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Welcome to Clinical anatomy of the head, neck and neuronal pathways
Lecture #10
Department of Anatomy
MUNI, MED
Alemeh Zamani, Ph.D.
Spring 2021
Today’s lecture will cover:
1- Somatosensory and Viscerosensory Pathways
2- Pain Pathways and Stress-Induced Analgesia
Today’s lecture will cover:
1- Somatosensory and Viscerosensory Pathways
2- Pain Pathways and Stress-Induced Analgesia
o Inform about objects around us through touch
o Inform about position and movements of our body parts (proprioception)
o Monitor the temperature
o Inform about painful, itchy and tickling stimuli
Somatosensory Pathways
Somathosensory Pathway
1st order somatosensory afferent neurons: pseudounipolar
neuron located in DRG
o Most somatosensory receptors are not specialized
receptor cells and are formed by the terminal endings
of the somatosensory 1st afferents.
Somatosensory Receptor Types
pain, temperature and crude touch
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Peripheral Somatosensory Axons
o Faster propagation of an action
potential in myelinated neurons
than that of unmyelinated
neurons.
Somatosensory Pathways
Lemniscal system
Medial Lemniscal Pathway
o perception with high discrimination ability
o discriminative touch, proprioception
Anterolateral system
Spinothalamic pathway
o perception with low discrimination ability
o crude touch, pain and temperature
Medial Lemniscal Pathway
2nd order neurons:
o Nucleus Gracilis (below T6)
o Nulleus Cuneatus (above T6)
3rd order neurons:
Ventroposterior lateral nucleus of thalamus
Anterolateral Pathway
2nd order neuron: Lamina I to V
3rd order neurons: Ventroposterior lateral nucleus of thalamus
o Anterior spinothalamic tract
o Lateral spinothalamic tract
o Spinotectal tract
o Spinohypothalamic tract
Sensory Homunculus Trigeminal Systems
Somatic sensory representation for most of head and all of face
Somatic sensory pathways:
o Lemniscal system
o Antrolaterlal system
o Trigeminal system
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Trigeminal Nerve (CN V)
Sensory Inputs to Trigeminal Nuclei:
Somatic sensory, tactile, pain and temperature
1st order neurons:
Trigeminal Ganglion cell
2nd order neurons:
Spinal nucleus of CN V & central sensory nucleus of CN V
3rd order neurons:
Ventral posterior medial nucleus of thalamus (VPM)
Trigeminal Pathways
Viscerosensory Pathways
o Carry information from the thoracic, abdominal and pelvic cavities, and from the cardiovascular system
o Utilize autonomic pathways to reach the CNS
o Participate in important reflexes
o Mostly end in the hypothalamus
Viscerosensory fibres go together with sympathetic and parasympathetic nerves.
pseudounipolar neurons in ganglia
visceral organ
dorsal horn or brain stem sensory nuclei (the solitary nucleus)
Visceral Pathways
1st order neurons:
Ganglion cell of spinal cord or inferior ganglion of CN IX or CN X
2nd order neurons:
Lamina I of dorsal horn or solitary nucleus in medulla
Signals are transmitted to reticular formation, hypothalamus and thalamus.
Today’s lecture will cover:
1- Somatosensory and Viscerosensory Pathways
2- Pain Pathways and Stress-Induced Analgesia
In the memory of
Christopher N Honda
University of Minnesota
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International Association for the Study of Pain (IASP):
“Pain is an unpleasant sensory and emotional experience
associated with actual or potential tissue damage or described in
terms of such damage“.
Pain is very personal and individualized
Noxious, adj.
from classical Latin noxius harmful, injurious, guilty (from noxa; harm, injury + -ius)
Noxious stimulus: A stimulus which is damaging to normal tissues
Nociceptor:
A sensory receptor neuron preferentially sensitive to a noxious stimulus or to a stimulus which would
become noxious if prolonged.
