These are a common exam question from year 2 to final year medical students.
The first important thing to note is that the spinal cord ends at L1.
This is known as the conus medullaris. The bundle of nerve roots that decend from the conus medullaris to supply the lower limb is called the Cauda Equina (horses tail)
Ascending Tracts (Sensory)
The two main tracts you should be aware of are the Spinothalamic Tract and Dorsal Columns.
They both perceive different modalities:
Spinothalamic Tract: Percieves Pain and Temperature
Dorsal Column: Proprioception, Fine Touch, Vibration Sense
First Order Neurone: Dorsal Root Ganglia
2nd order Neurone: cross the central canal in the midline where they run up in the spinothalamic tract and lateral lemniscus to terminate in the posterolateral nucleus in the thalamus. Note the Somatropic Arrangement of the spinal cord with the sacral fibres being most lateral on the outside of the cord. In a spinal cord lesion due to extrinsic compression there will be an initial loss of sensation in the sacrum, legs followed by torso and arms until a sensory level can be obtained. Note at this point pain and temperature fibres from the trigeminal nerve join the lateral spinothalamic tract.
3rd Order Neurones: Pass from the posterolateral nucleus of the thalamus to the somatosensory cortex in the parietal lobe.
- Patients with pathology whose central canal expands (e.g intradural intra-medullary tumours, syringomyelia) may experience a loss of pain and temperature because of the interruption of the spinothalamic tract.
- Patient with Brown-Sequard Syndrome will have contra-lateral pain and temperature loss due to the interruption of the spinothalamic tract whose fibres have already decussated
- The Spinothalamic tract is not affected in patients with Multiple Sclerosis
1st order Neurones: Dorsal Root Ganglia
2nd Order Neurones: They synapse at the fasciculus cuneatus and gracilis at the lower medulla.
As the internal arcurate fibres they decussate at the level of the medial leminiscus and maintain the somatotropic arrangement (sacral, lumbar, thoracic and cervical). They than synapse with order neurones in the posterolateral nucleus of the thalamus
3rd Order Neurones: thalamus to the somatosensory cortex
Lesions of the Tracts
Spinothalamic tract: Usually lack of awareness of pain and temperature are key. This may result in burns that patients are not aware of i.e Syringomyelia
Dorsal Columns: Impaired vibration sense, Proprioception, Two Point Discrimination and a Sensory Ataxia. Patients have loss of proprioception when eyes are closed (loss of conscious activty). These patients often have a stomping gait and when asked to perform finger-nose test when eyes are closed they often miss! Patients with dorsal column loss have a +ve Rombergs test. Other causes of dorsal column loss (not for Neurosurgeons) are alchol, vitamin B12 deficiency, MS, and Tabes Dorsalis.
A common medical school examination question is to ask candidates why they perform a Rombergs test in a cerebellar examination: The answer is not to assess for a cerebellar ataxia BUT TO distinguish it from a SENSORY ataxia. Please don’t forget to make this adjustment