Brainstem Flashcards
What is meant by the brainstem
Definition:
That part of the CNS, exclusive of the cerebellum, that lies between the cerebrum and the spinal cord
What are the three major divisions of the brainstem
Medulla oblongata
Pons
Midbrain
MRI- boundaries of brainstem
see diagram!
Where does the midbrain sit
In the posterior cranial fossa- in front of the cerebellum
What can be said about nearly every structure in the brainstem
Nearly everything in the brainstem is bilateral (one on each side) except for the pineal gland- sitting in the middle of the roof of the midbrain (hard to see while brain is intact- can only be seen once brain has been separated from the brainstem).
Importance for the maintenance of circadian rhythm- entrained by eyes (light/dark cycle- but not directly) and release of melatonin- leads to release of cortisol etc
Describe the superior and inferior colliculi of the midbrain
The midbrain contains the inferior and inferior colliculi defining its dorsal surface (or tectum).
Superior colliculi- involves in integrating and coordinating eye and neck movements (e.g following a tennis ball in a game of tennis)
Inferior colliculi- reflexive audiotry response- protective- look towards large noise to evaluate whether you need to run away or not.
What is the only cranial nerve to emerge from back of brainstem
the trochlear nerve- supplies the superior oblique muscles of the eye
Describe the dorsal surface of the pons
Floor of the 4th ventricle is in the dorsal surface of the ponse- diamond shaped recess
Describe the dorsal columns located on the back of the midbrain
Involved in sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints..
Medial dorsal column- from lower limbs- the gracile fasciculus, or gracile tract
Lateral dorsal columns- from upper limbs- the cuneate fasciculus or cuneate tract.
Where do nearly all cranial nerves emerge from
Brainstem
Except for the first two:
Olfactory nerve- through cribiform plate of ethmoid bone to the olfactory bulb
optic nerves- optic chaism
Describe the structures found on the dorsal surface of the midbrain
§ Optic chiasm.
§ The mammillary (memory and limbic system) bodies are part of the hypothalamus- base of hypothalamus- first cut in autopsy
§ Oculomotor nerve – conjugate eye movement.
Piuuitary stalk (infundibulum)- when cut exposes the pituitary gland at the base of the brain.
Cerebral peduncles- visible from the ventral surface- formed by massive projections from the cerebral motor cortex to targets in the brainstem and spinal cord- main motor tracts (corticospinal fibre tract that holds cerebrum to the brainstem).
Describe the midbrain nuclei lying in the ventral portion of the midbrain
Bipartite structure- the substantia nigra (pars reticulata and pars compacta) and the red nucleus
Describe the structures found on the ventral surface of the pons
§ Trigeminal nerve – muscles of mastication/chewing
Largest cranial nerve, exits the ventrolateral pons by traversing the fibres of the middle cerebellar peduncle
Cerebellum attached to the dorsal aspect of the pons by three large white mater tracts: the superior, middle and inferior cerebellar peduncles. Each of these tracts contains the efferent (superior and inferior) or afferent (inferior and middle) axons from or to the cerebellum.
Sensory- balance and posture
motor
Summarise the functions of the trigeminal nerve
Sensory nerve of the head and neck
3 main divisions- providing touch and sensations throughout the neck- some motor function (small root)- involves in chewing- motor function.
Summarise the pons
The pons (Latin, bridge) is rostral to the medullar and is easily recognised by the mass of decussating fibres that cross (bridge) the midline of the ventral surface, giving rise to the name of this subdivision (communicating fibres between the two hemispheres of the cerebellum- most communication is up or down from brain to body- but this crosses the midline- hence bridge- bridge of fibres obscures cerebellar peduncles
Describe the structures leaving the pontomedullary junction
§ Abducens nerve (VI) – lateral rectus muscle of the eye- move eye laterally
§ Facial nerve (VII) – musculature of face.
§ Vestibulocochlear nerve (VIII) – balance and hearing
Emerge in a medial to lateral manner- abducens nerve medially- and the vestibulocochlear nerve emerging most laterally.
Describe the landmarks seen on the ventral surface of the medulla
A series of swellings on the dorsal and ventral surfaces of the medulla reflects many of the major structures in this caudal part of the brainstem.
One prominent landmark that can be seen laterally is the inferior olive. Just medial to the inferior olives are the medullary pyramids, prominent swellings on the ventral surfaces of the medulla that reflect the underlying descending corticospinal tracts.
Describe the pyramids seen on the dorsal surface of the medulla
Seen as enlargements on the dorsal surface of the medulla.
