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PHOL0005 - Structure & Function Of The Nervous System > Reflexes & Motor Control (Theme B) > Flashcards

Reflexes & Motor Control (Theme B) Flashcards

1
Q

Brodmann areas define cortical territories. Which are the Brodmann areas for primary motor cortex and the supplementary motor area?

A

Primary Motor Cortex - Brodmann area 4

Supplementary Motor Area - Brodmann area 6

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2
Q

How can the corticospinal tract be both convergent and divergent upon its spinal targets?

A

Convergence:
Multiple cortical pyramidal neurones across a widespread cortical area project down to individual spinal motorneurones and / or interneurones

Divergence
Individual cortical pyramidal neurones have a single axon passing down the corticospinal tract but it branches to give multiple terminations upon multiple spinal motorneurones and / or interneurones

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3
Q

Does the primary motor cortex code for force or for something higher level, such as direction of movement?

A

An important current area of controversy…

Individual cortical pyramidal (M1) neurones have firing rates that correlate with the forces generated in the muscles they control - demonstrating force coding

But large numbers of cortical pyramidal (M1) neurones can be seen to have a population code for higher aspects: including movement direction coding

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4
Q

During a reaching movement, the premotor area is important for planning movement based on visual cues related to the object to be reached. Which inputs to the premotor area are important for providing the necessary visual information?

A

(Visual) Parietal cortical areas 5 & 7

Parietal area 5 - where in space the object is
Parietal area 7 - identification of object properties (e.g. how heavy)

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5
Q

Where do the majority of cerebellar dentate nucleus cells project to?

A

To the ventrolateral division of the contralateral thalamus

From there, the thalamic cells project to the primary motor cortex and the premotor area. There are no direct projections from dentate nucleus to the cerebral cortex.

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6
Q

Which cerebellar cortical cell type and which of its inputs are thought to be important for aspects of motor learning?

A

The Purkinje Cell.

Conjunctive activity of climbing fibre inputs from the inferior olive & mossy-parallel fibre inputs to the Purkinje cell results in LONG TERM DEPRESSION (a depression of efficacy of the parallel fibre synapses)

This may be the cellular basis of aspects of motor learning

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7
Q

Within the basal ganglia circuitry, why does activity through the direct pathway promote movement whereas activity through the indirect pathway suppresses it?

A

In the direct pathway:
2 x inhibitory synapses = disinhibition of outputs = excitation
Promotes movement

In the indirect pathway:
3 x inhibitory synapses = disinhibition (excitation) of final inhibitory synapse = inhibition of outputs
Suppresses movement

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8
Q

Why does dopamine acting on neurones of the striatum lead to promotion of movement through the direct and the indirect pathways?

A

In the direct pathway:
- Striatal cells that project to globus pallidus internal segment (GPi) & substansia nigra pars reticulata (SNr) express D1 receptors
- DA is excitatory upon D1 receptors
- Therefore DA inc excitation in direct pathway - promoting movement

In the indirect pathway:
- Striatal cells that project to globus pallidus external segment (GPe) express D2 receptors
- DA is inhibitory upon D2 receptors
- Therefore DA dec excitation in indirect pathway (decreases inhibition of movement) - promoting movement

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9
Q

All reflexes involve a receptor, an afferent limb, an integration centre, an efferent limb and an effector. Define these separate elements for the dynamic muscle stretch reflex.

A

In the dynamic muscle stretch reflex:

Receptor = the muscle spindle
Afferent limb = 1a afferents that enter the SC through the dorsal horn
Integration centre = single set of synaptic contacts to alpha motorneurones of the ventral horn of the SC
Efferent limb = alpha motorneurone axons in a motor nerve
Effector = the same (homonymous) muscle from which the spindle signal originated. (I.e., synergistic muscle of the homonymous muscle)

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10
Q

Define a motor pool

A

All of the motor neurones that project to a single muscle

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11
Q

Define a motor unit

A

A single motor neurone and the muscle fibres that it contacts. This is the basic unit of motor organisation.

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12
Q

Define the “size principle” of motor-neurone recruitment

A

A small motor unit has small motoneurones with high input resistance. These are more easily activated by weak synaptic drive than large motor units with large, low input resistance motoneurones.

Synaptic drive to the motor pool activates small motor units most easily, with increasing levels of synaptic drive necessary to activate larger and larger motor units.

