Anatomy and physiology of the Inner ear, and Motion Sickness

Welcome to the Presentation PREPERED BY Othman Abdikarim Othman . Let’s Begin Now!    Motor Function of the brain stem and cerebral cortex   

  

 Table of objectives
Introduction
Motion sickness
Reticular formation
Vestibular apparatus
Utricle and saccule Semicircular canals

Today’s Topic

  The brain stem is a complex extension of the spinal cord which performs sensory, motor and reflex function . Besides containing centres that regulate cardiovascular , respiratory and gastrointestinal functions, the brain stem plays major role in the control of eye movement in the support the body against the gravity.


Brain stem   

The reticular formation is a large structure occupying the core of the brain stem from caudal medulla to the rostral midbrain. It consist of areas of diffuse neurons of two types : Sensory & Motor Reticular Formation   

Sensory neuron

Sensory neuron are greater in number and make multiple connection with in the reticular formation it self.

Motor neurons

Motor neuron give rise to axons which divide into ascending branches and descending branches. The former( ascending branches) pass to the non-specific thalamic nuclei, to the basal ganglia and to the cerebral cortex via the thalamus . This is known as the Reticular activating system , which plays a major rule in the control of brain activity such as consciousness and alertness.    

 The descending branches, however, pass to the spinal cord to supply the anterior motor neurons these are the lateral and ventral reticulo-spinal tract. A number of nuclei function in association with the reticular formation of the brain stem. These are :
Vestibular nuclei
Red nuclei
Substantia nigra   

Vestibular apparatus

1. What are the three general parts of the ear?

2. Which parts of ear contain the sensory organs for hearing and balance ?

3. Which structures make up the inner ear or labyrinth?

 ANSWERS
1. The external ear, the middle ear(tympanic cavity) and inner ear( labyrinth)
2. The inner ear
3. Vestibule, cochlea and semi- circular canals    

  The internal ear or labyrinth is situated in the petrous part of the temporal bone, medial to middle ear it consist of : 
Bony labyrinth 
Membranous labyrinth
 
The bony labyrinth consist of three parts 
The vestibule 
Semi-circular canal 
Cochlea    

 Between the bony and membranous labyrinths is a space filled with fluid, the perilymph. Inside the membernous labyrinth there is also fluid, called the endolymph. The cochlear duct is the sensory organ for hearing while the utricle, saccule and semicircular canals contain receptors concerned with the control of equilibrium or balance.    

 The bony labyrinth is formed of the vestibule, a bony cochlea and three bony semi-circular canals 
The membranous labyrinth which consist of mainly of the cochlear duct (membranous cochlea), three membranous semi circular canals and two chambers : Utericle and Saccule.

Inside the Bony Labyrinth ,there is bony spaces and with it there is fluid called perilymph. 

With in the perilymph fluid there is floating membranous structure called Membranous labyrinth. 

With in the membranous labyrinth there is another fluid called endolymph. 

With in the endolymph fluid there is sensory epithelium. floating   

Bony structure
Membranous structure
Sensory epithelium Function
1. Cochlea Cochlea Duct Organ of corti Hearing
2. Vestibule Saccule & Utericle Macule Balance
3. Semi-circular canals Semi-circular Duct Cristae Balance  

Sensory epithelium of vestibular present on the floor of utricle and the wall of saccule and It is called maculae or otolithic. 

Sensory information from two maculae plus sensory information from sensory epithelium of semi-circular canal(cristae) they form Vestibular Nerve

Otoconia Sterocilia Tip links Kinocilium GELATINOUS LAYER Calcium carbonate crystals Each macule is formed of columnar epithelial cells and hair cells and is covered by gelatinous layer(gel like material) inside gelatinous layer are numerous small calcium carbonate stones(otoliths)(otoconia) The Otoliths/Otoconia lie on top of hairs or process that project from the apical ends of hair cells. Because of their weight, otoliths will bend the hairs in the direction of the pull of gravity Small hairs: Stereocilia Large hairs: Kinocilium    

Electron microscopy shows that very fine filamentous attachments called tip tinks connect the kinocilium to the nearest stereocilium. So when stereocilia bend towards kenocilium tip links pull stereocillium one by one this opens ion channels and cell is depolarized.   

When the sterocilia are bent in opposite direction( away from kinocilium) the pulling effect of the tip links on the sterocilia is reduced and ion channels closed and the cell undergo hyperpolarized.   

Semi-circular canals 

There are three semicircular canals on each side The three canals are:  the horizontal semicircular canal (also known as the lateral semicircular canal),  superior semicircular canal (also known as the anterior semicircular canal),  and the posterior semicircular canal (also known as the inferior semicircular canal).    

    Semicircular canals: Each semicircular canal opens at both ends into the utricle one of this ends is dilated to form an ampulla which houses the sensory epithelium called crista 

Contains Cilia of hair cells project into gelatinous cap called cupula

Enlargement of ampulla Crista ampullaris

Semicircular canals 

The cristae of the semicircular canals detect head rotation in any direction. This movement is called Angular acceleration.  Head rotation results in inertial movement of endolymph in opposite direction  Bends cupula which bends hair cells 
  
    Semicircular canals: sensory transduction  Steriocilia maintain directionality on both sides of the head  Bending towards kinocilium  opens ion channels  depolarization  Bending away from kinocilium  closes ion channels  hyperpolarization      

Motion sickness or kinetosis, also known as travel sickness, is a condition in which there is a disagreement exists between visually perceived movement and the vestibular system 's sense of movement.  Dizziness, fatigue, and nausea are the most common symptoms of motion sickness.    
 
What cause it ? 
  
The most common hypothesis for the cause of motion sickness is that it functions as a psychological defence mechanism against neurotoxins.  The area postrema in the brain is responsible for inducing vomiting when poisons are detected, and for resolving conflicts between vision and balance.  When feeling motion but not seeing it (for example, in a ship with no windows), the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still.  As a result of this Conflicts the brain will come to the conclusion that the individual is hallucinating and further conclude that the hallucination is due to poison ingestion. Then brain responds by inducing vomiting, to clear the supposed toxin.    

Advice for people who have motion sickness.

 One common suggestion is to simply look out of the window of the moving vehicle this helps to re-orient the inner sense of balance Take Scopolamine as a prophylaxis during travelling before 30 minutes. 
  
  That is all