Overview Of The Brainstem Flashcards
Midbrain (mesencephalon) overview
Links the brainstem and forebrain together
Contains cranial nerve nuclei for the following
- 3
- 4
- some of 5
Contains visual and auditory reflex centers, motor function centers, pain transmission centers and visceral functions
Pons overview
Contains the following cranial nerve nuclei
- 6
- 7
- parts of 5 and 8
Pontine nuclei form a relay station between cerebral cortex, cerebellum and descending motor fibers that travel to all spinal levels
Medulla oblongata overview
Contains the following cranial nerve nuclei:
- 9
- 10
- 12
- parts of 5 and 8
Contains ascending and descending tracts that move to and from the spinal cord
Possess essential nuclei for respiratory and HR regulation as well as visceral functions
Tegmentum (medial portion) contains
Section of the brain stem that is medial to the rectum and base
- is found in all 3 components of the brainstem
Contains all cranial nerve nuclei and reticular formations
Base (ventral portion) contains
Contains the long pathways or tracts that connect the spinal cord to the cerebrum
Is found in all 3 sections of the brainsteam
Tectum (dorsal portion) contains
Only found in the midbrain of the brain stem.
Where is the inferior olivary nucleus located?
ONLY in the medulla oblongata
What is the reticular formation
Cell groups found within the tegmentum
- are poorly defined ANS diffuse throughout the tegmentum
Has a rostral and caudal components
1) rostral
- continuous with certain diencephalon nuclei
- function to maintain an alert conscious state in the forebrain
- contains the pontomesencephalic reticular formation
2) caudal
- continuous with intermediate zone fo the spinal cord
- functions to carry out motor/reflex/autonomic functions with cranial nerve nuclei
Lateral third of the reticular formation
Called the “parvocellular region”
- contains small called nuclei that are responsible for receiving afferent fibers
Found in the pons and medulla
Medial third of the reticular formation
Called the “magnocellular region”
- functions to give rise to effector fibers
Found in the pons and medulla
Midline third of the reticular formation
Called the “raphe nuclei”
- they function to secrete serotonin that is wildly distributed throughout the brain and spinal cord
Found in all 3 areas of the brainstem
What is locus ceruleus?
A group of reticular nuceli that found in the pons and midbrain
- functions to give rise to noradrenergic projections
Peduncilopontine nuclei
A group of reticular nuclei that is found in the midbrain and pons areas
- functions to project cholinergic neurons to the forebrain
Pontomesencephalic reticular formation
Pathway which allows for alertness and consciousness of the brain
- found in rostral reticular formation
Projects to the following
- thalamus (intra laminar nuclei)
- hypothalamus
- basal forebrain
damage to the reticular formation, bilateral cerebral cortex or bilateral thalamus can cause coma since it disrupts this pathway
Nucleus of the solitary tract (NTS)
Is the site in the brainstem where the baroreceptors/chemoreceptors and cardiopulmonary sensory neurons make their 1st synapse
Located in the causal portion of the medulla
Central ventrolateral medullary depressor area (CVLM)
Relay nucleus for mediating baroreflex and cardiopulmonary reflex
Contains GABAergic neurons that project to RVLM causing a decrease in blood pressure and heart rate
Found in the caudal portion of the medulla
Rostral ventrolateral medullary presser area (RVLM)
Involved in maintenance of sympathetic tone and communicates with the intermediolateral cell column (IML) via direct monosynaptic projections
Found in the rostral portion of the medulla
Intermediolateral cell column (IML)
Location of sympathetic preganglionic neurons in the thoracolumbar spinal cord (T1-L2)
Found in spinal column only
Nucleus ambiguous (nAmb)
Receives direct or indirect projections from the nucleus of solitary tract.