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13
Q

Muscle spindles detect passive stretch but how do they continue to signal during active contraction & shortening of the muscle?

A

ALPHA-GAMMA COACTIVATION

Alpha-gamma coactivation is the organised activation of alpha motoneurones and gamma motoneurones.

Alpha motoneurones activate extrafusal fibres (responsible for generating force during muscle contraction) and gamma motoneurones control contraction of intrafusal fibres in the spindle.

During isotonic contractions (muscle contractions when the muscle changes length), alpha motoneurones stimulate contraction of extrafusal fibres - leading to generation of force for movement. Simultaneously, gamma motorneurones stimulate the intrafusal fibres (within the muscle spindle) to contract) - so that the muscle spindle doesn’t become slack and remains sensitive to changes in muscle length during contraction.

Alpha-gamma coactivation helps maintain a ‘length register’ between intrafusal & extrafusal fibres - so as the muscle shortens / lengthens during contraction, the muscle spindle (intrafusal fibres) remain appropriately stretched, allowing it to continuously monitor changes in muscle length.

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14
Q

What is reciprocal inhibition in the stretch reflex?

A

The 1a afferents that excite the alpha motoneurones of the agonist muscle also branch to excite 1a inhibitory neurones that project to the alpha motoneurones of the antagonist muscle.

Causing inhibition of antagonist muscle during excitation of agonist muscle.

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15
Q

What class of afferents is important for the flexion-crossed extension reflex?

A

A-delta fibres.
These are fast, nociceptive afferents.

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16
Q

Which muscles are inhibited on the contralateral side in the flexion-crossed extension reflex?

A

The contralateral limb flexors

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17
Q

The motor system controls 3 main types of movement. What are these?

A
  1. Reflexive
  2. Rhythmic
  3. Voluntary
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18
Q

What is the reflex arc?

A

Sensory receptor -> afferent limb -> integrating centre (e.g., interneurone) -> efferent limb -> effector organ

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19
Q

What are the 2 types of reflexes?

A
  1. Protective
  2. Regulatory
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20
Q

What are 3 examples of protective reflexes?

A
  1. Eye-blink
  2. Gag reflex
  3. Limb withdrawal reflex
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21
Q

What are 2 examples of regulatory reflexes? What do they do?

A
  1. Vestibulo-ocular reflex - maintains gaze
  2. Muscle-stretch reflex - helps maintain limb position
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22
Q

Skeletal muscle reflexes involve reflex pathways through which parts of the body?

A

Spinal cord or brainstem

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23
Q

The ventral horns of the spinal cord grey matter contain what type of motor neurones?

A

Alpha motor neurones (‘motoneurones’)

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24
Q

What are the locations of alpha motoneurones in the ventral horn of the SC organised by?

A

The muscles they control

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25
Q

Where are (alpha) motoneurones for proximal muscles (e.g., the trunk muscles) located in the ventral horn?

A

Medial

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26
Q

Where are (alpha) motoneurones for flexor muscles (e.g., biceps) located in the ventral horn?

A

Closer to the central canal

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27
Q

Where are (alpha) motoneurones for distal muscles (e.g., finger muscles) located in the ventral horn?

A

Lateral

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28
Q

Where are (alpha) motoneurones for extensor muscles (e.g., biceps) located in the ventral horn?

A

Further from the central canal and closer to the periphery of the grey matter

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29
Q

What is the motor neurone pool?

A

The total motor neurone projection to a single muscle
(Can span several spinal cord segments)

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30
Q

What is the motor unit?

A

A single alpha-motoneurone and the muscle fibres that it contacts (the basic unit of motor organisation)

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31
Q

What is the difference between small and large motor units?

A

In small motor units:
- a single motoneurone may contact 10-20 muscle fibres
- motoneurone will be small, with HIGH input resistance

In large motor units:
- a single motoneurone may contact >100 muscle fibres
- motoneurone will be large, with LOW input resistance

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32
Q

Why does relative input resistance change depending on the size of the motoneurone?

A

If we consider synaptic drive as current in Ohm’s law (V=IR), changes in membrane potential will be greater in smaller motor neurones than in larger ones in response to the same synaptic drive.

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33
Q

Muscle force is regulated by which 2 elements?

A
  1. Recruitment of motor units
  2. Firing rate (rate coding) of recruited units

Tension produced in the whole muscle is a function of the number of motor units recruited, and their firing rates

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34
Q

How does Henneman’s size principle of recruitment work?

