Motor Flashcards
1
Q
We use muscle contractions for more than just moving our limbs. Give 5 other uses.
A
- Moving the external world
- Moving yourself around
- To preventing movements: for example, maintaining equilibrium,
- Communication- speech/ gesture/ writing
- To move sensors: visual, somatosensory (active touch)
2
Q
How did the motor system first evolve
How was sense involved
A
simple organisms with bilateral columns of muscles on either side of the body which were activated alternately to produce swimming (e.g. fish).
sense organs in head provided signals that guided swimming behaviour towards/away from signal
3
Q
Describe the action of the spinal cord in early simple organisms with bilateral columns of muscle
A
largely operates autonomously (to generate swimming through the action of intrinsic spinal networks), and a guidance system from the brainstem adjusts the swimming appropriately
4
Q
Why did the basal ganglia and cerebellum originally develop
A
to assist the operation of of the guidance system from the brainstem which adjusts the swimming appropriately
5
Q
How did land-dwelling vertebrates elaborate on the motor systems of early aquatic vertebrates
What did the brainstem do here
A
by developing limbs, with new groups of muscles that act specifically on each limb appearing
controls the operation of this more sophisticated system in which
limbs need to be coordinated together inappropriate patterns for locomotion.
6
Q
What is the functional hierarchy of the motor system
A
IDEA (what is my goal?) PLAN (how do I achieve it?) PROGRAM (which muscles? scale?) EXECUTION (send motor commands) MOVEMENT
7
Q
How is the motor system defined in this lecture series? (may be useful for essays)
A
the system that plans and executes movements
8
Q
What is an advantage of the negative feedback systems
A
automatically compensate for
unpredicted deviations from the set point (e.g. caused by noise)
9
Q
What are the major limitations of negative feedback systems caused by
A
time delays in the feedback loop
(takes time to get to and from the brain)
leads to instability and oscillations in rapid movement control
10
Q
Why might we think there was an evolutionary attempt to regulate sensory and motor systems via negative feedback systems
A
t the very fast conduction of sensory and motor signals represents an evolutionary attempt to minimise biological delays, and therefore these problems, in negative feedback systems.
11
Q
Can we adjust ballistic movements as they are happening?
What does this mean?
A
no
a motor plan must be formulated in advance and then executed: there is no way that sensory information generated during the progress of the movement can influence its outcome
12
Q
What is an open loop system
A
where sensory information is used to generate a prediction of what is needed in the future
feedforward
13
Q
What does the body use to make the open- loop system work
How does this work
A
internal model system - : simulators that represents the mechanics of the body and the
behaviour of the external world
Such a system could learn to predict which motor commands are useful in a given situation and even to mentally rehearse movements before actually making them
14
Q
How does the internal model become accurate
A
by comparing the actual results with the desired movement:
differences can be used to adjust the
model so that in future it becomes more accurate.
15
Q
What is a system related to the internal model system that allows feedforward control
how does it work
A
internal feedback system via efference copies
efference copy is a prediction of the upcoming movement and so this can be compared to the desired result. the actual output is then corrected accordingly if predicted and desired outcomes do not match.
16
Q
What must be noted about the model systems
A
depend critically on the accuracy with which they simulate the mechanical world and as such are continually being adjusted and calibrated (they are continually learning).
17
Q
What is the major motor area of the spinal cord
A
ventral horn where alpha-motoneurons that innervate muscle fibres are located
18
Q
Where are most synapses on motor neurons
A
from spinal interneurons located in the intermediate zone grey matter
19
Q
What is a motorneuron pool?
Where is it located?
A
The group of 200-500 motoneurons that innervate a given muscle
close together in the ventral horn usually extending rostro-caudally over
several spinal segments.
20
Q
Are motor neurons arranged somatotopically in the spinal cord?
A
yes
distal muscles are represented laterally
21
Q
What is the basic unit of force production?
is there 1 motor neuron to 1 motor unit?
A
motor unit
no: Each motorneuron axon branches to innervate many muscle fibres which are distributed throughout the muscle.
22
Q
How may motor neurons innervate a single muscle
A
several hundred
23
Q
What is a key problem of motor control
how is this overcome
A
how the brain can generate both finely graded, low force contractions when precise control is needed and high forces when strength is needed
motor units can be divided into 3 different types with different properties
24
Q
What are slow motor units ideal for
what are fast fatigueable good for
A
Slow motor units are ideal for continuous generation of small forces,
whereas fast fatigueable
units produce high forces, but over a short period
25
Why can the body not just vary motor neuron firing rate instead of having different types of motorneuron
twitches generated by muscles fuse into a tetanus at quite low frequencies
26
What is the Size Principle
what does this ensure
motor units are recruited in an orderly sequence as force increases, the lowest force motor units first, the highest force units last
as you increase the force of a movement the next motor unit recruited is always the one that generates the lowest force
27
What is the mechanism underlying the size principle
developmental plasticity
28
What is developmental plasticity
motoneurons with low firing threshold innervate few muscle fibres and induce them to become slow twitch, low force and fatigue resistant.
Motoneurons with the highest firing thresholds (which are recruited last)
innervate many muscle fibres and induce them to become fast twitch and high force, but fatiguable
29
Does the brain have to control each motor neuron independently?
no - , inputs randomly distributed to all of the motoneurons innervating a given muscle will automatically recruit motoneurons in the order lowest force first to highest force last (Size Principle)
30
What does motor neuron damage lead to
flaccid paralysis
31
Give 3 ways to damage motor neurons and an example of when each would happen
Motorneuron degeneration - e.g. in ALS (motorneuron disease)
Peripheral nerve damage - (auto immune disease (e.g. Guillain-Barre Syndrome) or peripheral nerve damage (example in dog coming)
Spinal cord damage - e.g. vertebral column dislocation
32
What are the 3 sources of synaptic input to motor neurons
give examples of each
• muscle spindle afferents (ONLY from muscle spindles!)
•Descending fibres (direct pathways from brain stem or cerebral cortical structures, relatively
rare, with an important exception in primates)
•Spinal interneurons (most numerous, in most cases receiving inputs from both sensory
pathways and descending pathways from the brainstem and/or cerebral cortex)
33
What is the simplest form of motorneuron activation
through reflexes driven by sensory signals
34
How can spinal reflex actions be divided
distinctions between reflex actions targeted at specific small groups of muscles and involve in regulation of their force (examples are stretch reflexes and associated reciprocal and recurrent inhibition), and more complex reflexes that generate functional movements that involve multiple muscles (eg nociceptive withdrawal reflex).
35
What are the different types of proprioceptors
(the receptors from the surface of the body - cutaneous receptors - are exteroceptors,
those that sense the environment at a distance – eyes, ears & nose- are teloceptors
36
What are the 3 major groups of proprioceptors
* muscle spindle afferents are muscle stretch receptors
* Golgi tendon organ afferents are muscle tension receptors
* joint receptors signal joint position and movement, especially at the extremes
37
What are muscle spindles
spindle-shaped structures embedded in muscles that give rise to afferents that signal muscle length and change in muscle length.
Typically a muscle will contain between 20 and 100 spindles
38
Describe the structure of a muscle spindle
encapsulated bundle of small specialised intrafusal muscle fibres – (literally within-spindle muscle fibres).
