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SPINAL CORD
28. 04. 2014
Kaan Yücel
M.D., Ph.D.
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Dr.Kaan Yücel
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The nervous system comprises the central nervous system (CNS) and the peripheral nervous system (PNS).
The CNS is surrounded and protected by the skull (neurocranium) and vertebral column and consists of the brain
and the spinal cord. The PNS exists primarily outside these bony structures.
The entire nervous system is composed of neurons, which are characterized by their ability to conduct
information in the form of impulses (action potentials), and their supporting cells plus some connective tissue. A
neuron has a cell body (perikaryon) with its nucleus and organelles that support the functions of the cell and its
processes. Dendrites are the numerous short processes that carry an action potential toward the neuron’s cell body,
and an axon is the long process that carries the action potential away from the cell body. Many axons are ensheathed
with a substance called myelin, which acts as an insulator. Myelinated axons transmit impulses much faster.
The central nervous system consists of the brain and spinal cord, and the peripheral nervous system consists of
the sensory and motor nerves that are distributed throughout the body and that convey information to and from the
brain (via 12 pairs of cranial nerves) and the spinal cord (via 31 pairs of spinal nerves). The peripheral nervous
system is divided into the somatic nervous system and the autonomic nervous system.
The somatic nervous system is the part of the PNS that innervates the skin, joints, and skeletal muscles.
The autonomic nervous system (ANS) is the part of the PNS that innervates internal organs, blood vessels,and
glands. The neurons of the brain can be classified functionally into three major groups: afferent, motor, and
interneurons. Nerve cell bodies and axons are surrounded by glial cells. There are between 10 and 50 times more
glial cells than neurons in the CNS. Glial cells have other roles than processing information.
The spinal cord is a vital communication link between the brain and the peripheral nervous
system. Within the spinal cord, sensory nerves carry messages from the body to the brain for interpretation, and
motor nerves relay messages from the brain to the effectors. The spinal cord is also the primary reflex centre,
coordinating rapidly incoming and outgoing neural information.
The spinal cord begins as a continuation of the medulla oblongata (commonly called the medulla), the caudal part of
the brainstem In adults, the spinal cord extends from the foramen magnum to the level of the L1 or L2 vertebra.
Thus the spinal cord occupies only the superior two thirds of the vertebral canal. The distal end of the cord (the
conus medullaris) is cone shaped. A fine filament of connective tissue (the pial part of the filum terminale)
continues inferiorly from the apex of the conus medullaris. The bundle of spinal nerve roots arising inferior to the
L1 vertebra, known as the cauda equina (L. horse tail), descends past the termination of the spinal cord.
The spinal cord has two major swellings or enlargements in regions associated with the origin of spinal nerves
that innervate the upper and lower limbs. The cord possesses in the midline anteriorly a deep longitudinal fissure,
the anterior median fissure, and on the posterior surface a shallow furrow, the posterior median sulcus. Internally,
the cord has a small central canal surrounded by gray and white matter:
The spinal cord is a long tubular structure that is divided into a peripheral white matter (composed of myelinated
axons) and a central gray matter (cell bodies and their connecting fibers). When viewed in cross section, the gray
matter has pairs of horn-like projections into the surrounding white matter. These horns are called ventral horns,
dorsal horns, and lateral horns.
The arterial supply to the spinal cord comes from two sources: segmental spinal arteries, and two longitudinal
arteries; one anterior spinal artery and two posterior spinal arteries.
Spinal nerves initially arise from the spinal cord as rootlets; the rootlets converge to form two nerve roots. An
anterior (ventral) nerve root, consisting of motor (efferent) fibers passing from nerve cell bodies in the anterior horn
of spinal cord gray matter to effector organs located peripherally. A posterior (dorsal) nerve root, consisting of
sensory (afferent) fibers from cell bodies in the spinal sensory or posterior (dorsal) root ganglion that extend
peripherally to sensory endings and centrally to the posterior horn of spinal cord gray matter. A spinal cord segment
is the portion of the spinal cord that gives rise to a pair of spinal nerves. Thus, the spinal cord gives rise to 8 pairs of
cervical nerves (C1–C8), 12 pairs of thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral
nerves (S1–S5), and 1 pair of coccygeal nerves (Co1). As they emerge from the intervertebral foramina, spinal
nerves are divided into two rami: Posterior rami of spinal nerves supply nerve fibers to the synovial joints of the
vertebral column, deep muscles of the back, and the overlying skin in a segmental pattern. Anterior rami of spinal
nerves supply nerve fibers to the much larger remaining area, consisting of the anterior and lateral regions of the
trunk and the upper and lower limbs.