This is where the motor fibres decussate before continuing down the spinal cord in the corticospinal tracts.
Describe the cranial nerves of medullary origin
The glossopharygeal nerve and the vagus nerve are associated with the lateral medulla, whereas the hypoglossal nerve exits the ventromedial medulla between the medullary pyramid and the inferior olive.
What is special about the spinal accessory nerve
Does not originate in the brainstem- but, as the name implies, exits the lateral portion of the upper cervical spinal cord.
Summarise the structures of the ventral medulla
§ The pyramidal decussation is where fibre lines cross and give rise to contralateral control.
§ Glossopharyngeal nerve – tongue/pharynx function.
§ Vagus nerve – PNS innervation to viscera.
§ Accessory nerve – sternocleidomastoid & trapezius.
§ Hypoglossal nerve – intrinsic muscles of tongue.
Describe Bell’s palsy
One side of the face drooping- often at a result of infection and mistaken for stroke- facial muscles lose their innervation from the facial nerve.
How does the cerebral peduncles emerge from the pons
As the pyramids in the medulla
Compare pyramidal symptoms to extra-pyramidal symptoms
Extra-pyramidal symtpoms- due to motor problems in the cerebellum or other part of the brain
Extrapyramidal signs/symptoms are due to the side effects from dopamine blockade such as Dyskinesias (pseudoparkinsonism, dystonia, akathisia, tardive dyskinesia).
Pyramidal- due to motor problems in the pyramid and corticospinal tract
Pyramidal signs basically mean plantar extension and hyperreflexia
Ultimately, what is meant by special in the functional classification of cranial nerves
Related to the head and neck. Innervate skeletal muscles derived embryologically from the pharyngeal arches and not the somites.
Summarise the pharyngeal arches in embryology
In human embryology, six pharyngeal arches are designated, but the fifth pharyngeal arch never develops. Each of the pharyngeal arches that does develop is associated with a developing cranial nerve or one of its branches.
These cranial nerves carry efferent fibres that innervate the musculature derived from the pharyngeal arch
Innervation of the musculature derived from the five pharyngeal arches that do develop is as follows:
first arch- trigeminal nerve
second arch- facial nerve
third arch- glossopharyngeal nerve
fourth arch- superior laryngeal branch of the vagus nerve
sixth arch- recurrent laryngeal branch of vagus nerve
What does the sulcus limitans do
Helps to differentiate between the dorsal sensory and the ventral motor
Describe how the neural tube gives rise to an important distinction between dorsal and ventral identity
The neural tube also gives rise to an important differentiation of dorsal and ventral identity, with the dorsal gray mater establishing an alar plate and the ventral gray mater establishing a basal plate. The alar and basal plates are separated by a shallow longitudinal groove called the sulcus limitans, which extends the length of the spinal cord through the mesencephalon. The alar plates gives rise to the dorsal horn of the spinal cord and the sensory nuclei of the cranial nerves, and the basal plate gives rise to the ventral horn of the spinal cord and the motor nuclei of the cranial nerves.
Compare the division of sensory and motor in the spinal cord to that in the brainstem
Spinal cord- straightforward and easy to appreciate in transverse section (sulcans limitans)
In the brainstem, the enlargement of the ventricular system that generates the fourth ventricle contributes to the lateral displacement of the alar plate. Thus in the tegmentum of the metencephalon and myelencephalon, the derivatives of the alar plate are located laterally to the derivatives of the basal plate.
How else are the nuclei arranged in the brainstem
Differentiate into different types of nuclei (GSA etc) arranged in a medial to lateral progression with respect to the embryological derivatives that they innervate.
Summarise the cranial nerve nuclei in the brainstem
Cranial nerve nuclei within the brainstem are the targets of cranial sensory nerves or the source of cranial motor nerves. These nuclei are located in the tegmentum or central core of the brainstem between the ventricular system dorsally and the division-specific structures and long motor pathways located ventrally.
Where are the primary sensory neurons that innervate some of the brainstem sensory nuclei found
In ganglia associated with the cranial nerves, similar to the relationship between dorsal root ganglia and the spinal cord
What are the most medial motor nuclei in the brainstem
The general somatic efferents
visceral efferents more lateral
What are the general somatic efferent nuclei in the brainstem
Oculomotor- midbrain
trochlear- midbrain
abducens- pons
hypoglossal- medulla
What are the special somatic efferent nuclei found in the brainstem
trigeminal - pons
facial- pons
nucleus ambiguous- medulla
What is the role of the nucleus ambiguous
Output from a number of cranial nerves- muscles in laryngopharynx
What are the general visceral efferents in the brainstem
Edinger Westphal nuclei- pons- involved in parasympathetic innervation to the eye
salviatory nuclei in pons and medulla- secretomotor fibres from facial and glossopharyngeal nerve supplying the salivary glands
vagus nerve in midbrain
Describe the special visceral afferents in the brainstem
Nucleus solitaries- pons and medulla- taste
Describe the general somatic afferents in the brainstem
Trigeminal nerve- expands throughout the brainstem- to cervical spinal cord- topographically arranged.
