Ageing and Endings Cycle A LECTURE SPINAL CORD
Gross Anatomy of the Spinal Cord
The spinal cord is a cylindrical structure that occupies the vertebral canal of the
vertebral column. It is 42- 45 cms long and extends from the base of the skull to
approximately the level of the L1/2 intervertebral disc (it may be one vertebral
level higher or lower). Attaching along the sides of the cord are a series of fibre
bundles known as the dorsal and ventral roots of the spinal nerves.
The dorsal and ventral roots of the spinal nerves
attach along the dorsal and ventral surfaces
respectively of each side of the cord.
Dorsal is sensory and ventral are motor neurons.
The dorsal and ventral roots join together in a
segmental way to form 31 pairs of spinal nerves. The
spinal nerves emerge from the vertebral canal on each
side through the intervertebral foramina, between
each pair of vertebrae. In general spinal nerves are named according the level
where they exit the vertebral canal. There are 8 cervical nerves (C1-8) – but only
7 cervical vertebra as one begins between the first and the skull and the last
between C7 and T1. Below this all the nerves emerge below the corresponding
vertebra.
12 thoracic nerves (T1-12), 5 lumbar nerves (L1-5), 5 sacral nerves (S1-5) and I
coccygeal nerve. The spinal nerves divide the spinal cord into segments, a spinal
segment being defined as the area of the spinal cord that gives rise to one spinal
nerve. Because the spinal cord is shorter than the vertebral column, the spinal
segments, particularly the lower ones, do not lie opposite their corresponding
vertebra.
The spinal cord tapers to a conical-shaped ending called the conus medullaris
(at the level of the L1/2 disc), below which the vertebral canal is filled with the
rootlets of the spinal nerves that will exit the vertebral canal below L2. These
rootlets are collectively known as the cauda equina. (horses tail)
The diameter of the spinal cord is not uniform – it is greater in those regions
which give rise to the nerves that supply the limbs and is therefore described as
having a cervical enlargement (segments C5-T1) and a lumbosacral
enlargement (L3-S3) for the upper and lower limbs respectively.
Meninges The spinal cord and nerve roots (not the spinal nerves themselves)
are covered by 3 layers of meninges:
(i)
the pia mater (innermost), which adheres to the surface of the
cord & nerve roots,
(ii)
the arachnoid mater and
(iii) the dura mater (outermost).
The arachnoid lines the inner surface of the dura meter and they are often
collectively referred to as the dural sheath. The subarachnoid space lies
between the arachnoid and the pia mater and is filled with cerebrospinal fluid.
In the vertebral canal (unlike the skull) the dura mater is separated from
the surrounding bone by an epidural space, which is filled with fat and blood
vessels.
The Internal Structure of the Spinal Cord
The spinal cord is formed by a core of central core of grey matter (cell bodies),
surrounded by white matter (that is made up of the axons of nerves passing
towards or away from the brain or between different levels of the cord).
The core of grey matter is H-shaped in cross-section. It consists of a dorsal and a
ventral horn on each side, the grey matter between them forming the
intermediate zone. The ventral horn is formed primarily by the cell bodies of
motor neurons, whose axons pass out of the spinal cord to form the ventral
roots of spinal nerves. The dorsal horns are concerned primarily with the
processing of sensory information. The cells of the dorsal horn relay sensory
information from incoming primary sensory neurons to other parts of the spinal
cord or brain. Primary sensory neurons are specialised in shape, with their cell
bodies being located outside the spinal cord in the dorsal root ganglia (DRG’s).
They each have a peripheral process carrying information to the cell body from
peripheral receptors and a central process that carries information from the cell
body to the spinal cord via the dorsal root. (Note: there is NO synapse in the
DRG). The dorsal and ventral roots unite on each side to form the spinal nerves,
with each nerve containing both primary sensory and motor fibres.
Within the spinal grey matter, cells with similar connections functions tend to
cluster together into groups called nuclei. Similarly, in the spinal white matter
fibres with similar connections and function group together forming tracts.
These tracts may be ascending (carrying sensory information towards the brain)
or descending (carrying signals from the brain to the spinal nuclei). The major
ascending (sensory) tracts are the:
i.
spinothalamic tract – transmits pain and temperature information
from the spinal cord to the thalamus of the forebrain and
ii.
dorsal columns - transmit tactile (touch) and proprioceptive (from
joints and muscles) information to the brainstem.
The major descending (motor) tract is the corticospinal tract, which transmits
information from the cortex of the brain to the spinal motor neurons. These
tracts will be covered later in this course or in A & E next year.
Spinal Nerve (segmental) Distribution – Dermatomes and Myotomes
Each spinal nerve contains both motor and sensory fibres and has a specific area
of skin and specific groups of muscle that it supplies. The total area of skin
supplied by one spinal nerve is known as a dermatome and is named according
to the nerve (spinal segment) that supplies it. The group of muscles supplied by
one spinal nerve is known as a myotome, also named according to the nerve
that supplies it. The pattern of dermatomes and myotomes is referred to as the
segmental distribution and it reflects the embryonic development of the limbs.
