10/2 - Brainstem Nuclei in the Midbrain-Diencephalon Flashcards
BRAINSTEM AS AN INTEGRATIVE CENTER:
BRAINSTEM NUCLEI IN THE MIDBRAIN
TRIGEMINAL NUCLEUS – SENSORY COMPONENTS
Somatic sensory conveying information on touch, pain, proprioception from all parts of the face
The trigeminal nerve has a general sensory components that is divided among 3 nuclei. The neurons that carry these sensations, for the most part, are located within the trigeminal ganglion.
Axons carrying sensory information on touch or pressure terminate in the main sensory nucleus, which is located at the level of the brainstem where the axons enter in the mid pons.
Axons carrying information on pain and temperature enter at the mid pontine level, but then descend to the caudal medulla forming the spinal trigeminal tract. Axon leave this tract at all levels from the pons to the medulla and synapse on neurons in the underlying spinal nucleus of V.
The third nucleus is the mesencephalic nucleus of V. This is a bit unique as the neurons conveying this information are NOT located in the trigeminal ganglion, but rather are located within this nucleus. This is an exception to the rule that all sensory neurons are located outside the CNS. There also is a motor nucleus related to V that contains the neurons that innervate the muscles of mastication. These are derived from pharyngeal arches and thus are branchiomeric. Thus the motor nucleus of V will be located a bit more lateral and ventral compared to somatic motor nuclei. It is located medial to the main sensory nucleus of V.
Main sensory (principal) nucleus of V
touch, pressure
Located in mid pons at point of entry of V.
Spinal nucleus of V
pain & temperature
Extends from mid pons (point of entry of V) to rostral spinal cord.
Mesencephalic nucleus of V
proprioception from muscles of mastication and mechanoreceptors in teeth and jaw.
Long thin nucleus extending from caudal pons into midbrain
Neurons are within the CNS. The only sensory neurons that are not located in a peripheral ganglion.
Proprioception
from Latin proprius, meaning “one’s own,” “individual”; perception is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement.
The ability to sense stimuli arising within the body regarding position, motion, and equilibrium. Even if a person is blindfolded, he or she knows through proprioception if an arm is above the head or hanging by the side of the body.
General Sensation
Main Nucleus of V
Motor V
Trigeminal’s Motor innervation to muscles of mastication.
TRIGEMINAL NUCLEUS: MOTOR COMPONENT
Branchial Motor Component
Motor nucleus is located immediately medial to main sensory nucleus in mid pons at point of entry of V.
Innervates muscles of mastication
CAUDAL MEDULLA- DORSAL
Cross Section
We still see the Nucleus gracilis medially, the nucleus cuneatus lateral to it, and the spinal nucleus and tract of V, most lateral. 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; this was discussed in the presentation on the cranial nerves. Thus as we move, rostrally, we will encounter more and more of these axons.
CAUDAL MEDULLA- DORSAL
Cross Section
We see the Nucleus Gracilis medially, the Nucleus Cuneatus lateral to it, and the spinal nucleus and tract of V, most lateral. 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 up 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 and more of these axons.
CRANIAL NERVE NUCLEI
of the Rostral Medulla
In this schematic diagram, we see the location of cranial nerve nuclei that have motor components. The 4th ventricle is present. Starting with motor nuclei. On the midline of the cross section, there is a pair of hypoglossal nuclei. This corresponds to the hypoglossal trigone on the image of the gross brain. In the cross section, it is represented by the pair of nuclei located immediately adjacent to the midline called the hypoglossal nucleus. This is the nucleus of origin for the axons in the hypoglossal nerve. Just lateral to the hypoglossal trigone is the vagal trigone.
This corresponds to the dorsal motor nucleus of X. This is the nucleus that contains the neurons that give rise to axons in the vagus nerve that innervate smooth muscle; that is these are the preganglionic parasympathetic neurons. The one exception are the neurons that innervate the heart. These are located in another nucleus. Continuing laterally, we move into areas occupied by cranial nerve nuclei that relay sensory information to the thalamus. In this diagram, we see the relative position of cranial nerve nuclei that carry sensory information.
The axons that project to the nucleus form a dense bundle. The neurons are located around the tract forming a donut like structure. The donut hole contains the axons and the outer rim are the neurons. Finally, we can see another cranial nerve related nucleus at this level. It also is very long and is depicted as the brown structure in the diagram labeled sensory. This is the vestibular nuclear complex. It also extends from the medulla to the pons. We continue to see a nucleus and fiber tract we have been following from the caudal medulla this is the spinal nucleus and tract of V. We see more axons here as we are getting closer to the site where they enter the brainstem.
The rest of the tegmentum is called the reticular formation.