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The spinal dura mater is the outermost
meningeal membrane and is separated from the bones forming the 2
vertebral canal by an extradural space. The arachnoid mater is a thin delicate membrane. It is separated from the pia
mater by the subarachnoid space. The subarachnoid space between the arachnoid and pia mater contains CSF
(Cerebrospinal fluid-Beyin-omurilik sıvısı-BOS). The spinal pia mater is a vascular membrane that firmly adheres
to the surface of the spinal cord.
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Spinal cord
INTRODUCTION TO NERVOUS SYSTEM
The nervous system comprises the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS is surrounded and protected by the skull (neurocranium) and vertebral column and consists of
the brain and the spinal cord. The PNS exists primarily outside these bony structures.
The entire nervous system is composed of neurons, which are characterized by their ability to conduct
information in the form of impulses (action potentials), and their supporting cells plus some connective tissue.
A neuron has a cell body (perikaryon) with its nucleus and organelles that support the functions of the cell and
its processes. Dendrites are the numerous short processes that carry an action potential toward the neuron’s cell
body, and an axon is the long process that carries the action potential away from the cell body. Many axons are
ensheathed with a substance called myelin, which acts as an insulator. Myelinated axons transmit impulses
much faster than nonmyelinated axons.
One neuron communicates with other neurons or glands or muscle cells across a junction between cells
called a synapse. Typically, communication is transmitted across a synapse by means of specific
neurotransmitters, such as acetylcholine, epinephrine, and norepinephrine, but in some cases in the CNS by
means of electric current passing from cell to cell.
The central nervous system consists of the brain and spinal cord, and the peripheral nervous system
consists of the sensory and motor nerves that are distributed throughout the body and that convey information
to and from the brain (via 12 pairs of cranial nerves) and the spinal cord (via 31 pairs of spinal nerves). The
peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.
The somatic nervous system is the part of the PNS that innervates the skin, joints, and skeletal muscles.
The autonomic nervous system (ANS) is the part of the PNS that innervates internal organs, blood
vessels,and glands.
2. NEURONS & GLIAL CELLS
Information coming from peripheral receptors that sense the environment is analyzed by the brain into
components that give rise to perceptions, some of which are stored in memory. On the basis of this information,
the brain gives commands for the proper action (motor, emotional, autonomic, cognitive, etc. responses). The
brain does all this with nerve cells and the connections between them. Despite the simplicity of the basic units,
the complexity of behavior –evident in our capability for perception, information storage, and action-, is
achieved by the concerted signaling of an enormous number of neurons. The best estimate is that the human
brain contains about 100 billion neurons. Although nerve cells can be classified into perhaps as many as 10,000
different types, they share many common features.
A key discovery in the organization of the brain is that nerve cells with basically similar properties are
able to produce very different actions because of precise connections with each other and with sensory
receptors and muscle.
Nerve cells differ from other cells in the body because of their ability to communicate rapidly with one
another, sometimes over great distances and with great precision. The rapid and precise communication is made
possible by two mechanisms- axonal conduction and synaptic transmission. Synaptic transmission can be
electrical or chemical. At chemical synapses the pre-and post-synaptic elements are separated by a synaptic
cleft.
A typical neuron has four morphologically defined regions: the cell body [also called the soma,
consisting of the nucleus and perikaryon]; dendrites, axon and presynaptic terminals. Each of these regions has
a distinct function in the generation of signals. Nerve cells differ most at the molecular level. The cell body is
the metabolic center of the neuron. The cell body usually gives rise to two types of processes called the
dendrites and the axon. A neuron usually has several dendrites; the branch out in tree-like fashion and serve as
the main apparatus for receiving the input to the neuron from other nerve cells. Often the cell body is triangular
or pyramidal in shape.
