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  • Brainstem consists of the medulla oblongata, pons and midbrain. Brainstem begins at the foramen magnum
    • Contains vital control centres: breathing, heart activity, visceral reflexes like vomiting and coughing
    • Regulates consciousness and sleep cycles
    • Source of the cranial nerves (that supply muscles, skin, glands, structures of the head/neck
    • Motor and somatosensory fibres that travel down the spinal cord also pass through here
    • Essential for maintenance of life

Gross anatomy: ventral

  • Medulla: medially has two white columns called pyramids (these are white matter structures)
    • Formed by a bundle of fibres – the corticospinal tract
  • Transmits motor information from the cortex to the spinal cord
    • At the medulla-spinal cord junction, 90% of the fibres in each pyramid cross sides to enter the opposite side of the spinal cord
  • Forms a deviation in the midline called the pyramidal decussation (crossing over of the pyramids - marks the beginning of the medulla)
  • Next to the pyramids area the olives – rostral lateral swellings of the medulla. Underneath these swellings are some nuclei that are associated with the cerebellum (motor learning)
  • Cranial nerves:
    • CN9-11 arise in a row down the lateral side of the olive
    • CN12 arises as rootlets medial to the olive
    • CN6-8 arise from the pontomedullary junction
  • Above the medulla is the pons
    • The base of the pons (basis pontis) forms the ventral surface
      • Midline groove is known as the basilar groove which corresponds to the basilar artery
        • Lateral extensions on each side are the middle cerebellar peduncles (MCP) that enter the cerebellum (massive bands of white matter; 10 million fibres in each)
        • The largest CN, CN5: trigeminal nerve arises from the MCP; main nerve for sensation from the head
  • Above the pons is the midbrain
    • Ventrally formed by 2 white structures known as cerebral peduncles (crus cerebri)
      • Separated by an interpeduncular fossa (hypothalamus sits in front of this)
        • CN3: oculomotor nerve arises here

Brainstem1.PNG

Gross anatomy: sagittal

  • Area dorsal to the cerebral peduncle is the tectum, which has both superior and inferior colliculi
    • Superior colliculus = associated with vision
    • Inferior colliculus = associated with hearing
  • A band of white matter lies in the roof of the front of the fourth ventricle, which is known as the superior medullary velum
    • Fourth ventricle has openings on the side and in midline for CSF to escape. The midline one is the median aperture of the fourth ventricle (down the bottom of the cerebellum). These allows the CSF to reach the subarachnoid space
    • Choroid plexus produces CSF, and some of it sits in the roof of the 4th ventricle

Brainstem2.PNG

Gross anatomy: dorsal

  • Dorsal surface of brainstem, cerebellum removed (looking down into the 4th ventricle)
  • Medulla: has two paired white columns; pathways for touch from upper and lower limbs
    • Medially: gracile fasciculus (NB: fasciculus is another name for a tract)
      • Carries somatosensory information from the lower limb and lower trunk
    • Laterally: the cuneate fasciculus
      • Carries somatosensory information from the upper limb and upper trunk
      • These are enlarged at the upper ends forming the gracile and cuneate tubercles
  • Within these are the gracile and cuneate nuclei upon which the nerve fibres terminate (synapse)
  • At the top of the gracile fasciculus is the gracile tubercle (formed by gracile nucleus)
  • At the top of the cuneate fasciculus is the cuneate tubercle (formed by cuneate nucleus)
    • These are where the sensory ns from the periphery will synapse
    • More rostrally, the central canal (which formed from the spinal cord) opens out into the fourth ventricle
      • Lies between the medulla, pons and cerebellum
      • The caudal limit is the obex: where the central canal opens from the 4th ventricle

Brainstem3.PNG

  • Fourth ventricle; diamond shaped; below this the medulla has a central canal (like spinal cord)
    • Floor: rhomboid fossa
      • Has a median fissure in the midline (dorsal median sulcus)
      • More laterally, has the sulcus limitans – the fetal remnant of the basal (motor) and alar (sensory) plates (separates these two plates); lies laterally to the midline. It is landmark separates motor and sensory: medial to it is motor nuclei, lateral to it is sensory (the vestibular area - for balance)
    • Roof: superior and inferior medullary vela
      • Inferior is also known as the choroid plexus and creates CSF
    • Lateral boundaries: cuneate tubercle, inferior cerebellar peduncle (ICP), superior cerebellar peduncle (SCP)
      • SCP sits on top of MCP, attached to the inside. The SCP takes information away from cerebellum up to forebrain. MCP takes information from pons into cerebellum
        • SCP head up and medially, and will cross over in the midbrain; this is the decussation (crossing) of the SCPs
      • There is an inferior one, which is very difficult to see (ICP), joins onto the inside of the MCP, down below; connects cerebellum with spinal cord/medulla
    • Junction between the pons and midbrain is called the isthmus

Brainstem4.PNG

  • Midbrain dorsally is formed by paired superior and inferior colliculi
    • Together are known as the tectum
    • CN4 (trochlear nerve) emerges caudal to the inferior colliculus and winds around MCP to reach the ventral surface
  • Junction between the pons and midbrain is the isthmus (just below the colliculi, which from the roof/tectum of the midbrain)

Brainstem5.PNG

Internal features (parasagittal section)

