Spinal / Tracts

The spinal cord starts at the C1[…] level and ends at L1-L2[…] as the conus medullaris; below this lies the cauda equina[…] ("horse's tail").

Total spinal nerves: 31[…] — 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

The first cervical nerve (C1) exits ABOVE[…] the C1 vertebra. From T1 onward, each spinal nerve exits BELOW its corresponding vertebra.

Enlargements: the cervical[…] enlargement spans C5-T1 (BUE innervation); the lumbar[…] enlargement spans L2-S3 (BLE innervation).

As you descend the cord, gray[…] matter increases (more motor pools needed for limbs) and white[…] matter decreases (fewer ascending/descending fibers remain).

Gray matter forms the inner butterfly/starfish[…] (or H-shape), surrounded by white matter.

Gray matter zones: posterior gray horn[…] (somatic + visceral sensory), anterior gray horn[…] (somatic motor), and intermediate zone[…] containing the lateral gray horn[…] (visceral motor — only present at T1-L2 for sympathetic preganglionics, and S2-S4 for parasympathetic preganglionics).

The slab of gray matter connecting both sides is the gray commissure[…], with the central canal[…] running through it.

White matter columns (funiculi): dorsal/posterior, lateral, and anterior/ventral. The anterior white commissure[…] is the crossing point for STT fibers.

A group of neuron cell bodies in the CNS is a nucleus[…]; in the PNS, a ganglion[…].

Functions: discriminative touch, pressure, vibration, proprioception, and stretch[…].

PNS receptors: Meissner's corpuscles, Merkel's discs, Pacinian corpuscles, Ruffini corpuscles, peritrichial endings, plus muscle spindles (1a fibers[…]) and Golgi tendon organs (1b fibers[…]).

1st-order neuron: cell body in DRG[…] → posterior gray horn → ascends in the dorsal/posterior white column[…] IPSILATERALLY. Below T6 (legs): fasciculus gracilis[…]; above T6 (arms): fasciculus cuneatus[…].

2nd-order neuron: synapses at the nucleus gracilis or cuneatus in the medulla → fibers DECUSSATE as the internal arcuate fibers[…] → ascend as the medial lemniscus[…].

3rd-order neuron: synapses in the VPL[…] of thalamus → projects via posterior limb of internal capsule → primary somatosensory cortex (postcentral gyrus)[…].

Functions: crude touch, pressure, pain, and temperature[…].

PNS receptors: nociceptors, Merkel's discs, free nerve endings; carried by A-delta[…] fibers (sharp pain/cold) and C[…] fibers (dull pain/crude touch).

1st-order neuron: DRG.

2nd-order neuron: synapses in the posterior gray horn[…] → MOST fibers CROSS via the anterior white commissure[…] → ascend contralaterally. The lateral STT (lateral white column) carries pain/T; the ventral STT (anterior white column) carries crude touch.

STT collaterals project to reticular formation[…] (arousal), intralaminar thalamus (cortex "wake up"), parabrachial nucleus[…] → amygdala (emotional pain), midbrain tectum/superior colliculus, and PAG[…] (descending pain modulation).

3rd-order neuron: synapses in VPL[…] of thalamus → projects to primary somatosensory cortex.

All carry touch, pressure, and proprioception[…] to the cerebellum (unconscious proprioception).

Dorsal spinocerebellar tract (DSCT)[…]: carries IPSILATERAL info from C8-L2/3 (origin = Clarke's column[…] / nucleus dorsalis) → lateral white column → inferior cerebellar peduncle (ICP)[…] → cerebellum as mossy fibers.

Cuneocerebellar tract[…]: carries IPSILATERAL info from ABOVE C8 → ascends to the accessory cuneate nucleus[…] in the medulla → external arcuate fibers through the ICP[…] → cerebellum.