The cell body also gives rise to one axon, a tubular process with a diameter ranging that can ramify and
extend up to 1 meter. The axon is the main conducting unit of the neuron; it is capable of conveying
information great distances by propagating the signal and producing the transient electrical signal, called action
potential. Large axons are surrounded by a fatty insulating sheath called myelin, which is essential for highspeed conduction of action potentials.
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Near its end the axon divides into fine branches that have specialized swellings called presynaptic
terminals; these are the transmitting elements of a neuron. By means of its terminals, one neuron transmits
information about its activity to the receptor surfaces (for example dendrites) of other neurons. The point of
contact is known as synapse. The neuron sending out the information, therefore is called the presynaptic
neuron, the neuron receiving the information is called the postsynaptic neuron. The space separating the
presynaptic from the postsynaptic cell is called the synaptic cleft. Most presynaptic neurons terminate near the
postsynaptic neuron’s dendrites, but communication may occur with the cell body or, less often, with the initial
segment or terminal portion of the axons.
Neuron types:
On the basis of the number of processes that arise from the cell body, neurons are classified into three
large groups:
1) Unipolar neurons: have one primary process that may give rise to many branches. One branch is the
axon and other branches serve as dendritic receiving structures.
2) Bipolar neurons: have an ovoid soma and two processes; a peripheral process or dendrite which
conveys information from the periphery, and a central process or axon, which carries information toward the
CNS. Many bipolar cells are sensory.
3) Multipolar neurons: predominate in the vertebrate nervous system. These cells have a single axon and
one or more dendritic branches that typically emerge from all parts of the cell body. The size and shape of
cells vary. The larger dendritic tree of the Purkinje cell of the cerebellum receives approximately 150.000
contacts.
The neurons of the brain can be classified functionally into three major groups: afferent, motor, and
interneurons. Afferent or sensory neurons carry information into the nervous system both for conscious
perception and for motor coordination. Motor neurons carry commands to muscles and glands. Interneurons
constitute by far the largest class and consist of all the remaining cells in the nervous system that are not
specifically sensory or motor. Interneurons process information locally or convey information from one site
within the nervous system to another.
Glial cells
Nerve cell bodies and axons are surrounded by glial cells [Greek glia, “glue”]. There are between 10
and 50 times more glial cells than neurons in the CNS. Glial cells have other roles than processing information.
Some of the functions of the glial cells are as follows:
1- They serve as supporting elements, providing firmness and structures to the brain. They also separate
and occasionally insulate groups of neurons from each other.
2- Oligodendrocyte in the CNS forms myelin, the insulating sheath that covers most large axons.
3- Some glial cells remove debris after injury or neuronal death.
4- Some glial cells take up and remove chemical transmitters released by neurons during synaptic
transmission.
5- Some glial cells have nutritive functions for nerve cells.
Glial cells are divided into two major classes: microglia and macroglia. Ependymal cells are also
considered as glial cells.
TYPES OF NERVES
The PNS is anatomically and operationally continuous with the CNS. Its afferent (sensory) fibers
convey neural impulses to the CNS from the sense organs (e.g., the eyes) and from sensory receptors in various
parts of the body (e.g., in the skin). Its efferent (motor) fibers convey neural impulses from the CNS to effector
organs (muscles and glands). Nerves are either cranial nerves or spinal nerves, or derivatives of them.
SPINAL CORD
The spinal cord is a vital communication link between the brain and the peripheral nervous
system. Within the spinal cord, sensory nerves carry messages from the body to the brain for interpretation,
and motor nerves relay messages from the brain to the effectors. The spinal cord is also the primary reflex
centre, coordinating rapidly incoming and outgoing neural information.
