Spinal cord
Definition and function
A part of the central nervous system that is inside the spinal canal and sends
messages between the brain and the rest of the body.
Structure and segments
- It has an outer layer of white matter and a middle layer of gray matter.
- It is made up of different parts: the cervical part (8 sections), thoracic part (12
sections), lumbar part (5 sections), sacral part (5 sections), and coccygeal part (1
section).
Spinal nerves
There are 31 pairs of nerves that come from different parts of the spinal cord and
send messages to and from different areas of the body.These include 8 pairs
from the cervical part, 12 pairs from the thoracic part, 5 pairs from the lumbar
part, 5 pairs from the sacral part, and 1 pair from the coccygeal part.
Anatomy
The spinal cord is part of the central nervous system.It is inside the space formed
by the bones of the spine called the vertebral canal. When the body is growing,
the spinal cord stops growing earlier than the bones of the spine. The spinal cord
finishes growing by the age of 4, while the spine continues to grow until around
14 to 18 years old. That is why, in adults, the spinal cord only covers the top
two-thirds of the vertebral canal.
The spinal cord is connected to the brainstem.It starts at the top of the skull,
where it connects to the brain, and ends at the L1 or L2 vertebra, where it ends
as the conus medullaris. A thin strand called the filum terminale connects the end
of the spinal cord to the first coccygeal vertebra and holds the spinal cord in
place.
You can easily remember how far the spinal cord goes by using the mnemonic
'SCULL', which stands for 'Spinal Cord Until L2 (LL)'.
Along its length, the spinal cord has two swollen areas to help send messages to
the arms and legs.One is in the cervical area (for the arms), and the other is in
the lumbosacral area (for the legs).
Like the spine, the spinal cord is divided into five parts: cervical, thoracic, lumbar,
sacral, and coccygeal.Each section has one or more segments, and each
segment gives rise to one pair of spinal nerves. These nerves come out of the
spine through small openings called intervertebral foramina. There are 8 cervical
pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 coccygeal pair,
making a total of 31 pairs of spinal nerves.
Cross section
The spinal cord has both gray and white matter, similar to other parts of the
central nervous system. When you look at a cross section, you can see four main
areas: the front (anterior), the back (posterior), and two sides (lateral). These
areas have different features — the front has a groove called a fissure, while the
sides and back have shallow grooves called sulci.
The gray matter is the central part of the spinal cord and looks like a butterfly.It
contains the cell bodies of neurons. This part is divided into three sections: the
front (anterior), the side (lateral), and the back (posterior). The white matter is
found around the gray matter and is made up of axons, the long parts of nerve
cells. This white matter has pathways that connect the brain to the rest of the
body.
The spinal cord and the nerves that come out of it are covered by three layers
called meninges.The outer layer is called the dura mater, and it has two parts:
the periosteal layer and the meningeal layer. Between these two layers is the
epidural space. The middle layer is the arachnoid mater, and the innermost layer
is the pia mater. Between the arachnoid and pia mater is the subarachnoid
space, which is filled with cerebrospinal fluid.
Blood supply
The spinal cord gets its blood supply from branches of the vertebral artery and
the segmental arteries.
The vertebral artery forms the anterior and posterior spinal arteries. Segmental
arteries, like the deep cervical, ascending cervical, and posterior intercostal
arteries, give off 31 pairs of radicular arteries that supply the spinal nerve roots.
Veins that have the same names as the arteries go along with them.The anterior
and posterior spinal veins drain into radicular veins, which then empty into the
internal and external vertebral venous plexus. These veins eventually connect to
the veins in the neck and trunk. Blood supply is an important part of learning
about the spine.
Spinal cord tracts
The spinal cord has pathways, called tracts, that run through its white matter.
On each side of the spinal cord, the white matter is divided into three parts: the
front (anterior), the side (lateral), and the back (posterior).
The ascending tracts carry information from the body up to the brain.The
descending tracts carry signals from the brain down to the body. The spinal cord
is more than just a passageway—it also changes and combines the signals that
pass through it.
Key facts about spinal cord tracts
Anterior funiculus
Ascending tract:
- Anterior spinothalamic tract
Descending tracts:
- Anterior corticospinal tract
- Vestibulospinal tract
- Tectospinal tract
- Reticulospinal tract
Lateral funiculus
Ascending tracts:
- Posterior spinocerebellar tract
- Anterior spinocerebellar tract
- Lateral spinothalamic tract
- Spinotectal tract
- Posterolateral tract of Lissauer
- Spinoreticular tract
- Spino-olivary tract
Descending tracts:
- Lateral corticospinal tract
- Rubrospinal tract
- Lateral reticulospinal tract
- Descending autonomic tracts
- Olivospinal tract
Posterior funiculus
Ascending tracts:
- Gracile fasciculus of Golgi
- Cuneate fasciculus of Burdach
Spinal nerves
Spinal nerves are grouped into different types based on where they come from
on the spinal cord. These groups are called cervical (C1 to C8), thoracic (T1 to
T12), lumbar (L1 to L5), sacral (S1 to S5), and coccygeal (Co1). Each group
comes from a specific part of the spinal cord.
The spinal cord is divided into sections that match the time during a baby's
development in the womb when the spinal cord fills up the whole space inside
the spine.As people grow, the spine gets longer than the spinal cord, so each
section of the spinal cord is actually higher up than the bone it's connected to.
This difference is most noticeable in the lower parts of the spine, like the lumbar
and sacral areas. For example, the spinal cord segment L5 is actually at the level
of the L1 vertebra.
For the nerves that are further down the spine to come out, they have to travel
down through the space inside the spine.
The nerves from the lower part of the spine, like the lumbar and sacral areas,
have to travel the farthest to reach their exit points, so they are the longest. As
these nerves move down toward their exit points, they come together in a bundle
called the cauda equina, which means "horse's tail" in Latin.
Each spinal nerve has two parts: an anterior root and a posterior root.The
anterior root carries signals from the body to the brain and comes from the front
part of the spinal cord's gray matter. It exits the spinal cord through a groove on
the front side. The posterior root carries signals from the brain to the body and
comes from the back part of the spinal cord's gray matter. It exits through a
groove on the back side, and it has a small cluster of nerve cells called a sensory
ganglion attached to it.
Just before the nerves exit through the space between the vertebrae, the anterior
and posterior roots come together to form the main part of the spinal nerve.
This main part is very short, and as soon as it leaves the spine, it splits into four
branches: the anterior ramus, posterior ramus, communicating ramus, and
meningeal ramus.
Reflex arc
The spinal cord has a big role in controlling the body, and much of that control
comes from the brain, which sends and receives messages from different parts of
the body. But some reflexes happen in the spinal cord without the brain being
involved. These are called spinal reflexes, and they come in two main types:
monosynaptic and polysynaptic.
Monosynaptic reflexes involve just two nerve cells.One is a sensory neuron that
detects a stimulus, like stretching a muscle, and sends a signal through the
spinal cord. The other is a motor neuron that gets the signal and tells the muscle
to contract. This happens very quickly and directly. An example of this is the
stretch reflex, where a muscle tightens when stretched.
Polysynaptic reflexes are more complicated.They involve more than two nerve
cells. Along with a sensory neuron and a motor neuron, there are also
interneurons that help pass the message between them. These reflexes often
involve groups of muscles working together instead of just one muscle. A good
example is the withdrawal reflex, where you pull your hand away from something
hot.
