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Sensorimotor System Overview

Major Brain Regions Involved

• Sensorimotor and Premotor Cortex

• Dorsolateral Prefrontal and Lateral Orbitofrontal Cortex

• Limbic and Paralimbic Cortex, Hippocampus and Amygdala

Class Activity

• Circuits Drawing

  • Involved areas:

    • Thalamus

    • Cortex (Cn)

    • Basal Ganglia structures: SUBTHALAMIC NUCLEUS (STN), GLOBUS PALLIDUS INTERNA (GPi), GLOBUS PALLIDUS EXTERNA (GPe)

  • Types of Circuits to Draw:

    • (a) Motor Circuit

    • (b) Associative Circuit

    • (c) Limbic Circuit

Sensory Pathways

Pathways to Draw

• Cutaneous Mechanosensation

  • For lower limbs

  • For head/face sensation

Lower Motor Neurons and Motor Control

Anatomical Regions

• Cervical

• Thoracic

• Sacral

• Lower motor neuron circuit

Dorsal and Ventral Spinal Components

• Dorsal:

  • Somatosensory functions

  • Components:

    • Horn

    • Root

    • Column

  • Sensory Neuron:

    • Located in Dorsal Root Ganglion

    • Receives input from sensory receptors

• Ventral:

  • Motor functions

  • Components:

    • Ventral root

    • Horn

    • Spinal nerve

    • Final pathway to ventral muscle (Ventro- or Antero-column)

Lower Motor Neurons (LMN)

• Function:

  • Exit spinal cord/brainstem and send projections to skeletal muscles

  • Known as the “final common path” for initiating movement

• Local Circuits:

  • Primary source of synaptic input to LMN (sensory and descending inputs)

  • Essential for coordinating muscle movement between different groups for proper function

Upper Motor Neurons (UMN)

• Location:

  • Cell bodies located in the brainstem and cerebral cortex

  • Project to local circuit neurons and sometimes LMN

• Functions:

  • Necessary for voluntary and complex spatiotemporal movements

  • Involved regions include:

    • Primary Motor Cortex

    • Eye movement

    • Speech production (Broca’s area)

    • Emotional processing (facial expressions)

    • Brainstem (muscle tone and sensory integration)

Cerebellum

• Functionality:

  • Indirect control of motor movement through regulation of UMN

  • Acts as a servomechanism (feedback control system to UMN)

  • Motor Error: The difference between intended and actual motor performance; the cerebellum mitigates motor errors and is critical for motor learning

Basal Ganglia

• Components:

  • Group of brain regions including the Striatum, Globus Pallidus, Ventral Pallidum, Substantia Nigra, and Subthalamic Nucleus

• Functions:

  • Balancing between preventing UMN from initiating unwanted movements and preparing circuits for desired movements

  • Essential for transitioning between different movement patterns

  • Plays a role in habit formation, implicit learning, and motivated behaviors

Basic Organization of Movement Control

• Motor system hierarchy includes:

  • Motor Cortex

  • Basal Ganglia

  • Brain Stem

  • Cerebellum

  • Spinal Cord

  • Muscles

• Interaction styles:

  • Serial and Parallel processing

  • Key regions interact including Cerebral Cortex, Basal Ganglia, and Thalamus

Descending Systems Overview

• Upper Motor Neurons:

  • Originate in the motor cortex, responsible for planning, initiating, and directing voluntary movements

• Brainstem Centers:

  • Control stereotyped movements, postural control, and adjustments in gain

• Basal Ganglia:

  • Facilitate the initiation of intended movements while suppressing unwanted ones

• Cerebellum:

  • Coordinates ongoing movement

• Spinal Cord® and Brainstem Circuits:

  • Involved in sensorimotor integration and central pattern generation

Somatotopic Organization of LMN

• Lower motor neurons innervate muscle fibers in individual muscles

• Motor Neuron Pools:

  • All LMN that innervate a specific muscle

  • Somatotopic Arrangement:

    • Muscles arranged along medial-lateral and rostral-caudal axes

    • Arms: Cervical section

    • Legs: Lumbar section

    • Medial area for axial muscles; lateral for distal muscles

• Regional Enlargements:

  • Cervical and lumbar enlargements to accommodate upper and lower limbs

Types of Lower Motor Neurons (LMN)

• α Motor Neurons:

  • Innervate extrafusal fibers of skeletal muscles responsible for posture and movement

• γ Motor Neurons:

  • Innervate intrafusal fibers, regulating proprioceptive input by maintaining muscle spindles length

• Specifications for LMN Types:

  • Type/Class/Size:

    • α: Aα, 13–20 µm, Yes (Myelinated), Extrafusal fibers

    • γ: Aγ, 5–8 µm, Yes (Myelinated), Intrafusal fibers

Motor Units

• Definition:

  • Composed of a single α motor neuron and the muscle fibers it innervates

  • All fibers in a muscle are innervated by a single α motor neuron

• Characteristics:

  • Ensures even contraction and decreases motor deficits in case of damage to one α motor neuron

  • Motor Unit Category:

    • Smaller motor units generate less force compared to larger ones

Motor Unit Composition

• Categories:

  • Slow (S) Units:

    • Contract slowly, low force, fatigue-resistant, high in myoglobin, mitochondria, and capillaries

    • Low activation threshold; tonically active during sustained effort (e.g., standing).

  • Fast Fatigable (FF) Units:

    • Generate high force, fatigue quickly, pale muscle fibers

    • Higher activation threshold for activities like running or jumping.

