Somatosensory System

Anatomy and Physiology II

BIO 232

 

Function of Sensory Receptor

Transform natural stimuli into electrical signals which can be processed by the nervous system

Submodalities

      Touch-Pressure Sense

    Mechanical stimulation applied to the body surface

      Position Sense

    Via mechanical disturbances in muscle and joints

    Static limb position & kinesthesia

      Thermal Sense

      Pain Sense

Receptive Field

      That area on any receptive sheet which upon stimulation will either excite or inhibit the firing of the specific cell being monitored

Pacinian Corpuscle

      Mechanoreceptor

      Large encapsulated ending lying deep in the dermis of the skin

      Also found in mesentery

Somatosensory Pathways

      Dorsal Column-Medial lemniscal system

    Discriminative touch

    Vibratory sense

    Limb and joint position information

      Anterolateral System

    Pain

    Temperature

    Crude touch

Dorsal Column-Medial Lemniscal (DCML) System

 

 

 

 

Anterolateral System

 

Sensory Afferents

      Classified by fiber diameter & conduction velocity

      Type for muscle nerve:

    Aa - Large myelinated - fastest conduction

    Ab - Small myelinated

    Ad - Smaller myelinated

    C - Unmyelinated - slowest conduction

Fast Adapting Touch-Pressure Receptors

      Hair follicle receptor

      Meissner endings

    Glaborous skin (hairless)

    Encapsulated, in dermal papillae

    Each afferent fiber innervates several Meissner endings

      Pacinian corpuscle - dermis

Slowly Adapting Touch-Pressure Receptors

      Free nerve endings - hairy & hairless skin - C fibers

     Tickle & light or superficial touch

      Merkels Discs

     in both hairy and hairless skin

     Also in Touch domes

      Ruffini endings

     Hairy and some in hairless skin

     Responds to lateral stretch of the skin and steady pressure

Temperature Receptors

      Separate Warm and Cold receptors

      Free nerve endings

      Warm receptors - C fibers

    Respond to increasing temp

      Cold receptors - Ad fibers

    Respond to decreasing temperature

    3 - 4 times more cold receptors

Nocioceptors

      Free nerve endings

      C fibers - dull - burning pain

    Slow pain

    Multimodal receptors - chemical, mechanical & thermal stimuli

      Ad fibers - sharp pricking & stinging pain

    Fast pain

Central Processing of Somatosensory Information

      Movement of a brush across the skin

     FA receptors - greater increases in firing rate when the brush is moved faster than do slowly adapting receptors

    These phasic neurons encode information about he velocity of the stimulus

     Other receptors respond better to movement in one direction than to movement in the opposite direction

      Thus - somatosensory neurons can determine stimulus speed & direction

Discrimination of Detail by the Skin

 

 

 

 

 

Cortical Representations of Somatosensory Information

      Modern electrophysiological studies show that the body surface is mapped upon the cerebral cortex.

      There is in fact repeated representations in each of the two somatic sensory receiving areas of the cortex ( SI & S11 )

      That is there are multiple maps - Broadmans areas 3a, 3b, 1&2 . Each have a fairly complete map.

Human Cortex

      Wilder Penfield (1950 ) - Mapped the Human Cortex

Human Somatosensory Cortical Map

      Important Paradoxes arise when we consider carefully the sensory map.

    1. Resolution of the map is poor - yet we have remarkably precise tactile sensitivity.

    2. All submodalities seem to map at the same place - yet we can clearly distinguish superficial touch from deep sensation of touch & position sense.

      Thus something must be missing from the map.

What is Missing from the Map?

      1. Central cells - exhibit ongoing activity

      2. Each central cell has a well defined receptive field

      3. Different cells have different receptive fields - center-surround organization

    The size of which corresponds to the distortion of the body surface in the somatotopic map.

Receptive Fields of Central Neurons

 

 

 

 

Sub-modality Mapping & Cortex

      The cortex has a highly specific columnar organization - Broadmans areas 3a, 3b, 1 & 2 receive predominant projections from different receptors in the skin.

      Area 3a - receives predominant input from the stretch receptors of muscle.

      Area 3b - receives input from the superficial skin from both slowly adapting & rapidly adapting receptors

Sub-modality Mapping & Cortex

      Area 1 - receives superficial input primarily from rapidly adapting skin receptors

      Area 2 - receives input from receptors in deep tissue that signal joint position & deep pressure

Cortical Somatosensory Maps