Lectures 15, 16, 17 18 Motor System- upper and lower circuits, cerebellum and basal ganglia Flashcards
1
Q
what are the 4 types of muscle control?
A
voluntary, goal-directed, habit and involuntary
2
Q
which neurons govern motor control
A
upper and lower motor neurons
3
Q
describe the location of lower motor neurons
A
cell bodies in brainstem or spinal cord with projections to muscle
4
Q
describe the location of upper motor neurons
A
cell bodies in brain (motor cortex) and projections down to lower motor neurons (in brainstem/spinal cord)
5
Q
define M2
A
motor association cortex
6
Q
does M2 directly effect muscles?
A
no - sends output to motor cortex
7
Q
basic role ofcerebellum
A
fine motor control/ correcting errors
8
Q
do muscle fibres act in an all or non manner?
A
yes
9
Q
what causes muscle strength
A
the number and coordination of many muscle fibres
10
Q
describe antagonistic arrangement
A
combined or coordinated muscle action e.g. biceps and triceps
11
Q
are the number of muscle fibers in a person more dependent on training or genetics?
A
genetics
12
Q
if the number of muscle fibers is primarily genetic, what does training do?
A
gives you different types of muscle fibre
13
Q
how is a skeletal muscle attached to the bone?
A
tendon
14
Q
what is a skeletal muscle made up of?
A
several muscle fasciculi
15
Q
what is muscle fasciculus made up of?
A
several muscle fibres (cells)
16
Q
what is a muscle fibre made up or?
A
several myofibrils
17
Q
what do myofibrils contain?
A
protein filaments- actin and myosin microfilaments
18
Q
describe the sarcomere when muscle fibres contract
A
the muscle fibre is depolarised
the release of ACh causes a cascade of events resulting in the release of calcium inside muscle fibre,
the head of the myosin filaments attaches to the actin filament and pulls itself along by bending its head (contraction), which requires ATP and magnesium
ATP then breaks this bond, and the cycle starts again
19
Q
what causes rigor mortis?
A
oxidative metabolism (respiration) stops upon death, hence no more ATP is being reduced. This means myosin heads bound to actin filaments cannot break the bond causing whole body stiffness
20
Q
what ends rigor mortis
A
enzymes begin to disrupt the myosin-actin bond
21
Q
define a motor unit
A
a single alpha motor neuron and all the muscle fibres it innervates
22
Q
the fewer fibres a single motor neuron innervates…
A
the greater the movement resolution e.g. toungue/fingertips
23
Q
does activation of an alpha MN cause contraction of all muscle fibres in unit
A
yes (all or none)
24
Q
if more motor units fire, what’s the result
A
more fibres contracting hence more power
25
what 2 factors are effecting by the number of muscle fibres innervated by a single MN?
1- level of control (the fewer fibres a single MN innervates, the finer the level of control)
2- strength (the more fibres a single MN innervates, the greater the control)
26
what is the size principle
the idea that motor units are recruited in order of size- the smallest first (fine motor control requires lower force)
27
what are the 3 types of muscle fibre?
slow twitch
fast fatigue resistant twitch
fast fatigable twitch
28
what kind of actions do slow twitch fibres allow
maintenance of posture
29
what kind of actions do fast-fatigue resistant muscle fibres allow
longer/moderate activity e.g. hiking
30
what kind of actions do fast-fatigable muscle fibres allow
very quick and high energy movement
31
how does training effect muscle fibres
It changes the proportion of each muscle fibre type
32
describe the motor pool
all lower MNs that innervate a single muscle
it contains both alpha and gamma MNs which have cell bodies in spinal cord and project to muscle fibre
33
describe spinal cord mapping of motor neurons
lateral MNs in grey matter innervate more distal muscles, whereas medial MNs in grey matter innervate more proximal muscles
34
what 2 inputs innervate MNs in ventral horn?
