Neural pathway

Neural pathway

A neural pathway is a neural tract connecting one part of the nervous system with another, usually consisting of bundles of elongated, myelin insultated neurons, known collectively as white matter. Neural pathways serve to connect relatively distant areas of the brain or nervous system, compared to the local communication of grey matter.

Contents

Naming of neural pathways

The first named pathways were evident even in a poorly-preserved gross brain, and were named by the great anatomists of the Renaissance using cadaver material. Examples of these include the great commissures of the brain such as the corpus callosum (Latin, "huge body"), anterior commissure or posterior commissure.

Further examples of this (by no means a complete list) include the pyramidal tract, crus cerebri (Latin, "leg of the brain"), and cerebellar peduncles (Latin, "little foot of the cerebellum"). Note that these names describe the appearance of a structure but give one no information on its function or location. Later, as neuroanatomical knowledge became more sophisticated, the trend was toward naming things by their origin and termination. For example, the nigrostriatal pathway, which is degenerated in Parkinson's disease, runs from the substantia nigra (Latin, "black substance") to the corpus striatum (Latin, "striped body"). This naming can extend to include any number of structures in a pathway, such that the cerebellorubrothalamocortical pathway originates in the cerebellum, synapses in the red nucleus ("ruber" in Latin), on to the thalamus, and finally terminating in the cerebral cortex.

Sometimes, these two naming conventions coexist. For example, the name "pyramidal tract" has been mainly supplanted by lateral corticospinal tract in most texts. Note that the "old" name was primarily descriptive, evoking the pyramids of antiquity, from the appearance of this neural pathway in the medulla oblongata. The "new" name is based primarily on its origin (in the primary motor cortex, Brodmann area 4) and termination (onto the alpha motor neurons of the spinal cord).

Functional aspects

In general, neurons receive information either at their dendrites or cell bodies. The axon of a nerve cell is, in general, responsible for transmitting information over a relatively long distance. Therefore, most neural pathways are made up of axons. If the axons have myelin sheaths, then the pathway appears bright white because myelin is primarily lipid. If most or all of the axons lack myelin sheaths (i.e., are unmyelinated), then the pathway will appear a darker beige color, which is generally called gray (American English, or grey in British English).

Some neurons are responsible for conveying information over long distances. For example, motor neurons which travel from the spinal cord to the muscle can have axons up to a meter in length in humans; the longest axon in the human body is almost two meters long in tall individuals and runs from the great toe to the medulla oblongata of the brainstem. These are archetypical examples of neural pathways.

Corticospinal tract

The corticospinal or pyramidal tract is a massive collection of axons that travel between the cerebral cortex of the brain and the spinal cord.

The corticospinal tract mostly contains motor axons. It actually consists of two separate tracts in the spinal cord: the lateral corticospinal tract and the medial corticospinal tract. An understanding of these tracts leads to an understanding of why for the most part, one side of the body is controlled by the opposite side of the brain.

Contents

 

The motor pathway

The corticospinal tract originates from cells in layer V of the motor cortex. The neuronal cell bodies in the motor cortex send long axons to the motor cranial nerve nuclei mainly of the contralateral side of the midbrain (cortico-mesencephalic tract), pons (cortico-pontine tract), medulla oblongata (cortico-bulbar tract); the bulk of these fibers, however, extend all the way down to the spinal cord (corticospinal tract).

Despite which of these two tracts it travels in, the axon of a neuron which is part of this tract will synapse with another neuron in the ventral horn. This ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.

There is a common misconception that there is a precise somatotopic organisation of the motor cortex. The experiments performed by Penfield in the 1930's involved the stimulation of small areas of the motor cortex and mapping movements. With the information derived from over 400 neurosurgical patients, Penfield created a precise somatotopic map of the different body parts in the primary motor cortex with the leg area located medially (close to the midline), and the head and face area located laterally on the convex side of the cerebral hemisphere (motor homunculus). This map was somewhat fitting, as it showed a similar pattern of over-representation to the somatotopic maps of the somatosensory cortex. However, upon re-examination of the results obtained by Penfield, as well as new intracortical microstimulation mapping, a different picture was obtained.

