NS10 Lecture 10 of 63 in the Neuroscience Module "Sensory Receptors" [Physiology]

1. NS10 – SOMATOSENSORY PATHWAY
NS10 – SOMATOSENSORY PATHWAY
(corrections/clarifications are in blue)
I. OVERVIEW
A. Function of the somatosensory pathway:
1. To collect sensory information from sensory receptors in the skin, viscera, joints and muscles
2. To transmit sensory information to the brain for evaluation and integration.
B. Classification based on the source of sensory information
1. Proprioreception – information about the relative position and movement of body segments to each other
and in space – muscle tension, muscle length, muscle velocity
2. Exteroreception – information about the external environment
a. Mechanoreception: discriminative touch - form, texture, vibration, stretch
b. Nociception: pain - sharp pain, burning pain
c. Thermoreception: temperature - warmth, cold
3. Interoreception – information from the internal organs
C. Components of the somatosensory pathway
1. Sensory receptors: mechanoreceptors, nociceptors, thermoreceptors, proprioreceptors
2. Sensory nerve fibers
a. first order neurons – primary sensory afferents – unipolar neurons (dorsal root ganglion cells)
b. second order sensory neurons
c. third order sensory neurons (thalamus)
d. fourth order sensory neurons (somatosensory cortex)
3. Higher centers – cerebral cortex, cerebellum, periaqueductal gray
D. Organization of the somatosensory pathways
1. Dorsal spinal cord pathways
a. dorsal column/ medial leminiscal pathway (body; discriminative touch)
b. trigeminal pathway (face; depends on final target can be for touch, pain or proprioception)
c. spinocerebellar pathway (body; proprioception)
2. Ventral spinal cord pathways
a. spinothalamic pathway (body; pain & temperature)
b. spinoreticular pathway body; attentional, arousal)
c. spinomesencephalic pathway (body; emotional response)
E. Transmission and interpretation of somatosensory information
1. Receptor perception of stimuli
2. Receptive field
3. Sensory modality: an aspect of a stimulus; coding of information (e.g. mechanoreception, nociception0
4. Types of sensory afferents
5. Adaptation: ability of receptor to change its response to constant stimuli.
6. Dermatome: an area of skin innervated by a pair of dorsal root nerve. The body can be divided into
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2. NS10 – SOMATOSENSORY PATHWAY
regions (dermatomal map) that are supplied by a single nerve and this map can be used to identify sites of
nerve damage
E. Transmission and interpretation of somatosensory information
7. Modality segregation – sensory neurons are organized according to the type of information that they
transmit and become part of different ascending spinal tracts.
7. Somatosensory map - a complete map of the body exists in the somatosensory cortex.
II. SOMATOSENSORY RECEPTORS
A. MECHANORECEPTORS
TABLE 1: Comparison of cutaneous mechanoreceptors
RECEPTIVE GROSS VS. DETECTION AXON
STIMULI ADAPTATION
FIELD FINE OF STIMULI TYPE
large, >20 gross
Pacinian unlocaliized beginning &
rapid mm (70-
vibratiion end
corpuscle
deep 1000Hz)
small, 3-4
Meissner’s fine beginning &
skin stretch rapid mm
(10-200 Hz) end
corpuscle
superficial
myelinated
type Aβ
small, 3-4
Merkel’s fine
skin curvature slow mm sustained
(30 Hz)
disc
superficial
large, >10
Ruffini’s directional gross
slow mm sustained
skin stretch (15 Hz)
ending
deep
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3. NS10 – SOMATOSENSORY PATHWAY
B. THERMORECEPTORS & NOCICEPTORS
TABLE 2: COMPARISON OF THERMO AND NOCICEPTORS
NERVE ENDING AXON TYPE DETECTION OF RECEPTIVE FIELD
