Download Sheet 6 anatomy

6
Subject
Anatomy
…
Done by
Correction
Doctor
…
Mohammad Al-Salim
Reticulospinal Tracts (this sheet is based on the rec. from sec. 2)
(from its name, begins in the reticular
formation and ends in the spinal cord)
The reticular formation is a set of
interconnected nuclei that are located
throughout the brainstem, the neurons of the
reticular formation make up a complex set of
networks in the core of the brainstem that
extend from the upper part of the midbrain to
the lower part of the medulla oblongata. (from
wiki)
We have to differentiate between 2 types of
reticulospinal tracts, Pontine and Medullary
reticulospinal tracts because they differ in their
function.
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Pontine reticulospinal tract
-It begins in the reticular formation in the pons.
-axons of reticular function neurons descend uncrossed (fibers remain on the same side)
into the spinal cord and pass in the anterior white column.
-Also called Medial reticulospinal tract (MRST), (in a spinal cord section it is medial
while the medullary reticulospinal tract is lateral).
-Tonically active
We said before that the extrapyramidal tracts begin in structures in the brain stem as their
names suggest however, the cortex has an influence on them so a more precise naming
would be ex: cortico-reticulospinal tract and so on.
The pontine tract is continuously active and the effect of the cortex on it is inhibitory, so
if we remove the cortex’s influence on it, it will become hyperactive.
-Function: enhances the activity of axial and proximal limb extensors (anti-gravity
muscles) and inhibits the flexors in the leg (they enhance flexors in the arms)
Antigravity muscles are the muscles that have to primarily work to maintain upright
stance and are most important for movements such as jumps where you have to work
against gravity. And they are the extensors in the leg and flexors in the arm.
When you standing your joints like the knee and hip are fully extended to resist the effect
of gravity to allow you to stand upright but when you set these joints are flexed.
Medullary reticulospinal tracts
-It begins in the reticular formation in the Medulla.
- axons of RF neurons descend crossed and uncrossed into the spinal cord and pass in
the lateral white column.
-Also called the Lateral reticulospinal tract (LRST).
- NOT tonically active
Normally under stimulation from the cortex.
-Function: (opposite of pontine) Inhibit the axial and proximal limb extensors and
activates the flexors in the leg.
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The reticulospinal tracts (specifically Medullary tracts) also have descending autonomic
fibers providing a pathway by which the hypothalamus can control the sympathetic and
sacral parasympathetic outflow (the fibers descending from the hypothalamus to the
lateral horn are in the medullary tracts).
Vestibulospinal Tract
-It begins in the Vestibular nuclei which are found in the brain stem (in the pons and
medulla beneath the floor of 4th ventricle).
-The lateral vestibular nucleus receives afferent fibers from the inner ear through the
vestibular nerve (branch of the 8th cranial nerve) and also from the cerebellum.
-The fibers descend uncrossed through the medulla and through the length of the spinal
cord and pass through the anterior white column and synapse with a neuron in the
anterior gray column of the spinal cord.
-Function: (similar to pontine reticulospinal tract)
Facilitate the activity of extensor muscles and inhibit the activity of flexor muscles in
association with the maintenance of balance.
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Tectospinal tract
Tectospinal is a descending tract while
spinotectal is ascending.
(the Tectum is the posterior side of the midbrain
and has corpora quadrigemina or colliculi and
there is 2 superior colliculi and 2 inferior)
The 2 superior colliculi are related to visual
reflexes and the 2 inferior are related to auditory
reflexes.
-The axons descend crossed and in the anterior
white column close to Anterior median fissure,
the majority of fibers terminate in the anterior
gray column of upper cervical segments of the
spinal cord.
-Function: responsible for the reflex movement
of head & neck in response to visual stimuli. ( ‫لما‬
‫ابوك يرمي عليك كندرة وتشوفها جاية عليك ردة فعلك انك تزيح‬
‫)راسك‬
This reflex has an afferent limb (sensory) which is visual and an efferent limb (motor)
that causes you to move away, we call this visuo-spinal reflex.
The motor pathways to muscles are classified into
Medial Motor system:
-Axial & proximal muscles (closer to the midline) like the muscles of the vertebral
column and muscles of the trunk which are responsible posture.
