain

52
Q

What is convergence theory?

A

Pain is an integrated, plastic state represented by a pattern of convergent somatosensory activity within a distributed network

53
Q

What are the different types of nociceptors? What do they detect?

A

A delta fibres- fast, lightly myelinated, mechanosensitive and mechanothermal sensitive
C fiibres- slow, unmyelinated, polymodal (mechanical, thermal, chemical)

54
Q

What kind of receptor are nociceptors?

A

Afferents with free nerve endings

55
Q

How does the pain detected by A and C fibres differ?

A

A- fast, immediate pain ‘first pain’

C- slow, more diffuse, longer lasting, ‘second pain’

56
Q

What is TRPV1 and when is it activated?

A

Capsaicin receptor for A and C fibres

activated when temp is 45 degrees or above and by capsaicin- vanilloid which is the active component in chilies

57
Q

What are the two pathways that carry pain information to the brain?

A 
Sensory discriminative (spinothalamic tract)
Affective motivational
58
Q

What information does the sensory discriminative pathway carry?

A

Location, intensity and type of stimulus

59
Q

What information does the affective motivational pathway carry?

A

Signals unpleasantness and enables autonomic activation, fight or flight response

60
Q

What is the difference between the two pain pathways in terms of organistion?

A

SD- topologically preserved

AM- no topographic mapping

61
Q

What parts of the brain are used in the AM pathway?

A

Parabrachial nucleus- responds to painful stimuli anywhere on the body’s surface
Cingulate cortex- strong correlation of unpleasant experience
Limbic system

62
Q

What is hyperalgesia? What is the mechanism for this?

A

Increased response to a painful stimulus

Result of lowered nociceptor thresholds which heightens pain repsonse

63
Q

What is allodynia? What is the mechanism for this?

A

Painful response to a normally innocuous stimulus
Activity dependent local release of substances like prostaglandins
Relay neurons become sensitive to non-painful stimuli

64
Q

What is hyperpathia?

A

Variant of hyperalgesia and allodynia
Fibre/axonal loss or damage resulting in a higher detection threshold
So when threshold is reached excitability is much greater and pain is explosive

65
Q

What is central sensitisation?

A

When central pathways themselves are damaged

Diabetes, shingles, multiple scerosis, stroke

66
Q

What phenomena don’t fit into specificity theory?

A

Neuropathic pain
Referred pain
Central and external modulation of pain
Placebo effect

67
Q

What are upper motor neurons? Where are they found?

A

Neurons found in the brain that control motor function

Primary motor cortex, precentral gyrus

68
Q

How is the motor cortex somatotopically mapped?

A

Lower body is medial, upper boddy is lateral

Proportions reflect density of innervation and behavioral significance

69
Q

What do axial muscles control?

A

Trunk movement

70
Q

What do proximal muscles control?

A

Movement of shoulders, elbows, pelvis, knees

71
Q

What do distal muscles control?

A

Movement of hands, feet and digits

72
Q

What is a motor unit?

A

A motor neuron and all the muscle fibres it innervates

73
Q

How are motor pools organised in the spinal cord?

A

The motor pool for a single muscle are grouped in rod shaped clusters extending over several vertebral segments
Somatotopically organised- medio-lateral position indicates whether it innervates a more proximal or distal muscle

74
Q

What is a motor pool?

A

All the motor neurons that innervate a single muscle

75
Q

What descending tract is key for voluntary movement?

A

Corticospinal tract

76
Q

Where do the axons of the CST originate in the brain?

A

Large pyramidal cells in layer V of the motor cortex

77
Q

What layer of the motor cortex do main inputs come into?

A

Stellate cells in layer IV

78
Q

What is the difference between motor cortex upper neurons and brain stem upper neurons?

A

Motor cortex- primarily concerned with fine voluntary control of distal structures
Brain stem- project to medial motor pools primarily concerned with postural movement

79
Q

Where do axons of the CST cross the midline? Where does that mean they project?

A

Pyramidal decussation in the medulla

80
Q

Where do axons of the lateral CST synapse onto?

A

Laterally located lower motor neuron circuits that control distal muscles

81
Q

Where are upper motor neurons from the brain stem found?

A

Ipsilaterally in the vestibulospinal tract and the reticulospinal tact

82
Q

How do neurons in the vestibulospinal and reticulospinal tracts project? Where do they synapse onto?

A

Project medially in the spinal cord

Synapse on medially located motor neuron circuits that control axial muscles

83
Q

What are the ventromedial pathways?

A

Vestibulospinal tract

Reticulospinal tract

84
Q

What do neurons in the vestibulospinal tract control?

A

Head balance and turning

Introduction to neuroscience (8 decks)

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