CHAPTER 6. BRAIN STEM

Transcription

1 CHAPTER 6. BRAIN STEM Case IV: A Comatose Man Background Information, frame 1-42 Introduction The brain stem consists of the midbrain, pons, and medulla oblongata in descending order of the neuroaxis (Fig. 6-1). Descending tracts (e.g. corticobulbar and corticospinal tracts) travel from the internal capsule to (corticobulbar tracts) or through (corticospinal tract) the brain stem, whereas ascending tracts carrying afferent information to the cortex pass from or through the brain stem to the thalamus. Dorsal to the midbrain, pons, and rostral medulla is the cerebellum, which is NOT considered to be part of the brain stem. The cerebellum is connected to the midbrain, pons, and rostral medulla by the bilateral superior, middle, and inferior peduncles respectively. Superiorly, the cerebellum is separated from the cerebrum by the tentorium cerebelli: its free edge, the tentorial notch, encircles the lateral and posterior aspects of the midbrain. The ventral surface of the brain stem faces the bony wall of the basiocciput. Inferiorly, the caudal portion of the medulla oblongata passes through the foramen magnum, and continues as the spinal cord immediately inferior to the foramen. Thalamus Midbrain Cerebellum Pons Medulla Oblongata: Rostral portion Caudal portion Fig. 6-1 Components of the Brain Stem 40

2 In summary, the brain stem contains: 1. Nuclei (e.g. cranial nerve nuclei, visual & auditory reflex nuclei, pontine nuclei, reticular formation); 2. Descending tracts (e.g. from cortex to ventral horn; from hypothalamus to intermediolateral horn); 3. Ascending tracts (e.g. from the spinal cord and sensory cranial nerve nuclei); 4. Neurotransmitter nuclei (e.g. substantia nigra in the midbrain); 5. Reticular formation: A diffuse group of nuclei responsible for sleep, wakefulness, control of autonomic functions, and manufacturing of neurotransmitters that modulate pain, movement, and hypothalamic functions; 6. Cerebellar peduncles: axonal fibres travelling into (afferent) or leaving (efferent) the cerebellum; 7. Ventricular system: cerebral aqueduct (midbrain) and fourth ventricle (pons and medulla). The brain stem is vulnerable to displacement and compression by tumour growth and physical trauma. This is due to its location in a confined space and to the compactness with which its internal structures are arranged. Physical compression will also block the ventricular system (e.g. the narrow cerebral aqueduct) resulting in hydrocephalus proximal to the site of compression. Case 5: A Comatose Man Case Discussion, frame 1-5 Objective 1: Identify the COMPONENTS of the brain stem in the sagittal section. (Fig. 6-2) Midbrain. This is a small segment situated inferior to the bilateral diencephalic nuclei and the internal capsule. The cerebral aqueduct (of Sylvius) passes through the substance of the midbrain and forms a narrow conduit between the third and the fourth ventricles. The area dorsal to the cerebral aqueduct is the tectum. The area immediately ventral to the cerebral aqueduct is the tegmentum. The area ventral to the tegmentum is the crus cerebri. The tegmentum and crus cerebri together are known as the cerebral peduncle Pons is separated by the dorsally placed cerebellum by the fourth ventricle. To expose the dorsal surface, the cerebellum must be removed by severing the cerebellar peduncles which attach the cerebellum to the midbrain, pons, and rostral medulla Medulla Oblongata. Like the pons, the rostral portion of the medulla is open, separated from the cerebellum by the 4th ventricle; whereas the arrangement of gray and while matter in the caudal portion of the medulla is closed, i.e. it resembles the spinal cord. In reference to the fourth ventricle, the open portion of the fourth ventricle is dorsal to the rostral medulla, and the closed portion is within the caudal medulla. Frame