  • If we took sagittal section, there would be glial septum
  • This will show us internal features of brainstem
  • Brainstem consists of:
    • Central core of grey matter – reticular formation (extends through all levels) and cranial nerve nuclei; this is the tegmentum
      • Made up of a mixture of grey and white matter (reticular formation)
      • Involved in many functions including: regulation of some autonomic functions, pain transmission, regulation of sleep/wake cycles and some motor control. In the medulla = respiratory control centres (without, you need a respirator)
    • Anteriorly (left) there will be cerebral peduncles (sup) and pyramids (inf)
    • Ascending and descending pathways between the spinal cord, brainstem, cerebellum and forebrain
      • Largest: corticospinal tract, medial lemniscus – sensory transmission to the thalamus
    • Nuclei of cranial nerves
      • Spinal nucleus of the trigeminal nerve – replaces the dorsal horn of the spinal cord through the medulla
    • Other nuclei
      • Gracile and cuneate nuclei – caudal half of the medulla
        • Receive somatosensory information from corresponding fasciculi
    • Axons cross the midline as internal arcuate fibres and form the medial lemnisci that ascend to the thalamus
      • Inferior olivary nuclei – large, found in the rostral medulla (forms olive), anterior to the gracile and cuneate nuclei
        • Forms external swellings called the olives
      • Pontine nuclei – scattered grey matter in the base of the pons
        • Axons cross the midline forming the middle cerebellar peduncles entering the cerebellum
      • Superior and inferior colliculi – midbrain
        • Superior (visual reflex activity), inferior (auditory reflex activity)
      • Periaqueductal grey – surrounds the cerebral aqueduct (midbrain) (PAG = anterior to SC and IC)
        • Regulates pain transmission, involved in autonomic control and ?emotions?
      • Red nucleus – rostral part of midbrain
        • Involved in motor control
      • Substantia nigra – throughout midbrain (sits above the pons). Its cells contain dopamine, and is affected in parkinson's disease
        • Motor control
      • Above this is the red nucleus (looks vascular)

Brainstem6.PNG

Major tracts in the brainstem

medial leminiscus
corticospinal tract
spinothalamic tract
  • Medial lemniscus
    • Transmits:
      • Discriminative touch
      • Proprioception (joint sensations)
      • Vibration
      • Some light touch
    • Sensory axons from periphery
    • Cells of origin: gracile and cuneate nuclei (contralateral side), where they synapse
      • Axons cross in the caudal medulla, and ascend toward the thalamus (the medial lemniscus = the tract from the caudal medulla to the thalamus)
    • Terminates in the thalamus
    • Info from one side of the body travels on the ipsilateral side of the spinal cord, then crosses over in the caudal medulla
    • Fibres are organised depending on where they come from in the body
  • Corticospinal (pyramidal) tract
    • This is for voluntary movement. Skill, voluntary movements
    • Forms the pyramids
    • Passes via:
      • Base of the cerebral peduncle (midbrain)
      • Base of the pons
      • Pyramids of the medulla
      • Then crosses over, and ends up in ventral horn of spinal cord
    • 90% of fibres cross in the pyramidal decussation
    • terminates in spinal grey matter
  • Spinalthalamic tract
    • Transmits:
      • Pain
      • Temperature
      • Some light touch
    • Originates from the dorsal horn
      • Fibres cross soon after origin
    • Termination: thalamus (contralateral to origin)
      • Some fibres continue in brainstem through the reticular formation
    • Normally there is a chain of 3-4 neurons to get from skin to cortex (med lemniscus is 2nd in chain)
      • In this, it is different _______

Section of cervical spinal cord

Dorsal is toward top, ventral is bottom

  • Incoming sensory fibres (dorsal) will branch, some going to gracile, others to cuneate fasciculi
    • Corticospinal tracts in spinal cord, start in pyramids and then cross over
      • This slide is below that crossing, so 90% sit laterally and 10% sit ventrally
  • Higher up, there is a huge mass of grey matter as it crosses over in the medulla. This is the decussation (crossing of the pyramids)
  • Look at the levels of the sections to orientate yourselves
  • Median sulcus bends because the fibres are going to the other side, chopping up what was once the ventral and dorsal horn
  • The central canal is still there
  • Trigeminal nerve at this level takes over the function of the dorsal horn, so we call this the spinal trigeminal nucleus
  • Little bits of grey matter appear in the gracile/cuneate fasciculi, showing the beginnings of the nuclei
  • Moving up one level, just below level of olives, we can see the pyramids
    • This shows the reticular formation
    • At the back, the GN and CN nuclei are now light coloured, so we're arriving at grey matter (nucleus), no longer fasciculus
    • Axons cross the midline and form the medial lemniscus on opposite side
      • Swirly semicircular fibres around central canal (stained dark)
    • Spinothalamic fibres wrap themselves around the ventral horn. As they move up, they're squashed out to the side
  • When we go up, it will change because the 4th ventricle appears, as do the olives.

Rostral medulla

  • Trigeminal nerve = CN5, enters on sides of the MCP


Mid pons level

  • Medial lemniscus becomes horizontal (gets rotated at this level)

Midbrain

  • Substantia nigra stained white here
  • Crossing of superior cerebellar peduncles is visible
  • Corticospinal tract is within white matter of cerebellar peduncle
  • This section is below level of red nucleus
  • Just memorise the sections from the slides