The Spinal Cord II
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Spinal cord I | Main Anatomy Index | Blood supply of the spinal cord
Last updated 30 March 2006
Spinal Cord II
Spinal Grey Matter
The posterior horn consists mainly of interneurons.
The processes of these remain
within the spinal cord and of tract cells whose axons
collect into long ascending sensory pathways.
- This area of grey matter contains 2 prominent parts:
- The substantia gelatinosa;
- And the body of the posterior horn.
The Substantia Gelatinosa
This is a distinctive region that caps
the posterior horn at all spinal
In myelin-stained preparations this region looks pale compared with the rest of the grey
- It deals mostly with finely myelinated and unmyelinated sensory fibres that carry pain
and temperature information.
This is relatively pale staining area between the substantia gelatinosa and the surface
of the cord.
The Body of the Posterior Horn (Nucleus Proprius)
This consists mainly of interneurons and tract cells.
These transmit many types of somatic
and visceral sensory information.
In this respect it functionally overlaps parts of the intermediate grey matter.
This contains the cell bodies of the large
motor neurons that supply skeletal muscle.
- These alpha motor neurons (lower motor neurons) are the
only means by which the nervous
system can exercise control over body movements, whether voluntary
- Destruction of these neurons supplying a muscle, or interruption
of their axons causes complete paralysis of
- Lower motor neuron lesions cause paralysis
of a type called flaccid paralysis.
- Reflex contractions can no longer be elicited, and the muscle slowly atrophies.
- Alpha motor neurons occur in groups.
- They are separated from one another by areas of interneurons.
- The groups that innervate axial muscles are medial to those that innervate limb
- In the cervical and lumbar
enlargements, which contain the limb motor neurons,
the anterior horns are enlarged
- Smaller gamma motor neurons are interspersed
with alpha motor neurons.
- They innervate the intrafusal muscle fibres of muscle spindles.
Intermediate Grey Matter
This is intermediate to the anterior
and posterior horns.
It has some characteristics of both and also contains
the spinal preganglionic autonomic neurons.
In addition, at some levels it includes a distinctive region called the dorsal nucleus (of Clarke).
- From T1 through L2 or L3,
the preganglionic sympathetic neurons lie in a column of cells, the intermediolateral
- This forms the pointy lateral horn of the spinal grey
- Their axons leave through the ventral
- In segments S2 to S4, corresponding cells form the sacral parasympathetic nucleus but do not
form a distinct lateral horn.
- Their axons leave through the ventral roots and synapse on the postganglionic
parasympathetic neurons for the pelvic viscera.
Nucleus Dorsalis (Clarke's nucleus)
This is a rounded collection of large
cells located on the medial surface of the base of the posterior horn
from about T1 to L2 or L3.
It is particularly prominent at lower
- This is an important relay nucleus for the transmission of information to
- It also plays a role in forwarding proprioceptive information
from the leg to the thalamus.
- It is considered by many to be part of the posterior horn, due to its role in sensory
In 1952 Rexed devised a system for subdividing the grey matter
of the cat's spinal cord.
The same system has since been applied to the cords of other
mammals, including humans.
||Some spinothalamic tract cells
||Modulate pain and temperature
|Body of posterior horn
||Posterior spinocerebellar tract cells
|Sacral parasympathetic nucleus
||Preganglionic parasympathetics --> pelvic viscera
||Motor neurons --> trapezius and sternocleidomastoid
||Motor neurons --> diaphragm
Spinal White Matter
The nerve fibres in the white matter of the spinal cord
are of 3 general types:
- Long ascending fibres projecting to the thalamus,
the cerebellum, or various
- Long descending fibres projecting from the cerebral
cortex or from various brainstem nuclei to the
spinal grey matter;
- Shorter propriospinal fibres interconnecting various spinal cord levels, such
as the fibres responsible for the co-ordination of flexor reflexes.
- Fibres having similar connections tend to travel together forming the various
tracts of the spinal cord.
- Propriospinal fibres mostly remain in a thin shell
surrounding the grey matter called the propriospinal
tract or fasciculus proprius (L. fasciculus,
- Descending tracts are found primarily in the lateral
and anterior funiculi.
- Ascending tracts are found in all funiculi.
Information that reaches the thalamus is relayed to the
cerebral cortex and perceived
Information that reaches the cerebellum is used in the regulation of motor patterns; we are not
consciously aware of cerebellar activity.
Pathways to the Thalamus and Cortex
There are 2 traditionally important routes for somatic sensory information to reach the thalamus:
- The posterior column-medial lemniscus pathway;
- And the spinothalamic tract.
- Two generalisations may be made about ascending
somatosensory pathways of the spinal cord:
- Most somatosensory information travels in more than one pathway;
- Tracts in the posterior half of the cord ascend uncrossed whereas those in the anterior
half of the cord cross the midline as they
This refers to the entire contents of a posterior funiculus, exclusive
of its share of the propriospinal tract.
