Nerves in the human body control the movements that we carry out, and also carry sensory information from various areas of the body. 12 of the most important nerves innervate (provide a nerve supply to) parts of the head. These are the Cranial Nerves.
Cranial nerves differ from other nerves in the body by coming directly from the brain. Other nerves arise from another part of the nervous system, the spinal cord. Also, as they 'wire' parts of the head, they have to receive and send information to the most important structure, such as the eyes, muscles of facial expression and so on.
Before continuing with the entry, a few bits of jargon will have to be made clear.
Superior - Above a structure
Inferior - Below a structure
Medial and Lateral - You will need a mirror for this one. Look at yourself in the mirror, and draw an imaginary line through your nose and down the middle of your body. Structures which are nearest this mid-line are medial, and structures which are further away from this line are lateral. For example; your ear is lateral to your eye, which is in turn, medial to your ear.
Levator - Think of levitation. This term is often used to describe the action of a muscle. As the name suggests, the action would be lifting the structure which it is attached to.
Oblique - The anatomical term for diagonal
Constriction - Gets narrower
Dilation - Gets wider
Foramen - Latin for a hole. The plural for this term is foramina.
As said before, there are 12 nerves, each with different areas to supply and receive information. All of them have, at the most, three different types of fibres. If this is hard to imagine, think of these fibres as different types of wire within a big cable, carrying different types of information to and from the relevant area of the brain.
Sensory - This one is pretty self-explanatory. There are five senses: touch, hearing, sight, taste and smell1. This is what the fibres carry; the signals from the nerve endings go from the innervated area, ie areas of taste, olfaction (smelling), vision and or touch, to the brain, so that it can be analysed and you can decide what to do next.
Motor - Again, self-explanatory. This type of fibre carries signals from the brain to muscles which then move - hence these are called 'motor' fibres.
Parasympathetic - This one is slightly more difficult. The Parasympathetic Nervous System is part of the Autonomic Nervous System - the wires from the autopilot of the body; the brainstem and spinal cord. This governs all those things that you may take for granted eg, breathing, heart rate and blood pressure. So this sort of fibre will be part of your autopiloting system.
However, this is not to say that all unconscious activities are governed by the Autonomic Nervous System, and all conscious activities are controlled by a system known as the Somatic Nervous System; basically, the conscious nervous system. In the human body, nothing is as simple as it seems. Motor fibres can also be as involuntary as the autonomic fibres.
The best example of this is the response to a painful stimulus to the hand, say unintentionally putting your hand on a hot stove. The action of jerking your hand away from the heat source is involuntary, yet the muscles which enable you to do this are innervated by motor fibres, not fibres from the Autonomic System as you might think. Even the action of breathing, although governed by the Autonomic Nervous System, requires skeletal muscles that are in between the ribs, called intercostal muscles, to contract. This also requires the involvement of motor fibres. The same goes for blinking. The one aspect that connects the two types is that the signals originate from the same place; the brainstem and spinal cord. Signals sent from the brainstem may travel into the brain to be further analysed before any other action takes place; once a decision is made, then a signal will be sent to the place of action, via nerves.
The Cranial Nerves
Now, having got that all out of the way, here are the 12 nerves. They are numbered in Roman numerals, from I to XII.
Cranial Nerve I - Olfactory
The Olfactory Nerve, or Cranial Nerve I, is a purely sensory nerve. As you may have gathered from its name, it is mainly concerned with smelling, and hence innervates the nose. To be more precise, it innervates the olfactory epithelium. In plain English, that means the surface cells which are concerned with the detection and the discrimination of scent.
Cranial Nerve II - Optic
The Optic Nerve is, as its name suggests, the one which innervates the eye. Like the Olfactory Nerve, it is a purely sensory nerve. It receives signals from the retina; the optic equivalent of the olfactory epithelium; surface cells at the back of the eye which are specialised in vision.
Cranial Nerve III - Oculomotor
The names of the Cranial Nerves are very logical, and this theme is carried through to Cranial Nerve III, the Oculomotor Nerve. This has two functions; one is carrying motor fibres to several muscles that move the eyeball, the Superior, Inferior, Medial rectus muscles; the Inferior Oblique muscle, and Levator Palpebrae muscles. It also supplies parasympathetic fibres to the iris and the lens, enabling constriction and dilation of the pupil according to the brightness of the light and focusing on objects that are distant or near respectively.
Cranial Nerve IV - Trochlear
The name of this one is unfortunately, the exception to the rule of naming Cranial Nerves. The Trochlear Nerve is a purely motor nerve, and it supplies motor fibres to one muscle only: the Superior Oblique Muscle. This muscle is also involved in movement of the eyeball.
Cranial Nerve V - Trigeminal
The Trigeminal Nerve is called thus because it has three sensory divisions.
The Opthalamic Division of the Trigeminal Nerve - This carries sensory signals from the cornea (the surface of the eye), parts of the face and the front half of the scalp.
