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).
