Somatosensory Systems

Somatosensory System learning Goals

A. Transduction in Mechanoreceptors and Proprioceptors

B. More about Receptive Fields

C. Central Nervous System Pathways

D. Somatosensory Cortex

E. Plasticity of Somatosensory Cortex

Somatosensory System Overview

The somatic senses incorporate the perception of sensations from skin, joints, and internal organs. There are three main types of receptors:

1. Exteroception: Sensitivity to stimuli interacting with one’s body. Examples include cutaneous sensations such as touch, temperature, and pain.

2. Proprioception: The ability to sense position, location, and orientation of parts of one’s own body, primarily detected by receptors in skeletal muscles, tendons, and joints.

3. Interoception: These are signals originating from internal organs.

Together, these are referred to as the “general senses”.

• Trunk: Sensations are carried by dorsal root ganglia and spinal cord.

• Face and Head: Sensory information is carried by cranial nerves.

Connections from Periphery to Spinal Cord

Somatosensory afferents are responsible for conveying information from the skin surface to central circuits. Important components include:

• Dorsal Root Ganglia: Contains neuronal cell bodies innervating skin, muscles, joints, and organs in the trunk, characterized as:

  • Pseudounipolar

  • Afferent

  • Efferent

The peripheral processes of the nerve fibers involved in touch are typically encapsulated by specialized end organs, while pain and temperature sensations are conveyed through free nerve endings.

Spinal Cord Structure

Information carried by dorsal root ganglia enters the dorsal spinal cord in various anatomical regions:

1. Cervical

2. Thoracic

3. Lumbar

4. Sacral

5. Coccygeal (not shown)

Afferent Axonal Types

Somatosensory Afferents Table

Mechanotransduction in Mechanosensitive Fibers

Mechanoreceptors are cells that respond directly to mechanical stimulus (for example, touch). The following mechanisms contribute to transduction:

• Cellular Response: When the mechanoreceptor's nerve endings are subjected to stimuli, they respond by stretching the plasma membrane. If the stimulus is sufficient, it causes ion channels in the membrane to open. Consequently, ions enter the cell, leading to depolarization.

• Receptor Potential: Also known as generator potential, this is the depolarizing potential associated with stimulation of a sensory nerve. It functions as a graded potential, varying according to stimulus strength. A sufficiently strong stimulus can generate an action potential that occurs at the first myelinated region of the nerve.

Receptive Fields

Definition of Receptive Field

• Receptive field: It is defined as the part of the sensory field capable of eliciting neuronal responses when stimulated. The size and frequency of these fields can vary significantly across different sensory neurons.

Two-Point Discrimination Threshold

• Two-point discrimination tests measure the minimum distance by which two stimuli can be separated and still be perceived as distinct. This threshold varies with the density of innervation in different body regions:

  • Areas with higher density of receptive fields exhibit a lower two-point discrimination threshold.

  • Conversely, areas with lower density show a higher threshold.

• In general, if receptive fields are fewer and further apart, two points may be perceived as one stimulus.

Adaptation in Receptors

Adaptation refers to the decline of a receptor's electrical response over time, despite the continued presence of the stimulus. Two types of mechanoreceptors demonstrate distinct adaptive properties:

• Slowly Adapting Mechanoreceptors: Provide continuous information about a stimulus, indicating its prolonged presence.

• Rapidly Adapting Mechanoreceptors: Respond primarily to changes in stimulus, such as the onset or offset of a touch.

Types of Glaborous Skin Receptors

1. Free nerve endings: No end organs; possess small receptive fields.

2. Meissner corpuscle: Rapid adaptation; detects motion.

3. Merkel cell: Slow adaptation; sensitive to shape and texture.

4. Ruffini corpuscle: Slow adaptation; responsive to skin stretch.

5. Pacinian corpuscle: Rapid adaptation; responds to vibration.

Functional Implications of End Organs

The configuration of end organs alters interactions with nerve endings, consequently affecting signal transmission to the central nervous system.

Proprioception Details

Types of Muscles and Innervation

1. Skeletal Muscles: Consist of thousands of striated fibers, classified as:

   • Extrafusal fibers: Contractile fibers that enable muscle contraction.

   • Intrafusal fibers: Non-contractile fibers that detect stretch, which are associated with sensory receptors and play a role in proprioception.

2. Motor Units: Composed of a single motor neuron and the muscle fibers it innervates.

   • Intrafusal fibers are wrapped with sensory endings and are innervated by sensory (Ia, II) and motor (γ) fibers to maintain stretch-sensitive properties.

Golgi tendon Organs

• These organs collaborate with muscle spindles:

  • Muscle spindles signal changes in muscle length, while Golgi tendon organs monitor muscle tension.

  • Tendon organs are typically located at tendon junctures and assist in movement regulation by inhibiting contracting motoneurons and activating antagonists during muscle contraction.

Pathways for Mechanotransduction

1. Dorsal Column/Medial Lemniscal Pathway:

   • Mechanosensitive fibers enter the dorsal spinal cord and bifurcate, with one branch synapsing onto motor neurons and interneurons, while the other travels along the dorsal columns toward the medulla (gracile for lower body; cuneate for upper body).

   • In the medulla, signals decussate and proceed to the thalamus then to the primary somatosensory cortex (S1).

2. Proprioception Pathway:

   • Sensory info enters the dorsal spinal cord and bifurcates: one branch synapses on motor neurons/interneurons, while another travels to Clarke’s nucleus. The pathway ascends through the dorsal spinocerebellar tract to the cerebellum where proprioceptive data integrates with other sensory information.

Representation in the Somatosensory Cortex

• The Ventral Posterolateral (VPL) and Ventral Posteromedial (VPM) nuclei relay signals to the Primary Somatosensory Cortex (S1), which corresponds with the regions of the body and face.

• S1 is structured into Brodmann’s Areas 1, 2, 3a, and 3b, sending information to SII and other areas contributing to sensory processing.

Topographical Representation and Homunculus

• The organization within S1 demonstrates a point-to-point representation of the sensory organ (somatotopic), which is not uniform; regions with denser innervation occupy more cortical space, creating a homunculus map.

Functional Modules and Connectivity

• The sensory cortices and thalamus operate in modules, where sensory inputs from VPL and VPM are distributed to various S1 areas, with functional connectivity linking SII and higher cortical areas for advanced processing.

• Plasticity is evident, as seen in cortical adaptations in response to finger use and when digits are amputated, leading to reorganization of the cortical maps that represent the hand.

Implications of Somatosensory Plasticity

• Changes in the cortical representation showcase the brain’s adaptability to loss and functional restructuring based on usage patterns, indicating potential for recovery and rehabilitation strategies following injury or disability.

Conclusion

Understanding the intricacies of the somatosensory system, from receptor type and pathway to cortical representation and plasticity, is essential for both theoretical perspectives in neuroscience and practical applications in clinical settings.