Cranial Nerve Nuclei
Below is a summary of the brainstem structures in relation to the cranial nerve nuclei. This overview may be useful because the brainstem houses the nuclei that give rise to 10 of the 12 pairs of cranial nerves.
Overview of the Brainstem
The brainstem consists (from superior to inferior) of the midbrain, pons and the medulla oblongata. The midbrain is continuous, above, with the cerebral hemispheres. The medulla is continuous, below, with the spinal cord. Posteriorly, the pons and medulla are separated from the cerebellum by the 4th ventricle.
The brainstem has a ventral aspect (anterior surface) and a dorsal or posterior surface.
The posterior surface of the brainstem is formed by:
- the fasciculus gracilis (tracts from the posterior funiculus of the spinal cord) which ends superiorly as a rounded elevation called gracile tubercle
- the fasciculus cuneatus (also tracts from the posterior funiculus of the spinal cord) which ends superiorly as a rounded elevation called cuneate tubercle
These fasciculi and tubercles are at the lower half of the posterior surface of the medulla – thus the lowest portion of the brainstem (posteriorly).
At the midbrain level, the posterior surface of the brainstem is marked by four rounded swellings called the superior and inferior colliculi, with one pair (a superior- and an inferior colliculus) on each side of the midline, as well as a connecting ridge called the brachium. Most of the remaining part of the posterior surface of the brainstem, which includes the posterior surfaces of pons and the superior half of the medulla, is formed by the floor of the 4th ventricle.
The 4th ventricle is continuous, below, with the central canal, which traverses the lower part of the medulla, and becomes continuous with the central canal of the spinal cord. Cranially, the 4th ventricle is continuous with the cerebral aqueduct (of Sylvius), which passes through the midbrain to connect the 3rd ventricle [which is a median cavity into which, the 1st and 2nd (lateral) ventricles open, through the interventricular foramen (of Monro).
The fourth ventricle also has a tent-shaped roof, cavity and lateral walls. However, the floor is the most related part to the cranial nerve nuclei.
The floor of the fourth ventricle, often called the rhomboid fossa because of its shape, is divisible into an upper triangular part formed by the posterior surface of the pons; a lower triangular part formed by the upper part of the posterior surface of the medulla; and an intermediate part at the junction of the pons and medulla. The intermediate part is prolonged laterally over the inferior cerebellar peduncle as the floor of the lateral recess. Its surface is marked by the presence of delicate bundles of transeversely running fibres. Those bundles are the striae medullares.
The entire floor is divided into right and left halves by a median sulcus. Next to the middle line, there is a longitudinal elevation called the medial eminence. The eminence is bounded laterally by the sulcus limitans. The region lateral to the sulcus limitans is the vestibular area which overlies the vestibular nuclei (cranial nerve nuclei responsible for hearing and balance). The vestibular area lies partly in the pons and partly in the medulla.
The pontine part of the floor shows some features of interest in close relation to the sulcus limitans and the median eminence. The uppermost part of the sulcus limitans overlies an area that is bluish in color and this area is called the locus coeruleus. Somewhat lower down, the sulcus limitans is marked by a depression, the superior fovea. At this level, the medial eminence shows a swelling, the facial colliculus.
The medullary part of the floor also shows some features of interest in relation to the medial eminence and the sulcus limitans. The sulcus limitans is marked by a depression, the inferior fovea. Descending from the fovea, there is a sulcus that runs obliquely towards the middle line. That sulcus divides the medial eminence into two triangles – the hypoglossal trigone (which houses the cranial nerve nuclei called hypoglossal nuclei), medially; and the vagal trigone (housing the vagal nuclei), laterally. Between the vagal trigone, superiorly and the gracile tubercle, inferiorly, there is a small area called the area postrema.
The lowest part of the floor of the fourth ventricle is called the calamus scriptorius, because of its resemblance to a nib. Each inferolateral margin of the ventricle is marked by a narrow white ridge or taenia. The right and left taeniae meet at the inferior angle of the floor to form a small fold called the obex.
