The Cranial Nerves and the Brainstem

An earlier version of this material was contained in Smith (1997). It is repeated here as a succession of extracts, supported with hyperlinks.

The brainstem is a group name for the midbrain and the hindbrain combined, and the hindbrain, in turn, is a group name for the cerebellum, the medulla, and the pons. In general anatomical terms, the brainstem is a direct upwards continuation of the spinal cord. That is to say, the spinal cord automatically becomes the brainstem as it passes through the foramen magnum into the cavity of the skull. In so doing, it retains the basic dorsal-sensory-ascending, ventral-motor-descending, grey-cored, white-sheathed structure of the spinal cord. However, that basic structure is now complicated by the appearance of numerous additional nuclei, by frequent tract divisions and decussations, and by the need to communicate with the cerebellum, which is attached to the brainstem at the level of the pons. Because of these complications, cross-sectional views of the brainstem or its component structures need careful interpretation.

In cross-section, the central area of the brainstem, ventral to the neural canal, is known as the tegmentum. This is the brainstem's equivalent of the grey matter core of the spinal cord. The tegmentum includes among its structures the brainstem reticular formation. This is an elongated and distributed fibre-nucleus network, which stretches from midbrain to medulla, and is implicated in maintaining the arousal of brain structures at a higher level. It keeps the underlying electrical tone of the brain within acceptable limits. It does this by receiving collaterals (ie. side-branches) from the ascending sensory tracts. These collaterals enable it to "listen in" to ascending neural traffic. When the cerebral cortex is insufficiently aroused to process these ascending signals effectively, the reticular formation generates additional excitatory signals to "wake it up".

Many of our reflexes, especially the critical life support reflexes, are controlled by nuclei within the brainstem. The midbrain contains the cranial nerve nuclei for CN.III and CN.IV (see below). These are the eye movement nerves, and so the midbrain is a good place from which to control such reflexes as the pupillary reflex and the reflex shift of eye position which takes place whenever the head is rotated. The pons (CN.V-VIII) controls higher respiratory reflexes, and the medulla (CN.IX-XII) lower respiratory and other vital reflexes. The main surface features of the brainstem are shown from the ventral aspect in [picture], and the main internal features in [picture].

There are 12 left-right pairs of cranial nerves (CN). Each CN arises from a distinct grey matter nucleus buried within the white matter of the brainstem.

Traditionally, CNs are identified by the Roman numerals I through XII. This numbering system reflects the order of emergence cephalocaudally (with the exception of CN.XII, which actually emerges before CNs X and XI). CNs also have names indicating their specific function, and may be categorised as motor, sensory, or mixed. Five CNs are exclusively motor nerves, three CNs are exclusively sensory, and four CNs are mixed. CNs can carry both somatic and autonomic information, although the relative proportion of each varies from nerve to nerve. The 12 CNs are now described (the fibre counts being taken from Williams and Warwick, 1975):

The mesencephalon (or "midbrain") is the region of the brainstem between the pons (see below) and the diencephalon [picture]. It contains many important nuclei, including the source nuclei of some of the cranial nerves, some key nuclei of the extrapyramidal tract (particularly the red nucleus and the substantia nigra), key nuclei of certain sensory systems, and - as noted previously - a complex lacework of nuclei and decussating (crossing-over) fibres known as the brainstem reticular formation.

The ventral-lateral surface of the midbrain is characterised by two very large bundles of fibres descending from, and continuous with, the white matter bundles of the cerebral hemispheres. These bundles are known as the cerebral peduncles ("stalks"), and they are the main route for motor fibres leaving the cerebrum en route for the spinal cord. Sensory fibres pass more dorsally, en route for the thalamus. The medial lemniscus (carrying light tactile and kinaesthetic information upwards from the gracile and cuneate nuclei of the medulla) passes just laterally to the red nucleus, and the lateral lemniscus (carrying auditory information upwards from the cochlear nucleus) passes just beneath the inferior colliculus, giving off collaterals into that structure as it does so.

The cerebral peduncles are where the brainstem ceases to have a single central axis and starts to split into two, with each lateralised peduncle feeding only one of the two cerebral hemispheres. Viewed from the ventral aspect, therefore, the midbrain has a distinctive "Y"-shape. The dorsal surface of the midbrain, on the other hand, is known as the tectum (sometimes optic tectum). This contains two left-right pairs of nuclei known as the inferior colliculi and the superior colliculi (collectively, the corpora quadrigemina). These structures help integrate eye movement with head and body movement, and thus help stabilise visual input against all the other movements going on at the same time. The name optic lobe is more commonly used to refer to the tectum in lower vertebrates such as reptiles and birds, because it is relatively far larger in those species. Conversely, the auditory nuclei, the inferior colliculi, are much enlarged in echo-sounding species such as bats. Other nuclei of note are:

The midbrain is the site of many important reflexes, which are supported in various ways by the nuclei described above. The superior colliculi, for example, administer the pupillary reflex, which adjusts the size of the pupillary aperture to the amount of incoming light. In fact, light shown to one eye causes both pupils to constrict, so it is reasonable to assume that each lateralised colliculus communicates with its partner across the commissure between them. In a condition known as Argyll-Robertson pupil, the pupillary reflex is absent, and this deficit is taken by neurologists to be a sign of brainstem damage.

