Lecture notes: Spinal Cord IV – Sensory Systems & Pathways
James Bisley (jbisley@mednet.ucla.edu)
Learning objectives:
1. For the 2 main ascending sensory systems, you should know what information
they carry, where that information comes from and the pathway it follows to
cortex, with a particular emphasis on where information crosses from one side of
the CNS to the other.
2. Understand the basic concept underlying the way that pain information can be
centrally controlled at the level of the spinal cord.
Suggested reading:
Purves (4th ed) Chapters 9 & 10.
Overview
Multiple ascending (sensory) and descending (motor) systems are present within the
spinal cord, and each has a distinctive pathway. Knowledge of the pathways is necessary
for determining the location of lesions within these systems and for understanding their
effects.
One general principle is that most motor and sensory pathways cross the midline in
some region of the CNS. As a result, one side of the cerebral cortex will be associated with
function on the contralateral side of the body. A general rule of thumb is that the sensory
pathways cross at the level of the second order neurons’ cell bodies.
A second general principle is that there are topographic representations of the body at
all levels of the pathways. For instance, in primary somatosensory cortex, information
about the lower body is represented medially and information from the head is represented
most laterally. Usually the arrangement can be deduced from first principles.
Different types of somatosensory information are conveyed by different pathways. Such
organization of the somatosensory system has protective value for it helps guard against
loss of all sensation following localized damage. In this class, we focus on two distinctive
somatosensory systems: the dorsal column-medial lemniscal system and the anterolateral
system (frequently identified as the spinothalamic system).
The pathways described here and in the lecture represent the main route of information
from the periphery to the brain. However, it is important to remember that many of the
afferents entering the CNS have collaterals that synapse locally on motor neurons or
interneurons. These are often used in reflexes and can also be the start of other ascending
pathways, such as in Clarke’s Nucleus.
A. Dorsal Column-Medial Lemniscal System
This pathway conveys fine touch and position sense from the body to the cerebral
cortex.
From the body
1. Larger diameter afferents from receptors in the skin, joints and muscles have their cell
bodies in the dorsal root ganglia. They enter the dorsal horn through the medial
division of the dorsal root and their collaterals proceed to the dorsal columns without a
synapse.
2. Afferents from the lower limbs and lower trunk ascend within the medial column
(gracile fasciculus or tract) while those from the upper trunk and upper limbs ascend in
the lateral column (cuneate fasciculus or tract).
3. The fibers continue in the ipsilateral dorsal column until they reach the lower medulla
where they synapse on the second order neurons in the appropriate dorsal column
nucleus (gracile nucleus or cuneate nucleus).
4. Axons of these second order neurons curve ventromedially as the internal arcuate
fibers and cross the midline of the medulla in the decussation of the medial lemniscus.
The fibers are then grouped together in a bundle called the medial lemniscus that is
located along the midline of the ventral portion of the medulla.
5. As the medial lemniscus heads rostrally, it rotates so that the topographic
representation matches that in the thalamus.
6. The medial lemniscal fibers form synapses on the third order neurons in the ventral
posterior lateral (VPL) nucleus of the thalamus.
7. Fibers from the thalamus then project to the primary somatosensory cortex of the
parietal lobe (Areas 3a, 3b,1 & 2) via the posterior limb of the internal capsule.
Although not passing through the spinal cord, fine touch and position sense from the head
and face get processed in a similar way, but in the brain stem.
From the face
1. Afferents from receptors in the skin have their cell bodies in the trigeminal ganglion.
They then enter the brain stem via the trigeminal nerve. Afferents from receptors in the
joints and muscles also enter the brain stem via the trigeminal nerve, but their cell
bodies reside within the CNS in the mesencephalic nucleus of the trigeminal complex.
2. Independent of where their cell bodies are located, the afferents all synapse on second
order neurons in the sensory or principal nucleus of the trigeminal complex in the pons.
3. Axons from the second order neurons decussate in the pons and join the
trigeminothalamic tract, which runs adjacent to the medial lemniscus.
4. Axons in the trigeminothalamic tract form synapses on the third order neurons in the
ventral posterior medial (VPM) nucleus of the thalamus. This is medial to the VPL,
such that the whole body is represented in the VP complex.
