CNS: Spinal Cord Notes

The spinal cord is the bridge between brain and body. Before we trace tracts and dissect cross sections, anchor the gross picture: where it sits, where it ends, and how it widens to serve the limbs.

Gross Anatomy

Spinal Cord Basics

Shape: oval, slightly flattened anteriorly and posteriorly
Superior limit: continuous with the medulla oblongata at the foramen magnum
Inferior limit (adult): superior border of the second lumbar vertebra (L2)
Length: 16 to 18 inches
Maximum diameter: cervical region; smaller in thoracic segments and at the inferior tip

Developmental Note

Newborn: cord extends to L3 or L4
Early childhood: cord and column grow together as part of overall body growth
Cord elongation stops at age 4 to 5; the column continues to lengthen, leaving the cord progressively higher relative to the vertebrae

Two Enlargements

Cervical & Lumbar Enlargements

Where the cord widens, it is supplying a limb. Both enlargements reflect the extra neuronal traffic needed for the upper or lower extremities.

Enlargement	Levels	Supplies
Cervical	C4 to T1	Upper limbs
Lumbar	T9 to T12	Lower limbs

Memory cue: wider cord, wider job. The cervical enlargement powers the brachial plexus; the lumbar enlargement powers the lumbar and sacral plexuses.

Distal Anatomy

Conus Medullaris & Filum Terminale

Conus Medullaris

Tapered, conical termination of the spinal cord
Ends at the intervertebral disc between L1 and L2 in adults

Filum Terminale

Arises from the conus medullaris
Inferior extension of pia mater
Fuses with arachnoid and dura mater
Anchors the spinal cord inferiorly

Nerve Roots Below the Cord

Cauda Equina

Because the cord ends at L2 but nerves still need to exit at lumbar, sacral, and coccygeal levels, the lower roots travel inferiorly inside the vertebral canal before leaving through their respective intervertebral foramina.

Roots descend alongside the filum terminale, like wisps of hair
Named cauda equina because the bundle resembles a horse's tail
Clinical relevance: this is the safe zone for lumbar puncture

Spinal Nerves at a Glance

31 Pairs of Spinal Nerves

Spinal nerves are the paths of communication between the cord and the body. Each pair is a mixed nerve: the posterior root carries sensory axons; the anterior root carries motor axons.

Region	Pairs	Range
Cervical	8	C1 to C8
Thoracic	12	T1 to T12
Lumbar	5	L1 to L5
Sacral	5	S1 to S5
Coccygeal	1	Co1

Note: there are only 7 cervical vertebrae but 8 cervical spinal nerves. C1 through C7 exit above their corresponding vertebra; C8 exits below C7 and above T1. From T1 down, every nerve exits below its corresponding vertebra.

The cord sits inside three layers of protection: bone, connective tissue, and fluid. Each layer contributes a distinct kind of safety, and the spaces between the layers carry clinical weight.

Bony Shell

Vertebral Column

The cord sits within the vertebral canal, formed by stacked vertebral foramina
The surrounding vertebrae provide a sturdy shelter
Vertebral ligaments add reinforcement
Spinal nerves exit laterally through intervertebral foramina

Connective Tissue Coverings

Meninges Overview

Three layers, continuous with the cranial meninges. They cover the cord and continue along spinal nerves until those nerves exit the column.

Dura mater → Arachnoid mater → Pia mater

Mnemonic: Dura, Arachnoid, Pia. Outside to inside: tough shell, spider web, tender skin.

Outer Layer

Dura Mater

Most superficial; thick and strong
Composed of dense irregular connective tissue
Continuous with the epineurium, the outer covering of spinal and cranial nerves

Spaces Around the Dura

Epidural space: between dura and the wall of the vertebral canal; contains a cushion of fat and connective tissue (target for epidural anesthesia)
Subdural space: between dura and arachnoid; contains interstitial fluid

Middle Layer

Arachnoid Mater

Thin and avascular
Loosely arranged collagen and elastic fibers in a spider-web pattern
Continuous with the cranial arachnoid

Subarachnoid Space

Between arachnoid and pia mater
Contains cerebrospinal fluid (CSF)
Target for lumbar puncture

Innermost Layer

Pia Mater

Thin, transparent connective tissue layer
Adheres directly to the cord and brain surface
Composed of squamous to cuboidal cells with interlacing collagen and fine elastic fibers
Carries the blood vessels that supply oxygen and nutrients to the cord

Hydraulic Cushion

Cerebrospinal Fluid (CSF)

Buoyant liquid that suspends CNS tissue in a near-weightless environment
Acts as a shock-absorbing hydraulic cushion
Fills the subarachnoid space and the central canal of the cord
The central canal is continuous superiorly with the fourth ventricle of the brain

Clinical Correlation: Lumbar Puncture

Definition: withdrawal of CSF from the spinal canal for laboratory testing.

