# Lecture 15: The Spinal Cord and Spinal Nerves

## Anatomy and Physiology I

---

## Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the gross anatomy and protective structures of the spinal cord
2. Identify the regions and key landmarks of the spinal cord in cross-section
3. Explain the organization of gray matter and white matter in the spinal cord
4. Describe the formation, branches, and distribution of spinal nerves
5. List the major nerve plexuses and their principal nerves
6. Define a reflex arc and describe its components
7. Compare monosynaptic and polysynaptic reflexes with clinical examples

---

## Lecture Content

### I. Gross Anatomy of the Spinal Cord

The spinal cord extends from the foramen magnum to approximately the level of the **L1-L2** vertebra in adults. It measures approximately 42 to 45 centimeters in length and is roughly the diameter of a finger. Two conspicuous enlargements are evident: the **cervical enlargement (C4-T1)**, which supplies the upper limbs via the brachial plexus, and the **lumbar enlargement (L2-S3)**, which supplies the lower limbs via the lumbosacral plexus. The **conus medullaris** is the tapered, cone-shaped terminal end of the spinal cord at L1-L2. The **filum terminale** is a thin strand of pia mater extending from the conus medullaris to the coccyx, anchoring the cord inferiorly. The **cauda equina** is the collection of spinal nerve roots (L2 and below) that descend past the conus medullaris, resembling a horse's tail.

#### Spinal Cord Segments

The spinal cord comprises 31 segments, each giving rise to a pair of spinal nerves: 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal (Co1). Because the spinal cord is shorter than the vertebral column, lower spinal nerves travel progressively longer distances within the vertebral canal before exiting.

### II. Protective Structures

#### Meninges

Three meningeal layers protect the spinal cord. The **dura mater** is the tough, outermost layer, forming the dural sac that ends at approximately S2. Between the dura and the vertebral periosteum lies the **epidural space**, which contains fat and a venous plexus and is the site for epidural anesthesia. The **arachnoid mater** is the middle, delicate, web-like layer. The **subdural space** is a potential space between the dura and arachnoid, while the **subarachnoid space** contains cerebrospinal fluid (CSF) and is the site for lumbar puncture (performed below L3-L4 to avoid the cord). The **pia mater** is the innermost layer, thin and vascular, closely adherent to the spinal cord surface. The **denticulate ligaments** are lateral extensions of the pia mater that anchor the cord to the dura, providing lateral stability.

#### Cerebrospinal Fluid (CSF)

CSF is produced by the choroid plexuses in the brain ventricles. It circulates through the ventricles, the central canal of the spinal cord, and the subarachnoid space. Its functions include cushioning and protecting the CNS, providing buoyancy, and transporting nutrients and waste.

<image>A posterior view of the spinal cord within the opened vertebral canal. The diagram labels the cervical enlargement (C4-T1), thoracic region, lumbar enlargement (L2-S3), conus medullaris at L1-L2, the filum terminale extending to the coccyx, and the cauda equina fanning out below the conus medullaris. Spinal nerves are shown exiting through intervertebral foramina at each level. An inset cross-section through the vertebral column shows the three meningeal layers (dura mater, arachnoid mater, pia mater), the epidural space with fat and vessels, the subarachnoid space filled with CSF, and denticulate ligaments anchoring the cord laterally to the dura.</image>

### III. Cross-Sectional Anatomy of the Spinal Cord

#### Gray Matter (Central, Butterfly/H-Shaped)

The gray matter of the spinal cord is organized into several regions. The **posterior (dorsal) horns** contain interneurons and sensory relay nuclei and receive incoming sensory information from dorsal root ganglia. The **anterior (ventral) horns** contain motor neuron cell bodies (alpha and gamma motor neurons) whose axons exit via the ventral roots to innervate skeletal muscles. The **lateral horns** are present only in thoracic and upper lumbar segments (T1-L2), where they contain sympathetic preganglionic neuron cell bodies; parasympathetic preganglionic neurons are also present in sacral segments S2-S4. The **gray commissure** connects the two sides and surrounds the central canal.

#### White Matter (Peripheral)

The white matter is organized into three paired **funiculi (columns)**. The **posterior (dorsal) funiculus** contains ascending tracts carrying fine touch, proprioception, and vibration (the dorsal column-medial lemniscus pathway). The **lateral funiculus** contains both ascending tracts (spinothalamic and spinocerebellar) and descending tracts (lateral corticospinal). The **anterior (ventral) funiculus** contains ascending and descending tracts, including the anterior corticospinal and vestibulospinal tracts. The **white commissure** is where axons cross from one side to the other (decussation).

