Physiology of Somatosensation-Allard

Question 1

The different sensory receptors (skin mechanoreceptors) was discussed.

Answer 1

pacinian corpuscle: deep pressure, vibration

meissner’s corpuscles: fine tough

merkel’s receptors: pressure and texture

ruffini endings: skin stretch, deep pressure

Question 2

What are some of things that help to localize where on our body something is touching us and the fine detail of the thing touching us? What are the determinants of stimulus localization and sensory acuity?

Answer 2

sensory acuity: how acute are your senses able to discriminate the shape of something and the fine details of something that is impacting your somatosensory receptors

• Size of the Receptive Field
• Density of Innervation
• Convergence
• Lateral Inhibition

all of these affect sensory acuity

For the sensations of smell and sound, balance and taste, localization and acuity follow different mechanisms.
Sensory acuity refers to how well we can determine all the sensory attributes of a stimulus (modality, strength, size, color, frequency, shape, location, scent, taste)

Question 3

What is receptor field?

Answer 3

that spatial region that is innervated by the primary afferent neuron; area of skin that contains those tactile sensory receptors: some will have small or large receptive filed or some of them will be densely innervated on that skin (pack tightly in some areas of skin and may not on other areas of the skin)

Size of the receptive field influences the sensory acuity or the precision with which stimuli can be localized (spacial resolution) and its characteristics be defined.

Large RF = low resolution
Small Rf = high resolution

Question 4

Pacinian corpuscles

Answer 4

-senses touch
-structural refined to sense vibrating touch or repeating on or off touches
-has a large receptive field
in the image may only have 2 receptors so it is not densely innervated in the hand

Question 5

Meissner and Merkels

Answer 5

• purple dots represent the size of their receptive fields
• they have very small receptive fields!!
• they are densely packed in the tips of our fingers
• usually when you try to feel the texture of something you use the first two fingers and thumb, not as manu

how the function and size of receptive field changes and how it relates how they function in real world

Question 6

Stimuli applied where gives a larger impact on the receptor potential?

Answer 6

When something touches that receptor field it has greatest impact in the center of the receptor field it will fire a higher frequency of APs
-when stimuli is applied in the periphery the firing of APs is lower (as shown with blue)

Question 7

2-Point Discrimination: Sensory Acuity Test

Answer 7

how well are you able to determine two points close together are two separate points

the small dots represents the caliber

LARGER receptive fields:
in b you will not be able to tell that it is two points; it will feel like something larger is hitting you in the area and not two points

SMALLER receptive field when the caliber is father apart like in the red you can tell that is two distinct points

For larger receptive fields:
2 points must be separated by greater distance in order to be distinguishes as separate.

For smaller receptive fields:
2 points can be closer together and still be distinguished as separate.

demonstrates the size of the receptive field allows you to have better sensory acuity or 2 point discrimination

Question 8

Different Body areas have variable proportions of sensory receptors.

Answer 8

Receptive field size varies across the body surface; fingertips and peri-oral regions have smallest receptive fields

Areas with sensory receptors that have small receptive fields show better resolution/acuity in two-point discrimination.

Question 9

What would be a reasonable assumption about the size, density, convergence, divergence in the cheek compared to the belly?

Answer 9

cheek has much more sensory acuity than the belly

Question 10

Receptor Field Convergence

Answer 10

• means that somewhere along this pathway signals from two or more sensory neurons will converge onto another neuron
• Convergence and overlap decreases sensory acuity (it will combine all the receptive fields as ONE)

-Areas with small receptive fields and little overlap /convergence have the highest degree of sensory acuity.