The anterior surface of the brainstem is mainly marked by the left and right crus cerebri at the midbrain, the middle cerebellar peduncle at the level of the pons, and rounded elevations called olive and pyramid at the level of the medulla.
Within the brainstem, there is a remarkable number of tracts and grey matter. Along the entire length of the brainstem, there are areas not occupied by well-defined nuclei or nerve fibres, but consisted of a network of fibres within which scattered neurons are situated. Those areas are referred to as “the reticular formation of the brainstem”. These areas mainly occupy the dorsal part of the midbrain, pons and medulla. Such areas (reticular formation) are found at all levels of the central nervous system, and are functionally very important.
At the level of the midbrain and medulla, there is a grey matter region called central grey matter which surrounds the cerebral aqueduct and gives origin to some cranial nerve nuclei including the mesencephalic nucleus of the trigeminal nerve. Most of the cranial nerve nuclei that migrated from their original position (in relation to the floor of fourth ventricle) during embryonic development are situated in this grey matter region (central grey matter).
The cranial nerve nuclei are aggregate of cells (collection of cell bodies). Attached to these cell bodies are fibres called cranial nerves (bundles of axons). These nuclei are either sensory or motor but never both. However, cranial nerves can be sensory, motor or mixed nerves (when they have both sensory and motor functions).
The cranial nerve motor nuclei are further grouped according to their targets, that is, where their axons (attached cranial nerves) are sent or structures they innervate. Hence the cranial nerve nuclei with motor functions can be grouped according to the following functional components to which their fibres belong:
- General Somatic Efferents (GSE)
- Special Visceral Efferents (SVE)
- General Visceral Efferents (GVE)
Similarly, the cranial nerve sensory nuclei are grouped according to the information they receive, which constitutes the functional components to which their attached nerves belong. These functional components are:
- General Somatic Afferents (GSA)
- Special Somatic Afferents (SSA)
- General Visceral Afferents (GVA)
- Special Visceral Afferents (SVA)
The first cranial nerve (olfactory nerve – CN I) does not originate from a cranial nerve nucleus. CN 1 originates from the olfactory bulb, which is a structure located in the forebrain and controls olfaction. The second cranial nerve (optic nerve – CN II) originates from the retina and transmits visual impulses from the retina to the visual cortex (occipital lobe).
In the brainstem, there are about 18 cranial nerve nuclei comprising of 10 motor cranial nerve nuclei and 8 sensory cranial nerve nuclei. The functions of those cranial nerves are suggestive of the functions of the parts of the brainstem they are located. For example, the midbrain is involved in eye movement control and houses the oculomotor and trochlear nuclei which also have these functions. The pons control breathing, signal relay, and contains the trigeminal, abducens and facial nuclei.
In summary therefore, cranial nerve nuclei are either motor (efferent) or sensory (afferent), and both category can be somatic or visceral.
During embryonic development, the cranial nerve nuclei related to the various components – GSE, SVE, GVE, GSA, SSA, GVA and SVA, are arranged in vertical rows called columns in a definitive sequence in the grey matter related to the floor of the fourth ventricle. Each half (along the median sulcus) of the floor of the fourth ventricle is divided into a medial part called the basal lamina, and a lateral part called the alar lamina by the sulcus limitans.
The Efferent nuclei (motor nuclei) lie in the medial part, while the Afferent nuclei (sensory nuclei) lie in the lateral part – alar lamina. In both the medial and lateral parts, the visceral nuclei [special visceral (branchial) effernt, general visceral efferent, general visceral afferent, and special visceral afferent] lie nearer to the sulcus limitans than the somatic nuclei [GSE, GSA, and SSA]. Therefore, from the midline (median sulcus of the fourth ventricle) to the lateral aspects, the sequence of nuclear column is:
- GSE - lying closest to the median sulcus
- SVE - lying next to the GSE
- GVE - lying farthest from the median sulcus but next to the sulcus limitans
All of the above listed three nuclear columns are in the medial part (basal lamina) of the floor of the 4th ventricle.