A

Small motor units - small motoneurones with high input resistance - activated at lower levels of synaptic drive so get activated first

As synaptic drive increases - increases firing rate of recruited motor units & activates & recruits progressively larger motor units

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35
Q

What does Henneman’s size principle of recruitment allow for?

A

Progressive increases of tension in the muscle

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36
Q

Tension produced in the muscle is a function of what 2 things?

A
  1. The number of motor units recruited
  2. The firing rate of recruited motor units
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37
Q

What do the muscle spindles detect?

A

Length changes in the muscle

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38
Q

The muscle spindle is what type of receptor?

A

Proprioceptor

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39
Q

What shape are muscle spindle fibres?

A

Ellipsoid

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40
Q

What happens to the firing rate of muscle spindles as the muscle stretches (increases in length)?

A

Increases

41
Q

What do extrafusal fibres do? Where are they located? And what are they activated by?

A

Extrafusal fibres are the main, force-generating fibres of the muscle
They are NOT located in the muscle spindles
They are activated by alpha motoneurones from the ventral horn of the SC

42
Q

What are intrafusal muscle fibres? What do they do? Where are they located?

A

Specialised fibres located in the muscle spindles, which are sensitive to length changes - have mechanically sensitive nerve endings in the middle

Muscle spindle firing rates increase when the fibres are stretched

Gamma motor neurones stimulate them to contract when extrafusal fibres contract - ensuring that they don’t become ‘slack’ (alpha-gamma coactivation)

43
Q

Compare Ia and type II afferents (in motor control)

A

Ia afferents: Largest diameter, fastest-conducting axons in the PNS, respond to dynamic changes & report velocity of length change

Type II afferents: Smaller diameter, slower conducting, report static & slowly-changing lengths

44
Q

The Muscle Stretch Reflex can be tested as what type of reflex?

A

A Tendon Jerk Reflex (e.g., the Knee jerk reflex - by tapping the patellar tendon)

45
Q

What is the purpose of the muscle stretch reflex?

A

Help maintains limb position as loads vary

46
Q

What is the process of the dynamic stretch reflex?

A
  1. If a sudden load is applied, the muscle spindles stretch
  2. Fast-conducting Ia afferents synapse directly with the efferent alpha motoneurones which supply the same (homonymous) muscle (monosynaptic)
  3. Alpha motoneurones stimulate the muscle to contract
47
Q

What is the name of the only monosynaptic reflex in the nervous system?

A

The muscle stretch reflex

48
Q

Reciprocal inhibition of the antagonist muscle in the stretch reflex is provided by what?

A

The Ia inhibitory interneurone

49
Q

What does reciprocal inhibition of the antagonist muscle in the stretch reflex do?

A

Allows for the relaxation of the antagonist muscle during the stretch of the agonist

50
Q

Descending influences on the Ia inhibitory interneurone can modulate the level of reciprocal inhibition of the antagonist in the stretch reflex. What is an example of when this might be useful?

A

In instances where we want co-contraction (e.g., co-contraction at the knee when stepping down stairs)

51
Q

Why is co-contraction important?

A

It makes the joint stiff and resistant to sudden, unpredictable loads

52
Q

Clinical tests of the integrity of the muscle stretch reflex can test for what?

A

Spinal injury
Dorsal root / ventral root problems
Muscle disease & injury

53
Q

What are the Golgi Tendon Organs (GTOs), where are they located, and what do they detect?

A

GTOs are receptors located in the tendon, lie in series with the muscle fibres

Detect tension (force) created by muscular contraction & the loads supported

54
Q

What are the afferents of Golgi Tendon Organs?

A

Ib fibres (don’t have as large diameters as Ia)

55
Q

When does the Inverse Myotatic Reflex (tendon organ reflex) occur?

A

At very high tension forces (only)

56
Q

What is the function of the inverse myotatic reflex?

A

To prevent excessive tension in the muscle
Helps regulate force / tension in the muscle

57
Q

What is the function of the flexion withdrawal & crossed-extension reflex?

A

Allows limb to be withdrawn from a painful stimulus

58
Q

In the flexion withdrawal & crossed-extension reflex, which muscles are activated and which are inhibited?

A

On the affected side - the limb to be ‘withdrawn’
- Flexor muscle activated
- Extensor muscle inhibited

On the contralateral side
- Extensor muscle activated
- Flexor muscle inhibited

59
Q

What is the H-reflex?