Nociceptors
Nociceptors
o nociceptors of Aδ fibres (5-30 meters/sec)
o Aδ mechanical nociceptors
o Aδ thermal nociceptors
o nociceptors of C fibres (0.2-2.0 meters/sec)
o C polymodal nociceptors – react to thermal, mechanical
and chemical stimuli
o silent nociceptors (MIA = mechanically insensitive afferents)
- responsive after inflammation and tissue injury
Nociception
Nociception: all events following damage, or threat of damage to tissue
1. Begins with detection of noxious event by nociceptors
o influenced by non-neuronal cells (e.g. inflammatory and immune cells, keratinocytes)
2. Subsequent activity in CNS neurons and pathways
a. reflex and withdrawal behaviors
b. autonomic responses
c. activity of neurons in “pain” pathways and systems
d. activation of endogenous modulatory systems
3. Perception of pain
Factors that activate nociceptors
Spinoreticular tract
A. Spinothalamic Tracts (STT)
o neospinothalamic
o most cells of synapse in laminae I and V
o axons ascend contralateral
o thalamic terminations
a) VPL: somatotopic projection
b) Central Lateral (CL)
c) Posterior nuclei (PO)
o VPL projects primarily to primary somatosensory cortex
o Major role pain and temperature sensation (sharp, well localized pain)
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B. Spinoreticular Tract System (SRT)
o paleospinohalamic pathway
o many cells in lamina I and deep dorsal horn
o direct projections from spinal cord to reticular formation
a) medulla, pons, and midbrain (multiple levels)
o axons ascend contralateral (with STT)
o project to multiple thalamic and cortical regions
o Important in arousal and attention (diffuse, non-localized pain)
C. Spinoparabrachial Tract
o many cells in lamina I
o axons ascend through dorsal part of lateral funiculus
o axons terminate parabrachial nn. in pons
o parabrachial neurons project to thalamus
- also hypothalamus and amygdala
o cognitive, emotional, affective responses to pain
D. Direct spino-limbic Access
- spino-amygdaloid
- spino-hypothalamic
- spino-septal nuclei
o cells of origin in dorsal and ventral horns
o axons ascend through dorsal part of lateral funiculus
o many bilateral projections to
a. hypothalamus
b. amygdala
c. septal nuclei and nucleus accumbens
o motivational aspects of pain
E. Postsynaptic Dorsal Column Tract System (PSDC)
o most cells in laminae III-IV
o many respond noxious visceral inputs
o PSDC axons ascend ipsilaterally
o axons terminate in dorsal column nuclei
o DCN cells project to contralateral thalamus
- ventrobasal complex of the thalamus
o Important ascending visceral system
-effective for relief of extreme visceral pain
Westlund, Springer 2007
A. Primary Somatic Sensory Cortex (SI)
B. Secondary Somatic Sensory Cortex (SII)
C. Insular Cortex
D. Anterior Cingulate Gyrus
E. Pre-frontal cortex
F. Thalamus
H. Amygdala
Forebrain Regions Related to Nociception (Nociception Matrix)
Referred Pain
o Convergence theory
o Common dermatome hypothesis
o Facilitation or irritable focus
o Learned phenomenon
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Dental Pain
o Aβ + Aδ fibres –first and sharp pain, easily stimulated by application of cold
o C fibres – dull ache, sensitive to inflammatory mediators, changes in blood flow
o pulp + dentin – enormous number of free nerve endings
o Somatic pain: musculoskletal pain and visceral pain
o Neuropathic pain: inflammed pulp or periapex – peripheral and central sensitization hyperalgesia, allodynia and
spontaneous pain
o referred orofacial pain – both source and referral site
Modulating Systems of Nociceptive Pathway
Levels of modulation of nociceptive pathways
o spinal cord (“gate control theory”)- dorsal column medial lemniscal pathway
o peri-aquaeductal gray matter (PAG)
o locus coeruleus
o reticular formation of brain stem (gigantocellular reticular nucleus )
o raphe magnus nuclei
Ryan, Neurobiology 2009
Stress-Induced Analgesia
Pain suppression upon exposure to unconditioned or
conditioned stressful stimuli.
Reading List
Thank you very much for your attention