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Does the Spinoreticular Tract end in Brainstem?

Does the Spinoreticular Tract end in Brainstem?


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According to this book on Springer spinoreticular tract is :

As the name implies, the tract originates in the spinal cord and terminates in the reticular formation (RF) in the brainstem. While most of the fibers appear to terminate in the RF of the medulla and lower pons, some may ascend as far as the midbrain.

But according to Wikipedia :

The spinoreticular tract is an ascending pathway in the white matter of the spinal cord, positioned closely to the lateral spinothalamic tract. The tract is from spinal cord-to reticular formation- to thalamus.


What I am unable to understand is if Wikipedia is true then why do we consider this tract as ending in the brainstem rather than ending in the thalamus?

Also I am getting different information about the location of the origin of spinoreticular fibres,

My Textbook says:

The spinoreticular fibres begin from the spinal neurons mainly in lamina VII (also V and VIII).

But this site says:

The term spinoreticular tract refers to a fiber bundle that arises largely from lamina I, lamina IV and lamina V of the spinal cord.

Which source is correct?


Brainstem: Function and Location

The brainstem is the region of the brain that connects the cerebrum with the spinal cord. It consists of the midbrain, medulla oblongata, and the pons. Motor and sensory neurons travel through the brainstem allowing for the relay of signals between the brain and the spinal cord. Most cranial nerves are found in the brainstem.

The brainstem coordinates motor control signals sent from the ​brain to the body. This brain region also controls life-supporting autonomic functions of the peripheral nervous system. The fourth cerebral ventricle is located in the brainstem, posterior to the pons and medulla oblongata. This cerebrospinal fluid-filled ventricle is continuous with the cerebral aqueduct and the central canal of the spinal cord.


Contents

The DCML pathway is made up of the axons of first, second, and third-order sensory neurons, beginning in the dorsal root ganglia. The axons from the first-order neurons form the ascending tracts of the gracile fasciculus, and the cuneate fasciculus which synapse on the second-order neurons in the gracile nucleus and the cuneate nucleus known together as the dorsal column nuclei axons from these neurons ascend as the internal arcuate fibers the fibers cross over at the sensory decussation and form the medial lemniscus which connects with thalamus the axons synapse on neurons in the ventral nuclear group which then send axons to the postcentral gyrus in the parietal lobe.

The gracile fasciculus carries sensory information from the lower half of the body entering the spinal cord at the lumbar level. The cuneate fasciculus carries sensory information from the upper half of the body (upper limbs, trunk, and neck) entering the spinal cord at the cervical level. [4] The gracile fasciculus is wedge-shaped on transverse section and lies next to the posterior median septum. Its base is at the surface of the spinal cord, and its apex directed toward the posterior gray commissure. The gracile fasciculus increases in size from inferior to superior.

The cuneate fasciculus is triangular on transverse section, and lies between the gracile fasciculus and the posterior column, its base corresponding with the surface of the spinal cord. Its fibers, larger than those of the gracile fasciculus, are mostly derived from the same source, viz., the posterior nerve roots. Some ascend for only a short distance in the tract, and, entering the gray matter, come into close relationship with the cells of the dorsal nucleus, while others can be traced as far as the medulla oblongata, where they end in the gracile nucleus and cuneate nucleus.

The two ascending tracts meet at the T6 level. Ascending tracts typically have three levels of neurons, namely first-order, second-order, and third-order neurons, that relay information from the physical point of reception to the actual point of interpretation in the brain.

First-order neurons Edit

Periphery and spinal cord Edit

When an action potential is generated by a mechanoreceptor in the tissue, the action potential will travel along the peripheral axon of the first-order neuron. The first-order neuron is pseudounipolar in shape with its body in the dorsal root ganglion. The action signal will continue along the central axon of the neuron through the posterior root, into the posterior horn, and up the posterior column of the spinal cord.

Axons from the lower body enter the posterior column below the level of T6 and travel in the midline section of the column called the gracile fasciculus. [5] Axons from the upper body enter at or above T6 and travel up the posterior column on the outside of the gracile fasciculus in a more lateral section called the cuneate fasciculus. These fasciculi are in an area known as the posterior funiculus that lies between the posterolateral and the posterior median sulcus. They are separated by a partition of glial cells which places them on either side of the posterior intermediate sulcus.