The ends of these fibres are striated, but they are very small compared to the force-generating extrafusal fibres
Intrafusal fibres are
very small: their contractions generate no tension at the tendons but affect the sensitivity of sensory nerve endings in the spindle.
39
What are 2 morphological types of muscle spindle intrafusal fibre
bag fibres: swollen central region containing may nuclei and has contractile ends
chain fibre:
uniform diameter and uniformally contractile along its length
40
Which intrafusal fibre will a typical muscle spindle contain
several of each type
41
What are the 2 types of sensory fibre that attach to intrafusal muscle fibres
primary (Ia) spindle afferents
secondary (II) spindle afferents
42
Describe primary spindle afferents
```
very large (and thus very fast conducting) axons which have terminal branches that
end in coils (annulospiral endings) around the central region of the intrafusal muscle fibres
```
43
Describe secondary spindle afferents
Smaller, slower conducting afferent fibres end adjacent to the central region of the intrafusal muscle fibres.
44
```
true or false
Both primary (1a) and secondary (group II) sensory endings on muscle spindles are activated by stretch
```
true but in different ways
45
Describe the contractile properties of intrafusal bags
have contractile ends, which are
visco-elastic, but the central bag region is not
contractile, but is elastic.
46
What happens when rapid stretches are applied to bag fibres
What is the result
immediately elongate the central bag
region (providing a rapid strong activation of the
afferents at stretch onset - a dynamic response).
Subsequently this stretch is relieved as the viscous
ends the fibres elongate reducing stretch in the
central region.
during the stretch the central region is greatly elongated, generating a
strong initial response but this declines to be
distributed across the whole fibre at the end of the stretch
47
True or false
chain fibres have different mechanical properties across their length
What is the purpose
```
false
have uniform mechanical properties
throughout their length, so the sensory endings on them (especially secondary endings) signal muscle length
approximately linearly (a static response).
```
48
What do the properties of bag fibres allow it to signal
primary spindle endings on the
bag fibres a largely rapidly adapting response (i.e. have a dynamic sensitivity).
Thus, their responses signal changes in muscle length
49
Do primary spindle afferents connect to bag or chain fibres?
What do they therefore signal
both
have both static and dynamic components to their reponses
50
Do muscle spindles have efferent neural connections
yes
| there is an efferent control of the sensitivity of the terminals
51
What happens to muscle spindles when the motor neurons innervating them are activated
Since the ends of intrafusal muscle fibres are contractile, they shorten when activated by the motoneurons that innervate them
52
Which motor neurons innervate muscle spindles
In most cases intrafusal muscle fibres are innervated by a group of smaller and more slowly conducting motoneurons than the alpha motoneurons, the gamma motoneurons, the result of which is to stretch the central region of the fibres where the receptors are located. gamma motorneuron firing thus increases both the firing and the sensitivity the receptors: this complicates the sensory signal.
53
What does motor neuron innervation of the contractile parts of muscle spindles
provides a mechanism to allowing the receptor sensitivity to be adjusted – for example to allow the spindles to be able to signal length changes from different starting muscle lengths, rather than to become saturated when stretched and too short to signal on the muscle is contracted
essentially adaptation
54
What suggests motor neuron innervation of intrafusal muscle spindles are more complex than we realise
different gamma – motoneurons (dynamic and static) innervate bag and chain intrafusal fibres, which enhance the dynamic and
static responses of spindle afferents respectively.
55
What happens if the brain decreases the drive to gamma motoneurons to intrafusal fibres
How does this affect sensitivity
they will relax,
reducing sensivity to stretch
56
How will contracting the parent muscle affect the muscle spindle
spindle becomes slack
57
What are different things it could mean if a muscle spindle changes afferent firing
How does the brain differentiate
could be generated by muscle length changes OR by altered gamma motoneuron activity (or, more likely, by combinations of both!)
efference copy
58
What are Golgi Tendon Organs
a second type of
| proprioceptor located in tendons that are activated by tension in the tendon
59
How does passive stretch of a relaxed muscle affect a Golgi tendon organ
How does this differ from a muscle contraction?
tension in the tendon does not rise much due to the elasticity of the muscle fibres
generate tension directly in the tendon and make tendon organs fire briskly - i.e. they signal active tension (signal will be proportional to load on muscle)
60
Which signals underlie the stretch reflex
muscle spindle signals
61
What are stretch reflexed
do they work if the muscle is paralysed
Most muscles respond to being stretched by
contracting. This reflex is generated by the nervous system:
disappears if the muscle nerve is cut (or if the muscle is paralysed).
62
Why might we consider stretch reflexes to be homeostatic
maintaining muscle length in the face of an imposed stretch (stretch activates spindles, which excite alpha-motoneurons, which generate contraction that counteracts the stretch).
63
When might homeostatic stretch reflexes be useful
in postural control: if the body sways then the stretched
| leg muscles will automatically contract to counteract the sway, helping maintain an upright stance.
64
What is the neuronal circuit of the stretch reflex
Monosynaptic connections from muscle spindle afferents to the alpha-motoneurons that innervate the same muscle underly the stretch reflex
65
True or false
| there are multiple monosynaptic reflexes
false
the stretch reflex is the only monosynaptic reflex
all others involved spinal interneurons
66
Do all muscles have a stretch reflex?
The large majority of muscles have a stretch reflex, but eye and tongue muscles are notable exceptions.
67
Do muscle spindles only affect its own mother muscle?
muscle spindles also excite the motoneurons of close synergists (e.g. stretch of brachialis at the elbow produces a reflex in
biceps as well as brachialis).
also excite interneurons that inhibit antagonist muscles (reciprocal inhibition), preventing further stretch (e.g. stretch of brachialis and or biceps will produce inhibition of triceps motorneurons, and vice versa).
68
What is the benefit of a negative feedback system
How might this apply to the stretch reflex
they automatically compensate for deviation from a set point
```
compensate for
unexpected loads that
stretch a muscle: deviation
of the muscle length from a
pre-set value will be detected
by muscle spindle afferents
and will rapidly generate
excitation of motorneurons,
that could restore the pre-set
length
```
69
Why might eye muscles not have a stretch reflex
they do not encounter unexpected loads, which seems to support a role for the stretch reflex in counteracting unexpected loads.
70
Why is the idea of stretch reflexes being a negative feedback system attractive?
the brain needs to both calculate the geometry of movements (position and velocities of movement), but different loads will require different muscle forces to achieve these positions/velocities. If an automatic system produces the appropriate force levels regardless of load, all the brain need do is determine the muscle lengths needed – the spinal cord could take care of the forces needed.
