Physiology→Neurophysiology→Spinal cord and tracts

MMed Phase I · Neurophysiology · Lesson 11

Every pathway is described
by where it runs, and where it crosses.

Orientation

Rapid review

Prerequisites

What you should already have

Lesson 1 — organisation of the nervous system.

Estimated study time

About 70 minutes

Plus the time it takes to redraw this lesson’s figures from memory, which is the fastest way to find out what you have not understood.

Why it matters

Where this shows up

Every lesson from here to 16 is described against this cross-section. It is also the anatomy behind neuraxial blockade, the level of the conus, the artery of Adamkiewicz, and the reason a patient with an anterior cord injury can still feel where their legs are.

Learning outcomes

By the end of this lesson you should be able to:

Draw a labelled spinal cord cross-section showing grey matter horns, white matter columns and the principal tracts.
State the extent of the cord, the level of the conus medullaris and the implications for neuraxial blockade.
Describe the dorsal column-medial lemniscal and spinothalamic systems, giving the level of decussation for each.
Name the descending tracts, their origin and their function, and distinguish pyramidal from extrapyramidal.
Predict the sensory and motor deficit produced by hemisection, anterior cord and central cord lesions.
Describe the blood supply of the cord and explain why the anterior spinal artery territory is vulnerable.

Together these settle one syllabus objective: Spinal cord anatomy and the ascending and descending tracts. Tick it on the Physiology objective list once you can do all of the above without notes.

The minimum high-yield framework

Extent. Foramen magnum to the lower border of L1-L2 in the adult, ending as the conus medullaris; below that the canal contains the cauda equina. Dura and arachnoid continue to S2.
31 segments, each with a dorsal (sensory) and a ventral (motor) root. The dorsal root ganglion sits in the intervertebral foramen.
Grey inside, white outside — the reverse of the brain. Grey matter is H-shaped: dorsal horn sensory, ventral horn motor, lateral horn autonomic at T1-L2.
Rexed laminae I-X. I-VI dorsal horn, VII and X intermediate, VIII and IX ventral horn. II is the substantia gelatinosa; V holds the wide-dynamic-range neurons; VII holds the intermediolateral cell column.
Interneurons are 97% of cord cells, motor neurons about 2%.
Three decussation levels. Dorsal columns cross in the medulla; spinothalamic crosses in the cord, within one or two segments; corticospinal crosses in the medullary pyramids.
Blood supply — one anterior spinal artery supplying the anterior two-thirds, two posterior spinal arteries supplying the posterior third, plus radicular feeders, the largest being the artery of Adamkiewicz.

Gross anatomy

Extent, segments and the spaces around the cord

The anatomy that decides where a needle can safely go, and what a level means clinically.

The spinal cord runs from the foramen magnum, continuous with the medulla, to about the lower border of L1 or L2 in the adult, where it tapers into the conus medullaris. Below that the vertebral canal contains only the lumbar and sacral roots descending to their own foramina — the cauda equina. The filum terminale, a pial extension, anchors the conus to the coccyx, and the denticulate ligaments, also pial, tether the cord laterally.

There are 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Each gives a dorsal root, carrying afferent fibres whose cell bodies lie in the dorsal root ganglion, and a ventral root of efferent fibres from the anterior horn. The two merge into a mixed spinal nerve. Two enlargements mark the limb plexuses: cervical (about C4 to T1) for the upper limb, and lumbar (about L2 to S3) for the lower.

Layer or space	What it is	Anaesthetic relevance
Dura mater	Tough outer layer, continuous with cranial dura; ends at S2	The structure a Tuohy needle is felt to pass through, and the layer whose puncture causes post-dural-puncture headache
Epidural (extradural) space	Between dura and the vertebral canal wall — a potential space containing fat, the internal vertebral venous plexus and lymphatics	The target of epidural anaesthesia. Its venous plexus is valveless and engorges in pregnancy and with raised intra-abdominal pressure, which is why doses are reduced
Arachnoid mater	Thin avascular membrane applied to the inner dura; ends at S2	It, not the dura, is the principal barrier to drug diffusion
Subarachnoid space	Between arachnoid and pia; contains CSF, the cord and the roots	The target of spinal anaesthesia. Below L2 it contains only cauda equina, which is why a spinal is placed there
Pia mater	Adherent to the cord; forms the denticulate ligaments and the filum terminale	Vascular; not a barrier to drug diffusion

Why a spinal is placed below L2, and how the level is found

Because the cord ends at about L1-L2, a needle placed below that meets only the mobile roots of the cauda equina, which are far more likely to be pushed aside than pierced. The intercristal (Tuffier’s) line, between the highest points of the iliac crests, is conventionally taken as the L4 vertebral body or the L4-L5 interspace — but it is a surface landmark and it is known to be identified a level or more too high in a substantial proportion of patients, particularly in pregnancy and obesity. Counting up from the sacrum, or using ultrasound, is more reliable than assuming it.

