General
- Brainstem has 4 components
- Long ascending and descending tracts
- Cranial nerve nuclei and their fascicles
- Cerebellar nuclei and their connections
- Reticular neurons and their processes
The 4 rules of the brainstem
There are 4 structures in the ‘midline’ beginning with M.
- In the midbrain only the medial structures hold this rule. The lateral (side) one does not.
- Use Medial and lateral structures to figure out whether it is medial or lateral then use the motor deficit level to each cranial nerve to figure out which level the lesion is at:
- These pathways pass through the entire length of the brainstem and can be likened to ‘meridians of longitude’ whereas the various cranial nerves can be regarded as ‘parallels of latitude’.
- If you establish where the meridians of longitude and parallels of latitude intersect then you have established the site of the lesion.
The 4 medial structures and the associated deficit are:
- Motor pathway (or corticospinal tract): contra lateral weakness of the arm and leg.
- Medial Lemniscus: contra lateral loss of vibration and proprioception in the arm and leg.
- Medial longitudinal fasciculus: ipsilateral inter- nuclear ophthalmoplegia (failure of adduction of the ipsilateral eye towards the nose and nystagmus in the opposite eye as it looks laterally).
- Motor nucleus and nerve: ipsilateral loss of the cranial nerve that is affected (3, 4, 6 or 12).
There are 4 structures to the side beginning with S.
The 4 lateral structures and the associated deficit are:
- Spinocerebellar pathways: ipsilateral ataxia of the arm and leg.
- Spinothalamic pathway: contra lateral alteration of pain and temperature affecting the arm, leg and rarely the trunk.
- Sensory nucleus of the 5th: ipsilateral alteration of pain and temperature on the face in the distribution of the 5th cranial nerve (this nucleus is a long vertical structure that extends in the lateral aspect of the pons down into the medulla).
- Sympathetic pathway: ipsilateral Horner’s syndrome, that is partial ptosis and a small pupil (miosis).
There are 4 cranial nerves in the medulla, 4 in the pons and 4 above the pons (2 in the midbrain).
The 4 motor nuclei that are in the midline are those that divide equally into 12 except for 1 and 2, that is 3, 4, 6 and 12 (5, 7, 9 and 11 are in the lateral brainstem).
The 4 cranial nerves above the pons are:
- Olfactory: not in midbrain.
- Optic: not in midbrain.
- Oculomotor: impaired adduction, supraduction and infraduction of the ipsilateral eye with or without a dilated pupil. The eye is turned out and slightly down.
- Trochlear: eye unable to look down when the eye is looking in towards the nose.
- Note
- The 3rd and 4th cranial nerves are the motor nerves in the midbrain.
- Thus a medial brainstem syndrome will consist of the 4 M’s and the relevant motor cranial nerve, and a lateral brainstem syndrome will consist of the 4 S’s and either the 9–11th cranial nerve if in the medulla, or the 5th, 7th and 8th cranial nerve if in the pons
The 4 cranial nerves in the pons are:
- Trigeminal: ipsilateral alteration of pain, temperature and light touch on the face back as far as the anterior two-thirds of the scalp and sparing the angle of the jaw.
- Abducent: ipsilateral weakness of abduction (lateral movement) of the eye.
- Facial: ipsilateral facial weakness.
- Auditory: ipsilateral deafness.
- Notes
- The 6th cranial nerve is the motor nerve in the pons.
- The 7th is a motor nerve but it also carries pathways of taste, and using the rule of 4 it does not divide equally in to 12 and thus it is not a motor nerve that is in the midline.
- The vestibular portion of the 8th nerve is not included in order to keep the concept simple and to avoid confusion.
- Nausea and vomiting and vertigo are often more common with involvement of the vestibular connections in the lateral medulla.
The 4 cranial nerves in the medulla are:
- Glossopharyngeal: ipsilateral loss of pharyngeal sensation.
- Vagus: ipsilateral palatal weakness.
- Spinal accessory: ipsilateral weakness of the trapezius and sternocleidomastoid muscles.
- Hypoglossal: ipsilateral weakness of the tongue.
- Note
- The 12th cranial nerve is the motor nerve in the midline of the medulla.
- Although the 9th, 10th and 11th cranial nerves have motor components, they do not divide evenly into 12 (using our rule) and are thus not the medial motor nerves.
Topographic organisation of brainstem
Cranial caudal direction
- Mesencehphalon (midbrain)
- Pons
- Medulla oblongata
Anterior posterior direction
- Brainstem is organized into 4 Major Parts:
Ventricular cavity
- Contiguous with the central canal of the spinal cord.
- Comprises
- Central canal the cerebral aqueduct of Sylvius (mid-brain).
- 4th ventricle
- Foramen Margendie
Roof (tectum)
- Overlying the ventricular cavity is the roof of the brainstem,
- Comprises
- Choroid plexus and Tela choroidea of the fourth ventricle (medulla),
- Cerebellum (pons), and
- Tectum (midbrain).
- Consists of corpora quadrigemina
- Paired superior colliculi of the optic system
- Paired inferior colliculi of the auditory system.
Tegmentum
- Floor of the ventricular cavity
- Contains
- Cranial nerves and their nuclei
- Major long ascending tracts
- Reticular formation
- Together with the crux cerebri forms the cerebral peduncle
- Contains the brain stem nuclei
Base
- The basilar part of the brainstem
- Mesencephalon: Cerebral peduncles
- Comprises
- Pyramids of the medulla
- Ventral pons
- Crura cerebri of the midbrain
- Contains
- Major long descending tracts,
- Corticospinal
- Corticobulbar
- Corticopontine tracts
Functional organisation
Control centres
Nuclei for cranial nerves CN3-CN12: divided into 4 longitudinal columns
Motor nuclei
Somatic motor nuclei
- Nucleus CN3
- Nucleus CN4
- Nucleus CN6
- Nucleus CN11
- Nucleus CN12
Visceral motor nuclei
- Visceral motor nuclei
- Visceral oculomotor nuclei (Edinger westphal) CN3
- Superior salivatory nuclei CN7
- Inferior salivatory nuclei CN9
- Dorsal vagal nuclei CN10
- Nuclei of branchial arch nerve
- Trigeminal motor nuclei CN5
- Facial nuclei CN7
- Nucleus ambiguus CN9, CN10, CN11, cranial root is in fact considered to be part of CN10
Sensory nuclei
Somatic sensory and vestibulocochlear nuclei
- Sensory nuclei associated with CN5
- Mesencephalon nuclei:
- Special feature pseudounipolar ganglion cells (displace sensory ganglion)
- Provide direct sensory inner action for muscle of mastication
- Principal (pontine) sensory nuclei
- Spinal nuclei
- Nuclei of the vestibulocochlear nerve
- Vestibular part
- Medial vestibular nuclei
- Lateral vestibular nuclei
- Superior vestibular nuclei
- Inferior vestibular nuclei
- Cochlear part
- Anterior cochlear nucleus
- Posterior cochlear nucleus
Visceral sensory nuclei
- Nucleus of the solitary tract (nuclear complex)
- Superior part
- Special visceral afferents (taste): CN7, CN9, CN10
- Inferior part
- General visceral afferents from CN9, CN10
Coordination centre for motor control
- Red nucleus
- Substantial migration
Pontine nuclei
- Connected to cerebellum
Nuclei of the posterior funiculars
- Termination point of sensory pathways
Interconnection of auditory and visual stimuli
- Lamina tecti
Thoroughfare
Towards and away from the brain
Towards and away from cerebellum through
Away cerebellum
- Descending autonomic tracts
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