As we’ve seen in the previous article the spinal cord can maintain simple reflexes like a knee-jerk reaction or produce simple repetitive motion like walking or breathing. However to really control our body needs to be able to talk to the brain.
Let’s start with the basics. The white matter of the spinal cord consists mostly of myelinated axons that travel rostrally to the brain stem (ascending tracts) or caudally from the brain towards the spinal cord segments (descending tracts).
Ascending tracts carry certain type of sensory information to the brain. They begin with the unipolar neurons whose cell bodies are usually in the dorsal root ganglia. Axons of these neurons end up travelling up the spinal cord or synapsing on other neurons in the gray matter of the spinal cord.
Descending tracts carry motor commands or modulatory data to the spinal cord and the peripheral nervous system. They usually synapse on interneurons or lower motor neurons whose cell bodies are in the gray matter of the spinal cord and axons exit via the ventral roots.
There’s an easy way to tell if a spinal tract is ascending or descending:
- If the tract’s name begins with “spino”, then it is an ascending tract (e.g. spinothalamic)
- If the tract’s name ends with “spinal”, then it is a descending tract (e.g. tectospinal)
We call a tract ipsilateral if it ends on the same side of the body. We call a tract contralateral when at some point it crosses over (decussates) to the other side of the body. Some tracts decussate twice and end up being ipsilateral.
Neuronal Structure of Spinal White Matter
Axons of specific spinal tracts usually group in segments and can be located on the spinal cord cross section as in the image below.
Tracts marked with blue are ascending, tracts marked with red are descending, tracts marked with green go in both directions. Here’s a short summary of the spinal tracts and we’ll go through them individually later.
|Gracile fasciculus||Conscious proprioception, fine touch, pressure and vibration (lower body)|
|Cuneate fasciculus||Conscious proprioception, fine touch, pressure and vibration (upper body)|
|Spinocerebellar tracts||Unconscious proprioception|
|Spino-olivary tract||Unconscious proprioception (indirect pathway)|
|Spinothalamic tracts||Pain, temperature, crude touch and sustained pressure|
|Spinoreticular tract||Pain-related arousal and attention; pain that originates in internal organs|
|Spinotectal tract||Drawing attention to the area of skin that’s been touched; modulating pain|
The difference between the fine touch and crude touch is that with crude touch you know that something touched you (say, on the leg) but you can’t tell the exact location.
|Corticospinal tracts||Conscious motor commands to skeletal muscles|
|Rubrospinal tract||Conscious motor commands to upper limbs (arm swinging, baby crawling)|
|Reticulospinal tracts||Responsible for locomotion and anticipatory changes in equilibrium|
|Vestibulospinal tract||Maintains upright posture and stabilises head|
|Tectospinal tract||Head and eye movements in response to visual and auditory stimuli|
Bidirectional tracts contain axons that ascend or descend a few segments before synapsing:
|Lissauer’s fasciculus||Crude touch and pressure-related axons ascend or descend one or two levels here|
|Fasciculus proprius||Intersegmental branches of the axons in the posterior and lateral columns|
|Medial longitudinal fasciculus||Descending tectospinal and medial vestibulospinal tract fibres in the spinal cord|
Please note that there’s a great deal of redundancy in the central nervous system. Balance and postural control is achieved via multiple spinal tracts. Locomotion involves the central pattern generators of the spinal cord, pyramidal, reticulospinal and other tracts. In mammals other than humans the rubrospinal tract can almost entirely assume the role of the corticospinal tract if the latter is lesioned.
Neuronal Structure of Spinal Gray Matter
The function of the spinal gray matter neurons often depends on their location. Known the neuronal architecture of the gray matter makes it a lot easier to study the spinal cord tract.
We’ve already seen that the gray matter can be classified into the dorsal horn, ventral horn, lateral horn and the intermediate column:
|Dorsal horn||Receives sensory information from the body and transmits it to the brain|
|Ventral horn||Mostly motor neurons innervating skeletal muscles|
|Intermediate column / lateral horn||Innervates internal and pelvic organs|
For even better detail neuroscientists subdivide the spinal gray matter into nuclei or Rexed laminae. The latter was proposed by Bror Rexed in 1950s as an alternative to nuclei.
