UNITED INTERNATIONAL MEDICAL SCHOOLS
Spinal Tracts​​​
Brief Intro
A king cannot exert influence if his orders cannot be conveyed, nor can he make the right decisions without knowing the state of his subjects. We might make a lot of decisions with our heart, but at the end of the day, the brain is still the king, and without it, the decisions your heart makes won’t come into physical fruition. If you’re still here after that poor attempt at a joke, we’re going to discuss spinal tracts, which allow the brain to convey its commands and refine its commands as needed.This article will first review some neuroanatomy, discuss clinical relevance, and include some physical examination tips, so whether you're a beginner 1st year, a 4th year about to appear for their Neuro final, or a 6th year who has some time, this article is for you!
The Spinal Cord
Housed in the spinal canal of the vertebral column, the spinal cord is part of the central nervous system that connects the brain to the periphery. Beginning at the decussation of pyramids, and ending at L1/L2 as the conus medullaris and the filum terminale (collection of nerve roots), it sends off 2 spinal nerves at every segment.
Clinical Relevance: Lumbar puncture to sample CSF is usually done between L3/4. Amongst the indications are CSF testing for infection and subarachnoid haemorrhage (to relieve pressure). It is contraindicated if there is a risk of cerebral herniation.
Cross Section
Before we go into the tracts, we need to know what are the components of the spinal cord at each segment.Gray matter represents cell bodies (unmyelinated), whereas white matter represents axons (myelinated).The gray matter forms the central butterfly shape and can be divided into the anterior, posterior and lateral horns. The white matter is everything around the gray matter and can be divided into funiculi.
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To keep things brief, the anterior horn consists of lower motoneurons (LMN, the ‘UMN’s being in the cortex), part of the motor (descending) tracts; whereas the posterior horn consists of neurons part of sensory (ascending) tracts. Two of the nuclei of the posterior horn are: nucleus proprius and substantia gelatinosa
Sensory Tracts
Before we move on, we distinguish between protopathic (quick, coarse) and epicritic (fine) sensation. Protopathic is when you feel pain, but cannot pinpoint exactly where it is. Epicritic sensation allows you to identify where exactly it is. There is also proprioceptive sensation, which informs the cerebellum about muscle tension and position. A general rule of thumb in sensory tracts: the first neuron is ALWAYS in the dorsal root ganglion. Conscious sensation of a stimulus occurs if the last neuron terminates in the cortex.
Protopathic
The first neuron, as discussed, is located in the dorsal root ganglion of a particular segment. The peripheral limb collects temperature, pain, pressure and crude touch and relays it via the central limb to the nucleus proprius (or the substantia gelatinosa) of the same segment (sometimes it may be ±1-2 segments), ipsilaterally (on the same side). The axon of the proper nucleus then decussates (crosses the midline) and ascends as the anterior and lateral spinothalamic tracts. The 2nd axon synapses with the Ventral Posterolateral (VPL) nucleus of the thalamus, from where the 3rd axon ends in the somatosensory cortex (Brodmann areas 3,1,2), resulting in sensation! There exists also the spino-reticular tract, the 2nd axon of which synapses at the RF, which then sends efferents to BA 3,1,2.
Epicritic
Epicritic sensation is related to fine touch and pressure, like the feeling of an ant on your skin. The first axon, of the DRG, doesn’t synapse, nor decussates in the spinal cord; rather it ascends ipsilaterally to the medulla, where it synapses with the gracile and cuneate nuclei. Here, it decussates, ascends and synapses at the VPL, as was with the spinothalamic tract. The tract for epicritic sensation is called the dorsal column or spino-bulbar tract.
Clinical Relevance
Brown-Sequard syndrome [incomplete]This is when one half of the spinal cord and its tracts are damaged.
Motor
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There are two main divisions of motor pathways:
Pyramidal: responsible for voluntary motion
Extrapyramidal: responsible for involuntary motion, tone regulation and posture.
Pyramidal tracts
They originate from the motor cortex (BA4), pass through the internal capsule (white matter in between basal ganglia), decussate visibly as the decussation of the pyramids. They descend in the anterior funiculi as the anterior and lateral corticospinal tracts, and synapse with the alpha motoneurons (lower motoneurons).
Extrapyramidal tracts
They originate from various subcortical structures and mainly play a role in regulating skeletal muscle tone. They include the rubrospinal (from the nucleus ruber), vestibulospinal (from the vestibular nuclei, play a role in balance), tectospinal and reticulospinal tracts. Alongisde the alpha motoneurons, they also synapse with the gamma motoneurons.
Clinical Relevance
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1. UMN vs LMN Lesions
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Lesions of the UMN manifest as hyperreflexia, hypertonia amongst other symptoms. This is because the descending tracts from the UMN also play a ‘regulatory’ role, and ‘keep the LMN in check’. Conversely, amongst other symptoms, LMN lesions manifest as hyporeflexia, hypotonia and also muscle atrophy.
2. Anterior cord syndrome
Occlusion of the anterior spinal artery leads to damage of the anterior 2/3rds of the spinal cord. This causes loss of function of all the tracts mentioned above except the dorsal columnar tracts, which run in the posterior funiculi only. Thus, expected symptoms include bilateral paralysis, loss of temperature, pain and crude touch.
3. Posterior Cord syndrome
There are various etiologies that are responsible for posterior cord syndrome, the lesion that affects the dorsal column tracts. They include defective myelination, B12 (required for adequate myelination) deficiency, posterior spinal artery occlusion. Symptoms are characteristic for loss of epicritic: loss of vibration/fine touch sensation. Patients are romberg positive.