C1: Somatosensory

function of the sensory system is to provide information to the CNS about

external world (sound, light, smell, shape)

internal environment (pain, hunger O2, temperature, receptors)

position of body in space (determine where your limbs are when eyes are closed)

translation of information from the environment is the function of the ...

receptor organs (gated channel)

when the proper stimulus arises, channels open, depolarize the membrane to reach action potential

function of receptor organs (PNS)

convert mechanical, photic, chemical, and other forms of energy into action potentials (a form of information that can be used by the nervous system)

what does DCML stand for?

dorsal column medial lemniscus

what does ALS stand for?

anterolateral system (spinothalamic)

four somatic modailities

1. touch
2. proprioception
3. pain
4. thermal

what somatic modalities have encapsulated disk receptors?

1. touch
2. proprioception

mechanoreceptors (hair follicles)

what somatic modalities have free/bare nerve ending receptors?

1. temperature
2. pain

T/F distribution of receptors varies in different portions of the body

TRUE

areas are more sensitive than others (fingers vs. toes)

What do Ruffini's endings (corpuscles) respond to? What kind of nerve ending?

pressure

encapsulated

What do Pacinian corpuscles respond to? What kind of nerve ending?

pressure

encapsulated

What do krause's end bulbs respond to? What kind of nerve ending?

pressure

encapsulated

What do meissner's corpuscles respond to? What kind of nerve ending?

light touch

encapsulated

What do merkel disks respond to? What kind of nerve ending?

fine touch

encapsulated

What do root hair plexus respond to? What kind of nerve ending?

fine touch

encapsulated

touch is elicited by...

mechanical stimulation of body surface

encapsulated disk

proprioception is elicited by...

mechanical displacements of the muscles and joints

encapsulated disks

pain is elicited by...

noxious (tissue damaging) stimuli

free nerve endings

thermal is elicited by...

cool and warm stimuli

free nerve endings

transduction

converting environmental changes to action potentials

somatosensory receptors are specialized parts of a...

first order neuron

frequency coding

temporal summation

stimulus is proportional to frequency of AP (how things are perceived)

firing 100x/sec would cause more pain than firing 10x/sec

population coding

spatial summation

number of receptors increase the stimulus

pain signaled by 1000 receptors would cause more pain than if signaled by 5 receptors

rapidly adapting receptors

phasic

respond to CHANGING stimuli - doesn't matter if it's at, above or below baseline, only respond when it's changing


provide information about rapid changes

slowly adapting receptors

tonic receptors

sustained stimuli = sustained AP

any difference from baseline

examples of tonic receptors

nociceptors and proprioceptors

muscle spindles

examples of phasic receptors

receptors for pressure, touch, and smell

parallel organization

multiple paths that information can take getting to the brain

if there is a lesion in the main pathway, it can take a parallel pathway that will be less precise

hierarchal organizaiton

information transmitted sequentially via several order of neurons

processed at each relay nuclei

as information is processed, it is narrowed down to focus

first order neuron

1. remain ipsilateral

2. peripheral branches as their receptor organ (bare nerve or encapsulated disk)

3. cell body to dorsal root ganglion (psuedounipolar)

dermatome

Area of skin supplied by a single dorsal root (1st order neuron)

second order neuron

1. cell bodies in dorsal horn (ALS) or relay nuclei of medulla (DCML cuneate or gracile nuclei)

2. CROSSES THE MIDLINE

3. terminate in specific thalamic nuclei

third order neuron

cell bodies in thalamus
axon project to S1, from the CONTRALATERAL side

2 cell bodies:
ventroposterior lateral (VPL)
ventroposterior medial nucleus of thalamus

Termination of 2nd order from the VPL come from where?

trunk and extremities

Termination of 2nd order from the VPM come from where?

face

primary somatosensory cortex is located

post central gyrus of the parietal lobe

primary somatosensory cortex is concerned with?

discriminative aspects of reception and appreciation of somatic sensory impulses

organization of primary somatosensory cortex medial - lateral

leg
trunk
arms
face
teeth
tongue
esophagus

distortion of sizes and representation on the primary somatosensory cortex reflects?

difference in innervation density in different areas of the body

areas most sensitive to touch have the largest respresentation

disturbances within the somatosensory system can be localized clinically because of the direct relationship between ...