The column reaches the junction between the spinal cord and the medulla oblongata, where lower body axons in the gracile fasciculus connect (synapse) with neurons in the gracile nucleus, and upper body axons in the cuneate fasciculus synapse with neurons in the cuneate nucleus. [6]

First-order neurons secrete substance P in the dorsal horn as a chemical mediator of pain signaling. The dorsal horn of the spinal cord transmits pain and non-noxious signals from the periphery to the spinal cord itself. Adenosine is another local molecule that modulates dorsal horn pain transmission [3]

Second-order neurons Edit

Brainstem Edit

The neurons in these two nuclei (the dorsal column nuclei) are second-order neurons. [6] Their axons cross over to the other side of the medulla and are now named as the internal arcuate fibers, that form the medial lemniscus on each side. This crossing over is known as the sensory decussation.

At the medulla, the medial lemniscus is orientated perpendicular to the way the fibres travelled in their tracts in the posterior column. For example, in the column, lower limb is medial, upper limb is more lateral. At the medial lemniscus, axons from the leg are more ventral, and axons from the arm are more dorsal. Fibres from the trigeminal nerve (supplying the head) come in dorsal to the arm fibres, and travel up the lemniscus too.

The medial lemniscus rotates 90 degrees at the pons. The secondary axons from neurons giving sensation to the head, stay at around the same place, while the leg axons move outwards.

The axons travel up the rest of the brainstem, and synapse at the thalamus (at the ventral posterolateral nucleus for sensation from the neck, trunk, and extremities, and at the ventral posteromedial nucleus for sensation from the head).

Third-order neurons Edit

Thalamus to cortex Edit

Axons from the third-order neurons in the ventral posterior nucleus in the thalamus, ascend the posterior limb of the internal capsule. Those originating from the head and the leg swap their relative positions. The axons synapse in the primary somatosensory cortex, with lower body sensation most medial (e.g., the paracentral lobule) and upper body more lateral.

Discriminative sensation is well developed in the fingers of humans and allows the detection of fine textures. It also allows for the ability known as stereognosis, to determine what an unknown object is, using the hands without visual or audio input. This fine sensation is detected by mechanoreceptors called tactile corpuscles that lie in the dermis of the skin close to the epidermis. When these structures are stimulated by slight pressure, an action potential is started. Alternatively, proprioceptive muscle spindles and other skin surface touch receptors such as Merkel cells, bulbous corpuscles, lamellar corpuscles, and hair follicle receptors (peritrichial endings) may involve the first neuron in this pathway.

The sensory neurons in this pathway are pseudounipolar, meaning that they have a single process emanating from the cell body with two distinct branches: one peripheral branch that functions somewhat like a dendrite of a typical neuron by receiving input (although it should not be confused with a true dendrite), and one central branch that functions like a typical axon by carrying information to other neurons (again, both branches are actually part of one axon).

Damage to the dorsal column-medial lemniscus pathway below the crossing point of its fibers results in loss of vibration and joint sense (proprioception) on the same side of the body as the lesion. Damage above the crossing point result a loss of vibration and joint sense on the opposite side of the body to the lesion. The pathway is tested with Romberg's test.

Damage to either of the dorsal column tracts can result in the permanent loss of sensation in the limbs. See Brown-Séquard syndrome.

The cuneate fasciculus, fasciculus cuneatus, cuneate tract, tract of Burdach, was named for Karl Friedrich Burdach. The gracile fasciculus, the tract of Goll, was named after Swiss neuroanatomist Friedrich Goll (1829–1903).


Spinal cord tracts

Ascending tracts

The spinal cord consists of ascending and descending tracts. The ascending tracts are sensory pathways that travel through the white matter of the spinal cord, carrying somatosensory information up to the brain. They allow you to feel sensations from the external environment (exteroceptive) such as pain, temperature, touch, as well as proprioceptive information from muscles and joints.

The sensory pathways start from receptors located in our skin, organs, muscles, etc. These specialized sensory organs register physical and chemical changes in our body’s external and internal environment and convert these changes into electrical impulses. This afferent information then travels from these receptors, via peripheral nerves, to the CNS, where they join with the relevant ascending tract.