Ventral spinocerebellar tract (VSCT)[…]: carries info from L2/3 and BELOW → crosses via the anterior commissure → ascends in the contralateral lateral white column → enters via the superior cerebellar peduncle (SCP)[…] → CROSSES AGAIN in the cerebellum, ending up ipsilateral.

Rostral spinocerebellar tract[…]: cervical/upper extremity proprioception (analog of VSCT for arms).

Two mechanisms:

Gate control theory[…]: rubbing an injured area activates the DCML[…] pathway, whose collaterals stimulate inhibitory interneurons in the dorsal gray horn. These release GABA[…] onto the STT synapse — reducing pain perception.

Descending analgesic system[…]: the periaqueductal gray (PAG)[…] around the cerebral aqueduct and periventricular gray around the 3rd ventricle stimulate the locus ceruleus[…] (NE), raphe nuclei[…] (serotonin), and reticular formation.

These descend through the posterior white column → dorsal gray horn, where they stimulate inhibitory interneurons to release endorphins and endogenous opioids[…] — blocking pain transmission.

Pharmacologic relevance: SSRIs/SNRIs (e.g., duloxetine), TCAs (amitriptyline), and opioids all act on this system.

Origin: primary motor cortex (~30%), motor association cortex (~30%), and primary somatosensory cortex (~40%)[…]; cell bodies in the pyramidal (5th) cortical layer[…].

Pathway: centrum semiovale → corona radiata[…] → posterior limb of internal capsule[…] → crus cerebri[…] (cerebral peduncles) → pons (some fibers branch to pontine nuclei → ponto-cerebellar[…]) → pyramids[…] of medulla.

At the pyramidal decussation, ~85%[…] of fibers CROSS to form the lateral CST (limbs), and ~15% stay ipsilateral as the anterior CST (axial muscles).

Lateral CST runs in the lateral white column[…] → synapses on α/γ motor neurons in the anterior gray horn[…] → ventral root → muscles.

Anterior CST decussates at its target spinal level via the anterior white commissure[…] before synapsing.

Origin: primary motor cortex, motor association cortex, and primary somatosensory cortex[…]; on the homunculus, it sits laterally[…] (face area).

Pathway: centrum semiovale → corona radiata → genu of the internal capsule[…] → midbrain (medial crus cerebri) → synapses on motor nuclei of CN V, VII, IX, X, XI (cranial), XII[…].

Innervation pattern: MOST CN motor nuclei receive bilateral[…] corticobulbar input — explaining why UMN lesions cause only mild contralateral weakness for most CNs.

Exceptions (predominantly contralateral cortical input):
- Lower face (CN VII): UMN lesion → contralateral lower-face[…] weakness only (forehead spared).
- Tongue (CN XII): UMN lesion → tongue deviates AWAY[…] from the cortical lesion.

Pontine reticulospinal[…]: arises from pontine reticular formation → descends in the anterior white column[…] → activates extensor[…] muscles (assists the vestibulospinal tract).

Medullary reticulospinal[…]: arises from medullary reticular formation → descends in the lateral white column[…] → activates flexor[…] muscles AND inhibits extensor tone.

These two tracts antagonize each other: pontine = extensor drive; medullary = flexor drive + extensor inhibition.

The CST[…] normally projects collaterals to the medullary reticulospinal nuclei to keep extensor tone in check. Loss of CST in UMN lesions disinhibits the pontine pathway → spasticity[…] with extensor predominance.

Function: activates flexor[…] muscles, predominantly of the upper extremities (BUE > BLE).

Pathway: cortex sends corticorubral[…] fibers descending to the red nucleus[…] in the midbrain. The red nucleus also receives input from the cerebellum's globose and emboliform (interposed)[…] nuclei.

From the red nucleus, fibers DECUSSATE at the ventral tegmental decussation[…] → descend in the contralateral lateral white column → synapse on α/γ motor neurons in the anterior gray horn[…].

