Spinal Cord Organization

Question 1

The spinal cord: housed in, protected by, surrounded by, bathed and suspended in and anchored at caudal end by…

Answer 1

The spinal cord (SC) is housed in the vertebral canal, protected by bone, surrounded by meninges, bathed and suspended in cerebrospinal fluid (CSF) and anchored at its caudal end by the filum terminale, an extension of the pia mater.

Question 2

How many segments does the spinal cord have?

Answer 2

31 segments: There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.

Question 3

What five key things are included in each spinal cord segment?

Answer 3

Each segment consists of ventral roots, dorsal roots, a dorsal root ganglion, and the spinal cord gray matter (ventral horn and dorsal horn) and white matter at that level

Question 4

Which horns generally contain the somas of ventral horn motor neurons that project their axons to muscle?

Answer 4

Ventral horns

Question 5

What do the dorsal root ganlgions contain?

Answer 5

Dorsal root ganglions contain the somas of pseudounipolar sensory neurons whose peripheral axons carry information from peripheral skin and muscle receptors.

Their central axons carry that sensory information into the CNS through the dorsal root entry zone and many synapse on interneurons within the dorsal horn.

Question 6

Where are the large alpha motor neurons that innervate skeltal muscles contained?

Answer 6

Within the ventral horns

Question 7

Spinal cord cervical enlargement

Answer 7

The cervical enlargement lies between segments C5 ,C6, C7, C8 and T1 and gives rise to spinal nerves forming the brachial plexus.

Question 8

Spinal cord lumbosacram enlargement

Answer 8

The lumbosacral enlargement lies between segments L1 through L4 to form the lumbar plexus and L4 through S2 to form the sacral plexus.

Question 9

White matter of the spinal cord

Answer 9

The white matter regions of the SC are divided into three funiculi or columns which contain motor and sensory pathways: Dorsal (posterior) faniculus, lateral faniculus or lateral column and Ventral (Anterior) funiculus

Question 10

Where are the fasciculus gracilis and fasciculus cuneatus located?

Answer 10

In the Dorsal (posterior) funiculus (also called the dorsal column)

Question 11

Where is the dorsal faniculus the largest?

Answer 11

The dorsal funiculus is largest at the cervical levels because it contains ascending fibers of both the fasciculus gracilis and fasciculus cuneatus whereas the lumbar levels only contain the fasciculus gracilis. Progressively more sensory fibers accumulate at each level from the caudal to the rostral direction. (Think about the school bus analogy)

Question 12

Part of the lateral spinothalamic tract is located in which two funiculi?

Answer 12

Lateral funiculus and part is in the ventral funiculus

Question 13

The Lateral funiculus or lateral column of the SC white matter is the location for which four tracts?

Answer 13

Lateral corticospinal tract, rubrospinal tract, and dorsal and ventral spinocerebellar tracts.

Question 14

The Ventral (Anterior) funiculus or ventral column of the SC white matter is the location for which six features?

Answer 14

Anterior corticospinal tract, reticulospinal tracts, lateral vestibulospinal tract, tectospinal tract, medial longitudinal fasciculus, and part of the lateral spinothalamic tract.

Question 15

What accounts for the increased size of the cervical segments of the spinal cord?

Answer 15

The cervical segments are relatively large and ovoid in shape. The increased size is due to the large number of ascending and descending axons present at these levels (think of the school bus analogy) and the large number of motor neurons in ventral gray matter.

Question 16

Why are the thoracic segments of the spinal cord smaller than the cervical segments?

Answer 16

The thoracic segments are smaller because they do not contain as many motor neurons (not as much musculature to innervate in the trunk regions) and contain less axons compared to the cervical levels.

Question 17

Which region marks the location of cell bodies of preganglionic sympathetic (type B motor neurons)?

Answer 17

Lateral horn also called the intermediolateral cell column (present in all thoracic segments)

Question 18

What are some salient features of the Lumbar segments of the spinal cord?

