In one breath
Sensation climbs to the cortex along a chain of three neurons: a first-order neuron in the dorsal root ganglion, a second-order neuron that crosses the midline and reaches the thalamus, and a third-order neuron from the thalamus to the sensory cortex. In the dorsal column pathway (fine touch, vibration, proprioception, stereognosis) the first-order fibers climb the cord on their own side to the gracile and cuneate nuclei, and the second-order fibers cross in the medulla and run up the medial lemniscus to the VPL nucleus. In the anterolateral system the first-order fibers end in the dorsal horn, and the second-order fibers cross in the same segment and climb as the anterior (crude touch) and lateral (pain, temperature) spinothalamic tracts. So a lesion in one half of the cord takes dorsal column sensation on its own side, but pain and temperature on the other.
Builds on: Afferent fibers, dorsal root and spinal laminae · Sensory receptors and their classification · Leads to: Thalamus: nuclei, functions and thalamic syndrome · Somatosensory cortex · Pain pathways and perception
Arrangement of neurons
An ascending pathway is a chain of neurons linked in series. It carries sensation up to the brain, in most cases through the thalamus to the sensory cortex. The usual chain has three neurons; some pathways add a fourth.
| Neuron | Cell body | Ends on |
|---|---|---|
| First order | Dorsal root ganglion | Second-order cell in the cord or medulla |
| Second order | Spinal cord or brainstem | Thalamus of the opposite side |
| Third order | Specific thalamic nucleus | Specific area of sensory cortex |
| Fourth order | Sensory cortex | Sensory association cortex |
- First order: the primary afferent. It starts at the receptor, runs in a peripheral nerve and enters the cord by the dorsal root. It then either ends in the laminae of the cord or climbs the dorsal column to end in the medulla.
- Second order: relays the signal on to the thalamus. As a rule its axon crosses the midline, in the cord or in the medulla, so each half of the body reports to the opposite thalamus.
- Third order: runs from a specific thalamic nucleus to a specific cortical area. Sensations from the body all relay in the thalamus before they reach the cortex, which makes it the main sensory relay of the brain.
- Higher (fourth) order: usually left out of the pathway. When present it starts from the endings of the third-order neuron in the sensory cortex and carries the signal on, mainly to the sensory association area, for further processing.
Only the second-order neuron crosses
The first-order fiber always stays on its own side. What differs between the two systems is where the second-order neuron crosses: in the medulla for the dorsal column pathway, in the spinal cord for the anterolateral system.
Classification
Dorsal column pathway
- Made of the fibers that climb the dorsal column of the cord.
- Also called the lemniscal system, because in the brainstem its fibers run in the medial lemniscus.
- Six sensations: fine touch, vibration, proprioception, tactile localization, tactile discrimination and stereognosis.
Anterolateral system
- Anterior spinothalamic tract: crude touch.
- Lateral spinothalamic tract: pain and temperature.
Other ascending pathways
- Spinocerebellar tracts, dorsal and ventral
- Spinoreticulothalamic tract
- Spinocervicothalamic tract
Worth knowing, though not in your pages: the spinocerebellar tracts carry proprioception that never reaches awareness, to the cerebellum (see Cerebellar divisions, histology and connections).
Dorsal column pathway
- Formed by large myelinated afferents that enter the dorsal column and climb to the medulla.
- They carry the six dorsal column sensations listed above.
- In the cord they run as two bundles, the fasciculus gracilis and the fasciculus cuneatus, also known as the tracts of Goll and Burdach.
First-order neuron
| Fasciculus | Position | Carries from |
|---|---|---|
| Gracilis (Goll) | Medial | Lower limb, lower trunk |
| Cuneatus (Burdach) | Lateral | Upper limb, upper trunk |
- The cell body is in the dorsal root ganglion. The fiber climbs on the same side, with no relay in the cord.
- It ends in the nucleus gracilis or the nucleus cuneatus of the medulla.
- Worth knowing, though not in your pages: the fasciculus cuneatus is present only from about T6 upwards, so below mid-thorax the dorsal column is all gracile.
Second-order neuron
- The cell bodies lie in the nucleus gracilis and nucleus cuneatus, together called the dorsal column nuclei.
- Their axons cross the midline in the medulla, then climb in the medial lemniscus to the opposite thalamus.
- Worth knowing, though not in your pages: the crossing fibers are the internal arcuate fibers, and the crossing is the sensory decussation.
Dorsal column nuclei
- The nuclei are mapped by body part: your book places the face laterally and the trunk and hind limb medially.
- Two cell types have been described in them recently, and these divide the nuclei into two zones.
- Cluster region: the main part, with its cells grouped in clusters.
- Input comes directly from dorsal column fibers.
- Its second-order neurons cross to the opposite thalamus.
- Role: to tell the thalamus and cortex that the body has been stimulated.
- Non-cluster region: lies more rostrally.
- Input comes from dorsal column fibers and from descending cortical fibers.
- Output goes mainly to non-thalamic targets: cerebellum, tectum, pretectum, inferior olive, red nucleus and other motor-control areas.
- Role: to feed sensory signals straight to motor areas, so that movement can be corrected at once.
- Worth knowing, though not in your pages: no fibers from the face climb the dorsal column. Facial sensation relays in the trigeminal nuclei (see Trigeminal pathway and trigeminal neuralgia), whose spinal nucleus lies lateral to the cuneate nucleus in the medulla. Within the dorsal column nuclei themselves, the arm is lateral (cuneate) and the leg medial (gracile).
