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CRANIAL NERVES

The cranial nerves are the components of the peripheral nervous system that are attached to the brain, rather than the spinal cord. In fact, nerves I and II are not true peripheral nerves but fibre tracts of the brain.

Spinal nerves  are mixed carrying motor and sensory and autonomic fibres, but cranial nerves may be purely motor, purely sensory or mixed.  Some also carry fibres of the autonomic nervous system (as do some spinal nerves), some innervate muscles of branchial origin (The branchial system is typically used for respiration and/or feeding in many fish) , and some cranial nerve fibres are associated with special sense organs.

Cranial Nerve I – Olfactory (Sensory; SVA)

The cell bodies of the primary neurons are found in the nasal mucosa, axons pass through the cribiform plate of the ethmoid bone to the olfactory bulb, where they synapse.  The axons of the secondary neurons form the olfactory tract divide into medial and lateral olfactory striae.  The medial branch is a component of the limbic system and goes to septal area ( which is respnsible for emotional reaction to smell sensation and positive reinforcement of feeding and sexual behaviour).  The lateral branch goes to the primary olfactory area constituted by the uncus and dorsmedial amgydala. Tertiary  neurons extend to the olfactory association area or entorhinal cortex (area 28 on Brodmann’s map – the anterior  art of the parahippocampal gyrus).  There is also a relay via the thalamus to the orbitofrontal cortex (conscious odour perception).

The connections of the olfactory nerve are complex and include limbic structures (eg. hippocampus) and the hypothalamus and brain stem via the medial forebrain bundle.  In the brain stem there are reflex connections with other nuclei eg. salivatory nuclei, nucleus ambiguus, and autonomic centres (salivation for odour of food and nausea for unpleasant smells).

Cranial Nerve II – Optic (Sensory; SSA)
The rods and cones of the retina which receive light stimuli through the eyes are the first order neurons that connect with the bipolar cells of the retina – the second order neurons. These in turn synapse with the third order neurons – the Ganglion cells -near the surface of the retina. The myelinated axons of the ganglion cells form the optic nerve fibres.

At the optic chiasma, fibres from the nasal half of each retina cross to the opposite side, but fibres from temporal oral half continue as before.  Thus the optic tract contains fibres from the ipsilaterel halves of the retinas – constituting, in the case of the left optic tract, the right field of vision.  The optic tract runs to the lateral geniculate nucleus of the thalamus.  The visual fibres in the tract synapse and the fourth order neurons form the geniculocalcarine tract/optic radiation and pass to the primary visual cortex in the calcarine sulcus.

Other fibres in the optic tract have different functions. Light reflex fibres go to the pretectal nucleus and thence to the Edinger-Westphal nucleus and are responsible for simple and consensual light reflexes.  The parasympathetic efferent fibres run with the third cranial nerve to the ciliary ganglion, and from there to the constrictor papillae and ciliary muscles.

Other fibres in the optic tract pass to the superior colliculus and from thence to other cranial and spinal nuclei and are concerned with reflex movements of the eyes and head.

The accommodation reflex involves a relay from the calcarine cortex to the superior colliculus, possibly including a frontal lobe connection -

There are also fibres passing from the visual cortex to other cortical centres related to higher functions such as reading.

Cranial Nerve III – Oculomotor (Motor; CSE, GVE)

The motor fibres arise from a group of nuclei just ventral to the aqueduct (at the level of the superior colliculus) and run to supply the medial₁ superior and inferior rectus muscles and inferior oblique and levator palpebrae muscles.  The parasympathetic fibres arise from Edinger-Westphal and medial oculomotor nucleus and run via the ciliary ganglion to the constrictor  u illae and ciliar  muscles respectively. These fibres form the efferent components of the light and accommodation reflexes.

The oculomotor nuclei are connected to the other oculogyric nuclei and the vestibular nuclei via the medial longitudinal fasciculus.  This fasciculus continues caudally into the medial vestibulospinal tract and co-ordinates movements of the head, eves and neck

Cranial Nerve IV – Trochlear (Motor; GSE)

Motor fibres leave the trochlear nucleus which is just caudal to the III nucleus (at the level of inferior colliculus), decussate and emerge from the dorsal surface of the midbrain before passing to innervate the s~orobliuemuscle.

Cranial Nerve V .- Trigeminal (Mixed; GSA, SVE)

This is the largest cranial nerve.: It is sensory for the face, and motor to the muscles of the first branchial arch i.e. the muscles of mastication (and some small muscles associated with the ear and with swallowing).  Parasympathetic fibres from other cranial nerves also use branches of the V nerve to get to their effector organs.