The superior colliculi also help coordinate eye and head movement. This is needed because when the head is moved as part of normal behaviour it makes it difficult to fixate the eyes in a given direction. Leftwards movements of the head need to be counteracted with rightwards movements of the eyes, upwards with downwards, and so on. The neural circuitry by which this is accomplished is highly complex, and much of it is situated in the midbrain.

The pons (or Pons Varolii) is the region of the brainstem below the midbrain (see above) but above the medulla (see below), and is the point of attachment of the cerebellum (see below). It is characterised by a very heavy band of fibres crossing the ventral aspect of the brainstem from side to side (Latin pons = bridge). These fibres become the cerebellar peduncles as they turn dorsally towards the cerebellar hemispheres. The cerebral aqueduct at this point expands to form the fourth ventricle, from where it is continuous downwards with the neural canal of the spinal cord.

Like the midbrain, the pons, too, contains various important nuclei, including the source nuclei of CN.V to CN.VIII. A typical pontine nucleus is the cochlear nucleus, which contains the second sensory neurons of the auditory system. Another is the vestibular nucleus, which contains the second sensory neurons of the sense of balance. The brainstem reticular formation is continuous downward from the midbrain through the pons into the medulla.

The pons has an important role to play in controlling respiration, containing nuclei which rhythmically inhibit the inspiratory centres of the medulla (see below), thus allowing alternating inspiration and expiration.

The cerebellum is part of the hindbrain, and is the second largest component of the CNS. It comprises two lateralised hemispheric lobes arising from and directly dorsal to the fourth ventricle. It is separated from the occipital cortex by the tentorium, a heavy partition of meningeal dura mater. Like the cerebral hemispheres, the cerebellum has a surface layer of grey matter. This (the cerebellar cortex) is however more deeply and minutely folded than its cerebral counterpart. It overlays a central core of white matter shaped like the branches of a tree (hence the early neuroanatomists called it arbor vitae - the "tree of life"). Several distinctive types of neuron are found within the cerebellum, eg. basket cells, Purkinje cells, and granule cells. The cerebellum plays an important role in the coordination of skilled motor activity and in the maintenance of posture and equilibrium (see our associated Motor Systems page).

The medulla oblongata (or "medulla", or "bulb") is the lowest portion of the brainstem. It is situated immediately above the spinal cord, where it enters the cranial cavity through the foramen magnum. In cross-sectional layout, therefore, the medulla is initially very similar to the spinal cord. However, it is here that you begin to come across the additional nuclei and tract complexes which characterise the brainstem. The most important of these are described below. The term "bulb" is often seen in compound names, for example in the bulbospinal fibres, and in both bulbar and pseudobulbar palsy.

On the ventral surface of the medulla, there are two lateralised pairs of long oval prominences. The higher and more lateral of these are nuclei known as the olives, and the lower (somewhat larger and more oval, and lying medial to the olives) are white matter prominences known as the pyramids. The olives receive information from all levels of the spinal cord below, and from the cerebral cortex, thalamus, basal ganglia, and red nucleus above. They transmit information to the cerebellum, thalamus, and red nucleus. They are thus a key part of the extrapyramidal system (see our associated Motor Systems page). The pyramids are where the efferent fibres from the motor cortex pass close to the surface.

Immediately below the pyramids is a ventral midline structure known as the decussation of the pyramids. This is where the efferent tracts cross over to descend thereafter in the contralateral spinal tracts. The pyramids, therefore, constitute the last ipsilateral stage of efferent transmission. The tract in question is called the pyramidal tract. This is a descending (ie. motor) fibre tract, a large proportion of which is made up of corticospinal fibres arising from the pyramid cells (Layer V) of the primary motor area (Area 4). These are the upper motor neurons, and their axons travel without interruption through the internal capsule, the cerebral peduncles (see above), the pyramids, the decussation of the pyramids, and finally down into the lateral fasciculus of the spinal cord to synapse with the appropriate lower motor neuron at the appropriate spinal level. They support voluntary movement. For detail of what happens when the pyramidal tract is damaged, see our associated Motor Systems page.

The fact that there are pyramidal cells in Area 4, and pyramids in the medulla is an unfortunate terminological coincidence. The former were so named because they are neurons with a distinctive pointed shape under a microscope. The latter were so named because they stood out so clearly to the naked eyes of the early dissectors. The sensory pathways pass through the medulla dorsally to the reticular formation and the olives. The medulla contains five types of nuclei in addition to the olives.