Question 11

Lateral Inhibition

Answer 11

• you will get this concept again in the visual system
• happens in many of our sensory system that when something is hitting the sensory field of one neuron and the stimulus is really impacting the receptive field of neuron B but is also impact the field of A and C; so then all 3 of these neurons will fire at different frequencies

Neuron B sends collaterals to its neighbors and those collaterals are connected to their neighbors by inhibitory interneurons. Same thing with its neighbors inhibiting it as well

so your surrounding neurons inhibits its adjacent neighbor thus sharpening the signals

that neuron getting the brunt of the signal will have a much stronger inhibitory effect in its neighboring neurons than the neuron that is in the periphery (just getting a little bit of that stimulus)

• lateral inhibition increases the contrast between the receptive field that is getting most of the stimulus impact compared to those that are just getting a little bit
• so this helps our brain to determine border much more clearly of where the stimulus ends and where the stimulus starts

so lateral inhibition helps sensory acuity by helping us to more clearly define borders of where the stimulus ends and starts

-Capacity of an excited neuron to reduce the activity of its neighbors
Provides high localization precision by decreasing convergence and dissipation of signal.

-Surround (lateral) inhibition also enhances the ability to localize stimuli and perform spatial discriminations
This is really demonstrated in our visual system with horizontal cells providing inhibitory synapses.
Lateral inhibition provides contrast and allows our visual system to more clearly identify boundaries or edges.

Question 12

Thermoreceptors

Answer 12

• remember that pain and temp is also part of the somatosensory system
• allows us to know the temp of things imapcting uor skins
• the receptors are channels embedded in the peripheral free nerve endings
• their ion channels regulated by the temperature (open up when it gets cold or warm, etc)
• often times these receptors are activated by different chemicals (capsaicin pepper) and your tongue feels temp hot or menthol that has a cool feeling on your skin)
• some of these thermoreceptors are actually innervated by chemicals in our food

Question 13

Nociceptors

Answer 13

• are free nerve endings -there are channels associated with them that can be stimulated by temp, touch that is very intense and different chemicals
• generally (any damaging stimuli)
• unmyelinated fibers for slow or dull pain
• slightly myelinated fibers for initial sharp pain that we feel

Question 14

Proprioceptors: Muscle Spindle: Change in muscle length/stretch

Answer 14

Proprioception is concerned with position and balance
Kinesthesia is concerned with movement,

Fiber Type:
Ia (Aα), II (Aβ) = large and myelinated

Adaptation: low adaptive quality

Question 15

Proprioceptors: Golgi Tendon Organ

Answer 15

muscle tension/tendon stretch

Fiber Type: Ib (Aα)

Adaptation: very slow adapting

Question 16

Sensory Receptors of the Joint

Answer 16

• proprioceptive fibers in our joints that allows us to tell the angle and movement of joints
• help to determine the positional relationship of one body segment with another
• looks like and function like the tactile receptors in the skin
• the reason you can close your eyes and touch your nose with your finger without having to look where you arm is and so forth

-they are associ

Question 17

Somatosensory Fiber/Axon Types

Answer 17

Largest and fastest is Type I or Aalpha: 13-20 um, 75-120 m/sec

II, Abeta: 6-12 um, 35-75 m/sec
III, Agamma: 1-5 um, 5-35 m/sec
IV, C: less than 1.5um, 0.5-2.0 m/sec

-was related based on speed of conduction or the size (reason for the different notation)

Question 18

Somatosensory Receptors & Fiber Type

Answer 18

• muscle spindles: Ia/II, Aα/Aβ; Muscle length and velocity
• golgi tendon organ: Ib, Aα; Muscle tension
• joint: pacinian and ruffini: II, Aβ; Joint movement and angle
• Meissner’s corpuscles: II, Aβ; Stroking, fluttering
• Merkel disk: II, Aβ; Pressure, texture
• Pacinian corpuscle: II, Aβ; vibration
• Ruffini ending: II, Aβ; Skin stretch
• Hair-follicle: II/III, Aβ/Aδ; Stroking, fluttering
• Free nerve ending: III/IV, Aδ/C; pain, temperature, crude touch
• pretty much all of our tactile skin somatosensory fiber is associated with the same fiber type (Type II or Abeta)

Question 19

ASCENDING Sensory Systems (this is Gondre-Lewis lecture!!!)

