exteroception

Question 1

two divisions of somatosensory system and what types of informaton they carry

Answer 1

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

Question 2

Rapidly adapting vs slowly adapting sensory receptors

Answer 2

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.

Question 3

What is a receptive field

Answer 3

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.

Question 4

List 5 mechanoreceptors of skin

Answer 4

free nerve ending, Merkels disk, meissners corpuscle, pacinian corpuscle, hair follicle receptor

Question 5

Discuss the receptive field size and adaptation for meissners corpusscles, pacinian corpuscle, merkels disk and ruffinis/ free nerve endings

Answer 5

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.

Question 6

Sizes of receptive fields

Answer 6

small fields have sharp borders(2-8mm diameter), large fields have porrly defined fields (cover an entire finger or greater part of palm)

Question 7

Location of large receptive field receptors? Small receptive field?

Answer 7

Large are deep in the dermis and subcutaneous tissues. Small are closer to the surface

Question 8

function of small vs large receptive field afferents

Answer 8

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.

Question 9

Describe sensory detection in fast vs slow adapting receptors

Answer 9

fast- detect vibration. Slow detect steady state touch

Question 10

What do hair follicle receptors detect

Answer 10

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.

Question 11

What information do mechanoreceptors carry? Proprioceptors

Answer 11

mechanoreceptors: touch, pressure, vibration, hair bending. Proprioceptors: body position

Question 12

What type of axons do mechanoreceptors, proprioceptors have

Answer 12

Mechanoreceptors: large, myelinated axons (Aβ). Proprioceptors: large, myelinated axons (Aα)

Question 13

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

Answer 13

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

Question 14

fasciculus gracilis vs cuneatus

Answer 14

Afferents from the hind limbs travel in the fasciculus gracilis, while those from fore limbs run in the fasciculus cuneatus

Question 15

What is the ventrobasal complex

Answer 15

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

Question 16

What are nearest neighbor relationships

Answer 16

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
somatosensory system.

Question 17

What is somatotopy? Why some regions (e.g. mouth and hands) are enlarged compared to others in the somatotopic map in primary somatosensory cortex?

Answer 17

Somatotopy: map of body sensory locations on the cortex. Some regions are enlarged because they have a higher innervation density

Question 18

What are cortical barrels or columns?

Answer 18

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.

Question 19

Describe columnar organization of cortex

Answer 19

There are vertical arrangement of functionally related cells, barrels. Within each column, modality stays the same and cells have nearly identical receptive field location.

Question 20

How does columnar organization relate to the layers of the cortex

Answer 20

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.

Question 21

Discuss parallel somatotopic maps in different Brodmann’s areas in somatosensory cortex

Answer 21

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

Question 22

What kind of stimulation do cells in different Brodmann’s areas of primary somatosensory cortex respond to?

Answer 22

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

Question 23

describe hierarchical organization in cortex

Answer 23

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.

Question 24

What information does the trigeminal nerve carry

Answer 24

Afferent information regarding touch, proprioception and pain and temperature on the face and head flow through this nerve, plus motor info.

Question 25

Three branches of trigeminal nerve. What are located in the trigeminal ganglion?

Answer 25

ophthalmic, maxillary and mandibular. Soma of mechanoreceptors of skin are located in the ganglion

Question 26

Principal nucleus (main sensory nucleus)

Answer 26

afferents from trigeminal ganglion sub serving fine touch, vibration and proprioception synapse here.

Question 27

Describe trigeminal/lemniscal pathway

Answer 27

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

Question 28

Describe trigeminal proprioceptive afferents

Answer 28

These afferents have centrally located cell bodies (the only ones in adult CNS) that make up the trigeminal mesencephalic nucleus. These are cell bodies for muscle spindles in the muscles of mastication and some
mechanoreceptors in the mouth. These cells make monosynaptic connections with motorneurons
to the muscles of mastication in the motor trigeminal nucleus, analogous to stretch receptors in spinal cord