Medical Neurosciences
Spinal Cord
Spinal Cord
DAVID GRIESEMER, MD
Professor of Neurosciences and Pediatrics
Key Concepts:
1. In adults the spinal cord ends at vertebral level L2, and the nerve roots continue as the cauda
equina to exit at the appropriate vertebral level. The level of spinal cord termination allows a
lumbar puncture to be performed with greater safety at spinal level L4-5.
2. Cell bodies for sensory neurons carrying information to the spinal cord are located in dorsal root
ganglia, which are located outside the spinal canal.
3. Sensory information enters the spinal cord on the dorsal side.
4. The gray matter in the spinal cord contains motor neurons, neurons ascending to the brain,
interneurons, and glial cells.
5. Alpha motor neurons from segments C3, C4 and C5 travel via the phrenic nerve to innervate the
diaphragm.
6. Alpha motor neurons originate from lamina IX, with those innervating extensor muscles more
ventral than those innervating flexor muscles and those innervating distal muscles more lateral
than those innervating proximal muscles.
7. Gamma motor neurons, which innervate intrafusal muscle fibers necessary in the muscle stretch
reflex arc, also originate from lamina IX.
8. Autonomic fibers originate from the intermediolateral region of the gray matter. In the thoracic
and lumbar spinal cord these are preganglionic sympathetic neurons and in the sacral regions they
are preganglionic parasympathetic neurons. No autonomic neurons arise from the spinal cord at
the cervical level.
9. Axons that ascend in the posterior column of white matter have their cell bodies in the dorsal root
ganglia. These neurons mediate vibration, position sense, two-point discrimination, and feeling
for shape, pattern, or direction of stimulus on the skin.
10. The dorsal nucleus of Clarke, located in lamina V between T1 and L2, receives unconscious
sensory information about leg position. It has a vital role in coordination of walking.
11. Bladder control and micturition require coordination of signaling from somatic innervation and
from parasympathetic both derived from S2 – S4 and from sympathetic innervation derived from
T11-L2.
12. Supplemental alpha motor neurons from segments S1 – S4 innervate the anal and urethral
sphincters, as well as muscles necessary for male sexual function.
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Spinal Cord
INTRODUCTION
The spinal cord contains the first synapse in all
sensory pathways from the body and back of the
head. It also contains all the motor neurons that
innervate skeletal muscles of the body.
(However, it does not have a role in sensory
perception from the face or in motor control of
facial muscles.) It also plays a vital role in the
control of urination and the bladder.
EXTERNAL VIEW
The spinal cord is located in the vertebral canal. At birth the cord extends the full length of the spinal
column, but in the adult it extends from the foramen magnum to the L1 or L2 vertebral level. Most of the
cells in the spinal cord are present at birth, and it does not grow substantially. However, since the bony
vertebral column continues to grow for another 20 years, the adult spinal cord ends at the level of the L1
or L2 vertebra. As a result the spinal nerves associated with progressively lowers levels of the cord must
travel further downward within the canal to exit at the appropriate intervertebral foramen. This collection
of long spinal nerves extending beyond the conus medullaris, or tapering end of the spinal cord, is called
the cauda equina, or “tail of the horse.”
By way of review, the spinal cord is divided into 31 segments:

8
cervical

12 thoracic

5
lumbar

5
sacral

1
coccygeal
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Spinal Cord
Dorsal and ventral roots exit the spinal cord at each spinal level. Dorsal roots carry sensory information to
the spinal cord; their cell bodies are located in the dorsal root ganglia. The ventral roots carry motor and
autonomic inform from the spinal cord; their cell bodies are located in the gray matter of the spinal cord.
Both dorsal (somatosensory) and ventral (motor) roots fan into tiny rootlets that attach to the cord along a
vertical line at the dorsolateral and ventrolateral surfaces of the cord. Before exiting the spinal canal, the
dorsal and ventral roots join a short distance to form a spinal nerve.
CROSS SECTIONAL VIEW
Appearance. The spinal cord consists of gray matter that contains neuronal cell bodies (below, on left)
and white matter that consists of myelinated fiber tracts (below, on right). These are both ascending and
descending tracts. The gray matter is centrally located in the spinal cord. It is arranged in a “butterfly” of
H-shaped pattern that varies depending upon the level of the cord. The white matter surrounds the gray
matter and is therefore in the peripheral regions of the cord. The spinal cord is not perfectly symmetric,
the right side being slightly larger in most people. This occurs because of more descending fibers
originating from the left side of brain, regardless of handedness. The two halves of the spinal cord (where
not connected around the central canal) are separated by the ventral (anterior) fissure and the dorsal
(posterior) median septum.
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Spinal Cord
In some of the cervical and lumbar regions of the spinal cord there are additional neurons that innervate
muscles and extra skin receptors for the arms and legs. The addition of these neurons expands the gray
matter at the cervical enlargement (C5 to T2) and at the lumbar enlargement (L2 to S1). Ascending from
lower sacral segments to higher cervical segments of the cord, there is an increasing volume of white
matter. This is because:
 at progressively higher levels there is an accumulation of axons bringing sensory input from all
parts of the body;
 at progressively lower levels there is a decrease in the number of descending axons from the
brain, as they distribute into the gray matter along the length of the cord;
 some white matter tracts are not present below certain levels.
Organization of Gray Matter. The gray matter of the cord contains primarily the cell bodies of neurons
and glia. The neurons are of two basic types: primary neurons and interneurons. Primary neurons are
further divided into two broad categories: motor neurons that have axons leaving via the ventral nerve
root, and ascending neurons that carry information to the brain.
The traditional terminology divides the gray matter into four regions:
 dorsal horn (or, posterior column) – this region receives axons of dorsal root ganglia. It is
concerned with sensory function.

