System Neurosciences
PART I: Sensory Functions
1. The vestibular system
1.1. The vestibular system introduction
1.1.1. The vestibular system mechanism (exam)
The vestibular system is an input-output system:
OUTPUT
• Gaze stabilization
o Most important, needed to fixate targets e.g; chase an animal and catch it
• Orientation and navigation
• Balance
• Autonomic function
o Upon moving upright, the autonomous system needs to adjust heart rate to maintain appropriate
blood pressure. Also this is triggered by the vestibular system.
• Circadian rhythm
o Also depends on vestibular signals
INPUT
• Graviceptor organs
o The kidneys and intestine are tethered such that when moving, specific signals are fed to the brain to
indicate how we move
• Proprioceptive input
o Provide signals to the brain that indicate the position and tension of muscles, tendons,…
o E.g. sitting in the car and driving fast → pressure is felt on the back against the seat
, • Equilibrium system
o Faster, detects movements i.e. rotations and translations of the head and body
o To maintain or gaze stable during fast head movements
• Visual input
o Most dominant, but slow (100ms to process visual information)
o Sufficient to track slow moving targets, or when we move slowly
• Auditory information is also used that helps us to orient ourselves for example
All this information is processed in the brain to generate the appropriate reflexes.
When something is wrong in the input systems, for example due to a disease of the inner ear, or when something
is wrong in the brain itself, problems with balance may occur.
When a conflict arises between the different systems → motion sickness (NOT a disease)
Space motion sickness originates most likely in
1) Conflicting information coming from within the vestibular system where certain parts still work perfectly well, and
other partes no longer detect gravity
2) Conflicting information between the visual system and the vestibular and proprioceptive system
e.g. when floating upside down in the ISS, the visual system will tell the brain that you float upside down, while the
vestibular system has no clue of the direction of gravity due to the weightlessness. This generates a mental conflict
between different sensors, and may cause the brain to go into an error mode, causing nausea, palor, sweating and
perhaps ultimately vomiting. But after a couple of days, the brain adapts to the new strange situation, and the
space crew no longer experiences space motion sickness, in general.
BUT the brain adapts!
1.1.2. What constitutes the vestibular system?
• The entire balance system consists of different parts, located peripherally and centrally
• The peripheral part
o Bony labyrinth (filled with perilymph) [Na] [K]
▪ 3 semicircular canals
▪ Cochlea (auditory system)
▪ vestibule
o Membranous labyrinth (filled with endolymph) [Na] [K]
▪ 3 semicircular duct
→ Rotational 5sensory organs
▪ 2 otolith organs: saccule & utricle
→ Translational (H&V)
➔ constitutes the balance organs in both inner ears that transmit their signals through the VIII cranial
nerve (Vestibulocohlear nerve) to the vestibular nuclei in the brainstem.
, • The central part
From the vestibular nuclei, there are projections to several locations.
1) To the oculomotor nuclei
▪ Drive the eye muscles to generate the vestibulo-ocular reflex (VOR).
▪ This VOR ensures gaze stabilization upon head movement.
2) To the spinal cord through the vestibulo-spinal tracts,
▪ To ensure posture and balance
▪ = Vestibulo spinal reflex
= Reflex that keeps the body from falling (elderly have difficulties with these reflexes)
3) To the cerebellum
▪ To allow for motor coordination of movement and balance control
▪ = perception of self in space
4) To the autonomic medullary centers
▪ To generate the vestibulo-autonomic reflex that controls breathing and blood pressure.
▪ = vestibulo-autonomic reflex
5) To the thalamo-cortical structures, to enable the cognitive perception of motion - 'where am I in the
surrounding space' - ,spatial orientation and navigation.
1.2. The vestibulo-ocular reflex (VOR)
1.2.1. VOR mechanism
Head movement vs. index movement: Stretch your arm in front of you, put your index finger up and shake it quickly.
Your finger looks blurry. Now hold your finger steady and shake your head quickly while looking at your finger.
Normally, you should see your finger sharp and clear. In both cases, your finger and eyes move identical with respect
of each other but in the first case there was no clear image as opposed to the second case. This is because of the
vestibulo-ocular reflex (VOR).
= The reflex of the eyes to maintain the gaze at a fixed target when moving around
= very fundamental reflex needed by all moving mammals, all day
= A 3 neuron reflex compensating eye movement via push pull mechanism (see later)
The balance system measures precisely which head movements is made, and this information is used by the brain to
control the eyes to make perfectly compensating eye movements
→ Your eyes can voluntary follow slower movements, by fixating and following an object but is it slow
→ During fast movements your balance system takes over
• Primary task of the vestibular system: detection of head movements
• Its final goal : gaze stabilisation = result of the VOR
No coincidence that the eyes and the vestibular systel are housed within the same rigid bony structure
1.2.2. Other ocular mobility and stability systems
1. Smooth pursuit:
- Maintains image of a small moving target on the fovea (fovea = the center of the retina, the region of highest
visual acuity)
- e.g. when following a bee or fly nearby
, 2. Saccades:
− Rapid eye movement that brings the object of interest onto the fovea
− E.g. : looking from the screen of the pc to your keyboard.
3. Optokinetic:
− Stabilizes large moving images on the fovea
− E.g. watching the metro passing by in the tube station
4. Visual fixation:
− Maintains image of a stationary object on the fovea
− E.g. when walking down the road, you can read street names, or recognize faces of friends.
5. Vergence:
− Moves the eyes in opposite directions so that images are held simultaneously on both foveae
− E.g. Approaching your plate with e.g. a hamburger to grab and bite in it.
1.3. The human inner ear
1.3.1. Basic anatomy
- Both left and right inner ears contain a vestibular organ
- The posterior and anterior canals are approximately perpendicular to each other
- The right and the left semi-circular canals are orientated parallel to each other
- The horizontal canals are oriented approximately 30 degrees upward
The vestibular organ with its canals and otolith system is part of the
inner ear that also contains the cochlea for hearing.
