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No. 26
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Sensory Pathways (1)
Section 3 The Nervous Pathways
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Introduction:
As known, the basic way of activity of
nervous system is reflex. The
morphological foundation of completing
the reflex is called reflex arc.
A simple reflex only consists of two
neurons, i.e. afferent (sensory) and
efferent (motor) neurons. For example,
the reflex arc of knee jerk is merely
composed of two neurons.
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But a complicated reflex arc includes many
neurons. In view of this, the complicated reflex
activity is accomplished by the linkage consisting
of afferent, intermediate and efferent neurons.
We named the neuronal linkage as the nervous
pathway. It has characteristic with long
connected course, including afferent and efferent
portions.
The nervous pathways are the routes formed
by chains of neurons, through which sensory
awareness reaches the cerebral cortex and a
motor response is initiated.
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For convenience of study, the nervous
pathways are commonly classified into:
sensory (ascending) pathways,
motor (descending) pathways.
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Ⅰ. The Sensory (ascending) Pathways
In most sensory pathways there are three orders
of neurons involved:
① The lower (the first order) sensory neurons,
located in the ganglia, and their peripheral and
central processes;
② The intermediate (the second order) neurons
in the spinal cord or brain stem and their
processes;
③ The upper (the third order) sensory neurons,
which are the cells of the thalamus and the fibers
passing from them to the cerebral cortex.
The sensory pathways mainly include the deep
and superficial sensory, visual and acoustic
pathways.
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Ⅰ) The Proprioceptive (or Deep)
Sensory Pathways
It mediate the proprioceptive (deep)
sensations (including the sensations of
the body posture, movement, vibration)
and fine touch sensation (discriminatory
sensation).
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1. The conscious proprioceptive and
fine touch sensory pathway of trunk
and limbs
It consists of three orders of neurons.
(1) The first neurons and their fibers
The first neurons (pseudounipolar
neurons) are located in the spinal ganglia.
Their peripheral processes accompany the
corresponding spinal nerve to the
proprioceptive receptors of the muscle,
tendon, periosteum and joint, and the
fine touch sensory receptors.
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Their central processes enter the spinal cord as a
medial bundle (thick myelinated fibers) through
the dorsal roots of the spinal nerve. The longer
ascending fibers run upward in the ipsilateral
posterior funiculus, where they form the
fasciculus gracilis (fibers derived from below
the T5 spinal segments) and fasciculus
cuneatus (fibers derived from above the T4
spinal segments).
Both fasciculi end separately in the gracile and
cuneate nuclei, in which the second neurons of
this pathway begin.
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(2) The second neurons and their fibers
The bodies of the second neurons are in the
gracile and cuneate nuclei.
The majority of the fibers of the second neurons
sweep ventrally round the central gray matter as
the internal arcuate fibers, and then bend
medially to reach the median plane, where they
decussate with the corresponding fibers of the
opposite side, forming the decussation of
medial lemniscus. After decussation, the fibers
ascend along the two side of median line,
constituting the medial lemniscus.
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After that, as the medial lemnisci, they
ascend on each side through the medulla
oblongata, pons (at the anterior border of
tegmentum) and midbrain (lateral to the
red nucleus) to the ventroposterior lateral
nucleus of the thalamus which contains
the third neurons.
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(3) The third neurons and their fibers
The third neurons locating in the
ventroposterior lateral nucleus of the
thalamus send out axons to form the
thalamocortical tract, running through
the posterior limb of the internal capsule
to the cortex of the superior and middle
parts of the postcentral gyrus and to the
posterior part of the paracentral lobule.
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Interruption of pathways by lesions below
the level of the decussation of medial
lemniscus causes hemiplegia on the
affected side of the body, including the
loss of all the sensations conducted by the
path. An interruption by a lesion above
the level of the decussation produces
hemiplegia on the opposite side of the
body, including the loss of all the
sensations conducted by the path.
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2. The unconscious proprioceptive sensory
pathway of trunk and limbs
This pathway is only composed of two order
neurons.
(1) First order neurons
The cell bodies of the first order neurons are
located in the spinal ganglia.
