The Spinal Cord I
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of the brain | Main Anatomy Index | Spinal cord II
Last updated 30 March 2006
The Spinal Cord I
Barr M. L., Kiernan J. A. (1988) The Human Nervous System, An Anatomical Viewpoint
5th Ed. J. B. Lippincott Company, Philadelphia, Pennsylvania, USA.
- It and dorsal root ganglia are directly responsible for
innervation of the body, excluding
most of the head, however.
- Afferent sensory fibres enter the spinal cord through the dorsal
roots of spinal nerves.
- Efferent fibres leave by way the ventral roots
(the Bell-Magendie Law).
- Data originating in sensory endings can:
- Initiate a spinal reflex;
- Or are relayed to the brain stem
and cerebellum where they are used in various
- Sensory information is also transmitted to the thalamus
and then the cerebral cortex, where it becomes part of
the conscious experience --> emotional
- Motor neurons in the spinal cord are excited or inhibited by impulses originating from various levels of the
brain; from the medulla to the cerebral
- The spinal cord has a complex internal structure.
Gross Features of the Spinal Cord
and Nerve Roots
The spinal cord is a cylindrical structure.
It is slightly flattened dorsoventrally and located in
the spinal canal of the vertebral
- Protection for the cord is provided by:
- The vertebrae and their ligaments;
- And the meninges and a cushion of CSF.
- The innermost layer of pia mater
adheres to the surface of the spinal cord.
- The outermost layer of dura mater
forms a tube that extends from the level of the foramen magnum to S2.
- It is continuous with the dura mater around the brain.
- The arachnoid lies against the inner
surface of the dura mater and forms the outer boundary of the subarachnoid space.
The Denticulate Ligament
A denticulate ligament on each side suspends
the spinal cord in the dural sheath.
This ligament is in the form of a ribbon.
It is attached along the lateral surface of the cord midway between the dorsal and ventral roots.
- The lateral edge of the denticulate ligament is serrated.
- 21 points or processes are attached to the dural sheath at intervals between
the foramen magnum and the level at which the dura mater
is pierced by the roots of the first
lumbar spinal nerve.
This is a space filled with fatty tissue and contains a
It occupies the interval between the dural sheath and
the wall of the spinal canal.
Spinal Nerves and Gross Features
The segmental nature of the spinal cord is demonstrated
by the presence of 31 pairs of spinal nerves.
There is, however, little indication of segmentation in
its internal structure.
- Each dorsal root is broken up into a series
of rootlets that are attached to the cord along the corresponding
- The ventral root arises similarly.
- The spinal nerves are distributed as:
- The first cervical nerves lack dorsal roots in 50% of people, and the coccygeal nerves may be absent.
Development of Spinal Cord and
of the neural tube (neuromeres)
correspond in position with segments of the vertebral column (scleromeres) until
the 3rd month of foetal development.
The vertebral column elongates more rapidly than the
spinal cord during further foetal development.
- The cord is fixed at its rostral end and gradually advances.
- By the time of birth, the caudal end is opposite the disk between L2 and L3.
- This continues during childhood and the caudal end of
the cord in the adult is opposite the disk between L1 and L2.
- However, the caudal end of the spinal cord may vary to
be as high as the body of T12, or as low as L3.
- The rostral shift of the cord during development
determines the direction of the spinal nerve roots in the
- Spinal nerves from C1 through C7 leave the spinal canal
through the intervertebral foramina above the corresponding
vertebrae (the 1st and 2nd cervical nerves lie on the vertebral
arches of the atlas and axis,
- The 8th cervical nerve passes through the foramen between C7 and T1 because there are 8 cervical cord segments
and only 7 cervical vertebrae.
- From this point caudally, the spinal nerves leave the
canal through foramina immediately below the pedicles of
the corresponding vertebrae.
- All intervertebral foramina are slightly
rostral to the levels of the intervertebral disks.
Dorsal and Ventral Roots and Gross
The dorsal and ventral roots traverse the subarachnoid space
and pierce the arachnoid and dura
At this point, the dura becomes continuous with the epineurium.
- After a short course in the epidural space the roots
reach the intervertebral foramina.
- The dorsal root ganglia are located here.
- The dorsal and ventral roots
join immediately distal to the ganglion to form the spinal nerve.
- The length and obliquity
of the roots increase progressively in a posteroinferior direction.
