Divisions

Different neurons have different functions. This section will explain in more detail many of the terms in the definition sections and how that impacts the anatomy of the individual cranial nerves. We have already been exposed to the fact that there are sensory neurons and motor neurons. To better understand the structure of the cranial nerves we need to look more closely at the way the nervous system is organized. The most basic division is into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and the spinal cord. All of the nerves found in the body compose the PNS. The PNS consists of 12 pairs of cranial nerves that are the subject of this course and 31 spinal nerves which will be only briefly mentioned.

Within the PNS there are differences in the function of the fibers contained in the nerves. The main division used to classify them is into afferent fibers which carry sensory impulses towards the CNS and efferent fibers that carry impulses away from the CNS (Figure 5). Anatomically all sensory neurons have their cell bodies outside the CNS in ganglia. Each spinal nerve has a motor root and a sensory root adjacent to the area where they enter the spinal cord. The sensory ganglia are found attached to the sensory root of the spinal nerve so are in close proximity to the CNS. Cranial nerves are not as uniform, and we will have to look at each of them individually. They generally have their sensory ganglia in the cranial cavity where the root of the cranial nerve is found.

Figure 5- Central & Peripheral Nervous System

Figure 5- Central & Peripheral Nervous System

To make matters more complex sensory fibers in the cranial nerves can be further divided into general somatic sensory fibers, general visceral sensory and special sensory fibers. General sensations are those found throughout the body. Somatic refers to their being on the body surface. These sensations include touch, pressure, pain and temperature. Visceral sensations are from the internal organs and include not just the aforementioned but chemoreceptors, stretch receptors and baroreceptors. Special senses are those senses that are only found in specific, localized areas. These senses include sight, hearing, equilibrium, smell and taste, which not coincidentally, are all found in the head and neck. Their position in the head makes sense as these organs are taking in vast quantities of information from the environment so their close proximity to the brain is crucial for timely responses to external events.

On the motor side of the PNS there are also two types of innervating fibers. The motor fibers are divided into somatic motor fibers and autonomic motor fibers. The somatic fibers provide stimulation to skeletal muscle fibers. These fibers all have their cell bodies in the CNS. The axon leaving the CNS can be quite long as they must project from the spinal cord to the muscle they control. Each axon and its associated muscle fibers is known as a motor unit. Each nerve fiber controls a variable number of muscle fibers depending on what amount of fine control is needed. The eye muscles have small motor units of about ten muscle fibers per axon while postural muscles can have up to one hundred times that number.

Autonomic fibers control smooth muscle, glands and cardiac muscle. They are collectively known as visceral efferent fibers. They come in two varieties: sympathetic fibers and parasympathetic fibers. Each of these fibers have opposing actions and organs controlled by these nerves generally have dual innervation with the action taken by the organ depending not on the absolute level of either system but the degree to which one predominates over the other.

Sympathetic responses are geared to what is termed the “fight or flight” response. It turns down the digestive system activity while increasing the heart rate and respiratory rate, all of which act to increase oxygenated blood flow to the muscles. This response is strong but once the threat is gone the mediating neurotransmitters epinephrine and norepinephrine are rapidly eliminated. As an example of this, in dentistry we use epinephrine to constrict blood vessels in the area we are injecting the local anesthetic. This works because these blood vessels are not going to muscles and therefore are constricted by epinephrine as they are not crucial to providing oxygen to the muscles. Occasionally the epinephrine ends up in a blood vessel and can cause the patient to feel their heart racing as the epinephrine gets to the heart, stimulating it to beat faster. This clears up in a few minutes due to the rapid metabolism of the epinephrine.

Parasympathetic responses on the other hand are the opposite. The digestive system blood supply and motility are increased, and the heart rate and respiratory rate are slowed. This is mediated by a different neurotransmitter and does not dissipate as rapidly as the sympathetic response. This is the reason that patients often faint after the thing they are most afraid of is over rather than during the threat. The mechanism is as follows: in response to the threat the patient’s sympathetic system is activated, but in order to not look nervous, their parasympathetic system is also activated. Due to the fact that the sympathetic system only slightly predominates, the patient’s pulse and blood pressure are only slightly elevated. The procedure is accomplished, and the patient perceives the threat is now passed. The sympathetic outflow is turned off, but the parasympathetic outflow reacts more slowly. The heart rate and blood pressure plummet and the patient faints due to inadequate cerebral circulation.

Autonomic fibers anatomically are different than somatic fibers. All autonomic fibers that leave the spinal cord will synapse with a second neuron in the periphery that then innervates the target organ. The outflow from the spinal cord is also regionally restricted. All parasympathetic fibers leave the CNS only along cranial nerves III, VII, IX, X and spinal nerves S2-S4 while sympathetic fibers leave only from spinal nerves T1-L2. The sympathetic system covers the whole body by having a chain of ganglia that run along the spinal column bilaterally the entire length of the spinal cord. These interconnected ganglia allow fibers that enter the chain at any level to leave at a different level. In many cases the second neuron is in the chain and the fiber that leaves is known as a post-synaptic or post ganglionic fiber. Some pass through the ganglia as pre-synaptic fibers heading to the digestive system and synapse in ganglia in the abdomen. The sympathetic fibers in the head and neck are all post-ganglionic and generally join and travel with branches of cranial nerves to their target organ. They are often not recognized as separate nerves as they are small and often join other more substantial nerves on their way. We will not often specifically mention them as we are more concerned with fibers that originate in cranial nerves, but they are assumed to be following parasympathetic fibers in the head and following other nerves to the blood vessels in the head.

To be complete there is also a more recent finding that there are a number of autonomic ganglion in the gut that act as a reflex center for proper regulation of the many organs that are involved in proper function of the digestive system. Some authors regard this as a separate entity deserving equal billing with the CNS as it has numerous associative neurons which are not found in the PNS. Thus, it is believed to be largely autonomous though it does communicate with the CNS through cranial nerve X, which seems to modulate the response of the gut. Therefore, this cranial nerve may not have as much control of the digestive system as previously believed. As the digestive system itself is outside of the scope of this course we will not wade into the debate about whether it is part of the PNS or a whole separate entity but will note the contribution of cranial nerve X to its control.