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Neuro Greenspan


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Classifications of receptors based on microscopic structure
Unencapsulated Receptors
Free nerve ending and loses its myelin sheath and is only separated from tissue by the Schwann cell and a BM. Many of these respond to NOXIOUS stimuli and HIGH THRESHOLD mechanical stimuli.

Examples: thermoreceptors, nociceptor
Encapsulated Receptors
Consits of an elabortate arrangement of specialized cells that receive the peripheral endings of nerve fibers.

Examples: Pacinian corpuscle and Meissner's corpuscle
Specialized Receptors
ELECTROGENIC cells that become involved in the process of SENSORY TRANSDUCTION and their response depolarizes the attached nerve ending.

Ex: cochlear hair cells and photoreceptors
Sherrington's Classification of Receptors
detect EXTERNAL stimuli (5 senses = visual, touch, hearing, taste, smell)
detect INTERNAL stimuli (stomachache, visceral receptors)
Detect body position or movement (ex: muscle and joint mechanoreceptors, vestibular system)
Functional classification of receptors (based on physiological function)
nociceptor (** can respond to mechanial, thermal, or even chemical stimulation)
Auditory Sensory system
Modality: hearing
Stimulus energy: sound
Receptor class: mechanoreceptor
Receptor cell type: hair cells (cochlea)
Visual sensory system
Modality: vision
Stimulus energy: light
Receptor class: photoreceptor
Receptor cell type: rods, cones
Vestibular sensory system
Modality: balance
Stimulus energy: gravity
Receptor class: mechanoreceptor
Receptor cell type: hair cells (vestibular labyrinth)
Gustatory sensory system
Modality: taste
Stimulus energy: chemical
Receptor class: chemoreceptor
Receptor cell type: taste buds
Olfactory sensory system
Modality: smell
stimulus energy: chemical
Receptor class: chemoreceptor
Receptor cell type: olfactory sensory neurons
Touch [submodality]
stimulus energy: pressue
receptor class: mechanoreceptor
receptor cell type: cutaneous mechanoreceptors
Proprioception [submodality]
Stimulus energy: displacement
receptor: mechanoreceptor
Receptor cell type: muscle and joint receptors
Temperature sense [submodality]
stimulus energy: thermal
receptor class: themoreceptor
Receptor cell types: cold and warm receptors
Pain [submodality]
stimulus energy: chem, thermal, or mechanical
receptor class: chemo,thermo, mechanoreceptor
Itch [submodality]
stimulus energy: chemical
receptor class: chemoreceptor
Receptor cell type: chemical NOCICEPTOR
Rapidly adapting receptors
application of a stimulus may result in a few or only 1 action potential even if the stimulus is maintained.

ex: pacinian corpuscle
slowly adapting receptors
application of a stimulus results in a repetitive discharge in the primary afferent neuron as long as the stimulus is maintained.

ex: most nociceptors
Stimulus intensity
Refers to the strength of a stimulus. It is encoded at the receptor by FREQUENCY (nerve impulse/sec) and POPULATION (# of receptors activated).
Frequency coding
The strength of the stimulus is directly related to the firing rate of the sensory unit -- this is true for many slow adapting receptors.
Spatial or Population responses
more intense stimuli activate more sensory units at the point the stimulus is applied.
Why do large myelinated nerve fibers conduct action potentials more rapidly than smaller myelinated/unmyelinated fibers?
- increased diamater of myeline sheath produces a LOWER INTERNAL LONGITUDINAL RESISTANCE
-Myelinated fibers propagate impulses by SALTATORY CONDUCTION--> longer internodes so conduction is more efficient.
Relationship between conduction velocity and fiber diamater of myelinated fibers.
Conduction velocity increases by about 6 m/s per 1 micrometer increase in fiber diameter.
Fiber Type I
diameter: 12-20 microns
conduction velocity: 72-120 m/s
receptor associations: mechanoreceptors: muscle spindles; golgi tendon organs
Fiber Type II
diameter: 6-12 microns
conduction velocity: 36-72 m/s
receptor associations: mechanoreceptors: muscle spindles; Meissner, Pacinian, etc.
Fiber Type III
Agamma and Adelta
diameter: 1-6 microns
cond. velocity: 6-36 m/s
receptor associations: thermoreceptors (COOL); nociceptors
Fiber Type IV
C fibers
diameter: 0.4-1.2 microns
cond. velocity: 0.5-2.0 m/s
receptor associations: thermoreceptors (WARM); nociceptors
Fast-adapting Type I
Only on non-hairy part of hand
Small, sharp borders
43% meissner corpuscle; edge sensitive
Slow-adapting Type I
Irregular discharge
Edge sensitive
25% merkel cells
Found in hairy skin
Small, sharp borders
Fast-adapting Type II
Large, obscure borders
13% Paccini (Golgi-Mazzoni)
Large receptive field (deep in skin)
Slow-adapting Type II
Large receptive field
Deep in skin
Regular discharge
Sensitive to lateral skin stretch
19% Ruffini
Pacinian Corpuscles
Most sensitive to vibration, particularly in the 100-400Hz range. At 250 Hz vibration, PC can detect skin displacement of 0.10 micrometers.
Types of receptors found in the PDL
several types of modified Ruffini nerve endings and a variety of free nerve endings.

