Brainstem and Nuclei Condensed Grayscale Slides

7/27/2019 Brainstem and Nuclei Condensed Grayscale Slides

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Brainstem and NucleiBrainstem and Nuclei

Alex ForrestAssoci ate Profess or of For ensic Od ontol ogyForensic Science Research & Innovation Centre, Griffith UniversityConsultant Forensic Odontologist,Queensland Health Forensic and Scientific Services,

39 Kessels Rd, Coopers Plains, Queensland, Australia 4108

Oral Biology

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Learning ObjectivesLearning Objectives

You should have a general picture of the important

relationships between the major brainstem nuclei associated

with the cranial nerves.

You should understand and be able to describe and discuss

how the relationships between these nuclei will be useful in

diagnosis of neurological lesions you may encounter during

practice.

Brainstem


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BrainstemBrainstem

The brainstem unites the

brain (cerebrum and

cerebellum) with the

spinal cord. It comprises

three parts:

midbrain

pons

medulla oblongata

https://reader008.{domain}/reader008/html5/0417/5ad58ba34cb89/5ad58ba4869e2.jpg

BrainstemBrainstem

The brainstem transmits all

sensory and motor

pathways and information

between the brain and the

spinal cord.

In the brainstem, one finds

the central connections of

the cranial nerves and their

associated nuclei.

http://www-

unix.oit.umass.edu/~psyc335c/lectures/hindbrain.gif

Reticular FormationReticular Formation

Scattered deeply

throughout the brainstem

are loosely organized

groups of material

collectively known as thereticular formation, which

have important tasks in

relation of the level of

awareness or

consciousness, respiratory

and cardiac function, and

other things as well.http://www.biology.eku.edu/RITCHISO/reticularformation.gif

MidbrainMidbrain

The midbrain forms the

most superior part of the

brainstem.

It houses a canal which

connects a diamond-shaped space in the

medulla oblongata called

the fourth ventricle, which

connects with the third

ventricle, lying between

the cerebral hemispheres

of the brain.https://reader008.{domain}/reader008/html5/0417/5ad58ba34cb89/5ad58ba4e7321.jpg


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MidbrainMidbrain

This canal joining them is called the cerebral aqueduct, and is

continuous with the central canal of the spinal cord.

Netter, F. 1989, Atlas of Human Anatomy, Summit, New Jersey, Ciba-Geigy Medical, Plate 102.

MidbrainMidbrain

The part of the

midbrain posterior to

the aqueduct is called

the roof of the

aqueduct, or the

tectum, and on its

surface are four smallelevations called the

colliculi, arranged in

pairs.Netter, F. 1989, Atlas of Human Anatomy, Summit, New

Jersey, Ciba-Geigy Medical, Plate 109.

MidbrainMidbrain

The colliculi are important in reflex activity. The upper pair is

associated with vision, the lower pair with hearing and balance.


MidbrainMidbrain

Therefore, the dorsal part of the midbrain is mainly given over

to the visual and auditory systems, and in the ventral part are

nuclei for the oculomotor (III) and trochlear (IV) nerves.



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MidbrainMidbrain

The part anterior to the aqueduct is formed by the cerebral

peduncles which enter the cerebral hemispheres.

Netter, F. 1989, Atlas of Human Anatomy, Summit, New Jersey, Ciba-Geigy Medical, Plate 109. Netter, F. 1989, Atlas of Human Anatomy, Summit, New Jersey, Ciba-Geigy Medical, Plate 108.

PonsPons

The pons is the part of the brainstem between the midbrain

and the medulla.


PonsPons

It lies anterior to the connections of the cerebellum, and much

of its structure comprises fibres passing across the midline to

connect one cerebellar hemisphere with that on the other side.


PonsPons

Posteriorly, the cerebellum attaches to the pons by means of

the cerebellar peduncles.


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PonsPons

In diagrams of the brainstem, the cerebellum is removed to give

visibility, and these cut peduncles appear as ovoid masses.


PonsPons

The dorsal portion of

the pons contains

sensory and motor

tracts, and the nuclei of

the abducens, facial

and trigeminal nerves.

Ventrally, it contains

bundles of fibresconnecting the two

cerebellar hemispheres.

Netter, F. 1989, Atlas of Human

Anatomy, Summit, New Jersey,

Ciba-Geigy Medical, Plate 111.

PonsPons

Cranial nerves V VIII

exit the brainstem from

the pons, and many of

the cranial nucleiassociated with the

trigeminal nerve are

found here.




PonsPons

In the pons, it

becomes clear that

motor nuclei tend to

be situated more

medially than sensory

nuclei.

We also find that

nuclei with related

functions tend to be

grouped fairly close to

each other.