In the neonate and infant the cord ends lower, at about L3, because the vertebral column grows faster than the cord. A lumbar puncture in an infant is therefore placed lower still.

The drawing

The cord in cross-section

One figure that the next six lessons all refer back to.

Original teaching diagram

The spinal cord in cross-section: grey matter, columns and tracts

Conventional orientation, dorsal (posterior) uppermost. The grey matter is central and butterfly-shaped — cell bodies, dendrites and interneurons — with narrow, pointed dorsal horns reaching almost to the surface and broader, blunter ventral horns. The white matter around it carries the myelinated tracts. That arrangement is the reverse of the brain, and it is worth being able to state why: cord neurons stay where the neural tube formed them and grow their axons outward, whereas forebrain neurons migrate outward and leave their axons behind.

Ascending tracts are labelled on the left and descending on the right, which is a drawing convention rather than anatomy: every tract shown is present bilaterally. Three relationships carry most of the clinical weight. The dorsal columns are somatotopically arranged with the legs medial (gracile) and the arms lateral (cuneate), and they ascend ipsilaterally to decussate in the medulla. The spinothalamic tract decussates within one or two segments in the cord, so it is already contralateral by the time it ascends. And the lateral corticospinal tract has already decussated, in the medullary pyramids. Those three decussation levels are what make hemisection produce the dissociated deficit it does. Schematic: tract territories are shown as discrete blocks for legibility and are not drawn to relative cross-sectional area.

The grey matter is central and butterfly- or H-shaped, containing neuronal cell bodies, dendrites, unmyelinated axons and glia. The white matter surrounds it and contains the myelinated ascending and descending tracts. The proportion of white to grey rises as you move up the cord, because more tracts have joined by the time you reach the cervical region.

Horn	Present at	Contains	Function
Dorsal (posterior) horn	All levels	Second-order sensory neurons and interneurons; Rexed laminae I-VI	Receives and processes all afferent input
Ventral (anterior) horn	All levels	Alpha and gamma motor neurons; laminae VIII and IX	Final common pathway to skeletal muscle
Lateral horn	T1 to L2 only	The intermediolateral cell column, within lamina VII	Sympathetic preganglionic neurons — which is why the sympathetic outflow is thoracolumbar

The white matter is divided by the horns and roots into three columns (funiculi): dorsal, between the dorsal horns; lateral, between dorsal and ventral roots; and ventral, between the ventral horns.

Cytoarchitecture

The laminae of Rexed

Ten layers defined by cell size and packing. Three of them do most of the work you need to know about.

Lamina	Name	Principal input	Why it matters
I	Marginal nucleus (posteromarginal nucleus)	Nociceptive input from Aδ and C fibres	Origin of many spinothalamic projection neurons
II	Substantia gelatinosa	C fibre input; densely packed interneurons	The site of the gate control circuit and of much opioid action
III and IV	Nucleus proprius	Low-threshold cutaneous mechanoreceptors — Aβ	The large-fibre input that closes the gate
V	Neck of the dorsal horn	Cutaneous, muscle and joint mechanical nociceptors, and visceral nociceptors	Home of the wide-dynamic-range neurons, and therefore the anatomical basis of referred pain
VI	Base of the dorsal horn	Large-diameter afferents from muscle and joint, including muscle spindle afferents	Projects to cerebellum; modulates muscle tone
VII and X	Intermediate grey	Intermediomedial and intermediolateral nuclei; Clarke's column (nucleus dorsalis) at T1-L2; lamina X surrounds the central canal	Lamina VII contains the intermediolateral cell column — the sympathetic preganglionic neurons, T1 to L2
VIII	Ventral horn, medial	Motor interneurons; commissural nucleus	Coordinates the two sides of the cord
IX	Ventral horn, motor nuclei	Alpha and gamma motor neurons — medial groups to axial muscles, lateral to limb muscles	Includes the phrenic nucleus (C3-C5) and Onuf's nucleus in the sacral cord

Interneurons make up about 97% of the cells in the cord, motor neurons about 2%, and the remainder are the cells giving rise to ascending tracts. Interneurons are inhibitory — releasing GABA, glycine and enkephalins — or excitatory, releasing glutamate and neuropeptides. Within the dorsal horn there are three kinds of excitatory neuron: high-threshold neurons responding only to noxious stimuli, low-threshold neurons responding only to innocuous ones, and wide-dynamic-range neurons responding to both.

