Cortical Sensorimotor Systems Flashcards
how is the cerebral cortex organised?
- through hierarchal sensory-motor organization
- focuses on planning and programming in the premotor and motor cortex
- then sends sensory feedback into cortical areas
two main areas of the cerebral cortex
basal ganglia and cerebellum
how is the cerebral cortex divided?
into four main lobes (motor regions within the frontal lobe)
forms the outer surface of the forebrain (grey matter)
layers of the cerebral cortex
-has 6 distinct layers (of laminae )
- each layer has different cell types with specific output/inputs defining them as different regions
layer-specific inputs and outputs of the cerebral cortex
- Layer 4 is a key input layer
- Layers 5-6 key output layer
- Layers 3-5 have large calls called bat cells , executing movement
brodmann (1868-1918)
defined 52 areas of the cortex based on their distinct laminae profiles, to identify different relevant functions of different anatomic cortical areas
Brodmann critique
doesn’t take into account variability but still helpful in neuroimaging
where is the primary cortex located?
the frontal lobe, and this is one of the primary areas involved in motor function
what does the primary motor cortex (M1/BA4) contain?
betz cells in layer 5, which project from motor cortex to the spinal tract
only 5% projects motor neurons with the rest reaching spinal interneurons and project to the brainstem
Betz cells description
large pyramidal cells
- important for volitional control of actions in terms of muscle activity
what is the purpose of betz cells along the corticospinal tract?
- initiate, regulate, and control voluntary movement by innervating alpha/gamma motor neurones in the spinal cord
where does the corticospinal tract cross?
- at the medulla, hence limb movements are controlled by contralateral M1
- there isnt any obvious advantage to this cross
how can M1 be mapped?
by somatotopic representations
somatotopic representations
-different parts of the primary motor cortex send motor signals to different parts of the human body
- cortical territory isnt proportional to the size of muscles, instead related to how much fine-motor control is required
penfield (1940)
discovered electrical stimulation causes simple movements
sensory and motor maps
- we have a close mirror relationship between our motor (somatotopic) maps and our sensory (somatosensory) map
principle of organisation variations
- motor control/our somatotopic maps have adapted to vary based on ecological demands
- cats have a large rep for their whiskers
overlap between cortical motor maps
- idealized map may be unrealistic
- more overlap of neurons than expected when stimulated
- strict somatotopic maps suggests no overlap at all
Alternative organization principle
- two subdivisions that determined organisation of the primary motor cortex
- effector specific regions
- inter effective regions
effector specific regions
found that these effector specific regions are interdigitated with regions that show a very different connectivity, structure and function
Inter effector regions
- a more domain general region to motor control
- high connectivity to each other and connectivity to other regions in the brain that the primary motor cortex isn’t generally connected to
- active during planning of movement rather than execution
what was initially believed the motor cortex represents?
-simple contractions of contralateral muscles, seen in brief micro-stimulation of 50ms
- neurons just represent which kind of muscles will be activate
sustained stimulation longer than 500ms results in…
- complex goal-directed actions, such as climbing in monkeys, but these results have not been shown in humans
- shows there isnt a simple 1 to 1 mapping between motor cortex activity and simple muscle contractions - region also represents complex goal directed actions
precision grip in M1…
requires more M1 activity than power grip, but muscle activity remains the same
motor lesions result in…
-loss of individuated fine movements
- no effect on coarse/power actions
- result in ‘gross’ trunk movements
Frontal eye fields circuit
- key circuit for the connections between the frontal eye fields and the superior colliculus , allowing for the modulating of involuntary responses
- Allows us to e.g. read a book on a train where there’s constant intense sensory stimulation (control needed)
- Has a somatotopic map, with different parts of the frontal eye field representing different parts of visual space
Frontal eye field location
- small region in the frontal lobe, anterior to the primary motor cortex
- connected to the occipital cortex, receiving a lot of bottom up input of visual surroundings
-connected to regions in the prefrontal cortex allowing for modulation of eye movements based on tasks
Frontal eye field function
- critical for volitional control of eye movement in animals and humans
Superior colliculus
- part of the frontal eye field circuit
- important for volitional control of eye movements and contractions
- tiny structure
- salient changes are immediateley mediated by only the superior colliculus
Function of sensory motor areas of the frontal eye fields
- Not directly controlling movement
- active when processing motor control (e.g. selecting motor motions, planning, sequencing) even if no behaviour is carried out
- Dense connections between secondary motor areas + strong connection to the primary motor cortex
secondary motor areas
supplementary motor area (SMA)
premotor cortex (PMC)
- dorsal PM (PMd)
- ventral PM (PMv)
posterior parietal cortex (PPC)
what does PPC link?
frontal cortex decision-making with premotor planning areas, as it receives information from sensory regions
what is PPC important for?
determining potential actions given the environment
- the frontal cortex then decides which action to perform, and secondary areas plan for these
- damage = struggle to have an intuitive sense of what action to engage in
SMA regions
SMA proper learning
preSMA for execution
what is SMA involved in?
