The portion of cranial nerve III containing visceromotor nerve axons causes miosis.
Understanding Miosis and the Nerves Involved
Miosis, also known as pupillary constriction, is the narrowing of the pupil. This process is controlled by specific nerves that innervate the iris sphincter muscle. Let's break down which nerve is responsible.
The Key Player: Cranial Nerve III (Oculomotor Nerve)
A portion of cranial nerve III plays a crucial role in miosis. Here's how:
- Visceromotor Nerve Axons: These axons are part of cranial nerve III.
- Edinger-Westphal Nucleus: Along with the somatomotor portion, the visceromotor portion originates from the Edinger-Westphal nucleus.
- Ciliary Ganglion Synapse: These axons then synapse on neurons within the ciliary ganglion.
- Parasympathetic Innervation: The parasympathetic axons of these ciliary ganglion neurons innervate the iris sphincter muscle.
- Result: Miosis: When these parasympathetic fibers are activated, the iris sphincter muscle contracts, causing the pupil to constrict (miosis).
In summary, the visceromotor portion of cranial nerve III (specifically the parasympathetic fibers originating from the Edinger-Westphal nucleus that synapse in the ciliary ganglion) is the nerve that causes miosis.
