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.
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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).
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.
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.
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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 Loewenthal’s
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 Deiters’s
and Bechterew’s, 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 Dieters’s and Bechterew’s 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.
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.
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.
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).
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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)
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.
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.
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).
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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 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.
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.
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.
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.
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.
see upper motor neuron.
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.
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.