sensory information from muscle and descending information from the brain
35
what are the 2 proprioceptors in the muscle
golgi tendon organs and muscle spindles
36
describe golgi tendon organs
they sense intrinsic muscle tension, communicating level of force in a muscle- critical for both proprioception and preventing damage
37
describe muscle spindles
they detect stretch and are important in reflex, eg. patellar/knee-jerk, allowing stretched muscle to return to its previous status
38
describe the mechanism of a stretch reflex
muscle spindle comprised or intrafusal muscle fibers
when muscle is stretched, there's an increase in activity in 1a afferents and activity of alpha-motor neurons that innervate the same muscle. Whilst 1a afferents also excite the MNs that innervate synergetic muscles, to inhibit antagonist contraction so stretched muscle is contracted back to previous muscle status
39
describe muscle spindle feedback
muscle spindles detect how much a muscle is stretching with sensory fibers coiled around intrafusal fibres, innervated by gamma motor neurons which keep intrafusal fibres at a set length
40
describe the withdrawal reflex with reciprocal innervation using the example of standing on a pin
cutaneous nociceptors stimulation in the foot
sends sensory signal to spinal cord and synapses with motor neuron at SC level with motor neurons which contract flexor muscle on the leg of the painful stimuli and relax extensor muscle and vice versa on other leg, so the pain leg withdraws but the other leg straightens so balance is not lost
41
overview of cats landing on their feet
they have a righting reflex/vestibular righting reflex
when falling/not upright, the vestibular system detects that the body is not upright (orientation) and that animal is falling
vestibular, visual, somatosensory and proprioceptive inputs combes to result in a corrective pattern of motor behaviour to land on feet the right way up with the cerebellum involved in comparison between motor plan and actual situation
42
draw an overview of the sensorimotor system with descending and ascending pathways
see image in notes/lecture slide
43
Overview the involvement of brainstem structures in motor control
pathways and nuclei within the brainstem connect sensory input to motor output directly.
This is an evolutionarily ancient pathway but remains important.
it is involved in balance and postural control (vestibular system) and speech
44
describe how speech exhibits cooperation between the ancient and modern brain
the ability to vocalise is evolutionarily ancient with the cooperation between the respiratory, articulatory (mouth/tongue/jaw) and phonatory system (voice box/larynx) being well preserved to allow primitive vocalisations. In more modern evolution, in humans, these vocalisations receive sculpting commands from motor cortex and brainstem, allowing for the speech humans can produce
45
is it true, that the primary motor cortex (upper MNs) can be only 1 synapse away from lower MNs which innervate muscle cells
YES
46
briefly overview the descending projections from cortical MNs
the upper MNs originate in layer 5/6 grey matter of the motor cortex and travel down the pyramidal tract directly or indirectly (via brainstem) to spinal cord, where they synapse with lower MNs
47
are most cortical projections innervates contra- or ipsilateral motor units?
contralateral
48
what else modulates the motor cortex?
Basal Ganglia (inhibitory)
Cerebellum (excitatory)
49
what is the motor homunculus
the topographically mapped figure which indicates the space taken up my the motor movements of different body parts/muscles
50
why is the actual motor cortex more complex than the homunculus topographic map?
there is realistically lots of overlap and interaction between motor commands, with few motor actions requiring the activation of a single motor unit- for example hand grasping uses a range of overlapping units
51
what are the 2 main descending motor projections from the motor cortex and where in the spinal cord do they travel?
dorsolateral tract- travels at the back, laterally of the SC WM
ventromedial tract- travels at the front in the middle of the SC WM
52
what are the 2 dorsolateral tracts
dorsolateral corticospinal tract
dorsolateral Corticorubrospinal tract
53
describe the dorsolateral corticospinal tract
travels down to innervate contralateral distal limb muscles
54
describe the dorsolateral Corticorubrospinal tract
also innervates distal limb muscles, but has synapses with the red nucleus
55
In which part of motor cortex do dorsolateral tracts originate?
lateral
56
what are the 2 ventromedial tracts?
ventromedial corticospinal tract
ventromedial cortico-brainstem spinal tract
57
describe the ventromedial corticospinal tract
its primarily ipsilateral, with some bilaterality to innervate the trunk/proximal limb muscles
58
describe the ventromedial cortico-brainstem spinal tract
synapses on brainstem nuclei including tectum, vestibular nuclei, reticular formation and motor nuclei of cranial nerves, before travelling down SC, with both ipsi- and contralateral innervation
59
In which part of the motor cortex do ventromedial tracts originate
medial motor cortex
60
give 2 similarities between dorsolateral and ventromedial motor pathways
both contain direct corticospinal route
both contain an indirect route via brainstem nuclei
61
give 3 differences between dorsolateral and ventromedial motor pathways
- dorso-lateral brainstem pathways go via red nucleus, whereas ventromedial go via reticular formation, tectum, cranial nerve nuclei and vestibular nuclei
-dorsolateral innervates distal muscles and ventromedial innervates proximal muscles
-dorsolateral innervates the contralateral side of 1 segment of spinal cord, whereas ventromedial innervates both sides at several segments of spinal cord
62
what is/are basal ganglia
group of deep structures beneath the cortex acting as a gate-keeper/modulator of the motor system
63
what are the components of the basal ganglia?
caudate nucleus
globus pallidus
putamen
substantia nigra
thalamus
subthalamic nucleu
64
where does the BG receive input from, and what kind of input?
