The Brainstem

The Brainstem

1. Introduction

   • in POSTERIOR CRANIAL FOSSA
     • basal portion of occipital bone
   • 3 Main functions
     • conduit for ascending and descending pathways transmitting info between spinal cord and cerebrum
     • contains important reflex centres associated with control of respiration, CV system and control of consciousness
     • contains nuclei III-XII (3-12)
       • (1+2 direct into cerebral cortex)
   • Reticular Formation
     • several functions - control ver level f consciousness, perception of pain and regulation of CV and resp systems
     • extensive connections with CN nuclei, cerebellum, brainstem and spinal motor mechanisms
       • which it influences movement, posture and muscle tone

2. Midbrain

   • External Features
     • Dorsal Surface
       • Rostrally, continuous with diencephalon of forebrain
       • Cerebral aqueduct passes thoughout its length
         • surrounding it = PERIAQUEDUCTAL GREY
           • ventral par lie TROCHLEAR and OCULOMOTOR NUCLEI
       • Dorsal aspect has 4 paired elevations
         • Superior Colliculi
           • part of visual system
         • Inferior Colliculi
           • part of auditory system
           • Trochlear Nerve (IV) emerges imediately caudal to it
     • Ventral Surface
       • Consists of CRUS CEREBRI
         • in the midline, they are separated by INTERPEDUNCULAR FOSSA
         • continuous rostrally with INTERNAL CAPSULE
         • Contains CORTICOBULBAR and CORTICOSPINAL FIBRES
           • that have left hemisphere via internal capsule on their way to spinal cord
   • Internal Structure
     • dorsal portion = TECTUM
       • Consists largely of INFERIOR AND SUPERIOR COLLICULI
         • In the midbrain - IC = part of ASCENDING ACOUSTIC PROJECTION
         • in SC - main afferents are corticotectal fibres originating from visual cortex of occipital lobe and frontal eye field of frontal
           • part of visual system
           • controlling eye movements
           • corticotectal fibres = involved in ACCOMODATION REFLEX
     • Ventral portion = TEGMENTUM
       • bounded ventrally by CRUS CEREBRI
       • Central portion = RED NUCLEUS
         • Motor control
       • ventral = SUBSTANTIA NIGRA
         • PARS COMPACTA
           • synthesise DOPAMINE
         • PARS RETICULATA

3. Pons

   • External Features
     • Dorsal Surface
       • rostral pons, walls converge until at pontomesencephalic junction
         • 4th ventricle becomes continuous with CEREBRAL AQUEDUCT
           • (goes throughout midbrain)
     • Ventral Surface
       • Dominated by transverse system of fibres
         • TRANSVERSE PONTINE FIBRES or PONTOCEREBELLAR FIBRES
           • Pass through contralat MIDDLE CEREBELLAR PEDUNCLE to enter cerebellar hemisphere
           • originate from cells in ventral pons - PONTINE NUCLEI
             • receives CORTICOPONTINE FIBRES
               • Important connection between cereberal and cerebellar cortices
                 • involved in COORDINATION OF MOVEMENT
   • Internal Structure
     • Divided into ventral and basal portion
       • TEGMENTUM
         • Ventral portion
           • PONTINE NUCLEI
             • No. of CN nuclei
               • ABDUCENS - lat rectus muscle
               • FACIAL MOTOR - facial expression muscles
               • TRIGEM MOTOR NUCLEUS - muscles of mastication
                 • trigem sensory nucleus reaches its max extend in pons
     • Rostral 1/2 SUPERIOR CEREBELLAR PEDUNCLES form lat walls of 4th ventricle
       • SUPERIOR MEDULLARY VELUM - roof
       • contains ventral spinocerebellar tract

4. Medulla Oblongata

   • continuous with spinal cord just below foramen magnum
   • External Features
     • Dorsal Surface
       • Dorsal SA of medulla the midline is marked by DORSAL MEDIAN SULCUS
         • Continuous with spinal cord
       • Caudal part - DORSAL COLUMNS
       • Rostrally, ventral canal moves more dorsally = in rostral medulla = opens into 4th ventricle
         • rostral 2/3 of ventricular floor is made up of pons
         • Fourth Ventricle
           • widest at pontomedullary junction
             • LATERAL RECESS extends to lat margin of brainstem
               • Here FORMEN OF LUSCHKA provides a passage for CSF within 4th ventricle
                 • to reach subarachnoid space surrounding brain
           • lat walls of rostral part are made up of
             • Superior and Inferior Cerebellar Peduncles
               • connect brainstem with cerebellum
     • Ventral Surface
       • PYRAMIDS run on either side of ventral median fissure
         • Decussation of Pyramids
         • Pyramidal or Cortiocspinal Tract
         • Lateral to = OLIVE
           • inside = INFERIOR OLIVARY NUCLEUS
             • Involved in CONTROL OF MOVEMENT
               • receives afferents from motor and sensory cortices of cerebral hemi and red nucleus of midbrain
               • main efferent connection is the cerebellum via inferior cerebellar peduncle
             • axons originating from ION
               • CLIMBING FIBRES
                 • end in Dentate nucleus and Purkinje cells of cerebellar cortex
   • Internal Structure
     • Caudal Medulla
       • at transition from spinal cord to medulla - considerable rearrangement
         • dorsal horn is replaced by caudal part of TRIGEMINAL SENSORY NUCLEUS
           • (NUCLEUS OF SPINAL TRACT OF TRIGEMINAL NERVE
           • extends whole length of brainstem into upper Cx segments
             • Latter, caudal part is associated with modalities of pain and temp
           • attaches to PONS
       • In the ventral medulla
         • most pyramid fibres decussate
           • passing lat, dorsally and caudally = LAT CORTICOSPINAL TRACT
     • Rostral Medulla
       • beneath ventricular floor = HYPOGLOSSAL NUCLEUS
         • Dorsal to this = MOTOR NUCLEUS OF VAGUS
         • Ventromedial to this
           • MEDIAL LONGITUDINAL FASCICULUS
             • Links vestibular nuclei with nuclei supplying extraocular muscles and helps coordination of head and eye movements
       • Most caudal part of ventricular floor
         • Area Postrema
           • blood brain barrier is absent
           • vomiting area
       • lateral floor of 4th V = VESTIBULAR NUCLEI
       • dorsalat part = INFERIOR CEREBELLAR PEDUNCLE
         • Dorsal and lat aspects = COCHLEAR NUCLEI
         • Medially = NUCLEUS SOLITARIUS
           • Receives visceral afferent fibres in facial, glossopharyngeal and vagus nerves
           • ventral to this = NUCLEUS AMBIGUUS

5. Lesion

   • Unilateral brainstem lesion
     • caused by stroke, tumour or MS
     • = ipsilat CN dysfunction, contralat spastic hemiparesis, hyperreflexia and extensor plantar response (UMNL)
       • contralat hemisensory loss and ipsilat incoordination
   • Bilateral Brainstem Lesion
     • destroys 'vital' centres controlling breathing and circulation = coma and death