Chapter 13 study guide

The Spinal Cord: Anatomy and Protective Coverings

Location and Protection

The spinal cord is a vital structure of the central nervous system, enclosed within the vertebral column and surrounded by protective meninges. These coverings provide both physical protection and a supportive environment for neural function.

• Meninges: Three layers of tissue surround the spinal cord, from superficial to deep:

• Dura mater: The tough, outermost layer; separated from the vertebrae by the epidural space.

• Arachnoid mater: The middle layer; the subarachnoid space beneath contains cerebrospinal fluid (CSF), which cushions and nourishes the cord.

• Pia mater: The delicate, innermost layer; adheres directly to the spinal cord surface.

External Anatomy of the Spinal Cord

Regions and Landmarks

The spinal cord extends from the foramen magnum at the base of the skull to the level of the first or second lumbar vertebra (L1–L2). It is divided into four regions:

• Cervical

• Thoracic

• Lumbar

• Sacral

Key anatomical features include:

• Cervical enlargement: Supplies nerves to the upper limbs.

• Lumbosacral enlargement: Supplies nerves to the lower limbs.

• Conus medullaris: The tapered end of the spinal cord, usually at L1–L2.

• Filum terminale: Anchors the spinal cord to the coccyx.

• Cauda equina: Bundle of nerve roots continuing below the cord's end.

Spinal Nerves and Nerve Plexuses

Spinal Nerve Organization

There are 31 pairs of spinal nerves, named for the segment of the spinal cord from which they emerge:

• Cervical nerves: 8 pairs

• Thoracic nerves: 12 pairs

• Lumbar nerves: 5 pairs

• Sacral nerves: 5 pairs

• Coccygeal nerve: 1 pair

Spinal nerves are part of the peripheral nervous system (PNS) and are named and numbered according to their vertebral level.

Nerve Plexuses

Nerve plexuses are networks formed by the anterior rami of spinal nerves, containing both sensory and motor fibers. The four major plexuses are:

• Cervical plexus (C1–C5): Innervates neck muscles and the diaphragm.

• Brachial plexus (C5–T1): Innervates pectoral girdle and upper limbs.

• Lumbar plexus (T12–L4): Innervates pelvic girdle and lower limbs.

• Sacral plexus (L4–S4): Innervates pelvic girdle and lower limbs.

Internal Anatomy of the Spinal Cord

Gray and White Matter

The spinal cord's internal structure consists of gray matter and white matter:

• Gray matter: Contains cell bodies of neurons and glial cells; located centrally in the cord.

• White matter: Contains myelinated axons; surrounds the gray matter.

Two grooves run the length of the cord:

• Anterior median fissure: Deep and wide.

• Posterior median sulcus: Shallow and narrow.

The central canal is a small space in the center, filled with cerebrospinal fluid.

Functional Organization

• Posterior horn: Contains sensory nuclei.

• Lateral horn: Contains visceral motor nuclei (in thoracic and upper lumbar regions).

• Anterior horn: Contains somatic motor nuclei.

• Gray commissure: Area where gray matter crosses the midline.

White matter is organized into columns and tracts:

• Sensory tracts: Carry information toward the brain.

• Motor tracts: Carry commands from the brain to effectors.

Spinal Nerve Structure and Coverings

Connective Tissue Layers

Spinal nerves are protected by three connective tissue layers:

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds groups of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Branches: Roots, Nerves, and Rami

Roots

Roots are the initial branches emerging from the spinal cord:

• Dorsal (posterior) roots: Contain sensory fibers; associated with the dorsal root ganglion (cell bodies of sensory neurons).

• Ventral (anterior) roots: Contain motor fibers.

Spinal Nerves and Rami

• Spinal nerves: Formed where dorsal and ventral roots meet; mixed nerves containing both sensory and motor fibers.

• Rami: Branches off spinal nerves; contain both sensory and motor fibers. Posterior rami innervate muscles and skin of the back; anterior rami innervate limbs and anterior trunk.

Nerve Plexuses: Major Nerves and Distribution

Cervical Plexus

The cervical plexus innervates muscles and skin of the neck and the diaphragm. The phrenic nerve controls the diaphragm, a major respiratory muscle. Severing the spinal cord above the origin of the phrenic nerve results in loss of diaphragmatic function.

Brachial Plexus

The brachial plexus gives rise to all nerves innervating the upper limb, including:

• Musculocutaneous nerve

• Radial nerve

• Median nerve

• Ulnar nerve

Lumbar and Sacral Plexuses

The lumbar plexus innervates the pelvic girdle and anterior/medial thigh. The sacral plexus includes nerves such as the sciatic, gluteal, and pudendal nerves, innervating the lower limb and pelvic region.

Dermatomes and Intercostal Nerves

Dermatomes

A dermatome is an area of skin innervated by a single spinal nerve. Dermatomes are clinically important for diagnosing nerve or spinal cord injuries.

Intercostal Nerves

Intercostal nerves (T1–T12) innervate the muscles and skin of the thoracic wall.

Spinal Cord Injuries

Types of Paralysis

• Paraplegia: Spinal cord transected in the thoracic region; paralysis of lower limbs.

• Quadriplegia: Spinal cord transected in the cervical region; paralysis of all four limbs.

Spinal Reflexes

Reflex Arc

A reflex is a fast, involuntary response to a specific stimulus. The reflex arc is the pathway a nerve impulse takes to create a reflex, consisting of five components:

1. Stimulus activates receptor

2. Sensory neuron carries signal to CNS

3. Processing occurs in the CNS

4. Motor neuron carries signal to effector

5. Response by effector

Monosynaptic Reflexes

In monosynaptic reflexes, the sensory neuron directly synapses with the motor neuron. Example: Patellar (stretch) reflex, which prevents knee buckling while standing.

Polysynaptic Reflexes

Polysynaptic reflexes involve one or more interneurons. Example: Withdrawal reflex, which causes automatic withdrawal from a painful stimulus.

Summary Table: Spinal Cord Regions and Nerves

Region	Number of Nerves	Main Functions
Cervical	8 pairs	Neck, upper limbs
Thoracic	12 pairs	Thorax, intercostal muscles
Lumbar	5 pairs	Lower limbs, pelvic girdle
Sacral	5 pairs	Lower limbs, pelvic organs
Coccygeal	1 pair	Pelvic region

Key Terms and Definitions

• Spinal cord: Central nervous system structure transmitting information between brain and body.

• Meninges: Protective coverings of the CNS.

• Spinal nerve: Mixed nerve emerging from the spinal cord.

• Plexus: Network of nerves formed by anterior rami.

• Dermatome: Skin area innervated by a single spinal nerve.

• Reflex arc: Pathway mediating a reflex action.

Important Equations

While the spinal cord is primarily anatomical, the following formula is relevant for nerve conduction:

Nerve conduction velocity:

$\text{Velocity} = \frac{\text{Distance}}{\text{Time}}$

Where distance is the length the impulse travels, and time is the duration taken.