Exteroception Flashcards
Describe the sensory receptors of the skin that detect vibration
The fast-adapting Pacinian corpuscle type afferents will follow vibratory stimuli (at several 100 Hz) that move the skin by only 1 micron. Pacinian corpuscles have large receptive fields, lie deep in the dermis and subcutaneous tissues, and are rapidly adapting. Rapidly adapting means that they can measure frequency, so they are better for vibratory touch.
Describe the sensory receptors of the skin that sense fine touch
Meissner’s corpuscles are thought to correspond
to the rapidly-adapting afferents with small receptive fields, while the afferents innervating Merkel’s discs have small fields and are slowly-adapting. The small receptive field afferents must support the fine tactile sense of the fingertips, and innervation density studies support this proposal. The density of skin innervation by Meissner’s corpuscles and Merkel’s discs is approximately 100 units per cm2 in the very fingertip and 3 to 4-fold lower in the palm. This innervation density gradient is thought to underlie the high spatial acuity of touch in the fingertips. Slowly adapting is better for steady touch.
What is understood by receptive field?
That area of the skin in which a mechanical stimulus elicits a response from the cell. Meissner’s corpuscles have a small receptive field compared to Pacinian
How are sensory receptors of skin classified in
terms of receptive field?
Those with small fields (2-8 mm diameter) with sharp
borders, and those with large (covering an entire finger or the greater part of the palm), poorly
defined fields
Describe the sensory receptors of the skin that sense stretch
Ruffini’s endings have large receptive fields, lie deep in the dermis and subcutaneous tissues, and are slowly adapting. Ruffini’s endings provide information on how the skin is stretched. This information is important in evaluating one’s grip on an object, for example. Shear, or slip between skin and object, or between skin and deeper tissues results in stretch of the Ruffini’s endings.
Describe the flow of information (anatomical pathway) along the medial lemniscal and trigeminal lemniscal system
In the dorsal column nuclei in the medulla a synapse is made on second order cells whose axons then cross the midline to form the medial lemniscus; this fiber tract confers its name to the system. These second-order fibers are joined in the midbrain by second-order fibers crossing from the trigeminal nuclei. The medial lemniscus fibers ascend to synapse in the ventro-basal complex of the thalamus. The trunk and limbs are represented by cells in the ventral-posteriorlateral (VPL) nucleus and the head is represented in the ventral-posterior-medial (VPM) nucleus. Together these two, VPM and VPL, form the ventrobasal complex. Cells in the ventrobasal complex project to areas 3,1, and 2 on the posterior bank of the central sulcus (Primary Somatosensory Area, SI). From here cells send projections to primary motor cortex, to
Secondary Somatosensory Area (SII), and to nearby “association” somatosensory cortical areas in parietal cortex.
Define somatotopy and describe why some regions (e.g. mouth and hands) are enlarged compared to others in the somatotopic map in primary somatosensory cortex.
Neighboring cells within the lemniscal system would have receptive fields near to one another within the skin.
Areas of the body that are richly innervated by primary
afferents have relatively larger areas of cortical surface devoted to them. Thus, the homunculus has enlarged hands and face, especially lips, and relatively small trunk and proximal limbs.
Describe cortical barrels or columns
As one records the stimulus preference of successive
cells along a perpendicular electrode track through the cortex, one finds cells of similar modality, as well as having nearly identical receptive field location. I there appear to be vertical stacks, or columns of cells that are functionally related to one another, like the barrels that represent individual vibrissae in the rodent cortex. These columns are 0.1-0.3 mm in diameter.
The layers of the brain correspond with where message are sent.
Discuss the stimulation cells in different Brodmann’s areas of primary somatosensory cortex respond to
Brodmann area 4 is motor. Brodmnn area 3a is proprioception. Brodmann area 3b is touch. Brodmann areas 1 and 2 are for information processing and communication between motor and sensory cortex.
Discuss what is understood by receptive field and describe how the sensory receptors of skin are classified in terms of receptive field.
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Describe the parallel somatotopic maps in different Brodmann’s areas in somatosensory cortex
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