intermediolateral horn (part of the lateral column) -- this horn is limited to the thoracic and
upper lumbar regions of the cord and is concerned with autonomic nervous system function. It
consists of preganglionic sympathetic neurons which exit the spinal cord through the ventral
roots. In this equivalent region of the sacral region of the cord the S2, S3 and S4 segments
contain preganglionic parasympathetic neurons.

ventral horn (or, anterior column) -- this region contains motor neurons, axons from which exit
through the ventral nerve root.
A modern view of the spinal cord is based upon layers of axon termination, based on cytological criteria.
A series of ten layers, labeled I through X, was proposed by Bror Rexed. Details of the traditional and
modern classification system, together with a functional overview are outlined in the chart below (for
your reference only).
TRADITIONAL TERMINOLOGY
HORN
REGIONS
REXED
LAMINA
Posteromarginal nucleus
Substantia gelatinosa
I
II
Dorsal
Nucleus proprius
III-IV
Intermedio
lateral
Neck of Posterior horn
Base of Posterior horn
Intermediate zone
Commissural nucleus
Ventral horn
Grisea centralis
V
VI
VII
VIII
IX
X
Ventral
FUNCTION
Exteroceptive
sensation
Proprioceptive
sensation
Pain and temperature
Position sense,
vibration
touch,
pressure,
Position information from the legs
(includes dorsal nucleus of Clarke)
Relay between midbrain and cerebellum
Modulation of motor activity via γ motor neurons
Main motor nuclei of both α and γ motor neurons
Surrounds the central canal, and contains neuroglia
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Spinal Cord
Looking at a cross section of the cervical cord as an example, the figure on the left highlights cell groups
using the traditional nomenclature, while the figure on the right highlights synaptic layers identified by
Rexed laminae.
Regions of particular note include:
1

Lamina V. Of particular note is a region in this lamina between the T1 and L2 spinal segments.
A noticeable “bump” on the mediobasal margin of the dorsal horn in this region corresponds to
the dorsal nucleus of Clarke. This nucleus receives sensory data concerning position of the legs
and therefore has a role in coordination of walking.

Lamina IX. This is the primary motor area of the spinal cord. It contains large α motor neurons
that supply extrafusal muscle fibers and smaller γ motor neurons that supply intrafusal muscle
fibers.
o α motor neurons are arranged by function
 neurons innervating muscles causing flexion of joints are located more dorsally
 neurons innervating muscles causing extension of joints are located ventrally
 neurons innervating distal muscles (e.g. hand) are located more laterally
 neurons innervating proximal muscles (e.g. trunk) are located medially
o
α motor neurons from segments C3, C4 and C5 travel via the phrenic nerve. Axons of
this nerve innervate the diaphragm and are essential for breathing.
o
Onuf’s nucleus. Supplemental α motor neurons from segments S1 – S4 innervate the
anal and urethral sphincters. They are therefore essential for bowel and bladder
continence. In addition these motor neurons supply muscles essential for sexual function
in males.1
o
Interneurons. In addition to α and γ motor neurons, lamina IX contains interneurons,
including the Renshaw cell. This interneuron is part of a “negative feedback” loop for
the α motor neuron. It is stimulated by a collateral axon coming from the motor neuron,
but its output is an inhibitory signal to the dendrite going to the originating motor neuron
and neighboring motor neurons. Renshaw cell inhibition can therefore allow motor
neurons to influence their own activity and damp output of selected neurons.
Only these supplemental motor neurons are spared in motor neuron diseases like amyotrophic lateral sclerosis
(Lou Gehrig’s disease).
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Spinal Cord
Motor neurons that innervate muscles are arranged into vertical columns in the anterior horn of the spinal
cord. These columnar collections are considered nuclei, and they are analogous to nuclei for cranial
nerves that are located in the brainstem. There is a more complex arrangement in the cervical and lumbar
regions. During embryonic development, primitive muscles carry their original innervation with them,
generating a motor column that sends its axons through multiple nerve roots from multiple spinal cord
levels. Note that, while a muscle may be innervated by axons from multiple spinal nerve roots, those
axons arise from a single motor column.
Organization of White Matter. The white matter of the spinal cord contains primarily axons gathered
into ascending and descending fiber tracts. The H-shaped appearance of gray matter on cross-sectional
view allows division of the white matter into three symmetric, paired columns (or funiculi) which extend
the length of the spinal cord. Each column contains bundles of axons that channel specific sensory
information towards the brain or motor command signals from the brain. These are illustrated above.
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Spinal Cord