Each ascending pathway follows the same general structure as first-order, second-order and third-order neurons. First-order neurons are afferent in nature. The sensory input from the receptors is sent through the peripheral nerve to the spinal/dorsal root ganglion. The body of the first-order neuron, within the ganglia, projects its axons to the posterior gray horn of the spinal cord. Here, it synapses with second-order neurons that ascend along the spinal cord and project onto third-order neurons which are found in the subcortical structures of the brain, such as the thalamus. These third-order neurons pick up the neural impulse and carry it on to the cerebral cortex.

There are ten ascending tracts: posterior/dorsal column (fasciculus gracilis, fasciculus cuneatus), spinothalamic (anterior, lateral), spinocerebellar (anterior, posterior, Cuneo-), spinotectal, spinoreticular and spinoolivary.

Posterior/Dorsal column pathways

Let’s now take a look at each pathway more closely. The gracilis and cuneate fasciculi, also known as the dorsal/posterior columns, are two ascending pathways located side-by-side in the posterior funiculus of the spinal cord. They carry fine and discriminative touch as well as proprioceptive sensations. Together with the medial longitudinal fasciculus, these tracts form the so-called ‘dorsal column medial lemniscus pathway’ (DCML pathway), also known as the ‘posterior column medial lemniscus pathway’ (PCML pathway) .

First-order neurons ascend ipsilaterally (on the same side) through the spinal cord. They synapse in the gracilis and cuneate nuclei of the medulla oblongata, where the body of the second-order neuron lies. The axons of the second-order neuron immediately decussate (cross the midline) and ascend superiorly. At this point the posterior column pathway is renamed as the medial lemniscus, and the fibers continue to ascend until the thalamus. After synapsing in the thalamus, third-order neurons pass through the posterior one-third of the posterior arm of the internal capsule and project to the primary somatosensory cortex where the sensations are mapped out and the source pinpointed.

If you want to learn more details about how proprioception and discriminative touch reach the brain, take a look below:

Spinothalamic tracts

There are two spinothalamic tracts: anterior and lateral. The anterior spinothalamic tract transports course touch and pressure sensation. It is located in the anterior funiculus of the spinal cord. The lateral spinothalamic tract carries pain and temperature sensations. It is found in the lateral funiculus of the spinal cord.

The anterior spinothalamic tract begins with peripheral first-order neurons located in the spinal ganglion. Axons of the first-order neurons reach the posterior gray horn of the spinal cord through the posterior root of the spinal nerve. Fibers from the posterior grey horn (second-order neurons) ascend within the ipsilateral anterior funiculus for seven segments of the spinal cord, decussate, then travel on to the thalamus. Finally, third-order neurons project from the thalamus onto the primary somatosensory cortex.

The lateral spinothalamic tract travels in the lateral funiculus of the spinal cord and carries the sensations of pain and temperature. Similar to its anterior sibling, first-order neurons located in the spinal ganglion send axons to the posterior gray horn, specifically in the Rexed laminae regions I, IV, V and VI, where they synapse with second-order neurons. These decussate across the anterior white commissure and ascend in the (now contralateral) lateral spinothalamic tract. While crossing the medulla, these fibers join with those from the anterior spinothalamic and spinotectal tracts to form the anterolateral tract (spinal lemniscus). The second-order neurons of the lateral spinothalamic tract synapse in the thalamus and the subsequent third-order neurons, together with the anterior spinothalamic tract, cross through the posterior third of the posterior arm of the internal capsule. These neurons then project onto the primary somatosensory cortex, where the information about external stimuli is decoded and analyzed.

Spinocerebellar tracts

Now that we understand the tracts involved in somatosensation, how are they integrated with movement? For example, how can our fingers follow the rim of a glass, or how can we walk in a coordinated fashion? These actions occur with the help of our spinocerebellar tracts.

Spinocerebellar tracts sense proprioception from muscle spindles, Golgi tendon organs and joint receptors. As a result, they are involved in movement coordination and posture maintenance. There are two main spinocerebellar tracts that carry information from the lower extremities the posterior (dorsal) spinocerebellar and the (anterior) ventral spinocerebellar tracts. Whilst the cuneocerebellar and rostral spinocerebellar tracts carry information from the upper extremities.

The dorsal or posterior spinocerebellar tract (a.k.a. Flechsig's fasciculus) is specific for the lower limbs. The fibers originate from the posterior grey horn, travel posterolaterally through the white matter without decussating and project onto the cerebellar cortex by passing through the inferior cerebellar peduncle. Functionally, the posterior spinocerebellar tract conveys sensory information from the muscle spindles, Golgi tendon organs, as well as from touch and pressure receptors of the lower extremities.