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Spinal cord
The spinal cord is the major reflex center and conduction pathway between the body and brain. This
cylindrical structure, slightly flattened anteriorly and posteriorly, is protected by the vertebrae, their associated
ligaments and muscles, the spinal meninges, and the cerebrospinal fluid (CSF).
The spinal cord begins as a continuation of the medulla oblongata (commonly called the medulla), the
caudal part of the brainstem In adults, the spinal cord is 42-45 cm long and extends from the foramen magnum
in the occipital bone to the level of the L1 or L2 vertebra. However, its tapering inferior end, the conus
medullaris, may terminate as high as T12 vertebra or as low as L3 vertebra. Thus the spinal cord occupies only
the superior two thirds of the vertebral canal. In neonates, the spinal cord extends approximately to vertebra
LIII, but can reach as low as vertebra LIV. In the young child, it is relatively longer and ends at the upper
border of the third lumbar vertebra. The distal end of the cord (the conus medullaris) is cone shaped. A fine
filament of connective tissue (the pial part of the filum terminale) continues inferiorly from the apex of the
conus medullaris.
Although the spinal cord terminates at the level of first or second lumbar vertebra, the filum terminale
and the spinal nerve roots from the lumbosacral part of the spinal cord that form the cauda equina continue
inferiorly within the lumbar cistern containing CSF. This bundle of spinal nerve roots arising inferior to the L1
vertebra, known as the cauda equina (L. horse tail), descends past the termination of the spinal cord.
The spinal cord is not uniform in diameter along its length. It has two major swellings or enlargements
in regions associated with the origin of spinal nerves that innervate the upper and lower limbs. A cervical
enlargement occurs in the region associated with the origins of spinal nerves C5 to T1, which innervate the
upper limbs (brachial plexus). A lumbosacral enlargement occurs in the region associated with the origins of
spinal nerves L1 to S4, which innervate the lower limbs (lumbosacral plexus).
Inferiorly, the spinal cord tapers off into the conus medullaris, from the apex of which a prolongation of
the pia mater, the filum terminale, descends to be attached to the back of the coccyx . The cord possesses in the
midline anteriorly a deep longitudinal fissure, the anterior median fissure, and on the posterior surface a
shallow furrow, the posterior median sulcus. Internally, the cord has a small central canal (containing CSF)
surrounded by gray and white matter:
The gray matter is rich in nerve cell bodies, which form longitudinal columns along the cord, and in
cross-section these columns form a characteristic H-shaped appearance in the central regions of the cord;
The white matter surrounds the gray matter and is rich in nerve cell processes, which form large bundles or
tracts that ascend and descend in the cord to other spinal cord levels or carry information to and from the brain.
The spinal cord is a long tubular structure that is divided into a peripheral white matter (composed of
myelinated axons) and a central gray matter (cell bodies and their connecting fibers). When viewed in cross
section, the gray matter has pairs of horn-like projections into the surrounding white matter. These horns are
called ventral horns, dorsal horns, and lateral horns, but in three dimensions they represent columns that run the
length of the spinal cord.
The ventral horns contain the cell bodies of motor neurons and their axons. A collection of neuronal cell
bodies in the CNS is a nucleus. Axons of the ventral horn nuclei leave the spinal cord in bundles called ventral
roots. These motor fibers innervate skeletal muscles.
The lateral (intermediolateral) horns contain the cell bodies for the sympathetic nervous system at spinal
cord levels T1–L2 and for the parasympathetic nervous system at spinal cord levels S2–S4. The axons from
these neurons also leave the spinal cord through the ventral root and will synapse in various peripheral ganglia.
A collection of neuronal cell bodies in the PNS is a ganglion.
The dorsal horns receive the sensory fibers originating in the peripheral nervous system. Sensory fibers
reach the dorsal horn by means of a bundle called the dorsal root. The central axons of the sensory neuron enter
the dorsal horn of the gray matter. Some of these fibers will run in tracts (a bundle of fibers in the CNS) of the
white matter to reach other parts of the CNS. Other axons will synapse with intercalated neurons (interneurons),
which in turn synapse with motor neurons in the ventral horn to form a reflex arc.