Sensory

The ascending tracts

Modality, fibre type, and — the part that carries the clinical consequence — where each one crosses.

Tract	Modality	Fibres	Course and decussation	Note
Dorsal column-medial lemniscus	Fine touch, vibration, two-point discrimination, conscious proprioception	Aα and Aβ	Ascends ipsilaterally to the gracile and cuneate nuclei; second-order neuron decussates in the medulla as the internal arcuate fibres	Somatotopy: legs medial (gracile, present at all levels), arms lateral (cuneate, above T6)
Lateral spinothalamic	Pain and temperature	Aδ and C	Synapse in the dorsal horn; second-order neuron decussates in the anterior white commissure within one or two segments, then ascends anterolaterally	The tract divided in an anterolateral cordotomy. Already contralateral in the cord
Anterior spinothalamic	Crude touch and pressure	Aβ, Aδ and C	As for the lateral tract	Its overlap with the dorsal columns is why light touch is often preserved after a cord lesion that abolishes pain
Posterior (dorsal) spinocerebellar	Unconscious proprioception from the lower limb	Aα (group Ia and Ib)	Synapses in Clarke's column, then ascends ipsilaterally — does not cross	Enters the cerebellum through the inferior cerebellar peduncle
Anterior (ventral) spinocerebellar	Unconscious proprioception, whole-limb movement	Aα	Crosses in the cord, ascends, then crosses back within the cerebellum — so its information reaches the ipsilateral cerebellum	The double crossing is why cerebellar signs are ipsilateral to the lesion
Spinoreticular and spinomesencephalic	The affective and arousal component of pain	C	Largely bilateral projections to the reticular formation and periaqueductal grey	The paleospinothalamic system — poorly localised, slow, and the target of descending inhibition

Both of the two conscious systems use three neurons in series: first-order from receptor to the first synapse, second-order from there across the midline to the thalamus, third-order from thalamus to cortex. The full pathway diagram, drawn against shared anatomical levels, is in lesson 12.

Motor

The descending tracts

One pyramidal system for fine voluntary movement, and a set of extrapyramidal tracts for tone and posture.

Tract	System	Origin	Decussation	Function
Lateral corticospinal	Pyramidal	Primary motor cortex, premotor and somatosensory cortex	Decussates in the medullary pyramids — about 85-90% of fibres	Fine, fractionated, voluntary movement of distal limb muscles. A lesion above the decussation produces contralateral weakness; below it, ipsilateral
Anterior corticospinal	Pyramidal	As above	The 10-15% that do not decussate in the medulla; most cross at segmental level	Axial and proximal muscles, and posture
Rubrospinal	Extrapyramidal	Red nucleus, midbrain	Crosses immediately in the midbrain	Facilitates flexor tone in the upper limb. Its preservation is why decorticate posturing is flexor and decerebrate posturing, below the red nucleus, is extensor
Vestibulospinal	Extrapyramidal	Vestibular nuclei	Lateral tract uncrossed; medial tract largely bilateral	Extensor (antigravity) tone, balance and posture
Reticulospinal	Extrapyramidal	Pontine and medullary reticular formation	Largely uncrossed	Muscle tone, posture, locomotor pattern generation, and descending modulation of pain
Tectospinal	Extrapyramidal	Superior colliculus	Crosses in the midbrain	Reflex head and neck turning toward a visual or auditory stimulus

The pyramidal system — the corticospinal tract — plans, initiates and mediates voluntary movement, and is responsible for fine, precise, fractionated control of distal muscles. The extrapyramidal tracts arise in the brainstem and regulate muscle tone and posture. Motor control as a whole is lesson 14; what belongs here is where each tract runs in the cord, because that is what determines the deficit produced by a lesion at a given site.

Perfusion

Blood supply of the spinal cord

One artery supplies two-thirds of the cord, and it is the one that fails.

