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