Penfield's results show that stimulation of many different areas of the motor cortex would cause contraction of a particular muscle (nearly 50% of the cortex caused leg stimulation despite its relative under-representation). Now, there is evidence that the stimulation of even individual motor neurons causes stimulation, as well as inhibition, of spinal motor neurons leading to different muscles. It seems likely that groups of closely linked neurons in the cortex actually control co-ordinated muscle contractions and relaxations that lead to a specific movement. So, whilst the overall map given by Penfield's motor homunculus is correct, the principle behind the organisation of the motor cortex appears to be different.

The motor axons move closer together as they travel down through the cerebral white matter, and form part of the posterior limb of the internal capsule.

The motor fibers continue down into the brainstem. The bundle of corticospinal axons is visible as two column-like structures ("pyramids") on the ventral surface of medulla oblongata - this is where the name pyramidal tract comes from.

After the decussation, the axons travel down the spinal cord as the lateral corticospinal tract. Fibers that do not cross over in the medulla oblongata travel down the separate ventral corticospinal tract, and most of them cross over to the contralateral side in the spinal cord, shortly before reaching the lower motor neurons.

The motor neuron cell bodies in the motor cortex, together with their axons that travel down the brain stem and spinal cord, are referred to as upper motor neuron. In the spinal cord, these axons connect (most of them via interneurons, but to a lesser extent also via direct synapses) with the lower motor neurons (LMNs), located in the ventral horn of the spinal cord. In the brain stem, the lower motor neurons are located in the motor cranial nerve nuclei (occulomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial, accessory, hypoglossal). The lower motor neuron axons leave the brain stem via motor cranial nerves and the spinal cord via anterior roots of the spinal nerves respectively, end-up at the neuromuscular plate and provide motor innervation for voluntary muscles.

Pathways from the Brain to the Spinal Cord

The descending fasciculi which convey impulses from the higher centers to the spinal cord and located in the lateral and ventral funiculi.

  The Motor Tract conveying voluntary impulses, arises from the pyramid cells situated in the motor area of the cortex, the anterior central and the posterior portions of the frontal gyri and the paracentral lobule. The fibers are at first somewhat widely diffused, but as they descend through the corona radiata they gradually approach each other, and pass between the lentiform nucleus and thalamus, in the genu and anterior two-thirds of the occipital part of the internal capsule; those in the genu are named the geniculate fibers, while the remainder constitute the cerebrospinal fibers; proceeding downward they enter the middle three-fifths of the base of the cerebral peduncle. The geniculate fibers cross the middle line, and end by arborizing around the cells of the motor nuclei of the cranial nerves. The cerebrospinal fibers are continued downward into the pyramids of the medulla oblongata, and the transit of the fibers from the medulla oblongata is effected by two paths. The fibers nearest to the anterior median fissure cross the middle line, forming the decussation of the pyramids, and descend in the opposite side of the medulla spinalis, as the lateral cerebrospinal fasciculus (crossed pyramidal tract). Throughout the length of the medulla spinalis fibers from this column pass into the gray substance, to terminate either directly or indirectly around the motor cells of the anterior column. The more laterally placed portion of the tract does not decussate in the medulla oblongata, but descends as the anterior cerebrospinal fasciculus (direct pyramidal tract); these fibers, however, end in the anterior gray column of the opposite side of the medulla spinalis by passing across in the anterior white commissure. There is considerable variation in the extent to which decussation takes place in the medulla oblongata; about two-thirds or three-fourths of the fibers usually decussate in the medulla oblongata and the remainder in the medulla spinalis.

  The axons of the motor cells in the anterior column pass out as the fibers of the anterior roots of the spinal nerves, along which the impulses are conducted to the muscles of the trunk and limbs.

  From this it will be seen that all the fibers of the motor tract pass to the nuclei of the motor nerves on the opposite side of the brain or medulla spinalis, a fact which explains why a lesion involving the motor area of one side causes paralysis of the muscles of the opposite side of the body. Further, it will be seen that there is a break in the continuity of the motor chain; in the case of the cranial nerves this break occurs in the nuclei of these nerves; and in the case of the spinal nerves, in the anterior gray column of the medulla spinalis. For clinical purposes it is convenient to emphasize this break and divide the motor tract into two portions: (1) a series of upper motor neurons which comprises the motor cells in the cortex and their descending fibers down to the nuclei of the motor nerves; (2) a series of lower motor neurons which includes the cells of the nuclei of the motor cerebral nerves or the cells of the anterior columns of the medulla spinalis and their axiscylinder processes to the periphery.