Their peripheral fibers end in the proprioceptive
sensory receptors of the muscles, tendon,
periosteum and joint, and their central processes
enter the spinal cord and terminate in the
thoracic nucleus of segments C8-L2 and the
lateral part of layers Ⅴ--Ⅶ in the cervical and
lumbosacral enlargements of the spinal cord.
(2) Second order neurons
 The second order neurons are in the
thoracic nucleus and Ⅴ--Ⅶ in the cervical
and lumbosacral enlargements.
 The axons of the secondary neurons
ascend in the lateral funiculi as the
anterior or posterior spinocerebellar tracts
which enter the cerebellum by way of the
superior and inferior cerebellar peduncles
respectively.
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The connections are made with neurons of
the cortex of the cerebellum, where motor
impulses are returned to the segmental
levels of the spinal cord through part of
the extrapyramidal tracts to maintain the
coordination movement and equilibrium
and appropriate posture of the body.
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Ⅱ) The Pain, Thermal and Rude tactile
and Pressure (or Superficial) Sensory
Pathways
They include three orders of neurons and
convey the sensation of pain, temperature
and rude tactility and pressure from the
skin and mucosa to the centers.
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1. The pain, thermal and rude tactile and
pressure (superficial) sensory pathway of
trunk and limbs
(1) First neurons
The first neurons of the route are
pseudounipolar and located in the spinal
ganglia.
Their peripheral processes join the spinal
nerve to the cutaneous exteroceptors of
the trunk and limbs.
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Their central processes enter the spinal
cord.
The fine fibers (in the lateral part of
posterior root) conducting pain and
thermal sensation enter the spinal cord
through the dorsolateral fasciculus and
terminate in the second neurons.
The rude fibers (in the medial part of
posterior root) conducting rude tactile and
pressure enter the posterior funiculus and
then terminate in the second neurons.
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(2) Second neurons
The cell bodies of the second neurons are
mainly in the layersⅠand from Ⅳ to Ⅶ of gray
matter of spinal cord.
The second order fibers from the second neurons
run upward within 1–2 segments of spinal cord
and cross to the lateral funiculus and anterior
funiculus of the opposite side through the
anterior white commissure, forming the lateral
spinothalamic tract (conducting the pain and
thermal sensations) and anterior spinothalamic
tract (conducting the tactile and pressure
sensations).
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The spinothalamic tracts go upward lying
dorsolaterally to the inferior olivary
nucleus in the medulla oblongata, and,
dorsolaterally to the medial lemniscus in
the pons and midbrain, and terminate in
the ventral posterolateral nucleus.
(3) Third neurons
 The cell bodies of the third neurons are
located in the ventral posterolateral
nucleus.
 The third neurons send out axons to join
the central thalamic radiation (or
thalamocortical tract) and pass through
the posterior limb of the internal capsule
to the upper and middle parts of the
postcentral gyrus, and the posterior
part of the paracentral lobule.
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2. The pain, thermal and rude tactile and
pressure (superficial) sensory pathway of
the head and face
(1) First neurons
The cell bodies of the first neurons are
situated in the trigeminal ganglion.
Their peripheral processes join the
sensory branches of the trigeminal nerve
and terminate in the superficial receptors
in the skin of the head and face and
mucosa of the oral and nasal cavities.
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Their central processes enter the pons via
the sensory root of the trigeminal nerve.
The fibers conducting the impulses
associated with pain and thermal
sensation descends to form the spinal
tract of trigeminal nerve and terminate
in the spinal nucleus of trigeminal
nerve.
The fibers being concerned with tactile
and pressure sensation terminate in the
pontine nucleus of trigeminal nerve.
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(2) Second neurons
The second neurons, situated in the
spinal and pontine nuclei of the trigeminal
nerve, give rise to axons to opposite side
to join the trigeminal lemniscus running
through the pons and the midbrain to the
ventral posteromedial nucleus of the
thalamus.
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(3) Third neurons
The cell bodies of the third neurons are
in the ventral posteromedial nucleus of
the thalamus.
They send out fibers to form the central
thalamic radiation (thalamocortical
tract) coursing through the posterior limb
of the internal capsule to the inferior
part of the postcentral gyrus.
here.