- postural stability and planning and executing complex sequential movements
- organizes movement inti meaningful chunks
what does SMA initiate?
internally generated movements, that are not stimulus-driven
What motor other motor control do pre SMA seem to be involved in
- Pre SMA becomes active in tasks that require response inhibition
- unclear whether its involved in withholding an already initiated movement or reprogramming a motor task to stop
what is PMd important in?
- preparation of movement and learning conditional actions in response to external cues, e.g., red traffic light – foot on brake
- overall involved in action cues
what is PMv important in?
sensory guidance of movement, such as responding to tactile, visual, and auditory stimuli
involved in visuomotor control during grasping and contains mirror neurones
mirror neurons
- were first reported in the PMC
- these show similar activity when performing a goal directed action and when observing the same action by another agent (activity holds across modalities )
- important for observation learning and understanding intention
Mirror neurons study and criticism
- in monkeys found these neurons to fire to goal directed actions and over time when repeatedly exposed to novel relationships (plasticity)
- only suggestive evidence they exist in the human brain, no unequivocal evidence
neuroplasticity
- ability of the brain to form and reorganise synaptic connections, in response to learning, experience, or following injury. This can occur in all brain areas, but clear in M1.
- pronounced in childhood and decreases as we age because of synaptic pruning
rapid changes in the somatosensory or somatotopic maps after change in inputs:
- training expands + occupies more space in the map
- denervation/amputation reduces the map
- co-activation fuses the maps
Co activation and neuroplasticity
- Found fusion did not occur in the representation in the fingers that fused together (no change in distance)
- It was instead the representation of the other (unglued fingers) became more similar to each other, and dissimilar to the other body parts
- Not clear of the functional purpose of this reorganisation
Amputation and neuroplasticity
- Those that recently lost their hand had similar somatotopic maps to the control no clear evidence of organisation in relation to adjacent motor areas
- One handers= evidence that some face regions started to invade the areas, with other areas moving away from it
- suggests neuroplasticity is more profound in early developmental stages
how do changes in maps reflect neuroplasticity?
- long-term changes in functional connectivity e.g. growth of neurons
- branching/pruning of dendritic connections
- neurons compete for space in the cortex
what is synapse efficacy an example of?
- learning-based neural changes, as synapses enable neurons to communicate with each other more or less efficiently
- neuroplasticity at the synapse level
presynaptic (synapse efficacy)
increase vesicle volume, availability of vesicles, and increase release probabilitysy
synaptic cleft (synapse efficacy)
reduce reuptake mechanisms and gap dimensions
postsynaptic (synapse efficacy)
increase receptor density and area
growth ( synapse efficacy)
make new synapses
what causes long-term synaptic changes in plasticity ?
specific timed patterns of neuronal activity
- LTP + LTD
LTP
- activity-dependent persistent strengthening of synapses, which produces a long-lasting increase in signal transmission between two neurons
- learning of neurons
LTD
- activity-dependent reduction in the efficacy of neuronal synapses, producing a long-lasting decrease in signal transmission between two neurons
- forgetting of neurons
associative LTD induction involves… (step 1)
- NMDA channel is normally blocked by Mg+ and concurrent voltage channel drives this out
a. Achieved by glutamate binding to nearby AMPA receptors
associative LTD induction involves… (step 2)
- Glutamate binds to NMDA and AMPA receptors
a. Temporary change in shape of channel which opens it up
b. Calcium enters through the open, unblocked NMDA channel
associative LTD induction involves… (step 3)
- Ca+ entry triggers intra-cellular signalling cascade which results in…
a. Migration of AMPA receptors from intracellular stores to the cell membrane
b. Synthesis of more AMPA receptors
what does increasing AMPA receptors result in? in LTP
greater communication between neurones due to more output
key principles of LTP and importance to learning and memory
cooperation
associative
synapse specific
cooperation (LTP)
LTP requires simultaneous activation of large number of axons due to large depolarisation
can happen through external stimulation or large enough input during learning
Ensures only events of high significance that activate inputs will result in memory storage
associative (LTP)
Pairing weak synaptic input with strong, large depolarisation can cause LTP at synapse , making weak neuron stronger
Allows an event with little significance to have meaning, when associated with a significant event
synapse specific (LTP)
If particular synapse is not activated, then LTP will not occur even with strong post-synaptic depolarisation
Inputs that convey information not related to an event will not be strengthened in memory
where was LTD first identified?
- hippocampus and believed to be a major component of motor learning in the Cerebellum.
- Involves decrease in AMPA receptors, however this is not NMDA-dependent
- LTD doesn’t require magnesium + NMDA unlike LTP
how can LTP and LTD-like mechanisms be measured in humans?
- with TMS by stimulating shallow brain regions as a result of pulses caused by magnetic coils placed on the scalp, which cause muscle contractions.
- from this we can identify parts in motor cortex that induce specific muscle activity (somatotopic maps)
- can measure changes in the excitability of muscles with this method
Schones findings on arm amputation patients and neuroplasticity
- cortical representations of both the hand and lips remain stable before and after amputation.
- activation patterns of somatosensory cortex also remained stable
- no evidence of neuroplasticity, points to sensitive developmental stages where neuroplasticity really occurs