-Medial Motor system includes:
1-Anterior corticospinal tract (pyramidal tract, descends ipsilateral and crossing occurs at
the level of the target spinal segment).
2-Extrapyramidal pathway in general (responsible for balance, posture …..etc) except for
rubrospinal tract.
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Lateral Motor system:
-Distal muscles mainly, muscles of the upper and lower limb which are responsible for
fine and skilled movements like writing ..etc
-The lateral Motor system includes:
1-lateral corticospinal tract (pyramidal tract-85% of fibers are lateral-, crossing occurs at
the lower part of medulla oblongata)
55% of the fibers of this tract end in the cervical segments because fine movements are
mostly in the hand which is supplied from these segments, 20% in thoracic and 25% in
sacral.
2-Rubrospinal tract (descends from the red nucleus) responsible for distal muscles of the
limbs and mainly the flexors of the upper limb mainly (most fine and skilled movements
are actions of flexors).
Note:
We divided the grey matter into 10 lamina, the anterior horn which has the cell body for
motor neurons is mainly lamina 9, the medial part of the anterior horn is related to the
axial muscles and the lateral part is for distal muscles, so the classification Medial motor
system and Lateral motor system is related to their representation on the anterior horn.
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Notes:
-The lower motor neuron begins in the anterior horn and ends in the muscles.
if it is damaged you lose all motor function to the muscle (peripheral nerves like radial
nerve… contain lower motor neurons)
-Damage or a cut to the upper motor neuron will result in exaggerated reflexes
(Hyperreflexia) because usually, the effect of the cortex in general on the reflexes is
inhibitory.
Related to hyperreflexia is another phenomenon called hypertonia (the muscle tone
increases, muscle tone is a partial state of contraction in the muscle and it is important in
maintaining posture).
The opposite of these effects occurs in damage to lower motor neurons because you cut
all innervation to the muscle so hypotonia (muscle is flaccid) and hyporeflexia occur and
eventually atrophy of the muscle occurs (no atrophy in cut to UMN).
-Fasciculation (alternating contracting and relaxation in the same muscle as the twitching
of the eyelid) is present in the case of LMN lesion and absent in the UMN lesion.
One explanation for this is that when you cut the neuron levels of the neurotransmitter
start to decrease, so the receptors on the cell membrane of the muscle are upregulated and
this is thought to cause fasciculations.
-Paralysis occurs in both UMN and LMN lesions however the type is different if you cut
LMN the muscle becomes hypotonic and flaccid ( flaccid paralysis, the muscle is
relaxed), in UMN lesion the muscle becomes hypertonic and has exaggerated reflexes so
there is (spastic paralysis, the muscle is rigid\ clasp knife).
-Clasp knife reaction
when a clasp knife is opened “fully extended” and you try to close or open it, initially
there is resistance but after reaching a specific angle or point it closes suddenly.
And the same concept applies to the
rigidity that happens in the patient
(UMN lesion) where the patient would
have flexed muscles when a doctor
tries to extend the arm of the patient
initially there will be resistance but if
he persists and applies enough force
there will be “sudden release” and the
arm will extend.
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Explanation of why this happens is related to the 2 phenomena:
Initial resistance: Exaggerated stretch reflex (explained in the e-lecture the Dr. sent)
The muscle resists stretching, when you stretch a muscle it responds by contracting and
because UMN lesions cause exaggerated reflexes the effect is bigger.
Sudden release: Caused by activation of Golgi tendon reflex also called anti-stretch
reflex, which resists excessive contraction in the muscle
(After applying pressure, the tension in the muscle will increase and will be enough to
activate the Golgi tendon organs which will cause the relaxation).
Ex: (pay attention to the figure above)
There is a contraction in the quadriceps and this creates tension in its tendon and causes
firing of the Golgi tendon organs which are sensory receptor organs, one of the reflexes
that happen activates 2 interneurons in the spinal cord.
The first interneuron is inhibitory, it inhibits the lower motor neuron that is going to the
muscle that the signal originated from to stop it from further contracting.
This reflex is polysynaptic (inhibitory interneuron between the 2 excitatory neurons).
