Neuroanatomy Ch 7 276-287
Somatosensory Pathway Anatomy
Somatosensory refers to bodily sensations of touch, pain, temperature, vibration, and
proprioception (limb or joint sense). Two main pathways for somatic sensation:
1. Posterior column-medial lemniscal pathway – conveys proprioception, vibration, and
fine, discriminate touch
a. Crosses at the internal arcuate fibers in the lower medulla
2. Anterolateral pathway includes the spinothalamic tract and conveys temp, pain, and
crude touch
a. Crosses at the anterior commissure of spinal cord
Four types of sensory neuron fibers are classified according to diameter:
1. Aα – largest, myelinated fiber with receptors of muscle spindle/golgi tendon, and are
involved with proprioception
2. Aβ – myelinated fiber innervating muscle spindle, meissner’s corpuscle (superficial
touch), merkel’s receptor (superficial touch), pacinian corpuscle (deep touch/vibration),
ruffini ending (deep touch/vibration), and hair receptor (touch/vibration)
3. Aδ – myelinated fiber innervating bare nerve endings for pain, temp (cold), and itch
4. C – smallest, unmyelinated fiber: bare nerve ending for pain, temp (warm) and itch
Sensory cell bodies are located in the dorsal root ganglia which has a stem axon that bifurcates
resulting in one long process that conveys sensory information from periphery and one process
that carries information into the spinal cord
-peripheral region innervated by a single nerve root level is called a dermatome
Posterior Column-Medial Lemniscal Pathway – carry large-diameter, myelinated axons with
information about proprioception, vibration, and fine touch from dorsal root entry zone
-many of those axons enter ipsilateral posterior columns and ascend to posterior column nuclei
in medulla
-some axon collaterals enter spinal cord gray matter and synapse onto interneurons and
motor neurons
-posterior columns are organized somatotopically with fibers adding on laterally from higher
levels as posterior columns ascend:
-most medially you have the gracile fasciculus (legs/trunk), followed by the more lateral
cuneate fasciculus (upper trunk above T6/arms/neck)
-first-order sensory neurons in these fasciculi synapse onto second-order
neurons in the nucleus gracilis and nucleus cuneatus
-axons of the second order neurons decussate as internal arcuate fibers and form the medial
lemniscus on the other side of the medulla
-medial lemniscus is initially vertical but later becomes lateral and inclined as it ascends
-somatotopic organization in medial lemniscus assumes vertical position in medulla such that
feet are represented more ventrally and inclines again in pons and midbrain such that arms are
represented medially and legs more laterally
-reversal in somatotopic organization: for medial lemniscus in the pons and mibrain the
feet are lateral, while in the posterior columns the feet are medial
-next major synapse occurs when medial lemniscus axons terminate in ventral posterior lateral
nucleus (VPL) of the thalamus; neurons here project through posterior limb of internal capsule
in the thalamic somatosensory radiations to reach primary somatosensory cortex
-synaptic inputs to primary somatosensory cortex from face and body occur in cortical layers IV
and III
Spinothalamic Tract and Other Anterolateral Pathways – smaller-diameter and unmyelinated
axons carrying info about pain ant temp enter the cord via dorsal root entry zone, but first make
synapses immediately in gray matter of cord; in the dorsal horn marginal zone and deeper in
dorsal horn in lamina V
-some axons ascend or descend a few segments in Lissauer’s Tract before entering gray matter
-axons from second-order sensory neurons in central gray cross over in the anterior commissure
and ascend in the in the anterolateral white matter
-it takes 2-3 spinal segments for decussating fibers to reach the opposite side, so a lateral cord
lesion will affect contralateral pain and temperature sensation beginning a few segments below
the level of the lesion
-anterolateral pathways have a somatotopic organization where feet are most lateral and
higher trunk/arms added on medially through anterior commissure during ascension
-this is preserved as anterolateral pathways pass through brainstem
-when reaching medulla, anterolateral pathways are located laterally, running in the groove
between inferior olives and inferior cerebellar peduncles
-they enter the pontine tegmentum to lie lateral to medial lemniscus in pons and midbrain
-anterolateral pathway consists of 3 tracts: spinothalamic, spinoreticular, and
spinomesencephalic tracts
1. Spinothalamic Tract – mediates discriminative aspects of pain and temp sensation such as
location and intensity of stimulus, relays to the ventral posterior lateral nucleus (VPL) of
thalamus  information is conveyed to primary somatosensory cortex
a. Spinothalamic projections also go to other thalamic nuclei: intralaminar thalamic
nuclei and mediodorsal nuclei which participate with the spinoreticular tract in an
older pathway responsible for conveying emotional and arousal aspects of pain
2. Spinoreticular Tract – terminates on medullary-pontine reticular formation, which projects
to the intralaminar thalamic nuclei  projecting diffusely to entire cerebral cortex and
involved in behavioral arousal
3. Spinomesencephalic Tract – projects to midbrain periaqueductal gray matter and superior
colliculi; periaqueductal gray participates in central pain modulation
-in addition to pain and temp, anterolateral pathways also convey crude touch sensation
Somatosensory Cortex – from thalamic VPL and VPM nuclei, somatosensory info is conveyed to
the primary somatosensory cortex in the postcentral gyrus
-this is somatotopically organized with the face most laterally and leg most medially
-info from primary somatosensory cortex is conveyed to the secondary somatosensory