The posterior columns consist mainly of ascending collaterals of large myelinated
These carry impulses from various
kinds of mechanoreceptors.
This has traditionally been considered the major pathway by which information from low-threshold
cutaneous, joint, and muscle
receptors reaches the cerebral cortex.
- Many of these fibres have their cell bodies in the ipsilateral dorsal root ganglia.
- Where the dorsal root enters the spinal cord, it segregates into medial and lateral divisions.
- The medial division contains large
- The lateral division contains small,
finely myelinated or unmyelinated
First Order Fibres
Fibres of the medial division enter the posterior column.
Most of them give off numbers of collaterals to the spinal grey matter and finally terminate
at some spinal level.
Some, however, reach the caudal medulla and synapse there.
- Caudal to T6, each posterior column is an undivided bundle called the fasciculus
gracilis (L. gracilis, slender).
- Rostral to T6, fibres may leave the fasciculus gracilis, but few if any are added.
- Afferents entering rostral to T6 accumulate in a 2nd bundle lateral to the fasciculus gracilis, called the fasciculus cuneatus (L. cuneus, wedge).
- A glial partition, the posterior
intermediate septum, extends inward to partially separate the two.
Second Order Fibres
Those posterior column fibres that reach the brainstem synapse in the nucleus gracilis
or the nucleus cuneatus (the posterior column nuclei) in
the caudal medulla.
- Second order fibres arising in these nuclei cross the
midline and form the medial lemniscus (L. lemniscus,
- This is a flattened bundle of fibres that proceeds rostrally through the brainstem and terminates
in the thalamus.
Third Order Fibres
These fibres arise in the thalamus (specifically in the
ventral posterolateral nucleus of the thalamus, or VPL) and ascend through the internal capsule.
They synapse mainly in the cortex
of the postcentral gyrus.
This is the primary somatosensory cortex.
Fibres entering the posterior columns add on laterally
to those already present.
A lamination results, with layers of fibres from sacral levels most medial and layers from cervical
levels most lateral.
This arrangement is called somatotopic organisation and
is characteristic of most sensory
and motor pathways.
- When the primary afferents of the posterior
columns terminate in the posterior column nuclei,
they maintain this organisation.
- Fibres from sacral levels terminate in the most medial portions of the nucleus
- Fibres from cervical levels terminate in the most lateral portions of the nucleus
- A somatotopic arrangement is found throughout the rest of this pathway.
- In this way, information from sacral segments travels
through a particular part of the medial
lemniscus, projects to a particular portion of
the VPL, and then proceeds to a particular
region of the postcentral gyrus.
- This does not mean that the sacral-to-cervical
sequence remains along a medial-to-lateral line,
but rather that sacral information remains segregated from cervical information at
First Order Fibres
Collaterals of some touch and pressure-sensitive
fibres in the posterior columns, as well as those of mechanoreceptive, thermoreceptive,
and nociceptive fibres of the lateral
division of the dorsal root, enter the posterior horn.
They synapse in or near the substantia gelatinosa.
Second Order Fibres
Although these afferents end in the substantia gelatinosa, the latter
cells do not give rise to the spinothalamic tract.
Rather, the same afferents make additional
synapses on dendrites of neurons
whose cell bodies are located
deeper in the posterior horn or on the surface of the substantia
- These second order cells then send their axons across the midline with
a slight rostral inclination to form the spinothalamic tract.
- This is one alternate pathway by which mechanoreceptive input reaches the thalamus
and cerebral cortex.
- It is the primary pathway for pain
and temperature information.
- The tract occupies most of the anterior half of the lateral funiculus.
- New fibres join the spinothalamic tract at its ventromedial
edge, so that this tract, like the posterior columns, is somatotopically organised.
- Fibres from the most caudal segments occupy its most dorsolateral portion.
- Those from more rostral segments occupy more ventromedial portions.
- Although the spinothalamic tract carries some tactile
and pressure information, a great
deal also travels in the posterior column system.
- Destruction of the spinothalamic tract causes no significant
- There are, however, several types of sensation (in addition to pain and temperature) subserved more or less predominantly by the spinothalamic
- These are:
- Itch (and probably tickle) sensations;
- Pressure sensations from bladder and bowel;
- And sexual sensations.
- However, with the exception of itch, this information is carried bilaterally.
- However, as the spinothalamic tract is the principal pathway of somatic
pain sensations its destruction produces contralateral analgesia.
- An operation to destroy the tract (called a cordotomy or chordotomy) is sometimes performed on patients suffering from intractable
Pathways to the Cerebellum
Posterior Spinocerebellar Tract
First Order Fibres
Collaterals of posterior column fibres conveying tactile, pressure, and proprioceptive information (mainly the latter, from muscle
spindles and Golgi tendon organs) synapse on neurons of nucleus dorsalis.