Third-order neuron
- Starts in a specific thalamic nucleus, the VPL (ventral posterolateral) nucleus.
- Ends in the somatosensory areas of the cerebral cortex.
- Worth knowing, though not in your pages: the fibers pass through the posterior limb of the internal capsule to the postcentral gyrus, areas 3, 1 and 2 (see Somatosensory cortex).
Draw it: the dorsal column–medial lemniscus pathway
Stack three levels: a cord cross-section at the bottom, the medulla in the middle, and the thalamus with the postcentral gyrus at the top.
- First order: a DRG cell whose central branch enters by the dorsal root and turns straight up the dorsal column of the same side. Draw the gracile bundle medially (leg) and the cuneate bundle laterally (arm), ending in the nucleus gracilis and nucleus cuneatus.
- Second order: from those nuclei, draw the axon sweeping forward and crossing the midline in the medulla (label it “sensory decussation”), then climbing as the medial lemniscus to the opposite VPL nucleus.
- Third order: VPL → internal capsule → somatosensory cortex.
- Write the six sensations beside the tract, and put a clear “crosses here” label on the medulla.
Anterolateral system
- Older in evolution than the dorsal column system.
- Two tracts: the anterior spinothalamic for crude touch and the lateral spinothalamic for pain and temperature.
First-order neuron
- Mainly afferents from nociceptors, thermoreceptors and mechanoreceptors.
- The cell bodies are in the dorsal root ganglion, and the fibers enter by the dorsal root.
- They end in the dorsal horn on their own side, on second-order neurons that lie mainly in laminae I, II and V.
Second-order neuron
- The cell body lies in the dorsal horn.
- The axon crosses the midline in the same segment and climbs in the anterolateral funiculus of the opposite side to the thalamus.
| Tract | Where it runs | Carries |
|---|---|---|
| Anterior (ventral) spinothalamic | Anteriorly | Crude touch |
| Lateral spinothalamic | Laterally | Pain, temperature |
- Worth knowing, though not in your pages: the crossing fibers form the anterior white commissure, just in front of the central canal. Many pain fibers first run a segment or two up or down in Lissauer’s tract before they end.
Third-order neuron
- Starts in the VPL, midline and intralaminar nuclei of the thalamus.
- Ends in specific areas of the sensory cortex.
Draw it: the anterolateral (spinothalamic) system
Use the same three levels as for the dorsal column pathway, so the two can be compared.
- First order: a DRG cell whose fiber enters by the dorsal root and ends in the dorsal horn of the same side (mark laminae I, II and V).
- Second order: from the dorsal horn cell, draw the axon crossing in front of the central canal in the same segment, then turning up in the opposite anterolateral white matter. Show two bundles: crude touch in front (anterior spinothalamic), pain and temperature at the side (lateral spinothalamic). Carry them up through the brainstem to the thalamus.
- Third order: VPL, midline and intralaminar nuclei → sensory cortex.
- Label the crossing “same spinal segment”, in contrast to “medulla” on the dorsal column drawing.
The two systems compared
| Feature | Dorsal column | Anterolateral |
|---|---|---|
| Sensations | Fine touch, vibration, position | Crude touch, pain, temperature |
| First order ends | Gracile, cuneate nuclei | Dorsal horn, laminae I, II, V |
| Second order crosses | Medulla | Same spinal segment |
| Second order climbs in | Medial lemniscus | Anterolateral funiculus |
| Thalamic relay | VPL | VPL, midline, intralaminar |
| In evolution | Newer | Older |
Which side goes numb after a cord lesion
Dorsal column fibers climb on the side they entered, so a lesion in one half of the cord takes fine touch, vibration and position sense on the same side. Spinothalamic fibers have already crossed, so the same lesion takes pain and temperature on the opposite side. Hemisection of the cord is covered in Sensory lesions and sensory function tests.
Exam-answer skeleton: "Trace the dorsal column–medial lemniscus pathway with a labelled diagram" (long essay)
- Introduction: an ascending pathway is three neurons in series; the dorsal column (lemniscal) pathway carries fine touch, vibration, proprioception, tactile localization, tactile discrimination and stereognosis.
- First-order neuron: DRG cell with a large myelinated fiber; enters by the dorsal root and climbs on the same side in the fasciculus gracilis (medial, lower body) or cuneatus (lateral, upper body); ends in the nucleus gracilis or cuneatus.
- Second-order neuron: cell bodies in the dorsal column nuclei; axons cross in the medulla and climb in the medial lemniscus to the opposite VPL nucleus.
- Dorsal column nuclei: somatotopic map; cluster region to the thalamus, non-cluster region to the cerebellum and motor areas.
- Third-order neuron: VPL → somatosensory cortex.
- The labelled diagram, with the crossing in the medulla marked.
- Contrast with the anterolateral system: crosses in the cord, carries crude touch, pain and temperature.
- Applied: a cord lesion takes these sensations on the same side; dorsal column disease (tabes dorsalis, B12 deficiency) blunts vibration and position sense and gives a positive Romberg sign (see Abnormalities of dorsal column sensations).
Asked in exams
- Dorsal column–medial lemniscus pathway: Aug 2015, 4 marks · Jul 2018, 4 marks · Jan/Feb 2022, MCQ · Oct 2024, MCQ
- Anterolateral (spinothalamic) system: Jul/Aug 2014, 4 marks · Jan 2023, MCQ · Apr 2023, MCQ