Anatomy chapters
Spinal cord and its tracts
The external features, coverings and grey matter of the spinal cord, and the main ascending and descending tracts: where each starts, where it crosses and what it carries.
- In adults the cord ends at L1–L2 (conus medullaris); below this lie the nerve roots of the cauda equina.
- Dorsal columns (fine touch, vibration, position) cross in the medulla; spinothalamic fibres (pain, temperature) cross within one or two segments of entry.
- The lateral corticospinal tract carries voluntary movement; about 85–90% of its fibres cross at the pyramidal decussation in the medulla.
On this page
Overview
The spinal cord carries messages between the brain and the body and handles simple reflexes.
It lies in the vertebral canal, protected by the meninges and CSF. Its white matter contains tracts: bundles of fibres going up (sensory) or down (motor). Knowing where each tract crosses is the key to localising lesions.
External features
- Begins at the foramen magnum as the continuation of the medulla.
- Ends in adults as the conus medullaris at the level of the L1–L2 disc. In newborns it reaches about L3.
- Has 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
- Two enlargements for the limbs: cervical (C4–T1, for the brachial plexus) and lumbosacral (L2–S3, for the lumbosacral plexus).
- The filum terminale, a strand of pia, anchors the conus to the coccyx.
- The bundle of nerve roots below the conus is the cauda equina.
The cord ends at L1–L2 in adults, so a lumbar puncture is done at the L3–L4 or L4–L5 space to avoid injuring the cord.
Meninges and spaces
- Dura mater: tough outer layer; the epidural space outside it contains fat and veins.
- Arachnoid mater: under the dura.
- Subarachnoid space: contains CSF; it ends at about S2.
- Pia mater: on the cord itself; it forms denticulate ligaments that hold the cord in place.
A line joining the highest points of the iliac crests crosses the L4 spine, a guide for lumbar puncture.
Grey matter
The grey matter is H-shaped in cross-section.
| Part | Contains |
|---|---|
| Anterior (ventral) horn | Lower motor neurons to skeletal muscle |
| Posterior (dorsal) horn | Neurons receiving sensory input; the substantia gelatinosa handles pain |
| Lateral horn (T1–L2, and S2–S4) | Sympathetic preganglionic neurons (T1–L2); parasympathetic (S2–S4) |
Ascending (sensory) tracts
Dorsal columns (posterior column-medial lemniscus pathway)
- Carries: fine (discriminative) touch, vibration, and position sense (proprioception).
- Fasciculus gracilis: medial; from the lower limb and lower trunk.
- Fasciculus cuneatus: lateral; from the upper limb and upper trunk (above about T6).
- Route: first-order fibres go up on the same side to the nucleus gracilis and nucleus cuneatus in the medulla. Second-order fibres cross (the sensory decussation) and climb as the medial lemniscus to the thalamus, then to the sensory cortex.
Dorsal column fibres cross in the medulla, so a cord lesion causes loss of vibration and position sense on the same side.
Spinothalamic tracts
- Lateral spinothalamic: pain and temperature.
- Anterior spinothalamic: crude touch and pressure.
- Route: fibres enter the dorsal horn and synapse. Second-order fibres cross in the anterior white commissure within one or two segments, then ascend on the opposite side to the thalamus.
Spinothalamic fibres cross in the cord, so a cord lesion causes loss of pain and temperature on the opposite side, starting one or two segments below the lesion.
Spinocerebellar tracts
They carry unconscious position sense from muscles and joints to the cerebellum. The posterior tract stays on the same side; the anterior crosses twice, so both end on the same side of the cerebellum as the limb they come from.
The cerebellum controls the same side of the body. Spinocerebellar input therefore ends up ipsilateral.
Descending (motor) tracts
Corticospinal (pyramidal) tract
- Begins mainly in the motor cortex (area 4) and premotor areas.
- Passes through the posterior limb of the internal capsule, the midbrain (crus cerebri), the pons and the pyramids of the medulla.
- At the pyramidal decussation (lower medulla), about 85–90% of fibres cross and form the lateral corticospinal tract.
- The rest stay uncrossed in the anterior corticospinal tract and mostly cross at the level they end.
- Fibres end on anterior horn cells (lower motor neurons).
The lateral corticospinal tract controls fine voluntary movement, especially of the hands. A lesion above the decussation gives weakness on the opposite side; a lesion in the cord gives weakness on the same side below it.
Other descending tracts (extrapyramidal)
| Tract | From | Main job |
|---|---|---|
| Rubrospinal | Red nucleus (midbrain) | Helps flexor muscle tone |
| Vestibulospinal | Vestibular nuclei | Extensor tone, balance and posture |
| Reticulospinal | Reticular formation | Muscle tone and reflexes |
| Tectospinal | Superior colliculus | Turns the head towards visual stimuli |
Upper and lower motor neuron signs
| Sign | Upper motor neuron lesion | Lower motor neuron lesion |
|---|---|---|
| Tone | Increased (spasticity) | Reduced (flaccid) |
| Reflexes | Brisk | Reduced or absent |
| Plantar response | Extensor (Babinski sign) | Flexor or absent |
| Wasting | Little, late (disuse) | Marked, early |
| Fasciculations | Absent | Present |
Clinical relevance
Lumbar puncture
How it happens: a needle is passed in the midline between the spines of L3/L4 or L4/L5, through the supraspinous and interspinous ligaments, the ligamentum flavum (a "give" is felt), the dura and arachnoid, into the subarachnoid space.
What you see: CSF drips out; pressure can be measured and samples sent.
Dorsal column damage
How it happens: vitamin B12 deficiency (subacute combined degeneration) or tabes dorsalis (syphilis).
What you see: loss of position and vibration sense, unsteadiness worse in the dark, and a positive Romberg test.
For cord lesion patterns, see spinal cord lesions.
Most questions ask where a tract crosses, what it carries, and what is lost on which side. Also tested: the level at which the cord ends, the site of lumbar puncture, the level the subarachnoid space ends, and the location of sympathetic neurons in the lateral horn.
Quick revision
- Cord ends at L1–L2 (adults); subarachnoid space at S2.
- Lumbar puncture at L3–L4 or L4–L5.
- Dorsal columns: touch, vibration, position; cross in the medulla.
- Spinothalamic: pain and temperature; cross within 1–2 segments in the cord.
- Lateral corticospinal: crosses at the pyramidal decussation.
- Lateral horn T1–L2: sympathetic neurons.
- UMN: spastic, brisk reflexes, Babinski. LMN: flaccid, wasting, fasciculations.
Written by The House Job from standard neuroanatomy references (Snell's Clinical Neuroanatomy).
FAQ
Where does the spinal cord end in an adult?
At the level of the L1–L2 intervertebral disc, as the conus medullaris.
Which tract carries pain and temperature?
The lateral spinothalamic tract, which crosses within one or two segments of entry.
Where do the corticospinal fibres cross?
Most cross at the pyramidal decussation in the lower medulla.
What does the dorsal column carry?
Fine touch, vibration sense and position sense.