The Cauda Equina
The lumbosacral roots are the longest,
and constitute the cauda equina (L. horsetail) in the
lower part of the subarachnoid space.
The Filum Terminale
The cord tapers into a slender filament called the filum terminale.
This lies in the midst of the cauda equina and has a distinctive bluish-white colour.
- Dura invests the filum terminale opposite the 2nd sacrum segment.
- This results in the coccygeal ligament and it attaches
to the dorsum of the coccyx.
- The filum terminale consists of pia mater and neuroglial elements.
- It is a vestige of the spinal cord of the embryonic tail,
but in the adult it has no functional significance.
Spinal Lumbar Puncture
To obtain a sample of CSF from the subarachnoid
space, a spinal lumbar puncture is the
The needle is inserted between the arches
of L3 and L4.
- This avoids any damage to the spinal
Spinal Cord Enlargements
The spinal cord is enlarged in 2 regions for innervation of the limbs.
- The cervical enlargement extends from C4 through T1 segments.
- Most of the corresponding spinal nerves form the brachial plexuses for the innervation of the upper
- Segments L2 through S3 are included in the lumbosacral
- The corresponding nerves constitute most of the lumbosacral
plexuses for the innervation of the lower limbs.
- Individual segments are longest in the thoracic region.
- They are shortest in the lower
lumbar and sacral regions.
- The caudal end of the cord tapers rather abruptly and
forms the conus medullaris.
- It is from this the filum terminale arises.
The Surface of the Spinal Cord
This is mark off into dorsal, lateral
and ventral areas by longitudinal
- The deep ventral median fissure contains connective tissue of the pia mater
and branches of the anterior spinal artery.
- The dorsal median sulcus is a shallow
- The dorsal septum, composed of pial
tissue, extends from the base
of this sulcus almost to the grey
- The ventrolateral sulcus is indistinct.
- Its position is indicated by the zone of attachment of
the ventral roots, however.
- The dorsolateral sulcus marks the line
of attachment of the dorsal roots.
- The dorsal area on each side is divided above the midthoracic level.
- It contains a medial part that contains the fasciculus gracilis and a lateral part
that contains the fasciculus cuneatus.
- The intervening groove is called dorsal intermediate sulcus.
Grey Matter and White Matter
In a transverse section, the grey matter has a roughly H-shaped or butterfly outline.
The central canal is lined by ependymal
epithelium, and the lumen may be obliterated in places.
- The grey matter on each side consists:
- The dorsal horns;
- The ventral horns;
- And an intermediate zone.
- A small lateral horn containing sympathetic
efferent neurons is added in the thoracic and upper lumbar segments.
Neurons in the Spinal Grey Matter
There are 3 main categories.
- The smallest cells involved in local circuitry are the interneurons.
- Motor cells of the ventral horn supply the skeletal musculature and consist of alpha
and gamma motor neurons.
- The cells of the lateral horn and the sacral autonomic nucleus are preganglionic
neurons of the sympathetic and parasympathetic divisions of the ANS, respectively.
- The cell bodies of tract cells, whose axons constitute
the ascending fasciculi of the white matter, are located
mainly in the dorsal horn.
This consists of 3 funiculi (these are often called
"columns" but this word is more appropriate for longitudinally
aligned arrays of neuronal cell bodies in the grey matter).
- The dorsal funiculus is bounded by the dorsal septum and the dorsal grey horn.
- It consists of a medial fasciculus gracilis and a lateral fasciculus cuneatus above the midthoracic
- The fasciculus gracilis corresponds with the entire dorsal funiculus caudal to the midthoracic region.
- There is no anatomical demarcation between the lateral and ventral funiculi.
- The lateral funiculi are further subdivided into dorsolateral and ventrolateral funiculi.
- These are separated by a plane that passes through the central canal and the denticulate
- The dorsolateral tract (of Lissaner) occupies the
interval between the apex of the dorsal horn and the surface of the cord.
- The white matter consists of partially overlapping fibre bundles.
General Pattern of Grey and
The general pattern of grey matter and white matter is the same
throughout the spinal cord.
There are, however, some regional differences that are
apparent in transverse sections.
- The amount of white matter increases in a caudal-to-rostral direction because fibres
are added to ascending tracts and fibres leave descending
tracts to terminate in the grey matter.
- There is increased volume of grey matter in the cervical
and lumbosacral enlargements for innervation
of the limbs.
- The small lateral horn of grey matter is characteristic of the thoracic and upper