The Medulla
The medulla is the point where the pons meets the spinal cord. It is cone-shaped.
Structures and Connections
Anteriorly, the medulla is dominated by the pyramids, carrying descending corticospinal fibres from the cortex to the spinal cord. The inferior olives lie laterally of the pyramids.
Posteriorly, the fasiculus gracilis and cuneatus of the dorsal column of the spinal cord end in the nucleus gracilis (medial) and nucleus cuneatus (lateral), seen on the surface as tubercles.
nucleus gracilis and cuneatus
The medulla is the site of decussation of the dorsal column, which carries sensory information pertaining to touch, proprioreception, and vibration
The fasciculus gracilis and cuneatus, terminate in their respective nuclei, with second order arcuate fibres arching anteriorly and medially, crossing the midline at the sensory decussation
- form medial lemniscus, which still maintains somatotopy
- site of disparate pathway overlap,
- from here, fibres ascend to the thalamus (VPl)
Tracts and Nuclei
- pyramidal tracts
- inferior olive
- nucleus ambiguus
- dorsal motor nucleus
- solitary tract and nucleus
- hypoglossal nucleus
- spinal tract and nucleus of the trigeminal
- spinothalamic tract
- medial longitudinal fasciculus
- spinocerebellar tract
- reticular formation
Pyramidal tracts
- on the anterior side of the brainstem
- transmission of motor signals from the cortex to the spinal cord
- part of the corticospinal tracts; site of decussation of 85% (lateral corticospinal tracts)
Inferior Olive
the olive is a large, prominent nucleus in medulla
input
- motor cortex
- red nucleus (cerebellum-midbrain-medulla-cerebellum loop)
- spinal cord
- vestibular nuclei
output
- transmits to the cerebellum via the inferior peduncle via crossed olivocerebellar fibres
- give rise to 'climbing fibres' in the cerebellum which have important effects on cerebellar output
- Climbing fibres are glutaminergic axons arising from the inferior olivary nucleus that have a 1:1 relationship with Purkinje cells. Climbing fibres, which have about 200 synaptic contacts with their partners, generate massive EPSPs of about 40 mV, evoking a burst of spikes in Purkinje cells.
function
Medulla Cranial Nerves
- glossopharyngeal (9th)
- accessory (11th)
- hypoglossal nerve (12th) emerges between the olive and pyramidal nerve
- hypoglossal nucleus lies just beneath the hyopglossal trigone
- lower motor neurons exit medulla between medulla and pyramids
- lesion here causes the tongue to deviate ipsilaterally
nucleus ambiguus
- just above inferior olive in lateral medulla
- motor nucleus associated with CN IX, X
- outflow provides motor innervation to striated muscle of the palate, pharynx, larynx, and upper 1/3 of the esophagus
- problems can cause dysphagia
dorsal motor nucleus of vagus
- lies subjacent to vagal trigone
- provides parasympathetic innervation to thorax, abdomen, to proximal 2/3 of transverse colon
solitary tract and nucleus
- carries visceral sensory input derived from centrally distributed fibres from VII, IX, and X
- synapse in 2nd order neurons in surrounding nucleus
- taste (VII, IX) and visceral (IX, X) sensation
hypoglossal nucleus
- located posteriorly throughout most of medulla
spinal tract and nucleus of the trigeminal
- extends from CN V after entry to pons
- pain and temperature information for face, mouth, teeth, anterior 2/3 of tongue, scalp, and head from CN V)
- also receives pain and temperature input from VII, IX, and X
- becomes continuous with substantia gelatinosa of the spinothalamic tract, making a continuous series of nuclei dealing with pain and temperature
spinothalamic tract
continues superiorly, maintaining its anterolateral position just anterior to spinal tract of the trigeminal
medial longitudinal fasciculus
The MLF is involved in coordinating head and eye movements.
spinocerebellar tract
Proprioreceptive information traveling up the spinocerebellar tract leaves the brainstem in the medulla, entering the cerebellum via the inferior cerebellar peduncle.
reticular formation
- in centre of medulla; intermingled in gray and white matter