Sensorimotor Reflexes

Peripheral Sensorimotor Reflex Pathways

• Three main pathways in the spinal cord mediated by different sensory inputs:

  • Muscle Tone/Length Reflexes: Involves muscle spindles.

  • Muscle Contraction Force Reflex: Involves Golgi tendon organ (GTO).

  • Flexion Reflex: Triggered by pain receptors.

Key Structures of Reflexes

• Muscle Spindles:

  • Contain αδ-fibers that detect muscle stretch.

• Golgi Tendon Organs (GTO):

  • Located in the tendon, provide information about force of muscle contraction.

Reflex Pathway Mechanisms

Muscle Spindles and GTO Interaction

• Dorsal Root Ganglion:

  • Contains sensory neurons; relays signals to the spinal cord.

• Ventral Horn:

  • Houses motor neurons and interneurons (excitatory or inhibitory).

• Sensory Neurons:

  • Project to both interneurons and lower motor neurons for muscle activation and feedback.

Pain Reflex Pathway

• Dorsal Root Ganglion to Ventral Horn:

  • Pain sensory afferents only project to interneurons, not directly to motor neurons, influencing muscle contraction indirectly.

Proprioceptors

• Types:

  • Golgi Tendon Organ: Signals muscle tension.

  • Muscle Spindle: Signals muscle length.

• Function: Proprioceptors relay sensory input about muscle state to the spinal cord, enabling reflex actions.

Muscle Spindles vs Golgi Tendon Organs

• Action Potential Firing:

  • Both respond during passive stretch.

  • Muscle Contraction:

    • Spindles: AP firing decreases.

    • GTOs: AP firing increases.

• Significance: Firing rate corresponds to muscle length (spindles) and contraction force (GTO).

Reciprocal Innervation

• Mechanism:

  • Sensory afferents diverge upon reaching the spinal cord:

    • One path stimulates same muscle (contraction).

    • Another path stimulates inhibitory interneurons affecting antagonistic muscle (relaxation).

• Outcome: Coordinated movement through contraction of one muscle and relaxation of its antagonist.

Stretch Reflex Example

• Biceps and Triceps:

  • When holding an object (like a soda), added load triggers:

    1. Stretch of bicep muscle spindles.

    2. Increased spindle AP rate activated, signaling spinal cord.

    3. Bicep motor neurons activated.

    4. Triceps motor neurons inhibited (relaxation).

Golgi Tendon Organ Reflex

• Function:

  • Protects muscles by triggering relaxation when excessive tension is detected.

• Mechanism:

  • Inputs activate local neurons that inhibit motor neurons of the same muscle, leading to muscle relaxation.

Descending Pathways

• Cortex to Spinal Cord:

  • Involvement of upper motor neurons from various brain regions (e.g., motor cortex).

• Trajectories:

  • Lateral Corticospinal Tracts (90%): Major route for voluntary movement control.

  • Ventral Corticospinal Tracts (10%): Involved in posture and reflexive control.

Spinal Cord Organization

Medial to Lateral Maps

• a-motor Neurons:

  • Organized from medial (proximal muscles) to lateral (distal muscles) in the ventral horn.

Longitudinal Organization

• Motor Neuron Groups:

  • Neurons for specific muscles, like the bicep, spread over several spinal segments, leading to cervical and lumbar enlargements for arms/hands and legs/feet coordination.

Local Circuit Neurons

• Interactions:

  1. Medial local circuits coordinate broad movements (bilateral).

  2. Lateral circuits coordinate fine movements (unilateral).

Primary Motor Cortex

• Location: Precentral gyrus; responsible for movement initiation.

• Features:

  • Contains a somatotopic map of the body; areas needing fine control are more represented (e.g., hands, face).

Betz Cells

• Characteristics:

  • Large upper motor neurons in Layer 5 of the motor cortex with long axons projecting to spinal interneurons and lower motor neurons.

The Penfield Maps

• Significance:

  • Developed from electrical stimulation studies demonstrating precise localization of muscle contraction areas.

Organization of Motor Cortex

• Microstimulation:

  • Can invoke organized behaviors from multiple muscles, suggesting interconnected circuitry in the cortex for eliciting specific movements.

Purposeful Movements Research

• Study Insights:

  • Stimulation of the primary motor cortex results in complex, coordinated movements across joints/muscles, emphasizing brain's organization of purposeful actions.

Supplementary Motor Area (SMA)

• Role:

  • Involved in planning and organization of complex movements, responds to sensory input and contributes to movement intention.

Motor Cortex Participation in Movement

• Neural Activity Tracking:

  • Studies show motor cortex involvement in both planning and execution phases of movement, adjusting for force and duration based on task demands.

Blood Flow Studies in Motor Tasks

• Increased activity in motor and premotor areas correlates with task complexity, highlighting system dynamics in skilled movements.

Plasticity in Motor Cortex

• Functional reorganization observed in response to learning and injury; motor maps adjust accordingly.

Population Coding of Movement Direction

• Neurons in motor cortex exhibit collective firing patterns that predict movement direction, aiding in the development of neuroprosthetics.