Sensory Physiology Flashcards
how are nerves classified
contribution to a compound action potential based on fiber diameter, myelin thickness, and conduction velocity
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of sensory fiber: Aalpha
Ia and Ib
large (13-30)
fast (80-20)
primary muscle spindles, golgi tendon organ (proprioception)
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of sensory fiber: Abeta
II
slightly slower and smaller than Aa
Secondary muscle spindles, skin mechanoreceptors
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of sensory fiber: Adelta
III
Slightly larger and faster than C fibers
Skin mechanoreceptors, thermal receptors, and nociceptors
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of sensory fiber: C fibers
IV
small (0.2-1.5)
very slow 0.5-2
skin mechanoreceptors, thermal receptors, and nociceptors
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of motor fiber: Aaplpha
12-20
72-120
the fastest and largest
Extrafusal skeletal muscles
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of motor fiber: Ay
slightly smaller and slower than the Aalpha fibers
Intrafusal muscle fibers
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of motor fiber: B
larger than C but smaller than Ay
preganglionic autonomic fibers
Classification of afferent fibers, fiber diameter, conduction velocity, and receptor supplied of motor fiber: C
slowest and smallest of the muscle fibers
Postganglionic autonomic fibers
Meissner corpuscle
low threshold, rapidly adapting, found in glaborus skin
Touch and vibration less than 100 Hz. flutter and tapping
Pacinian Corpuscle
Low threshold, slowly adapting, found in both hairy and glaborus skin
Rapid indentation of the skin such as that during a high frequency vibration (100 to 400Hz) vibration
Ruffini corpuscle
Low threshold, slowly adapting, found in glaborous skin
and in hairy skin
Magnitude and direction of stretch and pressure and proprioception
Merkel cell
low threshold and slowly adapting, found in glaborus skin
Pressure
Hair follicle receptor
rapidly and slowly adapting
motion across the skin and directional of that motion
Tactile free nerve ending
high threshold, slowly adapting
Pain and temperature
Muscle spindles
Limb proprioception
Golgi tendon organ
limb proprioception
Receptive fields
areas of innervation where an individual mechanoreceptor fiber conveys information from that limited area of skin
vary in size:
fingertips have small area but are very receptor dense (fine discriminative touch)
back skin has a large area but are dont have a dense receptor base (no fine discriminative touch)
Two point discrimination
abillity to identify the site of stimulation and distingush between two stimuli that are at close distances
two point discrimination is better done by receptors with smaller receptive fields (fingertips) than large receptive fields (forearm)
Tactile acuity is important
allows for spacial resolution of detailed textures
Somatosensory area 1 (SI)
involved in the integration of the information for position sense as well as size, shape and discrimination
somatic sensory area 1 primary sensory cortex located in post-central gyrus first stop for most cutaneous senses crude identification of senses
Somatosensory area II (S2)
responsible for comparisons between objects, different tactile sensations and determining whether something becomes a memory
an assosciation area
located in the wall of the lateral fissure
receives input from S1
somatotopic representation is less maintained
important to cognitive touch
Parieto temporal occipital association area (PTO)
responsible for high level interpretation of sensory inputs
recieves input from multiple sensory areas
analyzes psatial coordinates of self in environment
identification of objects
many more functions
Large amount of association areas in our human brains is distinctive of being human compared to other mamals
Phantom limb pain
pain in the body part that is no longer present due to the law of projection
-states thhat regardeless of the place along the afferent pathway that is stimulated, the sensation is perceived to come from the place that the innervation arises
Pain
unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage
Nociception
the neural process of encoding noxious stimuli (a stimulus that is damaging or threatens to damage
consequences of encoding may be autonomic or behavioral
. pain sensation doesnt necessarily have to be implied
Hypersensitivity
increased responsiveness of nociceptive neurons to their input and or recruitment of a response to normally subthreshold inputs
Hyperaesthesia
increased sensitivity to stimulus excluding the special senses
Hyperalgesia
increased pain from a stimulus that normally provokes pain
Allodynia
pain due to a stimulus that does not normally provoke pain
classic example is the lay of sheets on skin that has been sunburned
biphasic response to pain
initial Adelta fibers (much faster and larger but receptive fields much smaller) provide the precise localization of pain
during phase two C fibers (slower smaller and respond to chemical and thermal stimuli and release substance P, glutamate, aspartate, calcitonin gene related peptide (CGRP) vasoactive intestinal polypeptide (VIP and nitric oxide) this generates a more dull and throbbing less localized pain
