Thirty Flashcards
What are the two sensory organs detecting linear acceleration? What is their orientation? When is each most important? What don’t they detect? What do they detect aside from linear acceleration? What part of these organs contains hair cells? What are the cilia of the hair cells surrounded by? What is on top of that? What makes them especially helpful for detecting gravitational changes? What happens to this system during acceleration? What happens when the acceleration stops? What is the function of these organs?
The Utricle (horizontal-important for standing) and Saccule (vertical-important for laying down) sensory organs– the MACULA-- detecting Linear Acceleration. The macula detects the orientation of the head with respect to the direction of gravity (linear acceleration). Each organ has a sheet of thousands of hair cells found in a spot (the macula) covering slightly over 2 mm in diameter whose cilia are surrounded by a gelatinous mass. This gel also has a clump of small crystals embedded in it, called otoliths or statoconia (fig. 3). The weight of the otoliths (3 times that of the surrounding tissues) gives the gel higher inertia, so that when you move to one side, the otolith-gel mass drags on the hair cells. Once you are moving at a constant speed, such as in a car, the otoliths come to equilibrium and you no longer perceive the motion.
A major role of the saccule and utricle is to keep you vertically oriented with respect to gravity. If your head and body start to tilt, the vestibular nuclei will automatically
compensate with the correct postural adjustments. This is not just something that happens only if the floor tilts; if you watch someone trying to stand still, you will notice small continuous adjustments to stay upright.
When the body is thrust forward suddenly (acceleration), the otoliths fall backwards on the hair cell cilia, relaying information to the CNS that makes the individual feel as though he/she were falling backwards. This causes the individual to lean the body forwards until the anterior shift of the otoliths caused by leaning exactly equals the tendency for the otoliths to fall backwards. At this point, the nervous system detects a state of proper equilibrium and therefore leans the body no farther forward. Thus, the maculae function to maintain equilibrium during linear acceleration in the same way as they operate in static equilibrium.
Note that the maculae detect linear acceleration, not linear velocity.
What is the firing of the hair cells like at rest? What is the cellular mechanism of hair cell transduction? What is the endolymph like? Perilymph? What is a kinocilium?
Even under “resting” conditions, most of the nerve fibers leading from the hair cells transmit a continuous series of nerve impulses (around 200 per second). Bending the cilia of the hair cell to one side causes a rise in frequency of impulses; bending to the opposite side causes a lowering in frequency of impulses. Therefore, as the position of the head changes, and the weight of the otoliths bends the cilia, appropriate signals are transmitted to the brain for equilibrium control.
Cellular Mechanisms of Hair Cell Transduction (and related vestibular disorders). Hair cells are laterally and vertically polarized. Laterally, there is a kinocilium displaced to one side, with sterocilia decreasing in height relative to the distance from that side. Vertically, the basal end is like any neuron (but no channels to elicit action potentials), but the apical end has the machinery for mechanical transduction.
Endolymph is high in potassium and perilymph is low in potassium. The cell is naturally a little depolarized because some potassium is always leaking in. When acceleration occurs, the otoliths are moved causing the gel to move causing the cilia to move. If the movement if toward the kinocilium, tip links open pores allowing potassium to flow in which increases the firing rate. If the movement is away from the kinocilium, tip links close the pores even more so than at rest which decreases the firing rate. Therefore, both directions can be detected.
What are some agents that cause ototoxicity? What is it? What is endolymphatic hydrops? What allows the potassium concentration of endolymph? Prognosis? What is BPPV?
Ototoxicity (damage to hair cells): aspirin, quinines, loop diuretics, aminoglycoside antibiotics, anti-neoplastics, environmental chemicals (butyl nitrite, carbon disulfide, carbon monoxide, heavy metals, organic solvents, styrene).
Endolymphatic hydrops stems from abnormal endolymph (change in volume or concentrations of sodium, potassium, chloride, and other electrolytes). Striae vascularis (capillaries) have potassium pumps which allow the endolymph to be high in K+. This condition will usually resolve with time.
Benign Paroxysmal Positional Vertigo (BPPV) is due to small crystals of calcium carbonate that are misplaced from the maculae due to head injury, infection, or other disorder of the
inner ear, or degenerated because of advanced age.
What is the function of the semicircular canals? How many are there? Why? Where are the hair cells? where do their cilia project? What are the 3 semicircular canals called? How are they arranged?
The Semi-circular Canals. The semicircular canals detect angular acceleration. There are 3
canals in each temporal bone, corresponding to the three dimensions in which you move, so
that each canal detects motion in a single plane. Each canal is set up as shown below, as a
continuous endolymph-filled hoop. The actual hair cells sit in a small swelling at the base
called the ampula.
The hair cells are arranged as a single tuft that projects up into a gelatinous mass, the
cupula (without otoliths!).
The three (bilateral) semi-circular canals are arranged at right angles to one another.
On each side of the head there is a lateral (or horizontal), an anterior and a posterior canal.
They represent all three planes in space. When the head is bent forward approximately 30o
the two horizontal canals are horizontal with respect to the surface of the earth. Thus, the
anterior canal on each side of the head is in a plane parallel to that of the posterior canal on
the opposite side of the head.