The anterior(ventral) spinocerebellar tract (a.k.a. Gowers fasciculus) also carries sensory information from the lower limb. However, while the posterior spinocerebellar tract conveys information about the muscle tone of synergistic muscles, strength and speed of movement from the lower extremities, the anterior spinocerebellar tract appears to relay information regarding their status (posture) during their movement. The organization of the anterior spinocerebellar tract is more complicated than the posterior, due to its numerous polysynaptic inputs and large receptive fields. The first order neuron is localized in the spinal ganglion. Its axon reaches the posterior horn through the posterior root and synapses with the second-order neurons. Their fibers immediately cross at the same level of the spinal cord through anterior commissural fibers and ascend contralaterally along the anterolateral funiculus. The majority of fibers from the second-order neurons reach the contralateral cerebellum by passing through the superior cerebellar peduncle and medullary velum. The fibers then cross over again, ending up in the ipsilateral cerebellar cortex. Therefore, the anterior spinocerebellar tract decussates twice, before synapsing in the vermal and paravermal regions of the cerebellum called the spinocerebellum.

The cuneocerebellar and rostral spinocerebellar tracts are the upper extremity homologs of the posterior/dorsal and the anterior/ventral spinocerebellar tracts, respectively. They carry proprioceptive information from the upper limbs and neck. Note that the "cuneo-" derives from the accessory cuneate nucleus, not the cuneate nucleus. These two nuclei are related in space, but not in function.

Spinotectal tract

Now that we’ve seen the major ascending tracts of the spinal cord, we can move on to the last three minor ones. The spinotectal tract, spinoreticular tract, and the spino-olivary tract.

The spinotectal tract (also known as the spinomesencephalic tract) is responsible for spinovisual reflexes, allowing you to turn your head and gaze toward a visual stimulus (e.g., a sudden flash of light). The fibers cross the spinal cord to travel in the anterolateral white column. They ultimately project on the superior colliculus, part of the tectum of the midbrain (mesencephalon).

Spinoreticular tract

The spinoreticular tract is involved in influencing levels of consciousness and provides a pathway from the muscles, joints and skin to the reticular formation of the brainstem.

The axons of the first-order neurons are localized within the spinal ganglion. They enter the spinal cord from the posterior root ganglion and synapse with second-order neurons in the posterior horn of the gray matter. The axons from these neurons ascend the spinal cord in the lateral white column, mixing with the lateral spinothalamic tract. Most of the fibers are uncrossed and synapse with neurons of the reticular formation in the medulla oblongata, pons and midbrain.

Spino-olivary tract

The spino-olivary tract (a.k.a. Helweg’s fasciculus) also transmits cutaneous and proprioceptive information to the cerebellum. Similar to other ascending pathways, the first-order neurons are located in the spinal ganglion. They synapse with second-order neurons in the posterior gray column. The axons of the second-order neurons cross the midline as they enter the spinal cord and ascend within the contralateral anterior funiculus to reach the accessory olivary nucleus. After synapsing with third-order neurons in the inferior olivary nuclei in the medulla oblongata, the axons cross the midline again and enter the cerebellum through the inferior cerebellar peduncle.

The inferior olivary nucleus is a source of climbing fibers to Purkinje cells in the cerebellar cortex. Thus, the spino-olivary tract may play a role in the control of movements of the body and limbs.

If you want to learn more about the spinal cord, take a look at these study units.

Descending tracts

Now that we understand how information travels up through the spinal cord, let’s see how information travels in the opposite direction by discussing the descending tracts of the spinal cord. These motor pathways travel through the white matter of the spinal cord carrying information from the brain to peripheral effectors, the skeletal muscles. The descending tracts are involved in voluntary motion, involuntary motion, reflexes and regulation of muscle tone.

The general structure of descending tracts is similar to the ascending tracts but in reverse. First-order neurons travel from the cerebral cortex or brainstem and synapse in the anterior gray horn of the spinal cord. Very short second-order neurons, called interneurons, transmit the impulse to third-order neurons which are also located in the anterior grey horn at the same spinal cord level.