Answer 18

• The lumbar segments have massive ventral and dorsal horns.
• The upper lumbar segments have a lateral horn (as do the thoracic segments).
• They also have a particularly well-developed nucleus dorsalis (also called Clarke’s nucleus or column which actually extends from C8-L2).
• It contains the cell bodies of neurons of the dorsal spinocerebellar tract.

Question 19

What are some salient features of the Sacral segments of the spinal cord?

Answer 19

• Relatively small cross sections with large amounts of gray matter.
• Large ventral and dorsal horns, and little white matter
• The ascending pathways are just beginning to collect axons at the more caudal segments (again think of the school bus analogy).
• Preganglionic parasympathetic fibers arise from Rexed’s lamina VII of the sacral segments (see below), but, unlike the case for the sympathetic system, there is no lateral horn present at that level.

Question 20

Spinal cord gray matter: Where are Rexed’s Laminae I through VI located?

Answer 20

Doral horn

Question 21

Spinal cord gray matter: Where is Lamina VII located?

Answer 21

Intermediate zone

Question 22

Spinal cord gray matter: Where are Laminae VIII and IX located?

Answer 22

Ventral horn

Question 23

Spinal cord gray matter: In which region is Lamina X located?

Answer 23

Central canal region

Question 24

Dorsal Horne Laminae (I-VI) axon pathway

Answer 24

Axons enter the dorsal horn of the spinal gray matter via the dorsal root entry zone and travel through Lissauer’s tract. These axons are considered afferent fibers and carry sensory information from the periphery.

Question 25

Where are the terminal branches of the C afferent (unmyelinated) and A-delta afferent (lightly myelinated) fibers located?

Answer 25

a region of the dorsal horn called the substantia gelatinosa, located in Rexed’s lamina II. These small fibers carry sensations of pain and temperature.

Question 26

Afferents terminating in Rexed’s lamina III and IV respond to what type of stimulus?

Answer 26

low intensity stimulation (e.g., light touch from hair receptors)

Question 27

What is a key feature of Lamina V?

Answer 27

It contains many interneurons

Question 28

Which lamina is especially prominent in the cervical and lumbosacral enlargements?

Answer 28

Lamina VI-Some branches of Group Ia and Ib muscle afferents synapse onto neurons in this region.

Question 29

Intermediate zone: Lamina VII

Answer 29

Lamina VII contains Clarke’s column. It extends primarily from C8 to L2 and cell bodies located there give rise to the dorsal spinocerebellar tract.

Question 30

Which lamina in the ventral horn contains terminations of many fibers descending motor pathways?

Answer 30

Lamina VIII

Question 31

Which lamina in the ventral horn is present in distinct clusters?

Answer 31

Lamina IX-This lamina is important because it is the location of cell bodies of large alpha motor neurons (and gamma motor neurons).

In general, motor neurons to more distal musculature are located laterally, whereas the motor neurons innervating axial muscles are located medially in the lamina IX clusters in the ventral horn.

Question 32

Central Canal Region: Lamina X (location and contents)

Answer 32

Located around the central canal and contains many interneurons that send axons across the midline.

Question 33

Lissauer’s tract-small fine fibers vs. large afferents entrances

Answer 33

As afferents enter the SC through Lissauer’s tract, the small fine fibers take a lateral position, whereas the large afferents enter more medially in the tract.

Question 34

What are the characteristics of the medial group within Lissauer’s tract?

Answer 34

The medial group contains thick, heavily myelinated (A-beta, Group Ia, Ib, II) afferents that transmit fine touch sensation and position sense.

Question 35

What are the characteristics of the lateral group within Lissauer’s tract?

Answer 35

The lateral group contains the thin, unmyelinated and lightly myelinated (Group C [also called Group IV] and A-delta [also called Group III]) fibers that transmit pain and temperature sensation.