Somatosensation and Pain Flashcards
Distinct classes of Peripheral Somatosensory Receptors
Epidermis (superficial to deep):
Free nerve endings - ONLY for Pain - changes in temperature and inflammation (temp and pain same system)
Meissner corpuscle - Form/Texture/Acute touch
Merkel cell (basement membrane b/w epidermis and dermis) - Grip/motion of skin/body
Dermis: (Merkel and)
Ruffini endings - Stretch
Subcutaneous: Pacinian corpuscle - vibration (very broad corpuscle, form follows function)
The five things that Sensory Systems do.
- Tranduce.
- Encode.
- Relay
- Represent
- Process
What is proprioception and what information does body use to perceive?
Sense of position of limbs and joints.
- Level of muscle contraction - Nerve endings in corpuscle capsules surrounding muscle fibers detect stretch and relaxation (they are themselves enervated muscle fibers) by detecting mismatch between corpuscle tone and muscle tone.
- Articulation of joints - Golgi tendon organ at connections between tendon and muscles - when deformed, capsule senses change of position.
(3) Mechanisms of molecular and cellular transduction of somatosensation
- Mechanotransduction - deformation of various corpuscles and their receptor cells. Perhaps sensitive to actual mechanical force that will open Na+ channels and depolarize.
- Nociception - distinct parallel pathway for painful stimuli (heat and chemicals) - (CAPSAICIN of hot peppers opens Na+/Ca++ channels for heat/pain)
- Local endogenous signals - from physical force OR tissue injury that release local chemical stimulants that act on (or are secreted from) free nerve endings to start repair or affect blood vessels.
How do neural circuits ENCODE with such specificity?
Must translate information about external stimuli into PATTERNS of electrical activity.
What is neural adaptation?
Receptor adjustment to different (sustained) levels of stimulus intensity, reflected by the pattern of encoded action potentials (usually fewer over time). Can happen quickly or more slowly.
Pain receptors hardly adapt at all. Other receptors do so more.
Tell me about nociception and neural adaptation.
Nocireceptors are the slowest to adapt. If they adapted faster, then the body would adapt more rapidly, and could potentially “ignore” threatening stimuli. Nocireceptors instead have a “threshold” for threatening stimuli, and don’t really react highly until that threshold is reached.
This is important for treatment because high stimulation (above threshold) can require local pain treatment. However, some pain-releiving treatments, like topical CAPSAICIN, can actually change the receptors to over-sense stimulants, and then those receptors actually die. This means that the person is no longer aware of threatening stimuli in that region.
How does neural circuit read spatial information? How does it know where a message is coming from?
All depends on how much afferent nerves are stimulated more than or less than their regular “background firing” activity.
Afferent nerves have a field of dendrites, called a receptive field. These receptive fields overlap with the R.Fs of nearby nerve cells, too. Within the center of the receptive field, receptors on afferent neurons fire at higher levels than background levels of firing. In the area surrounding the receptive field, the action potentials are actually down-regulated. This helps fine-tune perception of space.
What is two point discrimination and why is this clinically important?
Nearby nerves with overlapping receptive fields, the two point discrimination shows the relative size of the receptive fields. In areas on the hands and face, the two-point resolution is much finer, in order to sense with greater acuity.
Patient will only be able to feel the two distinct points if the separate parallel pathways (the CNS) are intact.
What are the parallel pathways in the somatosensory system?
- Dorsal Column/Medial Lemniscus (touch/pressure/proprioception) (faster messages) (crossover in spinal column happens farther up than entry)
- Anterolateral/Spinothalamic (pain/temp) (slower messages) (crossover in spinal column happens immediately)
How does neural system represent periphery in the brain?
For somatosensation, there is a point-to-point correspondence between peripheral receptors and the groups of nerve cells in the brain that receive information from those receptors. Creates a TOPOGRAPHY (map) of the body on the nerve cells in the cortex.
Neighbors on the skin will be neighbors in the brain!
Somatosensation topography and brain real estate.
Areas of the body with greater density of somatosensation receptors (face and hands) have greater amount of brain cortical space devoted to figuring out signals. More receptors means more nerves to figure it all out.
Describe what brain processing does with the signals.
Brain incorporates other regions than just the somatosensation gyrus to add information like, “What is this?”, “What should I do about this stimulus?”, “Should I just ignore this/make the rest of my brain ignore this?”, etc.
What is somatosensory plasticity?
Altered peripheral use can change central representations (cortical space devoted to somatosensory info from a specific area on the body). Often this is due to habitual stimulation, experience, or injury.