Because the second-order neurons are insignificant, we use only a two-order system for the descending (motor) tracts. This way, the first neuron in the pathway (the upper motor neuron) arises in the cerebral cortex or brainstem, descends along the spinal cord and synapses in the anterior gray horn. The second neuron in the pathway (lower motor neuron) leaves the spinal cord through the anterior(ventral) root. In the cervical, brachial and lumbosacral regions the anterior roots combine to form the so-called nerve plexuses. Peripheral nerves emerge from the distal aspect of these plexus, or in the case of the thoracic region directly from the anterior roots. These efferent neurons subsequently travel all the way to a specific skeletal muscle or muscle group (myotome), innervating them.

The descending tracts are named corticospinal, corticobulbar (or corticonuclear), reticulospinal, tectospinal, rubrospinal and vestibulospinal. The corticospinal and corticobulbar tracts form the pyramidal tract, which is under voluntary control. The remaining tracts are grouped together into the extrapyramidal system, which is under involuntary control.

Corticospinal tract

The corticospinal tract is involved with the speed and agility of voluntary movements. The tract originates mainly from the primary motor cortex of the precentral gyrus (Brodmann area 4) and consists of only two neurons rather than three. The first-order or upper motor neurons (UMN) descend until the medulla oblongata, where

90% of them decussate, forming the lateral corticospinal tracts. The un-decussated neurons travel ipsilaterally as the anterior corticospinal tracts. These decussate further down the spinal cord, below the level of the medulla oblongata.

The descending fibers of the anterior tracts travel through the anterior funiculus of the spinal cord, while those of the lateral tracts travel through the lateral funiculus. The fibers continue until the anterior grey horn, where they synapse with the second-order or lower motor neurons (LMN). The latter project onto peripheral effector (skeletal) muscles, resulting in movement.

The corticospinal tract received its alternative name, pyramidal tract, because it forms a pyramid while passing through the medulla oblongata.

Reading about visual concepts like the corticospinal tracts can be slightly confusing. Take a look at the learning materials given below that simplify and present the subject in a visual way.

Corticobulbar tract

The corticobulbar tract, otherwise known as the corticonuclear tract, influences the activity of the motor nuclei of both motor (oculomotor, trochlear, abducens, accessory, hypoglossal) and mixed (trigeminal, facial, glossopharyngeal, vagus) cranial nerves. Through these cranial nerves, this tract controls the activity of muscles of the head, face and neck. The corticobulbar tract connects the brain with the medulla oblongata, also referred to as the bulbus. Like the corticospinal tract, this tract also consists of only two neurons UMNs travel from the primary motor cortex, frontal eye fields and somatosensory cortex all the way to LMNs located in the brainstem. The LMNs are represented by the cranial nerve nuclei. The corticobulbar tract is also part of the pyramidal tract.

Reticulospinal tract

While several tracts, such as corticospinal, corticonuclear, are involved in motor functions, they must be regulated in order to be useful. This is the role of the extrapyramidal system.

The reticulospinal tract, which is part of this involuntary system, helps with motor regulation by facilitating or inhibiting voluntary and reflex actions. To put it in context, this tract helps maintain your posture by inhibiting the flexors and augmenting impulses to extensors in order for you to stand upright.

The uncrossed fibers of the reticulospinal tract originate from the reticular formation spanning the brainstem. They descend as the medial (pontine) and lateral (medullary) reticulospinal tracts through the anterior and lateral funiculi of the spinal cord white matter, respectively. These fibers synapse onto neurons in the anterior grey horns, in the anteromedial portion of laminae VII and VIII, where they influence motor neurons supplying paravertebral and limb extensor musculature.

In addition to its role of facilitating or inhibiting voluntary and reflex actions, the reticulospinal tract is also involved in breathing, it mediates the pressors and depressors of the circulatory system and, in conjunction with the lateral vestibulospinal tract, helps in maintaining balance and making postural adjustments. Muscle tone, balance maintenance and postural changes form a necessary background upon which voluntary movement is executed, which explains why these pathways have numerous synapses with the lower motor neurons.

Tectospinal tract

Thanks to the tectospinal tract, you are capable of moving your head swiftly towards the source of a sudden auditory or visual stimuli. Fibers of the tectospinal tract originate in the superior colliculus, which receives information from the retina and cortical visual association areas. These fibers then project to the contralateral (decussating posterior to the mesencephalic duct) and ipsilateral portion of the first cervical neuromeres of the spinal cord and to the cranial nerves responsible for eye movement (CN III, IV and VI), located in the brainstem. The tectospinal tract then continues to descend in the anterior funiculus of the spinal cord until it reaches the neurons within cervical laminae VI-VIII where the fibers synapse with lower motor neurons of the neck muscles.

