Neurophysiology: Somatosensory System

Neurophysiology: Somatosensory System

• The somatosensory system is responsible for our sense of body, both self and in relation to the world around us.
• It's a beautifully organized and conserved neurocircuit, making it highly suitable for study in humans.
• The word "soma" means body in Greek.

Sensory Experience

• Sensory experience is shaped by receptor systems encoding stimuli as electrical signals.
• Receptors respond to broad ranges of stimuli from different modalities.
• Receptor fields overlap, enabling a wide variety of sensory perceptions based on the various stimulus combinations.

Somatosensation Functions

• Interoception: Sense of one's organs, useful for monitoring function and potential damage. Mostly unconscious.
• Proprioception: Sense of one's body in time and space, both static and moving.
• Exteroception: Relationship between the body and the external world, eg., the warmth of the sun.

Sensory Receptors

• Sensory modalities are defined by the receptors encoding stimuli.
• Four major receptor types: photoreceptors, chemoreceptors, thermoreceptors, mechanoreceptors.
• Somatosensation utilizes all but photoreceptors.

Somatosensory Receptor Transmission

• Sensory receptors encode various stimuli as electrical signals, e.g., pressure changes.
• These signals are transmitted along spinal cord, brain stem, and CNS, generating thoughts and behaviors.

Types of Somatosensory Receptors

• Receptors are characterized by submodality, notably high-frequency vibration, and fiber groups.
• Mechanoreceptors are diverse, with "sheathed" and "unsheathed" types. Depth from surface is a major factor.
• Thermoreceptors and nociceptors exhibit exposed nerve endings and are unsheathed.

Mechanoreceptors: Ion Channels

• Mechanoreceptors are ion channels, crystolographically resolved.
• Each mechanoreceptor has 6 subunits forming a barrel-like structure, opening ion pores in response to mechanical deformations.
• Receptors are highly selective for the type of stimulus.

Types of Mechanoreceptors

• Opened through plasma membrane tension: E.g., cellular swelling.
• Opened by proteins linking to the ion channel: E.g., hair cells in the inner ear.
• Opened by secondary messengers: Activated by mechanical forces, e.g., pain receptors.
• Three types of mechanoreceptors differ by speed of mechanisms: fast, medium, and slow.

Touch Mechanoreceptors

• Touch mechanoreceptors vary based on morphology, location, receptive field, conduction velocity, and adaptation.
• Four types: Meissner corpuscles, Merkel cells, Pacinian corpuscles, and Ruffini endings.

Proprioceptors

• Mechanoreceptors that detect muscle movement.
• Found on muscle spindles.
• Wrap around muscles and open/close efficiently during stretching.
• Respond to stretch velocity and adapt to constant stretch and rate of change of stretch.

Nociceptors

• Pain receptors necessary for detecting current and past damage, often with intensity-dependent firing rates that adapt to constant force.
• Activated by mechanical, thermal, and chemical signals.
• Two classes: Αδ fibers and C fibers.

Thermoreceptors

• Ion channels opening in response to temperature (cold, cool, warm, and hot) sensations.
• Also respond to certain chemicals, explaining why some foods feel hot or cold.
• Highly sensitive but slower responding than mechanoreceptors.
• They can have overlapping or mixed responses.

Peripheral Nerves

• Bundles of sensory axons from multiple modalities.
• Classified by axon diameter and myelin amount surrounding each nerve for different conduction velocities.
• Bundling axons significantly enhances conduction velocity.
• Classified by Charles Sherrington into myelinated and unmyelinated types (Aα, Aβ, Aδ, and C).

Different Conduction Demands

• Skin, with a vast surface area, requires many nerve endings and thus less myelin and no A alpha fibers.
• Muscles, capable of rapid changes, demand more myelin and fewer nerve endings.

Compound Action Potentials

• Space demands limit the amount of myelination a nerve can have.
• Thus, to enhance fine discrimination, nerve endings must be exposed and nerve responses to stimuli are only so fast, depending on distance.
• If nerves are close, their action potentials can sum.
• Larger axons fire sooner and are more sensitive to the stimuli than smaller ones.

Spinal Nerves

• Peripheral nerves combine into spinal nerves to enhance signal transduction.
• 31 pairs of spinal nerves (two per side).
• Named according to the vertebral segments they enter the spinal cord (e.g., lumbar 4, thoracic 6).
• Their function depends on the organs they innervate.

Dermatomes

• Overlapping receptive fields bundled into a single spinal nerve.
• Large receptive fields mean less spatial resolution.
• Thus, spinal nerves detect large tissue areas.
• Clinically useful for locating spinal cord damage.

Dorsal Root Ganglion (DRG)

• Spinal nerves separate into sensory and motor fibers at the DRG.
• The DRG is a swelling containing multiple cell bodies outside the spinal column.
• Fibers further separate into modality-specific fibers before entering the spinal cord.

Separate into submodalities in dorsal horn

• Specific anatomical regions in dorsal horn receive specific modality-related sensory information.
• Three main tracts for distinct types of sensory information:
  • Dorsomedial columns: fine touch, proprioception, vibration.
  • Lateral spinothalamic tract: pain and temperature.
  • Anterior spinothalamic tract: light touch.

Local vs Distal Terminals

• Neurons terminate locally and distally.
• Local branches mediate pain, temperature detection through anterolateral systems, and involuntary movement (spinal reflexes) via A-delta and C fibers (anterolateral pathway).
• Distal branches support voluntary movement and complex involuntary pathways (e.g., dorsal column medial lemniscal pathway).

Stretch Reflex

• Responsible for simple involuntary movement.
• Proprioceptive sensory neurons from muscle spindles innervate excitatory motor neurons and inhibitory interneurons in the spinal cord.
• Excitation of extensor motor neurons and inhibition of flexor motor neurons cause a reflex, e.g., kicking.

Ascending Pathways

• Modality-specific fibers project to the midbrain through two tracts.
  • Dorsal Column Medial Lemniscal System: conveys proprioceptive and tactile sensations, synapses/decussates in medulla, fast Aα/δ fibers.
  • Anterolateral System: conveys thermal and painful sensations, generally slow C fibers.

Decussations

• Dorsal column medial lemniscal system decussates in medulla.
• Anterolateral system decussates in spinal cord.
• Decussation (crossing over) explains how one side of the body is represented by the opposite side of the brain.
• Somatic twist hypothesis: organs move to more protected side of the body.

Additional information

• Fiber addition in ascending pathways occurs progressively from outer to inner areas of the brain stem.
• Fibers differentiate and organize in the brain stem and thalamus to create a crude map of the body (somatotopically ordered).
• Full somatotopic maps are fully segregated in the cortex.

Conclusions

• Somatosensation informs the brain about body location and timing through highly sensitive receptors.
• Organized into peripheral nerves, then spinal nerves, and finally the brain for somatotopic mapping of body parts.