  The rubrospinal fasciculus arises from the large cells of the red nucleus. The fibers cross the raphé of the mid-brain in the decussation of Forel and descend in the formatio reticularis of the pons and medulla dorsal to the medial lemniscus and as they pass into the spinal cord come to lie in a position ventral to the crossed pyramidal tracts in the lateral funiculus. The rubrospinal fibers end either directly or indirectly by terminals and collaterals about the motor cells in the anterior column on the side opposite from their origin in the red nucleus. A few are said to pass down on the same side. Since the red nucleus is intimately related to the cerebellum by terminals and collaterals of the superior peduncle which arises in the dentate nucleus of the cerebellum, the rubrospinal fasciculus is supposed to be concerned with cerebellar reflexes, complex motor coördinations necessary in locomotion and equilibrium. The afferent paths concerned in these reflexes have already been partly considered, namely, the dorsal and ventral spinocerebellar fasciculi, and probably some of the fibers of the posterior funiculi which reach the cerebellum by the inferior peduncle.

 

The motor tract 

   The tectospinal fasciculus arises from the superior colliculus of the roof (tectum) of the mid-brain. The axons come from large cells in the stratum opticum and stratum lemnisci and sweep ventrally around the central gray matter of the aqueduct, cross the raphé in the fountain decussation of Meynert and turn downward in the tegmentum in the ventral longitudinal bundle. Some of the fibers do not cross in the raphé but pass down on the same side; it is uncertain whether they come from the superior colliculus of the same side or arch over the aqueduct from the colliculus of the opposite side. The tectospinal fasciculus which comprises the major part of the ventral longitudinal bundle passes down through the tegmentum and reticular formation of the pons and medulla oblongata ventral to the medial longitudinal bundle. In the medulla the two bundles are more or less intermingled and the tectospinal portion is continued into the antero-lateral funiculus of the spinal cord ventral to the rubrospinal fasciculus with which some of its fibers are intermingled. Some of the fibers of the tectospinal fasciculus pass through the red nucleus giving off collaterals to it, others are given off to the motor nuclei of the cranial nerves and in the spinal cord they terminate either directly or indirectly by terminals and collaterals among the nuclei of the anterior column. Since the superior colliculus is an important optic reflex center, this tract is probably concerned in optic reflexes; and possibly also with auditory reflexes since some of the fibers of the central auditory path, the lateral lemniscus, terminate in the superior colliculus.

  The vestibulospinal fasciculus (part of the anterior marginal fasciculus or Loewenthals tract) situated chiefly in the marginal part of the anterior funiculus is mainly derived from the cells of the terminal nuclei of the vestibular nerve, probably Deiterss and Bechterews, and some of its fibers are supposed to come from the nucleus fastigius (roof nucleus of the cerebellum). The latter nucleus is intimately connected with Dieterss and Bechterews nuclei. The vestibulospinal fasciculus is concerned with equilibratory reflexes. Its terminals and collaterals end about the motor cells in the anterior column. It extends to the sacral region of the cord. Its fibers are intermingled with the ascending spinothalamic fasciculus, with the anterior proper fasciculus and laterally with the tectospinal fasciculus. Its fibers are supposed to be both crossed and uncrossed. In the brain-stem it is associated with the dorsal longitudinal bundle.

  The pontospinal fasciculus (Bechterew) arises from the cells in the reticular formation of the pons from the same and the opposite side and is associated in the brain-stem with the ventral longitudinal bundle. In the cord it is intermingled with the fibers of the vestibulospinal fasciculus in the anterior funiculus. Not much is known about this tract.

  There are probably other descending fasciculi such as the thalamospinal but not much is known about them.

Sensory pathways

Extrapyramidal motor pathways

These are motor pathways that lie outside the corticospinal tract and are beyond voluntary control. Their main function is to support voluntary movement and help control posture and muscle tone.

Spinothalamic tract

The spinothalamic tract is a sensory pathway originating in the spinal cord that transmits information about pain, temperature, itch and crude touch to the thalamus. The pathway decussates at the level of the spinal cord, rather than in the brainstem like the posterior column-medial lemniscus pathway and corticospinal tract.

The neurons that make up the spinothalamic tract are located principally within the dorsal horn of the spinal cord. These neurons receive synaptic inputs from sensory fibers which innervate the skin and internal organs.