anatomical organization of the brain and specific perceptual and motor function

representation of the body map is _______________

use dependent corticoplasticity

can the shape/size of the representation of a particular body part be modified?

yes!

in response to peripheral injury, training, etc.

direct pathways

precise information flows directly from periphery --> CNS

what are the two direct pathways?

dorsal column medial lemniscus pathway (encapsulated) = DCML

anterolateral pathway (spinothalamic, bare nerve) = ALS

what information does the DCML carry?

fine touch
sensory
conscious proprioception
position in space
vibration

what information does the ALS system carry?

pain and temperature = bare nerve endings

indirect pathways

less direct, longer, less precise

NOT HELPFUL CLINICALLY

stereognosis

highly discriminative tactile sensation

ability to recognize object only by touch

what sensations are carried in the DCML pathway AND in parallel pathways?

vibration
static tactile discrimination
conscious proprioception

describe the path of the DCML

1st order: come from extremities, enter through dorsal column, travel up through spinal cord, and synapse at gracile (lower) and cuneate (upper) nucleus

2nd order: cross over in internal arcuate fibers and up through CONTRALATERAL medial lemnsicus to synapse in the VPL (trunk/extremities) or VPM (face)

3rd order: go from VPL or VPM to corresponding area in the somatosensory cortex

Draw the path of 1, 2, and 3 order neurons in the DCML pathway

do it

DCML is carried in what column

dorsal

where do the fibers of the DCML cross over?

internal arcuate fibers in the medulla oblongata

Where do 2nd order neurons in the DCML pathway synapse if its from the trunk/extremity?

ventral posterolateral nucleus of thalamus (VPL)

DCML first order neurons from the UPPER extremities travel through the spinal cord in the ____ to synapse at the ____

CUNEATE fasciculus

CUNEATE nucleus

DCML first order neurons from the LOWER extremities travel through the spinal cord in the ___ to synapse at the ___

GRACILE fasciculus

GRACILE nucleus

what are the receptors for the ALS system?

free nerve endings

ALS pathways ascend in the...

spinothalamic tract

T/F the pathways for the ALS system are segregated and do not run together

TRUE!

how would you test loss of function from the ALS system?

response to pin prick

thermal discrimination

describe the path of the ALS

1st order: come from extremities and synapse in the dorsal root ganglion (lamina II), remain IPSILATERAL

2nd order: enter spinal cord and immediately cross over and ascend through the lateral spinothalamic tract to the thalamus (do not go thru gracile or cuneate) (CONTRALATERAL)

3rd order: go from thalamus to corresponding area in the somatosensory cortex

pain and temperature sensations from the face and cranium are carried by

trigeminal system

second important function of the ALS

parallel channel for transmission of tactile information, including simple touch and static-discriminative modalities

paresethesia

abnormal sensations, numbness, tingling

CLINICAL CASE:
abrupt onset of pain followed by paresthesia
and loss of feeling in a rather circumscribed area along the lateral aspect of his left thigh.
a localized area of decreased perception of pinprick (pain), temperature, and touch, confined to this limited area. Results of the remainder of the examination were normal.

Level in NS?
Side of body?
Pathway(s) involved?
Wallerian Degeneration?

peripheral lesion

left side of body

ALS and DCML pathways

1st order

how would you know if a lesion is in the periphery?

the area affected is very localized and does not spread to other parts of the body

The distal axons of the peripheral sensory neurons mediating all types of afferent input are ______ in peripheral nerves

intermingled

A lesion that affects peripheral nerves would be expected to produce a variable sensory loss for?

all modalities

since motor axons (efferents) are also contained in the same peripheral nerves, weakness and a loss of muscle stretch related reflexes would also be noted.

CLINICAL CASE:
progressive loss of sensation in an area involving the shoulders and both upper extremities.
limited sensory loss involving only pain and temperature in this area.
no changes in motor performance, strength, or deep tendon reflexes.

Level in NS?
Side of body?
Pathway(s) involved?
Wallerian Degeneration?

1. central spinal cord where 2nd order from UE crossover so pathways from all other parts of body are intact

2. bilateral

3. ALS pathways (since DCML pathways would still be intact) - COMMISSURAL SYNDROME

4. 2nd order

Syringomyelia

abnormal longitudinal cavities in the spinal cord cause paresthesias and muscle weakness

commissural syndrome

bilateral pain/temp loss at LEVEL of lesion

specific type of segmental deficit that can result from a lesion involving the central regions of the spinal cord, usually over several segments

interruption of the second-order neurons as they decussate to form the spinothalamic tracts. As the lesion enlarges, other adjacent tracts become involved.

