Somatosensory Pathways (Glendinning) Flashcards
What are the negative (2) and positive (2) symptoms caused by sensory neuropathies?
Negative = loss of sensation (numbness, weakness, deadness)
- Analgesia: loss of pain
- Anesthesia: loss of touch
Positive = abnormal sensation
- Paresthesia: temporary mild pain (pins and needles, tingling, burning, prickling)
- Neuropathic pain/Central Pain Syndrome - chronic intense pain (shooting, stabbing, or electric shock-like jolts)
Elementary sensory attributes are encoded according to the following 4 criteria:
- Modality: subsystem for processing different kids of stimuli (physics/chemistry - odorant or taste molecule, receptor specificity, and labeled line/pathway)
- Intensity: strength of stimulus (i.e. how bright is the light)
- Timing (Receptor on vs off firing timing pattern)
- Slowly adapting (leg touching chair) vs. Rapidly adapting (finger brushing briefly on table) - Location (Receptor density, receptive field, and inhibitory mechanisms affect resolution)
- Fingertip has higher receptor density and therefore resolution than trunk
- Neurons in surrounding receptive field inhibited while neurons in center receptive field are stimulated
Name the 4 types of somatosensory modalities, their stimuli, receptor class, and receptor cell types
- Touch/Vibration
Stimuli: Pressure
Receptor Class: Mechanoreceptor
Receptor Cell Types: Cutaneous mechano- (i.e. skin) - Proprioception
Stimuli: Displacement
Receptor Class: Mechanoreceptor
Receptor Cell Type: Muscle and joint receptors - Temperature sense
Stimuli: Thermal
Receptor Class: Thermoreceptor
Receptor Cell Type: Cold and warm receptors - Pain
Stimuli: Chemical, Thermal, or Mechanical
Receptor Class: Chemo-, Thermo-, Mechanoreceptor
Receptor Cell Type: Polymodal, thermal, and mechanical nociceptors
What are the 3 major long pathways of the CNS, their MODALITY, and SITE OF DECUSSATION?
- Corticospinal
Motor
Pyramids (spino-medullary junction) - Dorsal Column-Medial Lemniscus System (DCMLS)
Sensory (vibration, proprioception, fine touch)
Internal arcuate fibers (lower medulla) - Spinothalamic Tract (STT)
Sensory (pain, temperature, crude touch)
Anterior commissure (spinal cord)
What is the purpose of having parallel pathways (DCMLS and STT)?
Compensation; Improves RELIABILITY and SPEED
What is a dermatome and why is there overlap?
An area of skin innervated by a single dorsal root ganglion
Overlap due to both convergence and divergence:
1) Axons making up a dorsal root originate from several different peripheral nerves
2) Individual peripheral nerves contribute axons to adjacent dorsal roots
Morphology and function of CUTANEOUS MECHANORECEPTORS (4) from superficial to deep:
Free nerve endings (nociceptors for pain)
- Meissner corpuscle
- Epidermis
- Just below hairless skin
- Surface, motion (can tell that you rubbed finger against table)
Merkel cell-neurite complex
- Epidermis
- At tip of epidermal ridge
- Edges, indentations
Ruffini corpuscle
- Dermis
- Aligned parallel with stretch lines
- Skin stretch (if you open your palm, stretch sensation picked up by ruffini corpuscle)
Pacinian corpuscle
- Subcutaneous layer
- Deep and with onion-like layers
- Vibration sense (test with tuning fork)
Morphology and function of PROPRIOCEPTORS
- Detect muscle length
- Detect muscle tension
Refer to Notes*
Neuron fibers and receptors associated with cutaneous and proprioceptive sensation
For each sensory function, name the receptor type and afferent axon type
Proprioception
- Muscle spindle
- Ia, II
Touch
- Merkel, Meissner, Pacinian, and Ruffini
- A(beta)
Pain, temp
- Free nerve endings (myelinated)
- A(gamma)
Pain, temp, itch
- Free nerve endings (unmyelinated)
- C
What are two clinical correlates that involve dorsal columns and the spinocerebellar tract?
TABES DORSALIS - cause: tertiary syphilis infection
Degeneration of dorsal columns -> impaired sensation and proprioception -> progressive sensory ataxia
SUBACUTE COMBINED DEGENERATION - cause: B12 or E deficiency
Demyelination of dorsal columns, lateral corticospinal tracts, and spinocerebellar tracts -> ataxic gait, paresthesia, impaired position and vibration sense
Romberg test - if you ask patient to close their eyes, they keep falling over
How does the somatosensory homunculus parallel the motor homunculus?
Legs located more medially with face more lateral
Explain how inputs from the VPL of the thalamus terminate in Brodmann’s areas of the primary somatic sensory cortex (SI).
SII CONNECTS TO LIMBIC SYSTEM - TACTILE LEARNING & MEMORY
VPL –> Terminate in areas 3a, 3b, 1 (chin/lip), and 2 (leg, foot, toes) - greatest density of projections in area 3b –> SII –> amygdala and hippocampus
3b projects heavily to areas 1 and 2
POSTERIOR PARIETAL CORTEX CONNECTS TO MOTOR CORTEX FOR ASSOCIATION/INVOLVED IN ATTENTION
2 projects to parietal areas 5, 7 -> motor and premotor cortical areas in the posterior parietal cortex -> motor and premotor areas of frontal lobe (voluntary muscle contraction)
Nociceptor free nerve endings are located in which dermal layer and what do they sense?
Axons associated with nociceptors fall into which two groups?
Epidermis/ pain and temperature
A(gamma) group of myelinated axons or
C fiber group of unmyelinated axons (slower conduction)
Do temperature receptors express multiple types of receptors? Can they adapt?
Only express one type of receptor (hot/cold)
Cold and warm receptors fire mostly during changes of temperature (you immediately feel change but then get used to it)