STIMULI
THERMORECEPTORS
free unmyelinated 30-45oC 1 -5 mm
Warm receptors
type C
free lightly myelinated 10 - 35 oC 1-5 mm
Cold receptors
type Aδ
NOCICEPTORS
lightly myelinated strong pressure
free 2-3 mm
Mechanical
type Aδ from sharp objects
nociceptor
lightly myelinated burning heat or
free 2-3 mm
Thermal
type Aδ extreme cold
nociceptor
lightly myelinated extreme in pH,
free 2-3 mm
Chemical
type Aδ irritants
nociceptor
combination of
Polymodal unmyelinated mechanical, thermal
free
type C (heat & cold) and
nociceptor
chemical stimuli
C. PROPRIORECEPTORS
TABLE 3: COMPARISON OF PROPRIOCEPTORS
RECEPTOR FUNCTION STIUMLI AXON TYPE
Ruffini’s ending
prevent
hyperflexion, stretch
JOINT RECEPTOR
Panciniform
hyperextension
corpuscles
encapsulated
myelinated
ending with a stretch change in muscle
type Aα
MUSCLE SPINDLE
primary and tendon tap length
secondary ending
terminals within
GOLGI TENDON change in muscle
collagen fibers of
tension
ORGAN stretch
tendon
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4. NS10 – SOMATOSENSORY PATHWAY
Table 4. Comparison of dorsal spinal cord pathways (pathways that are part of the learning objectives
are in red)
DORSAL SPINAL Dorsal column/ medial trigeminal Spinocerebellar
CORD lemniscal
PATHWAYS
body face body
REGION
conscious perception of touch, pain/temperature, subconsious proprioceptive
SENSORY
touch proprioception posture, balance,
INFORMATION
coordination of skilled
movement
mechanorecpetors of skin, mechanoreceptors, muscle and joint receptors
RECEPTORS
joints, muscle nociceptors, from lower body
thermoreceptors,
proprioceptors
dorsal root ganglia: cranial nerve ganglia dorsal root ganglia
FIRST ORDER
- fasciculus gracilis (T7 and
NEURONS
below; legs and trunk)
- fasciculus cuneatus
(above T7; arms and trunk)
brain stem or trigeminal nuclei synapse on neuron in
SECOND ORDER
caudal medulla: Clarke’s Nucleus
NEURONS
- gracilis nucleus
project through projects ipsilaterally via
- cuneate nucleus tigeminothalamic tract posterior spinocerebellar
tract
axons run ventrally = decussate
internal arcuate fibers; then
decussate to become
medial lemniscus
VPL (ventral posterior VPMN (ventral
THIRD ORDER
lateral) nucleus of the posteromedial) nucleus of
NEURONS
thalamus thalamus
pass through posterior limp
of internal capsule
somatosensory cortex somatosensory cortex cerebellum
HIGHER CENTER
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5. NS10 – SOMATOSENSORY PATHWAY
Table 5. Comparison of ventral spinal cord pathways
VENTRAL SPINAL anterolateral spinoreticular tract spinomesencephalic
CORD PATHWAYS (spinothalamic) tract
body body body
REGION
pain & temperature attentional mechanism & emotional response;
SENSORY
arousal; noxious stimuli noxious stimuli
INFORMATION
mechanoreceptors nocieptors nocieptors
RECEPTORS
(touch = anterior stt)
nociceptors
thermoreceptors
(pain & temp = lateral stt)
dorsal root ganglia dorsal root ganglia dorsal root ganglia
FIRST ORDER
NEURONS
dorsal horn dorsal horn dorsal horn
SECOND ORDER
NEURONS
decussate in spinal cord ascend as ipsilateral ascend as ipsilateral
spinoreticular tract spinoreticular tract
ascend in either anteror or
lateral spinothalamic tract
anterior and lateral tracts
run together as spinal
lemniscus in medulla
pass through pons and
midbrain
VPL (ventral posterior
THIRD ORDER
lateral) nucleus of the
NEURONS
thalamus
somatosensory cortex medullary, periaqueductal gray (PAG)
HIGHER CENTER
pontine, and in midbrain
mesencephalic reticular
formation mediates endogenous
analgesia & control of
aversive behaviour
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