Download TSM7 - Somatosensory Pathways

TSM7: SOMATOSENSORY PATHWAYS
29/09/08
BACKGROUND – SPINAL CORD ANATOMY
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A typical spinal cord cross-section shows two distinct areas:
o Grey matter – a central distorted ‘H’ shape
o White matter – the paler area surrounding the grey matter
The ventral median fissure is a useful orientation structure (ventral = anterior)
The grey matter comprises dorsal and ventral horns which are responsible for sensory and motor
transmission respectively
o Thoracic and upper lumbar segments also have lateral horns which are involved in the ANS
o The grey matter connecting the left and right sides is called the intermediate zone
The white matter is divided into three principal areas: the dorsal, lateral and ventral funiculi
o The dorsal funiculus is further divided into the gracile (medial) and cuneate (lateral) fasciculi
Rexed’s laminae are ten zones that describe different cellular regions of grey matter:
o Zones I to VI inclusively cover the dorsal horn, numbered sequentially dorsal to ventral
o Zones VII to IX make up the ventral horn
 Zone VII makes up the bulk; zone VIII represents a thin medial section
 Zone IX comprises three clustered separate ‘spots’ in the lateral ventral area
o Zone X is effectively the intermediate zone
LEARNING OUTCOMES
Describe the CNS pathways that serve somatosensation
MEDIAL LEMNISCUS (DORSAL COLUMNS)
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The ascending pathway for fine touch, proprioception and vibration
Primary afferents do not synapse in the spinal cord (see below)
Thick sensory fibres (Aα/β) enter the dorsal root medially at the dorsal funiculus and ascend
ipsilaterally through either the gracile or cuneate fasciculus
o The gracile fasciculus conveys sensory information from the lower limbs and trunk
o The cuneate fasciculus conveys sensory information from the upper limbs, trunk and neck
At the medulla the primary afferents terminate at the gracile or cuneate nucleus
o Second order neurons decussate to the contralateral side and group together
At the thalamus they converge at the ventral posterior lateral nucleus (VPL)
The sensory information finally reaches the primary somatosensory cortex (PSSC)
ANTEROLATERAL SYSTEM (SPINOTHALAMIC TRACT)
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The ascending pathway for crude touch, pain, temperature and pressure
Primary afferents synapse in the spinal cord (see below)
Thin sensory fibres (Aδ/C) enter the dorsal root laterally and synapse with cells in the dorsal horn
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Nociceptor afferents synapse in the substantia gelatinosa in the dorsal-most area of the horn
(Rexed’s laminae I to III)
o Other associated afferents synapse in Rexed’s lamina V with so-called lamina V cells
Second order neurones arising from the dorsal horn decussate and pass over to the contralateral
anterolateral funiculus
Passing through the medulla, they reach the ventral posterior lateral nucleus (VPL) in the thalamus
The sensory information finally reaches the primary somatosensory cortex (PSSC)
Distinguish between the structure and function of the dorsal column system and the anterolateral system
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See above
Describe the sensory portion of the trigeminal nerve and the CNS pathways that serve somatosensation from the
face
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Facial somatosensation is picked up directly by sensory fibres of the trigeminal nerve – the fifth (V)
cranial nerve
These fibres converge in the trigeminal sensory nucleus which spans the length of the brainstem into
the upper cervical region of the spinal cord
o Proprioception is picked up in its mesencephalic nucleus region which extends rostrally into
the midbrain
o Fine touch is picked up in its chief sensory nucleus region near the entry-point of CNV
o Pain and temperature are picked up in its spinal nucleus region which extends caudally into
the spinal cord
The mesencephalic and chief sensory nuclei are analogous to the dorsal columns
The spinal nucleus is analogous to the anterolateral system
Second order neurones arising from the trigeminal sensory nucleus decussate in the brainstem and
ascend along the trigeminal lemniscus
These neurones terminate at the ventral posterior lateral nucleus (VPL) in the thalamus
The sensory information finally reaches the primary somatosensory cortex (PSSC)