Why it is safe at lumbar levels: the cord ends at L2 but the spinal meninges and CSF extend to S2. A needle placed below L2 enters CSF without risking the cord itself. The roots of the cauda equina float aside.

Ideal location: between L3 and L4 or L4 and L5 in adults.

Target space: the needle enters the subarachnoid space.

Surface landmark: a line drawn between the highest points of the iliac crests (the supracristal plane) crosses the spinous process of L4.

Cut the cord across and you see a butterfly inside a circle. The butterfly is gray matter, organized by horn. The surrounding circle is white matter, organized by column. Function follows that geometry exactly.

Exam Scope: Anatomy vs. Physiology

For anatomy purposes, focus on structure, location, and the type of axon running through each region (sensory, motor, autonomic). Conduction mechanisms and pathway physiology are covered separately in the physiology unit.

Surface Landmarks

Two Grooves Divide the Cord

Two midline grooves split the white matter into left and right halves and serve as the principal surface landmarks on a cross section.

Anterior median fissure: deeper, ventral midline groove
Posterior median sulcus: shallower, dorsal midline groove

Inside the Butterfly

Gray Matter Overview

Gray matter forms the H-shaped or butterfly-shaped core. It contains dendrites, neuronal cell bodies, unmyelinated axons, and neuroglia.

Components

Gray commissure: the crossbar of the H; contains the central canal
Central canal: small CSF-filled space; continuous superiorly with the fourth ventricle
Anterior white commissure: connects white matter of the right and left sides

Three Horns

Gray Matter Horns

The projections of gray matter toward the cord surface are called horns. Each horn has a defined functional role.

Horn	Contains	Function
Posterior (dorsal)	Cell bodies and axons of interneurons; axons of incoming sensory neurons	Sensory input
Anterior (ventral)	Somatic motor nuclei: cell bodies of motor neurons	Skeletal muscle contraction
Lateral	Autonomic motor nuclei	Regulation of cardiac muscle, smooth muscle, glands. Present only in thoracic and upper lumbar segments.

Functional Clusters

Nuclei Within Gray Matter

Nuclei are clusters of neuronal cell bodies that share a function.

Sensory nuclei: receive input from receptors via sensory neurons
Motor nuclei: provide output to effector tissues via motor neurons

Tip: a horn is a region of gray matter; a nucleus is a functional cluster of cell bodies inside that region. Several nuclei can live in one horn.

Outside the Butterfly

White Matter Overview

White matter surrounds the gray. It is composed of bundles of myelinated axons traveling up and down the cord.

Two Levels of Organization

Columns: three large regions of white matter on each side, defined by the anterior and posterior horns
Tracts: bundles of axons within columns that share a common origin or destination and carry similar information (the CNS equivalent of nerves in the PNS)

Three Columns

White Columns (Funiculi)

On each side of the cord, white matter is divided into three columns:

Posterior (dorsal) white column: between posterior median sulcus and posterior horn
Lateral white column: between posterior and anterior horns on the side
Anterior (ventral) white column: between anterior median fissure and anterior horn

Tract Direction

Sensory (ascending) tracts: conduct nerve impulses toward the brain
Motor (descending) tracts: conduct nerve impulses away from the brain

Region by Region

Distinguishing Features by Spinal Region

Region	Cross Section	Gray Matter	White Matter
Cervical	Oval, relatively large diameter	C1 to C4: large posterior horn, small anterior horn. C5 and below: enlarged posterior horn, well-developed anterior horn (limb innervation).	Relatively large amount
Thoracic	Small diameter	Relatively small amount of gray. Lateral horn present (autonomic). Exception: T1 anterior and posterior horns are small.	Substantial
Lumbar	Nearly circular	Very large anterior and posterior horns. Small lateral horn present in upper segments.	Less than cervical

Sensory tracts ascend. They carry input from receptors in skin, muscle, joints, and viscera up to processing centers in the brain. The two big ones to know are the spinothalamic tract and the posterior columns.

Naming Convention

How to Read a Tract Name

The name of a tract typically tells you where it starts and where it ends. For sensory tracts, the cord (spino-) is the start and the destination is named second.

Spino (origin: cord) → thalamic (destination: thalamus)

This naming logic works for almost every ascending tract: spinocerebellar (cord to cerebellum), spinotectal (cord to tectum of midbrain), spinoreticular (cord to reticular formation), spinohypothalamic (cord to hypothalamus).