3 Midbrain: Tectum Tegmentum Crus cerebri Cerebral aqueduct Pons: Open 4 th ventricle Rostral Medulla: Open 4 th ventricle Caudal Medulla: Closed 4 th ventricle Fig. 6-2 Brain Stem and Ventricular System To expose the dorsal surface of the brain stem, the cerebellum must be removed by severing the three bilateral pairs of cerebellar peduncles and a thin layer of connective tissue which binds the cerebellum to the brain stem. The open portion of the fourth ventricle lies between the cerebellum and the dorsal surface of the pons and rostral medulla. Removal of cerebellum thus exposes a diamond-shaped area representing the fourth ventricle. The floor of the open fourth ventricle is formed by the dorsal surface of the pons and rostral medulla. Frame 10 42

4 Objective 2: Identify the SURFACE LANDMARKS and MAJOR DEEP STRUCTURES of the brain stem Midbrain (Fig. 6-3, 6-7) Frame 3, 6, The vertical striations on the ventral surface of the crus cerebri represent the descending corticopontine, corticobulbar, and corticospinal tracts, destined for the pons, the brain stem, and the spinal cord respectively. In the midbrain some fibres of the corticobulbar tract decussate and terminate (synapse) in the contralateral cranial nerve nuclei of the oculomotor (CNIII) and trochlear (CN IV) nerves. On the dorsal surface of the midbrain tectum are two pairs of surface elevations raised by nuclei in the midbrain tectum. The rostral pair is the superior colliculi (nuclei) responsible for visual reflex. The caudal pair is the inferior colliculi (nuclei) responsible for auditory reflex. Visual and auditory stimuli of a sudden and threatening nature (e.g. a fast approaching object or a loud noise) travel from the retina/cochlear to synapse in the nuclei within the respective colliculi. Post-synaptic fibres (tectospinal tract) descend to upper spinal cord segments and synapse with the alpha motor neurons to cause reflexive reaction of neck and upper limb muscles to the stimuli. In addition, neurons of the superior colliculi project to cranial nerve nuclei (III, IV and VI) responsible for movements of the extra-ocular muscles to direct visual attention to the source of the stimulus. The colliculi are collectively called corpora qudrigemina (bodies of the quadruplets). Trigeminal nucleus extends from the midbrain to the pons and medulla. The portion of the trigeminal nuclei located in the midbrain is called Mesencephalic portion of Trigmeinal Nuclei. Reticular formation, a bilateral strip of gray matter that contains numerous neurotransmitter and autonomic nuclei, also extends from the midbrain to the medulla oblongata in the tegmental area of the brain stem. Ventral view Sagittal Section Dorsal View Crus cerebrei Descending tracts Sup. Colliculi Cranial Nerve Nuclei III, IV, V Inf. Colliculi Reticular Formation Fig. 6-3 Anatomical Structures of the Midbrain 43

5 Pons (Fig. 6-4, 6-7) Frame 4, 7, 13 In the ventral portion of the pons, corticopontine tract terminates in the pontine nuclei which scatter in this portion of the pons. The axonal fibres of the pontine nuclei (ponto-cerebellar tract) cross the midline of the pons and travel beneath the surface of the contralateral pons to enter the cerebellum via the middle cerebellar peduncle. Therefore the horizontal striations seen on the ventral surface of the pons belong to the ponto-cerebellar tract. The corticobulbar and corticospinal tracts travel deep to the pontine nuclei and the ponto-cerebellar tract. In the pons, fibres of the corticobulbar tract decussate and terminate in the cranial nerve nuclei of the motor nucleus of trigeminal (CN V), abducent (CN VI), and facial (CN VII) nerve. The corticospinal tract passes through the pons and the medulla to the spinal cord. From the dorsal view, the surface of the pons (and rostral medulla) is marked by three longitudinal grooves and two bilateral rows of elevations. A median groove, the median sulcus, is flanked by the bilateral sulcus limitans. Cranial nerve nuclei deep to the elevations between the median sulcus and the sulcus limitans are derived from the basal plate of the primitive neural tube. Therefore these medial rows of nuclei serve motor functions (Abducent nucleus, motor nuclei of Facial and Trigeminal). Cranial nerve nuclei deep to the elevation lateral to sulcus limitans are derived from the alar plate, therefore these lateral rows of nuclei serve sensory functions ( Principal portion of Trigeminal nucleus, Vestibular nuclei, and Cochlear nuclei). Ventral view Sagittal Section Dorsal View Pontine nuclei Corticopontine tract Pontocerebellar tract Fig. 6-4 Anatomical Structures of the Pons Corticobulbar tract to Motor nuclei of CN V, VI, and VII Corticospinal tract Grooves: Median sulcus Sulcus limitans Cranial Nerve nuclei: Trigeminal Abducens Facial Vestibular and Cochlear 44