Anatomy, Summit, New Jersey, Ciba-

Geigy Medical, Plate 110.


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Medulla OblongataMedulla Oblongata

The medulla oblongata is

the most inferior part of

the brainstem, and is a

direct continuation of the

spinal cord as it passes

through foramen magnuminto the cranium.

https://reader008.{domain}/reader008/html5/0417/5ad58ba34cb89/5ad58ba6edec5.jpg

It begins at the foramen magnum by convention.

This is an arbitrarily chosen border and does not reflect an

anatomical change.

It ends at the inferior border of the pons.


On the lateral surface

there is a swelling on

each side known as the

olive, and anteriorly, a

second swelling close

to the midline called

the pyramid.

The pyramids are

separated by the

anteromedian fissure.


Grays Anatomy, Longmans, London, 38 th Ed 1989 p. 965

The grey matter and

white matter of the

spinal cord get

rearranged in this

area. The ventral grey

horns (motor) of the

spinal cord continue

into the regions of the

pyramids, where there

is extensive crossing-

over of fibres from one

side to the other

(pyramidal

decussation).


Netter, F. 1989, Atlas of Human Anatomy, Summit, New Jersey, Ciba-

Geigy Medical, Plate 108.


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Fibre tracts from the spinal cord continue into the medulla.

Above this, the architecture of the medulla bears l ittle

resemblance to that of the spinal cord, and you might expect

this, since there are nuclei scattered throughout the region,

and these will give rise to new tracts as they approach thebrain.


Clusters of nerve cell

bodies, called nuclei,

are also found here.

Nuclei of cranial nerves

9 12 are found in the

medulla, and the

nucleus of VIII is partly

in the medulla andpartly in the pons.





Close to the olive, we observe the exit of cranial nerves IX XII.


Grays Anatomy, Longmans, London, 38 th Ed 1989 p. 949.




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Trigeminal Nerve (V)

Trigeminal NerveTrigeminal Nerve

The cell bodies of

most of the somatic

sensory neurons are

located in the

trigeminal ganglion, a

ganglion that in many

ways resembles thedorsal root ganglion

found in spinal

nerves.

Modified from Grays Anatomy, Longmans, London, 38th Ed 1989 p. 1107


The central

processes of thesecells form the bulk

of the sensory root

of the trigeminal

nerve, and it enters

the side of the

pons.



Here, we need to recall what happens in a spinal nerve.

Recall that a sensory fibre terminates there in the dorsal horn.

In particular, fibres for pain and temperature terminate in thesubstantia gelatinosa.

Also recall that fibres need not terminate in the spinal segment

in which they originated, but rather may ascend or descend a

variable number of segments in the dorsolateral fasciculus

before reaching the dorsal horn.


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It turns out that there are similar structures in the brainstem

associated with the trigeminal nerve.

We find that there are three sensory nuclei which simulate

aspects of the dorsal gray horn, and a tract that simulates the

dorsolateral fasciculus.

In particular, the nucleus of the spinal tract simulates (and is

continuous with) the substantia gelatinosa of the spinal cord.

Histologically, we do not see any difference between the two.


The three general sensory nuclei, from superior to

inferior, are:

mesencephalic nucleus

chief sensory nucleus

nucleus of the spinal tract

The tract associated with these is called the spinal tract,

or the tract of the spinal nucleus.



These three nuclei run

from the medullaoblongata up to the mid-

region of the midbrain.

Modified from: Grays Anatomy,

Longmans, London, 38th Ed 1989 p. 953


The chief sensory nucleus

and the nucleus of the

spinal tract are nuclei of

common sensation.

In other words, it is here

that the cell bodies of

general sensory cells that

relay to higher centres are

located.




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The mesencephalic

nucleus is a very specific

nucleus.

It deals with

proprioception, and is

intimately involved with

reflex control ofmastication.




The need for speed in

this control is great, and

so this is the only

nucleus in the central

nervous system where

the peripheral sensory

cell bodies lie within theCNS.




Fibres from proprioceptive

organs in TMJ, periodontal

ligaments, muscle spindles

in the muscles ofmastication, and

proprioceptive organs in

the sutures of the maxilla

terminate here.







Central processes of

many of these

neurons pass directly

to the trigeminal motor

nucleus without

synapsing with an

interneuron.

This establishes rapid

reflex control over the

biting forces exerted

during mastication.


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Fibres from the TMJ

also relay to the chief

sensory nucleus to

provide conscious

information about the

joint position.




Trigeminal NerveTrigeminal Nerve Trigeminal NerveTrigeminal Nerve

The chief sensory

nucleus principally

subserves the function

of discriminative touch.




