Spinal Cord

The main nerve tissue column that is connected to the brain and lies within the vertebral canal from which the spinal nerves emerge.

It is a tubular bundle of nervous tissue that extends from the brainstem to the lumbar vertebrae and supports cells. The central nervous system is made of the spinal cord and the brain.

In the spinal cord thirty-one pairs of spinal nerves originate: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. The central nervous system consists of the spinal cord and the hippocampus. The spinal cord is formed by nerve fibers which transmit impulses to and from the brain. Like the brain, three connective-tissue envelopes, called the meninges, protect the spinal cord. Cerebrospinal fluid (CSF) fills the space between the outer and middle envelopes, a clear colorless fluid which cushions the spinal cord against jarring shock. Sometimes simply known as the chain. Also called simply as the cord. The below picture illustrates the schematic diagram of the human spinal cord running via the vertebral column.

Spinal Cord Anatomy and Its Structure

In this section spinal cord anatomy and spinal cord structure (spinal cord parts) is discussed in detail.

The spinal cord is the major part of CNS. This is found within the vertebral canal of the vertebral column. There is a disproportion in development between growth of the spinal cord and growth of the vertebral column. The spinal cord grows at age 4 while the vertebral column ends up rising at age 14-18. That is the reason why the spinal cord only occupies the upper two thirds of the vertebral canal in adults.

The spinal cord is an extension of the brain stem. This runs from the foramen magnum at the skull base to the L1 / L2 vertebra where it ends as the medullary cone (conus medullaris). A thin thread called filum terminale stretches all the way to the 1st coccygeal vertebra (Co1) from the tip of the conus medullaris and anchors the spinal cord in place.

The spinal cord displays two well-defined enlargements for the innervation of the upper and lower limbs along its length: one at the cervical (upper limbs) and one at the lumbosacral (lower limb).

Similarly, as the vertebral column, the spinal cord is divided into segments: cervical, thoracic, lumbar, sacral, and coccygeal. 

Spinal Cord Cross Section

Like other areas of the CNS, the spinal cord is composed of gray and white matter. This shows four surfaces: anterior, back, and two laterals. They include anterior (fissures) and sulci (anterolateral, posterolateral, and posterior).

The gray matter is the main component of the spinal cord which is shaped like a butterfly and consists of neuronal cell bodies. It shows horns that are anterior, lateral, and posterior. White matter encircles the gray matter and consists of axons. It includes pathways which connect the brain to the rest of the body.

Spinal Meninges

The roots of the spinal cord and the spinal nerve are wrapped within three layers known as meninges. The outermost is the dura mater, the arachnoid material underneath and the deepest is the pia mater. Dura mater has two layers (periosteal and meningeal) in between space is epidural space. The subarachnoid area is between the arachnoid and pia mater and it is filled with cerebrospinal fluid.

Blood Supply:

Branches of the vertebral and segmental arteries supply the spinal cord.

The vertebral artery is responsible for the formation of anterior and posterior spinal arteries. Segmental arteries, such as the deep cervical, ascending cervical, and posterior intercostal arteries, give rise to 31 pairs of radicular arterial branches that supply spinal nerve roots.

The arteries are surrounded by similar named veins. Anterior and posterior spinal veins drain into radicular veins, which then empty into the vertebral venous plexus (internal and outer). Eventually, this network empties through the vertebral (neck) and segmental (trunk) veins.

Spinal Nerves

The spinal nerves are mixed nerves forming the peripheral nervous system that originate from the spinal cord.

Number of Spinal Nerves

Human have 31 left–right pairs of spinal nerves, each roughly relating to a segment of the vertebral column: 

1. 8 cervical spinal nerve pairs (C1–C8)

2. 12 thoracic pairs (T1–T12)

3. 5 lumbar pairs (L1–L5)

4. 5 sacral pairs (S1–S5)

5. 1 coccygeal pair. 

Each spinal nerve begins in the form of an anterior (motor) and a posterior (sensory) root nerve. These roots arise from the spinal cord, and form a single spinal nerve at the intervertebral foramina.

Then the spinal nerve leaves the vertebral canal through the intervertebral foramina, then divides into two:

• Posterior Roots – Supplies nerve fibers to the vertebral column synovial joints, deep back muscles, and overlying s skin.

• Anterior Roots – Supplies nerve fibres, both motor and sensory, to most of the remaining body area.

The nerve roots L2-S5 originate from the distal end of the spinal cord, forming a bundle of nerves known as the cauda equina.                                                                                  

Spinal Cord Tracts

Spinal cord neural pathways are located within white matter of the spinal cord. The white matter is split into three funiculi on each side: anterior, lateral, and posterior.

Ascending tracts convey information into the brain from the periphery. On the other hand, the tracts coming down carry information from the brain to the periphery. The spinal cord is more than just a conduit, as the information that passes through it is also modified and integrated.

Spinal Cord Functions

A huge part of the function of the spinal cord is under brain influence. The main spinal cord function is to relay information from and to the periphery. However, there are many reflexes which are generated independently of the brain in the spinal cord. Spinal reflexes are either monosynaptic or polysynaptic.

Monosynaptic Reflexes

Two neurons are involved in this reflex arc: one sensory and one motor.

• The first-order neuron is sensory which  is in the spinal ganglion,

• The second-order neuron is motor which  is in the anterior horn of the spinal cord. 

The sensory neuron collects muscle impulses and sends this information to the motor neuron, which innerves the same muscle. Then, the motor neuron allows the innervated muscle to contract. Example: The stretch reflex.

Polysynaptic Reflexes:  Multiple Neurons Participate in This Reflex.

There are also one or more interneurons between them, together with one sensory and one motor neuron, which makes this communication indirect.

They are more complicated than monosynaptic reflexes as they involve muscle groups instead of a single muscle.

Summary: 

Spinal cord is the main vertebrate nerve tract, running from the base of the brain into the spinal column canal. It consists of nerve fibers which mediate reflex actions and transmits impulses to and from the brain.

A cross section of the spinal cord reveals white matter arranged around an area of gray matter shaped like a butterfly. The white matter consists of myelinated fibers, or axons, which form ascending and descending nerve tracts from the brain. The white matter consists of discrete sectors called funiculi. The gray matter contains cell bodies, non myelinated motor-neuron fibers, and interneurons which connect the two sides of the cord. Gray-matter cells form so-called horn projections.

Fibers from the dorsal and ventral horns that exit the spinal cord join in paired tracts to form the spinal nerves. Knowledge passes up neuronal ascending tracts, and is processed by the brain. Nerve impulses traveling down the descending tracts which stimulate motor neurons or initiate glandular secretion induce responses.