71
What are the 2 issues with the stretch reflex being a negative feedback system used to deal with unexpected loads
elaborate briefly on each
gain (For this to work there must be a compensatory contraction for any given stretch.)
delays (would lead to oscillations)
72
If the stretch reflex were designed to maintain muscle length in a negative feedback style, what would the gain have to be
what is gain in reality
The stretch reflex should have a gain of 1 (any given amount of stretch will elicit contraction to precisely counteract it with the same amount of contraction)
IRL = <1
73
What happens to the stretch reflex when the descending motor systems are damaged
What causes this
What can these changes be interpreted as
spasticity
the stretch reflex is exaggerated (a high gain) - brisk responses are evoked to tapping tendons and muscles have tonic contraction
• Oscillating muscle contractions follow stretch (MYOCLONUS or just CLONUS).
they are characteristic following motor cortex damage (eg stroke or cerebral palsy)
74
what does most of the evidence on the stretch reflex suggest about its role
despite its attractiveness,
the stretch reflex cannot be strong enough to fully compensate for unexpected loads as a perfect negative feedback system without becoming unstable.
75
Can you change the gain of a stretch reflex
yes - by adjusting sensitivity of the muscle spindle
76
What did Prochazka's recordings in cats suggest about the stretch reflex
the brain can control the strength of the stretch
reflex to suit the circumstances: in slow movements and situations where precision is needed then
gain may be set high (high spindle sensitivity and therefore a strong stretch reflex), whereas for
fast movements such a gain might generate clonus, so gain may be set low
77
What is kinesthesia
a sense of position,
| movement & effort
78
How is kinesthesia sensed by different parts of the body
highly integrated with major contributions from visual, vestibular, tactile, and efference copy information, as well as from proprioceptors.
Different parts of the body may use this information differently – for example eye position (which has to be know precisely for visuomotor control) is principally signalled by efference copy, whereas hand position depends to a great extent on cutaneous sensation.
79
What is the most important role of proprioceptors?
provide information for supraspinal motor systems, which are involved in predictive feedforward control of movement (e.g. cerebellum and motor cortex) using model systems
80
Where is the main projection of proprioceptors to
cerebellum via spinocerebellar pathways
81
What does proprioceptive information to the cerebellum do
informs internal model systems about the current state of play at the outset of movement (on which model predictions can be based), and assessment of the outcome after the movement (critical for modifying the model systems ensuring that they are accurate).
82
What is reciprocal inhibition
What does it parallel and what are the neurons involved
parallels the stretch reflex.
Muscle spindle primary afferents excite glycinergic inhibitory interneurons that inhibit motoneurons of antagonist
muscles when a muscle is stretched.
These are effects are mediated by a group of Ia Inhibitory interneurons associated with each motoneuron pool.
83
What mediates recurrent inhibition
Renshaw cells
84
What is recurrent inhibition
Motoneuron axons have branches (recurrent collaterals) within the spinal cord that innervate inhibitory Renshaw cells - inhibit the original motor neuron
85
What is the purpose of recurrent inhibition
regulate the timing of motoneuron firing, preventing synchrony
86
How are the stretch reflex, reciprocal and recurrent inhibition different from most reflexes
these apply to the muscle in which the stretch or activation occurs.
Most reflexes activate specific patterns of movement via groups of muscles, (like nociceptive withdrawal reflexes)
87
Give 4 general principles about spinal reflexes
most reflexes are:
1) context dependent
2) multi-joint responses
3) Different types of afferents can contribute to the same reflex pathway
4) Reflex pathways are controlled by descending pathways
88
What does it means to say that most reflexes are context dependent
the same stimulus may evoke different reflexes depending on the behavioural context
89
What are withdrawal reflexes
What interneurons mediate these reflexes
Noxious stimuli generate a coordinated pattern of muscle contraction that moves the stimulated part of the body away from the stimulus
interneurons that process information
relayed from the dorsal horn
90
What are 2 properties of withdrawal reflexes that reflect basic properties of reflexes generally
spatial and temporal summation - stimuli at adjacent sites or closely timed sum to give larger responses
local sign - different reflexes are evoked from different locations
91
Give an example to illustrate how withdrawal reflexes differ based on location
Stimulation of the plantar surface of the foot evokes leg flexion, stimulation of the foot dorsum evokes leg extension: both remove the skin surface from the source of the stimulus.
92
What does it mean to say tendon organ reflexes illustrate context dependence
Tendon organs generate reflexes via different groups of spinal interneurons, which are active depend on the motor state
In static or resting postures activation of tendon organs it evokes inhibition of the parent muscle, but during locomotion the effect is reversed to excitation, where it supports contraction against a load (in locomotion produced by the body weight).
93
Do tendon organs mediate the clasp-knife reflex
When can this specific reflex be seen
no
after a stroke
94
Name 3 neonatal reflexes that change with development
* Grasp reflex
* Babinski’s sign (plantar reflex)
* Reflex stepping
95
How does Babinski's sign differ between children and adults
scratch sole of the foot
infant: toes curl up
adult: toes curl down
96
Why is Babinski's sign clinically useful
Following brain damage changes in spinal reflex patterns occur, and neonatal reflexes may reappear
97
Give 3 examples of changes to spinal reflex patterns following brain damage
Exaggerated stretch reflexes (Spasticity). Muscles are tense and stiff. Stretch elicits strong reflexes and clonus (a negative feedback system oscillating)
* Babinski’s sign – toes turn up in response to plantar stimulation: reappearance of the neonatal reflex
* Clasp knife reflex the limbs snap into extension or flexion
98
True or false
| the spine can only produce simple motor outputs without the brain's input
false
Even after being separated from the brain, the spinal cord can produce complex motor
outputs.
Locomotion can be spinally generated in most vertebrates, including most
mammals
99
What are motor central pattern generators
Spinal circuits that have the capacity to
| generate a detailed motor pattern appropriate to produce stepping
100
Why do we think CPGs do not require brain input
Their activity can be seen generating a complex locomotor pattern after removal of the brain, indicating that this is not a brain-generated pattern. (headless chicken)
also survive the removal of sensory inputs, by cutting the dorsal roots of the spinal cord, indicating that an intrinsic spinal mechanism is responsible for this pattern
101
Can functional locomotion be generated after spinal transection in man
no
102
Can babies walk?
Although babies make stepping movements in the first few months after birth, these movements have a different pattern to the stepping that appears at about 12 months when walking first appears
103
What is the general pattern between encephalisation and spinally generated locomotion
greater encephalisation (a larger cerebral cortex) is associated with a weaker ability to generate locomotion in the spinal cord: nonmammalian vertebrates such as Fish and Amphibia routinely generate strong locomotion after removal of the brain.
Mammals are less prone to do so.
In young mammals this ability is stronger than in adults
104
What are the 2 major descending motor locations in the spinal cord?
both in white matter
dorsal lateral and ventromedial
105
Which of the descending motor pathways is evolutionarily older
What does this pathway do
VENTROMEDIAL PATHWAYS are evolutionarily ancient, and exist in all vertebrates
control axial (trunk) and proximal limb muscles and play a role in whole body movement (locomotion/posture). These tend to produce stereotypic whole-body movements and postures.
106
What is the largest of the descending motor ventromedial pathways
Where does this originate
Reticulospinal pathway
reticular formation
107
What is the reticular formation
a wide expanse of grey matter running throughout the entire brainstem
108
What is the reticulospinal pathway important for?
What is it controlled by?
principal route through which spinal
central pattern generators are activated and is also an
important pathway for characteristic whole body postures and movements
cerebral cortex
109
Where does the vestibulospinal system begin, and where does this origin receive input from?
originate from the vestibular nuclei, which receive inputs from the vestibular organs of balance.
