Voluntary Control

Balance

  • Balance: The ability to maintain postural equilibrium by controlling our Center of Mass (CoM) and Base of Support (BoS)

  • CoM: The point at which the whole body, or individual segment, mass is equally balanced

  • Center of gravity (CoG): Projection of CoM on the floor

  • BoS: Usually only the feet in contact with the ground, but could incorporate other objects used for balance

  • CoM must stay within the base of support (BoS)

  • If the CoM leaves the BoS, increased chance of falling

  • Anticipatory postural adjustment (APA):

    • Feedforward for anticipatory postural instability

    • Plan movements based on sensory input

    • Postural adjustments that occur prior to task

    • Stabilize in preparation for the task

    • Need to anticipate instability that occurs with upcoming movement

    • NO APAs in response to external perturbations

      • No time to anticipate the perturbation

  • Walking:

    • When walking, the BoS reduces down to one foot

    • Lifting one leg leads to instability in medial lateral direction

    • Must shift CoM to a location within the remaining BoS

  • Automatic postural response (APR):

    • Feedback for unanticipated postural instability (ex: external perturbations)

    • Muscle/skin/vestibular feedback signalling unplanned movements

Voluntary movement

  • Primary motor cortex (M1) – area 4

    • Movement execution

  • Premotor areas– both in area 6:

    • Premotor cortex – area 6

      • Movement preparation

    • Supplementary motor area – area 6

      • Movement planning/initiation

  • Primary somatosensory cortex (S1) – areas 3a, 3b, 1, 2

    • Proprioception – 3a & 2

    • Touch – 3b & 1

    • Sensory input of movement

  • Posterior parietal cortex – areas 5, 7

    • Sensory integration, including vision – current state of body

Primary motor cortex (M1) – area 4

  • Movement execution

    • Including: Direction, magnitude, force, speed of movement

  • M1 neurons respond to individual voluntary movements

  • M1 neurons fire before movement onset

  • Synapse directly with ⍺MN & interneurons of individual muscles, and groups of muscle around a joint via corticospinal tracts (medial & lateral)

  • Lateral corticospinal tract: Synapses on distal muscles (hand, foot)

  • Ventral-medial (anterior) corticospinal tract: Innervates axial muscles

  • Full myelination of corticospinal tract coincides with walking in infants, and pincer grip

 

Premotor cortex – area 6

  • Movement preparation through selection of appropriate motor plans

  • Premotor cortex neurons synapse with cells in M1 and signal preparation for movement & observing movement

  • Integrate sensory aspects for motor acts

    • Information from S1 important to structure movement

    • Respond to both making a movement, and watching another make a movement

    • Mirror neurons

  • Signal incorrect actions, and involved in motor learning skills

    • As a monkey learns a new task, neurons within premotor cortex alter firing in response to correct or incorrect rules being learned (Butch et al., 2006)

    • Are sensitive to behavioural context and instructions

  • Damage:

    • Causes difficulties in performing movements in response to visual and verbal commands

    • A loss of self-initiated movement- therefore, damage impacts APAs

 

Supplementary motor area (SMA) – area 6

  • Movement planning/initiation

  • SMA neurons respond to sequences/mental rehearsal of movement sequences and are involved in creating appropriate complex motor output (timing and gain

  • SMA is involved in the initiation of movements on the contralateral side of the boduy

  • A “readiness potential” is seen 0.8-1s before movement begins

 

Posterior parietal cortex - area 5 and 7 (of association complex)

  • Integrates sensory modalities for motor planning

  • Sensory integration, including vision – current state of body

  • Receives input from S1

  • Important for sensory integration

  • Area 5

    • Integrates tactile & proprioceptive information

  • Area 7

    • Integrates visual information

 

Prefrontal cortex (of association complex)

  • Working memory: Location of objects in space to guide movement

  • Memory of consequences of actions

 

Primary somatosensory cortex (S1) – areas 3a, 3b, 1, 2

  • Proprioception – 3a & 2

  • Touch – 3b & 1

  • Sensory feedback of movement

  • From dorsal column, peripheral sensory information projects, via 3 synapses, to S1

    • Synapse 1: Medulla (gracile + cuneate nucleus)

    • Synapse 2: Ventral posterior lateral (VPL) thalamus

      • Most thalamic fibers from VPL nucleus terminate in 3a and 3b

      • Fibers then project to areas 1 and 2

      • 3b & 1 = tactile (skin)

      • 3a & 2 = proprioceptive (skin, spindles, joints)

    • Synapse 3: Sensory cortex

Damage and therapy

  • Receptive field: Area of the skin that, when activated, elicits a response from a receptor

  • Projected fields: Area of the skin that a sensation is felt, following stimulation of the brain

  • Using cortical stimulation to influence sensation: Receptive fields > projected fields

  • Phantom limb: Following amputation, patients feel the phantom limb, try and move it, and experience tremendous pain

  • Cortex rewiring: After amputation, sensations in the phantom limb can be felt by stimulating the face, or the proximal limb

    • Ex: Hand input and face input rewired to go to same place/overlap

  • Reasons for phantom limb:

    • Peripheral nervous system

      • Damage/swelling in the axon of the nerve at the amputation site

      • Generates random action potentials

    • Central nervous system

      • Damage to pain fibers in spinal cord

      • Other nerves branch to same tracts to the brain, so some regular sensations could be perceived as pain

    • Cortical reorganization

      • Some areas take over other areas

      • E.g. the face takes over the hand area in M1 and S1

  • Pain persists as there is inconsistent sensory integration/feedback

  • Mirror therapy: Utilises vision to enable the sensory and motor feedback to align with the phantom sensations

  • Prosthetics:

    • After amputation, afferent and efferent pathways remain largely intact

    • A peripheral nerve interact is used to implement movement commands & transmit sensory feedback from a prosthesis