There are two main parts of the spinothalamic tract (STT):

The types of sensory information transmitted via the STT are described as affective sensation. This means that the sensation is accompanied by a compulsion to act. For instance, an itch is accompanied by a need to scratch, and a painful stimulus makes us want to withdraw from the pain.

Spinocerebellar tract

The spinocerebellar tract is a set of axonal fibers originating in the spinal cord and terminating in the ipsilateral cerebellum. This tract conveys information to the cerebellum about limb and joint position (proprioception).

Contents

Origins of Proprioceptive information

Proprioceptive information is obtained by Golgi tendon organs and muscle spindles.

Golgi tendon organs consist of a fibrous capsule enclosing tendon fasciculi and bare nerve endings that respond to tension in the tendon by causing action potentials in 1β (relatively large, myelinated, quickly conducting) afferent neurones.

muscle spindles fibres are complicated systems of tension monitoring within muscles which result in information being carried via 1α (larger and faster than 1β) neurones (from both nuclear bag fibres and nuclear chain fibres) and II neurones (solely from nuclear chain fibres).

All of these neurones are "first order" or "primary", are sensory (and thus have their cell bodies in the dorsal root ganglion) and pass through layers I-VI of the dorsal horn, to form synapses with "second order" or "secondary" neurones in the layer just beneath the dorsal horn (layer VII)

Subdivisions of the tract

The tract is divided into:

Information from muscle spindles in the hind limbs travel via the dorsal tract and golgi tendon organs in the hind limb travel via the ventral tract. Muscle spindle information from forelimbs travel via the spinocuneocerebellar tract and corresponding golgi tendon organ information travels via the rostral spinocerebellar tract.

Pathway for Dorsal and Spinocuneocerebellar Tracts

In the dorsal tract, the sensory neurones synapse in an area known as Clarke's nucleus or "Clarke's column".

This is a column of relay neurone cell bodies within the medial gray matter within the spinal cord in layer VII (just beneath the dorsal horn), specifically between C8-L3. These neurones then send axons up the spinal cord and form synapses in the accessory (lateral) cuneate nucleus, lateral to the cuneate nucleus in the medulla.

Below L3, relevant neurones pass into the fasciculus gracilis (usually associated with the dorsal column-medial lemniscal system) until L3 where they synapse with Clarke's nucleus (leading to considerable caudal enlargement).

From above C8, neurones enter the fasciculus cuneatus directly and again synapse with neurones in the accessory cuneate nucleus. This pathway is known as the spinocuneocerebellar tract.

The neurones in the accessory cuneate nucleus have axons leading to the ipsilateral cerebellum via the caudal cerebellar peduncle.

Pathways of the brain and spinal cord

A neural pathway, or neural tract, connects one part of the nervous system with another and usually consists of bundles of elongated, myelin-insulated neurons, known collectively as white matter. Neural pathways serve to connect relatively distant areas of the brain or nervous system, compared to the local communication of grey matter.

Naming of neural pathways

The first named pathways are evident to the naked eye even in a poorly-preserved brain, and were named by the great anatomists of the Renaissance using cadaver material. Examples of these include the great commissures of the brain such as the corpus callosum (Latin, "hard body"; not to be confused with the Latin word "colossus" - the "huge" statue), anterior commissure, and posterior commissure. Further examples of this (by no means a complete list) include the pyramidal tract, crus cerebri (Latin, "leg of the brain"), and cerebellar peduncles (Latin, "little foot of the cerebellum"). Note that these names describe the appearance of a structure but give one no information on its function or location.

Later, as neuroanatomical knowledge became more sophisticated, the trend was toward naming pathways by their origin and termination. For example, the nigrostriatal pathway, which is degenerated in Parkinson's disease, runs from the substantia nigra (Latin, "black substance") to the corpus striatum (Latin, "striped body"). This naming can extend to include any number of structures in a pathway, such that the cerebellorubrothalamocortical pathway originates in the cerebellum, synapses in the red nucleus ("ruber" in Latin), on to the thalamus, and finally terminating in the cerebral cortex.

Sometimes, these two naming conventions coexist. For example, the name "pyramidal tract" has been mainly supplanted by lateral corticospinal tract in most texts. Note that the "old" name was primarily descriptive, evoking the pyramids of antiquity, from the appearance of this neural pathway in the medulla oblongata. The "new" name is based primarily on its origin (in the primary motor cortex, Brodmann area 4) and termination (onto the alpha motor neurons of the spinal cord).

