9/25 - Internal-External brain relationships Flashcards
What are the functions of the brainstem?
- Contain nuclei that are related to cranial nerve function
- Integrative: Receive ascending and descending input; serve as relay
- Conduit: Ascending and descending tracts
What are internal structures (example: nuclei, tracts) correlated with?
“Bumps” on surface of the brain
The images we are looking at are processed with myelin stain. Why? What do the colors signify?
Why: Unable to distinguish structures without stain
Black or dark blue: Fiber tracts or axon bundles
Lighter colors: Nuclei
Each level of the brainstem, medulla, pons, and midbrain are shown and are related to a specific portion of the ventricular system.
The medulla is related to the central canal and the caudal part of the 4th ventricle.
The pons, is related to the major portion of the 4th ventricle and the beginning of the cerebral aqueduct.
The midbrain is related to the cerebral aqueduct.
Closed Medulla
Caudal Medulla
Caudal to Obex
Open Medulla
Rostral Medulla
Dorsal Surface Open To 4th Ventricle
the medulla may be divided into 2 parts
the medulla may be divided into 2 parts called the closed medulla which is located caudally and the open medulla which is more rostral. The terms open and closed refer to the portion of the ventricular system to which it is related.
Caudally, the dorsal aspect of the medulla surrounds the central canal which continues down into the spinal cord.
Rostrally, the central canal opens into the more expansive 4th ventricle. This portion of the medulla is referred to as the open Medulla.
The transition point between the central canal and the fourth ventricle is the obex.
EXTERNAL FEATURES OF VENTRAL MEDULLA
On the brainstem, there are various “bumps” caused by the presence of underlying nuclei or fiber tracts.
On the ventral surface of the medulla, there is a midline slit called the Anterior Median Fissure. Immediately adjacent to the fissure are 2 elevations called the Pyramids because of their shape. They are widest at the junction of the pons and medulla and they taper at the junction of the medulla and spinal cord. Near this latter junction, the anterior median fissure becomes somewhat obscured due to the fact that axons descending in the pyramids cross from one side to the other forming a Decussation or crossing. Immediately lateral to the pyramids is another depression called the Preolivary Sulcus. This is not as deep as the anterior median fissure. Continuing laterally, the next small round bump is called the Olivary Eminence. This structure is located around the mid-medullary level. Lateral to the olivary eminence is another depression called the Post-Olivary Sulcus. These structures are landmarks for several nerves that arise from the brainstem to innervate structures in the head. These are collectively referred to as cranial nerves.
CAUDAL MEDULLA – VENTRAL
Rostral to the Pyramidal Decussation
Focusing only on the ventral surface, we see clearly defined the axon bundles that make up the pyramids. At this level, the right and left pyramids are completely separated by a midline sulcus or depression. This can be clearly seen on the cross section where the axon bundles in the pyramids are clearly defined.
Look at a protuberance located just lateral to the pyramids called the Olivary Tubercle. It was given this name since it looked like an olive to early anatomists. Our cut is at the caudal most end of the olivary tubercle. On the cross section, see that this is a clear area and is formed by the beginning of a long nuclear group called the inferior olive. As these structures are on the ventral surface, they are related to the motor system.
DORSAL MEDULLA
CEREBELLUM REMOVED
In this view, the brainstem has been rotated 90 degrees so that we can look at the dorsal or posterior surface of the brainstem; the cerebellum has been removed to reveal the underlying medulla.
The large space on the dorsal surface of the medulla is the 4th ventricle. This is filled with cerebral spinal fluid (CSF). The distal end of the 4th ventricle is called the obex. It is an important landmark for structures in the caudal vs. the rostral medulla.
Lateral to the obex are paired “bumps” called the gracile tubercles. There are 2 other elevations adjacent to the gracile tubercles called the cuneate tubercles.
CAUDAL MEDULLA- DORSAL
Rostral to Pyramidal Decussation
Nucleus Gracilus and Nucleus Cuneatus Relay Fine Touch, Discrimination, Proprioception, etc. from the body
Spinal V Relays Pain and Temperature from the face.
Focus on the dorsal or Tegmental region of the brainstem. We still see the Nucleus gracilis medially, the nucleus cuneatus lateral to it, and the spinal nucleus and tract of V, most lateral.
The major change is that the nuclei are getting larger and the fiber tracts are getting smaller as they terminate in their respective nuclei. The fasciculus gracilis is almost completely gone, whereas there are still some axons remaining in the fasciculus cuneatus. The Spinal Tract of V is actually getting larger as we go rostrally due to the fact that it enters the brainstem at the level of the pons and descends to the medulla. Thus as we move rostrally, we will encounter more of these axons.