Second Order Fibres
These then send their axons into the lateral
funiculus of the same side.
This forms the posterior spinocerebellar tract.
- This tract is a curved band of fibres extending from
the dorsal root entry zone to the dentate
- It lies at the surface of the spinal
- Fibres in the tract project ipsilaterally to the vermis of the cerebellum
through the inferior cerebellar peduncle.
- Collaterals of some of these fibres end in the nucleus gracilis.
- This provides an important route by which nonprimary afferents transmit proprioceptive
information from the leg to the posterior column-medial lemniscus system.
- Since the nucleus dorsalis does
not exist caudal to L2 or L3, neither does the posterior spinocerebellar tract.
- However, afferents from segments
caudal to L2 or L3 ascend to that level in the fasciculus
gracilis to synapse in nucleus dorsalis.
- This probab1y explains why nucleus dorsalis is large at upper
lumber and lower thoracic levels
- The posterior spinocerebellar tract is principally concerned with the ipsilateral
- Most spinocerebellar-type afferents that enter in cervical and upper thoracic segments do
not project to nucleus dorsalis.
- They travel in the fasciculus cuneatus to a nucleus in the medulla
analogous to nucleus dorsalis called the lateral (or external) cuneate nucleus.
- It is located just lateral to the nucleus
- Axons of these cells form the cuneocerebellar tract.
- They also project ipsilaterally to the vermis of the cerebellum
through the inferior cerebellar peduncle.
Anterior Spinocerebellar Tract
Cells in the body of the lumbosacral
posterior horn, together with cells
on the lateral surface of the anterior
horn (spinal border cells) give rise to the anterior spinocerebellar tract.
- The inputs to these tract cells
are more complex than those of the posterior spinocerebellar tract.
- They come not only from group I
muscle afferent (mainly Golgi tendon organs), but also from a wide variety of cutaneous receptors
including spinal interneurons and from fibres
of descending tracts.
- The tract is crossed at the level of the spinal cord, in contrast to the posterior spinocerebellar
- Finally, the anterior spinocerebellar tract takes a roundabout route to the cerebellum.
- It ascends as far as the rostral
pons and then turns caudally and enters the cerebellum via the superior cerebellar peduncle.
- Here, most of its fibres recross the midline before
ending in the vermis of the anterior
- Thus, the fibres of the anterior spinocerebellar tract ultimately
end in the cerebellum on the ipsilateral side to their origin.
Lateral Corticospinal Tract
- The lateral corticospinal tract is a large, crossed, descending tract that
contains the 85% of fibres
from the contralateral pyramid that cross
in the pyramidal decussation.
- It is also known as the pyramidal tract.
- It occupies the posterior portion of the lateral funiculus medial to the posterior spinocerebellar tract.
- Its fibres originate in the cerebral
cortex (in the precentral gyrus and nearby
- They descend through the cerebral
peduncle, basal pons, and medullary
- They then decussate and end in the anterior horn or intermediate
- They terminate on the motor
neurons of the anterior horn or, more often, on smaller interneurons.
- These in turn synapse on motor
- Lateral corticospinal fibres are arranged somatotopically.
- Those destined for more caudal cord levels are located more laterally.
- The fibres of the lateral corticospinal tracts usually synapse on motor neurons or
interneurons that ultimately go to the distal muscles.
Anterior Corticospinal Tract
The 15% of the fibres in each pyramid
that do not cross in the pyramidal
decussation continue into the anterior funiculus.
This is located adjacent to the anterior
median fissure as the anterior corticospinal tract.
- These fibres also terminate on motor
neurons or interneurons of the anterior horn or intermediate
grey matter, mainly in cervical and thoracic segments.
- Many of them cross in the anterior
white commissure before synapsing.
- The term "pyramidal tract" refers to the combination of lateral and anterior corticospinal tracts.
- These fibres ultimately tend to go to the axial muscles.
This is an alternative route for the mediation
of voluntary movement.
It originates in the red nucleus
--> crosses to the other side of the midbrain --> descends
in the lateral part of the brainstem
tegmentum --> travels through the lateral funiculus
of the spinal cord in the company with the lateral
It is small and rudimentary
Lateral Vestibulospinal Tract
It arises in the lateral vestibular nucleus and
projects to all levels of the ipsilateral
It is located in the ventral part of the lateral funiculus.
- It is the principle route by which the vestibular system brings about postural
changes to compensate for tilts
and movements of the body.
Medial Vestibulospinal Tract
This arises mainly in the medial vestibulospinal nucleus
and projects bilaterally to the cervical
It is responsible for stabilising the head position as we walk around.
This tract only goes down to the midthoracic level.
- Many secondary vestibular fibres project directly
through the medial longitudinal fasciculus (MLF) to the motor neurons of the oculomotor,
trochlear and abducens nuclei.
- This forms much of the basis of the vestibuloocular reflex.
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entry under the Reticular Formation.