The tectospinal tract is responsible for controlling the movement of the head in response to auditory and visual stimuli. Therefore, it has been assumed this tract is responsible for head position and movement depending on visual input received by the superior colliculus.

Rubrospinal tract

The rubrospinal tract originates from the red nucleus located in the midbrain tegmentum. Its axons cross the midline and descend through the pons and medulla oblongata to enter the lateral funiculus of the spinal cord. The fibers terminate by synapsing with internuncial neurons in the anterior gray column at the level of laminae V, VI and VII, where it influences the lower motor neurons of the upper limbs.

The rubrospinal tract is considered to be responsible for the mediation of fine involuntary movement, along with other extrapyramidal tracts, including the vestibulospinal, tectospinal, and reticulospinal tracts. In other words, it coordinates the flexion/extension of muscle groups in order to execute large amplitude movements.

In humans, the rubrospinal tract is very small and its clinical importance is uncertain. It may participate in taking over motor functions after pyramidal (corticospinal) tract injury.

Vestibulospinal tract

Another pathway involved in balance is the vestibulospinal tract. By receiving information from the semicircular canals of the inner ear, this tract activates our body’s extensor muscles and inhibits the flexors, correcting our physical position in space and thus correcting our balance. The tract originates from the vestibular nuclei (CN VIII) of the brainstem and descends uncrossed through the anterior funiculus of the spinal cord, ending up in the anterior grey horn. At this level, the fibers synapse with interneurons and lower motor neurons responsible for antigravity muscle tone in response to the head being tilted to one side. Additionally, the activity of these neurons is indirectly influenced by the cerebellum and the labyrinthine system.

There are quite a lot of tracts to get your head around, right? Neuroanatomy is certainly not easy but with constant reviewing and testing, the information will be cemented into your brain. A good starting point would be the following study unit.

Do you struggle with remembering all the ascending or descending tracts? Try to improve your memory by better note-taking.


Clinical Relevance: Upper Motor Neurone Lesion

Upper motor neurone lesions are also known as supranuclear lesions.

Damage to the Corticospinal Tracts

The pyramidal tracts are susceptible to damage, because they extend almost the whole length of the central nervous system. As mentioned previously, they particularly vulnerable as they pass through the internal capsule – a common site of cerebrovascular accidents (CVA).

If there is only a unilateral lesion of the left or right corticospinal tract, symptoms will appear on the contralateral side of the body. The cardinal signs of an upper motor neurone lesion are:

  • Hypertonia – an increased muscle tone
  • Hyperreflexia – increased muscle reflexes
  • Clonus – involuntary, rhythmic muscle contractions
  • Babinski sign – extension of the hallux in response to blunt stimulation of the sole of the foot
  • Muscle weakness

Damage to the Corticobulbar Tracts

Due to the bilateral nature of the majority of the corticobulbar tracts, a unilateral lesion usually results in mild muscle weakness. However, not all the cranial nerves receive bilateral input, and so there are a few exceptions:

  • Hypoglossal nerve – a lesion to the upper motor neurones for CN XII will result in spastic paralysis of the contralateral genioglossus. This will result in the deviation of the tongue to the contralateral side.
    • Note: this is in contrast to a lower motor neurone lesion, where the tongue deviates towards the damaged side.
    • Facial nerve – a lesion to the upper motor neurones for CN VII will result in spastic paralysis of the muscles in the contralateral lower quadrant of the face.

    Damage to the Extrapyramidal Tracts

    Extrapyramidal tract lesions are commonly seen in degenerative diseases, encephalitis and tumours. They result in various types of dyskinesias or disorders of involuntary movement.

    Try again to score 100%. Use the information in this article to help you with the answers.

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    This article is about the descending tracts of the central nervous system. The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones. The lower motor neurones then directly innervate muscles to produce movement.

