Cortical Sensorimotor Systems Flashcards
how is the cerebral cortex organised?
through hierarchal sensory-motor organisation, that focuses on planning and programming in the premotor and motor complex, that provides sensory feedback into cortical areas
two main areas of the cerebral cortex
basal ganglia and cerebellum
basal ganglia
initiates movements
cerebellum
is important for fine-motor control
how is the cerebral cortex divided?
into four main lobes (motor regions within the frontal lobe)
forms the outer surface of the forebrain
layers of the cerebral cortex
has 6 distinct layers of laminae which define different regions of the cortex by their different cell types
example of layer-specific inputs and outputs of the cerebral cortex
5 and 6 are both involved in sending outputs to the muscles and spinal cord
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
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
95% of these cells instead reach spinal interneurons and project to the brainstem
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 to cause muscle contractions
where does the corticospinal tract cross?
at the medulla, hence limb movements are controlled by contralateral M1
how can M1 be mapped?
by somatotopic representations
somatotopic representations
different parts of the motor cortex send motor signals to different parts of the human body
- related to how much fine-motor control is required
penfield (1940)
discovered electrical stimulation causes simple movements
what do cortical motor maps reflect?
sensory-motor specialisation, seen in changes based on what is used most
- humans have big representations of hand/face as we have most fine-motor control of these
overlap between cortical motor maps
between these areas based on how much the muscles work together
what was initially believed the motor cortex represents?
simple contractions of contralateral muscles, seen in brief micro-stimulation of 50ms
sustained stimulation longer than 500ms results in…
complex goal-directed actions, such as defence, climbing, and reach-to-grasp in monkeys, but these results have not been shown in humans
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, and result in ‘gross’ trunk movements
what are frontal eye fields connected to?
the superior colliculus, which is a somatotopic area in the frontal lobe important for volitional control of eye movements
what is FEF connected to?
the occipital cortex and frontal regions important for decision making
why are there dense connections between sensory motor areas and M1?
secondary sensory motor areas are involved in executing actions vs M1 sends out motor commands
secondary motor areas
supplementary motor area
premotor cortex
- dorsal PM
- ventral PM
posterior parietal cortex
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
SMA regions
SMA proper learning
preSMA for execution
what is SMA involved in?
postural stability and planning and executing complex sequential movements
what does SMA initiate?
internally generated movements, that are not stimulus-driven
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
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 and observing the same action – important for observation learning and understanding intention
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.
rapid changes in the somatosensory or somatotopic maps after change in inputs:
- training expands map
- denervation/amputation reduces the map
- co-activation fuses the maps
how do changes in maps reflect neuroplasticity?
- long-term changes in functional connectivity
- branching 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
presynaptic
increase vesicle volume, availability of vesicles, and increase release probabilitysy
synaptic cleft
reduce reuptake mechanisms and gap dimensions
postsynaptic
increase receptor density and area
growth
make new synapses
what causes long-term synaptic changes?
specific timed patterns of neuronal activity
LTP
activity-dependent strengthening of synapses, which produces a long-lasting increase in signal transmission between two neurons
LTD
activity-dependent reduction in the efficacy of neuronal synapses, producing a long-lasting decrease in signal transmission between two neurons
associative LTD induction involves…
- NMDA channel is normally blocked by Mg+ and concurrent voltage channel drives this out
a. Achieved by glutamate binding to nearby AMPA receptors - 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 - 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?
greater communication between neurones due to more output
key principles of LTP and importance to learning and memory
cooperation
associative
synapse specific
cooperation
LTP requires simultaneous activation of large number of axons due to large depolarisation
Ensures only events of high significance that activate inputs will result in memory storage
associative
Pairing weak synaptic input with large depolarisation can cause LTP at synapse
Allows an event with little significance to have meaning, when associated with a significant event
synapse specific
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
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.
