Below
the diencephalon are the midbrain (or mesencephalon) and the hindbrain (or
rhombencephalon). The hindbrain consists of the pons (or metencephalon) and
the medulla oblongata (or myelencephalon). These parts of the brain stem are
characterized by three main features: (1) a roof plate superior to the
cerebral aqueduct and fourth ventricle; (2) a central core of gray matter,
known as the reticular formation; and (3)
a massive basal collection of fibers descending from the cerebral cortex to
the brain stem and spinal cord.
The roof plate of the midbrain is formed by two paired rounded eminences,
the superior and inferior colliculi. The superior colliculus receives input
from the retina and the visual cortex and participates in a variety of
visual reflexes, particularly the tracking of objects in the contralateral
visual field. The inferior colliculus receives both crossed and uncrossed
auditory fibers and projects upon the medial geniculate body, the auditory
relay nucleus of the thalamus (see above Diencephalon). In the hindbrain the
roof plate is formed by the cerebellum and a membrane containing the choroid
plexus of the fourth ventricle.
The reticular formation contains a core collection of cells of various sizes
that project to the thalamus, the cerebellum, and the spinal cord.
Surrounding this core are long ascending and descending tracts in which
various cranial nerve nuclei are embedded.
Fibers derived from the cerebral cortex lie on or near the ventral surface
of the midbrain, pons, and medulla. At the midbrain they gather into two
bundles called the crura cerebri; from there they descend into the pons,
where most terminate upon cell nuclei that project into the cerebellum.
These constitute the corticopontine tract. The other major tract, called the
corticospinal tract, forms the medullary pyramids before descending to the
spinal cord.
The midbrain contains the nuclear complex of the
oculomotor nerve as
well as the trochlear
nucleus; these cranial nerves innervate muscles that move the
eye and control the shape of the
lens and the diameter of the pupil. In addition, between the midbrain
reticular formation (known here as the tegmentum) and the crus cerebri is a
large, pigmented structure called the substantia nigra. This nucleus
consists of two parts, the pars reticulata and the pars compacta. Cells of
the pars compacta contain the black pigment melanin; these synthesize
dopamine and project to cells of either the caudate nucleus or the putamen
but not to both. By exercising an inhibitory action on large aspiny
cholinergic neurons in the neostriatum, the dopaminergic cells of the pars
compacta influence the output of the neurotransmitter GABA from spiny
striatal neurons. These spiny neurons in turn project to cells of the pars
reticulata, which, by projecting fibers to the thalamus, are in effect part
of the output system of the corpus striatum.
At the caudal midbrain, crossed fibers of the superior cerebellar peduncle
(the major output system of the cerebellum) surround and partially terminate
in a large, centrally located structure known as the red nucleus. Most
crossed ascending fibers of this bundle project to thalamic nuclei, which
have access to the primary motor cortex. A smaller number of fibers synapse
on large cells in caudal regions of the red nucleus; these give rise to the
crossed fibers of the rubrospinal tract (see below
The spinal cord: Descending spinal tracts).
The
pons consists of two parts: the tegmentum, a phylogenetically older part
that contains the reticular formation; and the pontine nuclei, a larger part
composed of masses of neurons that lie among large bundles of longitudinal
and transverse fibers.
Fibers originating from neurons in all major lobes of the cerebral cortex
terminate upon the pontine nuclei, which in turn project to the opposite
cerebellar hemisphere. These massive crossed fibers form the middle
cerebellar peduncle—in effect serving as the bridge that connects each
cerebral hemisphere with the opposite half of the cerebellum.
The reticular formation in the pontine tegmentum
contains multiple cell groups that exert facilitating influences upon motor
function. It also contains the nuclei of several cranial nerves. The facial
nerve and the two components of the vestibulocochlear nerve, for example,
emerge from and enter the brain stem at the junction of the pons, medulla,
and cerebellum. Motor nuclei for the trigeminal nerve lie in the upper pons.
Located on the periphery of the pons are long ascending and descending
tracts that connect the brain to the spinal cord.
The medulla oblongata is closest to the spinal cord, and is involved with
the regulation of heartbeat, breathing, vasoconstriction (blood pressure),
and reflex centers for vomiting, coughing, sneezing, swallowing, and
hiccuping.