Answer 19

Dorsal Column-Medial Lemniscus:

• Discriminative Touch, Vibration, CONSCIOUS Proprioception
• Aβ fibers
• in our skins and muscles

Spinothalamic:

• Crude Touch, Pain and Temperature
• Aδ and C fibers

Spinocerebellar:

• UNCONSCIOUS Proprioception
• Aalpha (really involved in refining movement)

Crude touch means it is difficult to localize.

Question 20

First, Second, and third order neurons

Answer 20

What is the number one thing secondary neurons do?
-decussate at the midline

-neurons to the cerebellum DO NOT do that as they receive information from the same side (ipsilateral)

First Order Neuron (1°) (Primary Afferent Neuron)
-Cell bodies located in dorsal root ganglia, trigeminal ganglion, or mesencephalic nucleus

Second Order (2°) Neuron
-Cell bodies located in ipsilateral, dorsal horn of spinal cord, ipsilateral medulla, or ipsilateral pons.
Cross the midline to the contralateral side.

Third Order (3°) Neuron
-Cell bodies located in the contralateral VPL or VPM nucleus of the thalamus.
Project to the somatosensory regions of the cerebral cortex.

Question 21

Dorsal Column-Medial Lemniscus Pathway vs Spinothalamic tract

Answer 21

Dorsal Column-Medial Lemniscus Pathway:

• First order neurons in the DRG send fibers through the gracile and cuneate fasciculus that will ascend to the medulla that have the cell bodies for the secondary neurons in the nucleus cuneatus or nucleus gracile; they will send fibers that will
• decussate as internal arcuate fiber to the contralateral side and the fibers will now be called the medial lemniscus

Spinothalamic tract

-same thing for primary afferent neurons found in the DRG and these will synapse as soon as they get into the SC and may travel one or two segments down or up (not like the dorsal column which travel all the way up to the medulla); secondary neurons decussates and goes to the VPL (ventroposterior lateral) nucleus in the thalamus which is holding our fine touch, pain, and temp information in the thalamus

Question 22

Primary Afferent Neurons: Body

Answer 22

Proprioception, Fine Touch, Pain and Temperature: all located in the Dorsal Root Ganglion

Proprioceptor fibers- mesencephalic

Question 23

1° Afferent Neurons: Face

Answer 23

-trigeminal ganglion hold the cell bodies of the primary afferent neurons for the face
as well as neurons for pain and temp in the face BUT for proprioception many cell bodies especially those of the phasic stretch receptors are found in the mesencephalic nucleus

Question 24

Sensory Pathways: Fine TOUCH from Face

Answer 24

• the trigeminal ganglion cells body
• second order neuron in the principal sensory nucleus of the V nerve and that one decussates and travels up just medial to the medial lemniscus pathway
• third order neurons travel from the ventro-posterior medial (VPM) nucleus of the thalamus which will send fibers to middle somatosensory cortex

Question 25

Sensory Pathways: Pain and Temp. from Face

Answer 25

trigmeinal ganglion=semilunar ganglion

will synapse in the spinal nucleus of trigeminal

General somatic nociceptors, thermoreceptors, and mechanoreceptors sensitive to crude touch and pressure from the face conduct signals to the brainstem over GSA fibers of cranial nerves V, VII, IX, and X. The afferent fibers involved are processes of monopolar neurons with cell bodies in the semilunar, geniculate, petrosal, and nodose ganglia, respectively. The central processes of these neurons enter the spinal tract of V, where they descend through the brainstem for a short distance before terminating in the SPINAL NUCLEUS OF V. Second-order neurons then cross over the opposite side of the brainstem at various levels to enter the ventral trigeminothalamic tract, where they ascend to the VPM of the thalamus. Finally, third-order neurons project to the “face” area of the cerebral cortex in areas 3, 1, and 2.