Corticospinal tract

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"Pyramidal tract" redirects here. This page refers to the nerve fibres underlying the pyramids. For the actual area of the brain, Pyramids, see Pyramid of medulla oblongata.

The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord.

The corticospinal tract mostly contains motor axons. It actually consists of two separate tracts in the spinal cord: the lateral corticospinal tract and the medial corticospinal tract. An understanding of these tracts leads to an understanding of why for the most part, one side of the body is controlled by the opposite side of the brain.

Also the corticobulbar tract is considered to be a pyramidal tract. The corticobulbar tract carries signals that control motor neurons located in cranial nerve brain nuclei rather than motor neurons located in the spinal cord.[1]

The neurons of the pyramidal tracts are pyramidal neurons, but that is not how the pyramidal tract got its name, as most of the pyramidal neurons send their axons elsewhere.[2] Instead, it got its name from the shape of the corticospinal axon tracts: when the pyramidal tract passes the medulla, it forms a dense bundle of nerve fibres that is shaped somewhat like a pyramid.[3]

The motor pathway

The corticospinal tract originates from pyramidal cells in layer V of the cerebral cortex. About half of its fibres arise from the primary motor cortex. Other contributions come from the supplementary motor area, premotor cortex, somatosensory cortex, parietal lobe, and cingulate gyrus. The average fibre diameter is in the region of 10μm; around 3% of fibres are extra-large (20μm) and arise from Betz cells, mostly in the leg area of the primary motor cortex.

Upper motor neurons

The motor neuron cell bodies in the motor cortex, together with their axons that travel down through the brain stem and spinal cord, are referred to as upper motor neurons.

Decussation and synapses

The neuronal cell bodies in the motor cortex send long axons to the motor cranial nerve nuclei mainly of the contralateral side of the midbrain (cortico-mesencephalic tract), pons (cortico-pontine tract), medulla oblongata (cortico-bulbar tract); the bulk of these fibers, however, extend all the way down to the spinal cord (corticospinal tract).

Whichever of these two tracts it travels in, a cortico-spinal axon will synapse with another neuron in the ventral horn. This ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.

From cerebral to motor neurons

The motor axons move closer together as they travel down through the cerebral white matter, and form part of the posterior limb of the internal capsule.

The motor fibers continue down into the brainstem. The bundle of corticospinal axons is visible as two column-like structures ("pyramids") on the ventral surface of medulla oblongata - this is where the name pyramidal tract comes from.

After the decussation, the axons travel down the spinal cord as the lateral corticospinal tract. Fibers that do not cross over in the medulla oblongata travel down the separate anterior corticospinal tract, and most of them cross over to the contralateral side in the spinal cord, shortly before reaching the lower motor neurons.

Lower motor neurons

In the spinal cord, the axons of the upper motor neuron connect (most of them via interneurons, but to a lesser extent also via direct synapses) with the lower motor neurons, located in the ventral horn of the spinal cord.

In the brain stem, the lower motor neurons are located in the motor cranial nerve nuclei (oculomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial, accessory, hypoglossal). The lower motor neuron axons leave the brain stem via motor cranial nerves and the spinal cord via anterior roots of the spinal nerves respectively, end-up at the neuromuscular plate and provide motor innervation for voluntary muscles.

Sensory pathways

Corticospinal tract damage

see upper motor neuron.

Extrapyramidal motor pathways

These are motor pathways that lie outside the corticospinal tract and are beyond voluntary control. Their main function is to support voluntary movement and help control posture and muscle tone. See extrapyramidal motor system.

Dissection of brain-stem. Lateral view.

Superficial dissection of brain-stem. Ventral view.

Functional aspects. In general, neurons receive information either at their dendrites or cell bodies. The axon of a nerve cell is, in general, responsible for transmitting information over a relatively long distance. Therefore, most neural pathways are made up of axons. If the axons have myelin sheaths, then the pathway appears bright white because myelin is primarily lipid. If most or all of the axons lack myelin sheaths (i.e., are unmyelinated), then the pathway will appear a darker beige color, which is generally called gray (American English, or grey in British English).

Major neural pathways

Some neurons are responsible for conveying information over long distances. For example, motor neurons, which travel from the spinal cord to the muscle, can have axons up to a meter in length in humans; the longest axon in the human body is almost two meters long in tall individuals and runs from the great toe to the medulla oblongata of the brainstem. These are archetypical examples of neural pathways.