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What are the features of Meissner corpsucles?
- found beneath epidermis in glabrous skin
- commin in lips and perioral
- layered schwann cells interdigetating myelinated axons
- low threshold (high sensitivity)
- low-freq vibrations
- RA (rapidly adapting)
- single afferent innervates multiple corpuscles
- small receptive field with "hot spots"
- perceive: bump, texture, grip from slip
What are the features of Pacinian corpsucles?
- variable size
- single axon
- low threshold (high-sensitivity)
- high freq vibrations
- RA (bec layered)
- Large receptive field
- perceive: fine textures, blowing on hand
What are Merkel's disks?
- mechanoreceptors just beneath epidermis
- contain vesicles of peptides that modulate nerve terminal
- low threshold (high-sensitivity)
- multiple per axon
- overlapping receptive fields
- perceive: Shape(firing) and force, coarse textures (braille)
what are Ruffini's corpuscles?
- vary in length
- slngle axon
- collagen thru and binds CT on ends
- low threshold (high sensitivity)
- large receptive field
- sensitive to cutaneous stretching
What is the difference between kinesthesia and proprioception?
- kinesthesia: awareness of body position and movement
- proprioception: sub-concious lnfO for feedback control of posture and precise movements
what are the features of muscle spindles?
- Stretch receptor: sensitive to length changes
- Gamma fibers
- la: all fibers, patellar reflex. dynamic (rate of change)
- II: nuclear chain fibers, static (absolute change)
Explain what happens in the knee-jerk reflex.
- 1. Tap stretches muscle-activates Ia afferent
- 2. tells alpha-motor neuron that muscle is too stretched
- l. alpha-motor increases extensor muscle activity
- 4. interneuron inhibits antagonist alpha to relax flexor
what does the golgi-tendon organ do?
- @ junction between muscle fibers and associated tendon
- contraction causes collagen to produce response
- Ib (large myel1nated) afferent
- SA (slow adapt)
- senses: change in tension
what does a joint receptor do?
- similar to Ruffini end organs in joints
- respond to joint angle over small movement ranges usually near extremes
What are the first order neurons in the DC-MlS?
- cell bodies in DRG
- pseudo-unipolar neurons
- enter dorsal horn of spinal cord ipsilaterally
- topographic representation: medial to lateral superiorly
- below T6: Gracile tract
- above T6: Cuneate tract
Where do the 2nd order neurons in the DC-MLS synapSe?
- caudal medulla,: Gracile and'cuneate Nucleus
- Axons form Internal Arcuate Fibers that decussate to contralateral medial Lemniscus
Where do the 2nd order axons terminate?
- In the Ventral Posterior Lateral(VPL) nucleus of the Thalamus
- VPL topograpy: legs = lateral, arms = medial
What is the facial sensory pathway?
- Pseudo-unipolar cell bodies in trigeminal ganglion
- EXCEPT PROPRIOCEPTION: MESENCEPHALIC nucleus in CNS (has motor outputs too jaw-jerk)
- 2nd order: principle sensory nucleus in PONS
- some decussate and join TRIGEMINOTHALMIC TRACT (adjacent to medial lemniscus)
- others(oral cavity insides) DONT decussate and join ipsilateral TRIGEMINOTHALMIC TRACT
- 2nd axons: teminate in VPM(Ventral Posterior Medial) nucleus of Thalamus
- project to PRIMARY SOMATIC-SENSORY CORTEX via Posterior Limb of Internal Capsule
What are the divisions of the Somatic Sensory cortex?
- Postcentral gyrus (sensory)
- 1: tactile (several digits), texture
- 2: tactile & proprioception (several digits),complex stimuli (specific shapes), coordiation, shape & size
- 3a: proprioception
- 3b: tactile (usually on single digit), texture & shape
Explain the somatosensory cortex topography.
- medial-sacral (1st in)
- lateral-trigeminal (last in)
- things highly used are more represented leading to better two-point discrimination (fingers, thumb, lip, cheek, nose, toe etc)
What are the features of free nerve endings?
- high threshold (low-sensitivity)
- tapping, squeezing, rubbing, pinching, stretching
- A-delta fibers
- non-discriminative touch
What are the features of thermal recptors?
