Cranial Nerves – A Reference for Vetrehabbers

For those of us who remember studying the cranial nerves for the first time, we likely gave them very little attention and focus.

First-year anatomy is, after all, overwhelmingly heavy and full, with many more important things to memorize. And that is true. In first year 😊

However, as Vetrehabbers in the field, you have likely realized that the nervous system is far more fascinating and complex than previously imagined.

So, for interest sake, let’s review and dive deeply into the cranial nerves.

 

Introduction

There are 12 pairs of cranial nerves that originate from the brain and the brainstem. They can carry motor, sensory, mixed, or autonomic nerve fibers to match their function.

Here, we discuss the 12 nerves, their origin, function, fiber type, and the reflexes that can be performed to test each one. 

The 12 cranial nerves are:

1. 1. • CN I: olfactory nerve
      • CN II: optic nerve
      • CN III: oculomotor nerve
      • CN IV: trochlear nerve
      • CN V: trigeminal nerve
      • CN VI: abducens nerve
      • CN VII: facial nerve
      • CN VIII: vestibulocochlear nerve
      • CN IX: glossopharyngeal nerve
      • CN X: vagus nerve
      • CN XI: accessory nerve
      • CN XII: hypoglossal nerve

Sensory and motor functions 

Peripheral nerves (like the cranial nerves) are either sensory, motor, a combination of the two, or autonomic.

Let’s briefly examine these terms and their relation to the cranial nerves.

Sensory neural input (or afferent signals) are divided into 3 categories:

somatic sensory, visceral sensory, and special sensory.

Somatic sensory input includes proprioception, pressure, touch, pain, temperature, and vibration signals, which come from an internal or external source.

Visceral sensory input (or visceral afferent) includes signals from the internal organs like the gastrointestinal tract, the heart and lungs, blood vessels, bladder, etc. They relay information relating to injury of the tissues they innervate, or the threat of injury such as stretch, or pressure. They can also influence autonomic outflow.

Special sensory input relates to our senses and is further divided into special visceral, and special somatic sensory.

Special visceral includes smell and taste sensory information, while special somatic includes sensory information related to hearing, vision, and balance.

Motor output or efferent signals are divided into two systems – the somatic and autonomic systems.

The somatic system relates to skeletal muscles – motor output fibers signal the activation of muscle fibers to elicit movement.

The autonomic system sends motor signals to smooth muscle, cardiac muscles, and glands.

The autonomic system is further divided into two systems – the sympathetic and parasympathetic systems. These two systems innervate the same organs but will have an opposing effect on those organs.

The autonomic system is controlled by the hypothalamus and the nuclei in the brainstem.

The parasympathetic system exits the nervous system at the cranium (as specific cranial nerves) and at the sacrum. It has a craniosacral outflow. The vagus nerve provides parasympathetic supply to the thoracic and abdominal cavities by traveling caudally from the head to the trunk via the vagosympathetic trunk. 

The sympathetic nerves exit at the thoracic and lumbar vertebrae – they have a thoracolumbar outflow. The head receives its sympathetic nerve supply from the first three thoracic spinal segments. These nerves project cranially along the vagosympathetic trunk to the postganglionic axon in the cranial cervical ganglion.

Cranial nerves that form part of the parasympathetic system with general visceral efferent fibers include the following:

• • CN III: oculomotor nerve
  • CN VII: facial nerve
  • CN IX: glossopharyngeal nerve
  • CN X: vagus nerve

This diagram provides us with a simplified visual summary of the area each cranial nerve innervates:

CN I: olfactory nerve

The olfactory nerve travels through the cribriform plate of the ethmoid bone via many foramina.

Instead of having one recognizable nerve trunk, it originates from the entire surface of the olfactory bulbs and terminates in the nasal cavity. The terminal nerve and vomeronasal nerve are included in the olfactory nerve.

The axons of the olfactory nerve are unmyelinated, and the fibres are sensory in function, detecting “smell” (scents)  from within the nasal cavity.

To test the olfactory nerve function, we test the patient’s sense of smell.

 

CN II: optic nerve

The optic nerve is not a true peripheral nerve, but instead forms part of the central nervous system.

It originates from the visual cortex of the occipital lobe, arches around the diencephalon in the optic tract and crosses over at the optic chiasm. It then exits the skull at the optic canal of the presphenoid bone, and travels to the eye.        

