Tract, fasciculus and funiculus in spinal white matter
In spinal white matter, a tract is one axon bundle, a fasciculus is part of a tract serving one body region, and a funiculus is a segment holding several tracts.
Remember
A funiculus holds several tracts, so impulses (the information axons carry) within it can travel in different directions. A tract carries them in one direction, and a fasciculus is the part of a tract serving one region of the body.
Memory aid
think of a cable. The funiculus is the whole cable holding several strands, a tract is one strand running one way, and a fasciculus is the part of one strand serving one region of the body.
Use the cable to keep the three terms in order of size: the whole cable, one strand, and the part of a strand.
White matter in the spinal cord is the outer shell of axons, with divisions based on where those axons are travelling to or from.
A tract is a defined bundle of axons that connects two regions of grey matter. Impulses travel along it in one direction. A tract is therefore either motor, carrying commands, or sensory, carrying sensation, and not both.
A fasciculus (plural fasciculi) is a smaller part of a tract, serving one region of the body.
A funiculus (plural funiculi) is a segment of white matter that holds several distinct tracts. With several tracts present, impulses can travel in different directions within it. A funiculus is involved when several pathways are interrupted at once.
In the posterior part of the cord, the dorsal column pathway carries fine touch and proprioception (both kinds of sensation). It is divided into the fasciculus gracilis, which serves the legs and lower body, and the fasciculus cuneatus, which serves the upper body and arms. A bundle that serves only the lower limb within that larger sensory pathway is a fasciculus.
Key facts
- Tract
- a defined bundle of axons that joins two regions of grey matter and carries impulses one way, either motor or sensory.
- Fasciculus
- a smaller part of a tract that serves one region of the body, such as the fasciculus gracilis for the lower limb.
- Funiculus
- a column of white matter holding several tracts, so impulses can travel in different directions within it.
- Funiculus columns
- a funiculus is a dorsal, lateral or ventral column of spinal white matter.
- Dorsal column fasciculi
- the fasciculus gracilis serves the legs and lower body, and the fasciculus cuneatus serves the upper body and arms.
- White and grey matter
- white matter, which contains the tracts, is the outer shell of axons, while nerve cell bodies form the central core of the cord as grey matter.
Exam traps
- A bundle serving one region of the body is not a funiculus, because a funiculus is a large segment containing several tracts, so damage to it would disturb several pathways at once.
- A tract does not mix motor and sensory axons, because a tract is functionally uniform, either motor or sensory.
- A fasciculus does not name a region that holds several tracts, because it is a subdivision of a single tract serving one distinct portion of the body.
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