Descending pathways_251021_183355 2.pdff

Anatomy cns

1. DESCENDING
PATHWAYS

2. wont ame oe

DESCENDING
PATHWAYS

Corticospinal tracts
Corticonuclear tract
Reticulospinal tracts
Rubrospinal

Tectospinal tracts
Vestibulospinal tracts
Olivospinal tract
Solitariospinal tract
Hypothalamospinal fibres

3. UPPER AND LOWER MOTOR NEURONS

+ The motor neurons situated in the anterior gray columns of the spinal cord send axons to
innervate skeletal muscle through the anterior roots of the spinal nerves. These motor
neurons are sometimes referred to as the lower motor neurons and constitute the final
common pathway to the muscles.

* The lower motor neurons are constantly bombarded by nervous impulses that descend
from the medulla, pons, midbrain, and cerebral cortex as well as those that enter along
sensory fibers from the posterior roots.

+ The nerve fibers that descend in the white matter from different supraspinal nerve centers
are segregated into nerve bundles called the descending tracts.

Reticulospinal fibers. ~

+ These supraspinal neurons and Revutospinal bers dd
their tracts are sometimes u)
referred to as the upper motor Canon ee

neurons, and they provide u
numerous separate pathways
that can influence motor
activity.

„Sonsory afferent über

Gamma offorent fiber

Neuromuscular spindle

+

Alpha eflrent fer of lower metor
neuron—he final common pathway

Renshaw feedback neuron

Motor end-ptate

4. CORTICOSPINAL AND CORTICO
NUCLEAR FIBERS

+ The majority of corticospinal and corticonuclear fibers
arise from cells situated in the primary motor cortex (area
4) and the premotor cortex (area 6). A small contribution
comes from cells in the postcentral gyrus (somatosensory
cortex; areas 3, 1 and 2) and the adjacent parietal cortex
(area 5). They constitute the first-order neuron.

+ These fibers descend progressively in: the corona radiata,
genu and posterior limb of internal capsule, basis
peduncli of the midbrain, basis pontis of pons, pyramid of
medulla.

+ The corticonuclear fibers end at the motor nuclei of
cranial nerves in the midbrain, pons and medulla.

+ At the medullo-spinal junction the majority of the
corticospinal fibers decussate in the pyramidal
decussation and descend in the lateral funiculus as the
lateral corticospinal tract down to the 4' sacral segment.

+ The minority uncrossed corticospinal fibers descend in the
ventral funiculus as the anterior corticospinal tract. These
fibers cross in the venteal white commissure to end on the
motor neuron in the cervical and upper thoracic segments

5. CORTICOSPINAL AND CORTICO
. NUCLEAR TRACTS
in Spinal Cord

+ Second-order neuron: contralaterally
on interneurones in the lateral parts of
laminae IV-VI and both lateral and
medial parts of lamina VII; some are
also distributed to lamina VIII
bilaterally.lon both Sider)

« Third-order neuron: a and y motor
neurones of lamina IX.(3)

+ In some instances, the axon of the first-
order neuron terminates directly on the
third-order neuron.

¢ Function: the corticospinal tracts are
the pathways concerned with voluntary,
discrete, skilled movements, especially
those of the distal parts of the limbs.

6. SECOND ORDER NEURON

Precentral corticospinal fibres, + Postcentral corticospinal fibres end
including motor and premotor mainly in the contralateral dorsal
areas 4 and 6, terminate mostly on horn, in the lateral parts of laminae
interneurones in laminae V-VIII, IV-VI and lamina VII, terminating
with the densest concentration | chiefly in laminae IV and V.
ending in the lateral part of lamina + They are congerned with the

. supraspinal modulation of the
They influence a and y motor transmission of afferent impulses
neurones of lamina IX, via these to higher centres, including the
interneurones. motor cortex.

As the widespread dendrites of
multipolar neurones in lamina IX
penetrate lamina VII, direct
monosynaptic axodendritic contacts
also occur on large a motor
neurones.

Direct termination on motor
neurones is most abundant in the
spinal enlargements.

7. Precentral sulcus SULCI AND GYRI OF
Superior frontal SUPEROLATERAL SURFACE

sulcus

Inferior frontal (FRONTAL LOBE)

sulcus

Anterior ramus
of lateral sulcus

Ascending ramus
of lateral sulcus

Superior frontal sulcus

Ascending ramus Precentral sulcus

Inferior

frontal sulcus \
Precentral gyrus
Superior frontal
gyrus
Middle frontal
gyrus
Inferior frontal
gyrus
* Pars orbitalis
+ Pars triangularis
+ Pars opericularis

Anterior
ramus

Posterior
Pars orbitalis
Pars triangularis

Pars opericularis
Superolateral surface

8. Sulci

Gyri
À

Postcentral
sulcus
Intraparietal
sulcus
Posterior end
of posterior
ramus of
lateral sulcus
Posterior end
of superior
temporal
sulcus

Postcentral
gyrus
Superior
parietal lobule
Inferior
parietal lobule
+ Supramarginal
gyrus

SULCI AND GYRI OF
SUPEROLATERAL SURFACE
(PAREITAL LOBE)

Postcentral sulcus Intraparietal sulcus

Inferior
parietal
lobule

Posterior end of
posterior ramus

Posterior end of
Superolateral surface superior temporal

9. Is located in the
precentral gyrus
and anterior
part of
paracentral
lobule; area 4.

Responsible for
volitional motor
activity on the
contralateral
side.