SPINO-MEDULLARY JUNCTIONAt Pyramidal Decussation
ventral
The brainstem at the point where the spinal cord transitions to become the caudal medulla = the spino medullary junction.
Pyramids: two cylindrical protuberances immediately adjacent to the midline. Since they are wider at their base (where they join the pons) and narrower where they join the spinal cord, they are called the pyramids. The pyramids are formed by a large group of axons coursing between the cerebral cortex and the spinal cord called the cortiocspinal tract. At their most caudal end, just above the spinal cord, the midline groove that separates the right and left pyramidal tract seems to blur and become less distinct. That is because the axons in this fiber tract cross the midline at this level forming the pyramidal decussation.
Look at a section taken through the medulla in the area of the pryamidal decussation. Much of this section is stained dark blue indicating myelinated axons. There are some clear areas which correspond to collections of neurons in nuclei.
The primary thing to point out is the pyramid on the ventral surface. We see the fiber tract that formed the protuberance on the gross brain. Remember, since this is on the ventral surface of the brain, these axons are related to the motor system.
Also seen on this section are axons leaving the pyramid and crossing from one side to the other. Note, these axons do not just cross the midline; as they cross, they move from a ventral to a dorsal position.
SPIN0-MEDULLARY JUNCTION – At Pyramidal Decussation
DORSAL
Visualize: The cerebellum has been removed and we are looking into the 4th ventricle.
The caudal, narrow point, of the 4th ventricle is called the obex. Below this point, the medulla is referred to as the “closed” medulla since it completely surrounds the continuation of the 4th ventricle called the central canal. Above the obex, the medulla is called “open”.
Look at some bumps on the dorsal surface. The most medial bumps are called the gracile tubercles. In the spinal cord, there is a tract called the fasciculus gracilis which carries touch information from the lower part of the body into the CNS. This tract is visible in the cross section. The axons from this tract synapse in the nucleus gracilis which forms the gracile tubercle.
Immediately lateral to the gracile tubercle is another bump called the cuneate tubercle. On the cross section, the underlying fiber tract is the fasciculus cuneatus. Remember, these are the axons that carry touch information from the upper part of the body into the CNS.
The most lateral bump is the trigeminal tubercle. On the cross section, the underlying structures are a collection of axons called the spinal tract of V and more prominent, is the spinal nucleus of V. These axons carry pain information from the face. Thus, at this level of the brainstem we see 3 nuclei that are related to carrying sensory information from the face, upper limb or lower limb.
We could draw a representation, or homunculus, of this sensory information on the dorsal surface of the brain. The lower part of the body is represented most medially, followed by the upper part of the body and the head.
Note: the head is separated from the body at this point because a different type of sensory information is carried by the spinal vs. the cranial nerve pathways. The nucleus gracilis and cuneatus relay fine touch, discrimination, proprioception, vibration, etc. whereas the spinal trigeminal nucleus relays information on pain & temperature from the face.
ROSTRAL MEDULLA – Rostral to Obex
VENTRAL
The pyramids are well defined and clearly separated. The inferior olivary nucleus has expanded and we can see how it forms the protuberance on the ventral surface of the medulla. One thing to note at this level is that the dorsal surface is beginning to open up as we are now just above the level of the obex.
DORSAL
The nucleus gracilis and cuneatus are no longer present, but there are some new nuclear groups demonstrated by the light areas on the dorsal surface of the medulla. We see 2 new elevations that are in the floor of the fourth ventricle. The medial one is called the hypoglossal trigone and the lateral bump is the vagal trigone. As you can imagine, the names are telling you that these bumps are related to cranial nerves X, the vagus, and cranial nerve XII, the hypoglossal.
CRANIAL NERVES RELATED TO MEDULLA
Cranial nerves innervate structures in the head. There are a total of 12 pairs of cranial nerves. They contain both motor and sensory functional components. 6 of the 12 cranial nerves are related to the medulla. From caudal to rostral these are:
Cranial nerve XII – Hypoglossal This is the only cranial nerve located in the preolivary sulcus.
Cranial nerve XI – Accessory. This nerve arises from the upper segments of the cervical spinal, enters the skull through a large opening at the base called the foramen magnum, and is joined by a few branches arising from the medulla.
Cranial nerve X – Vagus is located at the caudal end of the post-olivary sulcus.
Cranial nerve IX – Glossopharyngeal is also located in the post-olivary sulcus.
Cranial nerve VIII – Vestibulocochlear is arises from the lateral aspect of the brainstem at the junction of the medulla and pons.
Cranial nerve VII – Facial arises at the same level as Cranial nerve VIII, just a bit more rostral.
Cranial nerve VI – Abducens Located on either side of midline at the junction of the pons and medulla.
General Somatic Afferent Fibers (GSA)
Axons that transmit sensory information (touch, pressure, pain, temperature, muscle tension) from the body to the spinal cord.
