Explain spinal cord

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spinal cord anatomy cross section diagram

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The Spinal Cord

Overview

The spinal cord is the most caudal portion of the central nervous system (CNS). It extends from the foramen magnum (base of skull) to approximately the level of the L1–L2 intervertebral disc in adults (range: T12 to the L2–L3 disc). In neonates, it extends to about L3–L4, as the vertebral column grows faster than the cord postnatally.

The distal, cone-shaped end is the conus medullaris, from which a fine connective tissue filament — the filum terminale — continues inferiorly to the coccyx.

Gray's Anatomy for Students, p. 119

Gross Structure

Enlargements

The cord is not uniform — it has two bulges corresponding to limb innervation:

Enlargement	Segments	Innervates
Cervical	C5–T1	Upper limbs
Lumbosacral	L1–S3	Lower limbs

External Surface Markings

Anterior median fissure — runs the full anterior length
Posterior median sulcus — runs the full posterior length
Posterolateral sulci (bilateral) — where posterior rootlets enter the cord

Spinal Segments & Nerves

The spinal cord has 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)
1 coccygeal

Because the cord ends at L1–L2 but the vertebral column continues to the sacrum, the lumbar and sacral nerve roots descend within the spinal canal as the cauda equina before exiting.

Internal Organization

In cross-section, the cord has two distinct zones:

Gray Matter (Central)

Rich in neuronal cell bodies; forms a characteristic H-shaped (butterfly) appearance
Divided into:
- Dorsal (posterior) horn — receives sensory input
- Ventral (anterior) horn — contains motor neurons
- Intermediolateral (lateral) horn — present at T1–L2 (sympathetic preganglionic neurons) and S2–S4 (parasympathetic)
- Gray commissure — connects the two halves, enclosing the central canal

Rexed's Laminae

The gray matter is further divided into 10 cytoarchitectural zones (Rexed's laminae):

Lamina	Region	Key Function
I	Posteromarginal nucleus	Pain/temp modulation
II	Substantia gelatinosa (Rolando)	Pain modulation (gate control)
III–IV	Nucleus proprius	General sensory processing
V–VI	Base of dorsal horn / nucleus reticularis	Sensory convergence
VII	Intermediate zone (Clarke's nucleus)	Spinocerebellar pathways
VIII–IX	Ventral horn	Motor output
X	Around central canal	Gray commissure zone

The ventral horn contains two motor neuron types:

Alpha (α) motor neurons → innervate extrafusal skeletal muscle fibers
Gamma (γ) motor neurons → innervate intrafusal fibers of muscle spindles (stretch reflex receptors)
Somatotopic arrangement: flexor neurons are dorsal to extensor neurons; hand muscles are lateral to trunk muscles.

White Matter (Peripheral)

Surrounds the gray matter; rich in myelinated axons organized into three funiculi per side:

Funiculus	Location	Contents
Dorsal (posterior)	Between dorsomedian and dorsolateral sulci	Ascending sensory tracts
Lateral	Between dorsolateral and ventrolateral sulci	Mixed ascending + descending tracts
Ventral (anterior)	Between ventrolateral sulcus and anterior fissure	Descending motor tracts

Localization in Clinical Neurology, 8th ed., p. 242–244
Gray's Anatomy for Students

Ascending (Sensory) Tracts

Two major somatosensory pathways ascend to the cortex:

1. Posterolateral (Anterolateral) Pathway — Spinothalamic Tract

Modalities: Pain, temperature, crude touch, pressure
1st neuron: Dorsal root ganglion → enters posterior horn
2nd neuron: Crosses in the anterior commissure (contralateral, 2–3 segments above entry) → ascends in the lateral funiculus as the lateral spinothalamic tract
3rd neuron: Ventral posterior lateral (VPL) nucleus of thalamus → primary somatosensory cortex (S1)
The spinoreticular and spinomesencephalic tracts accompany this pathway for arousal and affective pain responses.

2. Posterior Column–Medial Lemniscal Pathway (Dorsal Columns)

Modalities: Fine/discriminative touch, vibration, conscious proprioception
1st neuron: Dorsal root ganglion → ascends ipsilaterally in the dorsal funiculus:
- Fasciculus gracilis (medial) — lower limb and trunk (below T6)
- Fasciculus cuneatus (lateral) — upper limb and neck (above T6)
2nd neuron: Crosses in the caudal medulla as internal arcuate fibers → ascends as the medial lemniscus (contralateral)
3rd neuron: VPL nucleus thalamus → S1 cortex

Key clinical rule: Spinothalamic decussates within the cord (contralateral signs at the level of injury); dorsal columns decussate in the medulla (ipsilateral signs at cord level).

