Somesthesis

Question 1

slowly adapting

Answer 1

-respond to enduring stimulus

Question 2

rapidly adapting

Answer 2

-respond only at onset and sometimes termination

Question 3

mechanoreceptors

Answer 3

• touch, pressure, vibration

- two major groups

Question 4

meissners

Answer 4

• encapsulated
• excited a large myelinated sensory nerve fiber
• adapt rapidly after they’ve been stimulated

Question 5

merkel receptors

Answer 5

-transmit an initially strong but partially adapting signal and then a continuing weaker signal that adapts only slowly

Question 6

ruffini’s

Answer 6

• located in deeper laters of the skin and tissues

- multi branched, encapsulated nerve endings that adapt very little

Question 7

pacinian corpuscles

Answer 7

• lie both immediately beneath the skin and also deep in the facial tissues of the body
• free nerve ending inside
• stimulate by very rapid movement of the tissues because they adapt in a few thousandths of a second
• based on unique lamellar accessory structure
• respond with AP for each phase of the stimulus

Question 8

mechanoreceptors-hair

Answer 8

• slight movement of any hair on the body stimulates another type of receptor that is made up of a nerve fiber entwining the base of the hair
• although each receptor has certain unique properties, the sensory experiences of touch, pressure, and vibration are quite different from the tactile sensations evoked by natural stimuli

Question 9

two point discrimination

Answer 9

• relationship between receptor density and receptive field size
• receptive field is portion of the skin directly innervated by the receptor terminals and the area of adjacent tissue through which stimulus can be conducted
• areas of skin that are most sensitive and have the greatest capacity for fine spatial discrimination have a high density of receptors
• these receptors have the smallest receptive fields and these areas of skin have the largest number of receptive fields per unit area
• fingers vs back

Question 10

resolving spatial temporal features

Answer 10

1. merkels, meissners, pacinian on a monkey
2. receptive field of a monkeys finger was stimulated on embossed dot array on a rotating drum:
   - response of the cell under study was examined
   - each AP represented by dot
   - they each had different abilities to tease out sensation

Question 11

major ascending pathways

Answer 11

• carry somatosensory information to the cerebral cortex- dorsolateral and anterolatera
  1. dorsal column-lemniscal mediates tactile sensations including vibration and proprioception
  2. anterolateral system carries information chiefly about pain and temp
• tactile in anterolateral is only crude touch and pressure, poor localizing, and little capability for fine intensity discrimination

Question 12

dorsolateral system

Answer 12

• mechanoreceptor axons lie in clusters in dorsal root ganglia
• once in the spinal cord they pass medially into the lateral margin of the dorsal column

Question 13

laminar organization

Answer 13

• medial branch turns upward in the dorsal columns and continues by way of the dorsal column pathway to the brain
• the lateral branch divides many times to give off multiple terminations in the cord
• participate in the mediation of local spinal activity
• very organized all the way up

Question 14

somatotopic organization of the dorsal columns

Answer 14

• segregation begins at the spinal cord
• afferent fibers of different receptors take specific routes to central projection
• within dorsal columns, modality segregated axons are also arranged in a highly ordered manner
• sacral axons near midline, then more lateral as you go up

Question 15

central ascending pathway

Answer 15

• nerve fibers pass up dorsal columns on ipsilateral side until caudal medulla
• synapse in cuneate and gracile nuclei (dorsal column nuclei)
• decussate to other side in the internal arcuate fibers
• then in medial lemniscus (also somatotropically organized) to VP nucleus of thalamus (lower to VPL, face to VPM)

Question 16

second order neuron receptive fields

Answer 16

• bigger circles around a cluster of smaller circles
• also have inhibitory neurons from outside excitatory circle
• receptive fields become larger because they receive convergent input from many primary neurons, each with a slightly different but overlapping receptive field
• they become more complex because unlike simple receptive fields, they have inhibitory

Question 17

lateral inhibition

Answer 17

• 1 point in the middle-punctate- that is turned down a little while lateral neurons are completely inhibited. primary unit influences its own secondary and nearest neighbors
• two point-two receptive fields stimulated separated by a 3rd inhibited one

Question 18

thalamic nuclei and projections

Answer 18

• medial lemniscus synapses on VP nucleus
• VP nucleus divided into medial and lateral division with tactile inputs from the trunk and limbs terminating in cells of the lateral division and input from the face projecting to the medial division
• VPL projects to medial side of cortex and VPM to lateral (homunculus- feet to face medial to lateral)
• post central gyrus

Question 19

representation of body in thalamus

Answer 19

• based on electrophysiological recordings, topographical separation for monkey
• sensory information about a particular modality from one part is processed by a collection of neurons that form discrete functional unit in the thalamus
• axons of medial lemniscus that subserve a particular modality from a restricted body part form a bundle as they enter the thalamus
• place represented mediolaterally and modality is anteroposteriorly

Question 20

cortical sites of somatosensory processing

Answer 20

• four distinct cytoarchitectonic regions
  3a
  -3b
  -1
  -2
  -most of the thalamic fibers terminate in areas 3a and 3b- which then project to 1 and 2

Question 21

monkey vs humans

Answer 21

• monkey was found leg medial to face latera
• similar representations were found in human cortex by wilder penfield during operations for epilepsy and other brain lesions
• penfield found that stimulation of the post central gyrus produced tactile sensations in particular parts of the opposite side of the body

Question 22

sensory homunculus

Answer 22

• some areas small some big
• proportional to number of specialized receptors in each respective peripheral area of the body
• figured out how to localize

Question 23

jacksonian seizure

Answer 23

• starts in hand goes up and goes down leg
• going medially on the post central gyrus
• more localization

Question 24

multiple body maps in SI

Answer 24

• four independent and fairly complete maps in SI one for each area 3a,b, 1,2
• 3b and 1 of a monkey each have their own cutaneous representation
• adjoining, parallel and correspond to each other in medial to lateral representation

Question 25

segregation of peripheral inputs to SI based on modality

Answer 25

1. within any one of the architectonic areas, input from another submodality tends to dominate
   - 3a respond to deep tissue, 3b to activation of rapidly or slowly adapting cutaneous receptors, 2 to deep pressure, 1 to rapidly adapting cutaneous receptors
2. processing in SI- the larger processing units of SI are place or topography mediolaterally and modality anteroposteriorly
   - thus, regions of SI are modality specific and columns within the regions are also modality specific
   - within any one area, on the microscopic level, there are several interrelated maps of place and modality

Question 26

direction sensitive neurons

Answer 26

• in most posterior portion, there are vertical columns of cells that respond only when a stimulus moves across the skin in a particular direction- 1 and 2
• other simpler cells are only motion sensitive, responding to stimulation in any direction, there are more complex cells, responding when stimulus is in an orientation
• other cortical neurons respond only when large numbers or receptors turned on simultaneously, edges of the object should be oriented in a direction- form, motion, texture
• processing of sensory information becomes even more complex the farther posteriorly in the posterior parietal cortex- somatic association areas

Question 27

fingers

Answer 27

• increased cortical territory and receptive field sized based on practice
• plasticity