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Neuro: 2:00 - 3:00
Monday, February 9, 2009
Dr. Weller
Somatosensory
Abbreviations: RF=Receptive Field; SA=Slowly Adapting; RA=Rapid Adapting
I.
Scribe: Taylor Nelson
Proof: Chaz Capra
Page 1 of 6
Key Points for Exam [S2]
a. Key points for the exam, meaning multiple choice questions will come from these topics
i. Somatosensory receptors
ii. Dorsal column/medial lemniscal pathway
iii. Traditional vs. current view of somatosensory areas
iv. General functions for some of these areas of cortex
v. Plasticity effects with deprivation,and the flip-side, increased use
II. Relevant Concept: Stimulus Location [S3]
a. One of the aspects of a stimulus in most of the sensory systems is knowing where it came from, location
b. Receptors and the neurons they are connected with have a receptive field, a particular area in the periphery
where application of a stimulus will cause the cell to respond
c. Cell body of a neuron that goes out into the periphery where it gets input – in this case from the skin, which is
the receptive field [S4]
d. For the somatosensory system that would be the portion of the skin innervated or contacted by the receptor or
accessory processes
e. The density of the receptors (how many you have per square inch) and the size of their receptive field
determines the special resolution or two-point touch discrimination
i. Two-point touch discrimination is the ability to distinguish individual points of contact when touched
simultaneously in two places
ii. [S5] Areas of the skin that have the smallest two-point touch discrimination or the greatest acuity for detail
in knowing where you’ve been touched have the greatest density of receptors (more receptors per square
inch) – humans are most sensitive on the bottom surfaces of the hand and parts of the face, and least
sensitive on back, calf, and thigh; meaning we have largest 2-point touch discrimination
iii. The receptors in areas with the greatest acuity for detail must have smaller receptive fields to fit them all in
III. Title [S6]
a. These receptive fields form topographic maps
b. For touch, theses maps are called somatotopic (somato meaning body) maps
i. Formed by orderly arrangement of receptive fields as they project into central parts of the somatosensory
system
c. These maps often have what are called congruent borders
d. They are not an exact replication of the skin surface because parts of the skin surface that we use to make fine
tactile discriminations are magnified or enlarged in the maps
e. [S7] These are two of the areas of the somatosensory cortex of an owl monkey
i. Owl monkeys are nice because they practically have no sulci, so it is easier to see things right on the
surface
ii. Two maps, areas 3b and 1 of the somatosensory cortex
iii. Roughly the skin of the opposite side of the body is represented from foot, leg, trunk, arm, hand, and face
iv. A congruent border is here, that means these maps often reverse at a common border
1. Reverse at the congruent border (congruent means shared)
2. The maps have a large area devoted to the hands and a large area for input from the face
3. Maps typically have a reversal, so when mapping the cortex you know you are going into another
area
f. [S8] Cartoon showing what animals would look like if the area devoted to cortex matched what was on the skin
surface – illustrates the concept of magnification
i. In the rabbit somatosensory areas the skin of the face is magnified or enlarged
ii. Cat – face, paws
iii. Monkey – face, hands
iv. Humans – hands, face, and especially intraoral surfaces and we think this relates to language
IV. 4 Perceptual Modalities [S10]
a. The somatosensory system actually encompasses four perceptual modalities, only two will be discussed
i. Tactile or cutaneous sense – how we commonly use the word touch, this is the sense by which you can
identify things when you run your hand over them, for discriminative touch – distinguishing size, shape, and
texture of an object
ii. Proprioception – refers to deeper receptors, these are receptors that are in joints and muscles and allow
you to know what your body position is when your eyes are closed and you are not moving (static) and
when you are moving, this information is very important in movement and maintaining posture; also used
with touch
Neuro: 2:00 - 3:00
Monday, February 9, 2009
Dr. Weller
iii. Will not discuss thermal or pain
Somatosensory
Scribe: Taylor Nelson
Proof: Chaz Capra
Page 2 of 6
V. Somatosensory Receptors [S11]
a. Mechanoreceptors
b. Receptors/receptor complex generate action potentials as well as doing the transduction
c. There is a key distinction between rapidly and slowly adapting kinds of receptors
i. These will be located in different parts of the skin and have different sizes of receptive fields
