Histological Organisation of the Brainstem Flashcards
Discuss the superior colliculi (function, afferents and efferents).
- mediate visual reflexes (visual reflexes happen when you move the eyes, head and neck in response to visual stimuli)
[PRO TIP: How do you remember this? Remember that the superior colliculus is connected to lateral geniculate body by superior brachium, and the lateral geniculate body plays an essential role in visual processing.] - afferents from the optic tract, visual cortices and spinal cord
- efferents to cervical cord [perhaps to control neck muscles], brainstem and olivary nucleus
Discuss the inferior colliculus (function, afferents, efferents).
- mediates auditory reflexes (auditory reflexes happen when you move the head, neck and trunk in response to auditory stimuli)
- also responsible for tonal discrimination and sound localisation
- Afferents: receives fibres from the lateral lemniscus (tract with second order neurons from the cochlear nucleus in the brainstem) and from the superior olivary complex
- Efferents: it projects to the medial geniculate body (through the inferior brachium). From the medial geniculate body, fibres are projected to the auditory area of the cerebral cortex. Efferents from the inferior colliculus also project to the superior colliculus of the same side. The superior colliculus, in turn, sends these auditory signals to the spinal cord via tectospinal tracts.
Discuss the crus cerebri.
contain descending fibers from the cerebral cortex which can be corticobulbar (from cerebral cortex to brainstem) [here there’s corticonuclear group functionally similar to corticospinal but controlling skeletal musculature innervated by cranial nerves] or corticospinal (controlling skeletal musculature innervated by spinal nerves), pyramidal (corticonuclear and corticospinal) or corticopontine (corticobulbar fibres which are part of corticopontocerebellar system)
NOTE:
Pyramidal fibres occupy the middle 2/3 of the crus cerebri. The medial portion of the middle 2/3 is occupied by corticonuclear, lateral portion of middle 2/3 by corticospinal.
The medial 1/6 is occupied by frontopontine [corticopontine from frontal cortex], lateral 1/6 by occipito, tempo, and parietopontine fibres.
The substantia nigra is a large motor nuclear strip dorsomedial to the crus cerebri. It is divided into a dorsal pars compacta and a ventral pars reticulata. What is important to note about the two? What’s the clinical relevance of the pars compacta?
Pars compacta - contains dopaminergic efferents to the neostriatum
Pars reticulata - GABAergic efferents to the thalamus
Clinical relevance of pars compacta: Injury to the dopaminergic neurons causes Parkinson’s disease.
Notes for context:
In molecular biology and physiology, something is GABAergic or GABAnergic if it pertains to or affects the neurotransmitter gamma-aminobutyric acid (GABA). For example, a synapse is GABAergic if it uses GABA as its neurotransmitter, and a GABAergic neuron produces GABA.
[Classification of neurons according to neurotransmitter.]
NB: Substantia nigra and ventral tegmental area are important in reward circuitry. Dopamine is released in this circuitry.
List the structures of the mesencephalic tegmentum at the level of superior colliculus that:
a) extend the whole tegmentum
b) are unique to the level of superior colliculus (upper midbrain)
a) extend the whole tegmentum
- periaqueductal grey matter
- lemniscal systems (medial lemniscus [vibration and conscious proprioception from the body], spinal lemniscus [pain, temperature, crude touch and pressure from the body] and trigeminal lemniscus [sensations from the head region except vision, smell, and hearing] are here.)
- medial longitudinal fasciculus - interconnects cranial nerve nuclei whose axons innervate the extraocular muscles
- mesencephalic nucleus of V
- reticular formation
b) are unique to the level of the superior colliculus
- pretectal nucleus
- motor nucleus of oculomotor
- Edinger Westphal nucleus
- oculomotor nerve roots
- red nucleus
Further notes:
~ PAG controls the responses to internal stressors e.g. pain and to external stressors e.g. threat.
~ The chief function of the medial longitudinal fasciculus is to coordinate movements of the eye, head and neck in response to stimulation of the vestibulochochlear nerve.
The red nucleus extends from the caudal region of the superior colliculus into the subthalamic nucleus.
a) What mainly contributes to its pinkish/reddish colour?
b) The red nucleus has complex afferent and efferent connections. In general, however, majority of the afferent connections are derived from the ____(I)____ nucleus (in the cerebellum) and these are, in turn, projected to the ____(II)____ (in the spinal cord), thalamus, and cerebral cortex, particular the motor cortex. Afferent fibres enter the red nucleus via the ____(III)____ cerebellar peduncle.
c) What are the roles of the red nucleus?
a) presence of iron-containing pigments which occur in many, but not all, cells of the nucleus
b) (I) dentate, (II) rubrospinal tract, (III) superior
c) roles of red nucleus:
- involved in regulation of movements: force, velocity and direction parameters
- helps in execution of learnt automated movements [Important for crawling in babies, forms rubrospinal tract controlling upper limb movements]
The pretectal nucleus is slightly superior and ventral to the superior colliculus and is located at the junction between the midbrain and the diencephalon.
