Motor systems - week 7 Flashcards
How is the motor system organised?
Hierarchically with the command centre at the top
what is the Posterior Parietal Association Cortex
PPAC transforms multisensory info into motor commands to plan movements
It receives and integrates info from visual, auditory and somatosensory systems.
Codes position of the body in relation to external objects by providing spatial information and directing attention
Where does PPAC output to?
Dorsolateral prefrontal association cortex ( collaboratively plan movements)
Frontal eye field (controls eye movement)
Secondary motor cortex
Dorsolateral prefrontal association cortex
DPAC receives major projection from PPAC and projects back to this area to generate motor plans
Evaluates likely outcomes of possible actions – feeds into motor plans
Damage / inactivation leads to illogical / disorganised behaviour
Identifies and responds to external stimuli
First neurons to fire in anticipation of motor activity
Initiates complex voluntary movements in collaboration with PPAC
Where does DPAC output to?
DPAC
Primary and secondary motor cortices
Frontal eye field
Secondary motor cortex SMC
Receives input from association areas and projects mainly to primary motor cortex PMC
Includes premotor cortex and supplementary motor area – SMA
When signals arrive at SMC from DPAC (and PPAC) the process of executing the planned response begins
Also, involved in planning and sequencing of complex movements
Damage to SMA results in inability to sequence correctly to perform behaviour, but not in the execution of movement (Gerloff et al., 1997)
Therefore, execution of movement must be controlled elsewhere
SMC also contains Mirror Neurons
Mirror neurons
A mirror neuron fires both when performing a behaviour and when observing the same action performed by another
Early evidence of mirror neurons in ventral premotor cortex from electrophysiological studies with Macaque monkeys (Rizzolatti et al., 1980’s and 1990’s)
Supported by human fMRI studies (e.g. Buccino et al., 2004)
Play vital role in imitation and understanding the actions and intentions of others
May explain the ‘contagious’ nature of yawning?
Facilitates learning by imitation?
Facilitates our understanding of others?
E.g. Emerging evidence using TMS suggests hypoactivity in mirror neurons may contribute to autistic spectrum disorders (e.g. Théoret et al., 2005)
Primary motor cortex
Located on precentral gyrus in frontal lobe
Somatotopically organised (Penfield & Rasmussen, 1950)
Contralateral outputs
Activation of some neurons in PMC cause movements of particular body parts
Body parts capable of more intricate movements cover greater area of PMC
Evidence from Graziano (2006) suggests activation of areas of PMC produce relatively complex action sequences rather than simple contractions of individual muscles
what are the 4 decending motor pathways?
2 dorsolateral –
Corticospinal tract
Corticorubrospinal tract
2 Ventromedial -
Corticospinal tract
Cortico-brainstem-spinal tract
Signals over these four pathways work together to control voluntary movement
Both corticospinal tracts descent directly to spinal cord
Dorsolateral
Corticospinal tract
Axons descend from PMC through medullary pyramids, cross, then down dorsolateral spinal cord
Contains Betz cells (pyramidal neurons)
Synapse on interneurons & motor neurons
Controls distal muscles (e.g. fingers and toes) (contralateral)
Dorsolateral Corticorubrospinal tract
Axons descend from PMC -> red nucleus, cross, down through medulla -> cranial nerves (control face muscles)
Or down dorsolateral spinal cord, where synapse on interneurons & motor neurons to control distal muscles (e.g. arms and legs) (contralateral)
Ventromedial Corticospinal tract
Axons descend from PMC -> spinal cord (ipsilateral)
Axons branch and innervate interneurons bilaterally to control trunk and proximal limb muscles
Ventromedial Cortico-brainstem-spinal trac
Axons descend from PMC -> brain stem structures, -> descends bilaterally to spinal cord.
Synapse on interneurons of different spinal cord regions to control trunk and proximal limb muscles (bilateral)
what are the 3 types of muscle?