In the early embryo the dermatomes form parallel bands around the trunk and
the myotomes correspond. The upper limbs develop from the region C5-T1 and
as they grow away from the trunk they take these dermatomes (skin and sensory
nerve fibres) and myotomes (muscle and motor nerve fibres) with them, such
that the C4 dermatome is the skin over shoulder, C5 the lateral side of the arm,
C6 the lateral side of the forearm and hand, C7 the middle of the hand, C8 the
medial side of the forearm and T1 the medial upper arm and axilla. Dermatomes
representing adjacent spinal segments (eg. C6 and C7) overlap quite a bit (and
this is why you will find some variation in different books) but those of
nonadjacent segments do not (eg. C6 and C8 in the forearm).
bands around the trunk and the myotomes correspond. The upper limbs develop from the region C5-T1
and as they grow away from the trunk they take these dermatomes (skin and sensory nerve fibres) and
myotomes (muscle and motor nerve fibres) with them, such that the C4 dermatome is the skin over
shoulder, C5 the lateral side of the arm, C6 the lateral side of the forearm and hand, C7 the middle of
the hand, C8 the medial side of the forearm and T1 the medial upper arm and axilla. Dermatomes
representing adjacent spinal segments (eg. C6 and C7) overlap quite a bit (and this is why you will find
It is not necessary to memorise the segmental innervation of each individual
some variation
books)
nonadjacent
segments
do notof(eg. C6 and C8 in the
muscle.
Rather,ina different
few simple
rulesbut
willthose
helpof
you
to remember
the pattern
forearm). innervation to the limbs:
segmental
1.
Most muscles are supplied by more than one spinal segment or
It is notalthough
necessary
to memorise
segmentalininnervation
of each individual muscle. Rather, a few
nerve,
there
are somethe
exceptions
the upper limb.
simple 2.
rules will
help you
remember
the pattern
of segmental
to the limbs:
Muscles
thattoshare
a common
primary
action on innervation
a joint are all
1.
Most
muscles
are
supplied
by
more
than
one
spinal
segment
or
nerve,
although
there are some
supplied by the same spinal segments
exceptions
in the opponents,
upper limb.sharing the opposite action, are likewise all
3.
Their
2. Muscles
thatsame
share segments
a commonand
primary
on a usually
joint arerun
all in
supplied
by the same spinal segment
supplied
by the
theseaction
segments
numerical
3.
Their
opponents,
sharing
the
opposite
action,
are
likewise
all
supplied
by
the same segments and
sequence.
these
run in in
numerical
4. segments
Jointsusually
more distal
the limbsequence.
are supplied by lower segments of
4.
Joints
more
distal
in
the
limb
are
supplied by lower segments of the spinal cord than more proxima
the spinal cord than more proximal ones.
ones
The key movements that you need to remember for the upper limb are:
Shoulder:
Abduct & laterally rotate C5
Adduct & medially rotate C6, 7, 8
Elbow:
Flex
C5, 6
Extend
C7, 8
Forearm
Supinate
C6
Pronate
C7, 8
Hand
intrinsic m’s
T1
Knowledge of the segmental pattern of innervation is useful in diagnosing the
Knowledge
of the
pattern
of innervation
is useful
in diagnosing
the location
(level) of
location
(level)
of segmental
lesions to the
spinal
cord or its nerve
roots,
eg. in a patient
in
lesionsthe
to the
spinal
cord
orsevered
its nerveatroots,
eg.level,
in a patient
in whom
thehave
spinal
cord was severed at the
whom
spinal
cord
was
the C7
the patient
would
normal
C7
level,
the
patient
would
have
normal
sensation
down
the
lateral
side
of
the
upper
limb (C5 and C6
sensation down the lateral side of the upper limb (C5 and C6 dermatomes) but
dermatomes)
but nothe
sensation
thethe
medial
of the
(C8 and T1This
dermatomes). This patient
no
sensation down
medialdown
side of
limb side
(C8 and
T1limb
dermatomes).
would
still
be
able
to
abduct
his
arm
(C5)
and
flex
the
elbow
(C5.6)
but
they
patient would still be able to abduct his arm (C5) and flex the elbow (C5.6) would
but be unable to exten
the elbow
or move
the fingers
(C8, T1).
they
would(C7,8)
be unable
to extend
the elbow
(C7,8) or move the fingers (C8, T1).
Once the spinal nerves leave the vertebral canal they rearrange themselves to
Once peripheral
the spinal nerves
vertebral
canal they
themselves
form peripheral nerves
form
nervesleave
eachthe
of which
includes
fibresrearrange
from more
than onetospinal
segment
. These
nervesfibres
will be
covered
your
lecture
on the
brachial
each of which
includes
from
more in
than
onenext
spinal
segment
. These
nerves will be covered in
plexus.
your next lecture on the brachial plexus.
E. Tancred 9/11
Spinal Cord Lecture AEA 2011
P