110
What are the vestibulospinal systems especially important for
antigravity actions
111
Where does the tectospinal pathway begin?
What is the purpose of this pathway?
from superior/rostral colliculus
pathways through which rapid sensory orientation movements are generated
112
What is the rostral/ superior colliculus important for
visual information processing
113
What are the main ventromedial descending motor pathways
reticulospinal pathway
vestibulospinal system
tectospinal pathway
114
What are the dorsolateral spinal systems important for
they are the most important route through which goal directed movements of the limbs are driven, especially movements of the hands, feet and face (and other prehensile structures, like the lips). These systems control the more individuated movements of the limbs, rather than whole body postures and movements
115
Where does the rubrospinal pathway originate
Red Nucleus in the midbrain
important in cats, dogs and monkeys
vestigial in humans
116
Where does the Corticospinal tract arise?
from motor areas of cerebral cortex
117
What is the largest descending motor spinal pathway in all mammals
How does this pathway's function vary between mammals
corticospinal tract`
In mammalian herbivores, this pathway primarily controls movement through a control of the reticulospinal and rubrospinal pathway is, whereas in man this pathway has evolved to dominate the function of the motoneurons in the spinal cord
118
Why is the vestibular system so complex in humans
the bipedal stance is inherently unstable
119
What are 3 systems involved in postural stability
balance of forces on each foot, stretch and other proprioceptive reflexes can tell us about the state of the muscles of supporting limbs.
The Visual System can tell us whether we are vertical with respect to the outside world
The Vestibular System can inform us of head position and movement.
120
Describe the vestibular system receptors
sensory hair cells located in the labyrinth in semicircular canals and the otolith organs (the saccule and utricle).
The hair cells are identical to the receptors in the cochlea hair cells have directional sensitivity and respond best to movement
in a specific direction.
121
Describe the arrangement of the hair cells in the utricle and saccule
hair cells project into a jelly-like mass on which gravity acts.
Different hair cells are arranged to have different preferred directional sensitivity, so specific sets of hair cells will be activated when the head is in different positions
122
What do the afferents from the hair cells in the utricle and saccule provide information on
provide information on the effective direction that gravity is acting (linear acceleration due to gravity):
when immobile this can be considered to be a head position signal.
When moving it signals head translation (up/ down, left/ right, backward/ forward)
123
How are hair cells associated with semicircular canals arranged
```
embedded
in a jelly-like mass that almost closes
the canal called the cupula. The
cupula is neutrally buoyant in the
fluid (endolymph) in the canals so
with the head stationary it is also
stationary
```
124
How are the hair cells in the semicircular canals activated
When would they be activated
activated when the
cupula is deflected.
This usually happens when the head rotates: the
fluid in the canals has inertia, so tends to remain stationary and since the cupula is fixed to the head, it is deflected.
125
What do the semicircular canals signal
How does the signal change
angular rotation of the head.
The greater the angular acceleration of the head, the greater the signal. The 3 different canals on each side are at right angles to each other so provide signals in 3 cardinal planes.
126
Are the labyrinths on either side of the head separate from each other functionally
no
canals on the 2 sides act as complementary pairs, with appropriate projections into the vestibular nuclei in the brainstem, through which their actions are mediated.
127
True or false
| the vestibular system is a proprioceptive system
true
| is a proprioceptive system, detecting head position and movement
128
Why is it important for vestibular responses to be fast
How do these signals reach the effectors
activation of vestibular receptors may indicate postural instability (movement of the centre of mass that precedes a fall)
vestibular signals generated by head movement therefore generate rapid and powerful vestibular correcting responses, through connections with the descending vestibulospinal pathways which principally influence extensor (antigravity)
129
True or false
| vestibular reflexes are negative feedback systems
false
these reflexes are feedforward motor commands: the vestibular signals that indicate postural instability generate predictive responses to restore stability. The learning of these responses relies on the cerebellum
130
We are usually unaware of vestibular reflexes. Does this mean they are weak?
no
Damage to the vestibular system is not uncommon (peripherally, e.g. through labyrinthitis, or centrally e.g., through brainstem stroke or cerebellar damage) and can produce dramatic problems, revealing the potency of everyday vestibular reflexes.
131
Can the symptoms of damage to the vestibular systems be improved?
Plasticity in the vestibular system and cerebellum reduces these pathological symptoms over time.
132
What is the issue with the vestibular system being in the head
While it is very valuable to have a system that detects head movement when the body sways, a person’s mass is mainly located in the torso. The system therefore needs to differentiate between vestibular signals generated by body sway (with the neck fixed) and those generated by head movement (with the body stationary)
133
Is the vestibular detecting head movement or body instability when we are moving
when we are moving the system is detecting both
body instability AND head movement
simultaneously
134
What are neck reflexes
Imposed movement of the neck produces actions that counteract the vestibular reflexes – reflexes that are exactly equal and opposite to the vestibular reflexes.
135
What exists in the adult human that differentiates between vestibular activation caused by one’s own movements and vestibular activation caused by body sway
a predicitive system using an efference copy of motor systems that move the head
this information can be used to generate a prediction of the expected head movements and therefore a prediction of the expected vestibular activation: if the predicted vestibular signals match the actual signals then the vestibular reflexes are cancelled.
136
give 3 special properties of the eye's motor system
* eye movement uses only 6 muscles
* the eye rotates in the orbit, essentially under predictable load
* in binocular species the eyes must move together (consensually).
137
Give 3 reasons eye movement is fundamental for visual function
a) decerning where movement of an image the object moving or person moving
b) The visual system is poor at resolving changing (moving images): a static image is important for visual acuity
c) binocular vision only works if the eyes move together in a predictable way
138
is the eye movement coordination control system based in the brainstem or motor cortex
Eye movements must be consensual, so the control system that coordinates the 2 sides is based
in the brainstem, not the motor cortex.
139
What do gaze fixing mechanisms achieve
Because the visual system is not good at resolving moving images, the eye needs to be kept fixed
relative to the outside world as much as is possible
140
What is the usual strategy for when the eye needs to move
saccades - move the eye quickly then stabilise the new image
141
What are saccades
Very rapid gaze shifting eye movements
Vision is suppressed during these movements
142
How do owls prevent vision blurring due to self movement
vestibulocollic reflexes can produce compensatory head movement
143
What does the vestibulo-ocular reflex (VOR) do
moves the eyes equal and opposite to the head (feedforward reflex)
144
How does the VOR work
What are the neural connections that are required
vestibular system detects head motion and drives equal and opposite movements of the eyes
direct connection between semicircular canal afferents, the vestibular nuclei
and the motoneurons of the oculomotor nuclei (via the MLF)
145
What is the MLF
medial longitudinal fasiculus, a fast conducting fibre tract in the brainstem
involved in eye movement
146
How is VOR used as a feedforward system
vestibular signals are used to generate a prediction of the eye movements needed to stabilize gaze
147
What part of the brain calibrates how much the eye needs to move in the VOR
cerebellum
148
What is the optokinetic system used by the eyes
What do optokinetic movements require
moves the eyes to follow slow movements of the visual field (when looking out of a train window)
detection of the slow movements of the visual image across the retina, which is done by the visual cortex
149
What is nystagmus
. The drift and saccade sequence seen with sustained
vestibular or optokinetic stimuli gives rise to a characteristic ‘sawtooth’ eye movement
ie When the eye deviates far from its axis within the orbit, fast resetting movements (saccades) move the eye back close to the central axis
150
How do physiological and pathological nystagmus differ
Physiologically nystagmus occur in response to sustained optokinetic or vestibular stimuli.
