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
how does M1 code direction?
M1 neurons are direction specific
neurons are preferentially active for different directions
How does the motor cortex innervate muscles?
can’t be 1:1 as this doesn’t give enough flexibility to code all the necessary information
1:many
neurons innervate many neurons in the spinal cord which innervate a number of muscle fibres
One muscle fibre can be innervated by many neurons - depending on which ones are firing at a given time is likely to encode the precise movement direction etc - the details of the action that need to be carried out.
Allows much more flexibility in the system which makes sense but makes it very hard to map these things
How can one neuron do different things
neurons are able to innervate multiple muscles at the same time because M1 codes for multiple things in parallel at the same time.
What is meant by rhythmical activity?
Movements are dynamic, changing over time
Activity of neurons changes over time as you move at different stages of the movement with changing patterns of activation
neurons that innervate the muscles that cause a specific movement are activated in an oscillatory signalling pattern.
plotting neural activity even for non oscillatory behaviours produces an oscillation
there is oscillatory activity going on at different frequencies within M1
As you go to move you get a decrease in activity at about 20-5Hz (Beta band)
What does the PPC do?
posteriot parietal cortex
highest order motor area
visuomotor integration
Involved in active touch and active sight
converging input to area 6
Area 5 - target of inputs from the primary somatosensory cortical areas 3, 1 and 2
Area 7 - target of higher order visual cortical areas such as MT
Lesions result in neglect, apraxia, agnosia and lack of body image
What does the PFC do?
prefrontal cortex
Decision making area - motivation
planning movement
highest order motor area
extensively interconnected with regions in the anterior frontal lobes that are thought to be important for abstract thought, decision making and anticipating the consequences of actions.
Inputs
Limbic (dorsomedial and anterior nuclei - thalamus)
Parietal and occipital (visuospatial signals)
Magnocellular dominated visual strea
Inferotemporal - object recognition stream
Outputs
FEF
M1
SMA
PPC
Cerebellum
Lesion (rod through head) - problems with inhibitory control and executive function and working memory
What are the main outputs of the motor areas
40% of corticospinal tract - through pyramidal
20% direct input to motor neurons (many:many)
Distal limb movements especially fine hand movements
Reticulospinal
Brainstem and reflex control
Striatum (basal ganglia)
Pons → cerebellum
Cranial nerve nuclei
Red nucleus (rubrospinal tract)
what does the premotor cortex do?
Planning of movement/ sensory guidance of movement
Lateral in area 6
Mirror cells - hypothesised that they are important for imitating movement so important for learning particularly language
Rosalanti
Main input from PFC and PPC and cerebellum (via VL thalamus)
Main output to M1, PFC PPC and cerebellum and corticospinal tract
what does the SMA do
Supplementary motor area
Medial in area 6
Planning of internally generated movement
Input from PFC and basal ganglia (via thalamus)
Output to M1 and corticospinal tract
Lesion results in difficulty doing dexterous tasks required both hands - lots of interconnectivity with the other hemifield / larger corpus callosum connections
what are the 6 layers of the cortex
Molecular Axons and dendrites - no somas Involved in getting afferents from other regions of the cortex and brainstem Effetrents to other areas External granular layer Densely packed stellate cells - interneurons Small pyramidal cells - excitatory Somas of the processes in the molecular layer External pyramidal layer Loosely packed stellate cells Bigger pyramidal cells (M) Same afferents and efferents Internal granular layer Densely packed pyramidal Many afferents from thalamus Internal pyramidal layer Largest pyramidal cells Contains betz cells Receives afferents form the brainstem Efferents to brainstem and spinal cord Multiform Pyramidal cells of all cells Some loosely packed stellate cells Efferent To the thalamus Functional organisation Established during development due to guiding glia cells
what is the corticospianl tract
Crossed pathway
Major role in fine control of distal limb musculature
Terminates in ventral horn
Complete decussation