In humans, the rubrospinal tract is functionally overshadowed by the CST — but it becomes prominent in decorticate[…] posturing (when cortex is gone but red nucleus is intact).

UMN tracts: CST[…] (cortex → anterior gray horn) and CBT[…] (cortex → CN motor nuclei).

UMN lesion signs:
- Spastic paralysis[…]: loss of CST input to medullary reticulospinal → less inhibition of α motor neurons → hypertonia.
- Hypertonia[…] that is velocity-dependent (more resistance with faster movement), in one direction, often with clasp-knife[…] phenomenon. Contrast with PD rigidity which is velocity-INDEPENDENT and bidirectional.
- Hyperreflexia[…], clonus, and positive Babinski sign[…].
- NO atrophy or fasciculations.

LMN lesion signs (anterior horn cell or peripheral motor nerve):
- Flaccid paralysis[…].
- Hypotonia[…], hyporeflexia[…], and absent Babinski.
- Atrophy and fasciculations[…].

ALS[…] uniquely shows BOTH UMN and LMN signs simultaneously.

For LP: palpate the iliac crest[…] (corresponds to the L4 spinous process); insert the needle just above it at the L3/L4[…] interspace — below the conus medullaris (L1-L2) to avoid cord injury.

Layers (deep to superficial):
- Pia mater[…]: attached to the spinal cord surface; ends as the filum terminale[…] anchoring to the coccyx.
- Subarachnoid space[…]: contains CSF; extends to S2 (large lumbar cistern is the LP target).
- Arachnoid mater[…].
- Subdural space[…]: potential space.
- Dura mater[…].
- Epidural space[…]: contains fat and venous plexus.

The denticulate ligaments[…] are pia mater offshoots that suspend the cord laterally within the dural sac.

Arterial supply: the vertebral arteries[…] and segmental radicular arteries feed one anterior spinal artery (ASA)[…] (supplies anterior 2/3 of cord) and two posterior spinal arteries (PSAs)[…] (supply posterior 1/3 — i.e., dorsal columns).

The artery of Adamkiewicz[…] is the largest radicular feeder, typically entering at T10-T12[…] on the left, supplying the lower thoracic and lumbar ASA territory.

Anterior Spinal Artery (ASA) Syndrome[…]: knocks out the anterior 2/3 of the cord:
- Lateral CST → BLE paraplegia[…] (or quadriplegia if higher).
- STT → loss of bilateral pain and temperature[…] below the lesion.
- Anterior gray horn (at the level) → LMN signs at that level.
- Lateral gray horn (if T1-L2) → autonomic dysfunction.

DCML/dorsal columns are SPARED[…] (PSA territory) — so vibration and proprioception are preserved.

Anterior gray horn lesion[…] (e.g., polio, West Nile, SMA): LMN signs at the level — flaccid paralysis, hypotonia, hyporeflexia, atrophy, fasciculations. ALS[…] shows additional UMN signs from CST involvement.

Central cord syndrome[…] (often from hyperextension injury in elderly or syringomyelia): hits the lateral CST and crossing anterior white commissure fibers. Features:
- Bilateral UMN signs[…] below the lesion, somatotopically affecting BUE > BLE (cervical/thoracic fibers are more medial).
- Bilateral cape-like loss of pain and temperature[…] at the level (crossing STT fibers).
- DDX: syringomyelia[…] — central canal dilation, often from Chiari I or cord injury.

Posterior cord syndrome (tabes dorsalis)[…]: hits dorsal columns — loss of vibration, proprioception, and fine touch[…] below the lesion, with sensory ataxia and a positive Romberg sign. Classic cause: neurosyphilis (tertiary)[…].

Brown-Séquard syndrome (cord hemisection)[…]: ipsilateral UMN paralysis and DCML loss BELOW the level; contralateral STT loss (pain/T) starting 1-2 segments BELOW[…] the lesion; LMN signs and ipsilateral total sensory loss AT the level.