VI. Principal Types of Tactile or Cutaneous Receptors [S12]
a. Most are Encapsulated – covered with some peripheral tissue, typically some kind of epithelial tissue to form a
complex
i. Meissner corpuscle
ii. Pacinian corpuscle
iii. Ruffini ending
b. Nonencapsulated
i. Merkel ending
VII. Mechanoreceptor distribution in hairy and glabrous skin [S13]
a. Hairy skin on the left (will not discuss), smooth or glabrous skin on the right
b. Some of these receptors are superficial in the skin and others are deeper in the skin
i. The two of the four receptors that are superficial in the skin are the Meissner corpuscle and the Merkel
disc/receptor
ii. The two that are deeper in the skin are the Pacinian corpuscle and the Ruffini ending
c. Of the four kinds of receptors superficial in the skin you will have one rapidly adapting and one slowly adapting
and deeper in the skin you will also have one rapidly adapting and one slowly adapting so everything is covered
in both locations
VIII. 2 Rapidly Adapting Touch Receptors [S14]
a. The rapidly adapting receptors respond to the onset of the touch and also usually the offset
b. Superficial Meissner corpuscle
i. Small RFs, 40% in hand, for touch, edges, flutter, texture as coarser objects moved across skin,
adjustment of grip
c. Deeper Pacinian corpuscle
i. Bigger RFs, 10-15% in hand, for vibration, movement of fine-textured objects across skin
IX. 2 Slowly Adapting Touch Receptors [S15]
a. Slowly adapting receptors are not just active when you touch something but also when you continue to have
contact with it
b. Superficial Merkel ending/epithelial cell complex
i. Small RFs, 25% in hand, for touch, static edges, shapes, rough texture
c. Deeper Ruffini ending
i. Bigger RFs, 20% in hand, for shape of grasped object, pressure
X. 4 Cutaneous Receptors [S16]
a. Nice slide that summarizes some of the features of the different kinds of receptors
i. Superficially in the skin we have Meissner’s corpuscle and Merkel cells, deep in the skin we have Pacinian
corpuscle and Ruffini endings
ii. Note the sizes of their receptive fields
1. It should come as no surprise that the receptors closer to the surface have smaller receptive fields
2. Receptors deeper in the skin have larger receptive fields, more diffuse
3. Of the superficial receptors in the skin one is rapidly adapting and one is slowly adapting, the bar
represents when the different receptors would fire during a constant stimulus – as stated earlier the
rapidly adapting will fire at the onset and offset of the stimulus and the slowly adapting will continue
to fire through the duration of contact, the same is true of the receptors deep in the skin
b. Summary [S17]
i. Superficial skin has Merkel endings (SA) and Meissner corpuscles (RA), both with small RFs; for detailed
stimuli like Braille dots
ii. Deeper skin has Ruffini endings (SA) and Pacinian corpuscles (RA); note: not “deep” as proprioceptors
which lie in the joints and muscles, all of these receptors are cutaneous so deep here refers to deep in the
skin
XI. Proprioception [S18]
a. Detects stationary body position and movement (kinesthesia); for evaluating shape of objects, controlling
movements
b. Just two main types of proprioceptors:
Neuro: 2:00 - 3:00
Scribe: Taylor Nelson
Monday, February 9, 2009
Proof: Chaz Capra
Dr. Weller
Somatosensory
Page 3 of 6
i. Muscle spindles – in muscles
ii. Golgi tendon organs – ends of the muscles in tendons
c. Can tell where they are by their names
XII. Muscle Spindles [S19]
a. Long, thin stretch receptors in striated (voluntary) muscle
b. Consist of few small intrafusal (meaning within the muscle) muscle fibers covered by central capsule
c. Ends of intrafusal muscle fibers attached to extrafusal muscle fibers (so connected in parallel)
d. Detect stretch or lengthening of muscle which lengthens the intrafusal fibers; if the muscle is extended, these
would be excited and fire
e. [S20] Intrafusal muscle fibers are located within the body of the muscle fibers and they contain the muscle
spindles
i. Receptor is connected in parallel with the other muscle fibers and it detects stretch
ii. [S21] Knee jerk reflex is due to activation of the muscle spindles
1. When the knee is taped with the hammer that stretches/lengthens the quadriceps which activates the
muscle spindle and you have a simple reflex in the spinal cord that causes contraction of the
quadriceps muscle, it shortens and that’s why you kick your leg out
XIII. Golgi Tendon Organs [S22]
a. Golgi tendon organs are found at the ends of the muscles so they are connected in series with the muscle
b. Sensory fibers kind of branch and become entwined with collagen tissue in the tendon
c. They detect muscle contraction
i. When the muscle is used or shortened that’s when the Golgi tendon organs are activated
ii. Opposite of muscle spindles
XIV.