On the other side of the sulcus limitans – that is the lateral part (alar lamina), the sequence is:
- GVA - lying next to the sulcus limitans
- SVA - lying next to GVA, laterally
- GSA - lying next to the SVA and finally
- SSA - lying farthest away from the sulcus limitans, laterally
As development continues, parts of those columns disappear from their position in relation to the floor of the fourth ventricle, so that each of the no longer extends the whole length of the brainstem, but is represented by one or more discrete nuclei. Some nuclei retain their original positions in relation to the floor of the fourth ventricle, but some others migrate deeper into the brainstem.
In the descriptions that follows, the cranial nerve nuclei originating from each of these columns, as well as their definitive positions in the brainstem are outlined.
General Somatic Efferent (Motor) Nuclei
The general somatic efferent column consists of the following nuclei that supply striated (skeletal) muscles of somatic origin.
- The oculomotor nucleus: This nucleus is located in the upper part of the midbrain at the level of the superior colliculus. The oculomotor nuclei (left and right sides) form a single complex that lies in the central grey matter, ventral to the cerebral aqueduct (of Sylvius). The oculomotor nucleus sends fibres (oculomotor nerve) to supply the superior, medial and inferior rectus, inferior oblique and levator palpebrae superioris muscles.
- The trochlear nucleus: The trochlear nucleus is located in the lower part of the midbrain at the level of the inferior colliculus. The nucleus lies anterior to the cerebral aqueduct in the central grey matter. Since the trochlear nucleus is a motor nucleus, it sends fibres to innervate the superior oblique muscle.
- The abducent nucleus: This nucleus lies in the lower part of the pons, deep to the facial colliculus in the floor of the fourth ventricle. It is situated in the grey matter lining the floor of the fourth ventricle near the midline. The abducent nucleus is a motor nucleus and sends its attached nerve to supply the lateral rectus muscle.
- The hypoglossal nucleus: The hypoglossal nucleus is situated near the midline and below the hypoglossal trigone (or triangle) in the floor of the fourth ventricle of the upper medulla. It is an elongated column extending into both the open and closed parts of the medulla. Fibres from this nucleus are motor, and they innervate the muscles of the tongue.
Special Visceral Efferent (Motor) Nuclei
These nuclei are also called Branchial Efferent or Branchiomotor nuclei. They supply striated (skeletal) muscles derived from the branchial arches.
- The motor nucleus of the trigeminal nerve: This cranial nucleus lies in the upper part of the pons, in the pons' dorsal part. It is situated in the lateral part of the reticular formation, medial to the main sensory nucleus of the trigeminal nerve. The motor nucleus of trigeminal nerve innervates the mastication muscles, mylohyoid muscle and tensor palati.
- The nucleus of the facial nerve: This nucleus lies in the lower part of the pons, deep and lateral to the facial colliculus, and occupies a position similar to that of the motor nucleus of the trigeminal nerve, because the spinal nucleus and tract of the trigeminal nerve lie lateral to it, as seen in figure 8 above. The facial nerve nucleus sends its nerve to the lacrimal gland for lacrimal secretion and salivary secretion.
- The nucleus ambiguus: The nucleus ambiguus lies in the medulla. It forms an elongated collumn lying deep in the reticular formation, both in the open and closed parts of the medulla. Inferiorly, it is continuous with the spinal accessory nucleus. The nucleus ambiguus is a composite nucleus and contributes fibres to the glossopharyngeal, vagus and accessory nerves.