A

A spinal reflex elicited by artificial (electrical) stimulation of the nerve

60
Q

In the H-reflex, why does the H-wave disappear at high stimulus strength?

A

All of the motor efferents are now activated & transmit orthodromically to the muscle - producing the M-wave

However, the motor efferents also transmit antidromically, which collides with the orthodromic transmission of the sensory Ia afferents

This prevents the sensory Ia afferents from transmitting to the SC, the monosynaptic reflex doesn’t occur & the H-wave disappears

61
Q

How do you measure the H-wave & M-wave to calculate the conduction velocity?

A

H-wave = (the distance between the point of stimulation to the SC) + (the distance between the SC to the muscle)
* NB: it depends which measurements you are provided as to which distance you use for the calculation

M-wave = the distance between the point of stimulation and the muscle

62
Q

What are strong modulators of the Ib inhibitory interneurones (used in the tendon organ reflex)?

A

Spinal inputs from the joints & skin
Descending influences from the brain

63
Q

What are the inhibitory interneurones used in:
1. Reciprocal inhibition of the antagonist muscle in the stretch reflex
2. The inverse myotatic reflex

A

Reciprocal inhibition of the antagonist muscle in the stretch reflex -> Ia inhibitory interneurone

The inverse myotatic reflex -> Ib inhibitory interneurone

64
Q

What are the 3 major brain regions for supraspinal motor control?

A
  1. Motor cortex
  2. Cerebellum
  3. Basal ganglia
65
Q

What are the lateral descending motor pathways?

A

Corticospinal tracts
Rubrospinal tracts

66
Q

What do the corticospinal and Rubrospinal tracts control?

A

Fine fractionated & mostly voluntary movements of the limbs & fingers

67
Q

Where does the corticospinal tract originate?

A

In the cortex (mostly from the motor cortex)

68
Q

Where does the Rubrospinal tract originate?

A

In the red nucleus
(In the tegmentum of the midbrain)

69
Q

What are the ventromedial descending motor pathways?

A

Vestibulospinal tracts
Tectospinal tracts
Reticulospinal tracts

70
Q

What do the vestibulospinal & tectospinal tracts control?

A

Posture of the head & neck

71
Q

What do the Reticulospinal tracts control?

A

Posture of the trunk & antigravity muscles of the limbs

72
Q

Where do the Reticulospinal tracts originate from?

A

From the reticular formation (brainstem), influenced by the motor cortex

73
Q

What are the motor regions of the cerebral cortex?
(In order of threshold for producing movement - lowest first)

A

M1: Primary Motor Cortex - Brodmann Area 4

PMS: Premotor Cortex - Brodmann Area 6
1. PM: Premotor Area
2. SMA: Supplementary Motor Area

S1: Somatosensory Cortex

Area 5: Brodmann Area 5 of Posterior Parietal Cortex
Area 7: Brodmann Area 7 of Posterior Parietal Cortex

Prefrontal cortex

74
Q

What is the location of the Primary Motor Cortex (M1 / Brodmann Area 4)?

A

Lies in the precentral gyrus

75
Q

What is the location of the Premotor Cortex (PMS / Brodmann Area 6)?

A

Immediately rostral/anterior to the precentral gyrus & Primary motor cortex

76
Q

The premotor cortex (PMS / Brodmann area 6) is made up of which structures?

A
  1. PM: Premotor area
  2. SMA: Supplementary motor area
77
Q

What are the 2 thalamocortical connections?

A

Ventral lateral nucleus (VL)

Ventral anterior nucleus (VA)

78
Q

What does the Ventral lateral nucleus (VL) of the thalamus do?

A

It is the major input to the Primary motor cortex (M1)

(Thalamocortical connection)

79
Q

What does the Ventral anterior nucleus (VA) of the thalamus do?

A

Supplies the Premotor Cortex (PMS) - made up of the Premotor area (PM) & Supplementary motor area (SMA)

80
Q

What do individual & populations of M1 (primary motor cortex) pyramidal neurones code for?

A

Individual - code for muscle force (firing frequencies increases as force increases in their associated muscles)

Population - potential role in coding higher aspects of movement (i.e., direction)

81
Q

Where does the premotor area (PM) of the premotor cortex (PMS) get inputs from?

What is it involved in?

