The trochlear nerve is the fourth cranial nerve and innervates the superior oblique muscle motor function. Together with the oculomotor nerve and the abducens nerve, it is involved in the movement of the eyeball. Double vision occurs when the nerve is paralyzed.
What is the trochlear nerve?
Cranial nerves are nerves with direct origin in the specialized nerve cell assemblies, called cranial nerve nuclei, of the brain or brain stem. Except for cranial nerves, all other body nerves originate in the spinal cord. Cranial nerves carry fiber qualities from somatosensitive to autonomic and somatomotor. The somatomotor nerve fibers innervate muscles and organs, giving them the ability to move at will. All somatomotric fibers are efferent nerves. One of the somatomotric cranial nerves is the fourth cranial nerve called the trochlear nerve. Together with the oculomotor nerve and abducens nerve, it enables the movements of the eyeball. The trochlear nerve is the only cranial nerve originating from the dorsal side of the brain and has its origin caudal to the inferior colliculi within the tectum in the mesencephalon. Like all motor nerves, it does not contain exclusively motor fibers, but also sensitive fibers for proprioception of the supplied muscles. Its area of supply is the contralateral side of the superior oblique muscle. The tendon of this muscle is deflected within the orbit by a rolling cartilage. This rolling cartilage is known as the trochlea and has helped give the trochlear nerve its name.
Anatomy and structure
The cranial nucleus of trochlear nerve corresponds to the trochlear nucleus and is located in the midbrain. Because the nerve is the only cranial nerve to exit dorsally of the brainstem, it crosses to the other side into the dorsal trochearis chiasm after exiting. In humans, the nerve exits the cranial cavity 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. Furthermore, of all the cranial nerves, it has the longest course within the skull. After breaking through the dorsolateral dura mater, running in the lateral wall of the cavernous sinus, and passing through the superior orbital fissure, the nerve passes laterally and cranially in the orbit at the origin of the eye muscles, called the common tendon. The nerve is connected to the motor endplate of the superior oblique muscle and transmits motor impulses from the central nervous system to the muscle at this point.
Function and tasks
The trochlear nerve, together with the oculomotor and abducens nerves, moves the eyeball. The precise and extensively targeted movement of the eyeball is possible for humans only thanks to the interaction of the three nerves. If one of the three nerves fails, the eye movement gets completely out of balance due to the failure of the paralyzed eye muscle and visual perception is impaired. The motor fibers of the trochlear nerve are responsible for transmitting centrally issued commands. They take care of the transmission of commands in the form of excitation at the motor end plate of the superior oblique muscle. In this way, the muscle fibers of the muscle are stimulated to contract so that the eyeball moves. The sensitive fibers of the somatomotor nerve transmit sensations from the muscle to the central nervous system. This process is mandatory for targeted muscle movements with appropriate contraction force, because without this feedback the nervous system cannot adequately assess the current contraction state of the muscle. Stimuli from the muscle are registered by receptors called proprioceptors, such as the muscle spindles and the Golgi tendon organ. Since the sensory conducting fibers transport excitation toward the central nervous system, they are also called afferent fibers. Thus, with its efferent fibers, the trochlear nerve is essentially involved in voluntary movements of the eyeball, while with its afferent fibers, it is involved in depth sensation in the region of the superior oblique muscle. The movement of the eyeball is relevant for humans, as eye-controlled creatures, also from an evolutionary-biological point of view.According to evolutionary biologists, in early times, visual perception provided the human species with the most reliable assessment of dangers in the environment, guiding responses to the environment far more than the other perceptual instances.
Diseases
Trochlear nerve palsy can occur when the trochlear nerve is damaged. This is defined as a loss of function of the contralateral portion on the superior oblique muscle. Since the nerve is not the only nerve that allows eyeball movement, such paralysis is not accompanied by a complete loss of mobility. Nonetheless, vision-impairing symptoms present themselves. The affected person squints and sees double images for this reason. Movement of the eyeball is restricted because the affected eye deviates upwards after paralysis of the nerve, which is also known as hypertrophy. At the same time, the eye rotates inward, causing esotropia. In the sagittal axis, the eye rolls outward, evoking excyclotropia. The vertical double images occur primarily when trying to look to the lower opposite side. To alleviate the discomfort, the patient usually tilts the head compensatorily to the healthy side, creating an ocular torticollis. If there is isolated unilateral damage to the supplying cranial nerve nucleus, the muscle on the opposite side is affected by the paralysis because of the crossing shortly after the nerve tracts exit.