The nucleus of the spinal

tract (also known as the

Spinal Nucleus) is, as we

have already said, partly

responsible for the

perception of simple touch

and pressure.

Crucially, it also relays the

modalities of pain and

temperature, but only

towards its more caudal

end.




The spinal tract (often but

wrongly called the trigeminal

spinal tract) is in many ways

equivalent to the dorsolateral

fasciculus in the spinal cord.

It is continuous with it, and

within it there is blending of

fibres from both spinal cord

and brainstem.




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Sensory axons, entering the

pons through the trigeminal

sensory root, combine with

other general sensory fibres

from other cranial nerves

with a sensory function (VII,

IX and X), and those from

the upper cervical spinal

segments that pass craniallyinto the medulla, to form the

spinal tract.

Modified from: Grays Ana tomy,


Some of the sensory fibres from cranial nerves also pass

through the foramen magnum and descend in what becomes

the dorsolateral fasciculus as far caudally as the third cervical

segment.


There is also a physical arrangement of sensory fibres fromthe different branches of the trigeminal nerve in the spinal

tract. The fibres of V3 lie dorsally, those of V1 lie ventrally, and

those of V2 lie in between.


The spinal tract, and therefore the spinal nucleus, serve as the

common centre for processing general sensory input from fourcranial nerves (V, VII, IX and X).

It is therefore inappropriate to refer to it simply as a trigeminal

nucleus, although many textbooks make this mistake.



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Because pain and temperature fibres from all of these nerves

terminate in the same portion of the spinal nucleus, there is a

possibility of referral of pain to areas subserved by other

nerves.

For example, pain in a lower posterior tooth (classically a

lower 8) may be felt as an earache as well, and may even

involve temporary deafness in the ipsilateral ear until the

inflammation is relieved.


The peripheral sensory fibres all synapse within the chief

sensory nucleus and the spinal nucleus, and CNS neurons

take the information to other destinations.

These include the trigeminal motor nucleus, the motor nucleus

of VII (you screw your face up if someone touches it lightly

and unexpectedly, and you close your eyes if the cornea of the

eye is touched). Connections therefore occur between these

nuclei at this level.


In fact, there is a tremendous amount of crosstalk betweennuclei of the brainstem, and this results in many of the effects

we see in living people.


Other destinations include the hypoglossal nucleus (tongue),

nucleus ambiguus (cough, gag and vomit reflexes), thethalamus (emotional response and relays to the cortex), the

reticular formation (awareness together with other centres,

including awareness of changes in external conditions such as

air movements, temperature etc.).



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The central motor fibres of

the trigeminal nerve

originate in other brain

centres and descend to

terminate in the trigeminal

motor nucleus.

This nucleus lies medial tothe chief sensory nucleus.




Here they synapse with

peripheral fibres, whose

cell bodies make up this

nucleus.

Their axons form the bulk

of the motor root of V,

which exits the cranium

through the foramen ovale

and connects with thesensory trunk of V3 shortly

afterwards.



Most of these motor fibres originate from the opposite side of

the brain, but not all of them, which means there is a certainamount of insurance if the tracts on one side are injured or

destroyed, for example, during a stroke.


Facial Nerve (VII)


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The nuclei of importance with respect to the facial nerve

include .

Facial NerveFacial Nerve

the facial motor

nucleus for the somatic

motor function of the

nerve, the superior

salivatory nucleus for

supply of the

parasympathetic

secretomotor fibres to

the salivary glands,





the lacrimal nucleus

for parasympathetic

secretomotor fibres to

the lacrimal gland, andthe nucleus of the

solitary tract which

supplies taste fibres.





Recall that general sensory functions of VII are subserved by

the nucleus of the spinal tract in the brainstem.



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The motor nucleus of the

facial nerve lies in the

pons medially to the

nucleus of the spinal

tract, and slightly inferior

to the chief sensory

nucleus of the trigeminalnerve.



Motor fibres originating

in the facial motor

nucleus pass towards

the abducens nucleus,

which they wind around

before exiting from the

brainstem en route forthe internal acoustic

meatus.


Modified from: Grays Anatomy, Longmans, London, 38th Ed

1989 p. 959.

The sharp angle made by

the fibres as they wind

around the abducens

nucleus is called the

internal genu, as opposed

to the external genu of the

nerve in the facial canal of

the temporal bone.




Learning ObjectivesLearning Objectives

You should have a general picture of the important

relationships between the major brainstem nuclei associated

with the cranial nerves.

You should understand and be able to describe and discuss

how the relationships between these nuclei will be useful in

diagnosis of neurological lesions you may encounter during

practice.


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The End

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