Flashcards in exteroception Deck (28):
two divisions of somatosensory system and what types of informaton they carry
dorsal column (posterior column) -lemniscal system detct fine, or discriminative, touch perception, and limb proprioception (perception of limb position). Antero-lateral system carriesg the sensations of pain and temperature and crude touch information
Rapidly adapting vs slowly adapting sensory receptors
Rapidly adapting receptors when stimulated with steady touch fire a few (in many cases only one) action potentials and then stop firing even though the skin is still being pressed. Slowly adapting receptors stimulated with steady touch will keep firing throughout the time the skin is pressed.
What is a receptive field
Area of the skin in which a mechanical stimulus elicits a
response from the cell.Area of the skin in which a mechanical stimulus elicits a
response from the cell.
List 5 mechanoreceptors of skin
free nerve ending, Merkels disk, meissners corpuscle, pacinian corpuscle, hair follicle receptor
Discuss the receptive field size and adaptation for meissners corpusscles, pacinian corpuscle, merkels disk and ruffinis/ free nerve endings
Meissners corpuscle: small receptive field, fast adaptation. Pacinian corpuscle: large receptive field, fast adaptation. Merkels disk: small receptiv field, slow adaptation. Ruffini's ending: large receptive field, slow adaptation.
Sizes of receptive fields
small fields have sharp borders(2-8mm diameter), large fields have porrly defined fields (cover an entire finger or greater part of palm)
Location of large receptive field receptors? Small receptive field?
Large are deep in the dermis and subcutaneous tissues. Small are closer to the surface
function of small vs large receptive field afferents
Meissners corpuscles and merkels discs (small receptive field afferents) must support the fine tactile sense of the fingertips. Pacinian corpuscles (large receptive field, fast) dtect vibratory stimuli. Ruffinis endings (large receptive field, slow) detect skin stretch.
Describe sensory detection in fast vs slow adapting receptors
fast- detect vibration. Slow detect steady state touch
What do hair follicle receptors detect
The dermal tissue around each hair follicle is penetrated by several myelinated axons, and these produce several unmyelinated branches. Some branches run up and down the hair follicle, others spiral around it. Bending of
the hair shaft activates the terminals, and they are rapidly-adapting.The dermal tissue around each hair follicle is penetrated by several myelinated axons, and these produce several unmyelinated branches. Some branches run up and down the hair follicle, others spiral around it. Bending of
the hair shaft activates the terminals, and they are rapidly-adapting.
What information do mechanoreceptors carry? Proprioceptors
mechanoreceptors: touch, pressure, vibration, hair bending. Proprioceptors: body position
What type of axons do mechanoreceptors, proprioceptors have
Mechanoreceptors: large, myelinated axons (Aβ). Proprioceptors: large, myelinated axons (Aα)
Understand the flow of information (anatomical pathway) along the medial lemniscal and trigeminal lemniscal system. Where is synaptic transmission in this pathway? Where do the axons cross the midline
Mechanoceptors/proprioceptors in skin/muscles > cell body in dorsal root ganglion with processes projecting in dorsal root and entering spinal cord > processes bifurcate (local branches project to dorsal horn for spinal reflexes, ascending branches enter dorsal columns > synapse on second order neurons in nucleus gracilis or cuneatus in medulla > second order axons cross midline as the medial lemniscus in the medulla > second order neurons from trigeminal nucleus join these fibers in midbrain > medial lemniscus fibers synapse in ventro-basal complex of thalamus > project to areas 3,1, and 2 on posterior bank of the central sulcus (Primary Somatosensory Area, SI) > cells project to primary motor cortex, secondary somatosnsory aree and association somatosensory cortical areas in parietal cortex
fasciculus gracilis vs cuneatus
Afferents from the hind limbs travel in the fasciculus gracilis, while those from fore limbs run in the fasciculus cuneatus
What is the ventrobasal complex
In 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. The VPL and VPM make up the ventrobasal complex
What are nearest neighbor relationships
Neighboring cells in the nucleus cuneatus, or within the thalamus, have receptive fields near to one another within the skin. This preservation of nearest neighbor relationships means that a coherent, contiguous map of the body surface can be found throughout all levels of the
somatosensory system.Neighboring cells in the nucleus cuneatus, or within the thalamus, have receptive fields near to one another within the skin. This preservation of nearest neighbor relationships means that a coherent, contiguous map of the body surface can be found throughout all levels of the
What is somatotopy? Why some regions (e.g. mouth and hands) are enlarged compared to others in the somatotopic map in primary somatosensory cortex?
Somatotopy: map of body sensory locations on the cortex. Some regions are enlarged because they have a higher innervation density
What are cortical barrels or columns?
Morphological specialization of cortex that reflects functional organization. circular arrangements of cells that run throughout the cortical depth which are projections of mechanosensitive afferents from rodent whiskers (vibrissa). Each vibrissa has one barrel, and all the cells in that barrel repond to movement of that whisker. If the whisker is removed early in life, one barrel dissapears from the cortex.
Describe columnar organization of cortex
There are vertical arrangement of functionally related cells, barrels. Within each column, modality stays the same and cells have nearly identical receptive field location.
How does columnar organization relate to the layers of the cortex
Layer IV receives input from the thalamus, layer VI projects back to the thalamus, layer V projects to other subcortical structures, while layers II and III project to other areas of somatosensory cortex. These interconnections imply that cortical columns serve as computational modules that transform information received from the thalamus and redistribute it to other regions of the brain.
Discuss parallel somatotopic maps in different Brodmann’s areas in somatosensory cortex
There are in fact multiple body maps within primary somatosensory cortex, for each sensory modality. Ie. Brodmanns area 3a for muscle stretch receptors and 3b for cutaneous receptors have parallel maps which are continuous and aligned, but separate for each modality. There are also maps in somatosensory area II and in somatosensory association areas. Multiple maps arise because the thalamus projects more widely than just the primary somatosensory cortex, and b/c hierarchical organization of cortex
What kind of stimulation do cells in different Brodmann’s areas of primary somatosensory cortex respond to?
3a: muscle stretch/proprioception. 3b: cutaneous receptors/ exteroception. 1: complex stimuli-direction and orientation. 2: complex stimuli: direction, orientation, shape, convergent input from 3a and 3b. 5 and 7: integrate several sensory modalities often related to movement, resulting in our perception of extrapersonal space
describe hierarchical organization in cortex
Removal of primary somatosensory cortex (SI) drastically
reduced the activity of cells in secondary somatosensory cortex (SII), but the converse surgery had no affect on SI. This implies that much of the information driving SII originates in SI and so supports the notion of serial
processing of information.
What information does the trigeminal nerve carry
Afferent information regarding touch, proprioception and pain and temperature on the face and head flow through this nerve, plus motor info.
Three branches of trigeminal nerve. What are located in the trigeminal ganglion?
ophthalmic, maxillary and mandibular. Soma of mechanoreceptors of skin are located in the ganglion
Principal nucleus (main sensory nucleus)
afferents from trigeminal ganglion sub serving fine touch, vibration and proprioception synapse here.
Describe trigeminal/lemniscal pathway
sensory receptors > trigeminal ganglion (cell body) > afferents synapse at main sensory nucleus (principal nucleus) in pons or branch into descending spinal tract/nucleus for reflexes > second order cells send axons across midline to join medial lemniscus (small fraction send axons ipsilaterally too) > Medial lemniscus fibers end in ventral posterior medial nucleus of thalamus > projects to face area of somatosensory cortex