6 6.2.3 Medullar Oblongata. (Fig. 6-5, 6-7) Frame 4, 8, The bulging vertical striations on the ventral surface of the medulla oblongata form the pyramids, which represent the descending fibres of the corticospinal tract. The bilateral pyramids are separated by the median sulcus. In the most caudal part of the pyramids (junction between medulla and spinal cord), 90% of the fibres in the corticospinal tract cross the midline at the decussation of the pyramid and continue to descend on the contralateral side of the spinal cord as the lateral corticospinal tract. The remaining 10% do not cross and descend ipsilaterally as the anterior corticospinal tract. From the dorsal view, the rostral portion of the medulla resembles the dorsal surface of the pons. The median sulcus and the bilateral sulcus limitans extend into this region. The medially placed nuclei include the motor nuclei of CN IX, X, and XI and the hypoglossal nucleus (XII). The laterally placed nuclei include the Descending portion of trigeminal nucleus, and the sensory nucleus of CN IX, X, and XI. The caudal portion of the medulla resembles the spinal cord, therefore the dorsal structures, similar to those in the spinal cord and midbrain, serve sensory function. The dorsal surface features four linearly arranged elevations. The medial pair, separated by the median sulcus, is named tubercle gracile; the lateral pair, tubercle cuneate. The elevations represent groups of nuclei within the tubercles. Ascending sensory fibres from the spinal cord synapse with cell bodies in these nuclei which in turn projects to tertiary sensory neurons in the ventral-posterior nucleus in the thalamus. In the medulla oblongata, the remaining fibres of the corticobulbar tract decussate and terminate in the cranial nerve nuclei of the glossopharyngeal (CN IX), vagus (CN X), accessory (CN XI), and hypoglossal (CN XII) nerves. Ventral view Sagittal Section Dorsal View Pyramid (corticospinal tract) Decussation of the pyramids Olive Fig. 6-5 Anatomical Structures of the Pons Corticobulbar tract to Motor Nuclei of CN IX, X, XI, and XII Rostral Medulla: Median sulcus Sulcus limitans Caudal Medulla: Tubercle Gracile Tubercle Cuneate 45

7 Objective 3: Describe the general composition of the three CEREBELLAR PEDUNCLES. Planning and execution of movement synergy require coordination by the basal ganglia, cerebellum, ideomotor, premotor and primary motor cortex. At the same time, the status of ongoing movements is monitored as proprioceptive information is relayed from the joints and muscle spindles to the cerebellum. The cerebellar peduncles are axonal bundles, the fibres of which are either afferent (towards the cerebellum) or efferent (away from the cerebellum). The following description of each peduncle includes only the major afferent or efferent tracts related to the generalized mechanism described above (Fig. 6-6): Inferior peduncle: It contains ipsilateral afferent fibres of: a) proprioceptive information from muscle spindles and joints via the dorsal spinocerebellar tract, and b) equilibrium information from the vestibular nuclei in the pons Middle peduncle: It contains afferent pontocerebellar tract from the contralateral pontine nuclei. Recall that the pontine nuclei receive projections form the cerebral cortex via the corticopontine tract (Section 6.2.2, page 43) Superior peduncle: It contains efferent information to the motor areas in the cerebral cortex via the midbrain reticular formation and thalamus. In addition, Efferent: Information from the cerebellum also projects to the vestibular nuclei which initiate movements via the vestibulospinal tract without cortical influence. Frame Inferior Peduncle conveys the following into (afferent fibres) the cerebellum: Proprioceptive information from spinal cord Equilibrium information from the vestibular nucleus M I S S I M Middle Peduncle conveys the following into (afferent fibres) the cerebellum: Intent to move information from the cerebrum via (corticopontine and) pontocerebellar tract Inferior Peduncle conveys the following from (efferent fibres) the cerebellum: Movement coordination information to the cerebrum via the thalamus 46 Fig. 6-6 Cerebellar peduncles

