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ASCENDING AND DESCENDING TRACTS
UNIT – 2: The Spinal Cord And Brain Stem
And Radio Imaging Techniques
DR. SHAIK ABDUL RAHIM
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’.
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
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 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
1
2
3
Second order neuron:
Has cell body in the spinal cord or medulla oblongata
Axon decussate &
Terminate on 3rd order neuron
Third order neuron:
Has cell body in thalamus
Axon terminates on cerebral cortex ipsilaterally
White Matter: Pathway Generalizations
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
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
Three major pathways carry sensory information
Posterior column pathway (gracile & cuneate
fasciculi)
Anterolateral pathway (spinothalamic)
Spinocerebellar pathway
Ascending Spinal Tracts
Dorsal white column
Lateral spinothalamic
Anterior spinothalamic
Anterior spinocerebellar
Posterior spinocerebellar
Cuneocerebellar
Spinotectal
Spinoreticulr
Dorsal Column 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 Carries impulses concerned
with pain and thermal sensations.
Anterior Spinothalamic Tract
Carries impulses concerned with non- discriminative touch and pressure
Spinocerebellar Tracts
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 tactile receptors to the cerebellum for the control of posture and coordination of movements
Somatic Motor Pathways
Motor Pathways 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
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
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
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
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: locomotion and postural
control
Descending Spinal Tracts
Pyramidal
Corticospinal
Extrapyramidal
Rubrospinal
Tectospinal
Vestibulospinal
Olivospinal
Reticulospinal
Descending
Autonomic Fibers
Corticospinal Tracts
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
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
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
Mediates reflex movements of
the head and neck in response
to visual stimuli
Origin: Superior colliculus
Vestibulospinal Tracts
Lateral Vestibulospinal
Tracts
Origin: lateral vestibular
(Deiter’s) nucleus
Has excitatory influences
upon extensor motor neurons,
control extensor muscle tone
in the antigravity maintenance
of posture
Vestibulospinal Tracts
Medial vestibulospinal
tract
Origin: medial vestibular
nucleus
Involved in movements of the
head required for maintaining
equilibrium
Reticulospinal Tracts
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