  • Fast Fatigue-Resistant (FR) Units:

    • Intermediate size, generate about 2x the force of S units, less than FF units

• Activation Profile:

  • Single Action Potential leads to forces throughout these motor unit types within the muscle.

Muscle Tension Regulation

• Size Principle:

  • Recruitment of motor neurons is based on size to create increasing muscle tension: S → FR → FF

• Action Potential Frequency:

  • Temporal summation occurs when successive muscle contractions happen before relaxation, leading to greater tension

• Fused Tetanus:

  • Achieved at the highest firing rate; the distinction between action potentials disappears

Motor Unit Plasticity

• Characteristics:

  • Muscles often contain a mixture of different motor unit types.

  • Increased exercise can lead to slower muscle contraction speed while increasing endurance and strength

  • Notable changes in motor unit plasticity occur in both the peripheral and central nervous system

Lower Motor Neuron Syndrome

• Definition:

  • Damage to LMN in the brainstem and spinal cord results in:

  • Paralysis (loss of movement)

  • Paresis (weakness)

  • Loss of reflexes (areflexia)

• Muscle Tone:

  • Dependent on reflex arcs connecting muscle spindles to LMN

• Additional Symptoms:

  • Fibrillations: Changes in excitability of a single denervated muscle fiber.

  • Fasciculations: Changes in excitability of a single denervated motor unit.

  • Long-term effect: Muscle atrophy due to lack of stimulation

Amyotrophic Lateral Sclerosis (ALS)

• Overview:

  • Also known as Lou Gehrig’s disease, is a neurodegenerative disorder affecting both LMN and UMN

• Statistics:

  • Prevalence of 0.05%; around 10% is familial.

  • Fatal with an average lifespan of ~5 years post-diagnosis

• Cognitive Functionality:

  • Typically remains intact despite physical degeneration

• Pathophysiology:

  • Excitotoxicity occurs when malfunction of EAAT2 leads to glutamate accumulation in synapses, activating receptors that lead to calcium-dependent pathways and free radical generation.

Local Circuits

• Function:

  • Major sources for synaptic input to LMN

  • Coordination of muscle movements necessary for proper function is critical

Reflexes Overview

• Definition:

  • Sensorimotor reflexes are automatic, unlearned responses to stimuli that do not require conscious effort. They are faster than responses requiring conscious evaluation.

Types of Spinal Reflexes

1. Stretch Reflex:

   • Triggered by passive muscle stretching.

   • Involves muscle spindles, a motor neuron, and coordinated movement of the agonist and antagonist muscles.

2. Golgi Tendon Reflex:

   • Responds to muscle tension through the Golgi tendon organ structure, enhancing muscle protection during high force levels.

3. Flexor Reflex:

   • Withdraws limb from pain through activation of ipsilateral excitatory interneurons and stabilization through contralateral responses.

Muscle Spindles

Composition:

• Nuclear Bag Fibers:

  • Types:

    • Dynamic (velocity-sensitive)

    • Static (length-sensitive)

• Nuclear Chain Fibers:

  • Static (length-sensitive)

• Governed By:

  • γ motor neurons which modulate response to stretch and regulate muscle tone in reaction to proprioceptive inputs.

Roles of γ Motor Neurons

• Activate & Modulate:

  • Increase firing during stretch and decrease during relaxation but do not fall silent

• Maintain Tension:

  • Adjust intrafusal muscle length to enhance proprioceptive input.

• Implications:

  • Increased γ activity leads to better correlation with feedback and muscle force generation.

Golgi Tendon Organ (GTO)

• Functionality:

  • Monitors muscle force through single Ib sensory afferent, attaches to extrafusal muscle fibers, generates feedback loops:

• Feedback Systems:

  • High Force Levels: Synapse onto GABA local circuit neurons inhibiting α motor neurons

  • Low Force Levels: Synapse onto excitatory local circuit neurons activating antagonistic α motor neurons

• Receive Inputs From:

  • UMNs, cutaneous receptors, muscle spindles, and joint receptors to maintain steady muscle tension and joint stability.

Differences Between Spindle Afferensts and Golgi Tendon Organs

• Muscle Spindles:

  • Discharge significantly during muscle elongation, less during contraction.

• Golgi Tendon Organs:

  • Increased firing during contraction, and monitor force generated by muscle activity.

Flexion Reflex Mechanism

• Activation Process:

  • Nociceptors stimulate dorsal horn neurons, causing limb withdrawal through activation of ipsilateral excitatory interneurons for flexor muscle contraction and inhibition of extensor muscles.

• Stabilization:

  • Contralateral response, activating the extensor muscle while inhibiting the flexor muscle.

Central Pattern Generators (CPGs)

• Definition:

  • Found within the spinal cord and brainstem; control programmed rhythmic motor behavior without sensory input (e.g., walking, swimming).

• Function:

  • Control timing and coordination of complex movement patterns while adjusting to changes.

CPG Regulatory Mechanisms

• Example in Quadruped Locomotion:

  • Involves stance (limb contact with ground) and swing phases (limb lifted).

CPGs and Independence from Cortical Processing

• Example of Independent Function:

  • Even when thoracic spinal cord is transected, hind legs retain coordinated movements, indicating independence from somatosensory input to CPGs.

• Bipedal Consideration:

  • More reliance on local circuits in UMN and cortical processing, possibly due to postural control requirements.

Review Questions

1. What are LMNs?

2. What is a motor unit?

3. How are motor units classified or categorized?

4. What is the recruitment order?

5. What constitutes a reflex?

6. What three reflexes were discussed?

7. Where are these reflexes located?

8. Explain Central Pattern Generators (CPGs).