At the same time there is activation of the 2nd interneuron which is excitatory and
activates the lower motor neuron that is going to the antagonist muscle (the muscle which
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works in the opposite direction to the original muscle) in this example it is the hamstring,
this is what’s called the law of reciprocal innervation; innervation so that the
contraction of a muscle or set of muscles (as of a joint) is accompanied by the
simultaneous inhibition of an antagonistic muscle or set of muscles.
It is impossible to have a full contraction of the biceps and triceps at the same time
because when there is a contraction in the biceps there is relaxation in the triceps and vice
versa (reciprocal innervation).
-Babinski sign (present in UMN lesion and not in LMN)
To understand it let’s explain the normal response first,
When a doctor stimulates the sole of the foot (specifically the lateral aspect) with a blunt
object, the normal response is flexion of the toes but in cases of UMN lesions what
occurs is the opposite (called Babinski sign), when stimulating the sole of the foot what
occurs is fanning of the toes, and the big toe is dorsiflexed rather than being flexed and
this is suggestive of UMN lesion.
Note: Upper motor neuron lesions most of the time affect both pyramidal and
extrapyramidal tracts, it is rare that only one of them is affected, however, the
explanation to most of the phenomenon that happens (clasp knife, hypertonia…etc) is
related to the extrapyramidal tract except Babinski sign which is explained by pyramidal
tract.
Note: in children below the age of 1-1.5 years, positive Babinski sign is normal because
full development and myelination of the pyramidal tracts happen after 1-1.5 years of age
(when the child stops crawling on 4 limbs and starts standing and moving on 2).
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-Clonus (another symptom of UMN lesion, not LMN)
In testing for clonus the doctor would
attempt to dorsiflex the foot and would
face resistance (remember what we said
above) and when he applies enough
force clonus happens which is rhythmic
contractions and relaxation of muscles
when they are subjected to sudden
sustained stretch caused by exaggerated
reflexes.
-Both Decerebrate and Decorticate rigidity are related to UMN lesions, the difference
between them is the level of the lesion if the lesion is higher than the red nucleus it’s
decorticate if the lesion was lower it’s decerebrate.
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The midbrain (will be discussed next week) is divided into two levels, level of superior
colliculus and level of the inferior colliculus, at the superior level there is a structure
called the red nucleus (from which the rubrospinal tract descends) a lesion above this
nucleus is called decorticate and below it is decerebrate.
In decorticate there is rigidity in the entire body and the lower limbs are extended while
upper limbs are flexed and rigid, in decerebrate, there is also complete rigidity and the
lower limbs extended but the upper limbs are extended as well.
Decorticate posture (lesion above red nucleus so you affected\removed the cortex, from
the name), remember what we said above about the pontine reticulospinal tract and that it
is tonically active and removing the cortex causes more activation so its effect is
overriding and more prominent and it causes activation of extensors in the leg and flexors
in the arm (antigravity muscles)
Decerebrate posture (lesion below the red nucleus) the rubrospinal tract is part of the
lateral motor system and is responsible for the flexion of muscles in upper limbs so if it is
lost there will be an extension of the upper and lower limbs.
The vital centers (related to CVS and RS) are present in the medulla oblongata and pons
(lower part of the brain stem) so a brain stem injury is fatal, if you lose part of the cortex
you lose some functions but brain stem injury is fatal, that is why decerebrate is worse
than decorticate because the lesion is closer to the vital centers so prognosis is worse.
-Clinical significance of lamination of the ascending tracts
In the posterior column system, the fibers closer to the midline are related to the lower
part of the body (sacral) and the cervical is most lateral, but in the spinothalamic tracts,
the cervical to sacral segments are located medial to lateral because the crossing of the
fibers occurred in the spinothalamic tract.
If the lesion started on the inside and moved to the
outside (we call it intramedullary tumor) the first
affected fibers in the spinothalamic tract are the
cervical and upper part of the body, If the lesion is
extramedullary like meningiomas the first affected
fibers are sacral.
Sacral sparing (sacral fibers are not affected) occurs
in intramedullary lesions because the lesion typically
doesn’t reach the sacral fibers.
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