08 27 2014 Somatosensory lecture Flashcards Preview

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What are the main somatosensory pathways?

posterior column- medial lemiscal pathway

anterolateral pathway (including spinothalamic tract)

Trigeminal Lemniscus--touch and vibration information from face to cortex.

Trigeminothalamic tract-- pain and temperature from face. (pathway descends 2 segments before turning and coming up).


Sensory Neuropathies--
Negative symptoms?

Analgesia - loss of pain
Anesthesia - loss of touch
"numbness, heaviness, weakness or deadness"


Sensory Neuropathies-- Positive symptoms?

Paresthesias-- temporary mild pain
Neuropathic pain/ central pain syndrome:
-chronic intense pain
- intense burning sensation interrupted by shooting, stabbing, or electric shock-like jolts.
-treatment: anti-convulsants


Encoding of Elementary sensory:

1. modalities
2. Intensity
3. Time
4. location


What are the modalities of sensation?

1. Touch/ vibration-- pressure -- cutaneous mechanoreceptor

2. proprioception -- detects displacement via a mechanoreceptor- muscle and joint sensation
-- muscle spindle and Golgi Tendon Organ (GTO)

3. Tempearture-- thermal-- thermoreceptor -- cold and warm receptors

4. Pain -- chemoreceptor, thermoreceptor, mechanoreceptor -- polymodal nociceptors, thermal nociceptors, and mechanical nociceptors.


Activation of cold receptor?

fire from 38 degrees C to 32 degrees C and below.


Activation of hot receptor?

fire from 32 degrees C to 38 degrees C and above.



Strength of stimulus -- depends on threshold. We can control our own threshold -- hot baking sheet and we refuse to drop it.


Timing? and two types of receptors?

receptors differ in the timing of responses to stimulus

1. slowly adapting-- fire quickly at first but then fire at a continuous and slow rate.
--detect long/static quality of stimulus

2. Rapidly adapting-- fire quickly at onset of stimulus but stop firing even though stimulus is still present



depends on receptor density, receptive field and inhibitory mechanisms affecting resolution.

Two point discrimination-- minimal distance required to perceive two simultaneously applied stimuli as distinct.

receptive fields determine resolution


Receptive fields

region in sensory space that elicits greatest AP response to specific stimulus.

the smaller the field the better because…

Area surrounding receptive field inhibits cortical neurons to help create the boundary of a shape

= shuts up neighbors so stimulus can be better heard.


Name the cutaneous mechanoreceptors:

Free nerve endings
Meissner corpuscles
Merkel cell-neurite complex
Ruffini corpuscle
Pacinian corpuscle


Free nerve endings

nociceptors - located in epidermis


Meissner corpuscles

just below epidermis -- touch
- rapidly adapting


Merkel cell-neurite copmlex

Tipe of epidermal ridge-- detects shapes, edges, indentations
-slowly adapting


Ruffini corpuscle

located in Dermis and is aligned parallel with stretch lines
- detects when skin is stretched


Pacinian corpuscle

deep in subcutaneous layer of skin
- vibrations


What structures help with proprioception?

muscle spindle and GTO (golgi tendon organ)


What are the fibers like for proprioception?

Receptor type: muscle spindle
Axon: Ia, II, Golgi Tendon organ

Thick axon with LOTs of myelin (13-20 microm)
= fast conduction


What are the fibers like for touch?

receptor type: Merkel, Meissner, Pacinian, and Ruffini cells

Axon: A-beta

Still a thick axon (not as thick as the muscle spindle-- proprioception). (6-12 microm)

Some myelin
= a little slower than receptors for proprioception


What are the fibers like for detection of pain and temperature?

receptor type: free nerve endings

Axon type: A- delta

Small diameter (1-5 microm) with small amount of myelin
- slower conduction


What are the fibers like for detection of pain, temperature and itch?

Receptor type: free nerve endings (UNMYELINATED)

VERY small diameter
- very slow conduction


Dorsal Column/ Posterior column- Medial Lemniscus pathway

Lower and upper body pathways

Ascending pathway
Touch, proprioception, vibration

Mechanoreceptors from lower body go into dorsal horn and go up the Gracile tract until it reaches the caudal medulla. At the caudal medulla it synapses with a 2ndary neuron, decussates via internal arcuate fibers and continues up the medial reminisces until it reaches the Ventral Posterior Lateral nucleus of the thalamus. There it synapses with a 3rd neuron which then takes it to the primary somatosensory cortex (medial).

Mechanoreceptors from upper body come in through DRG --> dorsal horn at cervical spinal cord levels. Neuron continues up cuneate tract (lateral to Gracile tract) to the caudal medulla. There, it too synapses with 2nd neuron and decussates across internal arcuate fibers and continues up the medial lemniscus where it too synapse to 3rd neuron --> Ventral Posterior Lateral Nucleus of thalamus (lateral).


Orientation of neurons along Dorsal Column-Medial Lemniscus pathway

In the posterior column (SPINAL CORD): upper trunk is lateral and lower trunk is medial

By the time in thalamus: lower trunk is lateral and upper trunk is medial.

In cerebelum, the paths cross again and the lower trunk is medial and upper trunk is lateral.


Pathway: Trigeminal Mechanosensory system

Ascending pathway

Touch, vibration from face

Sensation from opthalmic, maxillary, or mandibular goes to trigeminal ganglion --> principal nucleus of trigeminal complex located in MID-PONS. It synapses here with a 2nd order neuron, decussates and travels up the medial lemniscus. When it hist the MIDBRAIN, it continues up as the trigeminal lemniscus and synapse with a 3rd order neuron in the Ventral Posterior MEDIAL nucleus of the thalamus. 3rd order neuron continues to the primary somatic sensory cortex.


What is a test for large diameter sensory Neurons involved in proprioception?

Romberg test


Romberg test

ability to stabilize body -- proprioception and visual input.

-if they can't stabilize = proprioception is off = dagame to DCML and spinocerebellar tract



ability to detect what you are holding in your hand.



draw letters and numbers in patient's hands and you ask patient what letter or number it is.