what are the three modalities of nociceptors
Mechanical: response to mechanical forces ranging from moderate pressure with a blunt object to overtly tissue-damaging stimuli
chemical: response to endogenous or exogenous chemical compounds such as pro inflammatory mediators, acids, or capsaicin
THermal: response to noxious heat and cold will directly activate thermal receptors expressed by nociceptors
TRPV1
ligand gated noselective cation channel in nociceptive neurons
Found on many C-fibers and activated by vanilloid compounds (capsaicin) in addition to exogenous compounds, endogenous compounds (bradykinin) and heat greater than 43 degrees
activation causes a release of an action potential and release of neuropeptide (CGRP) and substance P which leads to vasodilation and activation of immune cells, this leads to pro-inflammatory mediator release which acts as a positive feedback loop activating more of these TRPV1 channels
found in, migraignes, dental pain, cancer pain, inflammatory pain, neuropathic pain, visceral pain, and osteoarthiritis
TRPA1
nociceptors ligand gated that are activated by: allyl isothiocyanate in mustard oil, wasabi, and horseradish
Anesthetics often have paradoxial pro-nociceptive effects acting through TRPA1
involved in inflammatory pain states: allergic contact dermatitis, chronic itch, painful bladder syndrome, migraine, irrritable bowel syndrome and pancreatitis
TRPM8
can be activated by both innocuous cooling and noxious cold temperatures as well as a number of cooling agents such as menthol which are commonly used for their analgestic properties
Rubbing the spot makes it feel better
No pain is sensed due to the inhibitory interneuron is blocking the nociceptive signal from continuing to move forward (closed gate)
Strong stimuli of C fiber (opens gate)
1) activate an Abeta fiber buy normal stimuli. the central process of this fiber branches in the dorsal horn and synapses on an inhibitory interneuron and release EAA
2) activated interneuron will release glycine and inhibits the secondary sensory neuron of the nociceptive pathway
3) rubbing an area of affected skin activates the Abeta fiber and reduces the sensation of pain
What is the descending inhibition of Pain receptors (specifically C fiber)
Periaqueductal grey are activated by EAA, opiates, and cannabinoids
then descending pathway travels to Locus Coeruleus (NE) and Raphe nucleus (serotonin)
serotonin and NE release into dorsal horns and activate inhibitory interneurons
local inhibitory interneruons release opiates (enkephalin)
these activate mu-receptors on presynaptic and post synaptic terminals of a C-fiber
results in reduction of SP from the C-fiber and reduces nociception
Central sensitization
Activity dependant synaptic plasticity in the spinal cord that generates post injury pain hypersensitivity together with cellular and molecular mechanisms responsible for this form of neuronal plasticity
- Reduced threshold of dorsal horn neurons to noxious stimulation
- commonly caused by chronic exposure to peripheral inflammation
- alterations in transcription and translation of various receptors and ion channels. changes the level of synaptic input produced by the afferent fiber
- receptive fields of the afferent expands
- involves persistant stimulation of EAA receptors, intracellular Ca++ and activation of various intracellular signaling cascades
Peripheral Sensitization
Neuroplastic changes relating to the function, chemical profile, or structure of the peripheral nervous system that encompasses changes in receptor, ion-channel, and neurotransmitter expression levels
- Neuroimmune activation can increase intensity and duration of pain
- at the site of inflammation, prostglandin E2 (PGE2) sensitizes the peripheral nociceptors via activation of receptors that are present on the peripheral terminals of nociceptors by reducing the firing threshold and increasing the responsiveness
- PGE2 may come from nearby mast cells, neutrophils, macrophages, and T-Lymphocytes following a peripheral injury
- adjacent non injured primary afferent nerve fibers can be come sensitized also
Peptidergic nociceptors
Express neuropeptides
-substance P
-CGRP (calcitonin gene related peptide)
Responsiveness to nerve growth factor
Most visceral afferents are peptidergic
-contributes to chronic visceral pain syndromes
Half of cutaneous afferents are peptidergic
Chronic inflammation upregulates neuropeptides
Non-peptidergic nociceptors
- Do not express CGRP or SP neuropeptides
- Responsive to GDNF (glial-derived neurotrophic factor)
- Very few visceral afferents are non peptidergic
- Half of cutaneous afferents are non-peptidergic
- involved in somatic chronic pain states such as that of diabetic neuropathy
Nociception distribution in the brain
S1 and S2: do recieve input from nocicieptors and play a role in localization of the pain
Insular cortex: particularly important in interpretation of nociception
- processes information about internal state if the body
- Contributes to autonomic response to pain
- integrates all signals related to pain
- damage causes asymbolia
- lesions in any single area alters the experience of pain but does not abolish it completely
Amygdala: important in the emotional component to pain
Hypothalamus and medula: integrating the physiological changes associated with visceral pain
