Introduction to Movement

How does movement begin?

sensory systems → continuously getting sensory feedback as you move

• internal representation that gets translated into motor commands

How is movement organized?

• higherarchically

  • cortex → brainstem → spinal cord

• Parallel

  • core pyramidal pathway

  • extrapyramidal pathway

Which functions are associated with the brainstem?

• respiration

• chewing

• facial expressions

• stereotyped behaviour

Which functions are associated with the spinal cord?

• protective reflexes

• locomotion

What makes up the extrapyramidal motor system?

The cerebellum and the basal ganglia nuclei

How is the frontal lobe organized along its rostral-caudal gradient?

• Anterior: planning and goals (executive function)

• Middle: translating goals into movements

• Posterior: sending commands to execute movement

Which areas make up the motor cortex?

• Area 4 (M1)

• Area 6 (lateral premotor cortex, SMA and pre-SMA)

What is the result of a lesion in the primary motor cortex?

Contralateral paralysis and increased muscle tone (spasticity), especially for dexterous movements

What deficits follow lesions to SMA vs. premotor cortex?

SMA: alien limb syndrome (loss of control of limbs)

Premotor: loss of ability to use sensory information to reach for objects

What did Wilder Penfield's work show about primary motor cortex?

It contains a map of the body musculature paralleling the somatotopic map in primary somatosensory cortex

What are larger regions of the map dedicated to? smaller regions?

larger regions:

• hands for fine motor control

• face for the complexity of facial muscles

smaller regions:

• torso

• legs

• feet

What are the firing patterns of neurons in the primary motor cortex?

fire before and during voluntary movements of contralateral muscles

• because activity in the brain precedes the movement

What is the role of pre-motor and supplemental motor areas?

organize the motor programs for complex voluntary movements

What do reciprocal parietal-premotor connections accomplish?

Sensory-motor transformations that let sensory information guide interaction with objects

• dorsal stream from the visual cortex is combined with somatosensory information in the parietal lobe

• axons from the parietal lobe then go to the lateral pre-motor cortex

• information is translated into correct command to enable movement

What network directs arm movements toward objects (reaching)? How was this shown?

• parietal lobe

• dorsal premotor cortex

• primary motor cortex

This was shown by recording neurons while monkeys reached out and grabbed objects

What do neurons in the parietal lobe respond to?

both tactile and visual stimuli with receptive fields that are spatially in register

• neurons are involved in contraction of a peripersonal spatial map used to guide goal directed movements

How do premotor neurons fire? What does this suggest?

fire during the movement and during an imposed delay prior to the movement

this suggests that they are involved in the planning and preparation to move

What happens if the premotor neuron is given the same cue but is preceded by instruction not to move?

the neurons do not fire during the delay

• shows that they are not sensory neurons → Because the same cue doesn't make them fire when no movement is required, they are not simply sensory neurons

• shows that they are connecting the sensory input to the appropriate action

How do premotor and primary motor neurons differ regarding which limb is used?

Premotor:

• neurons fire during preparation for a planned movement and during execution, regardless of which arm is used

Primary motor:

• neurons fire only during execution and only for the contralateral arm

the network that is involved in shaping the hand to grasp and object includes what structures? What is it indicative of?

• parietal lobe

• ventral premotor cortex

• primary motor cortex

as you reach out, hand takes shape which is indicative of two parallel pathways

What do ventral premotor neurons respond to?

they respond to preferred shapes

fires during cue and action

What are mirror neurons? What do they suggest?

Premotor neurons that fire when a monkey reaches for an object and when it watches someone else do so (but not to the object alone or non-goal-directed arm movements)

suggests that the monkey understands what the experimenter is doing

what is the SMA involved in?

internally generated movements → free will movements

learned complex movement sequences

What happens the more forward in the frontal lobe you go?

things are more directly involved in movement but things also become more abstract

What happens if the SMA and pre SMA are lesioned?

paradoxical effects on volitional movement including alien limb syndrome or less of spontaneous movement

• the mechanism for coordinated movement are still intact but the ability of the mind to voluntarily control has been decoupled

What is alien limb syndrome?

rare neurological disorder where a person's hand or limb moves involuntarily and purposefully, as if it has a mind of its own, while the person lacks conscious control over those actions

What might pre-SMA be involved in?

the connection between thought and action

When is SMA active?

active during complex learned movement sequences even when they are just imagined

What were the three tasks that demonstrated the time of activity of the SMA?

