Welcome to yet another instalment of my "Navigating Neuro" series where I shall attempt to explain, to the best of my abilities, basic concepts within neuroscience. Now, this topic seems to perplex, or rather, annoy a lot of students so my duty for today is to explain it in easy-to-understand terms in which even a person who knows nothing of the nervous system would be able to somewhat comprehend. Again, somewhat.
Before I begin, I must reiterate the fact that you must supplement whatever I've written here with textbooks, with what your lecturer says as well as journal articles. I cannot stress that enough. I'm not here to provide a sort of "neuro bible" for you as that title belongs to your textbook. Instead, my aim is to enable you to better understand something which many find confusing without further complicating something that's already so incredibly complex. And as with all of neuroscience, we can never be quite sure as to which paths specific neurons take to reach their destination. This is only a general outline of just a few of the many pathways within our nervous system. I must also warn you that this post will be content heavy so without further ado, let's get straight into it!
The ascending pathways within the spinal cord primarily deal with sensory information that's coming into the body via various receptors contained within the skin, sending it to the brain. Furthermore, there exists a sequence of neurons which make up all ascending pathways, each with a rather similar route as they travel from say your toe all the way up to the somatosensory cortex of your brain.
Sequence of Three Neurons
➪ First Order Neuron
- Enters the spinal cord (SC) via the dorsal root ganglion (DRG).
- Terminates via synaptic contact with the second order neuron.
➪ Second Order Neuron
- Cell body is located in either the spinal grey matter or in the medulla of the brainstem.
- Axon will decussate (cross-over to contralateral side of the CNS) and will terminate on a third order neuron located in the thalamus.
➪ Third Order Neuron
- Cell body is located in the thalamus.
- Axon synapses with somatosensory cortex in the parietal lobe of the ipsilateral (same side) cerebral hemisphere.
Somatotopic Organisation
Basically, adjacent receptors in the skin surface are connected to adjacent neurons. It's important you understand this arrangement when you acquaint yourself with the pathways below. I've attached an image (seen above) of sensory and motor homunculi.
AKA DORSAL COLUMN-MEDIAL LEMNISCUS (DCML) PATHWAY
➪ Nerve endings located on the surface of your skin (such as Meissner's Corpuscle and Ruffini Endings), which make up the first order neuron, may arise from lower or upper body portions.
➪ The cell bodies of primary afferent fibres (this is another way to describe neurons travelling towards the CNS, conversely, efferent means away from the CNS) are located within the ipsilateral DRG.
➪ Depending on where sensory information was initiated, primary afferent fibres will then travel to one of two different locations:
- Fibres from the lower body (from T6 and below) will enter the gracile fasciculus/fasciculus gracilis (to help you remember this: the ballerina is graceful on her feet, feet being a part of your lower body, of course) located in the dorsal column (hence, DC-ML Pathway) of the spinal cord.
- Terminates on a second order neuron that is located within the nucleus gracilis of the caudal medulla.
- Fibres from the upper body (T6 and above) will enter the cuneate fasciculus/fasciculus cuneatus which is lateral to the gracile fasciculus in the dorsal column of the spinal cord.
- Terminates on a second order neuron, but this time, its cell body is located within the nucleus cuneatus which is also located in the caudal medulla.
(NB: for each successive spinal cord level, fibres entering the posterior columns add on laterally.)
➪ Second order fibres which arise from second order neurons will then decussate (i.e. cross the midline) and form the medial lemniscus where it will traverse up the spinal cord (through the pons and midbrain, but you don't really need to know the specifics of its location, just know the main locations).
➪ Fibres arising from second order neurons will then terminate on a third order neuron located within the ventral posterolateral nucleus (VPL) of the thalamus where third order fibres will synapse in the primary sensory cortex of the postcentral gyrus.
LESIONS
Lesions to the DCML don't yield as devastating effects when compared to other pathways, which will be discussed later, however, the obvious loss of proprioception and the ability to distinguish between materials using fine touch does prove to be rather debilitating.
- Effects will occur on the ipsilateral side as fibres do not decussate until they reach the medulla.
GENERAL ANATOMY
Now, I don't like this sensory pathway because there's multiple ways in which neurons travel. For instance, we have the anterolateral pathway for pain and temperature and the spinocervical tract, not to mention the pathway for pain and temperature in the head. For now, we're going to deal with the lateral spinothalamic tract.
➪ Stimuli for pain is detected by nociceptors in the skin which sends a signal to the spinal cord.
➪ The cell body of the first order neuron is located within the DRG where axons will terminate on a second order neuron in the substantia gelatinosa that's located in the dorsal horn of the spinal cord.
- Second order fibres will ascend and descend for a few segments of the spinal cord before travelling in Lissaeur's Tract where it will cross over the ventral white commissure.
- Second order fibres then ascend in the lateral spinothalamic tract only to terminate on a third order neuron located in the VPL thalamus.
