somatic sensory system Flashcards
(27 cards)
what are the 2 major input components
-Mechanical stimuli (light touch, vibration, pressure and cutaneous tension)
-Painful stimuli and temperature
what does input and interpretation of these two components enable us to do
-To identify the shape and textures of objects
-To monitor the internal and external forces acting on the body
-To detect potentially harmful circumstances
-to have a sense of ourselves within our environment and so plan our actions accordingly
what are Meissner (or Tactile) Corpuscles
-Location: dermal papillae of skin, esp. palms, eyelids, lips, tongue etc.)
-Modality: Light touch, texture (movement)
-Sensitive to 30-50Hz
-Rapidly adapting
-Encapsulated Nerve Endings
what are Pacinian (or Lamellated) Corpuscles
-Location: dermis, joint capsules, viscera
-Modality: Deep pressure, stretch, tickle, vibration
-Sensitive to 250-350Hz
-Rapidly adapting
-Encapsulated Nerve Endings
what is Ruffini Corpuscles
-Location: dermis, subcutaneous tissue, joint capsules
-Modality: Heavy touch, pressure, skin stretch, joint movements. A kind of proprioceptor?
-Slowly adapting
what is Merkel (or Tactile) Discs
-Location: superficial skin (epidermis)
-Modality: Light touch, texture, edges, shapes
-Slowly adapting
-Unencapsulated Nerve Endings
what are Free nerve endings
-Location: widespread in epithelia and connective tissues
-Modality: Pain, heat, cold
-Unencapsulated Nerve Endings
what are Hair follicles
-Location: widespread in epithelia
-Modality: Varied according to type
-Both rapid and slowly adapting subtypes
what a cellular receptor
-a ‘device’ made of cells that detects changes in the body or the environment
whats a molecular receptor
-a molecule, usually located on the surface of a cell with a transmembrane linkage to the cytoplasm, that detects changes in molecular environment
why can Sensory receptors not always be neurons
-other sensory receptors that are a specialist cell type – for example photoreceptors, auditory & vestibular hair cells and taste receptors.
-In these cases, the specialist cell type synapses with a primary afferent neuron to relay the sensory information to the CNS
what are the Two different classes of mechanoreceptor response
-Rapidly adapting, or phasic receptors give information about changes in the stimulus – e.g. Pacinian Corpuscles- fires rapidly
-Slowly adapting or tonic receptors, continue to respond as long as stimulus is present (gives info about persistence of stimulus) – e.g. Ruffini Corpuscles
what are Primary afferent axon subtypes
-Classified according to conduction velocity, which broadly reflects diameter (faster = larger diameter)
-Axons coming from the skin are designated by letters (A, B, C; A = fastest/largest; C = slowest/smallest).
-The‘A’ group further broken down by Greek character (alpha, beta, delta; alpha = fastest; delta = slowest)
-Axons coming from the muscles designated by Roman numerals:
I, II, III & IV; I = largest; IV = smallest)
‘I’ group also further broken down (Ia, Ib etc ‘a’ faster, ‘b’ slower)
-pain fibres are slower than proprioceptors
how is sensory information organised
-layers in the dorsal horn of the spinal cord dorsal horn
-The cell bodies of different classes of sensory neuron are grouped in the DRG and their projections are organised to different layers of the dorsal horn
-also applies to different sensory modalities
-Sensory information remains spatially organised as it is carried into the brain by different pathways
what are the 2 main routes for sensory subtypes going to the brain
-The Medial Lemniscal tracts carry mechanoreceptive and proprioceptive signals to the thalamus
-The Spinothalamic tract carries pain and temperature signals to the thalamus
what are the 3 neurons that sensory information travels through to reach higher centres
-First-order neurons detect the stimulus and transmit to spinal cord
-Second-order neurons relay the signal to the thalamus, the “gateway” to the cortex
-Third-order carry the signal from the thalamus to the cortex
-2nd order axons cross the midline
how are the Axons of the medial lemniscal pathway organised
-topologically
whats the dorsal column nuclei
-1st order axons from the upper body follow the lateral (red) pathway and synapse on 2nd order neurons in the cuneate nucleus
-1st order axons from the lower body (below vertebra T6) follow the more medial (purple) pathway and synapse on neurons in the gracile nucleus
what do 2nd order axons cross
-midline and ascend in the medial lemniscus
-In doing so, their topology is reversed, relative to the midline, so that lower body axons are more lateral on reaching the thalamus
-Finally, 3rd order axons again reverse the topology so that lower body axons synapse on more medial cortical neurons, whereas upper body axons ‘map’ to the lateral cortex
what are dermatomes
-Each sensory ganglion innervates a specific region of skin called a dermatome
-These regions arise because the dermis of each region is derived from a specific embryonic structure called the somite
-Somites are iterated structures that give rise to the underlying musculature and skeleton
-In the embryo, each sensory ganglion (DRG) is associated with a specific somite and subsequently innervates the tissues arising from that somite
-This topological organisation is preserved in the spinal cord and the somatosensory projection
what does topographical mean
-relating to or representing the physical distribution of parts or features on the surface of or within an organ or organism
what does topological mean
-the way in which constituent parts are interrelated or arranged
A fine topographic map
-the area occupied by the different regions in the cortex is not proportional to their physical size
-there must also be a map in the thalamus, the ‘waystation’ between the 2nd and 3rd order neurons, and in the dorsal column nuclei
what are receptive fields
-Each sensory neuron has a receptive field
-The size of a receptive field can be measured by assessing the ability to discriminate two sharp points set apart at different distances
-If the subject feels two pin points then the distance between the points is larger than the receptive field
-The size of the receptive field for any particular neuron will vary depending on where it is in the body, some regions having denser innervation than others
