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Glossary of Neurosciences 2.02 -- Somatosensory Systems: Pain and Temp

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proprioception
sense of static and dynamic position of limbs and body
kinesthesia
ability to feel movement of the limbs and body
stereognosis
ability to recognize objects based on touch alone
what are the four main categories of pathways of the somatosensory system?
(1) spinocerebellar tracts

(2) trigeminal system

(3) dorsal column -- medial lemniscus

(4) lateral spinothalamic tract
spinocerebellar tracts
carry non-conscious proprioceptive info from muscles and tendons on one side of the body to the cerebellum on the same side
trigeminal system
conveys somatosensory info of all submodalities from the head and face to consciousness (neocortex)

info ends up contralateral to where it started
dorsal column -- medial lemniscus
transmits info about fine discrimination touch, vibration, pressure, kinesthesia, sterognosis, and conscious proprioception from the body to consciousness (cortex)

info ends up contralateral to where it started
lateral spinothalamic tract
info about pain and temp from the body to consciousness (cortex)

info ends up contralateral to where it started
which of the four main categories of somatosensory system pathways leads to ipsilateral path?
Spinocerebellar Tracts
which of Brodman's areas make up the somatosensory cortex?
3, 1, and 2
what are the 9 key generalizations to know about somatosensory pathways to consciousness?
Generalizations:

(1) Cell bodies of the primary afferents are located outside of the central nervous system.

(2) Cell bodies of the second order neurons are located either in the spinal cord or in the brainstem.

(3) The axon of the second order neuron crosses the midline (at the level of the second order neuronal cell body).

(4) Cell bodies of the third order neurons are located in the thalamus, within the Ventral Posterior Lateral (VPL) nucleus for the information from the body, and within the Ventral Posterior Medial (VPM) nucleus for information from the head and face.

(5) The thalamic neurons ascend ipsilaterally to the cortex.

(6) The fourth order neurons are located in Primary Somatosensory Cortex, Brodmans, 3, 1, and 2.

(7) If you lesion a pathway before it crosses the midline, there will be a loss of sensation ipsilateral to the lesion.

(8) If you lesion a pathway after it crosses the midline, you create contralateral deficits (loss of sensation contralateral to the lesion).

(9) If you lesion a pathway as it is crossing you create bilateral deficits.
Lateral Spinothalamic Tract
- receptors
- first-order neurons
- second-order neurons
- third-order neurons
- fourth-order neurons
receptors: free nerve endings

first-order neurons: pseudounipolar neurons in the dorsal root ganglion, with their central axon forming the dorsolateral fasciculus; all branches ultimately synapse on the second-order neurons in the dorsal horn

second-order neurons: multipolar neurons located in the dorsal horn of the spinal cord; axons cross at anterior white commissure and ascend contralaterally as the lateral spinothalamic tract

third-order neurons: VPL --> axons go through posterior limb of internal capsule to terminate in dorsal and medial parts of the primary somatosensory cortex

fourth-order neurons: in primary somatosensory cortex
desc the key elements of somatotopy
in lateral spinothalamic tract:

sacral segments most dorsal
lumbar ventral, etc., etc.

in thalamus:

lower body lateral VPL
upper body medial VPL
head VPM

in somatosensory cortex:

lower body medial
upper body lateral
collaterals of the lateral spinothalamic tract
axonal branches that come from the main axonal branch that is ascending towards the thalamus; given off throughout the brainstem and midbrain and synapse in the reticular formation in these structures, as well as in the superior colliculus

e.g. spinoreticular tract, spinomesencephalic tract, spinotectal tract
anterior spinothalamic tract
- general definition
carries a course or crude form of touch
where is the anterior spinothalamic tract in relation to the lateral spinothalamic tract?
just anterior to the lateral spinothalamic tract within the spinal cord and medulla
what are the receptors for the various trigeminal pathways?
free nerve endings, merkel's discs, hair receptors and receptors with encapsulated endings
what are the first-order neurons of the trigeminal pathways?
pseudounipolar cell bodies in the trigeminal ganglion, located outside the CNS
what is the only place within the CNS that contains primary afferent cell bodies?
the mesencephalic trigeminal nucleus
trigeminal pathways: ascending fibers
- project to where?
project to mesencephalic trigeminal nucleus and subserve non-conscious proprioception
trigeminal pathways: fibers
- project to where?
project direct to the principal (aka main/chief) trigeminal nucleus and subserve fine touch
trigeminal pathways: desc fibers
- project to where
project to the spinal trigeminal nucleus
where are the second-order neurons of the trigeminal pathways located?
in the trigeminal nucleus
the axons of the second-order neurons of the trigeminal pathways: main components project to the thalamus through 2 paths. name them
(1) most axons from the main sensory nucleus of V cross the midline and the ascend as the ventral trigeminothalamic tract (axons) to ultimately synpase in the VPM nucleus of the thalamus

(2) some axons project ipsilaterally as the dorsal trigeminothalamic tract, ultimately synapsing in the VPM nucleus of the ipsilateral thalamus
where are the third-order neurons of the trigeminal pathways?
dorsal and ventral trigeminothalamic tracts terminate on neurons in VPM nucleus of the thalamus
where are the fourth-order neurons of the trigeminal pathways located?
primary somatosensory cortex
syringomyelia
central cavitation of the cervical cord; damage to:

(1) ventral white commissure -- bilateral deficits in pain and temp fibers

(2) ventral horns -- flaccid paralysis of the intrinsic muscles of the hands
wallenberg's syndrome
aka lateral medullary syndrome

lesion of the lateral structures of the medulla, can occur as a result of occlusion of the posterior inferior cerebellar artery; results in symptoms of loss of pain and temp on the ipsilateral face, loss of pain and temp on the contralateral side of the body, and ataxia

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