• Pathologically, nystagmus follows damage to the systems that control gaze fixing – cerebellar or vestibular damage.
151
What generates saccades
the brainstem
152
What is the most frequent movement we make
When might we use this movement
saccades
2 /second
rapidly shift the gaze (fovea) to potential points of interest so detailed visual information can be got quickly.
153
What part of the brain organise saccades that move the eye to foveate visual stimuli
superior (rostral) colliculus
154
Describe the neural pathway which generates saccades to move the eyes to foveate visual stimuli
what is this organisation arranged to allow
retina projects to superior colliculus retinotopically
Deep layers of the colliculus project to regions of the
brainstem reticular formation which in turn project to
the oculomotor nuclei (control extraocular muscles)
that stimuli from any point in visual space will drive saccades that move the eye to
bring that point to the centre of the retina (fovea)
155
Are extraocular muscles the only muscles that move in saccades
no
Deep layers of the superior colliculus also project to the cervical spinal cord (via the tectospinal tract), so coordinated neck movements can accompany eye movements
156
Briefly describe how saccades are linked to the auditory system
Deep layers of the colliculus also receive auditory input and allow rapid orienting movement to sounds as well as to visual stimuli
157
What controls whether the superior colliculus is allowed to drive saccades to specific stimuli (2)
cerebral cortex and basal ganglia
158
are slow eye movements possible
yes but only when following objects
159
What type of system controls the oculomotor muscles when our eyes follow objects (smoot pursuit)
Which structures are involved in this system (4)
NOT feedback (too slow)
brain uses feedforward predicative control
regions of cerebral cortex (visual cortex and medial temporal cortex process visual signals)
regions of frontal lobe anterior to motor cortex (the - frontal eye fields)
brainstem regions, especially the cerebellum
160
Where is the motor cortex?
What does this general term apply to?
rostral to the central sulcus
applies both to a region immediately adjacent to the central sulcus, but includes a primary motor cortex (an area critical for the
execution of voluntary movements), and other areas rostral to this that are involved in particular with planning and preparation for action (motor association cortex).
161
What is the primary motor cortex critical for
execution of voluntary movements
162
What is the motor association cortex generally used for
planning and preparing for action
163
How does the size of the primary motor cortex compare to the motor association areas
As in sensory areas, the primary area is much smaller than the association areas.
164
Is the motor cortex important for motor function in all mammals?
yes but is very diverse between species
| eg very small in rodents and very large in primates
165
How would a mild unilateral stroke to the motor cortex affect movement
What about a more severe unilateral stroke
produces contralateral hemiparesis (one- sided weakness and partial paralysis)
contralateral hemiplegia (one-sided paralysis).
166
How can strokes in different regions affect different motor functions
anterior cerebral artery infarction affects lower limbs,
middle cerebral artery infarction affects upper
limbs and face.
Middle cerebral artery infarctions that affect the outflow of the motor cortex through
the internal capsule are devastating.
167
How does the motor cortex control movement
through exerting control on other descending pathways
from the brainstem, particularly the reticulospinal and
rubrospinal pathways.
168
Some mammals have a relatively small corticospinal tract. Does this affect the motor cortex's control?
no
| the motor cortex is important in controlling movement via this indirect pathway
169
How can the corticospinal tract affect reflexes
has a direct effect on interneurons
170
How does the motor cortex's role differ between different mammalian species (4)
* In most quadrupeds the motor cortex controls movement via the brainstem pathways
* In quadrupeds that use the forelimbs for manipulation (cats and rodents) there is a corticospinal tract as well
* In primates there is a large corticospinal tract as well
* In apes there is a direct pathway to motorneurons as well
(With evolution, new structures do not replace old ones but are added on top)
171
What is the most important route through which willed/ voluntary movements are mediated in mammals
corticospinal pathway
172
Name a voluntary movement that is an exception from being controlled via the corticospinal pathway
eye movement
the eyes need to move conjunctively (to avoid diplopia) – separate areas of cerebral cortex control eye movement
via the brainstem mechanisms rather than via the motor cortex.
173
Where does the corticospinal tract primarily arise from
primary motor cortex
174
Descibe how the corticospinal tract descends through the brain (6)
fibres descend through the ventral part of the brainstem.
In the forebrain these fibres lie in the internal capsule
In the midbrain the fibres form the cerebral peduncle
In the medulla the old name for the fibres is the pyramid
most fibres cross the midline in the motor decussation
fibres continue to the spine as the lateral corticospinal tract in the dorsolateral funiculus
175
What are corticobulbar fibres
What do they control (2)
cortical output fibres which branch to terminate in regions that control brainstem motornuclei
cranial nerve motor function
control brainstem descending pathways
176
Where is the motor decussation
low medulla
177
How did Hughlings Jackson describe neurological loss after the motor cortex in the descending pathway
loss of the ‘least automatic’
movements
This is particularly highly developed in apes and humans, where in addition to the corticospinal projection to spinal interneurons a new direct corticomotorneuron connection to motoneurons appears.
178
What do cortico-motorneuronal connections (allow in humans
control of precise, independent movement of the extremities (e.g. fingers)
179
What are corticomotorneuronal connections
direct, monosynaptic connections with motoneurons, bypassing the spinal interneurons
allow dexterity in humans
180
What is the evidence for cortico-motorneuronal connections underlying dexterity in humans (3)
Lesions of the corticospinal tract in the medulla leave permanent deficits that are most extreme in finger movement and manipulation.
• Comparative Neuroanatomy, projections to motorneurons appear in species that make independent finger movements (in primates) but are not seen in species without.
• Development: There is evidence that in man the direct projections post-natally, appearing
at about 9 months - when dexterity begins to develop.
181
Describe the corticospinal tract in rodents and marsupials
small pathway and
the motor cortex affects movement
largely through brainstem descending
pathways (rubro- and reticulospinal)
182
Describe the corticospinal tract in cats and new world monkeys
how does this compare to old world monkeys and apes
tract is larger but terminates in the spinal intermediate zone on spinal interneurons that control motorneurons
In some old-world monkeys direct
connections to motorneurons appear and in apes (like humans) these are widespread. These changes are accompanied by huge changes in the size of the motor cortex and number of axons descending to the spinal cord.
183
Motor cortex lesions are common in man. What do they usually result from?
What can result?
infarction in the middle cerebral artery, (supplies the lateral motor cortex regions that control the contralateral hand and face muscles)
A small stroke may cause a contralateral hemiparesis (weakness)
A larger stroke may cause a contralateral hemiplegia (paralysis)
184
Which disease appears to be unique to the corticomotorneuronal pathway in apes and man
ALS / motor neuron disease
185
What does 'upper motor lesion' refer to
motor cortex lesions that are complicated by spasticity (compared to “lower motoneuron lesions”, which cause flaccid paralysis).