A

Inputs from: visual parietal areas 5 & 7, the prefrontal cortex, the cerebellum (via thalamus)

Involved in: planning movements based on external (particularly visual) cues

82
Q

Where does the Supplementary Motor Area (SMA) of the premotor cortex (PMS) get inputs from?

What is it involved in?

A

Inputs from: prefrontal cortex, basal ganglia (via thalamus)

Involved in: planning movements based on internally generated strategies (e.g., learned sequences of movements)

83
Q

The Supplementary Motor Area (SMA) is involved in planning movements based on internally generated strategies (e.g., learned sequences of movements).

How has this been shown experimentally?

A

SMA is active during mental rehearsal (planning only) of a learned sequence, without any overt movements

84
Q

The Supplementary Motor Area (SMA) is involved in planning movements based on internally generated strategies (e.g., learned sequences of movements).

How has this been shown experimentally?

A

SMA is active during mental rehearsal (planning only) of a learned sequence, without any overt movements.

85
Q

The premotor cortex (PMS) - premotor area (PM) + supplementary motor area (SMA) is important for what?

A

Higher levels of motor planning

86
Q

Describe the convergence & divergence of corticospinal projections

A
  • Convergence → different regions of cortex can control the same muscle / muscle group
  • Divergence → Primary motor cortex pyramidal neurones project to several motoneuron & interneuron pools
87
Q

Describe the path of cerebellar outputs from the MEDIAL cerebellar cortex

A

Medial cortex → fastigial nucleus → vestibular nuclei → Vestibulospinal tract (to influence posture & balance, and via ascending connections, to influence eye movements)

88
Q

Describe the path of cerebellar outputs from the INTERMEDIATE cerebellar cortex

A

Intermediate cortex → globose & emboliform nuclei (humans) → red nucleus → Rubrospinal tract (influences ongoing movements)

89
Q

Describe the path of cerebellar outputs form the LATERAL cerebellar cortex

A

Lateral cortex → dentate nucleus → thalamus → motor cortex (M1 & PMC) → corticospinal tract (influence voluntary movements, especially of the limbs)

90
Q

The cerebellum controls which type of muscle interactions?
What is the effect of when this fails in cerebellar patients?

A

Controls agonist-antagonist interactions

Failure of these in cerebellar disruption underlies the intention tremor seen in cerebellar patients

91
Q

The cerebellum is located above which ventricle?

A

4th

92
Q

What are the afferent inputs of the cerebellum?

A

Mossy fibres from:
- Spinal cord
- Vestibular system
- Visual, auditory & other cortical areas, via pontine nuclei

Climbing fibres from:
- (only) the inferior olive in the medulla

93
Q

What is the hallmark sign of cerebellar injury / disease?

A

Ataxia → Loss of coordination in movement

  • Difficulty controlling the distance, velocity & force of movements
  • Relationships between synergist muscles break down
  • Control of agonist & antagonist muscles across joints is particularly affected
94
Q

What are the symptoms of cerebellar injury / disease?

A
  • Slurred speech
  • Trouble eating & swallowing
  • Deterioration of fine motor skills
  • Difficulty walking
  • Gait abnormalities (pattern of walking)
  • Eye movement abnormalities
  • Intention tremor - tremors during voluntary movements (not resting tremor)
95
Q

What are the components of the basal ganglia?

A
  • Striatum → Caudate + Putamen
  • Globus Pallidus → Internal + External segments
  • Subthalamic nucleus
  • Substantia nigra → Pars reticulata + Pars compacta
96
Q

Compare the Direct & Indirect pathway in the basal ganglia

A

Direct pathway → Overall excitation (disinhibition: 2 x GABA-ergic neurones)

Indirect pathway → Overall inhibition (3 x inhibition)

97
Q

What is the action of dopamine in the direct pathway ?

A
  • Striatal cells that project to GPi and to SNr express D1 receptors
  • DA is excitatory upon D1 receptors → therefore DA inc excitability of the direct pathway and promotes movements
98
Q

What is the action of dopamine in the indirect pathway?

A
  • Striatal cells that project to GPe express D2 receptors
  • DA is inhibitory upon D2 receptors → therefore DA dec excitability of the indirect pathway and promotes movement
99
Q

Hyperkinetic basal ganglia disorders such as ballism & HD indicate what about the basal ganglia?

A

Indicate that it may normally filter out unwanted movements

PHOL0005 - Structure & Function Of The Nervous System (19 decks)

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