Spinal Cords & Nerves

What is the spinal cord?

The spinal cord is a long structure composed of nervous tissue specifically neurons and associated cells that support neurons called glial cells. The spinal cord is part of the central nervous system (CNS) and connects the brain to the peripheral nervous system (PNS).

What are spinal nerves and vertebrae?

Spinal nerves extend out from the spinal cord towards the periphery. Bony structures called vertebrae surround the spinal cord. Vertebrae stack on top of one another to create the vertebral column.

What are the anterior and posterior parts of a vertebra?

The smooth side of the vertebrae (the body of the vertebra) is the anterior side while the side of the vertebrae with spiny processes (the spinous process of the vertebra) is the posterior side.

What is the vertebral foramen and vertebral canal?

Each individual vertebrae has an opening hole in the region of the spiny processes called the vertebral foramen. When the vertebrae are stacked on top of one another the successive holes create the vertebral canal. This canal is where the spinal cord is located.

What are the regions of the vertebral column?

From superior to inferior the curves are called the cervical thoracic lumbar sacral and coccygeal regions.

How are vertebrae and spinal nerves named?

The vertebrae in each of these regions will be called the cervical thoracic lumbar sacral and coccygeal vertebrae. The spinal nerves in each region will be called the cervical thoracic lumbar sacral and coccygeal nerves.

What structures protect and support the spinal cord?

The vertebrae provide protection for the delicate spinal cord tissue. This position is maintained by the meninges connective tissue layers that surround the spinal cord.

What are intervertebral foramina?

The spaces or openings between the vertebrae are called the intervertebral foramina. These openings are where the spinal nerves will exit the vertebral canal and extend out to the periphery.

How are spinal nerves numbered?

Spinal nerves are named based on where they EXIT the vertebral column (exit thru inter formina), NOT where they come out of the spinal cord.

Why is the vertebral canal longer than the spinal cord?

The spinal cord stops growing in length around ages 4–5 whereas the vertebral column continues to elongate until maximum height is reached around ages 18–22.

What is the function of the spinal cord?

The spinal cord is the communication link between the brain and the PNS. sensory info→ from the periphery into the spinal cord. motor info→ spinal cord out towards muscles and glands.

What additional functions does the spinal cord have?

It integrates incoming and outgoing information and produces responses. Specifically it helps determine where sensory information from the internal or external environment needs to travel after it has been collected from the periphery. It also creates motor responses.

Where does the spinal cord extend?

The spinal cord extends from the foramen magnum down to the top of the second lumbar (L2) vertebrae.

What is the foramen magnum?

The foramen magnum is a large circular-shaped hole in the base of the skull within the occipital bone that helps anchor the spinal cord longitudinally preventing superior and inferior movements.

What happens below L2?

Spinal nerves will continue down the rest of the vertebral canal after the L2 vertebrae and exit at their respective vertebral level through the intervertebral foramina.

How many spinal nerves are there?

In total 31 pairs of spinal nerves exit the spinal cord.

What is the shape of the spinal cord in cross-section?

slightly flattened on one side with a non-uniform diameter along its length.

What are the enlargements of the spinal cord?

There are two enlargements along its length: a cervical enlargement occurring between vertebrae C4 and T1 and a lumbosacral enlargement occurring between vertebrae T9 and T12.

What is the conus medullaris?

The spinal cord tapers into a cone-like shape just inferior to the lumbosacral enlargement that is called the conus medullaris.

What is the cauda equina?

The hair-like structure of nerves coming out of the bottom of the spinal cord is called the cauda equina because it looks like a horse’s tail.

What are meninges?

The spinal cord is surrounded by layers of connective tissue called meninges that are continuous with the meninges of the brain and provide protection.

What is the dura mater?

The dura mater is the most superficial meningeal layer and is composed of dense irregular connective tissue. It is the thickest and strongest meningeal layer.

What is the epidural space?

The space between the dura mater and the periosteum is called the epidural space. In the vertebral canal this space is filled with fat blood vessels and loose areolar connective tissue.

What is an epidural?

An epidural occurs when anesthesia is injected into the epidural space to block pain receptors exiting certain spinal cord levels.

What is the arachnoid mater?

Deep to the dura mater is the arachnoid mater a thin and avascular layer containing simple squamous epithelial cells and a delicate network of collagen and elastin fibres.

What is the subdural space?

Between the dura mater and the arachnoid mater is the subdural space a small space that contains a small amount of serous fluid.

What is the pia mater?

Deep to the arachnoid mater is the pia mater which is tightly adhered to the spinal cord and contains blood vessels that supply the spinal cord.

What are denticulate ligaments?

The pia mater has small extensions of itself that connect it to the dura mater called denticulate ligaments.

What is the filum terminale?

The filum terminale is another extension of the pia mater connective tissue that extends towards and anchors the spinal cord longitudinally to the coccyx.

What is the subarachnoid space?

Between the arachnoid and pia maters is the subarachnoid space which contains cerebrospinal fluid (CSF).