Let’s review nuclei first.
|Marginal Zone (MZ)||Neurons located here relay information related to pain and temperature|
|Substantia Gelatinosa (SG)||Relays pain, temperature and light touch; important role in pain modification (contains neurons capable of releasing opioids-like substance)|
|Nucleus Proprius (NP)||Involved in sensing light touch, pain and temperature both for a specific body surface area and visceral organs|
|Dorsal Nucleus of Clarke (NC)||Relays unconscious proprioceptory information to the brain (part of the dorsal spinocerebellar tract)|
|Intermediolateral nucleus (IML)||Sends sensory information from viscera to the brain and motor commands to the visceral organs|
|Motor neurons (MN)||These motor neurons innervate skeletal muscles and internal organs|
Notice how Nucleus Proprius delivers both visceral and somatic pain sensations to the brain. This convergence of somatic and visceral neurons may cause the phenomenon of “referred pain” to a particular body surface. It makes it difficult to recognise the actual source of pain. For example, pain arising in the heart muscle due to lack of oxygen can be sensed as pain in the chest wall, or upper back, left arm and hand pain.
Here’s a list of Rexed laminae.
|Lamina I||Pain and temperature (corresponds to MZ)|
|Lamina II||Pain, temperature and light touch; pain modulation (corresponds to SG)|
|Lamina III, IV||Somatic and visceral proprioception, light touch, pain and temperature (correspond to NP)|
|Lamina V||Somatic and visceral pain, proprioception|
|Lamina VI||Proprioception; also forms spinal reflexes, e.g. “fast pain” moves to lamina VI causing withdrawal|
|Lamina VII||Sympathetic innervation of the body (includes NC and IML)|
|Lamina VIII||Motor interneurons|
|Lamina IX||Motor neurons innervating skeletal muscles|
|Lamina X||Gray matter surrounding the central canal (gray commissure)|
Other than being divided into functional segments, the spinal cord white and gray matter often exhibit somatotopic arangement. Let’s consider the spinal tracts coursing through the white matter of the spinal cord.
The ascending posterior column-medial lemniscus pathway consists of the gracile fasciculus (medial) and the cuneate fasciculus (lateral). The more medial ascending axons deliver the sensory information from the sacral region of the spinal cord while the more lateral axons deliver stimuli from the neck and upper limbs. The spinothalamic tract carries pain, temperature and crude touch sensations from the sacral region of the spinal cord in the lateral part of the tract. The descending lateral corticospinal tract deliver motor commands to the neck and upper limbs in the medial part and lower limbs in the lateral part.
Lower motor neurons originating in the ventral horn of the gray matter are somatotopic as well.
Ascending Spinal Tracts
Ascending or sensory spinal tracts send sensations of pain, temperature, touch, pressure, vibration or proprioception to the forebrain (for sensations that we consciously perceive) or the brain stem and cerebellum (unconscious sensations).
- Posterior column-medial lemniscus pathway
- Spinothalamic pathways (including spinotectal and spinoreticular pathways)
Dorsal Column Tract
Dorsal or posterior column-medial lemniscus pathway sends conscious perception of fine touch, pressure, vibration and proprioception to the brain. Typical receptors in this pathway would be:
- Meisner’s corpuscles (located just below epidermis; sensitive to vibration)
- Pacinian corpuscles (receptors in subcutaneous tissue; extremely sensitive to vibration)
- Ruffini endings (in dermis; respond to sustained pressure, stretching)
- Merkel endings (found in basal epidermis; respond to pressure, static touch)
- Peritrichial endings (axon wrapped around hair follicles)
- Muscle spindles
This is a three neuron pathway. Neurons from the lower body ascend via the gracile fasciculus which is located medially in the dorsal column. Neurons from the upper body end up in the more lateral cuneate fasciculus. Their axons synapse on the second order neurons in the gracile and cuneate nuclei in the medulla. The second order neurons send their axons via the medial lemniscus pathway where they decussate and synapse in the ventral posterior lateral nucleus of the thalamus. The third order neurons send their axons to the primary somatosensory cortex in the postcentral gyrus of the telencephalon. This tract is contralateral.
Notice how the somatosensory cortex is somatotopic with areas arranged according to the different parts of the body they serve. The amount of cortex reserved for the body part is not proportional to its size but rather the amount of nerve fibre it receives. Therefore the projection of the body on the cortex becomes significantly distorted is is known as the cortical homunculus.
There are four spinocerebellar tracts. The dorsal, ventral and rostral spinocerebellar tracts, and the cuneocerebellar tract. The differences are summarised below. Note that this is a very simplistic approach, in reality these tracts may not have strict specialisation.
|Ventral spinocerebellar||Proprioception from joints, tendons and ligaments in the lower body|
|Rostral spinocerebellar||Same as above but in the upper body|
|Dorsal spinocerebellar||Proprioception from muscle, joints, tendons and ligaments in the lower body|
|Cuneocerebellar||Same as above but in the upper body|
In case you’re not overly familiar with the skeletal system, a joint is where two or more bones are joined together (think shoulder or knee). A tendon is connective tissue that attaches muscle to bone. A ligament is connective tissue that attaches bone to bone.
