Exteroception Flashcards
Somatosensory system definition
- collection of receptors whose ending transduce information about the body surface, the skin, some deeper tissues
Somatotopy definition
- precise and orderly mapping of body surface onto the cortex
Types of mechanoreceptors
- rapidly-adapting
- slowly-adapting
Rapidly adapting receptors
- stimulated with steady touch ==> fire a few AP ==> stop firing even though skin is still being pressed
Slowly-adapting receptors characteristics
- stimulated w/steady touch
- keep firing throughout the time skin is pressed
Receptive field definition
- area of skin in which a mechanical stimulus elicits a response from the cell
- small field receptors w/sharp borders
- large field receptors w/poorly-defined borders
- tend to lie in deeper tissues (dermis, subcutaneous)
Meissner’s corpuscles (adaptation, receptive field)
- rapidly adapting
- small receptive field
Merkel’s discs (adaptation, receptive field)
- slow-adapting
- small receptive field
Pacinian corpuscles (adaptation, receptive field)
- rapidly-adapting
- large receptive fields
Ruffini’s ending (adaptation, receptive field)
- slowly-adapting
- large receptive field
Small vs. Large Receptive fields jobs
- small receptive fields = fine tactile sens of fingertips
- high density gradient ==> high spatial acuity of touch @ fingertips
- large receptive fields = skin deformation over a wide area
Mechanoreceptors axon types
- A(beta)
- =large, myelinated axons
- conduct @ ~35-75 m/sec
Skeletal muscle proprioceptors axon types
- A(alpha)
- condution @ 80-120 m/sec
Location of cell bodies & central processes of mechanoreceptors
- cell bodies @ dorsal root ganglia
- central processes ==> dorsal root ==> spinal cord
- central processes bifurcate ==>
- local branches to dorsal horn in spinal reflexes
- ascending brances entering the dorsal (posterior) columns
- lower limbs = gracile fasciculus
- upper limbs = cuneate fasciculus
Course of sensory info from mechanoreceptors ==> cerebral cortex
- mechanoreceptors ==> DRG ==> spinal cord
- bifurcate ==> dorsal horn (spinal reflex) & ==> dorsal column
- axons @ dorsal column synapse @ nuclei @ medulla ==> cross midline = “medial lemniscus”
- medial lemniscus fibers ==> ventral-posterior-lateral (VPL) nucleus @ thalamus
- head = @ VPM nucleus
- VPM + VPL = ventro-basal complex ==> areas 3, 1, 2 @ primary somatosensory area
- projections ==> primary motor, secondary somatosensory area, and association somatosensory areas
“nearest neighbor” concept + characteristics of somatotopy
- neighboring cells @ cuneate nucleus or thalamus have receptive fields near to one another within the skin
- ==> somatotopic map/homunculus on cortical surface
- topographic arragement/homunculus is distorted due to different density of afferents @ different regions
- e.g. large lips and hands + small proximal limbs/trunk
- topographic arragement/homunculus is distorted due to different density of afferents @ different regions
Columnar organization @ somatosensory system characteristics
- = vertical arrangement of fxnly related cells
- @ somatosensory columnar organization = vertical segregation of cells by response modality
- cortical columns = computational modules that transform info from thalamus and redistribute it
Sensory modalities @ Broadmann’s area 3a, 3b, 1, 2
- 3a = muscle stretch receptors
- 3b = cutaneous receptors
- 1 &2 = respond to complex stimuli
- direction and orientation-sensitive
- some @ 2 = shape-sensitive
- 3a/3b project onto 1 & 2 to allow for complex stimulus processing and clues for fine motor
Lemniscal pathway/trigeminal nerve characteristics
- lemniscal system for head involves trigeminal nerve = ~DRG for the head
- trigeminal carries motor + touch + proprioception + pain + temp for face and head
- mechanoreceptor soma = @ trigeminal ganglion
- central processes terminate in different nuclei
- main sensory nucleus = principal nucleus ==> second order cells send axons across midline to join medial lemniscus ==> VPM