dorsal (posterior) column – lies medial to the dorsal horn of gray matter. It contains ascending
axons that carry somatic sensory signals to the brain. Sensory axons from inferior regions of the
body are located more medially in the posterior column than axons from higher regions. Dorsal
nerve roots that enter the spinal cord below the T7 segment are located medially in the posterior
column and form the gracile tract. Nerve roots that enter the spinal cord above the T6 segment
are located laterally in the posterior column and form the cuneate tract. Lesions in this region
manifest as a loss or diminution of the following sensations:
o Position sense
o Vibration sense
o Two point discrimination
o Touch
o Feel for shapes or patterns
o Discerning direction of stimulus on skin

lateral column – lies laterally to the gray matter between the dorsal horn and the ventral horn. It
contains ascending somatic sensory axons and descending motor control axons.

ventral (anterior) column – lies medial to the ventral horn of gray matter.
descending motor control axons.
It contains
TRACTS OF THE SPINAL CORD
While the posterior column contains only one ascending tract which conveys conscious perception to the
cerebral cortex, the lateral and ventral columns contain both ascending and descending tracts.

Ascending tracts. All of the clinically significant ascending tracts have cell bodies of origin in
the dorsal root ganglia. Not all, however, convey information of which a person is conscious.
There are four main ascending pathways:
o
dorsal (posterior) spinocerebellar tract – conveys to the cerebellum information about
strength, rate and phase of muscle contraction
o
ventral (anterior) spinocerebellar tract – conveys information about the effect of
descending signals and interneuronal activity. Dorsal and ventral spinocerebellar tracts
together provide unconscious information about position.
o
lateral spinothalamic tract – conveys pain and temperature information
o
anterior spinothalamic tract – conveys sensation of light touch and some pain.
Functionally the anterior and lateral spinothalamic tracts are one.
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
Spinal Cord
Descending tracts. While neurons of all ascending tracts are located in dorsal root ganglia,
axons in descending tracts come from neurons in several locations. Key pathways include:
o
corticospinal tract – conveys information essential for speed and agility of movement.
It does not by itself initiate movement.
o
rubrospinal tract – conveys information for correcting errors in movement.
o
lateral vestibulospinal tract – conveys information to activate extensor motor neurons
that maintain upright posture.
o
medial vestibulospinal tract – conveys information to control head position and activate
flexor motor neurons.
o
reticulospinal tract – axons arising from the pons facilitate extensor motor neurons, and
axons arising from the medulla facilitate flexor motor neurons
o
tectospinal tract – facilitates turning of the head in response to light
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Spinal Cord
CLINICAL APPLICATION: MICTURITION AND BLADDER CONTROL
From a clinical perspective, one of the most important functions of the spinal cord it to mediate control of
the bladder and the function of urination. The bladder had three sources of efferent and afferent signal
transmission:
 somatic innervation from Onuf’s nucleus at S2, S3 and S4 via the pudendal nerve
 sympathetic innervation from the intermediolateral cell column at T11 – L2 via the hypogastric
nerve
 parasympathetic innervation from the intermediolateral-like cell column at S2, S3 and S4 via
the pelvic nerve
Bladder filling is dependent upon tonic activity of somatic neurons and sympathetic neurons.
 Somatic stimulation causes contraction of the external urethral sphincter
 Sympathetic stimulation causes contraction of the internal urethral sphincter and relaxation of the
detrusor muscle that expels urine from the bladder
Bladder emptying is dependent upon all three innervations:
 Inhibition of somatic innervation to relax the external urethral sphincter
 Inhibition of sympathetic stimulation to relax the internal urethral sphincter
 Parasympathetic stimulation to stimulate detrusor contraction
Central nervous system control is also an essential element:
 Descending pathways originating in the pons coordinate starting and stopping of micturition
 Afferent information about bladder filling travels to the periaqueductal region of the midbrain.
At the point when it is filled enough to empty, the midbrain activates neurons in the micturition
center of the pons
 The medial preoptic area of the hypothalamus is involved in micturition
 The right inferior frontal gyrus and the right anterior cingulate gyrus of the cerebral cortex are
also involved in micturition.