CRANIAL NERVE NUCLEI
of the Rostral Medulla
In this schematic diagram, we see the location of cranial nerve nuclei that have motor components. The 4th ventricle is present. Starting with motor nuclei. On the midline of the cross section, there is a pair of hypoglossal nuclei. This corresponds to the hypoglossal trigone on the image of the gross brain. In the cross section, it is represented by the pair of nuclei located immediately adjacent to the midline called the hypoglossal nucleus. This is the nucleus of origin for the axons in the hypoglossal nerve. Just lateral to the hypoglossal trigone is the vagal trigone.
This corresponds to the dorsal motor nucleus of X. This is the nucleus that contains the neurons that give rise to axons in the vagus nerve that innervate smooth muscle; that is these are the preganglionic parasympathetic neurons. The one exception are the neurons that innervate the heart. These are located in another nucleus.
Continuing laterally, we move into areas occupied by cranial nerve nuclei that relay sensory information to the thalamus. In this diagram, we see the relative position of cranial nerve nuclei that carry sensory information.
The axons that project to the nucleus form a dense bundle. The neurons are located around the tract forming a donut like structure. The donut hole contains the axons and the outer rim are the neurons.
We continue to see a nucleus and fiber tract we have been following from the caudal medulla: the spinal nucleus and tract of V. We see more axons here as we are getting closer to the site where they enter the brainstem.
The rest of the tegmentum is called the reticular formation.
nucleus solitarius
The long one nucleus that extends from the rostral medulla into the pons is called the nucleus solitarius. On the cross section, it is located just lateral to the dorsal motor nucleus of X. It has a very characteristic appearance that allows it to be easily recognized.
CRANIAL NERVE NUCLEI
of the Rostral Medulla
nucleus ambiguus
Just dorsal to the inferior olive is a vague collection of neurons called the nucleus ambiguous for obvious reasons. This is a nucleus that contains motor neurons related to cranial nerves IX, X and XI. These neurons innervate branchial musculature in the larynx, pharynx, the neck (sternocleidomastoid and trapezius).
Also, the preganglionic parasympathetic neurons that innervate the heart are located in this nucleus. Remember, cardiac muscle is neither smooth or skeletal. It is its own special type of muscle.
CRANIAL NERVE NUCLEI
of the Rostral Medulla
vestibular nuclear complex
It is very long and is sensory. It extends from the medulla to the pons.
CRANIAL NERVE NUCLEI
of the Rostral Medulla
MIDBRAIN
This is the smallest part of the brainstem and extends from the rostral pons to the mammillary bodies.
What is the Roof of the Cerebral Aquaduct, or the Roof of the Midbrain?
the Tectum
What part of the ventricular system is associated with the midbrain?
the central aqueduct
What structures are associated with the Midbrain?
From those that start most Rostrally to those that begin most Caudally:
Brachium of the Inferior Colliculus
Oculomotor Nucleus
Periaqueductal Gray
Red Nucleus
Substantia Nigra
Superior Colliculus
Trochlear Nucleus
DORSAL MIDBRAIN, Tectum is associated with what nerve?
Trochlear Nerve
CN IV
TECTUM
Inferior Colliculus and Superior Colliculus
Inferior Colliculus
Relays auditory information
Superior Colliculus
Receives input from retina.
Reflex reactions.
Which structure is associated with Blind Sight?
Superior Colliculus
In Blind Sight, legally blind individuals walk without hitting obstacles. The visual cortex is not involved.
PONS-MIDBRAIN JUNCTION / CAUDAL MIDBRAIN
Cross Section through the caudal midbrain, at the junction of the pons.
On this section ventrally, there are some remnants of the rostralmost part of the basilar pons.
Laterally there is a large fiber tract called the cerebral peduncle. This fiber tract contains the axons that formed the corticospinal tract in the pons and the pyramids in the medulla. It is all one continual tract that originated in the cerebral cortex. We’re following it backward through the sections.
Dorsally, we are primarily seeing midbrain structures.
At this level, the trochlear nucleus related to cranial nerve IV is evident. Since it only innervates one muscle, it is a very small nucleus.
The 4th ventricle has closed down to form the cerebral aqueduct which is surrounded by grey matter called the periaqueductal grey. The function of this grey matter involves the pain pathway.
Other landmarks in the tegmentum include a very large fiber bundle called the superior cerebellar peduncle, which arose from the cerebellum and entered the midbrain. At this point the axons from the right and left cerebellum are crossing to the opposite side or decussating.
Another new addition is a roof over the midbrain: the tectum. At this level we see that it is fairly light indicating it is a collection of cell bodies. These are called the inferior colliculus, and they are part of the auditory system.
Periaqueductal Gray
Contains neurons that give rise to descending pain-control pathway. Neurons express numerous opiate receptors.
Activation of these receptors by opiates results in analgesia (the inability to feel pain).