VOL: 103, ISSUE: 41, PAGE NO: 26
Marion Richardson, BD, DipN, Cert Ed, RNT, RN, is principal lecturer, University of Hertfordshire
The third article in this four-part series discussed the ways in which the organs of the membranous labyrinth in th…
The third article in this four-part series discussed the ways in which the organs of the membranous labyrinth in the inner ear detect gravity and linear acceleration (the utricle and saccule) and rotation of the head (the semicircular ducts).
This final article discusses how the brain integrates the information it receives and how it and acts upon it. Some common problems with balance and equilibrium are also discussed.
The vestibular apparatus of the inner ear alerts the central nervous system (CNS) to changes in head movement and position. The CNS then initiates the appropriate compensatory mechanisms that keep the body balanced, weight evenly distributed and eyes focused on whatever was being looked at when the movement occurred (Allan, 2005).
VESTIBULAR PATHWAYS TO THE BRAIN
Sensory nerve fibres from the hair cells in the membranous labyrinth form the vestibular branch of the vestibulocochlear nerve (VIIIth cranial nerve). The fibres run to the four vestibular nuclei between the pons and medulla oblongata in the brain.
These vestibular nuclei are the major integrating centre for balance and receive input from the visual centres as well as from receptors in the joints and muscles of the neck as well as from the inner ears (Marieb, 2006). They then relay the information to various areas of the brain and spinal cord (Fig 1, Fig 2).
The vestibular nuclei have a number of functions that include:
- Integrating the sensory information on balance and equilibrium that arrives from the inner ears, eyes and muscle fibres on both sides of the head;
- Sending commands directly to the motor nerve endings in the brain stem and the spinal cord. These reflexes control eye, head and neck movements as well as the muscles of the body and enable us to respond very quickly to sudden changes in balance - for example, when stumbling. No other nerves from the special sense organs go directly to the reflex centres in the brain (Marieb, 2006);
- Relaying information to the cerebellum, which coordinates skeletal muscle activity and regulates muscle tone to maintain head position, posture and balance. The cerebellum is particularly responsible for the fine control of delicate postural movements (Marieb, 2006; Martini, 2005). Some fibres from the inner ear go directly to the cerebellum, without passing through the vestibular nuclei - this makes the reflex actions of the postural muscles of the body even quicker;
- Relaying information to the cerebral cortex, which gives us a conscious sense of our head position and movement. As yet, it is not clearly understood how vestibular sensation reaches conscious experience (Allan, 2005).
Maintaining balance despite the body’s sometimes unpredictable movements is an important physiological process.
In fact, the body is able to do rather better than simply respond to sudden changes. Information sent to the brain from the semicircular ducts is complex and signals a rate of change of the position of the head (see part 3), so the brain can actually predict the position of the head in advance of the movement. This is thought to be important in enabling the brain to alter the information to the postural muscles so we can move quickly without losing our balance and without having to think about it (Allan, 2005).
Keeping the eyes focused while we are moving is also important and information from the inner ear via the vestibular nuclei enables this to happen. Very fast automatic reflexes control and adjust the movement of the eyes and allow us to read while walking or riding in a car over a bumpy road, for example. The eyes can stay fixed on an object as if the head were not moving at all. If the head turns or spins rapidly, the eyes will fix on one point for a moment and then jump ahead to another in a series of short jerky movements.
This type of eye movement can occur even when the body is stationary if either the brain stem or the inner ear is damaged (Martini, 2005). This condition is called nystagmus and sufferers have problems controlling their eye movements.
PROBLEMS WITH BALANCE AND EQUILIBRIUM
Also known as travel sickness, this seems to occur when the brain receives conflicting information on the movement being detected by the eyes and that by the vestibular apparatus.
For example, visual information may say the body is sitting still within a cabin below deck at sea or in a car - but the labyrinthine receptors tell the brain that the body is moving. It occurs particularly when movement is erratic (Allan, 2005).
Motion sickness is unpleasant. Symptoms include headache, flushing of the face, sweating, nausea, vomiting and various changes in mental perspective - sufferers may go from a state of high excitement to one of deep despair in a matter of moments (Martini, 2005).
The symptoms usually stop once the movement is over. It is possible to control them to some extent by allowing the eyes to confirm the movement that the inner ears detect - for example, watching the horizon at sea rather than the inside of the cabin.
Some people rarely experience motion sickness while for others it makes travel by boat or plane almost impossible.
Drugs that help to prevent it usually depress activity at the vestibular nuclei in the brain, though sedatives can also be effective. Some people find wearing ‘sea bands’ that press on an acupuncture point near the wrist helpful but it is unclear how these work.
Vertigo is the sensation of spinning or of the surroundings moving, even though the body is completely still.
It can last for just a few minutes or can persist for many years (NHS Direct, 2007). Vertigo is worse than dizziness and can make daily life difficult.
Anything that disturbs the endolymph or the hair cells in the inner ear can cause vertigo. The most common cause is motion sickness but abnormal conditions in the inner ear or problems anywhere along the sensory pathway carrying equilibrium sensations will also cause vertigo.
Infections - bacterial or viral - or inflammation of the inner ear or central nervous system and high fever are also causes, as are excess alcohol consumption and certain drugs (NHS Direct, 2007; Martini, 2005). Acute, severe vertigo can also result from disorders of endolymph production, such as in Meniere’s disease.
This condition, in which there is excessive production of endolymph, can cause auditory disturbances as well as dizziness and nausea as a result of the impaired function of the vestibular apparatus. Another possible cause is rupture of the membranes that allow the perilymph and endolymph to mix, causing heightened stimulation of the hair cells. Sufferers commonly experience a ‘howling’ tinnitus - and may lose their hearing altogether - as well as spinning or rolling sensations, nausea and vomiting. Balance is so disturbed that standing upright is often impossible (Marieb, 2006).
Mild cases can usually be managed with anti-motion drugs. In severe cases, surgery may be required to drain the excess endolymph from the inner ear.
- This article has been double-blind peer-reviewed.