Organization
Of The Spinal Cord
Overall structure
The spinal cord lies within the vertebral canal and extends from the
foramen magnum to the lower border of the first lumbar vertebra.
•
It is enlarged at two sites (cervical and lumbar
regions) corre- sponding to the innervations of the upper and lower limbs (see
Chapter 2).
•
The lower part of the vertebral canal (below L1)
contains the lower lumbar and sacral nerves and is known as the cauda equina.
Sensory nerve fibres enter the spinal cord via the dorsal (poste-
rior) roots and their accompanying cell bodies are located in the
dorsal root ganglia, while the motor and preganglionic autonomic fibres exit
via the ventral (or anterior) root, together with some mostly
unmyelinated afferent fibres.
The motor cell bodies (or motor neurones [MNs]) are
found in the ventral horn of the spinal cord, while the preganglionic cell
bodies of the sympathetic nervous system are found in the intermediolateral
column of the spinal cord (see Chapter 3).
The neuronal cell bodies that make up the central grey matter of the cord
are organized into a series of laminae (of Rexed). The white
matter surrounding this is composed of myelinated and unmyelinated axons
constituting the ascending and descending spinal tracts.
Organization of sensory afferent fibres entering the spinal cord
Sensory information from the peripheral receptors is relayed by primary
afferent nerve fibres which terminate in layers I–V of the dorsal horn,
the site for termination being different for different receptors. However, in
reality, many afferent fibres divide (into an ascending and a descending
branch) as they enter the spinal cord so that synaptic contact can be made both
with many interneu- rones in the dorsal horn, and up and down the cord through Lis-
sauer’s tract.
Sensory processing in the dorsal horn
• Typically, a number of primary afferents make
synaptic contact with a single dorsal horn neurone.
• This convergence of input has the effect
of reducing the acuity (accuracy) of stimulus location, but the process of lateral
inhibition helps minimize this loss of acuity by promoting the inhibition
of submaximally activated fibre inputs and thus increasing spatial contrast in
the sensory input (see Chapter 22).
•
The dorsal horn receives a number of descending
inputs from supraspinal structures that are important in modulating the
processing of sensory information through the spinal cord (see, for example,
Chapter 32).
Ascending sensory pathways in spinal cord The major ascending
pathways of the spinal cord are (see also Chapter 22):
•
Spinothalamic tract (STT), also known as the
anterolateral system;
•
Spinocerebellar tracts;
•
Dorsal columns (dcs; sometimes called the dorsal
column- medial lemniscus system).
Each tract relays specific information in a topographical fashion, i.e.
the sensory information from different parts of the body is conserved in the
organization of the ascending pathways. Inputs from the more rostral parts of
the body (arm as opposed to leg) supply fibres that lie more laterally in the
ascending pathway.
Both the DC and STT decussate (fibres cross the midline) and therefore
the sensory information they relay is ultimately processed in the contralateral
cerebral hemisphere. However, the site at which this decussation occurs is
different for the two pathways, with the anterolateral system crossing the
midline in the spinal cord while the DCs decussate in the lower medulla after
synapsing in the DC nuclei and forming the medial lemniscus (see Chapters 31
and 32).
Spinal motor neurones
•
α- and γ-MNs are both found in the ventral
(anterior) horn.
• The α-MNs are some the largest neurones found in
the nervous system and innervate skeletal muscle fibres, while the γ-MNs inner-
vate the intrafusal muscle fibres of the muscle spindle (see Chapter 36).
• The cervical cord MNs innervate the arm muscles
while the lumbar and sacral MNs innervate the leg musculature.
• The MNs are arranged somatotopically across
the ventral horn such that the more medially placed MNs innervate proximal
muscles, while those located more laterally innervate distal muscles (see
Chapter 35–37).
Descending motor tracts
There are a number of descending motor pathways that are defined by their
site of origin within the brain (see Chapter 37):
• Corticospinal (Cost) Or Pyramidal Tract
Originates In The Cere- Bral Cortex;
• Rubrospinal Tract Originates From The Red
Nucleus In The Midbrain And Along With The Cost Innervates The Laterally Placed
Mns That Supply The Distal Musculature;
• The Vestibulospinal, Reticulospinal And
Tectospinal Tracts – Known As The Extrapyramidal Tracts – Innervate The More
Ventro- Medially Placed Mns That Control The Axial Musculature (See Chapters
35–37).
Clinical features of spinal cord damage (see Chapter 55)
A knowledge of the organizational anatomy of the spinal cord allows one to
predict the pattern of deficits with damage, which is of great value in
clinical neurology. Examples of specific spinal cord lesions and syndromes
illustrating this point are discussed in hapters 54 and 55.
