Anatomy Of The
Brainstem
The brainstem
begins at the foramen magnum and extends to the cerebral peduncles and
thalamus. It consists of the medulla, pons and midbrain and is
located anterior to the cerebellum to which it is connected by three cerebellar
peduncles. It contains the following:
• Nuclei for 10 of the 12 pairs of cranial
nerves (see Chapter 7), the exceptions being the olfactory and optic
nerves;
• Networks of neurones for controlling eye
movements, which includes the third, fourth and sixth cranial nerves (see
Chapter 56);
• Monoaminergic nuclei that project widely
throughout the central nervous system (see Chapter 18);
• Areas that are vital in the control of
respiration and the cardio- vascular system, as well as the autonomic nervous
system (see Chapter 3);
• Areas important in the control of consciousness
including sleep and associated monoaminergic nuclei (see Chapter 43);
•
Ascending and descending pathways linking the
spinal cord to supraspinal structures, such as the cerebral cortex and
cerebellum, some of which take their origin from the brainstem (see Chapters 9,
31, 32 and 35–39).
Important
structures in the brainstem
• The dorsal column nuclei represent the
primary site of termination of the fibres conveyed in the dorsal columns (DCs),
responsi- ble for light touch, vibration perception and joint position sense.
The relay neurones in this structure send axons that decussate in the lower
medulla to form the medial lemniscus, which synapses within the thalamus
(see Chapter 31).
• The pyramid which represents the
descending corticospinal tract (CoST) in the medulla, a pathway that decussates
at the lower border of this structure.
• The tractus solitarius and nucleus
ambiguus are associated with taste and the motor innervation of the pharynx
by the glossopha- ryngeal and vagus nerves (see Chapter 7).
• The inferior olive in the medulla
receives inputs from a number of sources and provides the climbing fibre input
to the cerebellum (see Chapters 40 and 49).
• The cerebellar peduncles convey
information to and from the cerebellum (see Chapter 40).
• The medial longitudinal fasciculus originates
in the vestibular nucleus and projects rostrally connecting some of the
oculomotor nuclei (third and sixth cranial nerves) as well as caudally to form
part of the vestibulospinal tract.
• The vestibular nucleus has important
connections from the balance organs
within the inner ear and projects to the spinal cord and cerebellum as well as
other brainstem structures (see Chapters 20, 40 and 49).
• The substantia nigra in the midbrain
contains both dopamine and γ-aminobutyric acid (GABA) neurones, forms part of
the basal ganglia and is involved in the control of movement (see Chapters 41
and 42). The loss of its dopaminergic neurones is the major pathological event
in Parkinson’s disease (see Chapter 42).
• The red nucleus in the midbrain is
intimately associated with the cerebellum, and is the site of origin for the
rubrospinal tract which, with the CoST, forms the lateral descending pathway of
motor control (see Chapter 37).
•
The periaqueductal grey matter of the
mesencephalon is an area rich in endogenous opioids and thus is important in
the supraspi- nal modulation of nociception (see Chapter 32).
• The central aqueduct of Sylvius running
through the midbrain connects the third to the fourth ventricle, and narrowing
of it (stenosis) can cause hydrocephalus (see Chapter 5).
• The cerebral peduncles contain the
descending motor pathways from the cerebral cortex to the spinal cord and
brainstem, especially the pons (see Chapter 35).
• The inferior colliculi in the midbrain
are part of the auditory system (see Chapter 28) while the superior
colliculi are more involved with visual processing and eye movement control
(see Chapters 25 and 56).
Thus, damage to the brainstem can have devastating
conse- quences, although small lesions can often be localized with great
accuracy because of the number of structures located within this small area of
the brain. The most common causes of lesions in this part of the brain are
either inflammatory (e.g. multiple sclerosis; see Chapter 62) or
vascular in nature (see Chapter 64). However, disorders of the brainstem can
also be seen with tumours (see Chapter 13) and a host of other conditions, and
if damage is severe and extensive then it can be fatal.
Testing specifically for brainstem functions is
undertaken to assess if an individual with extensive brain injury (e.g. massive
stroke or head injury) is brain dead, which has implications for further
interventional therapy and organ donation. This assessment involves looking
at reflex eye movements to head movement, eye movement responses to stimulation
of the vestibular system and spontaneous respiration.
