motor cortical areas Flashcards
How is the motor system organised?
Into motor maps
many in the motor cortical area, most prominently in the motor cortex
allows the motor system to be organised so that neurons controlling similar areas are next to each other
size of maps is related to how finely we can move them
it is an organisational property common across the majority of the motor areas that are plastic
what are the motor areas?
Primary motor cortex/ brodmann area 4 (anterior to the central sulcus) - primary output of the motor system from the brain
Supplementary areas
Premotor areas
Dorsolateral prefrontal cortex - allows us to plan out actions
Parietal regions allowing visuomotor integration
Premotor areas which have specific areas of control
what are the outputs of the motor system?
information is processed in parallel motor pathways
primary output from the motor cortex is the corticospianl tract
90% decassates
small portion remains on the ipsilateral side and is usually associated with the trunk and coarse movement
some outputs from non-primary motor areas, directly from the supplementary motor areas
how is sensorimotor processing organised?
simplified:
Areas involved in planning e.g supplementary motor area (SMA)
Programming - areas shaping movements
Brain stem and cerebellum integrate inputs and fine tune them
to the spinal column
into the muscles for execution
Back via the sensory system for online and offline feedback
gross oversimplification
Motor cortex does a bit of integration and possibly planning
Premotor cortex probably does some planning
what is the cortical homunculus?
Section through precentral gyrus of the primary motor cortex reveals areas controlling the foot up through the body to the hand and face with varying sizes of representation.
Size gives idea of how good we are at controlling a particular region, nonetheless they are plastic
what can animal models tell us?
Rats
motor maps are not specific to humans
stimulating motor cortex equivalent and recording elicited twitches revealed areas controlling the wrist and digits (and all other body parts basically)
skilled reaching tasks involving the motor cortex produces an enlargement in the representation of the wrist and digit area
Monkeys
inducing lesions in the previously mapped digit area disrupts function of hand
however after rehabilitation the function of affected limbs recovered as the areas innervating proximal muscles start to innervate the digits
what happens if the primary motor cortex is lesioned?
Lesion the primary motor cortex and record from ventral premotor cortex in squirrel monkey
loss of digit representation in primary motor cortex resulted in increased representation in the ventral motor cortex to compensate and improve function
the larger the representation the better the innervation
the brain is able to change and undergo plasticity in response to new needs.
How are cortical motor regions controlled and able to become plastic?
there is a decrease in local inhibition
decrease in GABA
allows the firing rate to increase
results in Hebian long term plasticity in the glutamatergic neurons
Silent synapses to other neurons controlling digits will become active if they become useful, becoming stronger the more its used
This is how a neuron that previously innervated a shoulder now innervates the digits
how do representations vary?
representations stay broadly stable over time (1 month on etc)
but look different between people
what is the significance of local GABA concentration?
changes
people with high levels of GABA have very discrete separate representations and are better at dissociating two fingers
what does cross species comparison tell us about the function of the motor cortex
implicated in dextrous movement
in humans the majority of output tracts come from the motor cortex
this is not true for mice as they don’t need the motor cortex for the majority of their movements.
mice recover very well if the motor cortex is lesioned because of this.
Humans have very different basic anatomy and skills
what do different stimulation patterns reveal?
brief electrical stimualtion: twitches long electrical stimulation: engages circuits representing full movements such as facial expressions or limb movements able to map these movements in M1 Central space manipulation Region for reach Region for defensive movements Hand to mouth etc Therefore you can representations of complex movements rather than just index finger twitch etc
what do we know about the motor cortex?
Know that it is a primary output
Important in human motor control (strokes etc - not able to move)
From looking at anatomy we can learn about inputs and outputs
Massive corticospinal pyramidal output - Important for dexterous movements in particular
reciprocal circuits in between the deep grey matter projecting to the cerebellum and lots come back from the thalamus
Loops of online corrections, smoothing movement
Very connected to other cortical regions and to subcortical area and to output
(Don’t know what it codes for and does)
What does long periods of recording in the motor cortex tell us?
See changes in patterns activity in the motor cortex over time
Different groups of neurons to a single muscle - neurons for movements rather than muscle (m1 codes movements)
Multiple representations innovating the deltoid that do different things (shoulder)
Distinct functional movements despite same muscle fibre contractions
Lot of evidence supports coding of whole movements
how does M1 code force?
rate coding signals muscle load activity as pulling motion occurs increasing load increases activity decreasing load decreases activity seen across muscle groups
