08 27 2014 Somatosensory lecture Flashcards
somatosensory
touch pain temperature vibration proprioception
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-convulsantsEncoding of Elementary sensory:
- modalities
- Intensity
- Time
- location
What are the modalities of sensation?
- Touch/ vibration– pressure – cutaneous mechanoreceptor
- proprioception – detects displacement via a mechanoreceptor- muscle and joint sensation
- - muscle spindle and Golgi Tendon Organ (GTO) - Tempearture– thermal– thermoreceptor – cold and warm receptors
- 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.
Intensity?
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
- slowly adapting– fire quickly at first but then fire at a continuous and slow rate.
- -detect long/static quality of stimulus - Rapidly adapting– fire quickly at onset of stimulus but stop firing even though stimulus is still present
Location?
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
Stereognosis
ability to detect what you are holding in your hand.
graphesthesia
draw letters and numbers in patient’s hands and you ask patient what letter or number it is.
What are the two regions that the primary somatic sensory cortex will project to?
- posterior parietal cortex – area 5 and 7 – projects sensory and motor projections. Also involved in EXTINCTION – give a double stimulus but only one is felt.
- Secondary somatic sensory cortex (CII) – tactile learning and memory.
- connected to limbic system
Amygdala
hippocampus
emotion and motivations
memory ( connect touch to emotions and memory)
Where do nociceptive afferents enter the spinal cord?
A-delta and C fibers
A-delta enter at laminal level I and V
C fivers enter in layer 2.
Enter spinal cord via dorsal root ganglion–Lissauer’s tract, goes through anterior white commissure and around grey matter. Travels in anterolateral tract up the spinal cord.
Pathway: Spinothalamc system
Ascending pathway
Pain, temperature, crude touch
Pain and temperature information from lower body goes through DRG and into the dorsal horn via LISSAUER’S TRACT. There it synapses with 2nd order neuron and cross the anterior white commissure and travels up the anterolateral system all the way to the Mid-pons. There it continues as the Spinothalamic tract through the midbrain –> Ventral posterior lateral nucleus of thalamus. Here it synapses in the primary somatic sensory cortex.
Pain and temperature from upper body (excluding face) enters via DRG into cervial spinal cord dorsal horn. There it synapses with a 2nd neuron that decussates via anterior tissue and travels up anterolateral system and continues up spinothalamic tract (after Mid-pons)
Trigeminal system for Pain and Temperature
Starts with trigeminal ganglion in mid pons and travels down to middle medulla where a collateral continues down to the caudal medulla.
The branch that comes off in middle medulla decussates in the middle medulla and travels up to the Mid-pons and continues as the trigemino-thalamic tract into the ventral posterior medial nucleus of the thalamus and continues to the primary somatic sensory cortex.
The branch that continues down the the caudal medulla (via the spinal nucleus of trigeminal complex). Here it synapses with a 2nd order neuron and decussates and travels up via the trigemino-thalamic tract to the ventral posterior medial nucleus of the thalami –> primary somatosensory cortex.
Somatic organization of STT in spinal cord
Upper limbs are medial/dorsal,
Lower limbs are latera/ ventral
What pathways end up in the Ventral posterior LATERAL nucleus of the thalamus?
Dorsal Column- Medial Lemnsicus
and the Spinothalamic tract
What pathways end up in the Ventral posterior MEDIAL nucleus of the thalamus?
Trigeminal Lemnisucs
and Trigeminothalamic tract
Brown-Sequard syndrome
loss of sensation and motor function (paralysis and anesthesia) that is caused by the lateral hemisection (cutting) of the spinal cord
– incomplete lesions
Brown- Sequard syndrome – symptoms
dorsal column- medial lemniscus tract
-Ipsilateral loss of vibration, proprioceptoin, fine touch
Spinothalamic tract ( decussates at level of spinal cord) –> contralateral loss of pain and temperature sensation
Corticospinal tract – loss of motor function ipsilaterally
- ipsilateral spasms
- babinski sign (ipsilateral)
- Abnormal reflexes
Why does phantom limb/ phantom pain occur
reorganization of somatosensory cortex in amputatee –> neurons that used to be inverted by limb now respond to stimulation of other body part
treatment : mirror therapy
