Structure and Course
- The trochlear nerve is the fourth cranial nerve.
- It innervates the superior oblique muscle of the eye.
- It is the smallest cranial nerve in terms of the number of axons it contains.
- It has the greatest intracranial length.
- It exits from the dorsal aspect of the brainstem.
- The trochlear nerve originates from a trochlear nucleus in the medial midbrain.
- It decussates within the midbrain before emerging from the dorsal midbrain.
- It courses on the contralateral side, passing laterally and then anteriorly around the pons.
- It enters the orbit through the superior orbital fissure.
- It innervates the superior oblique muscle.
- The trochlear nerve is derived from the basal plate of the embryonic midbrain.

Clinical Significance and Assessment
- Injury to the trochlear nerve can cause weakness of downward eye movement and vertical diplopia.
- Trochlear nerve palsy also affects torsion or rotation of the eyeball in the plane of the face.
- The characteristic appearance of patients with fourth nerve palsies includes tilting the head to one side and tucking the chin in.
- Causes of trochlear nerve palsy can be both peripheral and central lesions.
- Acute palsy can be caused by head trauma, while chronic palsy is often a congenital defect.
- The trochlear nerve is tested by examining the action of the superior oblique muscle.
- The patient is asked to look down and in, as the superior oblique muscle is most active in this movement.
- Trochlear nerve palsy can result in vertical diplopia, where the affected eye drifts upward relative to the normal eye.
- Patients with trochlear nerve palsy may tilt their head forward or to the opposite side to compensate for torsional diplopia.
- Other causes, such as torticollis, need to be ruled out when diagnosing fourth nerve palsies.

Homologous Trochlear Nerves in Animals
- Trochlear nerves are found in all jawed vertebrates.
- The unique features of the trochlear nerve are seen in the primitive brains of sharks.
- The trochlear nerve has a dorsal exit from the brainstem.
- The trochlear nerve has contralateral innervation.
- Homologous trochlear nerves exist in all jawed vertebrates.

Anatomy of the Trochlear Nerve
- The trochlear nerve has a cavernous portion.
- Traumatic trochlear nerve palsy can occur following minor occipital impact.
- The braincase morphology of the broadnose sevengill shark provides insights into the trochlear nerve.
- Dissection images show the origins of ocular muscles and nerves entering through the superior orbital fissure.
- Deep dissection views reveal the trochlear nerve from a superior perspective.

Additional Resources and Key Points
- Oxford Dictionary on Lexico.com provides a definition of trochlear.
- 'Orbit and accessory visual apparatus: trochlear nerve' is a section in Gray's Anatomy: The Anatomical Basis of Clinical Practice.
- 'Neuroanatomy, Cranial Nerve 4 (Trochlear)' is a publication in StatPearls.
- 'A Textbook of Neuroanatomy' by Maria A. Patestas and Leslie P. Gartner is a comprehensive resource.
- Various research papers and studies provide information on the trochlear nerve.
- NeuroNames provides information on hier-449 related to the trochlear nerve.
- eMedicine offers resources on trochlear nerve palsy.
- Loyola University Medical Education (MedEd) provides educational materials on the trochlear nerve.
- Grossanatomy/h_n/cn/cn1/cn4.htm is a website with information on the trochlear nerve.
- Additional external links related to the trochlear nerve can be found.
- The trochlear nerve is present in all jawed vertebrates.
- Sharks have primitive brains that exhibit unique features of the trochlear nerve.
- The trochlear nerve exits dorsally from the brainstem.
- Contralateral innervation is a characteristic of the trochlear nerve.
- Understanding the anatomy and function of the trochlear nerve is important in various medical contexts.

Trochlear nerve (Wikipedia)

The trochlear nerve (/ˈtrɒklɪər/), (lit. pulley-like nerve) also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a cranial nerve that innervates a single muscle - the superior oblique muscle of the eye (which operates through the pulley-like trochlea). Unlike most other cranial nerves, the trochlear nerve is exclusively a motor nerve (somatic efferent nerve).

Trochlear nerve
The trochlear nerve entering the orbit, seen from above, supplies the superior oblique muscle
The trochlear nerve (CN IV) seen with other cranial nerves. It is the only cranial nerve to emerge from behind the brainstem, and curves around it to reach the front
Details
Innervates	Superior oblique muscle
Identifiers
Latin	nervus trochlearis
MeSH	D014321
NeuroNames	466
TA98	A14.2.01.011
TA2	6191
FMA	50865
Anatomical terms of neuroanatomy [edit on Wikidata]

The trochlear nerve is unique among the cranial nerves in several respects:

• It is the smallest nerve in terms of the number of axons it contains.
• It has the greatest intracranial length.
• It is the only cranial nerve that exits from the dorsal (rear) aspect of the brainstem.
• It innervates a muscle, the superior oblique muscle, on the opposite side (contralateral) from its nucleus. The trochlear nerve decussates within the brainstem before emerging on the contralateral side of the brainstem (at the level of the inferior colliculus). An injury to the trochlear nucleus in the brainstem will result in an contralateral superior oblique muscle palsy, whereas an injury to the trochlear nerve (after it has emerged from the brainstem) results in an ipsilateral superior oblique muscle palsy.

The superior oblique muscle which the trochlear nerve innervates ends in a tendon that passes through a fibrous loop, the trochlea, located anteriorly on the medial aspect of the orbit. Trochlea means “pulley” in Latin; the fourth nerve is thus also named after this structure. The words trochlea and trochlear (/ˈtrɒkliə/, /ˈtrɒkliər/) come from Ancient Greek τροχιλέα trokhiléa, “pulley; block-and-tackle equipment”.