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Motor Nerves, Sensory Nerves: Clarifications

Updated: Jan 9, 2020

For some reason, what has been taught in most schools about motor and sensory nerves is not totally consistent with neuroanatomy. Most of us have learned that motor nerves flow from the CNS to the muscles and that sensory nerves flow to the CNS from the skin. I have made this mistake for years, and most of my students, both physiotherapists and osteopaths, do the same.

Basically, that's not untrue, but it's not totally true either.

All peripheral nerves are mixed nerves. That is, they ALL contain afferent neurons and efferent neurons that are unidirectional. This is regardless of the target of the nerve they are part of.

It is possible that a nerve ramus, for example the nerve to the psoas branching from the femoral nerve, may only reach a muscle, in this case the psoas, and not continue its path to the skin. It is no less a mixed nerve. Its sensitive part will consist of afferent neurons of the psoas muscle: some inform the CNS of muscle elongation (neuromuscular spindle cells fibres Ia and II), others carry information concerning the tension exerted on the muscle (Golgi tendon organ fibre Ib), other neurons sensitive to muscle contraction informs the CNS about muscle load (type III fibre) and finally others provide a biochemical representation of energy work or energy expenditure. (The latter are called metaboreceptors.) These neurons use the same peripheral nerve as the alpha and gamma motor neurons (efferent neurons to the muscle) that are involved in muscle contraction. This nerve is therefore not a "motor" nerve since it contains a large number of sensory neurons.

It is also possible that a nerve only reaches the skin but cannot either be said to be a purely sensory nerve. For example, the posterior cutaneous nerve of the thigh does not innervate any muscles, yet it is a mixed nerve that also contains motor neurons. These are the autonomic motor neurons that are responsible for the information coming from the CNS, through the pre- and post-ganglionic neurons, to arterioles (thermoregulation and blood volume management), sweat glands, hair follicles, mast cells (immune system involvement) and adrenal glands (fight or flight response). These neurons follow the same peripheral nerve as the sensory cutaneous neurons (Aß, A∂ and C fibres).

For all peripheral nerves, the afferent fibres reach the spinal cord through the dorsal root, the efferent fibres leave the spinal cord through the ventral root. For a short time, they form the spinal nerve which quickly splits into dorsal and ventral rami.

It should be noted that some cranial nerves are only motor nerves (common oculomotor nerve, trochlear nerve, abducens nerve, accessory nerve, hypoglossal nerve), and others are only sensory (olfactory nerve, optic nerve and vestibulocochlear nerve).

On the other hand, many cranial nerves are mixed nerves: the trigeminal nerve, the facial nerve, the glossopharyngeal nerve and the vagus nerve. The research of A. D. Craig shows that the afferences of the C and A∂ fibres as well as the afferences of the glossopharyngeal nerve and the vagus nerve are respectively the afferent sympathetic and parasympathetic counterparts of the autonomous nervous system, which is only efferent.

To express the reality of the nerve that only reaches the skin, or ends in the skin, we use the expression "cutaneous nerve", which does not take away its afferent function, but leaves a doubt as to its essential efferent role. Similarly, the use of the expression "motor nerve" to designate the nerve that goes to the muscle leads us to believe that its role is only motor and thus loses its so important afferent function.

I suggest that the term "mixed-muscle nerve" be used to emphasise the afferent as well as efferent function of the nerve that only reaches the muscle and to use the term "mixed-cutaneous nerve", which would add to the precision of the skin nerve's motor role. It's longer, but it has the merit of being clear.

Louise Tremblay

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