The arterial supply to the spinal cord comes from two sources. It consists of: longitudinally oriented
vessels, arising superior to the cervical portion of the cord, which descend on the surface of the cord; and feeder
arteries that enter the vertebral canal through the intervertebral foramina at every level; these feeder vessels, or
segmental spinal arteries, arise predominantly from the vertebral and deep cervical arteries in the neck, the
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posterior intercostal arteries in the thorax, and the lumbar arteries in the abdomen. After entering an
intervertebral foramen, the segmental spinal arteries give rise to anterior and posterior radicular arteries. This
occurs at every vertebral level.
The longitudinal vessels consist of: a single anterior spinal artery, which originates from the vertebral
arteries- passes inferiorly, approximately parallel to the anterior median fissure, along the surface of the spinal
cord; and two posterior spinal arteries, which also originate in the cranial cavity, usually arising directly from
a terminal branch of each vertebral artery. The right and left posterior spinal arteries descend along the spinal
cord, bracket the posterolateral sulcus.
Veins that drain the spinal cord form a number of longitudinal channels. These longitudinal channels
drain into an extensive internal vertebral plexus in the extradural (epidural) space of the vertebral canal.
.Spinal Nerves
Spinal nerves initially arise from the spinal cord as rootlets; the rootlets converge to form two nerve
roots. An anterior (ventral) nerve root, consisting of motor (efferent) fibers passing from nerve cell bodies in
the anterior horn of spinal cord gray matter to effector organs located peripherally.
A posterior (dorsal) nerve root, consisting of sensory (afferent) fibers from cell bodies in the spinal
sensory or posterior (dorsal) root ganglion that extend peripherally to sensory endings and centrally to the
posterior horn of spinal cord gray matter.
The posterior and anterior nerve roots unite, within or just proximal to the intervertebral foramen, to
form a mixed (both motor and sensory) spinal nerve, which immediately divides into two rami (L., branches): a
posterior (dorsal) ramus and an anterior (ventral) ramus. As branches of the mixed spinal nerve, the posterior
and anterior rami carry both motor and sensory fibers, as do all their subsequent branches. The terms motor
nerve and sensory nerve are almost always relative terms, referring to the majority of fiber types conveyed by
that nerve. Nerves supplying muscles of the trunk or limbs (motor nerves) also contain about 40% sensory
fibers, which convey pain and proprioceptive information. Conversely, cutaneous (sensory) nerves contain
motor fibers, which serve sweat glands and the smooth muscle of blood vessels and hair follicles.
Posterior rami are distributed to the synovial joints of the vertebral column, deep muscles of the back,
and the overlying skin. The remaining anterolateral body wall, including the limbs, is supplied by anterior rami.
Posterior rami and the anterior rami of spinal nerves T2-T12 generally do not merge with the rami of adjacent
spinal nerves to form plexuses.
Spinal (segmental) nerves exit the vertebral column (spine) through intervertebral foramina. Spinal
nerves arise in bilateral pairs from a specific segment of the spinal cord. The 31 spinal cord segments and the
31 pairs of nerves arising from them are identified by a letter and number (e.g., “T4”) designating the region of
the spinal cord and their superior-to-inferior order (C, cervical; T, thoracic; L, lumbar; S, sacral; Co,
coccygeal). A spinal cord segment is the portion of the spinal cord that gives rise to a pair of spinal nerves.
Thus, the spinal cord gives rise to 8 pairs of cervical nerves (C1–C8), 12 pairs of thoracic nerves (T1–T12), 5
pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of coccygeal nerves (Co1). The
spinal cord segments are numbered in the same manner as these nerves.
The first cervical nerve (C1) emerges from the vertebral canal between the skull and vertebra CI.
Therefore cervical nerves C2 to C7 also emerge from the vertebral canal above their respective vertebrae.