Question 26

Sensory Pathways: Proprioception from Face

Answer 26

secondary neuron fibers go into the the Principal sensory nucleus of the thalamus which sends fibers crossing over to go to the Mesencephalic nucleus of V or through the spinocerebellar pathway???

Question 27

The Somatosensory Cortex Has Three Major Divisions

Answer 27

-third order neurons go to the somatosensory cortex where we have integration of the information from other areas of the brain that impact the cortex to give us impact or meaning to us

Primary Somatosensory (SI)
 -in the postcentral gyrus

Secondary Somatosensory (SII)
-superior bank of the lateral fissure

Posterior Parietal Association
-Broadmann’s area 5 and 7

Question 28

The Sensory Homunculus

Answer 28

The primary somatosensory cortex is “topographically” organized.
The amount of cortex devoted to each body part is proportional to its level of sensory acuity.

-somatosensory cortex is arranged topographically
-somatosensory homunculus
so if we were to draw the body according to the amount of cortex dedication to processing info from the surfaces on our body

our brain thinks its much more important to process very accurately sensory stimulus that is impacting our lips, tongue, hands, and feet and not so much what is coming from the trunk of our body; sensory homunculus tells us about the sensory acuity in those areas of the body (density of receptors, size of receptive fields, densely packed areas, etc.)

Question 29

Primary Somatosensory Cortex (SI)Broadmann’s Areas

Answer 29

Area 3a receives input from proprioceptors in muscles

Area 3b mostly receives cutaneous input from slowly and rapidly adapting receptors (tactile receptors)

Area 1 receives input from rapidly adapting cutaneous mechano-receptors (pacinian corpuscle)

Area 2 mostly receives input from proprioceptive of our joints and deep tissues.

Question 30

Secondary Somatosensory Cortex (SII)

Answer 30

Innervated by neurons from SI (gets a lot of info from primary somatosensory cortex and projects to insula)

Projects to the insula cortex:

• Believed to be involved in
• Self-awareness
• Emotion (emotional context to touch) & Psychopathology
• Perception and Cognition

Insula innervates regions of the temporal lobe believed to be important for tactile memory.

Question 31

Posterior Parietal Cortex Broadmann’s Areas 5 and 7

Answer 31

• this area is very important for integrating all of that tactile information
• this is the area that allows us to pick up something and know what it is without looking (just feeling its edges and touching it around)

should allow the boy to pull out his favorite toy out of the bag without looking or someone drawing a 3 on your hand and you are aware even though you are not looking

Critical for higher order processing of somatosensory information.

The circuitry necessary for discriminative somesthesis must be sent to the posterior parietal association cortex for the meaning of the stimulus to be appreciated.

Stereognosis and graphesthesia require an intact posterior parietal cortex and are abolished without it, even if SI cortex is intact.

Question 32

Somatosensory Evoked Potentials (SSEP)

Answer 32

• are electrical potentials that can be recorded from the surface of your skin that is generated from the APs that are traveling in those neurons
• used by surgeons while they are doing an operation so they make sure they are not affecting any part of the somatosensory system; they want to see that the ascending conduction is normal and is there

Electrical responses to stimulation of peripheral nerves recorded over limbs, spine and scalp

Used to evaluate the integrity of the somatosensory pathways

Recordings obtained from all levels of the sensory system:

• Peripheral nerve
• Spinal cord
• Cortex

in the past we were measuring the integrity of the pathways (Seeing if the delay is normal or the amplitudes are normal)

Question 33

Somatosensory system (Touch) is Crucial for Development

Answer 33

Consequences of absence of significant touch experience early in life:

• Depression
• Bipolar disorder
• Skin ailment
• Immune dysfunction

Babies that grew up in orphanages (without touch in early periods of life) almost all of them ended up suffering from a lot of emotional and skin problems (depression, schizophrenia, immune dysfunction, etc.). Collaterals of the somatosensory system are important for immune system and psychological development.