Sensory & Motor
Pathways
Dr. Zeenat Zaidi
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There is a continuous flow of information between the
brain, spinal cord, and peripheral nerves. This
information is relayed by sensory (ascending) and
motor (descending) ‘pathways’.
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Generally the pathways:
 Consists of a chain of tracts, associated nuclei and
varying number of relays (synapses)
 Consist of two or three neurons
 Exhibit somatotopy (precise spatial relationships)
 Decussate
 Involve both the brain and spinal cord
 Are paired (bilaterally and symmetrically)
Somatic Sensory Pathways
Sensory Pathways
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Monitor conditions both inside the body and in the
external environment
Sensation-stimulated receptor passes information
to the CNS via afferent (sensory) fibers
Most sensory information is processed in the spinal
cord , thalamus, or brain stem. Only 1% reaches
the cerebral cortex and our conscious awareness
Processing in the spinal cord can produce a rapid
motor response (stretch reflex)
Processing within the brain stem may result in
complex motor activities (positional changes in the
eye, head, trunk)
Sensory Pathways
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Contain a sequence of THREE
neurons from the receptor to the
cerebral cortex
First order neuron: Sensory
neuron that delivers information
from the receptor to the CNS.
Cell body located in the dorsal
root ganglion. The Axon (central
process) passes to the spinal
cord through the dorsal root of
spinal nerve gives many
collaterals which take part in
spinal cord reflexes runs
ipsilaterally and synapses with
second-order neurons in the
cord and medulla oblongata
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2
1
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Second order neuron:
 Has cell body in the
spinal cord or medulla
oblongata
 Axon decussate &
 Terminate on 3rd order
neuron
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Third order neuron:
 Has cell body in
thalamus
 Axon terminates on
cerebral cortex
ipsilaterally
White Matter: Pathway Generalizations
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Ascending and descending fibers are organized in
distinct bundles which occupy particular areas and
regions in the white matter
Generally long tracts are located peripherally in the white
matter, while shorter tracts are found near the gray
matter
• The TRACT is a bundle of nerve fibers (within CNS)
having the same origin, course, destination &
function
• The name of the tract indicates the origin and
destination of its fibers
• The axons within each tract are grouped according
to the body region innervated
Tracts of the Spinal Cord
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Tracts that serve to join brain to the spinal
cord
 Ascending
 Descending
Fibers that interconnect adjacent or distant
segments of the spinal cord
 Intersegmental (propriospinal)
Intersegmental Tracts
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Extensive fiber connections
between spinal segments
Fasciculus proprius
 Short ascending &
descending fibers
 Both crossed &
uncrossed
 Begin and end within the
spinal cord
 Participate in
intersegmental spinal
reflexes
 Present in all funiculi
adjacent to gray matter
Intersegmental Tracts
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Dorsolateral tract of
Lissauer: Primary sensory
fibers carrying pain,
temperature and touch
information bifurcate upon
entering the spinal cord.
Their branches ascend and
descend for several spinal
segments in the dorsolateral
tract, before synapsing in
the dorsal horn
Intersegmental fibers, establishing connections with
neurons in the opposite half of the spinal cord, cross
the midline in the anterior white commissure
Ascending Spinal Tracts
Transmit impulses:
 Concerned with specific sensory modalities:
pain, temperature, touch, proprioception, that
reach a conscious level (cerebral cortex)
 Dorsal column funiculi
 Spinothalmic tracts
 From tactile and stretch receptors to
subconscious centers (cerebellum)
 Spinocerebellar tracts
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Three major pathways carry sensory information
 Posterior column pathway (gracile & cuneate
fasciculi)
 Anterolateral pathway (spinothalamic)
 Spinocerebellar pathway
Ascending Spinal Tracts
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Dorsal white column
Lateral spinothalamic
Anterior spinothalamic
Anterior spinocerebellar
Posterior spinocerebellar
Cuneocerebellar
Spinotectal
Spinoreticulr
Spino-olivary
Visceral sensory tracts
Dorsal Column
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Contains two tracts, Fasciculus
gracilis (FG) & fasciculus
cuneatus (FC)
Carry impulses concerned with
proprioception and
discriminative touch from
ipsilateral side of body
Contain the axons of primary
afferent neurons that have
entered cord through dorsal
roots of spinal nerves
FG contains fibers received at sacral, lumbar and lower
thoracic levels, FC contains fibers received at upper
thoracic and cervical levels
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Fibers ascend without
interruption where they
terminate upon 2nd order
neurons in nucleus gracilis
and nucleus cuneatus
The axons of the 2nd order
neurons decussate in the
medulla as internal arcuate
fibers and ascend through the
brain stem as medial
lemniscus.
