Named cranial nerve nuclei III–XII are found in the brainstem; they are bilaterally paired (Fig. 10.2). Their location relative to the sulcus limitans determines their function. Sensory cranial nerve nuclei are located dorsal to the sulcus limitans, parasympathetic (autonomic) nuclei are located lateral to it, and motor nuclei are ventral to it (Figs. 1.7, 2.3A) (Note: an exception to this is the location of the vestibular nuclei, which are sited dorsal and lateral to the sulcus limitans, despite having both sensory and motor functions). Remembering this anatomical arrangement is useful, as when the clinician is looking at a cross-section of the brainstem (e.g. MR image or tissue section), they can make an educated assumption about function of grey matter based on its dorsoventral position.

Fig. 10.2 The functional columns of grey matter in the spinal cord and their fragmentation to form nuclei of the cranial nerves in the brain stem.

Functional classification of cranial nerve nuclei

A nucleus is a collection of neurons, with similar function, that are clustered together and located in the CNS. Cranial nerve nuclei can have two different names depending on whether their function is somatic or autonomic (parasympathetic). For example, the oculomotor nucleus of CN III supplies some of the extraocular muscles, while the parasympathetic nucleus of CN III innervates smooth muscle of the eye and orbit. Some nuclei contain neurons of multiple cranial nerves yet all those neurons have a similar function. For example, the nucleus ambiguus, comprising neurons associated with CN IX, X and XI, innervates the striated muscle of the larynx and pharynx.

Cranial nerves may have one type of function only, or have several functions. It should be noted that even the ‘motor only’ nerves may contain proprioceptive afferent fibres. Motor fibres of cranial nerves form the lower motor neurons (LMNs) of the head region (Table 10.3). Note that CN VIII has traditionally been described as purely sensory, however efferent fibres have been described in both the vestibular and cochlear portions of CN VIII (Chapter 10, olivocochlear reflex).

Table 10.3 General functions of cranial nerves

Sensory only	I, II,
Motor only	III, IV, VI, XI, XII
Mixed sensory and motor	V, VII, VIII*, IX, X,
Autonomic (parasympathetic)	III, VII, IX, X

* efferent components have been described in CN VIII

Rather than considering the 12 cranial nerves in isolation, it is useful to group them by function.

Olfaction

Key points

Odoriferous substances stimulate olfactory neurons. Action potentials pass caudally into the cranial vault, synapsing in the olfactory bulb. Olfactory tract axons project to both sides of the forebrain for olfactory perception, stimulation of behavioural responses and to the brainstem for reflex function.

Input from the vomeronasal organ is via the olfactory nerve and may stimulate the Flehmen reaction during sexual activity in some species.

The name olfactory ‘nerve’ is actually a misnomer since it consists entirely of CNS tissue (see p1), however in humans the olfactory bulb is so diminutive as to resemble a nerve. In veterinary species, the olfactory bulb is prominent (Figs. 10.3, A2, A3, A10).

Fig. 10.3 Comparison of the olfactory bulbs of the dog (macrosmatic) and human (micromatic) brains, ventral aspects.

(Image of human brain courtesy of Dr. Henry Waldvogel, University of Auckland.)

The olfactory mucosa is located on the ethmoidal labyrinth and dorsal nasal septum in the dog. Grossly, the olfactory mucosa may be slightly yellowish compared with surrounding mucosa, due to pigment in the sustentacular cells. Afferent fibres from the olfactory mucosa and vomeronasal organ contribute to CN I, which is unmyelinated. Olfactory neurons are bipolar cells with 6–8 long cilia that project into the overlying mucus in the caudodorsal part of the nasal cavity. Olfactory receptors are also located in the vomeronasal organ on the rostral floor of the nasal cavity. These receptors may respond to pheromones, which are chemicals that trigger social responses between members of the same species. The vomeronasal organ may mediate the curling of the upper lip, the Flehmen reaction, in males scenting females with respect to mating suitability. Odoriferous substances dissolve in the mucus overlying the olfactory mucosa, or in the vomeronasal organ, stimulating the olfactory neurons. As odoriferous substances can be cytotoxic, olfactory neurons can be renewed from stem cells located at the base of the olfactory mucosa. The potential for harvesting olfactory stem cells and using them as a source of new neurons for a patient is currently an area of active research in neuroscience.