General Visceral Afferent Fibers (GVA)
Axons that transmit sensory information (distension, pressure, pain) from glands, blood vessels and the smooth muscle of organs to the spinal cord.
General Somatic Efferent Fibers (GSE)
Outgoing fibers carrying motor commands to skeletal muscles.
General Visceral Efferent Fibers (GVE)
Outgoing fibers carrying motor commands to smooth muscle, cardiac muscle and glandular tissue.
Special Sensory Afferent – (SSA)
Axons that transmit sensory information from sensations that are unique to the head (e.g., vision, hearing, taste, olfaction).
May be SSA (Special Somatic Afferents – physical stimuli; sound, light body position) or SVA (Special Visceral Afferents – chemically mediated; olfaction, taste)
Special Visceral Efferent – (SVE)
Axons projecting to muscles that are derived from pharyngeal arches. Mainly those related to face, oral cavity, pharynx, and larynx.
FUNCTIONAL COMPONENTS OF SPINAL AND CRANIAL NERVES
Both spinal and cranial nerves have sensory afferents that carry general sensory information from the body wall to the CNS. This would include sensations of touch, pressure, muscle tension, pain etc. Both have visceral afferents that carry similar sensory information from organs. Both have general efferent axons that innervate voluntary skeletal muscle, and both have general visceral efferent axons that innvervate smooth muscle in organs, cardiac muscle, and glands.
In addition to these 4 base components, cranial nerves also have special sensory afferents which transmit sensory information that is unique to the head – the special senses such as hearing, vision, balance, taste, olfaction.
They also have a subset of general somatic efferents called special visceral efferents. These axons innervate skeletal muscle that has a unique embryological origin from pharyngeal arches. These are muscles that are related to the oral cavity, the pharynx and the larynx.
If a cranial nerve has a motor component (somatic or visceral) the neuronal cell body will be in a ____ located ____.
If a cranial nerve has a motor component (somatic or visceral) the neuronal cell body will be in a NUCLEUS located INSIDE the CNS.
If a cranial nerve has a sensory component (special or general), the first order neuronal cell body will be in a _____ located ____ . There will be a nucleus within the CNS where that information is received before going on to the ____ for conscious awareness.
If a cranial nerve has a sensory component (special or general), the first order neuronal cell body will be in a GANGLION located OUTSIDE the CNS. There will be a nucleus within the CNS where that information is received before going on to the cortex for conscious awareness.
There are ____ nuclei related to cranial nerves than there are cranial nerves. That is because each component in a cranial ___ has a unique ____ in the brainstem related to it.
There are more nuclei related to cranial nerves than there are cranial nerves. That is because each component in a cranial nerve has a unique nucleus in the brainstem related to it.
Cranial nerve nuclei are not present in the caudal (closed) medulla. They are present in the ___, ___, ____and ____.
Cranial nerve nuclei are not present in the caudal (closed) medulla. They are present in the rostral medulla, pons, midbrain and diencephalon.
GENERAL RULES
If a cranial nerve has a motor component, whether it is somatic, including general and branchiomeric, or visceral, the cell body that gives rise to that axon will be located somewhere within the CNS.
In contrast, if a nerve has a sensory component, general or special, somatic or visceral, the cell body giving rise to the axon that detects that sensation will always be located in a ganglion located somewhere in the periphery.
For spinal nerves, these sensory detecting neurons are located in a dorsal root ganglion. For cranial nerves, they will be located in the ganglion on peripheral distribution of cranial nerves.
Because cranial nerves have multiple components, there are more nuclei related to cranial nerves than there are cranial nerves.
What ventricles is the medulla related to?
central canal and caudal 4th ventricle
What ventricles is the pons related to?
Most of the 4th ventricle, the beginning of the cerebral aquaduct
What ventricles is the midterm related to?
cerebral aqudect
closed medulla
caudal to the obex
open medulla
rostral to the obex
dorsal surface opens to the 4th ventricle
Medulla meaning
having an oblong shape; in the center
The midline slit down the ventral medulla is the _____________
Anterior Median Fissure
The midline slit down the dorsal medulla is the _____________
Posterior Median Fissure
Are the pyramids present on the Dorsal or Ventral surface?
The pyramids are present on the VENTRAL surface.
Is the Olive related to sensory or motor information?
Motor, because they present on the ventral side of the medulla.
Is the 4th ventricle seen on the dorsal or the ventral side of the medulla?
The 4th ventricle seen on the dorsal side of the medulla?
What kind of information do the Nucleus Cuneatus and the Nucleus Gracilis relay?
NG and NC relay Fine Touch, Discrimination, Vibration, Proprioception, etc. from the BODY