#### Roots and Rootlets

The **dorsal root** carries sensory (afferent) fibers into the spinal cord and contains the **dorsal root ganglion (DRG)**, which houses the cell bodies of sensory neurons (unipolar/pseudounipolar). The **ventral root** carries motor (efferent) fibers out of the spinal cord. The dorsal and ventral roots merge to form the **spinal nerve** at each segment.

<image>A detailed cross-section of the spinal cord at the thoracic level. The central gray matter is butterfly-shaped with clearly labeled posterior (dorsal) horns, anterior (ventral) horns, and lateral horns. The gray commissure surrounds the central canal. Surrounding white matter is divided into posterior, lateral, and anterior funiculi, each color-coded differently. The dorsal root with its dorsal root ganglion enters posteriorly, and the ventral root exits anteriorly. The two roots merge to form the spinal nerve, which then divides into dorsal and ventral rami. Major ascending tracts (dorsal columns, lateral spinothalamic, anterior spinothalamic, spinocerebellar) and descending tracts (lateral corticospinal, anterior corticospinal, vestibulospinal) are labeled within the white matter columns.</image>

### IV. Spinal Nerves

There are 31 pairs of mixed nerves (containing both sensory and motor fibers), formed by the union of dorsal and ventral roots. Spinal nerves are very short, quickly dividing into **rami** after exiting the intervertebral foramen.

#### Branches (Rami)

The **dorsal ramus** supplies the skin and deep muscles of the posterior trunk (back). The **ventral ramus** supplies the anterior and lateral trunk and the limbs and is larger than the dorsal ramus. The ventral rami of most spinal nerves form nerve plexuses, with the exception of T2-T12, which form the intercostal nerves. The **meningeal branch (recurrent meningeal nerve)** re-enters the vertebral canal to innervate the meninges, vertebral ligaments, and blood vessels. The **rami communicantes** connect spinal nerves to the sympathetic trunk (white and gray rami communicantes, discussed further with the ANS).

#### Dermatomes

A **dermatome** is the area of skin innervated by the sensory fibers of a single spinal nerve. Dermatomes form a map across the body surface that is clinically useful for localizing spinal cord injuries and nerve damage. Adjacent dermatomes overlap, so damage to a single spinal nerve may produce only reduced (not absent) sensation.

### V. Nerve Plexuses

#### Cervical Plexus (C1-C4)

The cervical plexus is located deep in the neck, lateral to the first four cervical vertebrae. Its most important nerve is the **phrenic nerve (C3-C5)**, which innervates the diaphragm and is essential for breathing. The plexus also supplies muscles and skin of the neck and posterior head.

#### Brachial Plexus (C5-T1)

The brachial plexus is located partly in the neck and partly in the axilla, formed by the ventral rami of C5-T1 and organized into roots, trunks, divisions, and cords. Its major nerves include the **musculocutaneous nerve**, which supplies the flexors of the arm (biceps brachii, brachialis); the **median nerve**, which innervates most forearm flexors and the thenar muscles (carpal tunnel syndrome results from its compression); the **ulnar nerve**, which supplies some forearm flexors and most intrinsic hand muscles (the "funny bone" nerve); the **radial nerve**, which innervates the extensors of the arm and forearm (damage causes wrist drop); and the **axillary nerve**, which supplies the deltoid and teres minor.

#### Lumbar Plexus (L1-L4)

The lumbar plexus is located within the psoas major muscle. Its major nerves are the **femoral nerve (L2-L4)**, which supplies the anterior thigh muscles (quadriceps) and the skin of the anterior thigh and medial leg, and the **obturator nerve (L2-L4)**, which supplies the medial thigh muscles (adductors).

#### Sacral Plexus (L4-S4)

The sacral plexus is located on the posterior pelvic wall. Its major nerve is the **sciatic nerve (L4-S3)**, the largest nerve in the body, which divides into the tibial and common fibular (peroneal) nerves and innervates the posterior thigh, leg, and foot. The plexus also gives rise to the **superior and inferior gluteal nerves**, which supply the gluteal muscles, and the **pudendal nerve (S2-S4)**, which innervates the perineum, external genitalia, and external anal and urethral sphincters.

<image>An anterior view of the human body showing the four major nerve plexuses and their principal nerves. The cervical plexus (C1-C4) is shown in the neck with the phrenic nerve descending to the diaphragm. The brachial plexus (C5-T1) is shown in the shoulder/axilla region with the musculocutaneous, median, ulnar, radial, and axillary nerves branching into the upper limb. The lumbar plexus (L1-L4) within the psoas muscle shows the femoral and obturator nerves descending into the thigh. The sacral plexus (L4-S4) on the posterior pelvis shows the sciatic nerve (the largest nerve in the body) descending into the posterior thigh and splitting into tibial and common fibular nerves. A dermatome map is overlaid on one side of the body showing horizontal bands for thoracic dermatomes on the trunk and the distribution pattern on the limbs.</image>

### VI. Reflexes

A **reflex** is a rapid, predictable, involuntary motor response to a stimulus. Reflexes may be **somatic reflexes**, which involve skeletal muscles, or **autonomic (visceral) reflexes**, which involve smooth muscle, cardiac muscle, or glands.