Axons from the olfactory mucosa and the vomeronasal organ pass through the cribiform plate into the cranial vault and synapse on the olfactory bulb neurons. Post-synaptic axons travel caudally in the olfactory tract of the olfactory peduncle. The tract splits into lateral, intermediate and medial stria (see Fig. A11). The axons of the lateral stria synapse in the olfactory tubercle and pass to the cortex of the piriform lobe for olfactory perception. Fibres also connect to the limbic system (see Chapter 11). More medial fibres pass to the septal nuclei, which are located between the rostral aspects of the lateral ventricles, and to the hypothalamus and reticular formation of the brainstem. Fibres also decussate via the rostral commissure and pass to the contralateral olfactory bulb. The rostral commissure also interconnects the two piriform lobes. Through the connections to the limbic system, cerebral cortex and hypothalamus, olfaction can be a potent stimulator of behaviour and emotional states. Connections to the brainstem permit olfacto-visceral reflexes such as salivation in response to olfactory stimulation (Fig. 10.4).

Fig. 10.4 Canine brain, ventral aspect, olfactory pathways and their connections.

Note that olfaction does not pass through the thalamus; this is different to all other afferent information which does pass through the thalamus en route to the cerebral cortex.

Lesions in one olfactory bulb lead to unilateral anosmia; this is hard to detect clinically. Bilateral lesions are required in the olfactory mucosa, or bulbs, peduncles or piriform lobes to cause complete anosmia.

Vision and CN II functions

Key points

The optic nerve is myelinated by oligodendrocytes. It is the only part of the CNS that can be observed on clinical examination.

Photoreceptors in the deepest part of the retina convert light into action potentials. The action potentials are modified by superficial layers of neurons, and exit the retina as axons in the optic nerve.

The majority of optic nerve axons decussate, but the degree of decussation depends on the type of animal and correlates inversely with the degree of binocular vision.

Caudal to the optic chiasm the majority of optic nerve axons travel to the lateral geniculate nucleus of the thalamus and then to the visual cortex for visual perception. The remaining axons course to the midbrain for reflex function.

General anatomy

The optic nerve is actually a tract of the CNS as it is myelinated by oligodendroglia, thus the term ‘nerve’ is a misnomer (p1). It is the only part of the CNS that can be observed on a neurological examination.

Photoreceptors (rods and cones) are the receptors of the visual system and convert light into receptor potentials. Rods are highly sensitive to light, therefore they are used during dim light conditions, while cones are less sensitive to light but respond to specific light frequencies, that is, colours. The presence of 20 times more rods than cones in the canine retina accounts for dogs’ good night vision. The vertebrate retina is inverted in the sense that the light-sensing cells sit at the back of the retina, so that light has to pass through layers of neurons before it reaches the photoreceptors. This arrangement ensures that incoming light only stimulates photoreceptors once. If the photoreceptors were at the front of the retina, then some of the incident light would stimulate them, while the remaining light could pass to the back of the retina. There it would be reflected back into the eye at a new angle stimulating different photoreceptors. Thus light from a single visual point would stimulate photoreceptors in different regions (incident versus reflected) causing loss of resolution and visual blurring.

There is a three-stage neuronal system in the retina. The photoreceptors form the deepest layer, the middle layer contains integrating neurons, while the superficial layer contains the multipolar ganglion neurons, the axons of which pass across the retina to collect at the optic disc and form the optic nerve. Thus light passes through the cornea, anterior chamber, iris, posterior chamber, ganglion and bipolar layers to reach the photoreceptors. Receptor potentials from the rods and cones undergo complex processing by integrating neurons of the retina but ultimately result in action potentials in retinal ganglion cells. Several important features of visual perception can be traced to the retinal encoding and processing of light.