    The motor tracts can be functionally divided into two major groups:

    • Pyramidal tracts - These tracts originate in the cerebral cortex, carrying motor fibres to the spinal cord and brain stem. They are responsible for the voluntary control of the musculature of the body and face.
    • Extrapyramidal tracts - These tracts originate in the brain stem, carrying motor fibres to the spinal cord. They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion

    There are no synapses within the descending pathways. At the termination of the descending tracts, the neurones synapse with a lower motor neurone. Thus, all the neurones within the descending motor system are classed as upper motor neurones. Their cell bodies are found in the cerebral cortex or the brain stem, with their axons remaining within the CNS.

    [caption align="aligncenter"] Fig 1 - Schematic of the motor nervous system. The descending tracts are represented by upper motor neurones.[/caption]

    Does the Spinoreticular Tract end in Brainstem? - Biology

    Somatosensory II: Pain and Temperature

    • Describe the physiological and structural characteristics of the two primary sensory neurons that transmit pain and temperature sensations to the central nervous system.
    • Compare the central connections within the spinal cord and the second order projections of A-delta- and C-fibers.
    • Appraise the organization of pars caudalis of the spinal nucleus of trigeminal nerve and the projections to the nucleus.
    • Associate specific higher brain centers with the behavioral responses to acute and chronic pain.
    • Provide examples of how higher brain centers modulate the transmission of pain stimuli within the CNS.

    To master the material presented in this lecture:

    Purves text, Chapter 10
    Haines pp 192, 198.

    • Pain (nociception) has both a sensory/discriminative aspect and a motivational/affective aspect (e.g. suffering). These different aspects of pain are localized to parallel systems that arise in the periphery and enter the spinal cord together but ultimately take separate paths (in part) and activate different systems within the CNS.
    • The sensation of pain and temperature is mediated by free nerve endings.
    • Free nerve endings have no obvious morphological end organ specializations, yet different free nerve endings are selectively sensitive to very different types of stimuli. Nociceptors (pain receptors) may be high threshold mechanoreceptors, high threshold thermoceptors, chemoceptors (sensitive to chemicals applied to the skin), or polymodal. Others are termed "silent", and do not appear to respond to any stimuli (other than shock which activates everything) until frank damage to tissue has occurred. Similarly, low threshold thermoceptors may be sensitive to warmth or coolness, but generally not both. Heat and cold are coded by subtypes of TRP (transient receptor potential) cation channels that are differentially sensitive to warmth or coolness, sensations that can be elicited by either capsaicin or menthol.
    • Cell bodies of the receptors are in the dorsal root ganglia (DRG). The central processes of the afferents enter the spinal cord via the lateral aspect of the dorsal root.
    • The peripheral (primary) afferent fibers of free nerve endings are all small diameter, and all are either thinly myelinated (A &delta , or group III fibers), or unmyelinated (C, or group IV fibers).
      • A &delta = myelinated, fast pain fibers (A &delta - conduction velocity

      • Describe the receptors, nerve, and pathways for perceiving pain on the body and face.
      • Describe the consequences of a lesion in the upper medulla that interrupts the spinal tract of the trigeminal nerve and the spinothalamic tract.
      • Describe the consequences of a lesion on the left side of the spinal cord at mid-thoracic levels with respect to the perception of touch and acute pain on the legs.
      • Describe the impact of myelination upon speed of axon conduction, and characterize the thickness of myelination for: 1. slow pain fibers 2. Golgi tendon organs 3. Pacinian corpuscles.
      • Name the thalamic nucleus receiving the medial lemniscus and spinothalamic tracts, and describe the somatotopic organization of the terminals from these tracts.
      • The secondary axon in an ascending sensory pathway tends to cross to the opposite side. For proprioception, this crossing takes place at what level of CNS? For pain, this crossing takes place at what level of CNS?
      • Describe the consequences of a lesion in the upper medulla that interrupts the spinal tract of the trigeminal nerve and the spinothalamic tract.
      • Describe the origins of the ventral trigeminothalamic tract and the synaptic destination in thalamus.

      Copyright © 1997- 2014 [University of Illinois at Chicago, College of Medicine, Department of Anatomy and Cell Biology]. Last revised: December 30, 2013.


      Physiology

      In this section, we will discuss the important functions of the reticular system in detail.

      Control of skeletal muscle

      The reticular formation plays an important role in regulating the activity of skeletal muscles. It does so by influencing the activity of the alpha and gamma motor neurons through the reticulospinal and reticulobulbar tracts.

