Med.Sci.532 Cranial Nerves
There are twelve pairs of cranial nerves in humans.
Their Roman numeral designations and names are:
  • I olfactory
  • II optic
  • III oculomotor
  • IV trochlear
  • V trigeminal
  • VI abducens
  • VII facial
  • VIII vestibulocochlear (auditory)
  • IX glossopharyngeal
  • X vagus
  • XI accessory (spinal accessory)
  • XII hypoglossal

The cranial nerves mostly innervate structures in the head and neck.

The cartoon codes for at least one major functional innervation of each cranial nerve.  This drawing is based on one done by Beatrice Humphris, R.N. published in Nursing 1984.
A general paradigm or model for cranial and spinal nerves' relationship to the central nervous system (CNS) is shown here. Cranial nerves attach to or join the brainstem, whereas spinal nerves carry information to (afferents) or from (efferents) the spinal cord.
  • In the afferent part of the model, a pair of neurons is shown. They are the first two in a series that will carry information from periphery to forebrain. It is conventional to speak of them, respectively, as 1st order and 2nd order neurons.
  • Notice that the cell body of the 1st order neuron may be either unipolar or - more rarely - bipolar. Their cell bodies are typically in sensory ganglia, which are parts of the peripheral nervous system (PNS).
  • The second order neuron is invariably multipolar and located in the CNS.
  • For the efferent part of the paradigm, there may be an outflow of information from the CNS over motor neurons - which innervate skeletal muscles - and over the autonomic system (sympathetic and parasympathetic) - which eventually innervates cardiac muscle, smooth muscle or gland cells.
  • Cell bodies of motor neurons are all in the CNS.
  • The autonomic nervous system has two neurons comprising it. The first is called preganglionic and has its soma in the CNS. It synapses on the second neuron - called postganglionic and located in an autonomic ganglion - which then innervates the cardiac or smooth muscle cell or the gland cell.
With regard to the location of the cell body (soma and perikaryon are synonyms) of the neuron, the spinal nerves perfectly fit the paradigm shown above. The cranial nerves fit it almost perfectly. The notable exception(s) being the trigeminal mesencephalic nucleus (and the optic nerve, not considered here because the retina and optic nerve are really entirely a part of the central nervous system or CNS).

The following chart gives the pattern of neuronal cell body locations for the various types of afferent and efferent neurons comprising cranial nerves.
Soma In CNS Soma In PNS Type of Neuron
trigeminal mesencephalic nucleus ALL (except one group) 1st order Afferent
ALL

none

 2nd order
ALL none skeletomotor
(to skeletal ms.)		 Efferent
ALL preganglionics

ALL postganglionics

 autonomic
(to cardiac ms., smooth ms., gland cell)
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Each cranial nerve - unlike nearly every spinal nerve - does not always have both afferent and efferent components.

The list below categorizes each cranial nerve in terms of whether it contains axons of afferent or efferent neurons or both. The listing is accurate for humans in terms of what tests are done to assess the integrity of the cranial nerves, but not rigorous in an absolute sense. For example, cranial nerve VIII actually does contain efferent fibers (i.e. the olivocochlear bundle), but those efferents are not routinely tested for clinically.

Afferent Only

Efferent Only

Afferent and Efferent

I olfactory

III*oculomotor

V trigeminal

II optic

IV trochlear

VII* facial

VIII vestibulocochlear

VI abducens

IX* glossopharyngeal
 

XI accessory

X* vagus
 

XII hypoglossal

  
* These four cranial nerves have an autonomic component. They carry the 'cranial' part of the craniosacral autonomic or parasympathetic outflow from the CNS.
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