Describe the special somatic afferents
vestibulocochlear- pons
Summarise the rostrocaudal organisation of the cranial nerve nuclei
The rostrocaudal organisation of the cranial nerve nuclei (all of which are bilaterally symmetric) reflects the rostrocaudal distribution of head and neck structures. The more caudal the nucleus, the more caudally located the target structures in the periphery.
For example, the spinal accessory nucleus in the cervical spinal cord and caudal medulla provides special somatic efferent innervation for neck and shoulder muscles, and the motor nucleus of the vagus nerve provides preganglionic PSNS innervation for many enteric and visceral targets.
In the pons- sensory and motor nuclei are concerned mainly with somatic sensation from the face (the principal trigeminal nuclei), as well as movement of the jaws and the muscles of facial expression (trigeminal motor and facial nuclei)
Describe the situation further rostrally
In the mesencephalic portion of the brainstem, are nuclei concerned primarily with eye movements (the oculomotor and trochlear nuclei) and preganglionic parasympathetic innervation of the iris (the Edinger-Westphal nucleus).
Summarise the brainstem identification of the midbrain
“Mickey mouse” looking.
Cerebral aqueduct only found here- connects 3rd ventricle to 4th ventricle.
Will also see:
§ Substantia nigra – thus named as it’s a pigmented nucleus (black) - obvious to see- neuromelanin- found in dopaminergic cells which project to basal ganglia- pathology in this area- responsible for motor patholiges of Parkinson’s- pale in parkinson’s and is damaged in Parkinson’s disease.
§ Cerebral peduncle – large fibre tracts from the motor corte
§ Inferior colliculus – mediates auditory responses.
Describe the brainstem identification of the pons
§ 4th Ventricle – the pons is the floor of the 4th ventricle.
§ Middle cerebellar peduncle – attachment of pons to cerebellum and also has a functional purpose.
§ Transverse fibres
Summarise the brainstem identification of the medulla
§ Inferior Olivary Nucleus – visually obvious in the medulla (squiggly).
Presence of pyramids
4th ventricle still open
Summarise the brainstem identification of the lower medulla
Start of central canal of spinal cord
dorsal columns (analogous to the spinal cord)
Pyramidal decussation- signifies that we are in the medulla- where 90-95% of fibres decussate
Shape and ventricles can help to identify which brainstem level we are looking at
Summarise lateral medullary syndrome
§ Thrombosis of vertebral artery or PICA (Posterior Inferior Cerebellar Artery).
§ Symptoms:
o Vertigo.
o Ipsilateral cerebellar ataxia.
o Ipsilateral loss of pain/thermal sense (face).
o Contralateral loss of pain/thermal sense (trunk and limbs) – spinothalamic tract affected.
o Horner’s syndrome – sympathetic tract affected.
o Hoarseness, difficulty swallowing.
Describe cerebellar ataxia
Loss of coordination and balance
Shaking/ tremoring in the feet- can’t stand on feet
What is Horner’s syndrome
Loss of SNS control- drooping eyelids, pinpoint eyes, lack of sweat
How can we explain the symptoms of lateral medullary syndrome
Blood clot- stopping blood flow to lateral medulla- damaging the structures there- see functional consequences
What are the consequences of damage to the vestibular nuclei
Vertigo- loss of balance and balance- unsteady on their feet
Consequence of damage to inferior cerebellar peduncles
Fibre pathway coming up from spinal cord - helping you in terms of balance- info from muscles (anti-gravity)- lose this- abnormal gait- ataxia
Consequence to spinal nucleus and trigeminal
Loss of thermal and pain sensation
Consequence to sympathetic tract
Difficult to locate precisely- horner’s syndrome
Consequence to nucleus ambiguus
musculature of throat- hoarse presentation
Consequence to spinothalamic tract
Pain and temp form other side of body- contralateral deficit
Why does this cerebral accident have so many consequences
Lots of important structures in one small space
Compare strokes in the different parts of the brain
stroke in cortex tends to be more focal (potential for recovery depending where it is)
brainstem stroke depends to be catastrophic and fatal.
