Cranial nerves 1,2, and 3 lecture

Are cranial nerves part of the central or peripheral nervous system?

They are considered part of the peripheral nervous system (PNS).

What type of axons can each cranial nerve have?

Motor (efferent) axons, sensory (afferent) axons, or both.

What is the function of the frontal lobe of the cerebral cortex?

Voluntary movement, planning, decision-making, speech, and personality.

What is the function of the parietal lobe?

Touch, pain, spatial awareness, and sensory integration.

What is the function of the temporal lobe?

Hearing.

What is the function of the occipital lobe?

Vision and visual perception.

What is the function of the insula?

Taste, internal sensations, and emotion.

what are the components of the basal nuclei (cerebral nuclei)?

Caudate nucleus, putamen, and globus pallidus

What is the function of the basal nuclei?

Regulate movement, muscle tone, and posture; help initiate and coordinate voluntary motor activity.

What structures make up the olfactory portion of the rhinencephalon?

Olfactory nerve (CN I), olfactory bulb, olfactory peduncles, and entorhinal cortex (piriform lobe).

What are the three main structures of the limbic system listed?

Amygdala, septum, and hippocampus.

What does the thalamus do with sensory information?

Receives all sensory information (except olfaction) and projects it to specific areas of the cortex.

What are some functions regulated by the hypothalamus?

Nervous and endocrine system regulation, heart rate, blood pressure, body temperature, water balance, food intake, circadian rhythms, and emotions.

What two hormones are directly secreted by the hypothalamus into the neurohypophysis?

Oxytocin and vasopressin.

What sensory functions does the midbrain (mesencephalon) receive and mediate?

Receives visual and auditory signals and mediates movement of the head in response to light and sound.

Which cranial nerves are involved in the motor portion of the midbrain for eye movement?

Oculomotor (CN III) and trochlear (CN IV), plus abducens (CN VI) from the medulla.

What cranial nerve arises from the pons, and what are its functions?

The trigeminal nerve (CN V) — facial sensation and masticatory muscle movement.

Besides the trigeminal nerve, what other functions are associated with the pons?

: Communicates with the cerebellum (regulation of movement), participates in respiratory control, and centers for integration/control of micturition.

Which cranial nerve nuclei are contained in the medulla oblongata?

Nuclei of cranial nerves VI to XII.

What sensory information does the medulla oblongata relay from the spinal cord?

Tactile, temperature, and pain sensation, plus proprioception

What vital functions are controlled by the medulla oblongata?

Cardiovascular and respiratory functions.

What is the olfactory epithelium located on, and how does its area in dogs compare to humans?

Located on the ethmoturbinate bones of the nasal cavity. In dogs, it occupies ~100 cm² compared with ~5 cm² in humans.

What type of cells are olfactory receptor neurons, and where are they located?

Bipolar sensory cells in the olfactory mucosa (beneath the cribriform plate).

What covers the cilia of olfactory receptor neurons, and what do the cilia contain?

Cilia are covered by mucus and contain receptor proteins that detect odor molecules → transduction of smell signal.

Describe the transduction of an olfactory stimulus step by step.

Odorant binds to receptor on cilia → G protein (G_olf) activated → adenylyl cyclase converts ATP to cAMP → cAMP opens Na⁺/Ca²⁺ channels → depolarization → Ca²⁺ opens Cl⁻ channels → more depolarization → action potential travels to olfactory bulb.

What are the small fascicles of axons formed by olfactory receptor cells called, and where do they synapse?

Fila olfactoria. They pass through the cribriform plate and synapse in the olfactory bulb with tufted and mitral cells.

What are anosmia and hyposmia?

Anosmia = loss of smell. Hyposmia = reduced ability to smell.

What are causes of anosmia listed in the notes?

Olfactory mucosa inflammation, viral infections (e.g. parainfluenza), head trauma damaging the olfactory nerve, and lesions in the olfactory bulb or nerve.

Why should irritating substances NOT be used when testing smell?

Because they can stimulate the trigeminal nerve in the nasal mucosa and cause a reaction even if the olfactory system is not working properly

What are the three layers of the eye?

