What is the trochlear nerve?
Cranial nerves are nerves of direct origin in the specialized neuronal aggregates, the so-called cranial nerve nuclei, the brain or the brainstem. Apart from the cranial nerves, all other body nerves arise from the spinal cord.
Cranial nerves lead fiber qualities from somatosensible to vegetative and somatomotor. The somatomotor nerve fibers innervate muscles and organs and thus give them the possibility of arbitrary movement. All somatomotric fibers are efferent nerves. One of the somatomotory cranial nerves is the fourth cranial nerve called the trochlear nerve. Together with the nervus oculomotorius and nervus abducens it allows the movements of the eyeball.
The trochlear nerve originates as the only cranial nerve on the dorsal side of the brain and has its origin caudal to the inferior colliculi within the mesencephalon tectum. Like all motor nerves, it contains not only motor fibers, but also sensory fibers for proprioception of the supplied muscles. Its supply area is the contralateral side of the superior oblique muscle.
The tendon of this muscle is deflected within the eye socket by a rolling cartilage. This cartilaginous cartilage is known as the trochlea and has given the trochlear nerve its name.
Anatomy & Construction
The cranial nerve nucleus of trochlear nerve corresponds to the nucleus nervi trochlearis and lies in the midbrain. As the nerve is the only cranial nerve to emerge dorsal to the brainstem, it crosses into the dorsal trochearic chiasm after exiting to the other side. The cranial cavity leaves the human nerve at the superior orbital fissure.
The somatomotor nerve is remarkable in many ways. For example, it is the weakest cranial nerve when it comes to the number of axons involved. In addition, the nerve of all cranial nerves has the sometimes longest course within the skull. After the dorsolateral opening of the dura mater, the course in the lateral wall of the cavernous sinus and the passage through the superior orbital fissure, the nerve in the eye socket extends laterally and cranially past the origin of the eye muscles, the so-called annulus tendineus communis. The nerve is connected to the motor end plate of the superior oblique muscle and at this point transmits motor impulses from the central nervous system to the muscle.
Function & Tasks
The trochlear nerve moves together with the nerves oculomotor nerve and abducens nerve the eyeball. The precise and extensively targeted movement of the eyeball is possible only thanks to the interplay of the three nerves. If one of the three nerves fails, the eye movement is completely out of balance due to the failure of the paralyzed eye muscle and the visual perception is difficult.
The motor fibers of the trochlear nerve are responsible for the transmission of centrally issued commands. They take care of the transmission of orders in the form of excitement at the motor end plate of the superior oblique muscle. The muscle fibers of the muscle are thus stimulated to contract, causing the eyeball to move.
The sensory fibers of the somatomotor nerve conduct sensations from the muscle into the central nervous system. This process is imperative for targeted muscle movements with adequate contraction force, because without this feedback, the nervous system can not adequately assess the muscle's current state of contraction.
The stimuli from the muscle are registered by receptors, the so-called proprioceptors, such as the muscle spindles and the Golgi tendon organ. Because the sensitively conductive fibers carry excitement towards the central nervous system, they are also referred to as afferent fibers. With its efferent fibers, the trochlear nerve is thus essentially involved in voluntary movements of the eyeball, while its afferent fibers are involved in the sensitivity to deep tissue in the region of the superior oblique muscle.
The movement of the eyeball is relevant for humans, as an eye-controlled living organism, also from an evolutionary biological point of view. Evolutionary biologists have made visual perception of the human species in early times the most reliable assessment of hazards in the environment and so much more directed reactions to the environment than the other perceptions.
If the trochlear nerve is damaged, trochlear nerve palsy may occur. As such, a loss of function of the contralateral portion of the superior oblique muscle is referred to. Since the nerve is not the only nerve that allows the movement of the eyeball, such a paralysis is not accompanied by complete loss of mobility.
Nonetheless, the symptoms are visually impaired. The affected people squint and see double images for this reason. It comes to movement restrictions of the eyeball, as the affected eye deviates upward after a paralysis of the nerve, which is also known as hypertrophy. At the same time, the eye turns inwards, causing an esotropia. In the sagittal axis, the eye rolls outward, causing an ex-cyclotomy. The vertical double images arise mainly when trying to look to the lower opposite side. In order to alleviate the discomfort, the patient tends to tilt the head compensatorily on the healthy side, resulting in an ocular torticollis.
If there is an isolated unilateral damage to the supplying cranial nerve nucleus, the crossing of the nerve tract shortly after the exit of the nerve tracts causes the muscle on the opposite side to be affected by the paralysis.