Physiology of the Motor Cortex

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Name and locate the motor areas of the cortex. Describe which pathways might be used in picking up a cup

  • Motor hierarchy - Prefrontal association areas send commands to premotor areas, which then send commands to primary motor areas
  • The premotor area is subdivided:
    • supplementary motor area - aka SMA; located at top of area
    • lateral premotor area - located at bottom of area
  • three cortical pathways involved in picking up a cup

Loop 1

  • route: primary somatosensory to primary motor
  • use: Simple acts like quickly regulating pressure on the cup
  • details
    • primary somatosensory: senses finger positions from afferents and pressure from touch receptors
    • primary motor: allows contraction of individual muscles

Loop 2

  • route: primary somatosensory to higher somatosensory to premotor to motor
  • use: for muscle synergy selection to lift the cup
  • details
    • premotor: selects the most appropriate synergy
    • higher somatosensory: involved in detecting texture and more complex features of cup

Loop 3

  • route: primary somatosensory to higher somatosensory to PTO association to prefrontal association to premotor to primary motor
  • use: when one reaches for the cup to pick it up; involves the integration of senses and attention

Describe the motor map in the primary motor cortex

  • mirrors the somatosensory map
  • is a distorted representation (homunculus) of body
  • the homunculus allows for fine control of muscles, since it is face- and hand- heavy
  • Old theory of the homunculus is that it represents a map of the body
  • New theory is that it represents a map of the movements of the body

Describe the function of the cortico-spinal tract

The cortico-spinal tract consists of two divisions:

lateral corticospinal tract

  • 70-90% of fibers
  • most highly developed in primates
  • originates from premotor cortex (6), primary motor cortex (4) and somatosensory cortex (3,2,1)
  • crosses at pyramidal decussation
  • projects to lateral ventral horn
  • allows for fine control of muscles/fingers

ventral corticospinal tract

  • used primarily for posture
  • remains uncrossed until spinal cord
  • bilateral and polysynaptic connections made on medial motoneurons of proximal/axial muscles

Compare the deficits produced by an "upper motor neuron" lesion vs a "lower motor neuron" lesion. What condition might produce spasticity and fasiculations?

Upper motor neuron lesions

  • lesion in the cortex or internal capsule
  • if the lesion is small, one may simply lose refined movements (like independent finger movements)
  • if it is large, it may manifest as flaccid paralysis (initially), then spasticity
  • symptoms include hypertonia, hyper-reflexia, Babinski sign, no fasciculations and no atrophy

Lower motor neuron lesions

  • lesion in the motoneurons or their axons
  • symptoms include weakness or paralysis of isolated muscles, which are flaccid

Describe the function of cortico-spinal neurons

There are two sets of corticospinal neurons:

in initiating a movement

  • Area 4 neurons initiate movements
  • preceded (by up to 800ms!) by input from area 6, where planning occurs
  • activates both α and γ motor units
  • frequency of firing determines strength of contraction (recruitment of cells)

in responding to an unexpected limb perturbation

  • Area 4 neurons also contribute to the stretch reflex
  • activates two responses: spinal monosynaptic stretch reflex (involuntary) and cortical long-loop response (voluntarily controled by cerebellum)
  • This would be considered an unexpected limb perturbation response

Using an every day example of a movement, describe the function of

the supplementary cortex

  • aka Supplementary Motor Area (SMA)
  • always contralateral
  • is activated when somebody plans or imagines a sequence of events
  • e.g., touching thumb to each finger in turn

the parietal association cortex

  • three functions:
    1. compares position of body to object (e.g., when reaching for something)
    2. compares different modalities (e.g., somatosensory vs. visual) for information about body
    3. allows for selection/attention of one object from among many

Compare the differences and similarities in the functions of the superior colliculus and frontal eye fields

  • The superior colliculus (SC) generates involuntary saccades to zoom to novel or flashing objects (aka visual grasp reflex), whereas the Frontal eye field (FEF) generates voluntary saccades as selected by attention or memory
  • Both SC and FEF:
    1. are activated by objects to the left
    2. project to the left PPRF
    3. left PPRF generates saccade to the left

Give an example of a deficit you might see in a patient with a lesion in the

Primary Motor Cortex (area 4)

Premotor cortex (lateral area 6)

Supplementary motor cortex (medial area 6)

parietal association area

Somatosensory cortex