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Chapter 10.
PYRAMIDAL AND EXTRAPYRAMIDAL SYSTEMS
10.1
OBJECTIVES
- 1. Define the brainstem and
the reticular formation.
- 2. Identify origin of the
pyramidal system and origins of the rubrospinal, reticulospinal,
tectospinal and vestibulospinal tracts.
- 3. Trace the path of the
corticospinal tract.
- 4. Compare and discuss
structural and functional characteristics of the pyramidal and the
extrapyramidal systems and explain why it is the extrapyramidal
system which is more important in motor control of the
animals.
- 5. Explain why many neurons
of descending tracts terminate not on the alpha but the gamma
motor neurons.
- 6. Name the four
extrapyramidal tracts and describe their functions.
- 7. Identify and define
major clinical signs after upper motor injury.
- 8. Localize the lesion
through examination of the motor deficiency.
- 9. Identify and define all
tracts covered.
- 10. Identify and define:
motor cortex, basal nuclei, caudate nucleus and corpus
striatum.
-
-
10.2
READING ASSIGNMENT
- deLahunta, pp
130-144
- Jenkins, pp 184-203;
214-217; 232; 292-293
-
-
10.3
CONTROL OF MUSCLE ACTIVITY
- 1. Reflexes
- 2. Lower motor
neurons
- 3. Upper motor neurons
(including their descending tracts):
- pyramidal
system
- extrapyramidal
system
- vestibular
system
- (cerebellum)
- 4. Autonomic nervous
system
10.4
BRAINSTEM AND RETICULAR FORMATION
- 10.4.1BRAINSTEM:
- 1. It extends from
diencephalon to myelencephalon and contains major components of
extrapyramidal system. Pyramidal tract also passes through
brainstem.
- 2. Brainstem contains upper
motor nuclei (extrapyramidal), cranial nerves' nuclei (sensory and
motor) and major relay centers. The central core of brainstem is
reticular formation.
- 10.4.2 RETICULAR
FORMATION:
- 1. A diffused and highly
complex mass of neuronal fibers and cells which forms the
"background" of brainstem.
- 2. Reticular formation
receives inputs from and gives outputs to the spinal cord,
cerebellum and other higher level portions of the
brain.
- 3. It is the control center
of animal's wakefulness, alertness and arousal, and the centers
for vital rhythmical and protective reflexes.
-
-
10.5
PYRAMIDAL SYSTEM
- This system controls
refined voluntary motor function. It is well developed in the
primates.
-
-
- 10.5.1ORIGIN:
- 1. Neurons concentrate in
the motor cortex of cerebrum where they are somatotopically
organized. In dogs, they are located in the pre- and post-cruciate
gyri.
- 2. Majority of pyramidal
motor neurons are in layer V of motor cortex (internal pyramidal
layer, especially its Betz cells). Other cortical neurons from
other layers may be involved.
- 3. Axons of pyramidal motor
neurons form the pyramidal tracts. They leave cerebrum by way of
the internal capsule and become a part of the crus cerebri in
diencephalon and mesencephalon.
-
-
- 10.5.2 PYRAMIDAL
TRACTS:
- 1. Corticospinal Tract -
terminates at the spinal LMNs.
- In brainstem, it is
identified as crus cerebri, longitudinal fibers of the pons,
pyramid and decussation of pyramid.
- In spinal cord, there are
two corticospinal tracts:
- a. Lateral corticospinal
tract - the main tract. After decussation, it is found in the
lateral funiculus of spinal cord. Most of its axons terminate with
LMNs through interneurons.
- b. Ventral corticospinal
tract - It is formed by uncrossed axons and runs in the ventral
funiculus. However, axons decussate before synapse with
LMNs.
- 2. Corticobulbar Tract -
terminates in brainstem LMNs. Mostly in cranial motor nuclei of
GSE and SVE. In animals, corticobulbar tract is better
developed.
- 3. Collateral of the
pyramidal axons reach basal nuclei, red nucleus, potine and caudal
olivery nuclei, as well as reticular formation.
-
-
10.6
EXTRAPYRAMIDAL SYSTEM
- 10.6.1GENERAL:
- 1. Its extend is not well
defined, but generally involves movement not under direct control
of the pyramidal system. It deals mostly with instinctive and
rhythmical movement and posture.
