Sensory & Motor
Pathways
faizan
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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
3
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)
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
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
Lateral Spinothalamic Tract
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Carries impulses concerned
with pain and thermal
sensations.
Anterior Spinothalamic Tract
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Carries impulses concerned
with non- discriminative touch
and pressure
Spinocerebellar Tracts
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The spinocerebellar system
consists of a sequence of
only two neurons
Two tracts: Posterior &
Anterior
Carry information derived
from muscle spindles, Golgi
tendon organs and tectile
receptors to the cerebellum
for the control of posture and
coordination of movements
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
Rubrospinal Tract
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Controls the tone of limb
flexor muscles, being
excitatory to motor neurons
of these muscles
Origin: Red nucleus
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
Vestibulospinal Tracts
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Lateral Vestibulospinal
Tracts
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Origin: lateral vestibular
(Deiter’s) nucleus
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
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
Are involved in control of
breathing
Origin: pontine & medullary
reticular formation