186
Why is the term 'upper motor neuron' misleading?
‘upper’ motoneurons do not innervate muscles
187
What is the motor homunculus
Penfield & Rasmussen found an organisation such that different parts of primary motor cortex would control different muscles.
The motor homunculus was
an attempt to simplify their findings and produce a single
canonical figure representing motor cortex organisation
188
why is the motor homunculus misleading
suggests that the motor cortex contains an orderly representation of individual muscles in a fractionated map. This is important because the homunculus suggests that a lesion at one location will affect a specific muscle or body part, which is not the case.
ie “does the brain think in terms of muscles or movements? “.
189
How can different groups of motor neurons in the motor cortex be thought of
as alternative libraries of different muscles synergies for specific movements
190
Describe the synergies that involve the spinal cord
How does this differ from the synergies of the cortex
simple synergies eg wrist extensors and finger flexors making a fist or power grip
Using the cortex allows much more flexibility, for example hand and mouth synergies for feeding, or alternative groupings of muscles. Sensory input to the cortex can regulate the operation of this synergy in a coordinated way
191
Does the motor cortex contribute to learning movement
yes - we don't understand how but have evidence it does
192
What suggests the motor cortex is involved in learning movement?
As with other areas of cortex, the cellular mechanisms of plasticity are known, and there is evidence for changes in cortex activity during the process of sensorimotor learning: note that the motor cortex has abundant connections from cerebellum and basal ganglia, both of which are also plastic.
193
Give 4 sources of input to the motor cortex
Motor association areas (direct cortico-cortical connections)
Cerebellum (via VL thalamus)
Basal Ganglia (via VL thalamus)
Sensory afferents (via VL thalamus and sensory cortex)
194
What does the rapid transmission of tactile and proprioceptive signals to the motor cortex allow?
How does this compare to those of the spinal cord?
rapid feedback correction of
ongoing movement
Although these
reflexes have longer pathways than their spinal equivalent, the flexibility provided by a route
through the cortex makes them very valuable.
195
What is an example of the modulation of ongoing movement that can be done by the motor cortex based off proprioceptive/ tactile afferents
What is initially used
How is this initial movement modified
regulation of grip force in delicate manipulation.
When you pick raspberries, too little force between finger and thumb and the berry slips from your grasp, too much and you squash it.
Motor cortex regulates grip force: initially an estimate of the appropriate force is used
Sensory signals from cutaneous tactile afferents detect any slippage between objects (the raspberry) and the fingertips and activate neurons in the motor cortex very rapidly (~20 ms). The output of these corticospinal neurons direct to the motoneurons corrects for the slip by strengthening the grip until the slip stops (the force increase begins as quickly as 60 ms)
196
What kind of system is involved in the modulation of grip force of a raspberry based upon sensory afferents to the motor cortex
Why can this system not work for everything
negative feedback controlling the whole group of muscles (not just one)
delays in the feedback loop limit the speed of these reflexes - still extremely useful in making manipulation of delicate objects efficient.
197
Where are the inputs to the motor association areas from (4)
higher cortical association areas (prefrontal and
parietal)
from the cerebellum and basal ganglia.
198
What are the connections to the output of the motor association areas (4)
What do these connections suggest about the role of this area
direct projections to primary motor cortex (through which they can control current commands for movement),
direct projections to spinal cord
connections to the cerebellum and basal ganglia (through which they can influence plans and preparation for future movement).
suggest a role in planning
199
What were the 2 regions of the motor association cortex that were initially recognised in primates
What has been discovered since about this cortex
What can it be compared to
lateral Premotor Cortex
Supplementary Motor Area (SMA)
these can be further subdivided and other areas also exist in macaque monkeys there are at least 7 ‘body representations’ in motor cortex, in man these areas are much larger
can be compared to the multiple representations of the world in the visual cortex - eg what vs where
200
What is the SMA important for
internally generated or self-paced movement (you decide when and how to move) and bimanual movements
201
What is the lateral premotor cortex important for
important for movements that are dependent on a sensory trigger signal, or when sensory information constrains the options (e.g. interacting with an external object – to catch something. You can only catch something if your hands are in the right place at the right time!).
202
What are the key inputs to
a) lateral premotor cortex
b) SMA
a) cerebellum
| b) basal ganglia
203
What does a lesion to the lateral premotor cortex lead to
an inability to appropriately incorporate sensory information (visual, tactile) into motor actions, particularly into grasping.
204
Describe an important visuomotor grasp circuit in primates that involves the lateral premotor cortex
How do lesions affect this
the location of objects in egocentric space is represented is an area of the posterior parietal cortex (area AIP - the apex of the ‘where’ visual stream). This projects to lateral premotor cortex, which in turn projects to primary motor cortex. There is a
heavy interconnection between these areas and the cerebellum
do not prevent attempts at movement, but this movement is often poorly organised in relation to objects in the outside world.
205
What are apraxias and what causes them
often involve getting elements of the movement in the wrong sequence, or failing to understand the spatial relationships between movement and the external world
caused by damage to motor association areas, together with the posterior parietal cortex (which provides a major input to the lateral premotor areas)
206
What are mirror neurons
what are they important for
neurons that fire in relation to making a grasping movement, but also when seeing another person making the same grasping movement – e.g. they contribute to action, but also 2 action observation.
may be useful in imitation, but much has been made of this beyond motor control
207
Where were mirror neurons first described
lateral premotor cortex
208
How do we know that the SMA is involved in internally generated self-paced actions
PET scans
209
What does activity in the motor association areas reflect
What does this suggest about the SMA
both perfomance and mental rehearsal of actions
that it helps predict the sequence of movements needed to achieve a goal and understanding consequences
210
Which brain areas are involved in both emotion and motion
Give an example of what they might be important for
areas and the medial part of the hemisphere (part of the cingulate cortex)
limbic laugh
211
What is the limbic laugh
voluntary facial paralysis in patients who suffer a primary motor cortex a stroke makes them unable to voluntarily smile or laugh (or pretend to laugh), yet they can laugh when told a genuinely funny joke
212
What is the posterior parietal cortex a major input for
to the lateral premotor cortex
213
What is ideomotor apraxia
an inability to make gestures or movements when instructed
214
what is ideational apraxia
inability to link the elements of a sequence of movements
215
what is constructional apraxia
an inability to link physical shapes/ pictures in an appropriate way
216
Which brain region is most associated with planning, cognitive and decision making skills
prefrontal cortex
217
Describe the different functions of the frontal lobe cortex, moving progressively rostral from the central sulcus
current motor output (primary motor cortex), plans for future movement (motor association cortex), and plans for future strategic goals on which future movements will be based (prefrontal cortex).