Task 1: single finger flexion → tell the person to tap index finger and thumb together

• motor region that corresponds to the fingers lights up

• somatosensory region that corresponds to the fingers lights up

Task 2: sequential finger movement → show complex movement sequence of fingers

• both previous regions light up

• SMA also lights up → complex sequence involved

Task 3: mental rehearsal of sequential movement → don’t actually move hand just imagine it

• SMA lights up

What are SMA neurons selectively responsive to?

select components of learned movement sequences

• for example in one case the neuron is active prior to the turning motion in a sequence of movements, but only when its followed by a pull movement

What is the readiness potential?

an EEG signal that is recorded from the medial frontal lobes (SMA) starting around 1 second before voluntary movement

• there is a build up of electrical activity that always precedes voluntary movement in the SMA

What did the early work by Wildren Penfield show?

the primary motor cortex contained a map of the body musculature that paralleled the somatotpic map in the primary somatosensory cortex

• extensive connections between the two regions

What does the somatotopy suggest about the primary motor cortex?

it is the final common pathway for motor output from the cortex

How much motor output does M1 contribute to the brainstem and spinal cord

only 1/3 of cortical motor output

Where does the remaining cortical output come from?

premotor cortex, SMA and S1

what do projections from motor cortex to the spinal cord form?

corticospinal tract

• evolutionarily advanced

What do projections from the motor cortex to the brainstem form?

Corticobulbar tract

• older pathway

What is the corticospinal tract important for?

moving the fingers

what is the pathway for motor fibres?

they pass within the internal capsule → form the cerebral peduncles and the pyramids

• some fibres from the corticobulbar tract end in the brainstem

• cotricospinal fibres project to the spinal cord

Where do the motor fibers cross over?

where the brainstem turns into the spinal cord at the pyramidal decusation → why left side of brain controls right side of body and vice versa

Where do the fibers start?

in layer 5 of the motor cortex

midbrain: corticospinal tract

Pons: corticospinal tract

rostral medulla: corticospinal tract (pryramids)

caudal medulla: corticospinal tract (pyramids)

what happens when the pyramids are cut?

no corticospinal tract intact

• moneys were fine for the most part after recovery → they could move arms and legs around after surgery

• loss control of individual fingers

What does the lesion in the pyramids indicate?

there is an alternate pathway through the brainstem → parallel pathway

what does the brainstem control?

• stereotyped movements of the head

• through descending connections with the spinal cord it, contributes to the control of voluntary movements of the body

what do some corticobulbar projections control?

cranial nerves involved in voluntary movements of the head

• facial expression

• tongue movement

• jaw movement

• eye movements

What do medial brainstem pathways innervate?

axial muscles that control posture and balance

what is the pathway of the medial brainstem fibres? Where do they terminate?

they travel down the ventral spinal cord white matter and terminate in the ventromedial regions of the cord gray matter

Where does the medial pathway originate from?

• tectospinal tract in the midbrain

• reticulospinal tract in the pons

• vestibulospinal tract in the medulla

where do the medial pathway fibres get input from?

• from the cortex

• cerebellum

• basal ganglia

What is the main lateral pathway?

rubrospinal tract

where does the rubrospinal tract originate?

red nucleus of the midbrain

how does the rubrospinal tracts descend and where does it terminate?

descend in the contralateral dorsolateral column of the spinal cord

terminates in the dorsolateral gray matter

What does the rubrospinal tract control?

voluntary limb movement

what does the complex circuity of the spinal cord gray matter enable?

rhythmic and coordinated movements of the body

Where do sescenting projections terminate in the spinal cord?

on premotor neurons or directly on ventral horn motor neurons

What are direct connections important for?

control of individual digits

what brain regions act in parallel with the central pattern generators?

• basal ganglia

• cerebellum