PNB 2XB3 Lecture 4: Somatosensory
System
RAPID-adapting mechanoreceptors - ✔️✔️Meissner and Pacinian
(dynamic phase QUICKLY becomes static phase)
dorsal root ganglion (DRG) - ✔️✔️(part of the PNS) cluster of cell bodies in the dorsal
root of a spinal nerve (when the action potential reaches the DRG, it splits into 2 - one
travels toward the cell body and the other continues down the axon)
dorsal horn - ✔️✔️the POSTERIOR column of grey matter in the spinal cord (shape
differs depending on spinal segment) - contains SENSORY neuron cell bodies
ventral horn - ✔️✔️the ANTERIOR column of grey matter in the spinal cord (shape
differs depending on spinal segment) - contains MOTOR neuron cell bodies
3 types of primary somatosensory afferent axons - ✔️✔️1) Aβ
2) Aδ
3) C
Aβ axon - sensory function - ✔️✔️touch (mechanoreceptive)
Aβ axon - receptor type - ✔️✔️Merkel, Meissner, Pacinian, Ruffini cells
Aβ axon - axon diameter - ✔️✔️6-12 microns
Aβ axon - conduction velocity - ✔️✔️35-75 m/s
Aδ axon - sensory function - ✔️✔️pain and temperature (thermoceptive and
nociceptive)
dermatome - ✔️✔️the area of skin supplied by the sensory axons of a single spinal
nerve
Aδ axon - receptor type - ✔️✔️free nerve endings (myelinated)
Aδ axon - axon diameter - ✔️✔️1-5 microns
Aδ axon - conduction velocity - ✔️✔️5-30 m/s
, C axon - sensory function - ✔️✔️pain, temperature, and itch (thermoceptive and
nociceptive)
C axon -receptor type - ✔️✔️free nerve endings (unmyelinated)
C axon - axon diameter - ✔️✔️0.2-1.5 microns
C axon - conduction velocity - ✔️✔️0.5-2 m/s
variation in conduction velocity exists because - ✔️✔️there is variation in myelination
and axon diameter (speed: Aβ > Aδ > C)
why are there TWO pain sensations? - ✔️✔️1) first sharp pain sensation arrives fastest
via Aδ axon
2) second duller pain sensation is delayed because it travels at a slower speed via C
axon
dorsal column medial lemniscus (DCML) pathway - ✔️✔️- decussates in the medulla
- synapses in the dorsal column nuclei, thalamus (VPL nucleus), and somatosensory
cortex
spinothalamic (anterolateral) tract (STT) - ✔️✔️- decussates in the dorsal horn
- synapses in the dorsal horn, thalamus (VPL nucleus), and somatosensory cortex
DCML pathways axons go as far ___ as they can - ✔️✔️medial
STT pathway axons go as far ___ as they can - ✔️✔️lateral
topography of somatosensory tracts - ✔️✔️DCML (dorsal):
CTLS SLTC
STT (ventral):
SLTC CTLS
Brown- Sequard syndrome - ✔️✔️caused by damage to one half of the spinal cord,
resulting in paralysis or loss of sensation on one side of the body
Microneurography - ✔️✔️technique used in which a recording electrode inserted into
the nerve measures action potentials as the skin is stimulated by a probe - used to
determine the receptive field sizes for different mechanoreceptors
receptive field of a sensory neuron - ✔️✔️the region of sensory space in which a
stimulus will modify the firing rate of that neuron
System
RAPID-adapting mechanoreceptors - ✔️✔️Meissner and Pacinian
(dynamic phase QUICKLY becomes static phase)
dorsal root ganglion (DRG) - ✔️✔️(part of the PNS) cluster of cell bodies in the dorsal
root of a spinal nerve (when the action potential reaches the DRG, it splits into 2 - one
travels toward the cell body and the other continues down the axon)
dorsal horn - ✔️✔️the POSTERIOR column of grey matter in the spinal cord (shape
differs depending on spinal segment) - contains SENSORY neuron cell bodies
ventral horn - ✔️✔️the ANTERIOR column of grey matter in the spinal cord (shape
differs depending on spinal segment) - contains MOTOR neuron cell bodies
3 types of primary somatosensory afferent axons - ✔️✔️1) Aβ
2) Aδ
3) C
Aβ axon - sensory function - ✔️✔️touch (mechanoreceptive)
Aβ axon - receptor type - ✔️✔️Merkel, Meissner, Pacinian, Ruffini cells
Aβ axon - axon diameter - ✔️✔️6-12 microns
Aβ axon - conduction velocity - ✔️✔️35-75 m/s
Aδ axon - sensory function - ✔️✔️pain and temperature (thermoceptive and
nociceptive)
dermatome - ✔️✔️the area of skin supplied by the sensory axons of a single spinal
nerve
Aδ axon - receptor type - ✔️✔️free nerve endings (myelinated)
Aδ axon - axon diameter - ✔️✔️1-5 microns
Aδ axon - conduction velocity - ✔️✔️5-30 m/s
, C axon - sensory function - ✔️✔️pain, temperature, and itch (thermoceptive and
nociceptive)
C axon -receptor type - ✔️✔️free nerve endings (unmyelinated)
C axon - axon diameter - ✔️✔️0.2-1.5 microns
C axon - conduction velocity - ✔️✔️0.5-2 m/s
variation in conduction velocity exists because - ✔️✔️there is variation in myelination
and axon diameter (speed: Aβ > Aδ > C)
why are there TWO pain sensations? - ✔️✔️1) first sharp pain sensation arrives fastest
via Aδ axon
2) second duller pain sensation is delayed because it travels at a slower speed via C
axon
dorsal column medial lemniscus (DCML) pathway - ✔️✔️- decussates in the medulla
- synapses in the dorsal column nuclei, thalamus (VPL nucleus), and somatosensory
cortex
spinothalamic (anterolateral) tract (STT) - ✔️✔️- decussates in the dorsal horn
- synapses in the dorsal horn, thalamus (VPL nucleus), and somatosensory cortex
DCML pathways axons go as far ___ as they can - ✔️✔️medial
STT pathway axons go as far ___ as they can - ✔️✔️lateral
topography of somatosensory tracts - ✔️✔️DCML (dorsal):
CTLS SLTC
STT (ventral):
SLTC CTLS
Brown- Sequard syndrome - ✔️✔️caused by damage to one half of the spinal cord,
resulting in paralysis or loss of sensation on one side of the body
Microneurography - ✔️✔️technique used in which a recording electrode inserted into
the nerve measures action potentials as the skin is stimulated by a probe - used to
determine the receptive field sizes for different mechanoreceptors
receptive field of a sensory neuron - ✔️✔️the region of sensory space in which a
stimulus will modify the firing rate of that neuron