CLINICAL CASE:
knife wound to the back.
weakness of the lower extremity
loss of pain and temperature perception on the right side from about the level of his navel downward
Vibration, joint-position sense, and discriminatory function (fine touch) are reduced in the left leg.
Touch (crude) was normal.

Level in NS?
Side of body?
Pathway(s) involved?
Wallerian Degeneration?

1. spinal cord (after ALS crosses over and before DCML crosses over)

2. left side of body

3. ALS & DCML --> BROWN SEQUARD SYNDROME

4. 1st of DCML; 2nd of ALS

brown-sequard's syndrome

a complete hemisection of the spinal cord

ipsilateral dorsal column deficit, ipsilateral motor deficit, and contralateral loss of pain and temperature senses.

Incomplete lesion stab wound

CLINICAL CASE:
complaining of a progressive difficulty in walking in the dark.
a decreased position sense in his arms and legs bilaterally, decreased vibratory sensation in both upper and lower extremities, and decreased ability to perceive discriminative tactile sensation bilaterally.
Pain and temperature sense were normal.

Level in NS?
Side of body?
Pathway(s) involved?
Wallerian Degeneration?

1. spinal cord

2. bilateral

3. DCML

4. 1st order (projecting to both gracile and cuneate)

what happens to the ALS and DCML pathways when there's a complete lesion in the dorsal columns

ALS = completely untouched

DCML = 1st order completely lost

tabes dorsalis

Degeneration of dorsal columns and dorsal roots due to tertiary syphilis, resulting in impaired proprioception and locomotor ataxia.

Associated with charcot's joints, shooting (lightning pain) , Argyll Robertson pupils (Reactive to accommodation but not light), absence of DTRs, positive romberg, and sensory ataxia at night.

lesions at the spinal level typically produce:

segmental sensory deficit

intersegmental sensory deficit

lesions at the spinal cord: segmental sensory deficit

sensory deficit limited to one level of the body and usually caused by involvement of the nerve roots or spinal nerves

lesions at the spinal cord: intersegmental sensory deficit

sensory deficit involving all the body below a specific level due to interruption of an ascending pathway

CASE STUDY:
hypertensive man awoke one morning noting that he was unable to feel anything over the entire left side of his body.
motor strength and reflexes were normal, as were the visual field
did not respond to pinprick, temperature, or touch over the left side of his face, trunk, and extremities
could not perceive joint motion or vibration in his left arm and leg.

Level in NS?
Side of body?
Pathway(s) involved?
Wallerian Degeneration?

1. thalamus --> CNS

2. left side

3. DCML and ALS

4. 3rd order

** symptoms in the face indicate a brainstem lesion

in a thalamic lesion, why can't it be a spinal cord or medial lemniscus lesion?

spinal cord = because the face is involved

medial lemniscus = no loss of motor strength

lesions in the thalamus

lesion affecting a specific sensory nuclei of the thalamus causes a complete loss of all forms of general somatic afferent sensation in the contralateral face, trunk, and limbs.

lesions at the brainstem level

characterized by a contralateral loss of sensory function in the trunk and limbs because of interruption of the ascending pathways

lesions at the level of the medulla are often associated with

a sensory loss (usually pain and temperature) over the ipsilateral face because of segmental involvement of CN V (trigeminal) or its descending tract and nucleus

suprathalamic lesions

lesions that involve the thalamo-cortical projections or the cortex itself
associated with somatosensory deficits to the contralateral side of the body
are associated with only minimal involvement of pain, temperature, touch, and vibratory sensibility but a severe deficit in joint position sense, touch location, two-point discrimination, and stereognosis

intact thalamocortical pathways are needed for the full appreciation of ______ whereas the primary modalities of superficial sensation are perceived and integrated at the ____ level.

discriminative sensations

thalamic

are you aware of a stimuli before it reaches the cortex?

yes

where are the branches of the first order neurons?

peripheral nervous system

at what order do DCML and ALS pathways cross the midline?

second order

third order neurons that synapse in the ventral posterlateral (VPL) complex of the thalamus come from

trunk and extremities

third order neurons that synapse in the ventral posteromedial (VPM) complex of the thalamus come from

the face