a) It receives afferent fibres from the ____________.
b) Efferent fibres go to the ____________ nuclei (bilaterally).
c) State the function of the pretectal nucleus.
a) optic tract and superior colliculus
b) Edinger-Westphal
c) mediate pupillary light reflexes (pupillary constriction when light is shone on it) - both direct and consensual/indirect types
List the structures of the mesencephalic tegmentum at the level of inferior colliculus that are unique to the level of inferior colliculus (lower midbrain). (3)
- trochlear nucleus
- trochlear nerve roots
- decussation of superior cerebellar peduncles
- [Diagram]
NB:
The superior cerebellar peduncle have afferent and efferent fibres passing through them either going to cerebellum from midbrain (afferent) or to midbrain from cerebellum (efferent).
- afferent fibres are those of ventral spinocerebellar tract
- efferent fibres are those going to dentate nucleus (dentatorubral tract and dentatothalamic tracts)
State the cranial nerve nuclei (CNN) within midbrain. (Hint: 1 sensory, 3 motor)
Sensory nucleus: mesencephalic nucleus of V
Motor nuclei:
- motor nucleus of oculomotor
- Edinger-Westphal nucleus/accessory nucleus of oculomotor
- trochlear nucleus
What would your reasons be for histologically identifying the brainstem segment as the midbrain?
- cerebral aqueduct
- substantia nigra
- crus cerebri
- boxer shorts appearance
The pons has two histological zones, namely __________________ (ventrally) and ___________________ (dorsally).
basal pons (ventrally) and pontine tegmentum (dorsally).
What is contained in the basal pons?
- dispersed pyramidal tracts
- scattered pontine nuclei
- decussating pontocerebellar fibres (they form middle cerebellar peduncle)
- middle cerebellar peduncle
Discuss the cortico-ponto-cerebellar fibre system.
- consists of corticopontine fibres that arise from the cerebral cortex to the pontine nuclei in the basal pons
- from pontine nuclei, pontocerebellar fibers decussate and enter the cerebellum via the contralateral middle cerebellar peduncle
- terminate in the cerebellar cortex as part of the mossy fibre system
NB:
The type of afferents going to the cerebellum are grouped into:
a) climbing fibers: fibers coming from the inferior olivary nucleus (these fibers form the olivocerebellar tract)
b) mossy fiber system: any other fibre system there, usually terminates at the granular layer
The pontine tegmentum is the relatively compact zone adjacent to the 4th ventricle. It is continuous with the mesencephalic tegmentum. State the structures found in the pontine tegmentum.
- vestibular nuclei
- cochlear nucleus
- facial nucleus
- abducens nucleus
- medial longitudinal fasciculus
- spinal nucleus of V
- medial lemniscus
- spinal lemniscus
- trigeminal lemniscus
- reticular formation
- trapezoid body
[Diagram for reference: transverse section through upper part of pons]
[Diagram for reference: transverse section through lower part of pons]
[Diagram labelling the nuclei and tracts in the pons]
[What causes the bulge on the facial colliculus? The facial nucleus lies ventral to the abducens nucleus. Fibres of facial nerve coming from the nucleus arch dorsal to the abducens nucleus, forming the facial colliculus and making it to bulge.]
Note:
1. The trapezoid body is part of the auditory pathway where some of the axons coming from the cochlear nucleus decussate, or cross over to the other side before traveling on to the superior olivary nucleus. This is believed to help with localization of sound.
2. The medial nucleus of the trapezoid body (MNTB) is a distinct nucleus in the superior olivary complex that transforms excitatory input from the cochlear nucleus into a widespread inhibitory output to distinct auditory brainstem nuclei.
3. As you check the diagram, note how the lateral lemniscus is close to the superior olivary nucleus. The lateral lemniscus is part of the auditory tract of the brainstem. It starts at the level of the superior olivary complex and terminates at the inferior colliculus. The fibers of the contralateral dorsal cochlear nucleus reach the central nucleus of the inferior colliculus directly via the lateral lemniscus.
State the CNN within pons. (Hint: 3 sensory nuclei and 4 motor nuclei)
Sensory nuclei:
- chief sensory nucleus of V [upper pons]
- cochlear nuclei [lower pons]
- vestibular nuclear complex (has around 5 nuclei) [lower pons]
[Note that cochlear nuclei and vestibular nuclear complex are at the pontomedullary junction.]
Motor nuclei:
- trigeminal motor nuclei
- motor nuclei of facial
- abducens nucleus
- superior salivatory nucleus
- inferior salivatory nucleus (at pontomedullary junction)
[Diagram: transverse section through upper part of pons]
[Diagram: transverse section through lower part of pons]