General Visceral Efferent (Motor) Nuclei
The nuclei of this column gives origin to preganglionic fibres that contribute to the cranial parasympathetic outflow. These fibres end in peripheral ganglia. Postganglionic fibres arising in those ganglia (peripheral ganglia) supply the smooth muscles and glands. The nuclei from the GVE are as follows:
- The Edinger-Westphal nucleus (Accessory Oculomotor nucleus): This nucleus lies in the midbrain. It is closely related to the oculomotor complex. Fibres arising in this nucleus pass through the oculomotor nerve, and relay in the ciliary ganglion to supply the sphincter pupillae and the ciliaris muscle (see the “Visual Pathway”).
- The Salivary (salivatory) nuclei: The superior and inferior salivary nuclei lie in the dorsal part of the pons, just above its junction with the medulla. They are located a little above the upper end of the dorsal nucleus of the vagus nerve. The superior salivatory nucleus sends fibres into the facial nerve and these fibres relay in the submandibular ganglion to supply the submandibular and sublingual salivary glands. The inferior salivary nucleus sends fibres into the glossopharyngeal nerve. Those fibres relay in the otic ganglion to supply the parotid gland. The parotid gland may also receive some fibres from the superior salivatory nucleus, through the submandibular ganglion.
- The dorsal or motor nucleus of the vagus nerve (dorsal vagal nucleus): This cranial nerve nucleus is long and it lies vertically in the medulla. Its upper end lies deep to the vagal trigone in the floor of the fourth ventricle. When traced backwards, it extends into the closed part of the medulla where it lies in the lateral part of the central grey matter. Fibres arising fom this nucleus supply the heart, lungs, bronchi, oesophagus, stomach, small intestine and large intestine up to the right two-thirds of the transverse colon. Those fibres end in ganglion (nerve plexuses) closely related to those organs – heart, lungs, stomach, etc.
General and Special Visceral Afferent (Sensory) Nuclei
The general and special visceral afferent collumns are represented by:
- The nucleus of the solitary tract
- The commissural nucleus of vagus
- The gustatory nucleus
The nucleus of the solitary tract is present in the medulla. Its cells (nucleus of solitary tract) form an elongated column lying deep in the reticular formation. The upper part of the nucleus of the solitary tract lies ventrolateral to the dorsal nucleus of the vagus (dorsal vagal nucleus). When traced backwards, it extends into the closed part of the medulla. There it lies dorsomedial to the vagal nucleus. The lower ends of the nuclei of the two sides fuse to form the commissural nucleus of the vagus.
The nucleus solitarius receives fibres carrying general visceral sensations through the vagus and glossopharyngeal nerves. Through those afferents, and through connections with the reticular formation, the nucleus of solitary tract plays an important role in reflex control of respiratory and cardiovascular functions.
Fibres of taste – special visceral afferent, carried by facial, glossopharyngeal and vagus nerves end in the upper part of the nucleus of the solitary tract. This upper part of the nucleus is referred to as the gustatory nucleus.
General Somatic Afferent (Sensory) Nuclei
These nuclei give rise to the following sensory nuclei:
- The main or superior sensory nucleus: This nucleus lies in the upper part of the pons, in the lateral part of the reticular formation. It lies lateral to the motor nucleus of the trigeminal. The superior sensory nucleus is mainly concerned in mediation of proprioceptive impulses, touch and pressure.
- The spinal nucleus: The spinal nucleus is another sensory cranial nerve nucleus which extends from the main nucleus (superior sensory nucleus) in the pons down into the medulla, and into the upper two segments of the spinal cord. The lower end of the spinal nucleus is continuous with the substantia gelatinosa of the spinal cord. The spinal nucleus receives general somatic sensations carried by the facial, glossopharyngeal and vagus nerves. Functions of the spinal nucleus includes mediation of pain and thermal sensibility. The spinal nucleus is divisible (cranio-caudally) into three sub-nuclei, the oralis, interpolaris, and caudalis.
