Skilled Performance and Motor Learning

Introduction to Sensory Systems

Understanding how sensory systems contribute to skilled movement is essential in anatomy and physiology. Sensory processing and integration allow the nervous system to monitor and control body position, movement, and coordination.

• Key Sensory Systems: Visual, Vestibular, Somatosensory

• Role: Provide information for motor control, error correction, and movement planning

Visual System

Function of the Visual System

The visual system provides critical information about the environment and the body’s position within it. It is divided into exteroceptive and proprioceptive functions.

• Exteroceptive Sense:

  • Identifies objects in space

  • Determines movement of objects

• Visual Proprioception:

  • Monitors body position in space

  • Assesses relation of body segments

  • Detects motion of the body

Higher Order Visual Cortex

After initial processing in the visual cortex, information is distributed via two main pathways, each specialized for different aspects of visual processing.

• Dorsal Stream ("Where"):

  • Processes spatial visual cues

  • Projects to posterior parietal region

  • Concerned with action and spatial awareness

• Ventral Stream ("What"):

  • Handles pattern discrimination and recognition

  • Projects to temporal cortex

  • Concerned with perception and recognition of objects

Example: The dorsal stream helps guide reaching movements, while the ventral stream helps identify objects being reached for.

Vestibular System

Function of the Vestibular System

The vestibular system is essential for balance, spatial orientation, and coordination of head, neck, and eye movements.

• Balance Control: Vestibulo-spinal reflexes maintain posture

• Head and Neck Positioning: Vestibulo-colic reflexes adjust head position

• Eye Movement Control: Vestibulo-ocular reflex stabilizes gaze during head movement

• Main Structures: Semicircular canals (detect angular acceleration), Otolith organs (detect linear acceleration)

• Neural Pathway: Signals carried via vestibulo-cochlear nerve (CN VIII) to ipsilateral vestibular nuclei

Vestibular Nuclei Outputs

• From brainstem to cortex (via thalamus)

• Integrated with visual and somatosensory information for perception of head motion and orientation

• Reciprocal projections with cerebellum for balance, eye control, and movement coordination

Somatosensory System

Proprioception and Kinesthesia

Proprioception is the sense of limb position and movement, while kinesthesia refers specifically to the sense of movement.

• Receptors: Detect changes in muscle length, tension, and joint position

• First-order Sensory Neurons: Transmit information from peripheral receptors to the spinal cord

• Transduction: Conversion of mechanical stimuli into electrical signals

Dorsal Column-Medial Lemniscus System

This pathway transmits fine touch, vibration, and proprioceptive information to the brain.

• First-order (primary) neurons:

  • Bifurcate: Ascending axons in dorsal column (ipsilateral)

  • From periphery (receptor) to ipsilateral brainstem (medulla)

• Second-order (secondary) neurons:

  • Medial lemniscus system

  • From brainstem (medulla) to contralateral thalamus

• Third-order (tertiary) neurons:

  • Thalamus to ipsilateral somatosensory cortex

Spinothalamic (Anterolateral) Pathway

This pathway transmits pain, temperature, and crude touch information.

• Comprised of:

  • Spinothalamic (thalamus)

  • Spinoreticular (pons)

  • Spinomesencephalic (midbrain)

• Primary Sensory Neuron: Synapse in ipsilateral dorsal horn

• Second-Order Sensory Neuron:

  • Dorsal horn → cross midline (over 2-3 segments) → thalamus

  • Ascend in anterolateral white matter

• Third-order neurons: Thalamus to ipsilateral somatosensory cortex

Somatosensory Cortex

The somatosensory cortex is the site of complex sensory processing and integration, essential for coordinated movement.

• Somatotopic Map (Homunculus): Neighboring neurons represent neighboring body parts

• Integration: Processes input from contralateral side of the body

• Multimodal Association Cortex: Parietal cortex integrates sensory information from visual, proprioceptive, vestibular, and other systems for movement planning

Sensory Contributions to Motor Control

Role of Sensory Input in Movement

Sensory input is used by the nervous system to control and refine movement. It helps set parameters, trigger commands, and correct errors.

• Timing: Visual input is used after ~200 ms; somatosensory input is used sooner

• Natural Movements: Both visual and somatosensory inputs are used for error correction in movements longer than 200 ms

• Task-Specific Use:

  • Somatosensory input for fine, controlled movements (e.g., handwriting, piano playing)

  • Visual input for movements to targets in extrapersonal space

  • Weighted combination of sensory information when both are available

Sensory Organization Test

This test assesses how different sensory systems contribute to balance by varying the quality of sensory information.

• Six conditions: Normal, unreliable, or absent visual and/or somatosensory inputs

• Instability increases from condition 1 to 6

Somatosensory vs. Vestibular Contributions

Different sensory inputs are used for different components of postural responses.

• Somatosensory Information: Triggers the response (timing)

• Vestibular Information: Scales the response (amplitude)

Summary Table: Sensory Pathways and Functions

Key Equations and Concepts

• Neural Transmission: Sensory information is encoded as electrical signals in neurons.

• Somatotopic Organization: The "homunculus" represents the mapping of body parts in the somatosensory cortex.

• Weighted Sensory Integration: For movement correction, the nervous system uses a weighted sum of available sensory inputs: where and are the weights assigned to visual and somatosensory inputs, respectively.

Summary

• Movement requires monitoring the position of body segments using multiple sensory systems.

• Visual information is processed via dorsal and ventral streams for action and recognition.

• Somatosensory information travels through spinal cord and higher brain centers for reflexes and integration.

• Vestibular information is essential for balance and coordination.

• Sensory integration is crucial for skilled movement, and the use of different inputs varies by task and timing.