5. Fibers from the thalamus project to the same areas of primary somatosensory cortex.
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B. Anterolateral System
This pathway conveys the sensations of pain, temperature and coarse touch from the
periphery to the cortex.
From the body
1. Smaller diameter fibers from the skin and deeper structures have their cell bodies in
the dorsal root ganglia and enter the spinal cord through the dorsal roots and may
ascend or descend 2-3 segments in Lissauer's tract (the dorsolateral tract) before
penetrating deeper into the dorsal horn.
2. The fibers then form synapses on the second order neurons in several lamina of the
dorsal horn in the spinal cord.
3. The axons of the second order neurons cross the midline of the spinal cord in the
anterior white commissure and assemble in the spinothalamic or anterolateral tract on
the contralateral side.
4. The fibers of the anterolateral tract remain in lateral locations throughout the brainstem
and are eventually located dorsolateral to the medial lemniscus.
5. Upon reaching the thalamus, the axons form synaptic connections with third order
neurons in the ventral posterior lateral (VPL) nucleus of the thalamus. Note that in
addition, some axons also have synapses in other thalamic nuclei that include the
intralaminar and posterior nuclei.
6. Neurons in the ventral posterior lateral nucleus project to the primary somatosensory
cortex.
Note that on their way through the brainstem, many fibers of the anterolateral system send
collaterals into the reticular formation and, by such connections, are believed to influence
the level of arousal. Other fibers in this system terminate within the brainstem rather than
continuing to the thalamus. Some fibers terminate in the reticular formation and others
terminate in the mesencephalon (tectum and periaqueductal gray). Thus, they are referred
to as the spinoreticular and spinomesencephalic tracts.
As for the dorsal column-medial lemniscal system, there is an equivalent pathway for pain,
temperature and coarse touch from the head and face in the brain stem.
From the face
1. Afferents have their cell bodies in the trigeminal ganglion and the ganglia associated
with the Facial (VII), Glosso-pharyngeal (IX) and Vagus (X) nerves and enter the brain
stem via those nerves.
2. Afferents from the trigeminal nerve descend in the spinal trigeminal tract to synapse on
second order neurons in the spinal nucleus of the trigeminal complex (primarily the pas
caudalis) in the medulla. Afferents from IX and X enter through the medulla and
synapse in the same nucleus.
3. Axons from the second order neurons decussate in the medulla and join the
anterolateral tract (although one text states that they travel separately from the
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anterolateral system in the anterior trigeminothalamic tract, we will just refer to it as the
anterolateral tract).
4. Fibers in the anterolateral tract containing information from the head and face form
synapses on the third order neurons in the ventral posterior medial (VPM) nucleus of
the thalamus. Thus, the VP complex contains information from the whole body about
all these senses
5. Fibers from the thalamus project to the same areas of primary somatosensory cortex.
C. Spinocerebellar Pathways
The spinocerebellar tracts will not be traced in detail at this time. However, it is good to
appreciate their general organization, i.e. where they are located in the spinal cord and
how they reach the cerebellum. The major features of the pathways are outlined below:
1. Both dorsal and ventral spinocerebellar tracts are located in the lateral part of the lateral
columns of the spinal cord.
2. These pathways transmit proprioceptive information (non-conscious) from muscle
spindles and Golgi tendon organs and some exteroceptive information from cutaneous
mechanorecptors to the cerebellum.
3. For both tracts, there are slight differences in the specific pathways for the upper and
lower parts of the body. The details of these pathways can be found in the Nolte text.
D. Descending pathways for pain modulation
While ascending pathways such as the anterolateral system convey pain sensation to
higher levels of the nervous system, descending pathways can modulate the sensation of
pain. Multisynaptic, descending pathways from the periaqueductal gray in the midbrain to
the dorsal horn of the spinal cord (or the spinal nucleus of the trigeminal complex) are
capable of decreasing painful sensations. The pathways produce their analgesic effects by
inhibiting the nociceptive transmission in the dorsal horn. Local connections from tactile or
proprioceptive afferents can also affect transmission of nociceptive information in the
dorsal horn.
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