Smooth
Cardiac
Skeletal - bundle of muscle fibres attach to the skeleton by tendons
A key function of skeletal muscles is to move joints which moves limbs
Flexor muscles - limb bends in (flexion)
Extensor muscles - limb extends out (extension)
Synergistic muscles – two contracting muscles that produce the same movement
Antagonistic muscles – two muscles that act in opposition (e.g. Biceps)
Neuromuscular junctions
Terminal buttons of motor neurons synapse on motor endplates of muscle fibres
When stimulated Acetylcholine (ACh) released by motor neurons at neuromuscular junctions - binds to receptors on motor end-plate - depolarises endplate -> causes muscle contraction - moves limb
A single motor neuron provides input to (“innervates”) multiple muscle fibres (neuron and associated fibres known as a “motor unit”)
Strength of muscular contraction depends on number of associated motor units that fire and their firing rate
Damage to the neuromuscular junction transmission mechanism can impair muscle control / movement
E.g. Myastheniagravis
Sensory feedback from muscles
Proprioceptors – receptors that provide information about joint angle, muscle length and tension
This information is integrated to inform about the position of the limb in space
Muscle activity is monitored by 2 key proprioceptors:
Muscle spindles – embedded in muscle tissue. When muscle lengthens, spindle stretches – firing increases à sends signals to spinal cord. Respond to changes in muscle length
Golgi tendon organs – embedded in tendon. Monitor how hard muscle pulls on tendon – increase their firing in response to increases in muscle tension – send signals to spinal cord. Respond to increases in muscle tension
Both provide their info to CNS and movement adjusted as required
Monosynaptic Stretch Reflex
Proprioceptors are used to adjust muscles during reflex actions
Monosynaptic Stretch reflex
Serves to maintain limb stability
E.g. adjustment to load
Muscle stretch - muscle spindle fires - signal carried to spinal cord by sensory neuron - synapses with & stimulates motor neuron - sends signal back to muscle to contract (compensatory response)
E.g. patellar tendon reflex
Polysynaptic Reflexes
More complex reflexive responses involving several pathways and often altering activity in more than one muscle
E.g. Pain withdrawal response
Cerebellum
Complex structure which modulates motor action
Receives information from a number of sources (e.g. motor cortex, brain stem, and vestibular and somatosensory systems), compares it, and fine tunes/corrects movements that deviate from optimal
Plays major role in motor learning and sequencing, needed to enact motor plans
Involved in maintenance of posture, balance, gait, speech and control of eye movement
Basal ganglia
Group of subcortical nuclei (caudate nucleus, putamen (collectively =striatum) & globus pallidus
Key involvement in control of movement
Looped projections from multiple areas of cortex
striatum (via thalamus)-> cortex
Monitors sensory info. & receives info. about planned movements then fine tunes movements by sending inhibitory and excitatory messages to the cortex
Disruption to the DA signals in BG
profoundly disturb movement and
causes Parkinsonian-like symptoms
Parkinsons disease – PD
symptoms
PD symptoms include muscular rigidity, slowness of movement, tremor, postural instability
Difficulty in initiating and terminating behaviours
Difficulty executing sequences of movements required to carry out a motor plan
Caused by degeneration of DA neurons connecting substantia nigra with striatum of BG (nigrostriatal pathway)
PD - Impact on the motor systems
Reduced DA input to BG alters the normal balance between excitatory and inhibitory pathways:
Excitatory circuits (and their cortical projections) suppressed
Inhibitory circuits (+ cortical projections) enhanced
Therefore inhibitory responses dominateà poor motor function associated with PD
Treatments aim to re-establish the balance by either increasing DA transmission from the SN or suppressing the BG inhibitory pathways / outputs (removing motor inhibition)
PD Treatments
Drug treatments e.g. L-Dopa (precursor to DA) boosts production of DA in the remaining DA neurons
Varied side effects including dyskinesia and dystonia, hallucinations, delusions
Progressive nature of neuronal loss means potentially not long term solution
Deep brain stimulation (DBS) – stimulates areas of BG with implanted electrodes
Long term effectiveness is promising (see Fruend, 2005)
Cell replacement therapy –transplant cells to boost declining numbers of DA neurons (e.g. Wijeyekoon & Barker, 2009)
Still in its infancy, but a promising area for future research
