Motor systems - week 7

Question 1

How is the motor system organised?

Answer 1

Hierarchically with the command centre at the top

Question 2

what is the Posterior Parietal Association Cortex

Answer 2

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

Question 3

Where does PPAC output to?

Answer 3

Dorsolateral prefrontal association cortex ( collaboratively plan movements)

Frontal eye field (controls eye movement)

Secondary motor cortex

Question 4

Dorsolateral prefrontal association cortex

Answer 4

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

Question 5

Where does DPAC output to?

Answer 5

DPAC

Primary and secondary motor cortices

Frontal eye field

Question 6

Secondary motor cortex SMC

Answer 6

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

Question 7

Mirror neurons

Answer 7

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)

Question 8

Primary motor cortex

Answer 8

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

Question 9

what are the 4 decending motor pathways?

Answer 9

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

Question 10

Dorsolateral
Corticospinal tract

Answer 10

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)

Question 11

Dorsolateral Corticorubrospinal tract

Answer 11

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)

Question 12

Ventromedial Corticospinal tract

Answer 12

Axons descend from PMC -> spinal cord (ipsilateral)
Axons branch and innervate interneurons bilaterally to control trunk and proximal limb muscles

Question 13

Ventromedial Cortico-brainstem-spinal trac

Answer 13

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)

Question 14

what are the 3 types of muscle?

Answer 14

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)

Question 15

Neuromuscular junctions

Answer 15

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

Question 16

Sensory feedback from muscles

Answer 16

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

Question 17

Monosynaptic Stretch Reflex

Answer 17

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

Question 18

Polysynaptic Reflexes

Answer 18

More complex reflexive responses involving several pathways and often altering activity in more than one muscle

E.g. Pain withdrawal response

Question 19

Cerebellum

Answer 19

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

Question 20

Basal ganglia

Answer 20

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

Question 21

Parkinsons disease – PD
symptoms

Answer 21

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)

Question 22

PD - Impact on the motor systems

Answer 22

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)

Question 23

PD Treatments

Answer 23

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