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