Ruffini: slowly adapting mechanoreceptors
Threshold sensitivity
Often tested with monofilaments; varies across body; determined by sensitivity of RAPIDLY adapting mechanoreceptors (although perception of PRESSURE is mediated by slowly adapting mechanoreceptors).
Types of input of DC-ML system
slowly and rapidly adapting cutaneous mechanoreceptors as well as muscle and joint mechanoreceptors
Which encapsulated mechanoreceptor is the FAST-adapting one(s)?
Pacinian corpuscle, then Meissner.
Which encapsulated mechanoreceptor is found on GLABROUS skin?
Meissner's corpuscle
Which encapsulated mechanoreceptor functions for the stretching of skin?
Ruffini's corpuscle
DC-ML pathway: where are the second order neurons?
gracile nucleus: MEDIAL; fibers that convey infromation from LOWER limbs

cuneate nucleus: LATERAL: fibers that convey information from UPPER limbs, trunk, neck.
Internal arcuate tract
Right after contacting the 2nd order neurons; axons project in the dorsal portion of each side of the lower branstem; subsequently cross the midline.
Medial Lemniscus
Elongated dorsoventrally (after the internal arcuate axons cross the midline).

Info. from lower limbs located ventrally; info. from upper limbs located dorsally.
When does hte medial lemniscus rotate 90 degrees laterally?
when it ascends through the pons and midbrain.

Upper body: medial
Lower body: lateral
DC-ML pathway: 3rd order neurons
cells of ventral posterior lateral (VPL) nucleus of thalamus.
-these are mechanosensory receptors from FACE
-trigeminal ganglion
-principal nucleus (2nd order)
-medial lemniscus
-trigeminal lemniscus (trigeminothalamic tract)
-VPM (ventral posterior medial) nucleus of thalamus
Primary somatic sensory cortex
aka SI
Located in the postcentral gyrus of the parietal lobe and comprises 4 regions:
Broadmann's areas 3a, 3b, 1, and 2.
Which laminae do the Adelta fibers and C fibers synapse in?
Adelta: laminae 1 and 5
C: laminae 1 and 2
The MAJOR ascending pathway for information abotu pain and temperature (for below face)
Spinothalamic tract
anterolateral system
At what level of the brainstem does the first-order axons enter? This is for noxious and thermal stimulation of the FACE.
Enter at the PONS, then descend to the MEDULLA forming the spinal trigeminal tract.
First-order axons for pain/thermal stimulation for FACE originate where?
trigeminal ganglion cells and from ganglia associated with VII, IX, X

[geniculate gang, sup/inf gang, sup/inf gang]
What are 2 subdivions of the spinal [descending] nucleus of V?
Pars interpolaris
Pars caudalis
What is the major ascending pathway for inforamtion about pain/temp from the FACE?
trigeminothalamic tract
Neurons in the parabrachial nucleus project where?
Hypothalamus & amygdala --> motivation and affect; also sends projections to the periaqueductal gray matter which has to do with descending modulation of the pain circuits.
Where do projections of affective-motival pain pathway NOT in the parabrachial nucleus go?
Other projections go to older parts of the thalamus such as the INTRALAMINAR nad MEDIAL THALAMIC NUCLEI (medial to ventral posterior nucleus)
Projections from the anterolateral system to the medial thalamic nuclei provide nociceptive signals to which areas?
Frontal lobe, insula, and cingulate cortex
Nociception vs. pain
Sensibility is the ability to detect noxious stimuli. Sensation is how the brain interprets that sensory information. Generally, sensation has two components: recognition of the location, form and intensity of the stimulus and the affective-emotional responses to the noxious stimulus.
how does capsaicin create pain?
capsaicin activates responses in a subset of nociceptive polymodal C fibers by opening ligand-gated ion channels (like VR-1) that permit the entry to Na+ and Ca2+.
abnormal unpleasant sensations
characteristics of persistent pain (7):
referred pain
trigger zones
pain in the absence of detectable tissue damage
pain in a region of sensory deficit
summation and after-reaction with repetitive stimuli
The analgesic effects of stimulating the periaqueductal gray are mediated through which brainstem sites?
parabrachial nucleus, medullary reticular formation, locus coeruleus, raphe nuclei
Referred pain: anginal pain (pain arising from heart muscle that is not adequately perfused with blood)
upper chest wall, radiation into left arm and hand.
referred pain: gallbladder pain
scapular region
referred pain: esophogeal pain
chest wall
referred pain: ureteral pain
lower abdominal wall
referred pain: bladder pain
referred pain: inflamed appendix
anterior abdominal wall around the umbilicus
Gate control theory: activation of the small fibers
leads to inhibition of SG (substantia gelatinosa) fibers which are inhibitory interneurons and this leads to increased activation of teh transmission cells and opening of the gate.
Gate control theory: activation of the large fibers along with the small fibers
leads to excitation of SG neurons and more inhibition of the T cells and closing of the gate.
3 families of endogenous peptides and their receptors
endorphin: mu
enkephalin: delta
dynorphin: Kappa
Serotonin pathways of descending control originate where?
medulla: mainly the nucleus raphe
Norepinephrine pathways of descending control originate where?
in the pons locus coeruleus and subcoeruleus regions.
Chemical mediators of CENTRAL sensitization
excitatory amino acids
opioid peptides
substance P
CGRP (calcitonin gene-related peptide)
Management of hyperalgesia and pain following TISSUE injury (peripheral + central)
This is INFLAMMATORY hyperalgesia.
Peripheral: local anesthetics & NSAIDS
Central: opioids & NMDA receptor antagonises
Management of hyperalgesia and pain following NERVE injuery (peripheral + central)
This is NEUROPATHIC hyperalgesia.
Peripheral: Local anesthetics & capsaicin analogs
Central: Opioids, NMDA receptor antagonists, and tricyclic antidepressants.
Chemical mediators for descending control (3)
5-HT (serotonin)
What are the EAA (excitatory amino acid) receptors of central sensitization?
NMDA - Ca++ influx
AMPA - Ca++ influx (?)
Metabotropic glutamate

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