Relation of Modalities to Afferent Fiber Size [S24]
a. How do these receptors send information on into the nervous system
b. Different kinds of nerves carry different information
i. Discovered by Henry Head, 1920’s
ii. He cut his own peripheral nerves and noted that there was a difference in the time course of recovery of
cutaneous versus pain and temperature
iii. This suggested that this kind of information was carried by different nerves
c. Erlanger and Gasser, 1927, used a oscilloscope to show the conduction velocity or the speed of action
potentials and found that cutaneous touch was carried by thicker myelinated axons and transmitted faster
compared to pain and temperature which is carried by thinner and commonly unmyelinated axons
d. Nomenclature – fastest to slowest: A to C or I to IV
XV.
Generalities [S26]
a. Only talking about the somatosensory pathway for touch and proprioception (pain and temperature will come
later)
b. For all pathways should know where the first, second, and third neurons are
i. Know where the cell bodies are, where the synapses occur, where crossing occurs, etc.
c. For this pathway the primary neuron’s cell body is in the dorsal root ganglia for spinal nerves or for input from
the head (cranial structures) in the trigeminal ganglion
d. The cell body is a pseudounipolar neuron that branches and has one part going out to the periphery and the
other acts as the real axon sending information centrally into the spinal cord
e. Input for the neurons comes from regions of the skin called dermatomes [S27]
i. A spinal nerve would get input from a strip of skin
ii. Dermatomes running down the arms and legs are mainly longitudinal whereas the dermatomes of the trunk
are horizontal
iii. If you ever have had a pain or sciatica, the pain would follow these dermatomes
XVI.
Fine touch or dorsal column/medial lemniscal pathway [S28]
a. Named such because it travels up the dorsal column of the spinal cord and once it crosses over at the level of
the brainstem it travels up the medial lemniscus
b. Axons from primary neurons enter spinal cord via dorsal roots and ascend ipsilaterally in dorsal columns which
are also called medial gracile (lower body) and lateral cuneate (upper body) fasciculi
c. Synapse on 2nd order neurons at the level of the lower medulla in gracilis and cuneatus nuclei, the cuneate
fasiculi go to the cuneatus nucleus and the gracile fasiculi go to the gracilis nucleus
d. Axons of 2nd order neurons cross and ascend in medial lemniscus to synapse on 3 rd order neurons in
Ventroposteriorlateralis (VPL) nucleus of thalamus
e. Axons of 3rd order neurons ascend via internal capsule to somatosensory cortex in parietal lobe
i. Above pictured in [S29]
Neuro: 2:00 - 3:00
Scribe: Taylor Nelson
Monday, February 9, 2009
Proof: Chaz Capra
Dr. Weller
Somatosensory
Page 4 of 6
XVII. Details [S31]
a. What about input from the skin of the head and the face (input from the trigeminal nerve)?