1. Fibrous tunic (sclera and cornea) — support & protection

2. Vascular tunic/uvea (iris, ciliary body, choroid) — pigmented & vascularized

3. Neuroepithelial tunic (retina) — innermost sensory layer

What is the difference between rods and cones in the retina?

Rods have flattened membranous sacs; detect dim light and shades of gray. Cones have membranous invaginations; responsible for color vision.

How does color vision differ between humans and dogs?

Humans have trichromatic vision (three types of cones: red, green, blue). Dogs have dichromatic vision (cones for blue and yellow only) and cannot distinguish red and green (red-green color-blind).

What is the optic disc, and why is it the blind spot?

The optic disc is about 1–2 mm in diameter, has no photoreceptors, and is where the optic nerve exits the eye — representing the blind spot.

Which retinal fibers cross at the optic chiasm, and which remain on the same side?

Fibers from the medial (nasal) retina cross at the optic chiasm; fibers from the lateral (temporal) retina remain on the same side.

What three nuclei do the optic tracts terminate in?

Lateral geniculate nucleus, rostral colliculus, and pretectal nucleus.

where does the oculomotor nerve (CN III) originate, and what are its two nuclei per side?

It originates from nuclei in the rostral (anterior) ventral area of the midbrain. There are 2 nuclei on each side: 1 for eye movement (somatic) and 1 for iris constriction (parasympathetic).

What are the functions of the oculomotor nerve (CN III)?

Motor control of iris constrictor muscles (parasympathetic — pupillary constriction) and somatic motor innervation of some medial rectus and retractor bulbi muscle.

Describe the subcortical pathway of the pupillary light reflex.

A small percentage of optic nerve fibers synapse in the pretectal area (midbrain) → signals sent to Edinger-Westphal nucleus → preganglionic parasympathetic fibers exit via CN III → synapse in ciliary ganglion → postganglionic fibers innervate sphincter pupillae → pupil constriction.

What clinical sign results from damage to CN III or its somatic nucleus?

Ventrolateral strabismus.

What are the effects of a lesion of the left optic nerve (Lesion 1)?

Blindness of the left eye. Shining a light in the left eye does not induce the PLR in either eye, whereas shining a light in the right eye induces both direct and consensual responses.

What are the effects of a lesion of both optic nerves (Lesion 2)?

Blindness of both eyes. There is no PLR in either eye.

What are the effects of a lesion involving one optic tract (Lesion 3)?

Partially impairs vision but does not affect the pupillary response of either eye.

What are the effects of a lesion of the oculomotor nerve (Lesion 4)?

Induces lateral strabismus, ptosis (narrowed palpebral fissure), and pupillary dilation. There is no PLR in the left eye.

Where is the nucleus for the trochlear nerve (CN IV) located, and what is its function?

Caudally in the mesencephalon (midbrain). Motor innervation of the dorsal oblique muscle — helps keep the visual world "steady" by counter-rotating the eye when the head tilts to the side

How is CN IV dysfunction observed in cats and horses (species with slit-like pupils)?

A dorsolateral rotation of the affected eye can be observed. In dogs, lesions are difficult to diagnose (use a fundoscope to look for rotation).

Where are the nuclei of the trigeminal nerve (CN V) located, and what is the large sensory ganglion called?

Nuclei (motor and sensory) are in the pons. The large sensory ganglion is the trigeminal (or Gasserian) ganglion, containing primary nerve cell bodies for the sensory component.

What regions are innervated by each sensory branch of the trigeminal nerve?

Ophthalmic branch — ocular region and medial canthi. Maxillary branch — nasal region and lateral canthi. Mandibular branch — buccal area.

What is the function of the motor component of the trigeminal nerve?

Innervates the muscles involved with mastication.

How is the palpebral reflex tested as part of the CN V clinical examination?

Stimulate the medial canthus (ophthalmic branch), lateral canthus (maxillary branch), or skin over mandible (mandibular branch) and look for a blink in response (normal).

What clinical signs indicate motor loss of the trigeminal nerve?

Masseter and temporalis muscle wasting, and loss of jaw tone.

What clinical signs indicate sensory loss of the trigeminal nerve?