- The mesencephalic nucleus: This is also called the mesencephalic nucleus of the trigeminal nerve. It extends cranially from the upper end of the main sensory nucleus in the pons into the midbrain. In the midbrain, the mesencephalic nucleus lies in the central grey matter lateral to the aqueduct. Functionally, this nucleus appears to be similar to sensory ganglia of the cranial nerves, and to the spinal ganglia, rather than to afferent nuclei. The processes (dendrites) of the neurons of this nucleus are believed to carry proprioceptive impulses from muscles of mastication, and possibly also from muscles of the eyeballs, face, tongue and teeth. The mesencephalic nucleus is the centre for jaw jerk.
Special Somatic Afferent (Sensory) Nuclei
This column gives rise to:
- The cochlear nucleus
- The vestibular nucleus
There are two cochlear nuclei – dorsal and ventral cochlear nucleus. They are respectively positioned dorsal and anterior to the inferior cerebellar peduncle at the level of the junction between the pons and medulla. The two nuclei are continuous with each other, being separated only by a layer of nerve fibres.
The vestibular nucleus lies in the grey matter underlying the lateral part of the floor of the fourth ventricle. The vestibular nucleus is situated partly in the medulla and partly in the pons. Four distinct nuclei of the vestibular nucleus are recognized. These nuclei are the medial, lateral, inferior and superior vestibular nuclei. The lateral nucleus is also called the Deiter's nucleus.
Connections Of Some Cranial Nerve Nuclei
Column of motor nucleus of the trigeminal nerve, nucleus of the facial nerve, nucleus ambiguus, and oculomotor, trochear, abducent, hypoglossal nuclei
The nuclei of the GSE and SVE connect with the skeletal muscles, to which they send their nerves. Those nuclei (GSE and SVE) receive sensory fibres through which they also make connections with the cerebral cortex, the tectum (connections of vision and of hearing), vestibular nuclei (vestibular impulses), sensory cranial nuclei, the red nucleus and the reticular formation.
Connections of the cochlear nuclei
The dorsal and ventral cochlear nuclei are major parts of the partway of hearing. They receive fibres connecting them with the spiral ganglion and sends their attached fibres, some of which relay in the trapezoid body, before reaching the nucleus of trapezoid body and the superior olivary nucleus.
Connections of the vestibular nuclei
The vestibular nuclei are connected to the cerebellum to which they send and receive fibres. These nuclei also have connections with the spinal cord, through fibres, mainly, originating in the medial vestibular nucleus. These fibres descend in the fasciculus.
Connections of the nucleus of the solitary tract
The nucleus of the solitary tract receives fibres from the sensory ganglia of the seventh, ninth, and tenth cranial nerves, and mainly send fibres to the hypothalamus (solitario-hypothalamic tract), the thalamus and cerebral cortex.
Connections of the sensory nuclei of the trigeminal nerve
The main sensory nucleus of the trigeminal nerve and the spinal nucleus of trigeminal nerve, both receive fibres, through which, they connect with the trigeminal ganglion. These nuclei (main sensory and spinal nucleus) send fibres to the thalamus and cerebral cortex.
The combined developments of ventilatory machines and transplantation surgery have underlined the need for defining the criteria for making a correct diagnosis of brain death – the irreversible cessation of brainstem function. In comatose patients with irremediable structural brain damage due to a disorder that can lead to brain death, brainstem reflexes must be shown to be absent in order to establish brain death. The following are the criteria (anatomical basis) normally adopted:
- Fixed pupils, not reacting to light – testing midbrain function
- No corneal reflexes – testing pontine connections between trigeminal and facial nerve nuclei
- No vestibulo-ocular reflexes – testing the connections between the vestibular nerve and eye muscle nerves
- No gag reflex or response to bronchial stimulation by a catheter passed down the trachea – testing vagal connections in the medulla
- No motor responses in any cranial nerves on adequate nociceptive stimulation of any somatic area – testing for facial grimacing from pressure on the supraorbital margins (connections between trigeminal and facial nerve nuclei) or on the bases of fingernails (connections between cervical spinal cord and facial nerve nuclei)