The Spinal Cord I

General topography of the brain | Main Anatomy Index | Spinal cord II

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:

1. Initiate a spinal reflex;
2. Or are relayed to the brain stem and cerebellum where they are used in various circuits.
3. Sensory information is also transmitted to the thalamus and then the cerebral cortex, where it becomes part of the conscious experience --> emotional response.

• 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 cortex.
• The spinal cord has a complex internal structure.

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• The spinal cord is a cylindrical structure.
• It is slightly flattened dorsoventrally and located in the spinal canal of the vertebral column.

• Protection for the cord is provided by:

1. The vertebrae and their ligaments;
2. 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.

• 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 venous plexus.
• It occupies the interval between the dural sheath and the wall of the spinal canal.

• 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 segment.
• 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.

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• Segments 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 subarachnoid space.

1. 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, respectively).
2. The 8th cervical nerve passes through the foramen between C7 and T1 because there are 8 cervical cord segments and only 7 cervical vertebrae.
3. 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.

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• The dorsal and ventral roots traverse the subarachnoid space and pierce the arachnoid and dura mater.
• 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 lumbosacral roots are the longest, and constitute the cauda equina (L. horsetail) in the lower part of the subarachnoid space.

• 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.

• To obtain a sample of CSF from the subarachnoid space, a spinal lumbar puncture is the preferred method.
• The needle is inserted between the arches of L3 and L4.

• This avoids any damage to the spinal cord.

• 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 limbs.

• Segments L2 through S3 are included in the lumbosacral enlargement.
• 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.

• This is mark off into dorsal, lateral and ventral areas by longitudinal furrows.

• 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 midline furrow.
• The dorsal septum, composed of pial tissue, extends from the base of this sulcus almost to the grey matter.

• 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.

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• 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:

1. The dorsal horns;
2. The ventral horns;
3. And an intermediate zone.

• A small lateral horn containing sympathetic efferent neurons is added in the thoracic and upper lumbar segments.

• 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 level.
• 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 ligament.

• 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.

• 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.

1. 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.
2. There is increased volume of grey matter in the cervical and lumbosacral enlargements for innervation of the limbs.
3. The small lateral horn of grey matter is characteristic of the thoracic and upper lumbar segments.

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