Spinocerebellar tract transmits unconscious proprioceptory information from skeletal muscles (muscle spindles), tendons and ligaments (Golgi tendon organs) and joints (fibrous capsules) to the cerebellum. The cerebellum then integrated this information with other sensory input to help maintain balance, eye movements, assist in locomotion and keep movements smooth and precise. When a doctor asks you to touch your nose with your finger, they are actually testing the function of your cerebellum. All spinocerebellar tracts are ipsilateral (terminate on the same side of the body) though ventral spinocerebellar tract decussates twice before ending up on the ipsilateral side.
The dorsal spinocerebellar tract (DST) starts with sensory neurons delivering proprioception from the muscles and Golgi tendon organs of the lower body. The axon synapses on the second order neuron in the Clarke’s nucleus (column). This second neuron sends an axon to the cerebellum via the inferior cerebellar peduncle in the medulla. Its upper body counterpart is called the rostral spinocerebellar tract (RST). The ventral spinocerebellar tract (VST) has proprioceptory information from the joints, tendons and ligaments sent down the sensory neuron in the dorsal root that synapses on the second order neuron in the spinal border cells.
Cuneocerebellar tract sends proprioceptory information it receives from muscle spindles and Golgi tendon organs in the upper body to the cerebellum as seen in the image above. Unlike the other spinocerebellar tracts it contains three neurons with the axon of the second neuron synapsing on the third neuron in the accessory cuneate nucleus in the caudal medulla.
The anterolateral system includes the lateral and ventral spinothalamic, spinotectal and spinoreticular tracts.
The spinothalamic tract sends conscious sensations of pain, temperature, crude touch and firm pressure from skin to the thalamus and then to the primary somatosensory cortex.
Fast, stabbing pain is picked up at MZ (lamina I) where many Aδ fibres synapse. It then travels to the ventral posterior nucleus of the thalamus via the lateral division of the spinothalamic tract. Thalamus sends the pain signals to the somatosensory cortex. This is called the neospinothalamic pathway (fast pain).
Slow pain (throbbing, soreness, burning) is delivered via C-fibres synapsing on SG (lamina II) or NP (laminae II-VI). NP also receives projections from SG and fibres carrying visceral pain. The pain signals are sent to the periaqueductal gray (PAG) and the superior colliculi (spinotectal tract) and to the reticular formation (spinoreticular tract). The spinoreticular tract terminates in the medullary-pontine reticular formation however fibres from the RF send the pain signal to the intralaminar nuclei of the thalamus that diffusely project the pain signal to various areas of the cerebral cortex. This is the paleospinothalamic pathway (slow pain).
Here’s a simplified version of how pain is processed:
- Neospinothalamic pathway makes you quickly aware of pain and its location
- Spinoreticular pathway (to the medullary and pontine nuclei) helps to inhibit pain, assists in motor responses to pain
- Spinotectal pathway (to the superior colliculi) makes you unconsciously look at the source of pain
- Spinotectal pathway (to the PAG) helps to inhibit pain
- Paleospinothalamic pathway (via the spinoreticular tract) plays a role in pain-related attention and memory
The pain is inhibited via axons projecting to the dorsal roots (areas of the SG and NP). These axons release opioid-like neurotransmitters stopping the pain signals.
The hypothalamus sends dopamine projections, the nucleus raphe magnus (stimulated by the PAG) sends serotonergic projections, and the locus ceruleus sends noradrenergic projections.
The spinotectal tract (also known as spinomesencephalic tract)
The spinotectal tract also travels via the anterolateral system but terminates in the colliculi and the periaqueductal grey (PAG) of the midbrain. The colliculi assist in direction attention to the stimuli (e.g. touch on the skin). PAG, on the other hand, projects to raphe nuclei in the brainstem which in turn release opioid-like neurotransmitters (5-HT or serotonin) in the area of substantia gelatinosa of the spinal cord thereby inhibiting pain sensations.
The spinoreticular tract is displayed in the image below. It utilises the same anterolateral pathway however it is a 4-neuron pathway. The third order neuron begins in the reticular formation of the medulla and pons and synapses on the fourth order neuron in the thalamus. The spinoreticular tract projects diffusely to the entire cerebral cortex and is though to play a role in pain-related attention and memory.
Descending Spinal Tracts
Descending spinal tracts send commands from the brain to the muscles of the body via the spinal cord. This muscle control can be conscious (corticospinal and rubrospinal) and unconscious (vestibulospinal, tectospinal and reticulospinal). Descending tracts can also be classified as pyramidal and extra-pyramidal.