excitatory input from cerebral cortex
65
what is the primary output or the BG and what kind of output
inhibitory output to the thalamus (which then inputs to the cerebral cortex)
66
what makes up the striatum?
caudate and putamen
67
which 2 parts of the BG are primarily involved in releasing BG inhibition?
substantia nigra and striatum
68
describe the selection problem and links to BG
BG is involved in solving the selection problem.
we have multiple command systems which are processing information in parallel, however, they all have a shared motor output path, so we are constantly choosing which is the most important at that time e.g. between, eating, drinking or escaping predators. In order to perform one of these motor tasks, the other tasks must be supressed- this is done by the BG which inhibits the systems we don't 'want' to use
69
describe a flow chart starting with BG at rest
the striatum is inactive--> removes inhibition on globus pallidus so GP is Tonically active--> sends inhibitory output to the thalamus--> reduces excitatory output of thalamus to cerebral cortex --> reduced motor output to lower MNs
70
describe a flow chart starting with BG excitation
when the striatum receives excitatory input from the cortex (and dopaminergic input from substantia nigra), it transiently excites the striatum, which sends transient inhibition to globus pallidus, which disinhibits the inhibitory output from GP to thalamus--> this increases the excitatory output from thalamus to cortex--> allows increased excitatory signals and activation of lower MNs from motor cortex
71
as well as motor loops, what other loops is the basal ganglia involved in?
prefrontal loop and limbic loop
72
briefly overview the role of the cerebellum
acts as a parallel processor with the cerebral cortex to enable smooth, coordinated movements.
it may also be involved with a range of cognitive tasks
73
does the cerebellum have any direct projection to lower MNs
no (like basal ganglia), instead it modulates uppers MNs in motor cortex
74
what proportion of upper motor neurons does the cerebellum project to?
almost all
75
describe the 3 major inputs to the cerebellum
spinal cord- proprioceptive information (muscle spindles)
cerebral cortex- motor cortex, somatosensory cortex and visual areas of parietal lobe
vestibular inputs- rotational and accelerating head movement (semicircular canals etc)
76
what's the output of the cerebellum?
to motor cortex via the thalamus
77
what is the primary function of the cerebellum
to compare the current motor command with actual body position/movement and project error corrections to make these align back to motor cortex
78
what other roles is the cerebellum thought to have?
motor learning and non-motor tasks
79
draw a picture of the main nuclei of the basal ganglia
see picture in notes/slides
80
which part of basal ganglia receives input?
striatum- putamen and caudate nucleus
81
which part of basal ganglia delivers output?
internal globus pallidus and the substantia nigra pars reticulata
82
describe the striatum cellular organisation
primarily composed of D1 and D2 receptor type dense medium spiny neurons at around a 50:50 ratio
D1 sends output to GPi and SNpr and D2 to GPe
83
draw the wire diagram of the basal ganglia showing the stop and go pathways
see slides/notes
84
outline the go pathway
excitatory input from the cortex to D1 neurons in striatum. This increases the inhibitory (GABA) output from striatum to GPi and SNpr. This inhibits inhibitory output from GPi and SNpr hence transiently removing inhibition on the thalamus, which increases the thalamic excitatory (Glu) output into cortex
GO
85
outline the stop pathway
excitatory (Glu) output from cortex primarily to D2 receptor neurons in striatum, this increases inhibitory (GABA) output to GPe which reduces it's inhibitory output onto the subthalamic nucleus which has excitatory output, hence increases excitatory output from STN to GPi and SNpr, which maintains their tonic inhibition of the thalamus, hence inhibiting excitatory output to the cortex
STOP
86
describe evidence supporting the Stop and Go pathways?
optogenetic research where channel rhodopsins labelled D1 direct and D2 indirect cells. When D1 were activated, mouse running speed increased (Go pathway) and when D2 were activated running speed decreased (Stop pathway)
87
what is the name of the key hypothesis of the BG function?
Action Selection Hypothesis
88
briefly describe the action selection hypothesis
the BG is acting like a switch to enable some motor behaviours and supress others dependent on predisposing conditions such as sensory stimuli.