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What happens in the semicircular canals when rotation occurs? After sustained rotation? After sustained rotation stops?
When the head suddenly begins to rotate in any direction (angular acceleration) the
endolymph in the canals tends to remain stationary, while the canals move. This causes
relative fluid flow in the canals in the opposite direction to the rotation of the head. Fig. 6
below shows a record from a single hair cell in the crista ampullaris. Great stimulation (up to
800 impulses per second) occurs at the beginning of rotation, but then within 20 seconds the
tonic level of discharge is restored. Upon cessation of rotation, the afferent impulse
frequency is suddenly reduced to zero. The transduction by the semicircular canals is
therefore characterized by short-latency responses to the onset of motion but the response is
rapidly adapting.
When you turn your head in the plane of the canal, the inertia of
the endolymph causes it to slosh against the cupula, deflecting the hair cells. Now, if you
were to keep turning in circles, eventually the fluid would catch up with the canal, and there
would be no more pressure on the cupula. If you stopped spinning, the moving fluid would
slosh up against a suddenly still cupula, and you would feel as though you were turning in the
other direction. This is the explanation for the phenomenon you discovered when you were a
toddler.
How do both sides of semicircular canals work together? What happens if one side is damaged? What will eventually happen if one vestibular nerve is cut? Which side will be excited with a counterclockwise movement or movement to the left? To which side will you stumble in a unilateral damage?
Naturally, you have the same arrangement (mirrored) on both sides of the head. Each tuft
of hair cells is polarized - if you push it one way, it will be excited, but if you push it the
other way, it will be inhibited. This means that the canals on either side of the head will
generally be operating in a push-pull rhythm; when one is excited, the other is inhibited
(see below). It is important that both sides agree as to what the head is doing. If there is
disagreement, if both sides push at once, then you will feel debilitating vertigo and
nausea. This is the reason that infections of the endolymph or damage to the inner ear
can cause vertigo. However, if one vestibular nerve is cut, the brain will gradually get
used to only listening to one side - this can actually be a treatment for intractable vertigo.
With any rotation, the side that the movement is towards will be excited and the other inhibited. Also, you will stumble to the ipsilateral side in unilateral vestibular damage.
Explain the vestibulo-ocular reflex? What other reflex helps us stabilize the eyes? Why is it harder to concentrate on something that is moving than to concentrate on something when we are moving? How do we know that visual information is very important for maintaing equilibrium? Will someone be able to maintain equilibrium with an impaired vestibular aparatus? What conditions will make it hard for them?
When a person is in motion with his/her eyes open, the eyes can keep a stable image focused
on the retina. This compensatory mechanism is due in part to signals from the semi-circular
canals that cause the eyes to move in an equal and opposite direction to the rotation of the
head. This reflex, transmitted from the semi-circular canals through the vestibular nuclei and
the medial longitudinal fasciculus to the ocular nuclei, is called the vestibulo-ocular reflex
(VOR). Recall that the eye is controlled by three pairs of muscles; the medial and lateral
rectus, the superior and inferior rectus, and the inferior and superior oblique. You may also
remember that their directions of motion seemed to be at crazy diagonals. Those same crazy
diagonals are matched closely by the three planes of the semicircular canals, so that a single
canal (in general) interacts with a single muscle pair.
If you nod, shake, and swivel your head, you will find that you have no trouble staying
focused on this page. But hold a piece of paper in front of you and shake it around, and your
eyes will not be able to keep up with the quick movements. The reason is that the
semicircular canals exert short-latency control over the eyes to compensate for head
movements, as long as the transduction of the head motion doesn’t rapidly adapt. However,
even though the visual control of eye movements (optokinetic reflex) does not adapt, it has a very long latency so
cannot respond quick enough to shaking the paper before your eyes.
Visual information is still, however, very important itself in maintaining equilibrium.
How do we know this? After destruction of the vestibular apparatus, a person can still use
the visual mechanisms effectively for maintaining equilibrium. When a body movement
shifts the visual image on the retina, this information is relayed to the equilibrium control as
long as their eyes are open and as long as they perform all motions slowly. Rapid movement,
or when the eyes are closed, will reveal great deficiencies in equilibrium control.
What are 3 other factors aside form visual or vestibular that help us maintain equilibrium and how do they do it?
- The Neck Proprioceptors
The vestibular apparatus detects orientation and movements of the head only. The CNS
needs to have information about the position of the rest of the body with respect to the
position of the head. The joint receptors of the neck provide by far the most important
proprioceptive information. When the neck is bent, these receptors transmit signals that
exactly oppose the signals transmitted from the vestibular apparatus.
- The Neck Reflexes.
If the vestibular apparatus is damaged, bending the neck causes immediate muscular
reflexes called neck reflexes. For example, bending the head forward causes both arms to
relax – but this effect does not occur when the vestibular apparatuses are intact. Equilibrium
works for the entire body so it makes sense that the vestibular and neck reflexes must
function in opposite directions.
- Proprioceptors from Other Parts of the Body.
a. Pressure sensations from the soles of the feet.
b. In a running person air pressure against the front of the body causes the person to lean
forward to oppose this.