#### Components of a Reflex Arc

Every reflex arc consists of five components. The **receptor** detects the stimulus (such as a stretch receptor in a muscle spindle or a pain receptor in the skin). The **sensory (afferent) neuron** transmits the impulse from the receptor to the CNS. The **integration center** comprises one or more synapses in the CNS and may or may not involve interneurons. The **motor (efferent) neuron** transmits the impulse from the CNS to the effector. The **effector** is the muscle or gland that carries out the response.

### VII. Somatic Reflexes

#### Stretch Reflex (Monosynaptic)

The **patellar (knee-jerk) reflex** is the classic example of a stretch reflex. The stimulus is the stretching of a muscle (for example, tapping the patellar tendon stretches the quadriceps), and the receptor is the muscle spindle (a stretch receptor within the muscle). The pathway proceeds as follows: stretching activates the muscle spindle; the sensory neuron carries the impulse via the dorsal root to the spinal cord; the sensory neuron synapses **directly** on an alpha motor neuron in the anterior horn (no interneuron is involved, making this a monosynaptic reflex); the motor neuron stimulates contraction of the stretched muscle (the quadriceps contracts); and simultaneously, an interneuron inhibits the antagonist muscle (reciprocal inhibition causes the hamstrings to relax). This reflex maintains muscle tone and posture by automatically correcting changes in muscle length.

#### Withdrawal (Flexor) Reflex (Polysynaptic)

The withdrawal reflex is a protective reflex that pulls a limb away from a painful stimulus. A nociceptor detects the painful stimulus (such as stepping on a tack), the sensory neuron enters the spinal cord and synapses on multiple interneurons (making this a polysynaptic reflex), motor neurons activate the flexor muscles of the affected limb (withdrawal), and motor neurons simultaneously inhibit the extensor muscles of the same limb. The **crossed extensor reflex** occurs simultaneously on the opposite side: extensors contract and flexors relax on the contralateral limb to support body weight.

#### Golgi Tendon Reflex

The Golgi tendon reflex uses Golgi tendon organs (located in tendons) as receptors to detect excessive muscle tension. When tension is too great, sensory signals cause inhibition of the contracting muscle (relaxation) and activation of the antagonist muscle. This reflex serves a protective function, preventing muscle or tendon damage from excessive force.

<image>A diagram showing three spinal reflexes side by side. Panel A (Stretch reflex): A cross-section of the spinal cord showing a muscle spindle in the quadriceps detecting stretch from a patellar tendon tap. A sensory neuron enters via the dorsal root and synapses directly (monosynaptic) on an alpha motor neuron in the ventral horn, which causes the quadriceps to contract. An inhibitory interneuron simultaneously relaxes the hamstrings (reciprocal inhibition). Panel B (Withdrawal reflex): A nociceptor in the foot detects a painful stimulus (tack). Sensory neurons enter the cord and synapse on multiple interneurons (polysynaptic), activating flexors and inhibiting extensors on the ipsilateral side. Panel C (Crossed extensor reflex): Occurring simultaneously with the withdrawal reflex, interneurons cross to the contralateral side via the white commissure and activate extensors while inhibiting flexors on the opposite limb to support body weight. Each panel labels all five components of the reflex arc: receptor, sensory neuron, integration center, motor neuron, and effector.</image>

### VIII. Clinical Correlations

#### Lumbar Puncture (Spinal Tap)

A lumbar puncture involves inserting a needle into the subarachnoid space between L3-L4 or L4-L5 (below the conus medullaris) to collect CSF or measure CSF pressure. The procedure is safe because it avoids the spinal cord; the needle passes through cauda equina fibers, which are pushed aside rather than damaged.

#### Epidural Anesthesia

Epidural anesthesia involves injecting anesthetic into the epidural space to block nerve roots. It is commonly used during labor and delivery.

#### Reflex Testing

Clinical assessment of reflexes tests the integrity of the reflex arc and specific spinal cord segments. Absent reflexes (areflexia) may indicate damage to sensory or motor neurons or to the spinal cord itself. Hyperactive reflexes may indicate upper motor neuron damage, reflecting the loss of inhibitory input from the brain. The Babinski sign — dorsiflexion of the great toe and fanning of the other toes upon stroking the sole of the foot — is normal in infants but abnormal in adults, where it indicates an upper motor neuron lesion.