What are the connective tissue layers of nerves?

Each axon is surrounded by three connective tissue layers: endoneurium perineurium epineurium.

What are spinal nerve counts?

There are 8 pairs of cervical nerves 12 thoracic 5 lumbar 5 sacral and 1 coccygeal.

What are rami?

Shortly after forming the spinal nerve splits into posterior (dorsal) ramus and anterior (ventral) ramus.

Function of posterior ramus?

The posterior ramus innervates the dorsal trunk muscles and back skin.

Function of anterior ramus?

The anterior ramus becomes intercostal nerves or forms plexuses.

What are plexuses?

Plexuses are nerve networks that mix signals from multiple spinal cord levels.

What is quadriplegia?

Complete transection in the cervical region results in loss of function in both upper and lower limbs called quadriplegia.

What is paraplegia?

Injury to the thoracic region results in loss of lower limb function called paraplegia.

What are the two major portions of the spinal cord?

The spinal cord has two major portions an outer white matter portion divided into two halves and an inner gray matter portion that is butterfly-shaped or H-shaped.

What are fascicles and columns?

These nerve tracts or fascicles are grouped together to create columns that allow information to be transmitted in an organized fashion throughout the spinal cord.

What are the white matter columns?

There are three white matter columns mirrored on both sides of the spinal cord the posterior lateral and anterior columns.

What are white commissures?

Connecting the right and left sides of the spinal cord in the white matter are the anterior and posterior white commissures which are axons that cross from one side of the body to the other to send signals.

What are the gray horns?

The gray matter is organized into three horns the lateral horns posterior horns and anterior horns.

Where are lateral horns found?

The lateral horns are very small and found only in the thoracic and lumbar spinal cord regions. - This horn contains the cell bodies of neurons of the autonomic nervous system.

Function of posterior horns?

The posterior horns are where axons of the sensory neurons enter the spinal cord. - Sensory neurons either synapse with interneurons or combine with a nerve tract in the white matter to ascend or descend the spinal cord.

Where are sensory neuron cell bodies located?

Sensory nerve cell bodies are found in the dorsal root ganglion outside the CNS

Function of anterior horns?

The anterior horns also called motor horns are the largest horns and contain cell bodies for somatic motor neurons that travel to skeletal muscles.

What are gray commissures?

Connecting the left and right sides of the spinal cord in the gray matter are the gray commissures which are axons that cross from one side of the body to the other.

What is the central canal?

In the middle of the gray commissures is the central canal which is continuous with the ventricles of the brain and contains CSF.

What are rootlets?

At each spinal level are rootlets that either leave or enter the spinal cord and are part of the PNS.

What are ventral roots?

On the anterior side there are ventral roots where all motor neurons exit the spinal cord.

What are dorsal roots?

On the posterior side there are dorsal roots containing pseudo-unipolar sensory neurons that enter the spinal cord.

What are dorsal root ganglia?

Within the dorsal root are the dorsal root ganglia which contain groups of the sensory neuron cell bodies outside the CNS.

What is a spinal nerve formation?

When the dorsal and ventral roots merge together a spinal nerve is formed. - All spinal nerves have both sensory and motor functions.

What is polio?

Polio is caused by the poliovirus and attacks motor neuron cell bodies in the anterior horn resulting in paralysis.

What is ALS?

Amyotrophic Lateral Sclerosis ALS results from degeneration of anterior horn neurons and motor pathways causing loss of movement speech swallowing and breathing.

What surrounds each axon?

Schwann cells surround each axon and form the myelin sheath.

What is endoneurium?

The endoneurium surrounds each individual axon and separates axons from each other.

What is perineurium?

The perineurium surrounds a nerve fascicle and holds bundles together.

What is epineurium?

The epineurium is the outer layer surrounding the spinal nerve and is continuous with the dura mater.

What is special about C8?

There are 8 cervical nerves but 7 cervical vertebrae so C8 exits below C7. whereas everything else (ex. C2 nerve exits above C2 vertebrae)

Cervical plexus

• Roots: C1–C4

• Function: Innervates head, neck, and upper shoulders

• Important nerve: Phrenic nerve (C3–C5) → controls the diaphragm (breathing)

Brachial plexus

• Roots: C5–T1

• Function: Innervates shoulders and upper limbs

• Major nerves:

  • Axillary

  • Radial

  • Musculocutaneous

  • Median

  • Ulnar

• mnemonic (MURAM)

Lumbar plexus

• Roots: L1–L4

• Function: Innervates parts of the lower abdomen, pelvis, and legs

• Major nerves:

  • Femoral

  • Obturator

Sacral plexus

• Roots: L4–S4

• Function: Innervates lower limbs and pelvic region

• Major nerves:

  • Tibial

  • Common fibular (peroneal)

• Forms the sciatic nerve (largest nerve in the body)

Coccygeal plexus

• Roots: S5–Co

• Function: Innervates pelvic floor muscles and skin over the coccyx