89
describe central switching vs distributed connectivity and which one has the advantage/ represents the BG
central switching is whereby all action requests arrive to a single device (BG) which decides motor outcome. This is likely the role of the BG and is advantageous to distributed connectivity where all nodes are interconnected making it less likely for a single outcome to be produced
90
What are the 6 pieces of evidence/hypotheses supporting the action selection hypothesis for the BG
- research into damaged BG
- it receives input from a wide range of brain regions
- it has a mechanism to switch selected resources on
- there's a flow of information between command centres, BG and motor areas
- it can detect the urgency of each request
- there is evidence of neuronal selection mechanisms in BG
91
discuss damage to the BG as evidence for the action selection hypothesis
damage to the BG causes a number of diseases/disorders such as:
PD- difficulty initiating movement
HD- difficulty supressing movement
Dystonia- spasmodic movement
Tourette's- Involuntary movement/vocalisations
these all indicate that the BG is important in controlling motor output and when damaged there is loss of this control
92
describe the aspect of the action selection hypothesis whereby the BG must receive input from a wide range of brain regions
If BG is the action-selection control, it must receive input from much of the brain in order to collate all information such as predisposing factors in order to decide action output.
This seems to be the case as it receives input from almost all the cortex and some subcortical nuclei e.g. superior colliculi (via thalamus)
93
describe the aspect of the action selection hypothesis whereby the BG must have a mechanism to to switch on selected resources (and supress those not selected)
This is important as in order to act as an action-selector, it must be able to select the action required
Evidence of this seen in the Go-Stop pathways (optogenetic evidence) and the disinhibition gating hypothesis whereby for actions there is usually the tonic inhibition of the thalamus which is supressed when the BG selects an action to output, which has been supported by electrophysiological research in monkeys whereby excitatory glutamate injected into striatum increases striatal neural activity, then a drop in SNpr activity, then an increase in thalamic activity, releasing the cortex to increase activity
94
describe the aspect of the action selection hypothesis whereby the BG must be part of a flow of information between command centres and motor centres
there must be pathways of activity between the BG, command centres and motor areas. We know there are at least 5 loops in which the basal ganglia is involved- motor, oculomotor, prefrontal 1, prefrontal 2 and limbic
with cortical and subcortical input and some topographic mapping of the body in discrete channels of the BG
95
describe the aspect of the action selection hypothesis whereby the BG must be able to detect the saliency/urgency of each request
it is important for the BG as a central command centre to be able to differentiate between the urgency of requests in order to favour one. Medium spiny neurons of the striatum (input to BG) have around 50,000 synapses from which the salience of action requests is thought to be received
96
describe the aspect of the action selection hypothesis whereby the BG must have neuronal selection mechanisms
this is important in which to explain how the BG favours particular pathways despite so much input. In the striatum, medium spiny neurons are either in up or downstate, whereby they are closer or further from threshold for AP. This allows groups of neurons in the striatum to be in upstate and be activated, whilst laterally inhibiting their neighbours to favour pathways. Similar is also happening at the output nuclei to favour actions.
97
What are the 2 broad categories of eye movements?
stabilising and target
98
what are the 2 types of stabilising eye movement?
Vestibulo-Ocular Reflex VOR
Optokinetic Reflex OKR
99
describe VOR
when the head rotates the eyes move in the opposite direction at the same rate to keep visual world stable
prevents world appearing blurry
e.g. used when walking
100
describe the optokinetic reflex
stabilising eye movement when the visual scene is moving, the eyes follow the surround movement so the image remains stable
101
what are the 3 target eye movements?
saccades, smooth pursuit and vergence
102
describe saccades
fast flicks of the eye (around 3 per second when awake) to foveate visual targets which we determine as interesting from our wide field view
103
describe smooth pursuits
following slow moving targets to foveate them- mostly involuntary
104
describe vergence
the only movement where eyes move in different directions in order to look at targets moving towards away from you can be slow like target pursuit or fast like saccades
105
describe the mechanism of VOR
head movement detected by semicircular canals
sensory neurons signal head movement info to vestibular neurons which synapse onto oculomotor neurons in brain step
oculomotor neurons send eye movement commands to eye muscles and orbital tissue and output is eye movement
106
what kind of control system does VOR use?
feedforward
107
define feedforward
where the output does not affect the input
108
what's different between the mechanism of OKR and VOR?
OKR input is from the retina and pretectum which signal retinal slip (instead of semicircular canals signalling head movement in VOR)
OKR is under feedback control, where he output of moving eyes to adjust to the moving world is seen by the retina and any further retinal slip continues to be corrected
109
why do we need 2 stabilising reflexes (OKR and VOR)?
because VOR can respond to high frequency movement/changes from head movement but semicircular canals cannot detect slow longer movements well, whereas OKR feedback has some delay so the system works to detect lower frequency movement
hence need to work together to cover both high and low frequency changes
110
how does VOR know how big of an eye movement to make?
it's learned- through the cerebellum so can change with growing/ageing/injury
111
explain the mechanism of VOR with input from the cerebelum
a copy of the eye movement command is send to the cerebellum as well as the retinal slip, and the cerebellum can provide feedback (so a slow feedback system) which over time can correct any errors
