Difference Between Endolymph and Perilymph

The main difference between endolymph and perilymph is that the endolymph is the fluid found in the membranous labyrinth whereas the perilymph is the fluid that surrounds the endolymph, located inside the bony labyrinth. Furthermore, endolymph is rich in potassium ions and generates electrical impulses from sound waves while perilymph is rich in both sodium and chloride ions and transmits sound waves to the endolymph.

Endolymph and perilymph are two types of cochlear fluids in the inner ear. They are in close contact and the Reissner membrane serves as the selective barrier between the two. The waves of the endolymph occur in response to the waves in the perilymph.

Key Areas Covered

1. What is Endolymph
     – Definition, Composition, Function
2. What is Perilymph
     – Definition, Composition, Function
3. What are the Similarities Between Endolymph and Perilymph
     – Outline of Common Features
4. What is the Difference Between Endolymph and Perilymph
     – Comparison of Key Differences

Key Terms: Composition, Endolymph, Nerve Impulses, Perilymph, Location, Sound Waves

What is Endolymph

Endolymph is the fluid of the membranous labyrinth of the inner ear. It is also called the Scarpa’s fluid. The membranous labyrinth is surrounded by the perilymph. Outside the perilymph, the bony labyrinth is present. Endolymph consists of a significantly higher amount of potassium ions (150 mM) than sodium ions (1 mM). Therefore, the electric potential of the endolymph is ~80-90 mV. The capillary loops and small blood vessels called stria vascularis secrete potassium ions into the endolymph. The ion concentration within the endolymph facilitates the formation of nerve impulses in the hair cells. Generally, the electric potential of the hair cells is -50 mV. The potential difference between endolymph and hair cells is around 150 mV and it is the largest potential difference in the human body.

Figure 1: Inner Ear Anatomy

Fluid waves of the endolymph stimulate the hair cells, which convert the fluid waves into nerve impulses. The nerve impulses are transmitted through the auditory nerve to the brain.

What is Perilymph

Perilymph is the fluid between the endolymph and the bony labyrinth. It is found in both tympanic duct and vestibular duct of the cochlea. The ionic composition of the perilymph resembles the extracellular fluid or cerebrospinal fluid as the perilymph is continuous with the cerebrospinal fluid of the subarachnoid space via the perilymphatic duct. Hence, the major cation found in the perilymph is sodium (140 mM). Chloride ions (130 mM) are also present in the perilymph. Potassium ion concentration in the perilymph is 4-5 mM.

Figure 2: Endolymph (green) and Perilymph (blue)

Perilymph surrounds and protects the endolymph. Most importantly, the sound waves are transmitted from the perilymph to the endolymph.

Similarities Between Endolymph and Perilymph

• Endolymph and perilymph are the two types of cochlear fluids found in the inner ear.
• They are filled inside compartments of the cochlea.
• Both help in the transmission of sound waves to the hair cells.

Difference Between Endolymph and Perilymph

Definition

Endolymph refers to the fluid in the membranous labyrinth of the inner ear whereas perilymph refers to the fluid between the membranous labyrinth of the ear and the bone which encloses it.

Location

Endolymph is present inside the membranous labyrinth while perilymph surrounds the endolymph, located inside the bony labyrinth.

Found in

Endolymph occurs inside the cochlear duct while perilymph is the fluid found in the tympanic duct and vestibular duct of the cochlea.

Composition

Endolymph is rich in potassium ions and resembles the intracellular fluid while perilymph is rich in both sodium and chloride ions and resembles the extracellular fluid.

Electric Potential

Endolymph has a more positive electric potential when compared to perilymph.

Transmission

Endolymph transmits sound waves to the hair cells while perilymph transmits sound waves to the endolymph.

Conclusion

Endolymph is the fluid found inside the membranous labyrinth while perilymph is the fluid that surrounds the endolymph, located inside the bony labyrinth. The conversion of sound waves into the nerve impulses occurs inside the endolymph while the perilymph transforms the sound waves into the endolymph. Endolymph consists of a suitable environment for generating an electric impulse. The main difference between endolymph and perilymph is their location and function.

Reference:

1. Delprat, Benjamin. “ COCHLEAR FLUIDS.” Cochlea, Journey into the World of Hearing, 10 Nov. 2016, Available Here

Image Courtesy:

1. “Blausen 0329 EarAnatomy InternalEar” By BruceBlaus. Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia  
2. “Cochlea-crosssection” By The original uploader was Oarih at English Wikipedia. – Transferred from en.wikipedia to Commons. (CC BY-SA 3.0) via Commons Wikimedia