The axons are myelinated and carry sensory information from the eye to the brain, conveying sight.

To test this nerve, we need to test the patient’s visual acuity, which we do in several ways. These evaluations include the menace response and by assessing whether an animal can follow a moving object with both eyes.

 

CN III: oculomotor nerve

The oculomotor nerve originates in the rostral mesencephalon in the midbrain and contains somatic efferent neurons (or motor neurons) that innervate the extraocular muscles.

It also contains parasympathetic nerve fibres which innervate the ocular smooth muscles and regulate pupil dilation and constriction.

You can test this nerve through pupil size regulation and through the pupillary light reflex. 

 

CN IV: trochlear nerve

The trochlear nerve originates in the tegmentum of the midbrain and consists of motor fibres that innervate and control the dorsal oblique muscle of the eye.

To test this nerve, we ask the patient to look down.

 

CN V: trigeminal nerve

The trigeminal nerve originates from the brainstem at the junction between the caudal mesencephalon and the pons, extending bilaterally to both sides of the head and dividing into three branches – the ophthalmic, maxillary, and mandibular branches.

The ophthalmic and maxillary branches contain only sensory fibres, while the mandibular branch contains sensory and motor fibres.

The ophthalmic branch innervates the upper parts of the face and skull, particularly the eyes, upper eyelids, and forehead. This branch is tested by touching the medial canthus of the eye to elicit a blink response.

The maxillary branch provides sensation to the middle part of the face, including the cheeks, nose, lower eyelids, upper lip, and gums. This branch is tested by touching the upper lip lateral to the canine tooth.

The mandibular branch provides sensation to the lower parts of the face, including the jaw, lower lip, and gums. It also provides the motor innervation to bite, chew and swallow. This branch is tested by touching the lower lip lateral to the canine tooth.

 

CN VI: abducens nerve

The abducens nerve originates from the brainstem and contains motor nerves that innervate the lateral rectus muscle of the eye.

This muscle moves the eye horizontally, and is tested by encouraging the patient to move their eye to the side as they follow a treat, while their head is held steady.

 

CN VII: facial nerve

The facial nerve originates from the medulla oblongata and contains somatic and visceral sensory and motor nerves.

It provides motor innervation to the submandibular, sublingual and lacrimal glands. It also provides sensory innervation to the anterior part of the tongue and the soft palate.

To test this nerve, we can pinch the inner surface of the pinna.

 

CN VIII: vestibulocochlear nerve

The vestibulocochlear nerve originates from the brainstem at the junction between the pons and the medulla oblongata, and contains sensory fibres.

It is divided into the vestibular nerve, which is responsible for maintaining balance, and the cochlear nerve, which is responsible for hearing.

This nerve is tested in two ways – firstly, by their reaction to sound, and, secondly, by looking for nystagmus of the eye when the head is moved.

 

CN IX: glossopharyngeal nerve

The glossopharyngeal nerve originates from the medulla oblongata and contains sensory, motor and parasympathetic nerve fibres.

It innervates the back of the tongue, pharynx, tonsils, middle ear and soft palate. It also acts to regulate blood pressure in the carotid sinus, moderating saliva production, providing a sense of taste at the back of the tongue, controlling the pharynx.

It is tested by stimulating the gag reflex.

 

CN X: vagus nerve

The Vagus nerve originates from the ventrolateral aspect of the medulla oblongata and contains sensory, motor and parasympathetic nerve fibres. It travels from the medulla oblongata, down the neck along the vagosympathetic trunk, through the thoracic inlet to the thoracic and abdominal cavities.

It provides sensory innervation to the heart, lungs, trachea, bronchi, larynx, gastrointestinal tract, and external ear. It also provides motor innervation to the heart, lungs, palate, pharynx, larynx, bronchi, and gastrointestinal tract.

It further aids in controlling and regulating heart rate and the gastrointestinal tract, and providing sensory information from the organs to the brain.

 

CN XI: accessory nerve

The accessory nerve originates from the lateral aspect of the medulla oblongata and contains motor nerve fibres.

It innervates the brachiocephalicus and trapezius muscles of the neck and shoulder.

 

CN XII: hypoglossal nerve

The hypoglossal nerve originates in the medulla oblongata and contains motor fibres that control the tongue.

It is closely related to the glossopharyngeal nerve and is tested by encouraging the patient to lick or to retract their tongue.