4.4 Somatosensory system and proprioception

The somatosensory system and proprioception are crucial for our ability to feel and interact with the world. They involve various receptors in our skin, muscles, and joints that detect touch, pressure, temperature, pain, and body position.

These sensory inputs are processed in the brain, allowing us to perceive our environment and control our movements. Understanding this system helps us grasp how we navigate and respond to the world around us.

Cutaneous Receptors

Types of Cutaneous Receptors

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• Touch receptors detect mechanical pressure, vibration, and texture
  • Includes Meissner's corpuscles (detect light touch), Pacinian corpuscles (detect deep pressure and vibration), Merkel's discs (detect sustained pressure), and Ruffini endings (detect skin stretch)
• Pain receptors, called nociceptors, respond to potentially damaging stimuli
  • Includes mechanical nociceptors (respond to intense pressure), thermal nociceptors (respond to extreme temperatures), and polymodal nociceptors (respond to multiple types of noxious stimuli)
• Temperature receptors detect changes in skin temperature
  • Includes cold receptors (respond to decreases in temperature) and warm receptors (respond to increases in temperature)
• Dermatomes are areas of skin innervated by a single spinal nerve
  • Allows for mapping of sensory input to specific regions of the spinal cord and brain (somatotopic organization)

Transduction and Transmission of Cutaneous Stimuli

• Cutaneous receptors transduce mechanical, thermal, or chemical stimuli into electrical signals (receptor potentials)
• Receptor potentials trigger action potentials in sensory neurons, which transmit the information to the central nervous system
• Sensory neurons from cutaneous receptors have cell bodies in the dorsal root ganglia and synapse in the spinal cord or brainstem
• Axons of second-order neurons decussate (cross to the opposite side) and ascend to the thalamus via the spinothalamic tract (pain and temperature) or the dorsal column-medial lemniscal pathway (touch and pressure)

Proprioceptors

Types of Proprioceptors

• Proprioceptors are sensory receptors that detect the position, movement, and tension of muscles, tendons, and joints
• Muscle spindles are proprioceptors embedded within skeletal muscle fibers
  • Detect changes in muscle length and rate of change, providing information about muscle stretch and contraction velocity
  • Consist of intrafusal muscle fibers (sensory) and extrafusal muscle fibers (contractile)
• Golgi tendon organs are proprioceptors located at the junction between muscles and tendons
  • Detect changes in muscle tension during contraction, providing information about muscle force
  • Respond to excessive muscle tension, triggering the inverse myotatic reflex to prevent injury

Proprioceptive Feedback and Motor Control

• Proprioceptive information is essential for maintaining posture, balance, and coordinated movement
• Proprioceptive feedback is integrated with other sensory inputs (vision, vestibular) and motor commands in the cerebellum and motor cortex
• Proprioceptive reflexes, such as the stretch reflex and inverse myotatic reflex, help maintain muscle length and tension
  • Stretch reflex (knee-jerk reflex) is a monosynaptic reflex that causes muscle contraction in response to stretch, maintaining posture and resisting gravity
  • Inverse myotatic reflex is a polysynaptic reflex that causes muscle relaxation in response to excessive tension, preventing injury

Somatosensory Processing

Somatosensory Cortex

• The somatosensory cortex, located in the parietal lobe, is the primary cortical area for processing touch, pressure, pain, temperature, and proprioception
• Organized somatotopically, with different body parts represented in proportion to their innervation density (sensory homunculus)
  • Highly sensitive areas (hands, face) have larger cortical representations than less sensitive areas (back, legs)
• Consists of the primary somatosensory cortex (S1) and the secondary somatosensory cortex (S2)
  • S1 receives thalamic input and is involved in discriminative touch, proprioception, and initial processing of pain and temperature
  • S2 integrates information from S1 and other cortical areas, contributing to tactile learning, memory, and object recognition

Higher-Order Processing and Perception

• Somatosensory information is further processed in association areas, such as the posterior parietal cortex and insular cortex
• These areas integrate somatosensory input with other sensory modalities (vision, audition) and cognitive processes (attention, memory, emotion)
• Higher-order processing allows for the perception of complex tactile features, such as texture, shape, and size
• Somatosensory perception is modulated by top-down influences, such as attention, expectation, and past experience
  • Phantom limb sensation is an example of how central processes can influence somatosensory perception in the absence of peripheral input