Cranial Nerves of Horse
Cranial Nerves of Horse
Throughout the body is a complex network of nerves that run from the spinal cord out into the body and limbs of the horse. Most of these major nerves branch off of the spinal cord before making their way out into the body where they help send signals to and from the brain. But there is a special set of nerves that originate not from the spinal cord but from the brain itself inside the cranium of the skull. These nerves are referred to as Cranial Nerves and are numbered using Roman numerals. These series of nerves help in the function of many systems of the horse and problems in any of these nerves can affect the whole horse.
The Cranial nerves all have their unique functions and will always be found in pairs that branch to each side of the body. Some of the Cranial nerves are for sensory input and will relay messages to and from the brain. Other nerves are used for motor function to help control muscle movement. Some pairs of nerves serve both sensory and motor functions. All the thirteen nerves but Cranial nerve IV exit the brain on the bottom (ventral) surface and then pass through small openings in the skull called foramen. Cranial nerve IV exits the brain on the top (dorsal) surface but immediately follows down to the bottom of the brain to join the other nerves.
The first two Cranial nerves both deal with the sense of smell. The first nerve is CN 0 and is called Nervus Terminalis. It is associated with sensing pheromones and helps to trigger mating behavior. CN I is the Olfactory Nerve and is responsible for the horse’s sense of smell. Both these nerves are sensory nerves and only relay information about smell to the brain. The signals from these nerves do not travel to the thalamus of the brain so there is no delay in the sensory input. Thus this information can instantly trigger the fight or flight process and evolved for survival.
Cranial nerves II, III, IV, and VI all are associated with vision and the eyes. The Optic Nerve, CN II, is responsible for vision and completely a sensory nerve. The Oculomotor nerve (III), the Trochlear nerve (IV), and the Abducent nerve (VI) are a motor nerve and responsible for eye movement, pupil size, and focusing the lenses. CN II transmits sensory information from the retina of the eye on to the brain. Possible issues in these nerves will affect the horse’s vision which may present with the horse bumping into things and spooking on the affected side.
One of the most important Cranial nerves to remember is CN V or the Trigeminal Nerve. It is complex nerve that has three branches that travel out into the head of the horse. This nerve is both responsible for sensation to the head and face, but also motor function to the chewing muscles. The information sent to the brain covers everything from temperature, pressure, proprioception, and pain. The three branches of this nerve are the ophthalmic, maxillary, and mandibular portions. Pain stemming from dysfunction in this nerve can cause behavioral issues such as head shaking, head tossing, chewing on the bit, difficulty bridling/haltering, sensitivity to the head/poll area, tensed muscles of the jaw, or a number of other unwanted behaviors.
Three nerves are associated with the tongue and muscles of the face. These are the Facial nerve (VII), Glossopharyngeal nerve (IX), and the Hypoglossal nerve (XII). The Facial nerve is responsible for the motor function of the facial muscles, but also sensory input from the tongue for taste. The Hypoglossal nerve is responsible for motor of the muscles of the tongue. The Glossopharyngeal nerve (IX) is responsible for swallowing and some motor tongue movement. Issues with these nerves can show up as facial asymmetry, drooling out one side of mouth, difficulty drinking/eating, or partial paralysis of the tongue.
The Vestibular nerve (VIII) is responsible for hearing and the horse’s balance. This nerve sends sensory input from the inner ear to the brain. Where for balance and proprioception it is motor control coming from the nerve. This is a key nerve to look at for horse’s having hearing problems, head tilts, or any balance issues.
The Vagus nerve (X) is the only Cranial nerve that extends beyond the head and neck of the horse. It is mainly a sensory nerve but does have some motor functions. This nerve helps with sensory of the gastrointestinal tract and respiratory tract, motor to the larynx, and parasympathetic motor to the abdominal and thoracic organs. This nerve can play a role in horses who have issues breathing like roaring, along with horses with digestive issues.
The Spinal Accessory nerve (VI) is a motor nerve for the muscles of the neck and withers. This is not a sensory nerve but provides motor control for movement of the neck in the horse. While there is no sensory input, this nerve can be compromised by poor saddle fit, improper neck positioning under saddle, and horses overworking the neck muscles.
All of the Cranial nerves are vital to a horse’s day to day activities and when not fully functioning can affect the whole horse. It is key to have a full veterinary workup if any horse is showing issues that might relate to the Cranial nerves. Once they have ruled out any true diseases or medical conditions an owner can call in an alternative practitioner to access the horse. Cranial Sacral therapy is a great starting place to help restore proper function to these nerves and balance to the whole nervous system.