Sensory fibres arise from unipolar cells in the semilunar or trigeminal ganglion.  They supply sensation via 3 nerves – the ophthalmic, maxillary and mandibular.

There are 3 nuclei associated with the afferent fibres of the trigeminal nerve, and they also receive input from other cranial nerves. These nuclei are the chief sensory nucleus, the spinal tract and nucleus, and the mesencephalic nucleus.  They subserve discriminatory touch,  pain and temperature, and proprioception respectively.  The  Proprioceptive fibres pass via the motor root of V to the muscles of mastication.

The trigeminothalamic tract passes centrally from the chief sensory and spinal nuclei of the fifth nerve to the thalamus.  A large number of collateral fibres run from this tract to the reticular activating system.

The motor fibres pass from the motor nucleus at the midpontine level and accompany the mandibular division   These fibres supply the muscles of mastication, tensor tympani and tensor veli palatini and the anterior belly of digastric.  The motor fibres are mainly crossed but significant numbers are uncrossed.  The motor nucleus receives bilateral (but mainly crossed) cerebral connections from the corticobulbar and reflex connections from the spinal tract of V.  Because of the bilateral cortical representation it is unusual for chewing to be paralysed following a C.V.A.

In the corneal reflex, the afferent limb is the ophthalmic division of the trigeminal nerve and the efferent limb is the facial nerve (VII).

Cranial Nerve VI – Abducens (Motor/GSE)

The VI nerve nucleus lies in the floor of the fourth ventricle in the lower pons and the motor (uncrossed) fibres emerge from the ventral pons and have a long course to the orbit to supply the lateral rectus muscle.

Cranial Nerve VII – Facial Nerve (Mixed, mainly motor; SVE, GVE, SVA, GSA)

The facial nerve is motor to the muscles of the second branchial arch – the muscles of facial expression (plus stapedius, stylohyoid and the posterior belly of digastric).  The parasympathetic fibres derive from the superior salivatory nucleus and pass via the nervus intermedius to the glands and mucous membranes of the pharynx, palate, nasopharynx and  paranasal sinuses; and to the submaxillar  and sublingual salivary  glands and the lacrimal glands.

The facial nerve also carries taste fibres from the anterior two-thirds of the tongue via the chorda tympani.  The cell bodies lie in the geniculate ganglion.  Central branches pass via the nervus  intermedius to the nucleus of the solitary tract

With respect to the cutaneous sensory component (GSA), the fibres are distributed to the skin of the external ear and behind the ear, and outer surface of the tempanic membrane.

The facial nerve motor fibres constitute the efferent limb of the corneal reflex.

Cranial Nerve VIII – Vestibulocochlear (composite sensory nerve; SSA)

This nerve consists of 2 separate parts.

Cochlear or Auditory Nerve

Fibres from bipolar cells in the spiral ganglion consist of peripheral branches that end in the organ of Corti and central branches that end in the ventral and dorsal cochlear nuclei.

From the cochlear nuclei many second-order neurons cross the midline as the trapezoid body and ascend as the lateral lemniscus to the inferior colliculus. (Some fibres synapse in the superior olivary nucleus).  The inferior colliculus is the. area for reflex connections to sensory nuclei and other motor nuclei of cranial and spinal nerves via tectobulbar and tectospinal tracts.

The auditory fibres pass from the inferior colliculus via the inferior brachium to the medial Geniculate nucleus of the thalamus.  From this area the auditory radiation projects to the auditory cortex of the temporal lobe and the association cortex. The auditory pathway is bilateral above the cochlear nuclei, although mainly crossed.

Vestibular Nerve

Fibres from bipolar cells in the vestibular ganglion consist of peripheral branches that pass to the semicircular canals, and central branches that terminate in the vestibular nuclear complex.  The vestibular nuclei have reciprocal connections to the archicerebellum; some vestibular nerve fibres  enter the cerebellum directly.

The  vestibular nuclei have direct paths to the spinal cord – the lateral vestibulospinal tract which is found throughout the whole cord and the medial vestibulospinal tract which is a continuation of the medial longitudinal fasciculus into the cervical cord.

In the brainstem the vestibular nuclei connect with the oculogyric nuclei through the medial longitudinal fasciculus, producing co-ordinated movement  of the head and eyes. There are reciprocal connections with the reticular formation, and centres (nausea, vomiting).