The corticospinal tract is responsible for the voluntary motor control of skeletal muscles of the limbs and trunk. In the corticospinal tract the majority of fibres originate in the primary motor area, premotor and supplementary motor areas. There are some fibres originating in the primary somatosensory area as well. Fibres controlling the face, head and neck are part of the corticobulbar tract.
Fibres of the tracts descend through the internal capsule, the cerebral peduncles of the midbrain, the pons and then the medulla. The corticobulbar tract fibres synapse with motor nuclei of cranial nerves while fibres of the corticospinal tract continue into the spinal cord. The majority (~90%) of the corticospinal tract fibres decussate in the caudal medulla to form the lateral corticospinal tract while the remaining fibres form the anterior corticospinal tract.
The lateral corticospinal tract mostly controls limbs while the anterior corticospinal tract controls mostly muscles of the trunk.
Another tract facilitating voluntary movement is the rubrospinal tract. It begins in the magnocellular red nucleus, crosses along the ventral tegmental decussation in the caudal midbrain and travels to the spinal cord via the lateral funiculus, right next to the lateral corticospinal tract. The red nucleus itself gets input from the cerebellum, cortex, subtantia nigra and the globus pallidus.
The rubrospinal tract terminates in the cervical segments of the spinal cord and therefore only innervates upper limbs. It is believed that in humans it is responsible for arms swinging while walking or baby crawling motion. It is more developed in quadrupeds and can assume the duties of the corticospinal tract if the latter is damaged.
The vestibulospinal tract is an extrapyramidal tract relaying information from the brain stem nuclei to motor neurons in the spinal cord. The function of this tract is to correct postural instability by altering muscle tone and changing the position of the limbs and head.
The vestibular nerve conducts impulses carrying information from the vestibular system which registers the force of gravity, the rotational and linear movements with the utricle, the saccule and the three semicircular canals. In order to maintain balance this and other information flows to the vestibular nuclei as well as various parts of the brain and the spinal cord.
The image below shows only the descending pathways relevant for the vestibulospinal tract. However there are other pathways, for example fibres run from the vestibular nuclei to the colliculi to make sure eyes stay on target even if the head moves.
The vestibulospinal tract consists of two pathways.
The medial vestibulospinal tract begins in the medial and inferior vestibular nuclei and runs along the medial longitudinal fasciculus to the cervical segments of the spinal cord where it synapses on the interneurons in the laminae VII and VIII. This tract innervates neck muscles and helps with the head coordination.
The lateral vestibulospinal tract begins in the lateral vestibular nucleus and runs along the anterior portion of the lateral funiculus. Its fibres run the entire length of the spinal cord and, just like the medial vestibulorspinal tract fibres, synapse on the interneurons. This tract particularly innervates extensor muscles in the legs helping to maintain balance.
Vestibulospinal tract is part of the vestibulospinal reflexes that help the body to maintain balance and posture. The “righting reflex” is one example of such reflexes. It helps to return the head and body back to their normal position following a significant change in their position or environment. This is not a pure vestibulospinal reflex, though, as it uses information provided by the visual and somatosensory systems, as well.
The reticulospinal tract begins in the reticular nuclei of the pons as the medial reticulospinal tract and medulla as the lateral reticulospinal tract. The reticulospinal tract is responsible primarily for the anticipatory changes in equilibrium, but also helps with locomotion, modulates pain signals, mediates some autonomic functions (breathing, circulatory system).
The medial pathway powers extensor (antigravity) muscles of the legs to help maintain postural control. The lateral pathway inhibits extensors and stimulates flexors.
The reticulospinal tract can initiate balance correcting movements in routine, basic, familiar situations whereas the corticospinal tract takes control in less familiar or more complex circumstances requiring better cognitive control and attention.
The tectospinal tract (see image above) begins in the colliculi of the mesencephalon. It descends to the cervical segments of the spinal cord. Along the way in the brain stem it branches out into the tectobulbar tract. Together, these pathways coordinate head and eye movements in response to visual or auditory stimuli (e.g. a bright light, sudden movement, loud noise, etc).
- Spinoreticular tract image. The lateral spinothalamic tract is somatotopic however lower body is represented laterally, not medially.
- Images for anterolateral tracts. Fast pain fibres synapse on MZ, slow pain synapse on SG, slow pain visceral synapse further towards NP (it also gets projections from SG).
- Images for anterolateral tracts. Anterior part of the spinothalamic tract transmits slow nociceptive signal (reticulospinal tract). Lateral part transmits pain, temperature, crude touch and pressure.