The medial lemniscus
terminates in the ventral
posterior nucleus of the
thalamus upon 3rd order
neurons, which project to the
somatosensory cortex
(thalamocortical fibers)
Spinothalamic Tracts
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Located lateral and ventral to
the ventral horn
Carry impulses concerned
with pain and thermal
sensations (lateral tract) and
also non- discriminative
touch and pressure (medial
tract)
Fibers of the two tracts are
intermingled to some extent
In brain stem, constitute the
spinal lemniscus
Fibers are highly somatotopically arranged, with those Information is sent to the
for the lower limb lying most primary sensory cortex on
superficially and those for the the opposite side of the body
upper limb lying deeply
Lateral Spinothalamic Tract
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Carries impulses concerned
with pain and thermal
sensations.
Axons of 1st order neurons
terminate in the dorsal horn
Axons of 2nd order neuron
(mostly in the nucleus
proprius), decussate within
one segment of their origin, by
passing through the ventral
white commissure & terminate
on 3rd order neurons in ventral
posterior nucleus of the
thalamus
Thalamic neurons project to
the somatosensory cortex
Anterior Spinothalamic Tract
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Carries impulses concerned
with non- discriminative touch
and pressure
Axons of 1st order neurons
enter cord terminate in the
dorsal horn
Axons of 2nd order neuron
(mostly in the nucleus
proprius) may ascend several
segments before crossing to
opposite side by passing
through the ventral white
commissure & terminate on 3rd
order neurons in ventral
posterior nucleus of the
thalamus
Thalamic neurons project to
the somatosensory cortex
Spino-reticulo-thalamic System
The system represents an additional route
by which dull, aching pain is transmitted to
a conscious level
 Some 2nd order neurons terminate in the
reticular formation of the brain stem,
mainly within the medulla
 Reticulothalamic fibers ascend to
intralaminar nuclei of thalamus, which in
turn activate the cerebral cortex
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Spinocerebellar Tracts
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The spinocerebellar system
consists of a sequence of
only two neurons
Two tracts: Posterior &
Anterior
Located near the
dorsolateral and
ventrolateral surfaces of the
cord
Contain axons of the second
order neurons
Carry information derived
from muscle spindles, Golgi
tendon organs and tectile
receptors to the cerebellum
for the control of posture and
coordination of movements
Posterior Spinocerebellar Tracts
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Present only above level
L3
The cell bodies of 2nd
order neuron lie in
Clark’s column
Axons of 2nd order
neuron terminate
ipsilaterally (uncrossed)
in the cerebellar cortex
by entering through the
inferior cerebellar
peduncle
Ventral Spinocerebellar Tracts
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The cell bodies of 2nd order
neuron lie in base of the dorsal
horn of the lumbosacral segments
Axons of 2nd order neuron cross to
opposite side, ascend as far as
the midbrain, and then make a
sharp turn caudally and enter the
superior cerebellar peduncle
The fibers cross the midline for a
second time within the cerebellum
before terminating in the
cerebellar cortex
Both spinocerebellar tracts convey
sensory information to the same
side of the cerebellum
Spinotectal Tract
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Ascends in the anterolateral
part in close association with
spinothalamic system
Primary afferents reach dorsal
horn through dorsal roots and
terminate on 2nd order neurons
The cell bodies of 2nd order
neuron lie in base of the dorsal
horn
Axons of 2nd order neuron
cross to opposite side, and
project to the periaquiductal
gray matter and superior
colliculus in the midbrain
Spino - olivary Tract
Indirect spinocerebellar pathway (spinoolivo-cerebellar)
 Impulses from the spinal cord are
relayed to the cerebellum via inferior
olivary nucleus
 Conveys sensory information to the
cerebellum
 Fibers arise at all level of the spinal cord
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Spinoreticular Tract
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Originates in laminae IVVIII
Contains uncrossed
fibers that end in
medullary reticular
formation & crossed &
uncrossed fibers that
terminate in pontine
reticular formation
Form part of the
ascending reticular
activating system
Somatic Motor Pathways
Motor Pathways
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CNS issues motor commands in response to
information provided by sensory systems, sent by
the somatic nervous system (SNS) and the
autonomic nervous system (ANS)
Conscious and subconscious motor commands
control skeletal muscles by traveling over 3
integrated motor pathways
The corticospinal pathway – voluntary control of
motor activity
 Corticobulbar tracts
 Corticospinal tracts
The medial and lateral pathways – modify or direct
skeletal muscle contractions by stimulating,
facilitating, or inhibiting lower motor neurons
Motor Pathways
• Contain a sequence of TWO
neurons from the cerebral
cortex or brain stem to the
muscles
• Upper motor neuron : has cell
body in the cerebral cortex or
brain stem, axon decussates
before terminating on the
lower motor neuron
• Lower motor neuron: has cell
body in the ventral horn of the
spinal cord, axon runs in the
ipsilateral ventral root of the
spinal nerve and supply the
muscle.