Because there are only seven cervical vertebrae, C8 emerges between vertebrae CVII and TI. As a
consequence, all remaining spinal nerves, beginning with T1, emerge from the vertebral canal below their
respective vertebrae. Cervical spinal nerves (except C8) bear the same alphanumeric designation as the
vertebrae forming the inferior margin of the IV foramina through which the nerve exits the vertebral canal. The
more inferior spinal (T1 through Co1) nerves bear the same alphanumeric designation as the vertebrae forming
the superior margin of their exit. The first cervical nerves lack posterior roots in 50% of people, and the
coccygeal nerve may be absent.
Although the dorsal root is essentially sensory and the ventral root is motor, the two roots come together
within the bony intervertebral foramen to form a mixed spinal nerve (i.e., it contains both sensory and motor
fibers). The spinal cord is defined as part of the CNS, but the ventral and dorsal roots are considered parts of
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the PNS. Outside the intervertebral foramen, the mixed nerve divides into a ventral ramus (from the Latin for
“branch”) and a dorsal ramus.
The larger ventral ramus supplies the ventrolateral body wall and the limbs; the smaller dorsal ramus
supplies the back. Since the ventral and dorsal rami are branches of the mixed nerve, they both carry sensory
and motor fibers.
The term “peripheral nerve” such as sciatic nerve, ulnar nerve etc. should not be confused by the spinal
nerve. Peripheral nerve is the last product of these somatic networks; somatic plexuses.
The anterior rami form plexuses (network). All major somatic plexuses (cervical, brachial, lumbar, and
sacral) are formed by anterior rami (ramus=branch, rami=branches).
The peripheral nervous system contains two systems; one working voluntarily; somatic nervous system
(soma, in ancient Greek, body), and one involuntarily, as it name implies, autonomic nervous system.
The unilateral area of skin innervated by the sensory fibers of a single spinal nerve is called a
dermatome; the unilateral muscle mass receiving innervation from the fibers conveyed by a single spinal nerve
is a myotome. Generally, at least two adjacent spinal nerves (or posterior roots) must be interrupted to produce
a discernible area of numbness.
As they emerge from the intervertebral foramina, spinal nerves are divided into two rami
 Posterior (primary) rami of spinal nerves supply nerve fibers to the synovial joints of the vertebral column,
deep muscles of the back, and the overlying skin in a segmental pattern. As a general rule, the posterior rami
remain separate from each other (do not merge to form major somatic nerve plexuses).
 Anterior (primary) rami of spinal nerves supply nerve fibers to the much larger remaining area, consisting of
the anterior and lateral regions of the trunk and the upper and lower limbs. The anterior rami that are distributed
exclusively to the trunk generally remain separate from each other, also innervating muscles and skin in a
segmental pattern. However, primarily in relationship to the innervation of the limbs, the majority of anterior
rami merge with one or more adjacent anterior rami, forming the major somatic nerve plexuses (networks) in
which their fibers intermingle and from which a new set of multisegmental peripheral nerves emerges. The
anterior rami of spinal nerves participating in plexus formation contribute fibers to multiple peripheral nerves
arising from the plexus; conversely, most peripheral nerves arising from the plexus contain fibers from multiple
spinal nerves.
Spinal meninges
The spinal dura mater is the outermost meningeal membrane and is separated from the bones forming
the vertebral canal by an extradural space. Superiorly, it is continuous with the inner meningeal layer of cranial
dura mater at the foramen magnum of the skull. Inferiorly, the dural sac dramatically narrows at the level of the
lower border of vertebra SII and forms an investing sheath for the pial part of the filum terminale of the spinal
cord. This terminal cord-like extension of dura mater (the dural part of the filum terminale) attaches to the
posterior surface of the vertebral bodies of the coccyx.
The arachnoid mater is a thin delicate membrane against, but not adherent to, the deep surface of the
dura mater. It is separated from the pia mater by the subarachnoid space. The arachnoid mater ends at the level
of vertebra SII.
The subarachnoid space between the arachnoid and pia mater contains CSF (Cerebrospinal fluid-Beyinomurilik sıvısı-BOS). The subarachnoid space around the spinal cord is continuous at the foramen magnum
with the subarachnoid space surrounding the brain.
The spinal pia mater is a vascular membrane that firmly adheres to the surface of the spinal cord.
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