b. Trigeminal input goes to principal trigeminal nucleus in the pons instead of the gracile and cuneate nuclei
c. 2nd order neurons’ axons cross and ascend in medial lemniscus to 3rd order neurons in ventroposteriormedialis
(VPM) of thalamus (saw the VPM of thalamus in taste lecture)
d. VPM neurons project to somatosensory cortex in the parietal lobe
e. All structures have somatotopic maps with magnification of particular body parts where you have more receptors
XVIII. Thalamus: Ventroposterior Complex [S33]
a. When we call it the ventroposterior complex, we use complex because it has two parts
i. Lateral part which gets input from skin on the opposite side of the body
ii. Medial part that gets input from skin on the opposite side of the head
b. [S34] Named ventroposterior because its ventral and a little posterior
i. Lateral part is VPL
ii. Medial part is just medial to VPL
XIX.
Cartoon [S35]
a. “Mammunculus”
b. How the skin of the mammal would be represented in VPL and VPM of thalamus
i. Head is represented in VPM medially, it’s magnified
XX.
How areas of cortex defined: multiple criteria [S37]
a. If an area of cortex does something unique it will have a distinctive architecture
i. [S39] Lateral view of a monkey brain
1. If we took a section of tissue from primary visual cortex and stained it with crystal violet you
would see something like this – the cortex forms horizontal layers, you will have denser
accumulations of cells and the cells may differ in size of cell bodies.
2. This histological appearance is called cytoarchitecture because it’s looking at cell body location
3. Brodmann used this to come up with his map of the cortex [S40]
a. He identified about 52 areas for the human cortex
b. Somatosensory areas are areas 3,1,2,5,and 7
c. Brodmann used horizontal sections through the cortex to identify different areas, so in
general the areas at the top have low numbers and the areas on the bottom have high
numbers
b. Will have a certain map or representation of the periphery
c. Will have a unique set of neural properties
d. Will have a unique pattern of connections with other areas of cortex and subcortical structures
e. All of this will culminate by making a unique contribution to function
XXI.
Traditional View of Sensory Cortex [S38] – Contrast traditional and Current views of somatosensory
cortex = EXAM QUESTION
a. Woolsey and collaborators (1937, 1941) used evoked potentials or macroelectrode recording of large regions of
cortex
b. Found single “S1” map encompassing 4 of Brodmann’s (1909) cytoarchitectonic areas: 3a, 3b, 1, and 2
(puzzling) – came to this erroneous conclusion due to crude methods that did not allow them to see a lot of
detail
c. We believe there are at least 200 different areas of cortex that do different things so this result is going in the
wrong direction as Woolsey said that there is one map that covers four of Brodmann’s areas
d. This map formed basic somatotopic organization, used the term “-unculus”, with medial (tail) to lateral (head)
organization
e. Differential magnification in different areas for different species
f. [S42] Adrian (1941) reported ventral “S2”, in parietal operculum or insula (bottom of Sylvian or lateral fissure)
g. Woolsey did somatosensory mapping in a range of mammals and concluded they all had only S1 and S2
i. This is the traditional view, that all animals have just two areas of somatosensory cortex
h. Penfield and collaborators (1950, 1954) “confirmed” SI homunculus in humans using electrodes on epilepsy
patients
i. Reported genitals represented on the medial wall by the feet
XXII. Current View [S44]
a. Arose when people used delicate microelectrodes and picked the right animal to study (smooth-brained owl
monkey)
b. Not just 2 somatosensory areas of cortex, but 5 in lower mammals, > 8-10 in primates, including humans
c. Woolsey’s “SI” not 1 but 4 areas in primates, corresponding to Brodmann’s areas 3a, 3b, 1, and 2 (puzzle
resolved) – Brodmann was right!