Sends information directly to the heart and controls rythum
Found in the caudal medulla
What’s are the 2 main aggregates of respiratory-related neurons
Dorsal respiratory group (DRG)
- receives input from pulmonary stretch receptors
- Found in the rostral medulla
Ventral respiratory group (VRG)
- cVRG subsection contains Expiratory neurons
- rVRG subsection contains inspiratory neurons
- botzinger complex contains Expiratory neurons
- pre-botzinger complex contains pacemaker for respiration
- all VRG is found in the rostral medulla except the cVRG, this is in the caudal medulla*
Descending tracts
Long projections that originate from the cerebral cortex and various other centers of the brainstem
Represent motor pathways to the periphery and sensory/motor regulatory pathways for the spinal cord
The following are descending tracts:
- lateral corticospinal
- anterior corticospinal tract
- reticulospinal
- vestibulospinal
- tectospinal
- rubrospinal
Reticulospinal tracts specifics
Arise from the reticular formation
Contains 3 functionally different fiber systems
- 1st fiber system = mediates motor functions
(Includes lateral reticulospinal and medial reticulospinal tracts)
- 2nd fiber system = mediates autonomic functions by taking fibers from the ventrolateral medulla and projects to the IML.
- 3rd fiber system = mediates pain impulses that ascend the spinothalmic tract via two limbs
Lateral reticulospinal tract
One of the two tracts associated with the 1st fiber system of the reticular formation (motor functions)
Originates in the medulla from the nuecli in the nucleusreticularisgigantocellularis (NRGC)
Projects bilaterally to all levels of the spinal cord to powerfully suppress extension spinal reflex activity (suppresses extension muscles)
Medial reticulospinal tract
One of the two tracts associated with the 1st fiber system of the reticular formation (motor functions)
Originates in the pons from the nuecli in the nucleus reticularis pontis caudalis (NRPC) and the nucleus reticularis pontis oralis (NRPO)
Project ipsilaterally to the entire spinal cord and functions to promote extensor spinal reflexes (promotes extensor muscles)
* DOES NOT decussate*
Third fiber system for reticulospinal tracts specifics
Modulates pain impulses that ascend in spinothalamic tract to cerebral cortex
1st limb: consists of enkephalinergic neurons located in the PAG of the midbrain
- neurons project to the serotonergic neurons isn’t he nucleus raphe magnus of the medulla
2nd limb: consists of projections from the raphe magnus neurons to the dorsal horn of the spinal cord where they meet another set of enkephalinergic neurons. This set moves the impulse to primary afferent pain fibers
- antagonizing this fiber system can help ignore pain for small periods of time*
** the locus ceruleus noraderenrgic neurons follow a similar pathway to also modulate the pain impulses**
Lateral and medial vestibulospinal tracts
Originate from the vestibular nuclei of the rostral medulla (very close to pons junction)
Lateral:
- is uncrossed (does not decussate) and descends the entire length of the spinal cord
- innervates interneurons for motor neurons of ipsilateral trunk extensors and limb extensors
- primary function = control muscles that maintain upright posture and balance ipsilaterally
Medial:
- is crossed and uncrossed at the same time (projects bilaterally) * note when they decuss, it is at the medulla level*
- ends in the cervical spinal cord
- primary function = adjust the position of the head in response to postural changes (i.e keeps the brain stable and centered while walking or running)
Rubrospinal tract
Originate from the midbrain and within the nuclei of the red nucleus
Fibers decussate in the midbrain and descend to lateral funiculus of the spinal white matter
Primary function = facilitate flexor motor neurons and inhibit extensor motor neurons
Tectospinal tract
Originates from the midbrain within nuceli of the superior colliculus in the tectum.
Fibers decussate in midbrain and projects to cervical spinal cord.
Primary function = participates in orientation of head in response to visual(and some auditory) stimuli.
Decorticate vs decerebrate posturing
Indicators of levels of Brainstem impairment in comatose patients (primarily with herniation syndromes)
Decorticate:
- excessive action or the rubrospinal tract
- signifies that above the midbrain is damaged, NOT the midbrain (Supratentorium)
- cuts off the corticospinal tract
Decerebrate:
- the Supratentorial pathology has moved down into the caudal portion of the midbrain
- cuts off the corticospinal tract, as well as all descending cortical systems and the rubrospinal tract
- results in hyperactivity of extensor muscles on all 4 extremities (since the lateral vestibulospinal tract is active without opposing tracts)
Pontine arteries
Paramedian branches
- enter pons and supplies medial portion of points
Short circumferential branches
- supply wedge shaped are in the ventrallateral pons (essentially lateral to the paramedian branches)
Long circumferential branches
- supples the rostral pons, caudal midbrain and superior surface of the cerebellum