- free nerve endings in dermis (A-delta)
- no mechanical stimulation
- Heat (35-45 C) and cold 17-35 C increasing reponses for extreme temps
What are the features of mechanical nociceptors?
- free nerve endings in dermis (myelinated A-delta)
- multiple "hot spots" single afferent
- mechanical injury associated w tissue damage
- also have thermo-nociceptors >45, <17 SAME RECEPTOR
What are the features of C-polymodal nociceptors?
- dermis, 1-2mm diameter 1 or 2 hot spots single afferent
- mechanical, thermal and chemical
- unmyelinated C fibers
- note:chemonociceptors respond to bradykinin, H+ and foreign irritants (venom)
Are A-delta and C fibers only in the dermis?
- No, also in pulp of tooth
- they are smaller and slower than A-beta and alpha
What kind of dental pains are transmitted by A-delta and C fibers?
- A-delta = sharp dental hypersensitivity, smaller receptor areas acute
- C: dull, throbbing (tooth ache)
Explain the features of the Vanilloid receptor (VR-1).
- TRP (transient receptor potential family
- respond to heat >45, H+ and capsacin
- capsaicin desensitizes recptor
What does substance P do?
- released from pain fiber stimlation to vasodialate and activate immune response (mast cell or neutrophil release histamine -> + feedback on nociceptor)
- positive feedback can lead to allergic reaction
Exlain the central pain pathway: sensory discriminative component.
- pseudo-unipolar neuron cell bodies in DRG
- enter dorsal horn or spinal cord
- branch into ascending and descending collaterals to form DORSOLATERAL TRACT of LISSAUER
- then synapse in gray matter of dorsal horn (2nd order)
- 2nd order: decussate through the anterior white comissure immediately and form SPINOTHALAMIC TRACT or anterolateral tract
- neurons terminate in the VPL (ventral posterior lateral) nucleus of thalamus
- VPL: lower = lateral
What is the topography of the spinothalamic tract?
- lateral is sacral
- medial is neck (newest push oldest lateral)
What is the consequence of multiple levels of decussation in the anterolateral?
- a spinal hemi-lesion will create a particular pattern of sensory loss
- one side will have reduced pain and temperature sensation
- while other side witll have reduced two-point discrimination, vibration or proprioception
What is the facial nociceptor pathway?
- 1st: pseudo-unipolar bodies in Trigeminal ganglion and ganglia associated with VII, IX, and X
- enter brain stem and DESCEND to SPINAL TRIGEMINAL TRACT to the medulla where they synapse onto neurons of SPINAL NUCLEUS of TRIGEMINAL COMPLEX primarily the pars caudalis
- 2nd order decussation
- pars caudalis topography (upside down compared to anatomy of face) mouth rostral, back head (caudal)
- 2nd order axons dessucate immediately and join the ascending TRIGEMINOTHALAMIC TRACT to terminate int he VPM of thalamus
- 3rd order carry pain info to Primary and Secondary somatic-sensory cortex
- Cortex: localization
- Sub-cortical: perception
- Paleospinothalmic: suffering component so you avoid in future (reduced by benzodiazepines)
- axons may terminate elsewhere in INTRALAMINAR NUCLEI(thalamus), RETICULAR FORMATION(PARABRACHIAL NUCLEUS) and the PERI-AQUEDUCTAL GREY)
Where do projections from the intralaminar nuclei go?
- INSULA and CINGULATE CORTEX: higher cortical functions
- help quantify, worth and behavior of pain
Where do projections from the parabrachial nucleus go?
- HYPOTHALAMUS and AMYGDALA (emotional part of pain)
- involved with motivation and affect
- PERI-AQUEDUCTAL GREY: involved in the descending control of pain, gray matter around 3rd and 4th ventricles
- filters to RAPHE NUCLEI: inhibitory interneuron decrease transmission of pain from C-fiber via endogenous opiod (enkephalin)
How is pain locally controlled?
- through interactions with dorsal column and anterolateral system regulates pain perception
- stimulation of dorsal column ANTIDROMICALLY induce analgesia (rub injury)
- A-alpha and A-beta excite interneurons that reduce the transmission of pain information
- Descending fibers excite interneurons that reduce the transmission of pain information.