Download doc Watt lecture 2 notes

Watt lecture 2
Vestibular system: It can measure head movement to help us stay up right. Because of the mechanical
integration, we end with a signal that goes into brain stem that is proportional to angular velocity.
Otolith organs:There are hair cells that have projected cilia and on there are dense gelatinous crystals
above that are more dense
-How to stimulate? 1)Head tilting and letting gravity will cause the otolith to displace and this will
deflect the hair cells
2)Accelerating the head forward: This is simple inertial reaction and the otolith will move to the left.We
don’t only tilt our heads or only do translation, we do both at same time therefore the otolith senses the
vector sum of both gravitational and linear acceleration.
Key:The vectors that cause stimulus are identical in both cases in both cases and the stimulus to the
otolith is that same in both cases. So how do we distinguish between a gravitational acceleration vs.
translation?See below..We don’t want to react that same way to gravity then to a car hitting us and
causing a linear acceleration!
-In normal life, we get don’t only get a separate tilt stimuli and a separate translation signal. However,
the nervous system uses other ways to distinguish between tilt and translation (linear acceleration).
Otioloth organs are linear accelerometers and take the vector sum of both linear and gravitational
acceleration.
Experiment: Doing a functional lesion-Normally, the kinds of head movements performed are quick and
under these conditions, the canal functions well and transduces the angular velocity of the head well. If
you rotate steadily, you will see the world spinning after. Why does this happen?
Experiment-A person is in the dark and you spin them at a constant velocity. The subjective velocity is
the person telling you how fast they are turning. Initially, you get a sensation of rotation but eventually
the cupula will straighten out and you will no longer feel like you’re rotating at all since there is no
acceleration and the otolith will not give input.Then at the end of rotation, the endolymph will overshot
in the opposite direction resulting in angular acceleration but in the opposite direction giving a false
sensation of rotation in the opposite direction. You will also get vestibular spinal reflexes and they will
compensate for a rotation that doesn’t exist.The eyes will also and try to track the movement you are
filling although you’ve stopped and this is why you get dizzy and the world spins around you.
Making the otiloth organ smaller will make the optimal frequency of head movement lower. Canals
primarily fail when you go below normal range for head movement frequency.
Otolith’s problem: Gravity and linear acceleration are the same thing. When we have a steady linear
acceleration applied to us, we assume it is vertical. We always sense the vector sum of both linear
acceleration and gravity and we always assume that this is the vertical although it may not be. This is a
problem in aviation. When the plane accelerates, the resultant is a force that is tilted backward but you
perc eive that as down so you feel like the aircraft is going up and therefore you will aim the plane down
and will crash. If the plane deccelates and you will think the plane is going down and you will aim the
plane up and you can crash the plane.
Proprioception: Vestibular measure how the head is moving in space and tells you how you are moving
in the outside world but it is not effective at all in compensating in problems associated with skeletal
muscle. Proprioreception will take care of this and will know what all the muscles are doing
Muscle spindles: -It has a capsule and inside has intrafusal fibers
-They are attached at two ends by extrafusal fibers (all the other muscle)
-They are about 2mm in length
-Widely scattered throughout all skeletal muscle
-The number of spindles per muscle is tightly linked to precision that you need to control that muscle.
More spindles means more precision
-They have both afferent innervations (1a and 2) and they have an efferent innervations (gamma motor
neurons)
-Inside the spindle there are 2 or 3 kinds of intrafusal fibers: nuclear bag fibers and the nuclear chain
fibers
-In all these cases, there is a transducer region I the middle and its not contractile but senses something
but the outer region is contractile and is like normal skeletal muscle
-Group 1a:Come from transducer region of all kind of intrafusal fibers
-Group 2:Come from mainly nuclear chain fibers
-There are two gamma neurons:gamma static and gamma dynamic. Gamma dynamic innervates
primarily the nuclear bag fibers and gamma static innervate the nuclear chain fibers
-The 1a’s come from all different kinds and they have both static and dynamic sensitivity and they
transducer both length and rate of change of length
-The group 2’s have only static sensitivity
Golgi tendon organs:Smaller than spindles and they are at the end of the muscle
-They have no efferent info and they have afferent 1b’s
-They measure force that the muscle is generating but their function is just as important
Diagram: what happens to the different patterns of different afferents given different events
-First case: contraction of muscle
-Second case: contraction of polar region of spindle which is just a gamma activation
-Third case: passive stretch
-Intrafusal and extrafusal fibers are in parallel and the golgi tendon organs are always in series with
them.
-Case 1: Contraction of whole muscle will shorten (squish it) the intrafusal fibers and this will cause the
firing rate of 1a’s and 2’s to decrease. The golgi tendon organ will increase firing in 1b’s from the golgi
-Case 2:Activate only gamma. Gamma will contract the ends of the intrafusal fibers and that will stretch
the central portion and this will cause more firing from the 1a’s and 2’s. The golgi tendon organs will not
detect it since the force of the intrafusal fibers is not enough
-Case 3: This is an external load to the muscle which is like pulling it down. If you stretch the muscle out
by pulling on the tendon, you will stretch the muscle and the intrafusal fibers and therefore increase the
firing rate. But, you are also stretching the golgi tendon organs and this increases the firing rate of 1b’s
but not very much and this is increase is hard to predict. The active contraction of the muscle produces a
much larger increase in activation of the golgi tendon organ than a passive stretch. They are more
sensitive to internally generated forces than external generated forces and this is not really
understood well.
Basic stretch reflex: When muscle is stretched, this will stretch the spindle and this will increase the 1a
firing rate and this will excite motor neurons and it will recruit more neurons or increase the firing rate
of an individual neuron. This will cause contraction and this will resist the stretch. If the muscle were to
shorten, you would get the reverse effect. This design has one problem: it has a very slow delay and this
is a setup for oscillation(i.e. you cannot keep your muscle perfectly still). An oscillation is a series of
overshots in contraction and relaxation. You want the muscle to behave in a particular way and you
notice that it has gone too far I contraction so you relax but the information is late coming in so you
have already over contracted before you relax. What is a way of keeping this under control?
Every 1a fiber presynaptically inhibits other 1a fibers and therefore the more info that comes in, the lets
gets through and this prevents the system from being overwhelmed.