- 2. Extrapyramidal system
may be said to originate in the motor cortex. However, cortical
influence may be modified at one or several levels before motor
signals reach LMNs.
-
-
- 10.6.2:
Origin
- 1. Motor
cortex.
- 2. Basal nuclei (ganglia) -
They are subcortical motor nuclei.
- caudate nucleus
- putamen
- globus pallidus
- The above 3 nuclei and the
intersecting internal capsule are called corpus striatum. Corpus
striatum receives inputs from the cortex, thalamus, pyramidal
tracts mostly by the caudate nucleus and putamen. Its output
mostly originates from globus pallidus, and projects to the red
nucleus, substantial nigra, reticular formation and
thalamus.
- 3. Red nucleus (n. ruber) -
in the midbrain at the level of CN III. Origin of the rubrospinal
tract.
- 4. Tectum - dorsal portion
of the mescencephalon, consists of corpora quadrigemina. Origin of
the tectospinal tract.
- 5. Vestibular nuclei - in
the hindbrain above the sulcus limitans. Origin of the
vestibulospinal tracts.
- 6. Reticular formation -
origin of the reticulospinal tracts.
-
- 10.6.3. EXTRAPYRAMIDAL
TRACTS:
- 10.6.3.1.
Rubrospinal tract
- This is a very important
motor tract in animals. Nuclear ruber contains neuron cell bodies.
Their axons once leave red nucleus immediately decussate and
descend to the lateral funiculus of the spinal cord.
- In the spinal cord,
rubrospinal tract partially intermingled with lateral
corticospinal tract. Most of rubrospinal fibers terminate on
fusimotor LMNs, but termination on alpha motor neurons is
possible.
- Some of the tract fibers
terminate in motor nuclei of the cranial nerve.
- 10.6.3. 2.
Reticulospinal tracts - consist of:
- medullary reticulospinal
tract - in the lateral
- funiculus
- pontine reticulospinal
tract - in the ventral funiculus
- The tracts are
phylogenetically old. They are complex and difficult to
trace.
- 10.6.3. 3.
Tectospinal tract - originates from the rostral and the caudal
colliculi. Fibers crossover to contralateral ventral funiculus.
Most of them terminate on fusimotor LMNs via interneurons.
Tectospinal tract reaches only the cervical spinal
cord.
- 10.6.3 4.
Vestibulospinal tract - originates from the lateral
vestibular nucleus. Most of its fibers run in the ipsilteral
ventral funiculus, and terminate on alpha LMNs. The tract is
important in reflex postural movements of head and
body.
-
-
10.7
CLINICAL SIGNS OF THE UPPER MOTOR NEURON INJURY
- 1. Paresis - incomplete
paralysis; impairment of muscular activity. Severity of the
paresis increases as the location of the lesion descends in the
brainstem and spinal cord.
- 2. Hypertonia - increased
resistance to passive manipulation of the limb.
- 3. Myotatic reflexes are
normal or hyperreflexia. Hyperreflexia and hypertonia are caused
by the release of normal myotatic control.
- 4. Atrophy of affected
skeletal muscle does not occur. No muscular
fasciculation.
10.8
LOCALIZATION OF THE LESION
- 10.8.1
TERMINOLOGY:
- monoparesis/monoplegia
- paraparesis/paraplegia
- tetraparesis/tetraplegia
(quadriplegia)
- hemiparesis/hemiplegia
-
- 10.8.2 LESION IN
BRAINSTEM: consider one unilateral lesion
- rostral to the red
nucleus
- in the
mesencephalon
- in the
rhombencephalon
- in the caudal
medulla
-
- 10.8.3 LESION IN
SPINAL CORD (CANINE):
-
- Lesion Site Neurological
Sign Affected Body Part
- C1 - C5 UMN Thoracic &
pelvic limbs
- C6 - T1,2 LMN Thoracic
limb
- UMN Pelvic limb
- T2,3 - L4 UMN Pelvic
limb
- L5 - S1 LMN Pelvic
limb
- S1 - S3 LMN Anus;
Bladder
- Cd1 - LMN
Tail