A hair strand is a complex structure composed of three distinct parts: the medulla, cortex and cuticle. This article describes each one in detail and how they work together to give you healthy hair.

Cuticle

Cuticle is the outermost layer of the hair shaft, and it consists of overlapping scales that protect the cortex and medulla. The cuticle layer also contributes to shine and color by reflecting light off of its surface.

The cuticle is comprised of dead cells that are embedded in the cortex. The cuticle is like a shield that protects the hair shaft from external factors, such as chlorine and other chemicals found in swimming pools, sun exposure, styling tools and more. When the cuticle layer becomes damaged or disrupted it can lead to frizzy hair that tangles easily.

Cortex

The cortex is the middle layer of your hair, and it's responsible for its strength and color. The cortex contains keratin, which makes up about 90% of your hair shaft. Keratin is a protein that forms natural waterproofing on top of your skin to keep water from entering into your body. It also gives strength to nails and hooves in animals like horses or cows!

The second most important ingredient found inside the cortex is melanin (a pigment), which helps give color to both humans and animals alike!

Medulla

The medulla is the innermost part of your hair. It's not visible in a strand of hair, but it can be seen if you cut open a piece of scalp or pluck out some strands from near your scalp. The medulla consists of a core of dead keratin cells surrounded by live ones, which gives it its whitish appearance. The medulla contains no melanin; instead, its structure makes it look white under magnification due to air bubbles between cells and their irregular shapes (like those found in bone marrow).

Hair smoothing treatments impact on hair anatomy

A smoothing treatment is a process that permanently changes the structure of your hair. It's often used on curly or wavy hair, and it can be done in salons or at home.  Treatments like salon type Brazilian keratin treatments work on the outside of the hair strand by pressing it into place.  Japanese straighteners work by penetrating the hair strands and breaking down the bonds in the hair that hold the curl in shape, yielding a straight strand.

Some treatments work by penetrating the cuticle and restructuring the bonds in the hair, without breaking them down.  Some of these use proteins, nutrients and vitamins to actually improve the health of the hair.  They actually plump up the hair strand to make the hair smoother and shinier.  While these are less damaging to the hair, they do not yield as straight of a result, but they do make the hair smoother and less frizzy.

Conclusion

The anatomy of a hair is complex and smoothing/straightening treatments use different chemistry.  The more aggressive the treatment, the straighter the hair, but also a greater risk of damage.  If you are seeking as straight as can be without concern for the chemicals, Brazilian treatments and Japanese type straighteners might be a good choice.  You should discuss these with a professional stylist to get the safest and best result.  If you are seeking an at home smoothing treatment, look for one that is formaldehyde free and packed with proteins, nutrients, amino acids and vitamins.  These are better for your hair but will not make it pin straight.