In Biology, We come to know that The nervous system is the most complicated system within the human body that helps in the coordination of action. There are several nerves that are interconnected to one another and helps in the transmission of signals in various parts of our bodies. It is a mode of communication between the various parts of the body and the brain and spinal cord. Our body reacts to a number of internal and external factors. The human body reacts to these changes at a very fast rate. It is because the nervous system instantaneously transmits electric impulses from one part of the body to the other part along neurons, which are specialized nerve cells. The Central Nervous System (CNS) and the Peripheral Nervous System (PNS) are the two most important divisions pertaining to the nervous system.
In order to know about the Trochlear Nerve, it is important to have some idea about the Cranial Nerves. There are around 12 pairs of cranial nerves in our peripheral nervous system that is responsible for the functioning of our head and neck. This nerve emerges directly from the brain whereas the spinal nerve emerges from the spinal cord segments. This nerve helps in transmitting messages between the brain and the different parts of the body.
The fourth paired nerve in the cranial segment is the trochlear nerve. It has the longest course of inter-cranial transmission but is the smallest cranial nerve. It is formed totally by the axons and is the only motor cranial nerve that is formed like this. The axons cross the midlines before exit. This nerve is also denoted as CN IV. The nerve innervates a single muscle and that is the eye’s superior oblique muscle. The unique characteristics of this nerve can be therefore stated as below:
i. It is entirely formed of axons.
ii. It is the smallest nerve in relation to the number of axons it contains.
iii. It has the longest intracranial length.
iv. This nerve is the one and only cranial nerve that exits from the rear side of the brain.
Anatomically this nerve arises from the brain's trochlear nucleus that emerges from the rear side of the midbrain. This nerve runs within the subarachnoid space both inferiorly and anteriorly before penetrating the dura mater. The nerve then passes along the cavernous sinus walls laterally and then enters the eye orbit through the superior orbital fissure. The trochlear muscle is responsible for controlling the superior oblique muscle and acts as a connection to the annular tendon. This, therefore, helps in procession the brain signals to move upward, downward and outwards.
This nerve is therefore responsible for the physical movement of the eyeball. The unique feature about the function of this nerve is that this nerve does not transmit any kind of sensory signals to all the body, rather operates as a motor nerve and transmits a signal to the only superior oblique muscle. The superior oblique muscle is also responsible for making quick and precise eye movements for focusing and tracking any objects that are seen.
Another term for the Trochlear nerve is a motor nerve, which is the only nerve that exits the brain dorsally. Only one muscle is supplied by the trochlear nerve, termed as superior oblique. The origination of the fourth cranial nerve from cell bodies can be identified in the ventral section in the brainstem of the trochlear nucleus.
This nerve will impact the functionality of the superior oblique nerve. It will lead to a problem in the vision. The damage to the trochlear nerve resulting in the loss of function is termed as palsy. Palsy to this nerve will impact muscle control, which will drift the eyes upward. An acute palsy that usually results from a trauma in the head does not last long. The duration of an acute palsy ranges from a week to about two months. The effects of an acute palsy involve blurry vision or double vision. Another type of damage in the trochlear nerve that is congenital in nature and occurs because of malformation of the trochlear nerve is termed as chronic palsy. This is usually noticed during childhood only but in certain cases it remains unnoticed till adulthood. This can be cured surgically.
The trochlear injury can be segregated into two distinct parts – Unilateral trochlear nerve injury and bilateral trochlear nerve injury. The unilateral trochlear nerve injury is caused by impacts in the front-lateral part of the nerve, whereas bilateral trochlear nerve injury can occur because of injury in the mid-front part. The injury in this nerve causes a hyper-deviation in the part of the eye that gets affected. This deviation is caused while looking at the contra-lateral side or when the head is tilted towards the side of the affected area. Diplopia is caused because of the damage in this nerve.
Vertical diplopia causes weakness in the movement of the eyes downward. It causes double vision. The patient can see two visions from each eye that are separated vertically. The patients suffering from this tilt their head forward in order to adjust the vision. In the case of Torsional diplopia, the rotation of the eyeball is affected. In this case, the eyes rotate in the opposite direction equally resulting in two different visual fields that are tilted with respect to one another. In order to compensate for this kind of vision, one needs to tilt their heads in the opposite direction of the vision for combining the vision together.
How paralysis and palsy related to this nerve affect the overall nervous system function.
The trochlear nerve is very thin and long. Hence it is very much vulnerable to traumatic injuries. Any injury in the Trochlear nerve can be termed as the fourth cranial nerve palsy that can cause paralysis in the superior oblique muscle. In the case of this kind of paralysis, some complications arise in the other parts of the body and the nervous system as well. In order to rectify the impaired and separated vision, the patients usually tilt their heads, which causes a deformation in the muscle growth. The upper portion of the tilted face does not grow proper muscles because of this, whereas the lower portion has proper growth of the nerves and muscles. The face starts looking asymmetrical.