Gray's Anatomy for Students; Localization in Clinical Neurology, 8th ed.

Descending (Motor) Tracts

Lateral Motor System

Tract	Origin	Decussation	Location in cord	Function
Lateral corticospinal tract	Motor cortex (UMN)	Pyramidal decussation (caudal medulla, ~85%)	Lateral funiculus	Voluntary fine motor, especially distal limb
Rubrospinal tract	Red nucleus (midbrain)	Ventral tegmental decussation	Lateral funiculus	Flexor facilitation; minor in humans

Medial Motor System

Tract	Origin	Projection
Anterior corticospinal tract	Motor cortex (~15% non-decussated)	Descends ipsilaterally; crosses at segmental level; controls axial/proximal muscles
Tectospinal tract	Superior colliculus	Cervical cord; reflex postural responses to visual stimuli
Vestibulospinal tract (medial + lateral)	Vestibular nuclei (pons/medulla)	Postural tone, balance, extensor facilitation
Reticulospinal tracts	Pontine + medullary reticular formation	Gait, posture, autonomic modulation

Gray's Anatomy for Students, p. 1327–1332

Autonomic Outflow

Sympathetic: Preganglionic cell bodies in the intermediolateral column, segments T1–L2 → exit via anterior roots → synapse in paravertebral or prevertebral ganglia
Parasympathetic (sacral): Preganglionic neurons in the S2–S4 intermediate gray → form the pelvic splanchnic nerves → innervate pelvic viscera (bladder, bowel, genitalia)

Blood Supply

The spinal cord is supplied by three longitudinal arteries:

Artery	Number	Origin	Territory
Anterior spinal artery (ASA)	1 (midline)	Vertebral arteries at craniocervical junction	Anterior 2/3 of cord — corticospinal, spinothalamic tracts, anterior horn
Posterior spinal arteries (PSA)	2 (posterolateral)	PICA or vertebral arteries	Posterior 1/3 — dorsal columns

These are reinforced by radiculomedullary arteries segmentally from the aorta. The most important is the artery of Adamkiewicz (artery of the lumbar enlargement), arising from the lower thoracic/upper lumbar aorta (usually T9–T12, left side), which is the dominant supply to the lower thoracic and lumbosacral cord.

Blood supply is sparse in the mid-thoracic region (T4–T8), making it a watershed zone vulnerable to ischemia.

Anterior Spinal Artery Syndrome: Infarction of the ASA territory (aortic dissection, surgery, embolism) causes:

Bilateral motor paralysis below the lesion (corticospinal tract)
Loss of pain and temperature below the lesion (spinothalamic tract)
Preserved vibration and proprioception (dorsal columns, supplied by PSA)
Autonomic dysfunction (bladder/bowel incontinence, hypotension)
Miller's Anesthesia, 10th ed., p. 8016–8018

Key Images

Spinal cord cross-section with labeled tracts

Cross-section of the spinal cord showing major ascending and descending tracts with somatotopic lamination (C = cervical, T = thoracic, L = lumbar, S = sacral). — Bailey & Love's Surgery, 28th ed.

Spinal cord cross-section showing dorsal (sensory) and ventral (motor) roots, dorsal root ganglion, formation of the spinal nerve, and rami communicantes to the sympathetic chain.

Clinical Summary: Spinal Cord Syndromes

Syndrome	Lesion	Features
Complete transection	All tracts	Complete loss of motor + all sensation + autonomic below level
Brown-Séquard	Hemisection	Ipsilateral motor loss + proprioception; contralateral pain/temp loss
Anterior cord syndrome	ASA infarction	Bilateral motor + pain/temp loss; proprioception spared
Central cord syndrome	Central gray/commissure	Upper > lower limb weakness; dissociated sensory loss
Posterior cord syndrome	Dorsal columns	Loss of proprioception + vibration; motor/pain preserved
Conus medullaris	L1–L2	Mixed UMN/LMN; bladder/bowel prominent
Cauda equina	Below L2	Pure LMN; radicular pain; saddle anesthesia

Sources: Gray's Anatomy for Students | Localization in Clinical Neurology, 8th ed. | Costanzo Physiology, 7th ed. | Miller's Anesthesia, 10th ed. | Bailey & Love's Surgery, 28th ed.

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