It is the most caudal
segment of the brain stem, and appears as a conical expansion of the spinal
cord. Both the pons and the medulla are separated from the overlying
cerebellum by the fourth ventricle, and cerebrospinal fluid entering the
fourth ventricle from the cerebral aqueduct passes into the cisterna magna,
a subarachnoid space surrounding the medulla and the cerebellum, via
foramina in the lateral recesses and in the midline of the ventricle.
At the transition from the medulla to the spinal
cord, there are two major decussations, or crossings, of nerve fibers. The
corticospinal decussation is the site at which 90 percent of the fibers of
the medullary pyramid cross and enter the dorsolateral funiculus of the
spinal cord. Signals conveyed by this tract provide the basis for voluntary
motor function on the opposite side of the body (see below
The spinal cord: Descending spinal tracts). In the other decussation,
sensory fibers ascending in the fasciculus gracilis and fasciculus cuneatus
of the spinal cord terminate upon large nuclear masses on the dorsal surface
of the medulla. Known as the nuclei gracilis and cuneatus, these masses give
rise to fibers that decussate above the
corticospinal tract and form a major ascending sensory pathway known as
the medial lemniscus. Present at all brain-stem levels, the medial lemniscus
projects upon the somesthetic relay nuclei of the thalamus.
The medulla contains nuclei associated with the hypoglossal, accessory,
vagus, and glossopharyngeal cranial nerves. In addition, it contains
portions of the vestibular nuclear complex, parts of the trigeminal nuclear
complex concerned with pain and thermal sense, and solitary nuclei related
to the vagus, glossopharyngeal, and facial nerves that subserve the sense of
taste.
Of several medullary relay nuclei that project to the cerebellum via the
inferior cerebellar peduncle, the largest is the inferior olive.
The
cerebellum is the second largest part of the brain, after the cerebrum. It
functions for muscle coordination and maintains normal muscle tone and
posture. The cerebellum coordinates balance.
It overlies the posterior aspect of the pons and medulla and fills the
greater part of the back portion of the skull. It consists of two paired
lobes, or hemispheres, and a midline portion known as the vermis. Cerebellar
cortex appears very different from cerebral cortex in that it consists of
small, leaflike laminae, referred to as folia. Structurally the cerebellum
consists of a three-layered, gray cellular mantle called the cerebellar
cortex and a core of white matter containing four paired intrinsic (i.e.,
deep) nuclei, the dentate, globose, emboliform, and fastigial. Three paired
fiber bundles—the superior, middle, and inferior peduncles—connect the
cerebellum with the midbrain, pons, and medulla, respectively.
On an embryological basis the cerebellum can be divided into three parts:
(1) the archicerebellum, related primarily to the vestibular system; (2) the
paleocerebellum, or anterior lobe, concerned with control of muscle tone;
and (3) the neocerebellum, known as the posterior lobe. Receiving input from
the cerebral hemispheres via the middle cerebellar peduncle, the
neocerebellum is the part most concerned with coordination of voluntary
motor function.
The three layers of the cerebellar cortex are an outer synaptic layer (also
called the molecular layer), an intermediate discharge layer (the Purkinje
layer), and an inner receptive layer (the granular layer). Sensory input
from all sorts of receptors are conveyed to specific regions of the
receptive layer, which consists of enormous numbers of small nerve cells
(hence the name granular) that project axons into the synaptic layer. There
they excite the dendrites of the Purkinje cells, which in turn project axons
to portions of the four intrinsic nuclei and upon dorsal portions of the
lateral vestibular nucleus. Because most Purkinje cells are GABAergic and
therefore exert strong inhibitory influences upon the cells that receive
their terminals, all sensory input into the cerebellum results in inhibitory
impulses being exerted upon the deep cerebellar nuclei and parts of the
vestibular nucleus. Cells of all deep cerebellar nuclei, on the other hand,
are excitatory (secreting the neurotransmitter glutamate) and project upon
parts of the thalamus, red nucleus, vestibular nuclei, and reticular
formation.
The cerebellum thus functions as a kind of computer, providing a quick and
clear response to any set of sensory signals. It plays no role in sensory
perception, but it exerts profound influences upon equilibrium, muscle tone,
and the coordination of voluntary motor function. Lesions of the cerebellum
produce a constellation of disturbances, including intention tremor, ataxia,
hypotonus, easy fatigability, and disturbances of speech.
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