      In this way, the reticular formation can modulate muscle tone and reflex activity. It also brings about reciprocal inhibition for example, when the flexor muscles contract, the antagonistic relaxation of extensors is due to reticular formation.

      Control of Muscle Tone and Balance

      Along with the vestibular
      apparatus of the inner ear and the vestibular spinal tract, the reticular
      formation plays an important role in maintaining the tone of the antigravity
      muscles when standing.

      Control of Respiratory Muscles

      The respiratory centers
      of the brainstem that control the respiratory muscles are also considered to be
      a part of the reticular formation.

      Control of Facial Expressions

      The reticular formation also plays a role in controlling the muscles of facial expression when associated with emotion. For example, when you smile or laugh in response to a joke, the motor control to your facial muscles is provided by the reticular formation on both sides of the brain.

      Control of Somatic and Visceral Sensations

      Because of its strategic
      central location in the cerebrospinal axis, the reticular formation can influence
      all ascending pathways that pass to higher levels. This influence can be
      facilitative or inhibitory. In particular, the

      reticular formation is
      considered to have a key role in the gating mechanism, a mechanism for the
      control of pain perception.

      Control of the Autonomic Nervous System

      It is considered that the higher control of the autonomic nervous system, from the cerebral cortex, hypothalamus, and other subcortical nuclei, can be exerted by the reticulobulbar and reticulospinal tracts, which descend to the sympathetic

      outflow and the
      parasympathetic craniosacral outflow.

      Control of the Endocrine Nervous System

      The reticular formation is considered to influence the synthesis or release of releasing or release-inhibiting factors. In this way, it controls the activity of the hypophysis cerebri. The reticular formation can do this either directly or indirectly through the hypothalamic nuclei.

      Influence on the Biologic Clocks

      The reticular formation
      also influences the biologic rhythms of the body by means of its afferent and
      efferent pathways to the hypothalamus.

      The Reticular Activating System

      The level of consciousness and arousal are controlled by the reticular formation. The ascending pathways carrying the sensory information to the higher centers are channeled through the reticular formation. The reticular formation, in turn, projects this information to different parts of the cerebral cortex. This causes a sleeping person to awaken.

      It is considered that
      the state of consciousness is dependent on the continuous projection of sensory
      information to the cortex. Different degrees of wakefulness also depend on the
      degree of activity of the reticular formation.

      The activity of the reticular formation is strongly increased by the incoming pain sensations. This greatly excites the cerebral cortex.


      Mid Pons

      The trigeminal nerve is the largest cranial nerve and enters the brain stem at mid pontine levels. The ventral trigeminothalamic tract, which contains the crossed 2° afferents of the trigeminal system, remains in close association with the medial lemniscus and spinothalamic tract. The spinothalamic tract is located near the lateral tip of the medial lemniscus. The spinoreticular fibers and 2° spinal trigeminal afferents continue to give off branches to the pontine reticular formation.


      Spino-olivary tract

      Fibers of this tract arise from all levels of the spinal cord and are somatotopically organized. These fibers convey information from cutaneous and proprioceptive organs. The second order neurons are present in the posterior gray column.

      The axons cross the midline and ascend in the anterior and lateral white columns more precisely at the intersection between the two columns. They end by synapsing with the inferior olivary nuclei in medulla oblongata. The axons of the third order neurons cross the midline and enter the cerebellum via inferior cerebellar peduncle.


      Avian Neuroanatomy Revisited: From Clinical Principles to Avian Cognition

      Several significant advances in understanding brain-behavior development have made a critical contribution to clinical assessment of companion birds. First, psychobiological health and its dysfunctions now are understood as the product of nature and nurture and therefore exquisitely sensitive to stressors effected by altered socio-ecological conditions within and across generations. Second, discoveries associated with avian brain evolution and ethology show that emotional and cognitive capacities of birds are comparable to mammals. This article presents an overview of these new perspectives and, following, discusses specific, clinically relevant anatomy of the avian central nervous system. By understanding the location of these tracts and their function and the location of the cranial nerves and their nuclei in the brain stem, the clinician can understand and perform the neurological examination, better interpret findings, and localize lesions.

      Portions of this work appeared originally in Orosz SE, Principles of avian clinical neuroanatomy. Semin Avian Exotic Pet Med 19965(3):127–39 reprinted with permission.


      Watch the video: Ascending Tracts. Spinothalamic Tract (February 2023).