Is
somatotopically
organized as the
motor

, homunculus.

PRIMARY MOTOR CORTEX

10. area4onthe ,
superolateral
surface (area
6).

It prepares
the primary
motor cortex
for specific
movements
in advance of
their
execution.

snterorto. PREMOTOR CORTEX

a

11. « Is located in the PRIMARY

postcentral syrus SOMATOSENSORY
and the posterior

part of the uses
paracentral

lobule; areas 3,1,
and 2.

+ It receives
discriminative
tactile, vibration,
form, pressure,
position, and
movement senses.

+ Is somatotopically
organized as the
sensory
homunculus.

12. Primary visual cortex, area 17, is located in both banks of
calcarine sulcus on the medial and superolateral surfaces
of the hemisphere. VISUAL

Secondary and tertiary visual cortices areas (18, 19). CO RTI E ES

Primary Suppl À
First somatic motor SA lementary
sensory area area motor area

Primary
visual
area

Entorhinal
area

Uncus

Olfactory cortex

13. MOTOR HOMUNCULUS

* The cortical
representation of
the body parts in
the precentral
gyrus constitutes
what is known as
the motor
homunculus.

14. VESTIBULOSPINAL
TRACTS

The lateral vestibulospinal tract arises from the
lateral vestibular nucleus.

Fibers descend ipsilaterally the whole length of the
spinal cord in the anterior white funiculus.

Fibers end in the medial part of the ventral horn in
lamina VIII and lamina VII.

The medial vestibulospinal tract arises mainly from
the medial vestibular nucleus.

Fibers descend in the mediai longitudinal fasciculus
into the anterior white funiculus .

The tract contains both crossed and uncrossed
fibers.

E ages not extend beyond the mid-thoracic cord
level.

Fibers of the medial tract end in lamina VIII and
lamina VII.

Function: strong excitatory influence upon the
antigravity muscleséhat include the epaxial muscles
of the vertebral column and the extensor muscles of
the lower limbs

15. RETICULOSPINAL TRACTS

+ Pontine reticulospinal tract : originates from the
pontine tegmentum.

+ The fibers, mostly uncrossed, end in lamina VIII
and lamina VII.

+ Medullary reticulospinal tract: arises from the
nucleus gigantocellularis, which lies dorsal to the
inferior olivary complex.

+ The fibers, uncrossed and crossed end in lamina
Vill and laminae VI and Vil and also directly. on a
and y motor neurones.

+ Function: The pontine reticulospinal tract is
excitatory to the antigravity muscles, including
the epaxial muscles of the vertebral column and
the extensor muscles of the lower limbs.

+ The medullary reticulospinal tract is inhibitory to
antigravity muscles.
+ The cells of the medullary reticulospinal

tract receive input from the corticospinal
tract and the rubrospinal tract.

16. TECTOSPINAL TRACT

+ The tectospinal tract arises from the superior colliculus of the midbrain.

+ It crosses ventral to the periaqueductal grey matter in the dorsal tegmental decussation
and descends in the anterior funiculus of the spinal cord.

. Cres of the tract project only to the upper cervical cord segments, ending in laminae VI,

, VI.

+ Function: Fibres of the tract make polysynaptic connections with motor neurones serving
muscles in the neck, facilitating those that innervate contralateral muscles and inhibiting
those that innervate ipsilateral ones.

+ In animals, unilateral electrical stimulation of the superior colliculus causes turning of the
head to the contralateral side, an effect mainly mediated through the tectospinal tract.

Descending pathways
TE a
Deco cs
ER namen {a ss)
cig ps vera octet at
= Foca cma
— — panda ps
Venen
pudre av pr tcs
spnochar vc

17. RUBROSPINAL TRACT

+ The rubrospinal tract arises from the red nucleus.

+ Rubrospinal fibres cross in the ventral tegmental decussation in the midbrain and
descend in the lateral funiculus of the cord, where they lie ventral to, and
intermingled with, fibres of the lateral corticospinal tract.

+ The tract projects only to the upper three cervical cord segments.
+ Rubrospinal fibres are distributed to the laminae V-VIl of the spinal grey matter.

+ Function: Animal studies demonstrate that the effects of rubrospinal fibres on a and y
motor neurones are similar to those of costicospinal fibres.