The Midbrain
The midbrain is an important conduit to the cortex, also containing several important cranial nerve nuclei.
Structures and Connections
The midbrain is an extension if the pons inferiorly. Superiorly, it makes direct connection with the thalamus, the internal capsule, and the basal ganglia (I think...)
The cerebral aqueduct runs through the midbrain, separating the posterior tectum from the anterior tegmentum.
The midbrain is characterized by the paired superior and inferior colliculi on its posterior surface and the cerebral peduncles (basis pedunculi) on its anterior surface.
The oculomotor (III) nerve emerges between the two cerebral peduncles
The trochlear (IV) nerve leaves the midbrain-pons junction just below the inferior colliculus.
The periaqueductal gray
The PAG is an area of gray matter surrounding the cerebral aqueduct, if you can believe it. It is an important part of the decending pain control system.
Superior Colliculus
location and structure
- contains Edinger-Wesphal nucleus adjacent to oculomotor nucleus, with parasympathetic neurons for pupillary constriction
function
- coordinates head and eye movements during tracking
inputs
- retinal ganglion cells via optic tract
- visual cortex
- somatosensory input via spinomesencephalic tract
outputs
- lateral geniculate nucleus of thalamus
- tectobulbar pathway: lower motor neurons of CN III ,IV, and VI
- tectospinal tract: LMNs of cervical spinal cord
- reticular formation
Inferior Colliculus
location and structure
- on the posterior midbrain, the inferior colliculi are paired bumps
function
- auditory relay (see brainstem fig for connections)
inputs
- auditory pathway
outputs
- superior colliculus
- via inferior brachium, to the medial geniculate nucleus in the thalamus
cerebral peduncles
large collection of fibres on the anterior midbrain, receiveing information from the internal capsule and transmitting it to the medullary pyramids.
- includes corticospinal tract and motor fibres from corticopontine tract
Tracts and Nuclei
Red Nucleus
The red nucleus is a large, richly vascularized nucleus. It is important in motor coordination.
It acts as a relay and modifying station, receiving information from the cerebellum via the superior cerebellar peduncle (which decussates in the midbrain) and sending it to the thalamus and contralateral spinal cord via the rubrospinal tract. This is important for tone of flexor muscles. It also sends signals to the inferior olive which then return to the cerebellum
Ventral Tegmental Area
The VTA lies medially of the subtantia nigra pars compacta, and comprises of dopaminergic neurons with projections to the prefrontal cortex (mesocortical) and parts of the limbic system, ie amygdala (mesolimbic)
- associated with reward, as well as drug addiction and schizophrenia.
Substantia Nigra
The substantia nigra is an important dopaminergic mediator of movement and emotion.