8 Objective 4: Locate the approximate POSITIONS OF CRANIAL NERVE NUCLEI in the brain stem. (FYI only. EXCEPTION: Cochlear Nucleus and Auditory Pathway Page 46) As indicated earlier, nuclei of cranial nerve III (oculomotor) and IV (trochlear) are located in the midbrain, those of CN V (principal portion of trigeminal, which also extends into the midbrain and medulla), VI (abducens), VII (facial), VIII (vestibular and cochlear) in pons, and those of CN IX (glossopharyngeal), X (vagus), VI (accessory), and XII (hypoglossal) in medulla oblongata. Note that nuclei of CN III to XII are arranged in a rostral to caudal direction in the brain stem (Fig. 6-7). (Descending Nuc. V) Fig. 6-7 Location of the cranial nerve nuclei in the brain stem. It should be noted that while CN I, II, IV, VI, VIII, XII each have only a single type of functional fibre as they emanate from the surface of the brain, the remaining have two or more functional types (e.g. CNIII carries both somato-motor fibres to the extra-ocular muscles and parasympathetic preganglionic fibres to the ciliary ganglion). Each type of functional fibre has its own discrete nucleus within the brain; therefore a multi-functional cranial nerve, although seen as a single nerve as it emanates from the brain, is a collection of nerve fibres originating from (efferent) or entering (afferent) different nuclei which are located in close approximation in the brain. In some instances, cranial nerves with similar functions share the same nucleus specific for that function. A study on the approximate location of the cranial nerve nuclei in the brain stem requires a thorough understanding of 1) functional classification of the nervous system, and 2) the development of the cerebellum and of the brain stem from the neural tube, particularly that of the pons and rostral medulla. Please review these topics before proceeding to the next section. 47

9 6.4.1 The result of alar and basal plates development in the pons and rostral medulla regions. Frame As a result of the rhombic lips developing into the cerebellum which occupies an area dorsal to the fourth ventricle and the pons and rostral medulla, the alar and basal plates in these regions are displaced to lie in a linear arrangement: alar plates are placed laterally and basal plates, medially. The boundary between the basal plates is marked on the dorsal surface by the median sulcus, and the boundary between the alar and basal plates is marked bilaterally by the sulcus limitans. Basal plates give rise to motor (efferent) nuclei whereas alar plate, sensory (afferent) Arrangement, functions and projections of the cranial nerve nuclei in the brain stem. Recall that in the T1 L2 and S 2-4 segments of the spinal cord, autonomic (visceral) functions are served by cell bodies in the intermediolateral column which is placed between the somato-sensory (dorsal horn) and somato-motor (ventral horn) areas. Similarly in the brain stem, autonomic functions are served by nuclei developing in the region beneath the sulcus limitans, i.e. between the alar and basal plates. Based on the above, the general arrangement of cranial nerve nuclei on one side of the pons and rostral medulla region, as indicated by the functions they serve, can be illustrated as follows: (Lateral) (Sulcus Limitans) (Median Sulcus) Somatic Afferent Visceral Afferent Visceral Efferent Somatic Efferent Fig. 6-8 General arrangement of cranial nerve nuclei in the pons and rostral medulla. Frame 21 For both somatic and visceral nuclei, there is a further differential classification in the origin of the nuclei: A. AFFERENT (nuclei arrangement from lateral to medial in the brain stem): Frame 22 A.1 General Somatic Afferent Frame Trigeminal nucleus: CN V, IX and X convey peripheral information of proprioception, touch, pain and temperature from the face, tongue, pharynx, and larynx to the elongated trigeminal nucleus (Fig. 33). This nucleus extends along the length of the brain stem: the portion in the midbrain, pons, and medulla is named mesencephalic, principal, and descending (or spinal tract) nucleus respectively. Upon entering the pons and medulla, fibres of CN V, IX, X travel alongside the trigeminal nucleus and enter all three parts of the nucleus. After synapsing in the nucleus, axons of the post-synaptic neurons decussate to the contralateral side and project to the thalamus. A.2 Special Somatic Afferent Frame 25 CNVIII conveys auditory and equilibrium information from the cochlea and semicircular canals to the separate cochlear nucleus and vestibular nucleus in the pons. a. Cochlear nucleus (Fig. 6-9): From the cochlear nucleus, post-synaptic fibres 48