Pain, Pressure, Temperature

Spinothalamic Tract

Sensations carried: pain, pressure, temperature, crude touch
Origin: spinal cord
Destination: thalamus, then onward to somatosensory cortex
Located in the lateral and anterior white columns

Tip: if you cannot feel a hot stove or a pinprick on one side of the body after a cord injury, suspect spinothalamic involvement on the opposite side of the cord (this tract crosses near its level of entry).

Discriminative Touch & Position

Posterior Columns

Sensations carried: discriminative touch, light pressure, vibration, joint position sense (proprioception)
Two components:
- Fasciculus gracilis: medial; carries information from the lower limbs and trunk below T6
- Fasciculus cuneatus: lateral; carries information from the upper limbs and trunk above T6
Located in the posterior white column

Memory cue: gracilis sounds like grace, used for the legs. Cuneatus is the upper one.

Unconscious Proprioception

Spinocerebellar Tracts

Anterior (ventral) spinocerebellar tract
Posterior (dorsal) spinocerebellar tract

Function

Carry information about muscle and joint position to the cerebellum
This input is processed below conscious awareness and is used for coordination and balance

Other Ascending Tracts

Less-Emphasized Pathways

Spinoreticular tract: influences arousal and consciousness
Spinotectal (spinomesencephalic) tract: contributes to reflexive head and eye movements
Spinohypothalamic tract: conveys information that influences autonomic and emotional responses
Spinoolivary tract: indirect input to the cerebellum
Tract of Lissauer (dorsolateral fasciculus): short ascending and descending pain and temperature fibers near the dorsal horn entry zone

Quick Reference

Sensory Tract Summary Table

Tract	Modality	White Column Location
Spinothalamic	Pain, temperature, pressure, crude touch	Lateral and anterior
Fasciculus gracilis	Touch, vibration, position from lower body	Posterior (medial)
Fasciculus cuneatus	Touch, vibration, position from upper body	Posterior (lateral)
Posterior spinocerebellar	Unconscious proprioception	Lateral
Anterior spinocerebellar	Unconscious proprioception	Lateral

Motor tracts descend. They divide into two families: direct pathways from the cerebral cortex (voluntary movement) and indirect pathways from the brain stem (postural and reflexive movement). The name almost always tells you the origin and destination.

Two Families

Direct vs. Indirect Pathways

	Direct (Pyramidal)	Indirect (Extrapyramidal)
Origin	Cerebral cortex	Brain stem nuclei
Function	Voluntary skeletal movement (especially fine, skilled movements)	Automatic movements: posture, balance, muscle tone, coordination of body movement with visual stimuli
Members	Lateral corticospinal, anterior corticospinal, corticobulbar	Rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, medial reticulospinal

Direct Pathway

Lateral Corticospinal Tract

Origin: cerebral cortex
Destination: spinal cord (anterior horn motor neurons)
Location in cord: lateral white column
Function: precise, voluntary movements of the limbs (especially distal: hands, feet, fingers, toes)
Crosses (decussates) in the medulla

Direct Pathway

Anterior Corticospinal Tract

Origin: cerebral cortex
Destination: spinal cord (anterior horn motor neurons)
Location in cord: anterior white column
Function: voluntary movements of the trunk and proximal limb muscles (postural control during voluntary action)
Does not cross in the medulla; crosses at the level of innervation

Direct Pathway

Corticobulbar Tract

Origin: cerebral cortex
Destination: motor nuclei of cranial nerves in the brain stem (the "bulb" refers to brain stem)
Function: voluntary control of skeletal muscles in the head and neck (face, jaw, tongue, larynx, pharynx)

Why it matters: this is the cortical link to the cranial nerves you studied separately. It connects voluntary movement intent in the cortex to the cranial nerve nuclei that drive the face and head.

Indirect Pathway

Rubrospinal Tract

Origin: red nucleus (Latin ruber, red) of the midbrain
Function: contributes to control of distal limb muscles, especially flexor activity

Indirect Pathway

Tectospinal Tract

Origin: superior colliculus of the tectum (midbrain)
Function: coordinates head, neck, and eye movements in response to visual and auditory stimuli

Indirect Pathway

Vestibulospinal Tract

Origin: vestibular nuclei in the medulla and pons
Function: maintains balance and posture by adjusting muscle tone in response to head movements detected by the inner ear

