Primary Motor Areas: PMA & SMA

• Primary motor areas include the primary motor cortex, premotor area (PMA), and supplementary motor area (SMA).
• Key readings include the Sanes & Donoghue (2000) article, Lundy-Ekman (6th ed) chapters 14 & 27, and SC, 6th ed., Ch. 3 (only the Action Systems section).

Course Objectives

• Analyze the peripheral and central nervous system components involved in movement execution, directional tuning, and force production.
• Explain the motor control role of different motor neurons.
• Apply knowledge of peripheral and central nervous system structures in neuromuscular control to patient care.
• Predict physiological and functional losses from damage to the peripheral and central nervous system.

Session Objectives

• Describe contributions of primary motor cortex, Premotor Area (PMA), & Supplementary Motor Area (SMA) to movement.
• Define major descending tract systems.

Key Concepts to Remember

• Distributed control
• Need for fractionation facilitates the ability to move body parts independently and involves inhibition & facilitation i.e. co-contraction
• Goal-directed movement

Cerebral Cortex Areas

• The premotor area is the lateral portion of the premotor cortex.
• The supplementary motor area is the medial portion of the premotor cortex.

Motor Systems

• Lower motor neurons (LMN) can be found in LE Chapter 13
• Upper motor neurons (UMN) synapse on lower motor neurons.
• LMN axons synapse directly on muscle fibers but UMN cell bodies are in the cortex.
• LMN cell bodies are in the spinal cord (SC).
• UMN axons can project to LMN/IN in SC or brainstem LMN.
• LMNs are alpha and gamma types.
• Alpha: cell bodies in ventral horn SC, extrafusal muscle, large cell bodies, myelinated axons.
• Gamma: ventral horn SC, intrafusal, smaller

Primary Motor Cortex (M1)

• Most neurons are in the corticospinal and corticobrainstem tracts (corticobulbar or corticonuclear).
• As the primary source of the corticospinal tract, the motor cortex controls voluntary, contralateral movement.
• M1 has a unique role in precise hand and face movement.
• There is a homunculus in the motor cortex with a large proportion devoted to the hand and face.
• Input to M1 comes from anatomical connections PMA, SMA, Primary SSC, BG, and Cbm via the thalamus, and functional connections with the prefrontal cortex.
• Output from M1 goes to the spinal cord, brainstem cranial nerve nuclei (CNs III, IV, V, VI, VII, XI, XII), Cbm via pons, BG, red nucleus, reticular formation nuclei, and vestibular nuclei.

Cytoarchitectonics

• These structures are NOT on the exam
• The cortex contains 6 layers
• Layer 1 is the Molecular layer containing fibers and dendrites.
• Layer 2 is the External granular layer consisting of small pyramidal & stellate cells.
• Layer 3 is the External pyramidal cell layer consisting of also small pyramidal cells
• Layer 4 is the Internal granular with stellate cells
• Layer 5 is the Internal pyramidal with large pyramidal cells called Betz.
• Layer 6 is a Multiform layer with spindle-shaped cells

Additional Notes on Architectonics

• Columns are functionally related
• Input & output layers -Superficial Layers (I – III or 1-3)
  • Input
  • Output: project to other cortical areas for intracortical communication -Layer V (Layer 5): Pyramidal layer -Major output layer consisting of axons of pyramidal cells that create the corticospinal tract (20%), also gives output to subcortical structures and other cortical areas (80%).

Primary Motor Cortex (M1)

• Layer 5 axons contribute to the corticospinal tract.
• 40% of CST axons originate in M1; the rest come from PMA/SMA, and SSC.
• Output travels via the internal capsule to midbrain (cerebral peduncle), then to the pons to medulla, crosses at decussation (80-90%), goes to the lateral column of spinal cord, lateral ventral horn, and synapses on LMNs for distal extremity limb movements.
• Corticomotoneurons (CM) are monosynaptic projections from layer 5 to spinal motor neurons and Ia inhibitory interneurons, important for fractionation.
• Axons also terminate on Interneurons (INs) which are part of the circuitry of stepping pattern generators.

Sanes & Donoghue (2000) Findings

• M1 controls contralateral voluntary movement.
• It contains functional subregions (legs, arms, head).
• Coding includes direction, 'actions', and 'synergies'.
• Output of M1 is influenced by force requirements.

Directional Tuning

• Motor cortical neurons are broadly tuned to movement direction, firing preferentially in certain directions.
• Multiple neurons firing together produce a direction of movement.
• Movement relies on multiple neurons firing based on direction and force.

Additional M1 Results

• Whether an individual corticomotoneuronal (CM) cell is active depends on the motor task.
• Maps within M1 show plasticity and rapid reorganization with amputation and SCI capable of modifying the maps.
• UMNs in M1 with axons forming the corticospinal tract have rich horizontal connections within their subregion: long term potentiation or LTP.
• Neurons can be influenced by thought and instruction playing some role in cognition.
• MI neurons change firing rate with imagery and mental rotation.
• The motor cortex and LCST axons are critical for fractionated movement.
• The motor cortex codes for direction and force (not by individual muscles).

Premotor (PMA) vs. Supplementary (SMA) Areas

• Apraxia is a neurological disorder characterized by the loss of carrying out skilled movements and gestures, despite desire/physical ability.
• It typically involves deficits with UMN lesion
• Use of PMA vs. SMA in motor planning depends on the task.
• Both project to M1 for final planning stages and sending out signals for motor execution.

Upper Motor Neuron (UMN) Tracts

• LCST (pyramidal) originates in the cortex: M1, SMA, PMA, SSC.
• The LCST controls contralateral movement, fractionation of movement, and hand movement, and enables fast, goal-directed actions.

Motor Tracts

• Medial (anterior) corticospinal originates in the cortex (M1, SMA, PMA).
• It controls the neck, shoulder, trunk, and APAs, and decussates at spinal levels to affect muscles bilaterally.
• Reticulospinal originates in the pons (reticular formation).
• It controls postural muscles and gross limb movements.
• It is also involved with APAs and orients the body to sound/moving objects
• It decussates at spinal levels to affect muscles bilaterally.
• Medial/lateral vestibulospinal originate in the medulla and pons (vestibular nuclei).
• The medial vestibulospinal project bilaterally to influence neck/upper back muscles.
• The lateral vestibulospinal tract influences extensors and limb extensors (anti-gravity) ipsilaterally.
• APA refers to anticipatory postural adjustments and prepares the body for movement.

Distributed Control: Reach to Grasp

• Phases
• Transport Phase: moving arm towards the cup, acceleration
• Contact Phase: preparing to grasp, deceleration
• Hand shaping: aperture widens during transport; then narrows preparing to grasp
• Premotor (Planning)
• Posterior Parietal Cortex (object location/shape)
• Somatosensory Cortex (body)
• Primary Motor Cortex (execution)
• Grasp
• Postural muscles
• The LCST is critical for goal-directed, fractionated limb movement; no other tract controls precise hand movements.
• There are four main tracts for postural control/gross limb movement.
• When doing an upright task, all of these tracts are activated to some degree.

Explore the primary motor cortex, premotor area (PMA), and supplementary motor area (SMA). Understand their contributions to movement and the descending tract systems. Review key concepts like distributed control and fractionation for precise body movement.