218
What are the most important inputs the cerebellum provides for limb movements
what pathways do these form
inputs to both primary and motor association cortex, forming pathways through which information from many areas of cerebral cortex connect with the frontal lobe motor areas
219
Briefly what the connectivity with the hugely expanded prefrontal cortex of humans from the basal ganglia and cerebellum underlies
high level and cognitive functions (with the caveat that these are ultimately evolved for motor control)
220
What does cerebellum mean
little brain
221
What is the most common form of cerebellar lesions in domestic species
What are the humans equivalaents
neural degeneration leading to cerebellar abiotrophy
spinocerebellar ataxias
222
Who was Gordon Holmes
WWI neurologist
studied soldiers who received focal cerebellar
lesions from gunshot wounds. These lesions did not produce an obvious intellectual or sensory
loss, but he described a series of motor disorders, with the global term Cerebellar Ataxia (bad movement)
223
What were some specific elements of the cerebellar ataxia described by Gordon Holmes (5)
Hypotonia - weakness
Dysmetria - inappropriate displacement e.g. overreaching
Dysdiadochokinesis - inability to make rapid alternating movement,
Decomposition of movement - lack of co-ordination of different joint movements
also observed that: ‘after a cerebellar lesion……it is as if each movement is being performed for the first time’.
224
What kind of movements does the cerebellum make a major contribution to
What happens to these movements after a cerebellar lesion
fast movements that require feedforward control
profoundly affect movement leaving it uncoordinated. The construction of movements from appropriately timed, scaled and patterned contractions of muscles is disordered
225
Holmes said:
‘after a cerebellar lesion……it is as if each movement is being
performed for the first time’.
Can fast movements that require feedforward control be relearnt
no
226
Give an overview of how the cerebellum can create a model system for feedforward motor control
cerebellum enables accurate estimations of what is really happening in the world to be made based on the relatively imprecise signals available from sensory systems, and from copies of motor commands (efference copy): these predictions can be used to learn and refine appropriate movements. Over repeated iterations the motor system can thus learn how to interpret specific sensory signals to allow appropriate movements to be generated automatically.
227
what is the macroscopic structure of the cerebellum
comprises a superficial sheet of tissue (cerebellar cortex) that buries the output structures (deep cerebellar nuclei)
228
Which cerebellar cortex cells are for output
Purkinje
229
describe the structure of Purkinje cells
These have dendrites that are planar
| tree-like in the sagittal plane, but narrow in the coronal plane
230
Where do cerebellar Purkinje cells project to and what do they do
project to and inhibit (GABA) cells in the cerebellar nuclei, which are the output neurons of the cerebellum and project to motor structures
231
What is the largest cerebellar nucleus
Where does it project to
dentate
he parts of the thalamus (VL & VA) that supply motor and motor association areas of the cerebral cortex
232
How many types of cell are there in the cerebellar cortex
5
233
Which cerebellar cells are excitatory
ONLY granule cells
234
What is the most numerous type of cell in the brain
What do these cells give rise to
cerebellar granule cells
to parallel fibres that excite Purkinje cells
235
give 3 features of the cerebellar cortex
* highly ordered in a geometric organisation
* uniform over the whole cortex and (unlike the cerebral cortex, all areas look the same)
* conserved between species (all vertebrates have a similar cerebellum)
236
What is the sequence of innervation starting from a mossy cell in the cerebellar cortex
mossy fibres excite granule cells
granule cell axons become parallel fibres which excite all other cerebellar neurons
Purkinje cells can listen to the inputs of ~200,000 parallel fibres (parallel fibres excite Purkinje fibres)
Purkinje fibres inhibit deep cerebellar neurons
Climbing fibres excite Purkinje fibres
237
Where do mossy fibres arise from
many brain structures, some from sensory pathways, and (particularly in man), from the pons which is a cerebro-cerebellar relay
238
What is the input to the pons
include information on activity in motor cortex and motor association cortex (efference copy information on motor commands), as well as inputs from sensory areas.
239
Where do climbing fibres come from
inferior olive (in the medulla)
240
What is the ratio of climbing fibre to Purkinje fibre
1:1
241
True or false
| There is only one climbing fibre to each Purkinje fibre so the interaction is not very important
false
each Purkinje cell receives input from a single climbing fibre. This connection is, however,
massive and always makes the Purkinje cell discharge at least one action potential.
242
What is a major complexity in trying to understand the cerebellum
there is an evolutionarily ancient part, which has been successively built upon so there is a large evolutionary diversity between different species
243
Describe the cerebellum in fish
What is the cerebellar connections in non mammalian species
What does it control
has a strong relationship to the vestibular system.
connects to brainstem descending motor pathways (reticulospinal and vestibulospinal), thus controls axial whole-body movements.
This medial region may be referred to as spinocerebellum
244
How does the cerebellum look different in mammals from non mammalian species
What is this part referred to as
In mammals the cerebral cortex appears, and a new region of cerebellum evolves in parallel which has outputs to the dorsolateral descending motor pathways (motor cortex – corticospinal tract and rubrospinal tract)
connections to the motor cortex pass through the thalamus
intermediate cerebellum
245
How does the cerebellum differ in humans from other mammals?
In species that evolve a large motor association cortex (primates and humans) a new region of cerebellum evolves with output connections to the motor association cortex (again passing through the thalamus).
246
Why is the relationship between the cerebellum and motor association areas poorly understood
the boundaries between frontal lobe association cortex and motor association
cortex is not well defined
247
What is the cerebrocerebellum
what is it also called
a cerebellar region with output connections to the motor association cortex (in humans and primates)
lateral cerebellum
248
What do the different evolutionary parts of the cerebellum control in motor systems
most ancient medial parts of the cerebellum control whole-body posture and movement through ventromedial descending motor pathways
the intermediate part of the cerebellum evolves in relation to motor cortex, which controls descending pathways sending out commands for current fractionated movements of individual limbs,
lateral regions control motor association cortex which is planning future movements
249
What did Marr and Albus suggest about the cerebellum
it was a learning device
250
How does the cerebellum act as a teaching device
ckimbing fibres teach Purkinje cells which parallel fibres are most important, thus mkaing it so that the cerebellar circuitry could learn an association which outputs (which control movement) are appropriate to specific inputs (represented in mossy fibre activity)
251
Which cerebellar cells mediate synaptic plasticity in the cerebellar cortex
give an example of a study that supports this
climbing fibres
Long Term Depression (LTD) of parallel fibre to Purkinje cell synapses following conjunctive activation of parallel fibres and climbing fibres
252
How does the long term depression of parallel fibres differ from LTP in the hippocampus
similar principles underlie cerebellar LTD (change in AMPA receptors at synapses) to those
underlying LPT in the hippocampus, but there are different mechanisms (cerebellar LTD involves removal of receptors and does not involve NMDA receptors)
253
Give an example of cerebellar learning in action
calibration of movement based on sensory signals, particularly when different types of information have to be interrelated
eg VOR
254
How does VOR demonstrate cerebellar learning
a given head movement must generate an exactly equal and opposite eye movement, or the image will ‘slip’ across the retina. Head movement signals, initiated in the vestibular system, must generate equal and opposite eye movements. A region of cerebellum called the flocculus is involved. If the wrong amount of head movement is generated then the retinal slip will generate climbing fibre signals in the flocculus, which in
turn mediate cerebellar plasticity
in the flocculus, modifying its output
255
What are examples of cerebral calibration
calibration vestibular reflexes generated by one’s own head movements and visuo-motor adaptation to prisms that shift the visual image
256
What is eyeblink conditioning?
what is it an example of
eg blowing in someone's eye at the same time as a noise repeatedly then playing the noise without the puff and eye will blink
skeletomotor conditioning
257
What does the cerebellum compute
How do basal ganglia's function compare to this
the PARAMETERS of movement (force, speed and timing)
involved in other types of learning, selection of movement patterns and triggering of movements: ACTION
SELECTION.
