Brain Stem Anatomy and Reflexes

Question 0

Name all the discernible landmarks on (and around) the brain stem posteriorly and describe functions

Answer 0

Superior colliculus
Inferior colliculus
Trochlear IV - This marks the transition from Midbrain to Pons. The trochlear nerve supplies the Superior Oblique muscle
Superior cerebral peduncles
Middle cerebral peduncles
Inferior cerebral peduncles
Rhomboid fossa - This is part of the 4th ventricle. The RF is mainly within the pons however does enter part of the superior medulla oblongata.
Sulcus limitans - Transverse white fibres that mark the floor of the 4th ventricle. The Vestibular area (4 Vestibular nuclei and 1 Cochlear nucleus) lies within this region and is consequently the origin of the Vestibulospinal tract. This marks the transition from the Pons to the Medulla Oblongata.
Gracile tubercle - contains gracile nucleus
Cunate tubercle - contains cunate nucleus (part of the Dorsal Column/ Medial Leminiscal pathway)

Question 1

Name all the discernible landmarks on (and around) the brain stem anteriorly and describe functions

Answer 1

1. Mammillary bodies
2. Cerebral crus - Contain Corticospinal tract fibres from the cortex
3. Pons - Contains Reticular Formation neurons. Contains cranial nerve V, VI, VII and VIII nuclei. The Pons contains transverse fibres due to decussation to the contralateral side that goes to the cerebellum. Fibres enter the cerebellum via the large middle cerebral peduncle.
4. Medullary pyramids - This is point at which 85% of corticospinal tract fibres decussate to form the lateral corticospinal tract fibres.
5. Olives

Question 2

Name all the discernible landmarks on the transverse section of the midbrain at the level of the superior colliculi

Answer 2

Superior colliculi - This contains cell bodies of the Rectospinal tract and also contains pretectal nucleus.
Pretectal nucleus - Where some optic fibres synapse with interneurons that bilaterally innervate Edinger-Westphal nuclei.
Cerebral aqueduct - CSF travels from the 3rd Ventricle inferiorly to the 4th Ventricle via the CA. Also, the CA marks the Tegmentum (anterior) and Tectum (posterior).
Edinger-Westphal nuclei - Parasympathetic nucleus, the fibres of which travel with the Oculomotor nerve. These fibres synapse at the cillary ganglon of the nasocillary nerve (a branch of the opthalmic nerve). The postganglionic neuron travels via short cillary nerves to supply sphincter pupillae muscles and cillary muscles to constrict and focus the lens.
Periaqueductal Gray Mater - Important in the descending modulation of pain.
Internal arcuate fibres of the Dorsal Column / Medial Leminiscal Pathway
Red Nucleus - This area is very vascular and looks red on newly cut specimens. This is the origin of the Rubrospinal tract.
Medial Longitudinal Fasiculus.
Substantia Nigra - Contains dopaminergic neuronal cell bodies. Important for the modulation of signals within the Basal Ganglia.
Cerebral Crus - Contains myelinated axons of descending Corticospinal tract fibres.

Question 3

What are the functions of the Brain Stem?

Answer 3

1. Intergrating functions - this includes respiratory centres, cardiorespiratory centres, heart rate centres, control of arousal, sleep centres.
2. Cranial Nerve Nuclei - Motor, Sensory and Parasympathetic nuclei
3. Contain long tracts

Question 4

What nuclei are situated within the brain stem?

Answer 4

Motor nuclei: III, IV, V (motor), VI, VII (motor), Nucleus ambiguus, XII.
Sensory nuclei: V (complex), Nucleus solitarius, Vestibular (4 nuclei), Cochlear.
Parasympathetic: Edinger-Westphal, Dosal motor, Superior/inferior salivatory nuclei.