Absent palpebral reflex and lack of sensation in the face.

Where is the nucleus of the abducens nerve (CN VI) located, and what is its function?

nucleus is in the dorsal medulla (brainstem). Motor — innervates the lateral rectus and retractor bulbi muscles (also innervated by oculomotor nerve

What clinical signs are seen with CN VI dysfunction?

Medial strabismus, loss of lateral gaze, and inability to retract the globe.

What are the functions of the facial nerve (CN VII)?

Motor and sensory. Motor branches innervate facial muscles. Sensory branches innervate the palate and cranial two-thirds of the tongue (taste and pain/proprioception). Also parasympathetic innervation of glands (submandibular salivary, lacrimal).

Where are the multiple specific nuclei of the facial nerve located?

In the medulla.

How is the facial nerve (CN VII) clinically examined?\

Observe facial asymmetry, droopy eyelids and lips, inability to blink properly. Observe Menace Response and Palpebral Reflex. For palpebral closure, brush a finger across the palpebral commissure and observe how tightly the animal can close its eye.

What does the vestibulocochlear nerve (CN VIII) consist of?

Two branches: the cochlear (auditory) nerve — sensory pathway for hearing; and the vestibular nerve — involved in orienting the head and body

What structures make up the vestibular portion of CN VIII, and what are they responsible for?

The vestibular nerve and four vestibular nuclei in the brainstem (medulla). They are responsible for normal posture and coordination of the eyes, head, neck, trunk, and limbs.

Where is vestibular information projected?

to CN III, IV, and VI via the medial longitudinal fasciculus (MLF), and also to the cerebellum.

What does the oculocephalic reflex test, and what is a normal response called?

Tests the vestibular system. Moving the head from side to side should produce symmetrical, coordinated eye movement — called NORMAL nystagmus.

What is abnormal nystagmus, and what are its types?

Eye movement in a stationary animal, repeatedly back and forth. Types: horizontal, vertical, rotatory (or pendular).

What signs indicate vestibular disease?

Vestibular ataxia, circling, head tilt, and abnormal nystagmus.

What is the function of the glossopharyngeal nerve (CN IX)?

Both motor and sensory (mixed). Motor innervation of pharyngeal muscles and parasympathetic innervation to parotid salivary glands. Sensory: tongue (caudal third), taste (caudal third), and pharynx mucosa.

Where are the glossopharyngeal nuclei located?

In the ventrolateral aspect of the medulla.

How is CN IX clinically examined?

History of dysphagia or regurgitation. Some clinicians check the "gag" reflex.

What is the major function of the vagus nerve (CN X) regarding the viscera?

Supplies parasympathetic innervation to all the thoracic and abdominal viscera and carries visceral sensory fibres from those tissues.

What motor and sensory functions does the vagus nerve have in the head/throat?

Motor and sensory innervation of the muscles of the pharynx, larynx, and palate; sense of taste (much overlap with CN IX).

How can the parasympathetic activity of the vagus nerve be evaluated clinically?

The Oculocardiac Reflex may be employed — pressure on the eyeballs results in a decreased heart rate.

What are potential signs of vagus nerve dysfunction?

Dysphagia, salivation (historical), regurgitation of undigested food, and chronic indigestion (vagus indigestion) in cattle.

What muscles are innervated by the spinal accessory nerve (CN XI), and where does it originate?

Motor innervation of neck muscles including the trapezius. Originates from ventral roots of cervical segments and enters the skull through the foramen magnum before exiting through a foramen.

How is CN XI clinically examined, and what is an example of clinical disease?

Examine for neck muscle atrophy or asymmetry. Clinical disease: neck muscle atrophy (not often seen).

What muscles are innervated by the hypoglossal nerve (CN XII), and where does it originate?

Motor innervation of intrinsic and extrinsic muscles of the tongue. Originates in the hypoglossal nucleus in the medulla.

How is CN XII clinically examined, and what is a sign of dysfunction?

Observe the function and position of the tongue relative to the rest of the mouth. Paralysis and atrophy of the tongue (uni- or bilateral) is a sign of dysfunction; lateral deviation of the tongue may be caused by damage.