258
What are the basal ganglia
a collection of large subcortical forebrain nuclei
259
What are the many nuclei of the basal ganglia
Putamen & Caudate and the globus pallidus
260
Why are the e Putamen & Caudate and the globus pallidus referred to as the striatum
Because of the appearance of these nuclei (with stripes of fibres crossing grey matter)
261
Which structures are connected to the e Putamen & Caudate and the globus pallidus as a functional unit
2 midbrain nuclei, (substantia nigra & subthalamic nucleus)
262
What are the major inputs to the basal ganglia
the cerebral cortex (all lobes)
263
What is the output of the basal ganglia
Is it stimulatory or inhibitory
directed to the parts of the thalamus that supply the frontal lobes.
The ultimate output is inhibitory to the thalamus.
264
How are the basal ganglia interconnected with each other generally
by sequential inhibitory connections (GABA).
265
What are the basal ganglia output neurons to the thalamus
Describe their firing
therefore what does inhibition of these neurons lead to
internal globus pallidus
fire continuously, inhibiting the thalamus and therefore preventing movement
Inhibition of these inhibitory neurons will ‘release’ the thalamus from inhibition (disinhibition) allowing it to respond to excitatory inputs and activate motor cortex.
266
How can the circuitry of the thalamus be divided in to 3 parts
direct circuit
indirect circuit
hyper direct circuit
267
describe the direct circuit of the basal ganglia
what are the connections and what is the outcome of the circuit
corticostriate fibres activate neurons in the caudate or putamen nuclei.
These inhibit neurons in the internal globus pallidus
Increased activity in this pathway generates disinhibition in the thalamus, allowing movement
268
Describe the indirect circuit of the basal ganglia
what are the connections and what is the ultimate output of this output
Corticostriate inputs activate different caudate or putamen neurons that in turn inhibit neurons in the external globus pallidus, which inhibits the subthalamic nucleus.
Subthalamic neurons excite the internal globus pallidus.
Increased activity in this pathway disinhibits the subthalamic nucleus allowing it to increase activity in internal globus pallidus, increasing inhibition in the thalamus and preventing movement.
269
Describe the hyper direct pathway in the basal ganglia
Connections from the motor cortex directly excite the subthalamic nucleus, which in turn excites the internal globus pallidus. Activity in this pathway will abruptly inhibit the thalamus, to stop movement (an emergency brake).
270
If the output from the basal ganglia is a brake, how do the direct and indirect pathways affect the brake
direct pathway will release the brake,
increased activity in the indirect pathway will enhance the brake.
The balance of activity between these 2 pathways thus determines whether or not the
motor cortex is allowed to generate movements.
271
How is the substantia nigra involved in the activity of the basal ganglia's circuits
provides
dopaminergic projections to the caudate and putamen
differentially modulates the direct pathway (facilitating its activity) and the indirect pathway (depressing its activity)
272
How is the substantia nigra involved in the motor system overall
its position in the basal ganglia allows it to be in a controlling position to either permit or forbid activity
in the motor cortex which generates movement.
273
What is the effect of basal ganglia dysfunction generally
either an excess or a paucity of movement
274
What is hyperkinesia
movements are produced not as random twitches, but often well coordinated – but are produced at inappropriate times
275
What causes Parkinson's
dopaminergic neurons in the substantia nigra degenerate. Loss of dopamine causes imbalances in activity in the different pathways.
276
How does dopamine act in the basal ganglia (2)
i) activates the inhibitory direct pathway to the internal globus pallidus via D1
receptors
ii) suppresses the inhibitory projections to external globus pallidus (part of the indirect pathway) via D2 receptors
277
What effect does a loss of dopamine have on the direct and indirect pathways in the basal ganglia
reduced activity in the direct pathway and more activity in the indirect path, ultimately resulting in excessive activity in internal globus pallidus and increased inhibition in the thalamus
278
What are the treatments for Parkinsonism (2)
what is the aim
controlled lesions of the Globus Pallidus (pallidotomy) or, more common
nowadays, deep brain stimulation in the subthalamic nucleus
to disrupt the excessive inhibition of the thalamic neurons
279
What is the frequency of the tremor in Parkinson's
What does this reflect
10Hz
a poorly understood set of neurons in the basal ganglia which discharge in an oscillatory pattern during awake behaviour and these rhythmic discharges have different frequencies in different behavioural conditions
280
Describe the parallel circuits in the basal ganglia that have different in and outputs to the main pathways
Some are clearly concerned with motor function – e.g. those ending in the primary motor cortex and supplementary motor areas controlling limb movement and those projecting to the superior colliculus and controlling eye movements (the motor and oculomotor circuits). Others though are not directly ‘motor’, for example projecting to prefrontal cortex
281
Why are basal ganglia disorders referred to as 'extrapyramidal'
Based on the dramatic differences between the symptoms of motor cortex/pyramidal tract lesions (e.g. stroke) where the pyramid degenerates and basal ganglia lesions
282
Why is the term extrapyramidal for basal ganglia lesions misleading
Extrapyramidal assumes an alternative descending pathway
but the main way that basal ganglia influence movement is via the motor cortex and pyramidal tract.
Basal ganglia motor symptoms reflect abnormal patterns of activity through an intact pyramidal tract
283
What is the key physiological role of the basal ganglia which tends to be ignored in pathological studies
decision making - whether to make an action or not
284
What is the action selection problem
We are all faced constantly with the problem of choosing whether or not to make a movement, and if we do want to make a movement which one should we choose from an infinite potential range .
285
It would be maladaptive if all
| visual stimuli always drove saccades. How does the brain prevent this
the superior colliculus is tonically inhibited by basal ganglia output (substantia nigra/globus pallidus).
Cerebral cortex can activate appropriate cells in the caudate/putamen to inhibit the output cells, which disinhibits the superior colliculus allowing the saccade to occur. The basal ganglia thus determine whether and when the saccade occurs: selection of an action and triggering it
286
Give an overview of the physiological role of dopamine in the basal ganglia
supervising learning appropriate selection of actions.
mediates plasticity of the cerebral corticostriate inputs to the caudate and putamen. If an output from the basal ganglia (a decision to move) is successful, then the release of dopamine reinforces the synapses that were active to generate the successful decision, so that when those synapses are activated again the appropriate movement decision is generated again.
This mechanism is thought to underlie habit learning (stimulus-response
relationships)
287
What structures are some of the non motor outputs from the basal to
```
many parts (nucleus accumbens,
ventral striatum) connect to prefrontal areas of cortex
```
288
What are the prefrontal areas of the cortex thought to be concerned with
How does the the basal ganglia interact with them
high level executive decisions on strategy rather than implementation of specific movements
selecting between strategies or potential behaviours, parallel to the role in the motor system of selecting between different movements
289
What are the non-motor roles of dopamine
reward processes – the activation dopaminergic systems by reward processes supports its role as an instructive signal for learning what are good decisions