10 ascend bilaterally as lateral lemniscus through a series of nuclei in the brain stem before reaching the medial geniculate nucleus in the thalamus and the finally the auditory cortex in the ipsilateral temporal lobe. Details of the ipsilateral pathway are as follows: Case VI: Left-sides Bruises Background Information, frame 7-8 Signal from cochlea ipsilateral ventral cochlear nucleus superior olivary nucleus inferior colliculus medial geniculate ganglion ipsilateral primary auditory cortex. Detailed knowledge of the contralateral pathway, unless required in other courses, is not needed in this course. Auditory Cortex (Temporal lobe) Auditory Cortex Midbrain/thalamus Midbrain Pons Ventral brain stem Dorsal brain stem, cerebellum removed Pons/medulla Ventral Fig. 6-9 Auditory pathway 49

11 b. Vestibular nucleus: From the vestibular nuclei, signals are relayed to the cerebellum (vestibulo-cerebellar tract for coordination of on-going movements), spinal cord (vestibulo-spinal tract for reflexive adjustment of movements), and cerebral cortex (for cognitive awareness). Knowledge on the details of these pathways is not necessary for this course. A.3 Specific Visceral afferent Frame 26. Title should read Cranial Nerve Nuclei: VISCERAL Afferent CN VII (facial), IX (glossopharyngeal) and X (vagus) carry special sensation of taste to the gustatory nucleus in pons/medulla. A.4 General Visceral Afferent Frame These fibres carry impulses from the visceral structures (i.e. hollow organs and glands) within the thoracic, abdominal cavities via CN IX and X to the solitary nucleus. B. EFFERENT (nuclei arrangement from lateral to medial in the brain stem): Frame 29 B.1 General Visceral Efferent: Frame These nuclei provide parasympathetic preganglionic fibres to the peripheral ganglia for the innervation of smooth and cardiac muscles: a. Edinger-Westphal nucleus: sends fibres via CN III to ciliary ganglion (pupil dilation and accommodation). b. Superior salivatory nucleus sends fibres via CN VII to TWO ganglia. i. Sphenopalatine/pterygopalatine ganglion (lacrimal and nasal mucous secretion). ii. Submandibular ganglion (submandibular, sublingual, and lingual mucous glands). c. Inferior salivatory nucleus sends fibres via CN IX to otic ganglion (parotid glands). d. Dorsal motor nucleus of X sends fibres via CN X to cardiac and pulmonary plexus, and plexus responsible for the caudal part of foregut and the entire midgut. B.2 Special Visceral Efferent Frame 32 The efferent fibres of these nuclei innervate muscles developed from the mesoderm of the branchial arches in the cranial portion of the foregut where conscious control of the musculature is possible, they are: a. First Arch: Motor nucleus of V sends fibres via CN V3 to muscles of mastication, mylohyoid, and anterior belly of digastric muscle. b. Second Arch: Facial nucleus sends fibres via CN VII to muscles of facial expression, stylohyoid, and post belly of digastric muscle. c. Third Arch and Fourth Arch: Nucleus ambiguus sends fibres via CN IX, X, and XI to muscles of pharynx and larynx. 50