Vestibular cortical connections are uncertain – may be parietal lobe behind general sensation for the head, or superior temporal gyrus in front of auditory cortex.

Cranial Nerve IX – Glossopharyngeal (Mixed; SVA, GV:A, GSA, SVF, GVF)

Sensory components – The glossopharyngeal nerve has 3 different sensory components. It has special visceral afferent fibres subserving taste for the posterior  one third of the tongue and the pharynx.  These fibres end centrally in the solitary nucleus.  It has general visceral afferent fibres supplying receptors in the carotid body  and sinus.  Finally it has general fibres supplying general sensation for the back of the tongue, pharynx, soft palate, tonsils and Eustachian tube. These fibres pass centrally to the spinal trigeminal nucleus.

Motor components – Branchiomotor (special visceral efferent) fibres from the nucleus ambiguus got to stylo-pharyngeus.

parasympathetic(general visceral efferent) fibres pass from the inferior salivator  nucleus via  ganglion to the parotid gland.

The pharyngeal or gag reflex depends on the ninth nerve for its sensory component, as does the carotid sinus reflex-

Cranial Nerve X – Vagus (Mixed; SVA, GVA, GSA, SVE, GVE)

Sensory components – The vagus nerve has the same sensory components as the ninth nerve.  It has a few taste fibres (SVA) from visceral afferent fibres from the aortic body and sinus.  However in addition to the latter it carries visceral sensation from the larynx, trachea. oesophaous  thoracic and abdominal viscera.  Finally  it has general somatic afferent fibres from the external auditory meatus, auditory canal and tympanic membrane.  The central connections of the afferent fibres of X are as for IX.

Motor components – Special visceral efferent or branchiomotor fibres pass from the nucleus ambiguus to the muscles of the soft palate, pharynx, external larynx and upper oesophagus.

Cranial Nerve XI – Accessory (Motor; SVE)

The cranial branch of the accessory nerve passes from the nucleus ambiguus to the intrinsic muscles of the larynx and soft palate.

The spinal branch runs from the accessory nucleus to innervate trapezius and sternocleidomastoid muscles.

Cranial Nerve XII – Hypoglossal (Motor; GSE)

Motor fibres from the hypoglossal nucleus in the medulla form the twelfth nerve and pass to the muscles of the tongue

Alternative terms:

Somatosensory   -                           General somatic afferent (GSA)

Somatomotor      -                           General somatic efferent (GSE)

Branchiomotor   -                           Special visceral efferent (SVE)

Parasympathetic -                          General visceral efferent (GVE)

TABLE 1

Components and Functions of Cranial Nerves

TABLE 2

SUMMARY OF REFLEXES

Recommended Readings:

Cranial Nerves: Functional Anatomy:Stanley Monkhouse

Cranial nerves can be a challenge for many medical students especially with regards to the understanding how one nerve relates to another and their clinical significance. Monkhouse lays out the basics in a clear and easy to understand manner and then considers each nerve based on their functional relations to one another in this wonderful little book. Line diagrams are used throughout the text and while they aren’t as fancy as 3D diagrams used in other books, they are very easy to understand and draw in an exam (particularly helpful in short notes/essays). Embryological and clinical bits are also included to further reinforce one’s understanding of the subject. All in all, this is an excellent book that’s easy to carry around and entertaining to read!

The Man Who Mistook His Wife for a Hat (Picador):Oliver Sacks

The book is a fascinating compendium of neurological case studies, classified into four parts: Losses, Excesses, Transports, The World of the Simple. Mr. Sacks takes us on a journey through a series of neurological disturbances with extreme effects.This book is written in a beautifully accessible and entertaining style; it is also moving, funny and tragic in equal measures. Consisting mainly of short stories relating patient ‘oddities’ that the author has treated in his long career as a neurologist it manages to come across as anything but a list of dry case histories. The inclusion of the emotions of the patient as they deal with their difficulties and the reactions of the author keep this book human rather than academic.

Cranial Nerves in Health and Disease :Linda Wilson-Pauwels

This second edition presents a thorough revision of Cranial Nerves. The format reflects the shift in teaching methods from didactic lectures to problem-based learning. It maintains the first edition’s approach of blending the neuro- and gross anatomy of the cranial nerves as seen through color-coded functional drawings of the pathways from the periphery of the body to the brain (sensory input), and from the brain to the periphery (motor output).