One anterior spinal artery, formed from branches of both vertebral arteries and running in the anterior median fissure. It supplies the anterior two-thirds of the cord — which includes the corticospinal tracts, the spinothalamic tracts and the anterior horns.
Two posterior spinal arteries, from the posterior inferior cerebellar arteries, running medial to the posterior roots. They supply the posterior third — essentially the dorsal columns.
Radicular (segmental) feeder arteries from the vertebral, deep cervical, ascending cervical, posterior intercostal, lumbar and lateral sacral arteries. The longitudinal vessels alone cannot supply the whole cord; the feeders are essential rather than accessory.
The artery of Adamkiewicz (arteria radicularis magna) is the largest feeder and supplies the low thoracic and lumbar cord. It arises on the left from a posterior intercostal artery between T8 and L3 in about 80% of people, and at about T5 in about 15%.

Anterior spinal artery syndrome, predicted from the anatomy

Occlusion or hypoperfusion of the anterior spinal artery produces paraplegia, from damage to the corticospinal and vestibulospinal tracts, and loss of pain, temperature and deep touch, from damage to the spinothalamic tracts — while light touch, proprioception and vibration are preserved, because the dorsal columns are supplied by the separate posterior spinal arteries. That dissociation is the syndrome, and it is entirely predictable from which vessel supplies which column.

It is the injury feared during thoracoabdominal aortic surgery, where cross-clamping interrupts the segmental feeders. The physiological principle behind the protective measures is the same one as for the brain: perfusion pressure is a gradient, so CSF drainage to lower the downstream pressure, and maintaining the upstream pressure, both act on it.

Prediction, not memorisation

The classical lesion patterns

Each of these is derivable from the cross-section and the decussation levels. Work them out rather than learning them.

Syndrome	Deficit	Anatomical explanation	Typical cause
Brown-Séquard (hemisection)	Ipsilateral loss of fine touch, vibration and proprioception below the lesion; ipsilateral upper motor neuron weakness; contralateral loss of pain and temperature, beginning one or two segments below	The dorsal columns and corticospinal tract have not yet crossed at cord level; the spinothalamic tract already has	The classical demonstration that decussation level is the whole story
Anterior cord syndrome	Loss of motor function and of pain and temperature below the lesion; dorsal columns preserved	Anterior spinal artery territory — the anterior two-thirds of the cord	Aortic cross-clamping, aortic dissection, severe hypotension. Proprioception and vibration survive because the posterior spinal arteries are separate
Central cord syndrome	Cape-like bilateral loss of pain and temperature at the level, with sacral sparing; upper limbs affected more than lower	The decussating spinothalamic fibres cross close to the central canal and are damaged first; corticospinal somatotopy places arm fibres medially	Syringomyelia; hyperextension injury in a stenotic cervical canal
Posterior cord syndrome	Loss of vibration, proprioception and fine touch, with sensory ataxia; pain and temperature preserved	Dorsal columns	Subacute combined degeneration from B12 deficiency; tabes dorsalis; posterior spinal artery territory
Complete transection	Spinal shock initially — flaccid areflexic paralysis below the level — evolving over weeks into spasticity, hyperreflexia and extensor plantars	Loss of all descending input, then reorganisation and denervation supersensitivity	Autonomic dysreflexia becomes possible once reflexes return, for lesions above T6

Viva prompt

A patient has weakness and loss of vibration sense in the right leg, and loss of pinprick in the left leg. Where is the lesion, and why?

Answer it out loud, then reveal the model answer

This is a right-sided hemisection — Brown-Séquard syndrome — and the side is given by the motor and dorsal column findings, not by the pain findings.

The corticospinal tract has already decussated in the medullary pyramids, so below the lesion it is supplying the same side it runs on: a right cord lesion gives right-sided weakness. The dorsal columns have not yet crossed — they ascend ipsilaterally to the medulla — so a right cord lesion abolishes vibration and proprioception on the right. The spinothalamic tract, by contrast, crossed in the cord within one or two segments of entry, so the fibres running in the right cord are carrying information from the left side of the body: hence contralateral loss of pain and temperature.

The pinprick level begins one or two segments below the lesion precisely because of that delayed crossing — fibres entering at the level of the lesion have not yet reached the midline and are spared.

At the level of the lesion itself I would expect a narrow band of ipsilateral loss of all modalities, from destruction of the dorsal horn and entering root fibres, and lower motor neuron signs in that myotome from anterior horn cell damage — flaccidity and wasting, in contrast to the upper motor neuron pattern below.