➪ Third order neurons will project topographically to primary sensory cortex in the postcentral gyrus.
LESIONS
Loss of pain and temperature sensation will exist on the contralateral side of the body as decussation occurs within the spinal cord.
GENERAL ANATOMY
This pathway is a bit iffy but please deal with me here. The main constituents are as follows:
➪ First order neuron arises from sensory receptors in the periphery where its fibres will enter the spinal cord via the DRG (in which its cell body resides, NB: for almost all sensory pathways, including the ones I've described, the neuron type located within the DRG is pseudounipolar, hence why it has projections from sensory organs as well as to the spinal cord itself).
- First order fibres will ascend a couple of segments before synapsing on a second order neuron located within the nucleus proprius of the dorsal horn of the spinal cord.
➪ Second order fibres will decussate and join the ventral spinothalamic tract (some might join the ipsilateral gracile or cuneate tracts but for our purposes, we're not going to bother with that).
Fibres continue to ascend the spinal cord to form the spinal lemniscus where it will eventually terminate on a third order neuron that is located within the VPL thalamus.
➪ Third order fibres will synapse on ipsilateral primary sensory cortex of the postcentral gyrus.
The descending pathways primarily send motor information from the brain to lower motor neurons (LMN) which directly innervate muscles to produce movement. NB: LMN are in direct contact with striated muscle whereas upper motor neurons (UMN) descend from the cerebral cortex to terminate on an LMN.
Descending Pathways are Functionally Divided into Two Groups
➪ Pyramidal Tracts
- Responsible for voluntary control of musculature of the body and face.
- Originate in cerebral cortex.
➪ Extrapyramidal Tracts
- Responsible for involuntary and automatic control of all musculature i.e. balance and locomotion.
- Originate in the brainstem.
CORTICOSPINAL TRACTS
➪ Deals with skilled voluntary movement of lower extremities.
➪ UMN fibres originate in the precentral gyrus (primary motor cortex) and will pass through the internal capsule.
- Fibres then descend through the cerebral peduncles (crux cerebri) of the midbrain.
➪ UMN fibres will continue into the pyramids of the medulla where they will follow one of two paths:
- Lateral Corticospinal Tract: ~85% of fibres will decussate at the spinomedullary junction (called the pyramidal decussation) to join the lateral corticospinal tract which deals with fine voluntary movement of lower extremities.
- Anterior Corticospinal Tract: ~15% of fibres will remain ipsilateral, that is, they will not decussate at the spinomedullary junction. This pathway deals with postural adjustments which compensate for said voluntary movements occurring at lower extremities.
➪ Termination of UMN fibres on LMNs occur in two different ways, again, depending on which path they follow:
- Lateral Corticospinal Tract: terminates on LMN located within the anterior horn of the spinal cord.
- Anterior Corticospinal Tract: continues ipsilaterally in the anterior fasciculus and will decussate at the level at which its LMN resides.
LESIONS
UPM lesions will produce deficits to contralateral side of the body granted that the incision/injury is unilateral.
- Hypertonia: increased muscle tone.
- Hyperreflexia: increased muscle reflexes.
- Clonus: involuntary muscle contractions.
VESTIBULOSPINAL TRACT
Medial
Origin: Medial vestibular nuclei.
Target: LMN in spinal cord.
Decussation: Bilateral to neck muscles.
Function: Postural adjustment in response to changes in balance.
Lateral
Origin: Lateral vestibular nuclei.
Target: LMN in spinal cord.
Decussation: Ipsilateral.
Function: Stimulates flexors and inhibits extensors; postural adjustments.
RETICULOSPINAL TRACT
Origin: Reticular formation within the pons.
Target: LMN in spinal cord.
Decussation: mostly ipsi, has some bilateral projections.
Function: Changes in muscle tone.
RUBROSPINAL TRACT
Origin: Red nucleus.
Target: LMN in spinal cord.
Decussation: Midbrain.
Function: Flexor movements in upper limb.
TECTOSPINAL TRACT
Origin: Tectum i.e. superior colliculus.
Target: LMN in cervical spinal cord.
Decussation: Bilateral.
Function: Orientation of head/neck in response to eye movements.
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WOAH. That was a lot of information to take in, huh? I know, I got a bit lazy towards the end when describing the extrapyramidal pathways (basically, they're pathways which do not go through the medullary pyramids), but honestly, curriculum-wise, they're not that important. Besides, it's easy to figure out the origin and target locations because they're literally given in the name!
e.g. Reticulospinal = reticulo is the reticular formation and because this comes before spinal—that is, the spinal cord—we can infer that the reticulospinal tract originates in the reticular formation and terminates in the spinal cord.
With enough practice (that is, I recommend you draw out these pathways over and over again), you'll be getting HDs in no time ;)
Studiously,
P.S. Most, if not all, information was obtained from Nolte's "The Human Brain: an Introduction to Its Functional Anatomy" which you can purchase here.
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