Neuro: 2:00 - 3:00
Scribe: Taylor Nelson
Monday, February 9, 2009
Proof: Chaz Capra
Dr. Weller
Somatosensory
Page 5 of 6
d. Not just 1 area, “SII”, in parietal operculum, but at least 2 areas (SII, Parietal Ventral or PV)
e. Also areas behind area 2: areas 5 and 7
f. [S45] Photo/diagram of areas described above
g. Many areas in humans because of specialization of function, presumably the different areas make different
contributions to somatosensation
XXIII. Details about “SI” [S46]
a. Areas 3b and 1 have clearest maps because they get mainly light touch cutaneous input
b. 3b and 1 maps share a congruent border and are roughly mirror reversals
c. Differential magnification of important body parts, with species differences
i. Whiskers of rats vs. squirrels; paws of raccoons vs. cats; rays of star-nosed mole; incisors of naked mole
rat; ventral hand, face, tongue of humans
d. Correctly identified the location of the genitals, not medial to the feet but in order where they belong
XXIV. Somatosensory specializations result in magnification in maps [S48]
a. Various examples of magnification resulting from an expanded map
b. [S49] Mouse or Rat unculus with a 1:1 correspondence of each whisker and its representation in the brain
XXV.
[S50]
a. Areas 3b and 1
i. Cutaneous input (minor pain and temperature)
ii. With removal of 3b and 1 in monkeys, deficits in fine touch
b. Areas 2 and 3a
i. Proprioceptive input from muscle spindle and Golgi tendon (also some cutaneous input from cortex)
ii. Should know where these are located, 3a in front of 3b and area 2 is behind area 1, so straddling the
cutaneous areas
c. Damage to 3a, 3b, 1, and 2: deficits in tactile discrimination and proprioception
XXVI. [S52]
a. SII and PV are involved in more complex somatosensory abilities involving tactile touch learning, transfer
between the hands, etc.
XXVII. Cortical Areas Consist of Columns [S53]
a. All of the neurons in a column would do the same thing, they would get inputs from the same receptive field
b. Discovered first in the somatosensory system in the 1950s
c. Columns exists because neurons in a vertical penetration mainly make connections up and down
XXVIII. Details about Connections [S55]
a. Serial and parallel processing
b. 1 cortical area receives input from multiple thalamic sources, 1 thalamic nucleus projects to several cortical
targets
XXIX. Plasticity in the Somatosensory System [S57]
a. Traditional dogma: plasticity only occurs in young brains, old brains are rigid and cannot adjust to injury
i. Came from studies done in rats – if a row of vibrissae or whiskers in a newborn rat areas of cortex that got
input from that removed row would be totally taken over by input from adjacent vibrissae; this was not
found if you did the same in an adult rat
b. [S61] New view: plasticity also in mature brains
i. If a nerve is cut in an owl monkey you will see little differences right away but if you waited some months
you would see the map was reorganized and some of the other areas would have taken over the input to
the lost nerve
1. [S63] Two stages in the changes seen
a. Small changes right after the nerve cut
b. Greater changes long term
i. 2 implied different mechanisms
1. Immediate: unmasking of latent inputs, disinhibition
2. Delayed: sprouting, growth of connections, synaptic modifications
ii. [S64] Also found variability in hand maps in different normal owl monkeys: implies each brain unique
iii. Increased usage induces changes in maps
1. Repetitive finger stimulation, nursing rats[S65]
iv. [S66-67]Silver springs monkeys – big scale change, nerves cut to entire hand thus the expansion was too
great for the cortex to cover so found changes all along the way
v. [S69] Findings in Humans
1. People who have lost an arm have reported that when the skin of their face is touched it feels like the
missing arm is being touched
a. The cortical face area has taken over the input that should have gone to missing arm
Neuro: 2:00 - 3:00
Scribe: Taylor Nelson
Monday, February 9, 2009
Proof: Chaz Capra
Dr. Weller
Somatosensory
Page 6 of 6
2. Changes due to surgery for syndactyly
a. Compressed representations of fingers expanded
3. Changes due to increased use
a. Functionally: Ebert, Taub et al. (1995), increased cortical representation of fingers D2-5 in left
hand of stringed instrument players
b. Structurally: Gauthier et al. (2008), increased cortical area devoted to stroke-affected arm
after constraint-induced (CI) therapy
[END 54:18]