Likely follow-ups

Why does the pinprick level start a segment or two below the lesion?
What would you expect at the level of the lesion itself?

Retrieval check

A patient develops a cape-like loss of pain and temperature across both shoulders, with normal vibration sense. Where is the lesion?
Say it out loud, then check
Central, around the canal — a syrinx. The spinothalamic fibres decussate in the anterior white commissure just anterior to the central canal, so an expanding central lesion catches them as they cross, bilaterally and at the levels involved. The dorsal columns run posteriorly and are spared, and the sacral spinothalamic fibres run most laterally, hence sacral sparing.
Name the three decussation levels that determine every cord lesion pattern.
Say it out loud, then check
Dorsal columns cross in the medulla, as the internal arcuate fibres. Spinothalamic crosses in the anterior white commissure of the cord, within one or two segments of entry. Corticospinal crosses in the medullary pyramids.
Why does the anterior spinal artery supply a disproportionate share of the cord?
Say it out loud, then check
There is only one of it, and it supplies the anterior two-thirds — the corticospinal tracts, the spinothalamic tracts and the anterior horns — whereas the posterior third has two arteries. The anterior territory is therefore both larger and less redundantly supplied, which is why cord ischaemia characteristically spares the dorsal columns.

Anaesthetic and clinical application

Where cord anatomy changes practice

Neuraxial blockade

Everything about a spinal or epidural is an application of this lesson. The needle is placed below L2 because the cord has ended. The arachnoid, not the dura, is the main diffusion barrier. The epidural venous plexus is valveless and engorges with raised intra-abdominal pressure, reducing the epidural space and the dose required. And the order in which modalities are lost follows fibre size and myelination rather than tract position — sympathetic B fibres first, then temperature and pinprick, then touch, then motor, as set out in lesson 2.

Autonomic dysreflexia

A complete cord lesion above T6 disconnects the sympathetic outflow below the lesion from brainstem control. A noxious stimulus below the level — a distended bladder, a blocked catheter, surgery — produces a massive unmodulated sympathetic discharge from the intact intermediolateral cell columns below the lesion, causing severe hypertension. The baroreflex responds with bradycardia and with vasodilatation above the lesion only, producing the characteristic flushing and headache above a pale, sweating body below. The reason the threshold is T6 is that a lesion above it isolates the splanchnic outflow (T6-L2), which is where the capacity for major vasoconstriction lies.

Spinal cord monitoring

The tract anatomy dictates what each monitor sees. Somatosensory evoked potentials travel in the dorsal columns — posterior circulation territory — so they can be normal while an anterior spinal artery injury is occurring. Motor evoked potentials travel in the corticospinal tract, in the anterior two-thirds, and therefore monitor the territory actually at risk during aortic surgery. Using only the first is a monitoring strategy that watches the wrong third of the cord.

Consolidation

The lesson in one paragraph

The spinal cord runs from the foramen magnum to about L1-L2, ending as the conus medullaris, below which the canal contains the cauda equina; dura and arachnoid continue to S2. It has 31 segments, each with a dorsal sensory root whose cell bodies lie in the dorsal root ganglion and a ventral motor root from the anterior horn. In cross-section, grey matter is central and H-shaped and white matter peripheral — the reverse of the brain. The grey matter has a dorsal horn for sensory processing, a ventral horn containing alpha and gamma motor neurons, and, from T1 to L2, a lateral horn containing the sympathetic preganglionic neurons of the intermediolateral cell column. It is divided into ten Rexed laminae: I-VI dorsal horn, with lamina II the substantia gelatinosa and lamina V the wide-dynamic-range neurons; VII and X intermediate; VIII and IX ventral. Interneurons are about 97% of cord cells. The ascending tracts are the dorsal column-medial lemniscal system, carrying fine touch, vibration and proprioception ipsilaterally to decussate in the medulla; the spinothalamic tracts, carrying pain, temperature and crude touch, which decussate in the cord within one or two segments; and the spinocerebellar tracts for unconscious proprioception. The descending tracts are the pyramidal corticospinal tract, decussating in the medullary pyramids and responsible for fine voluntary movement, and the extrapyramidal rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts, responsible for tone and posture. The cord is supplied by one anterior spinal artery serving the anterior two-thirds and two posterior spinal arteries serving the posterior third, supplemented by radicular feeders of which the artery of Adamkiewicz is the largest.