UMN
LMN
Descending Spinal Tracts
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Originate from the cerebral cortex & brain
stem
Concerned with:
 Control of movements
 Muscle tone
 Spinal reflexes & equilibrium
 Modulation of sensory transmission to
higher centers
 Spinal autonomic functions
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The motor pathways are
divided into two groups
 Direct pathways
(voluntary motion
pathways) - the
pyramidal tracts
 Indirect pathways
(postural pathways),
essentially all others the extrapyramidal
pathways
Direct (Pyramidal) System
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Regulates fast and fine (skilled) movements
Originate in the pyramidal neurons in the
precentral gyri,
Impulses are sent through the corticospinal
tracts and synapse in the anterior horn
Stimulation of anterior horn neurons activates
skeletal muscles
Part of the direct pathway, called corticobulbar
tracts, innervates cranial nerve nuclei
Indirect (Extrapyramidal) System
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Complex and multisynaptic pathways
The system includes:
• Rubrospinal tracts: control flexor muscles
• Vestibulospinal tracts: maintain balance and
posture
• Tectospinal tracts: mediate head neck, and eye
movement
• Reticulospinal tracts
Descending Spinal Tracts
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Pyramidal
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Extrapyramidal
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Corticospinal
Rubrospinal
Tectospinal
Vestibulospinal
Olivospinal
Reticulospinal
Descending
Autonomic Fibers
Corticospinal Tracts
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Concerned with
voluntary, discrete,
skilled movements,
especially those of distal
parts of the limbs
(fractionated movements)
Innervate the
contralateral side of the
spinal cord
Provide rapid direct
method for controlling
skeletal muscle
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Origin: motor and sensory
cortices
Axons pass through corona
radiata, internal capsule, crus
cerebri and pyramid of
medulla oblongata
In the caudal medulla about
75-90% of the fibers
decussate and form the
lateral corticospinal tract
Rest of the fibers remain
ipsilateral and form anterior
corticospinal tract. They
also decussate before
termination
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Distribution:
 55% terminate at
cervical region
 20% at thoracic
 25% at lumbosacral
level
Termination: Ventral horn
neurons (mostly through
interneurons, a few fibers
terminate directly)
Corticobulbar tracts end
at the motor nuclei of CNs
of the contralateral side
Rubrospinal Tract
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Controls the tone of limb
flexor muscles, being
excitatory to motor neurons
of these muscles
Origin: Red nucleus
Axons course ventromedially, cross in ventral
tegmental decussation,
descend in spinal cord
ventral to the lateral
corticospinal tract
Cortico-rubro-spinal pathway
(Extrapyramidal)
Tectospinal Tract
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Mediates reflex movements of
the head and neck in response
to visual stimuli
Origin: Superior colliculus
Axons course ventro-medially
around the periaqueductal gray
matter, cross in dorsal
tegmental decussation,
descend in spinal cord near the
ventral median fissure,
terminate mainly in cervical
segments
Cortico-tecto-spinal pathway
(Extrapyramidal)
Vestibulospinal Tracts
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Lateral Vestibulospinal
Tracts
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Origin: lateral vestibular
(Deiter’s) nucleus
Axons descend ipsilaterally in
the ventral funiculus
Terminate on ventral horn
cells throughout the length of
spinal cord
Has excitatory influences
upon extensor motor neurons,
control extensor muscle tone
in the antigravity maintenance
of posture
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Vestibulospinal Tracts
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Medial vestibulospinal
tract
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Origin: medial vestibular
nucleus
Axons descend bilaterally in
the ventral funiculus, with the
medial longitudinal fasciculus
Most of the fibers end in the
cervical region, some reaching
upper thoracic segments
Involved in movements of the
head required for maintaining
equilibrium
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Reticulospinal Tracts
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Influence voluntary movement,
reflex activity and muscle tone by
controlling the activity of both
alpha and gamma motor neurons
Mediate pressor and depressor
effect on the circulatory system
Are involved in control of
breathing
Origin: pontine & medullary
reticular formation
Medial (pontine) reticulospinal
tract descends ipsilaterally
Lateral (medullary) reticulospinal
tract descends bilaterally
Both tracts located in the ventral
funiculus
Descending Autonomic Fibers
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The higher centers
associated with the control of
autonomic activity are
situated mainly in the
hypothalmaus
The fibers run in the
reticulospinal tracts
Terminate on the autonomic
neurons in the lateral horn of
thoracic & upper lumbar
(sympathetic) and sacral
segments (parasympathetic)
levels of the spinal cord
Love
nature