In most domestic mammals, axons of retinal neurons are unmyelinated until they coalesce at the optic disc. Thus the opaque nature of myelin does not interrupt the passage of light through the retinal layers to the photoreceptors. However, there are no photoreceptors at the optic disc, resulting in a blind spot in the visual field. Myelination of the optic nerve fibres makes the optic disc appear creamy white on fundic examination. In some species, such as rabbits, there is some myelination of retinal ganglion axons. This may be observed clinically as a white streak extending from the optic disc.

The optic chiasm and decussation

The optic nerve extends caudally from the retina, through the optic foramen in the presphenoid bone into the neurocranium. It joins the ventral aspect of the diencephalon at the optic chiasm, just rostral to the hypophysis (pituitary gland). In general, the majority of axons decussate, but the degree of decussation depends on the type of animal. In fishes and birds, all fibres decussate (Fig. 10.10). In mammals, there is partial decussation (ungulates about 80–90%, dogs 75%, cats 65%, primates 50%).

Light also crosses in the lens of the eye, thus axons decussating at the optic chiasm come from the nasal retina and temporal field of view, while the temporal retinal axons receiving the nasal field of view, remain ipsilateral (see Fig. 10.5).

Fig. 10.5 (A) The optic pathway and transmission of light stimuli to the visual cortex. (B) Examples of optic axes of the sheep, dog and cat, identified using metal probes placed in the optic canals. The angles between the axes for the sheep, dog and cat are 100°, 70° and 58°, respectively; the decreasing angles indicate increasing overlap of the visual fields.

The visual pathway continues caudal to the optic chiasm as the optic tracts. In animals in which there is partial decussation at the optic chiasm, each optic tract contains axons from the contralateral nasal field and the ipsilateral temporal field. Thus the optic tract contains axons from retinal areas of the two eyes that are receiving input from the same part of the visual field (Fig. 10.5A).

The placement of the eyes and decussation of the optic chiasm is functionally significant. Animals with laterally placed eyes, e.g. ungulates/prey animals (Fig. 10.5B) have wide fields of view and a high percentage of fibres decussate at the optic chiasm. Thus the majority of visual input is processed in the contralateral visual cortex. A wide field of view permits these animals to see behind them; this is useful to detect danger such as stalking predators. But depth of field/binocular vision requires processing of visual input from both eyes in the same visual cortex, thus prey species have poor binocular vision. Conversely, animals with eyes that face forward, like the cat, have smaller fields of view, but have better binocular vision due to decreased decussation at the optic chiasm. This results in the animal receiving input from each eye on both sides of the brain. For example in Fig. 10.5A, input from the orange diamond located in the left side of the visual field will project to the nasal retina of the left eye and the temporal retina of the right eye. The input from those retinal areas will both end up in the same visual cortex – that is the right side one. Thus the same region of the brain processes two sets of information seen by the separate eyes, about the same object. This dual perspective results in binocular vision which is essential for depth perception. Depth perception is required for accurate judgement of distance and how fast an object is moving, therefore it is essential for predatory animals. The ability to perform coordinated, conjugate eye movements, including convergence (focussing both eyes on the same object) is also greater in animals with forward-facing eyes and reduced decussation.

The total field of view combined from both eyes in the horse is approximately 320°. That comprises a field of view for each eye being about 200°s, with about 65° of overlap. The overlapping visual field is especially well developed ventrally for viewing objects on the ground. However, there is not overlap immediately in front of the nose, creating a blind spot. Most dogs have a total visual field of 250°. The degree of binocular overlap is about 75° for long-nosed dogs to 85° for short-nosed breeds. Comparatively, humans have a maximum total horizontal field of view of approximately 180°, 120° of which comprises the binocular field of view, flanked by two uniocular fields of approximately 40°.