12 Spinal accessory nucleus is the continuation of nucleus ambiguus in upper segment of the spinal cord. Spinal accessory nerve from this nucleus ascends through foramen magnum to join the cranial accessory nerve; it only innervates trapezius and sternomastoid which are not of branchial origin. Frame 33 B.3 General Somatic Efferent: These nuclei innervate skeletal muscles developed from the mesoderm of the somites (as opposed to mesoderm of the branchial arches). Specifically, they are the extraocular and tongue muscles: a. Oculomotor nucleus sends fibres via CN III (oculomotor) to five extraocular muscles b. Trochlear nucleus sends fibres via CN IV (trochlear) to superior oblique muscle. c. Abducens nucleus sends fibres via CN VI (abducens) to lateral rectus muscle. d. Hypoglossal nucleus sends fibres via CN XII to intrinsic (longitudinal, vertical, and transverse on the surface) and extrinsic (genioglossus, hyoglossus, and styloglossus) muscles of the tongue. Objective 5: Locate the exact position of the cranial nerve nuclei and the emergence of cranial nerves from the brain stem. (FYI ONLY) SEFL-DIRECTED LEARNING: Please review Fig Frame Fine details in the relative arrangement of the white and gray matters in the brain stem are strictly for future reference in understanding the symptoms in brain stem lesion. 51

13 Objective 6: Describe the pathways for LIGHT REFLEXES and CONJUGATED MOVEMENTS OF THE EYE Light Reflex A simple test to examine the integrity of the midbrain is to shine a light in the eye. The normal responses would be pupil constriction in the eye where the light is shone (direct light reflex), and also in the opposite eye (consensual light reflex). The neural pathways when a light is shone into the left eye are as follows (Fig. 6-10): A. Direct Light Reflex: a. Signals from the left optic tract project directly to the left pretectal area of the midbrain where the projection synapses with an interneuron. b. The interneuron travels into the left Edinger-Westphal portion of the Oculomotor nucleus and synapses with the cell bodies of the preganglionic parasympathetic fibres. c. Preganglionic parasympathetic fibres leave the Edinger-Westphal nucleus via CN III to reach the ciliary ganglion, from which postganglionic fibres travel with ciliary nerves of CN V1 to the constrictor pupillae of the left eye. B. Consensual Light Reflex: a. Fibres from the left optic tract decussate to the right pretectal area. From here, the pathway mirrors that of the direct light reflex pathway. Retina Optic tract Pretectal area Edinger-Westpahl Nucleus Optic nerve Ciliary ganglion Constrictor pupillae Fig Horizontal section of the midbrain. 52

14 6.6.2 Saccadic Eye Movement Coordination of eye movement requires the integral collaboration of the cortical association areas and cranial nerve nuclei in the midbrain and pons. It is essential for the following tasks: 1. Fast conjugate tracking of moving objects (saccadic). 2. Slow tracking of moving objects (pursuit). 3. Alignment of both eyes to track a target that moves closer or farther away (vergence). 4. Usage of vestibular signals to move the eyes in an equal and opposite direction if the viewer's head or body moves (vestibulo-ocular). Frame The neural pathway in coordinating non-visually guided saccadic eye movement is as follows: 1. Association areas in cortex (right side, for example) send descending fibres to synapse in 2. the contralateral paramedian pontine nucleus of reticular formation (PPRF) in the brain stem. From PPRF, an interneuron relays signal to. 3. the left abducens nucleus which in turn sends signal simultaneously to: a. Left lateral rectus muscle to abduct the left eye, and b. Right oculomotor nucleus via the medial longitudinal fasciculus (MLF); the right oculomotor nucleus in turn signals i. the right medial rectus muscle for adduction of the right eye. For visually guided saccadic movement, the initial command comes from the visual areas of the occipital lobe (e.g. telling the patient to stare at the tip of the ball-point pen on the left side without head movement). 53