The Maxillary Division of the Trigeminal Nerve - This carries sensory fibres from parts of the face, including the skin on the upper jaw.
The Mandibular Division of the Trigeminal Nerve - Like its name suggests, this carries sensory fibres from parts of the face, including the skin on the lower jaw and temples of the head.
The trigeminal nerve also has a motor function, and that is to provide fibres to the muscles of mastication, or chewing. It also supplies motor fibres to a muscle called Tensor Tympani, which keeps the tympani (eardrum) stretched as tight as its musical namesake.
Cranial Nerve VI - Abuducens
This is also a motor nerve, and supplies fibres to a muscle which moves the eyeball, the Lateral Rectus Muscle.
Cranial Nerve VII - Facial
This nerve contains two types of fibres. The sensory fibres receive taste information from the front two-thirds of the tongue.
The motor fibres supply the muscles of facial expression, the muscle which are involved in blinking and also a muscle which controls tension on the small bones in the ear, the Stapedius Muscle.
It also provides parasympathetic fibres to salivary glands and tear ducts. This nerve is probably the best known, as it is the one which is involved in a condition known as Bell's Palsy. Its symptoms can be attributed to what the nerve supplies; the loss of taste from the interruption of signals from the tongue, the frozen expression and loss of the reflex from the paralysis of muscles which govern facial expression and blinking respectively, drying of the eyes and mouth from the lack of saliva and tears, and ringing in the ears from the loss of tension on the Stapedius Muscle.
Cranial Nerve VIII - Vestibulocochlear
This nerve, like the Trigeminal, has divisions. Only this one has two, to account for the different areas of innervation:
The Vestibular Division - This is a sensory division, and innervates the semi-circular canals in the ear; the part of the body which is mainly concerned with balance. The semi-circular canals could be called the gyroscope of the body.
The Cochlear Division - Again, another sensory division, but this time concerned with hearing. This innervates a small snail shell-like structure called the cochlea (part of the ear called the Inner Ear) which is lined with cilia that move to every sound vibration that is received. This generates signals which can travel to the brain, via this branch.
Cranial Nerve IX - Glossopharengeal
This nerve innervates the tongue and the pharynx (the back of the throat). It receives sensory information from the back third of the tongue and the pharynx, as well as the Eustastian Tube2 and the Middle Ear (the bit beyond the eardrum, but before the Inner Ear). It also supplies sensory stretch receptors to the arteries in the neck; the Carotid arteries. The degree of stretch on these nerve ends measure how full the arteries are, and give the body an indication of blood pressure. It also supplies motor fibres to the muscle involved in swallowing, the Stylopharyngeus muscle, and provides parasympathetic fibres to the salivary glands.
Cranial Nerve X - Vagus
As with all aspects of anatomy, there is always one exception to the rule. The Vagus Nerve is such an exception. It does receive sensory information from the pharynx, larynx (voicebox) and the bit of the ear that you can see, and also from other structures not in the head such as the oesophagus (gullet), aorta (the largest artery in the body) and organs in the chest and abdomen.
It also supplies motor fibres to the soft palate, the pharynx, larynx and the upper bit of the oesophagus. A busy nerve, it also supplies parasympathetic fibres to organs in the chest and the abdomen.
Cranial Nerve XI - Accessory
After the workhorse that is the Vagus, comes the relatively simple Accessory Nerve. This is a pure motor nerve, and supplies motor fibres to two muscles which are involved in the movement of the head and the shoulders; the Trapezius and the Sternomastoid.
Cranial Nerve XII - Hypoglossal
Finally, there is Cranial Nerve XII. The Hypoglossal nerve is also a pure motor nerve. This innervates the muscles of the tongue.
It is important to remember that there is a pair of each Cranial Nerve, one branch supplying one side of the body. Cranial Nerves I, III to XII are ipsilateral ie, the right side is supplied by the same-sided branch of that nerve. The exception to the rule is the Optic Nerve, Cranial Nerve II, which is contralateral, so that the right branch supplies the left eye and vice-versa.
Getting Out of Your Skull
As the Cranial Nerves originate from the brain, which is within the skull, they need to find some way out of the skull in order to reach the structures that they supply. This is a bit difficult for the nerves, as the skull acts as a kind of all-enclosed helmet to protect the delicate brain. So they find their way out via small foramina in the floor of the vault of the skull. If this is difficult to picture, think of the skull as a large dome that you are currently in. The floor on which you are standing on would be the aforementioned, floor of the vault of the skull.
So here is a table of where they all leave the safe confines of the skull:
|Foramina||The nerve(s) which use this exit|
|Internal Acoustic meatus5||Facial
|Foramen Ovale6||Mandibular Division of the Trigeminal|
|Foramen Rotundum7||Maxillary Branch of the Trigeminal|
|Superior Orbital Fissure||Trochlear
Opthalamic Division of the Trigeminal
|Optic Canal||Optic Nerve|
|Cribriform Plate||Olfactory Nerve|