Indirect Pathways

Reticulospinal Tracts

Lateral reticulospinal tract: originates in the medullary reticular formation; generally inhibits extensor motor neurons
Medial reticulospinal tract: originates in the pontine reticular formation; generally excites extensor motor neurons
Function: regulate skeletal muscle tone, sustain contraction of postural muscles, and contribute to autonomic and reflexive movements

Quick Reference

Motor Tract Summary Table

Tract	Family	Origin	Function
Lateral corticospinal	Direct	Cerebral cortex	Voluntary distal limb movement
Anterior corticospinal	Direct	Cerebral cortex	Voluntary trunk and proximal limb movement
Corticobulbar	Direct	Cerebral cortex	Voluntary head and neck (cranial nerve targets)
Rubrospinal	Indirect	Red nucleus (midbrain)	Distal flexor activity
Tectospinal	Indirect	Superior colliculus (midbrain)	Head and eye reflexes to vision and sound
Vestibulospinal	Indirect	Vestibular nuclei (medulla/pons)	Balance, postural muscle tone
Lateral reticulospinal	Indirect	Medullary reticular formation	Inhibits extensors
Medial reticulospinal	Indirect	Pontine reticular formation	Excites extensors; maintains posture

Three clinical themes recur on cord questions: where to safely access CSF, why phrenic origin matters for breathing, and how cord level mismatch with vertebral level shapes injury patterns. Anchor these and the rest follows.

Lumbar Puncture in Detail

Indication: withdrawal of cerebrospinal fluid for laboratory analysis (suspected meningitis, subarachnoid hemorrhage, demyelinating disease, intracranial pressure measurement).

Safe zone rationale: the spinal cord ends at L2 in adults. The dural sac and CSF continue to S2. Below L2, only the cauda equina is present, and these mobile roots can drift away from the needle.

Adult target: intervertebral space at L3 to L4 or L4 to L5.

Pediatric note: in newborns, the cord may extend to L3 or L4, so the puncture site is moved more inferiorly.

Layers traversed by the needle (superficial to deep): skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura mater, arachnoid mater, then into the subarachnoid space.

Surface landmark: the supracristal line, drawn between the highest points of the iliac crests, typically crosses the L4 spinous process.

Cord vs. Column Levels

Why Levels Mismatch

Because the cord stops growing around age 4 to 5 while the column continues to lengthen, the cord level and the vertebral level are not the same below the cervical region. By adulthood:

Cord ends at the L1 to L2 disc
L1 through coccygeal nerve roots travel inferiorly inside the canal as the cauda equina before exiting their respective intervertebral foramina

Clinical implication: a vertebral injury at, for example, the T12 vertebra often damages cord segments below T12 (lumbar and sacral cord), producing lower-extremity deficits.

Phrenic Origin

Cervical Cord and Breathing

The phrenic nerve, which drives the diaphragm, originates from cervical roots C3, C4, and C5.

Clinical Significance

Complete cord transection above C3: respiratory arrest. The phrenic nerves no longer receive descending input and the diaphragm stops working.
Injury at or below C5: diaphragm typically spared; intercostal weakness possible
External pressure on the phrenic nerve from mediastinal tumors (tracheal, esophageal) can also impair diaphragmatic function

Classic mnemonic: "C3, 4, 5 keeps the diaphragm alive."

Cord Injury Pattern Logic

How to Predict a Deficit from a Cord Lesion

Three quick questions reveal what is lost when a cord lesion is described:

What level? Everything served at and below that level is at risk.
Which side of the cord? Most descending motor pathways have already crossed by the time they enter the cord, so a cord lesion produces motor loss on the same side as the lesion. Spinothalamic input crosses near its level of entry, so pain and temperature loss appears on the opposite side of the body below the lesion.
Which column? Posterior column injury wipes out vibration and proprioception on the same side. Lateral column injury affects the lateral corticospinal tract (motor) and the spinothalamic tract on the opposite side.

This three-step logic explains classic cord syndromes (Brown-Sequard, anterior cord, central cord). The full syndromes are reserved for the physiology and pathology unit.

Study Anchors

If You Remember Nothing Else

Cord ends at L1 to L2; CSF reaches S2; safe LP at L3 to L4 or L4 to L5.
Cervical and lumbar enlargements widen because they serve the limbs.
Posterior root: sensory only. Anterior root: motor only. Spinal nerve: mixed.
Lateral horns are present only in the thoracic and upper lumbar cord and house autonomic motor neurons.
Direct motor pathways start in the cortex; indirect pathways start in the brain stem.
Spinothalamic tract carries pain, temperature, pressure. Posterior columns carry vibration, position, fine touch.
C3, 4, 5 keeps the diaphragm alive.