association cortex inside the sylvian fissure along the superior margin called the parietal
operculum
-secondary somatosensory cortex is also somatotopic
-further processing occurs in the superior parietal lobe
-lesions of this cortex and adjacent regions cause deficits referred to as cortical sensory loss
Central Modulation of Pain – pain modulation involves interactions between local circuits at
level of spinal cord dorsal horn and long-range modulatory inputs
-in gate control theory – sensory inputs from large, nonpain Aβ fibers reduce pain
transmission through dorsal horn
-periaqueductal gray receives inputs from hypothalamus, amygdala, and cortex, and inhibits
pain transmission in dorsal horn via a region in pontomedullary junction called rostral ventral
medulla (RVM)
-RVM contains serotonergic neurons of the raphe nuclei that project to spinal cord,
modulating pain in the dorsal horn
-RVM also sends inputs mediated by substance P to the locus ceruleus in the pons,
which in turn sends noradrenergic projections to modulate pain in dorsal horn of spinal
cord
-Histamine also contributes to modulation of pain through H3 receptors
-Opiates like morphine exert analgesic effects through receptors found throughout nervous
system, including those on peripheral nerves and in dorsal horn
-opiate receptors and endogenous opiate peptides such as enkephalin, β-endorphin, and
dynorphin are found in high concentrations in pain modulatory pathways
-Enkephalin and Dynorphin neurons are in periaqueductal gray, RVM, dorsal horn
-β-endorphin neurons are in hypothalamus that project to periaqueductal gray
The Thalamus – processing center in brain which relays nearly all pathways that project to
cerebral cortex through synaptic connections
-often thought of as major sensory relay station, but it also conveys motor inputs from
cerebellum, basal ganglia, limbic inputs (sleep/wake)
-thalamic nuclei receive dense reciprocal feedback connections from cortical areas to which they
project; corticothalamic projections outnumber thalamocortical projections
-thalamus is part of the diencephalon with hypothalamus and epithalamus
-diencephalon is rostral to the midbrain
-hypothalamus is just ventral to the thalamus
-epithalamus has small nuclei including habenula, parts of pretectum, and pineal body
-thalamus is divided into medial nuclear group, lateral nuclear group and anterior nuclear
group by a Y shaped white matter structure called internal medullary lamina
-nuclei within internal medullary lamina are called intralaminar nuclei
-midline thalamic nuclei are an additional thin collection of nuclei adjacent to the 3rd ventricle
-thalamic reticular nucleus forms an extensive but thin sheet enveloping lateral aspect of
thalamus
Relay Nuclei – most of the thalamus is made up of these, which receive inputs from numerous
pathways and then project them to the cortex; most nuclei lie in the lateral thalamus… all
sensory modalities besides olfaction have relays in the lateral thalamus
1. Ventral posterior lateral nucleus (VPL) – receives input from medial lemniscus,
spinothalamic tract and relays to somatosensory cortex:: functions to relay somatosensory
inputs to cortex
2. Ventral posteromedial nucleus (VPM) – receives input from trigeminal lemniscus,
trigeminothalamic tract and taste inputs and relays to somatosensory and taste cortex::
functions to relay somatosensory cranial nerve inputs and taste to cortex
3. Lateral Geniculate Nucleus (LGN) – main input from retina and relays to primary visual
cortex
4. Medial Geniculate Nucleus (MGN) – receives input from inferior colliculus and sends output
to primary auditory cortex to relay auditory inputs
5. Ventral Lateral Nucleus (VL) – inputs from internal globus pallidus, deep cerebellar nuclei,
substantia nigra and pars reticulate and sends output to motor, premotor, and
supplementary motor cortex to relay basal ganglia and cerebellar inputs to cortex
6. Ventral Anterior Nucleus (VA) – inputs from substantia nigra pars reticulate, internal globus
pallidus, deep cerebellar nuclei and sends output to frontal lobe, including prefrontal,
premotor, motor, and supplementary motor cortex to relay basal ganglia and cerebellar
inputs to cortex
7. Pulvinar – receives input from tectum (extrageniculate visual pathway) and sends signals to
parietotemporooccipital association; functions as behavioral orientation toward relevant
visual stimuli
8. Mediodorsal Nucleus (MD) – receives input from amygdala, olfactory cortex, limbic basal
ganglia and relays the input to frontal cortex (functions in limbic pathways, major relay to
frontal cortex)
9. Anterior Nucleus – receives input from mammillary body, hippocampal formation and relays
signal to the cingulate gyrus to function in limbic pathways
Intralaminar Nuclei – are inside internal medullary lamina and receive inputs from multiple
pathways and have reciprocal connections with cortex; main inputs/outputs are from basal
ganglia, and can be divided into caudal intralaminar nucei and rostral intralaminar nuclei
1. Centromedian Nucleus – part of caudal intralaminar nuclei, receives inputs from globus
pallidus, (ascending reticular activating systems) ARAS, and sensory pathways and delivers
them to striatum, cerebral cortex; functions as a motor relay for basal ganglia
2. Rostral intralaminar nuclei relay deep cerebellar, ARAS, and sensory pathways to cerebral
cortex + striatum to maintain alert consciousness, motor relay for basal ganglia/cerebellum
Reticular Nucleus – forms thin sheet lateral to the rest of the thalamus and medial to internal
capsule
-only nucleus that does not project to the cortex
-receives inputs from other thalamic nuclei and projects them back to thalamus
-consists of almost pure GABAergic neurons for inhibition
-regulates thalamic activity