Extraocular muscles and their innervation : Brief Notes
Reading: Chapter 8 of RHSC2.
Gross anatomy:
What's wrong with the picture on the cover of RHSC book?
muscle tendon attachments to the globe not on equator...
What is the size of the adult human eye? 1 inch diameter = 25 mm
What is the length of eye muscles? about 50 mm in human.
There are six extraocular muscles on each eye. Five out of six insert around
foramen, which the optic nerve passes through; room is nneded there for optic
nerve to "stretch" 30°? either way--engineering the flexibility?
inferior oblique inserts near nose, and is the only muscle in the body arranged
as a pulley.
Direction of pull of EO muscles: note that the EO muscles can only pull back.
Because the eyeball is constrained in a socket, it will rotate only, but there is
some net force pulling the eyeball back. However, during stress, all eye muscles
can contract, forcing the eyeball slightly forward (a fact cartoonists sometimes
make use of when drawing bug-eyes on stressed-out characters...Wile E. Coyote, for
example:)
Muscle fiber types in EO muscles:
chpt 8 RHSC2, first part: slow glycolytic, fast oxidative (SG, FO) and mixed are
there. SG are slower, less easily fatigued muscle fibers than FG, but both participate
in all eye movements, and no fatigue or slowing down of saccades is measurable.
ref: Mark Binder, U. Wash. Seattle.
Spindles: Extraocular muscles appear to contain spindles and Golgi tendon organs, yet when recording from oculomotor motoneurons, no stretch response can be seen (no change in discharge if eye is moved passively in the dark).
Oculomotor nuclei and their projections
The motoneurons that contract eye muscles are not located in the spinal cord,
but in three brainstem nuclei.
III: Oculomotor (4 EOM's)
IV: Trochlear (superior oblique)
VI: Abducens (lateral rectus)
Optic nerve is cranial II
Auditory nerve carrying vestibular signals is VIII.
G-rated mneumonic for the 12 cranial nerves:
On Old Olympus' Towering Top A Finn And German Viewed A Hop.
Connections of motoneuron axons to muscles.
Four projections are ipsilateral (same side),
Two are contralateral (crossed)-trochlear is to superior oblique and OMN to
superior rectus
optic nerve from retina is nerve II; it bifurcates (decussates) at the optic
chiasm.
superior and inferior recti insert medially, about 23 deg off axis
Firing patterns of OMN neurons
chapter 8: single unit recording from brainstem nuclei of alert primates:
see §8.4.2 page 200 RHSC2, examine fig 8.10
The equation
describes the firing rate (discharge) of a motoneuron moving the eye theta degrees
at a speed of theta_dot deg/sec. Note degrees here, not radians, in Robinson's early
work. K averages 4 over a range 1-15; R averages 0.6 over a range 0.3 to 5. Is there
a recruitment order? Choosing the averages, note that 0.6/4 = .150, the time
constant of the eyeball in its orbit. Laplace transform of average motoneuron:
Note that here you can think of θdesired as
the input the to OMN, and θ as the output position of the eye
itself. D(t) is the spikes per second sent from the OMN to the eye muscle.
Once the eye stops moving, then K θ is the firing rate to maintain new fixation. Since the mechanical load never changes, and the eye muscle never fatigues or strengthens, then K θ sent back to the CNS represents the position of the eye without the need for proprioception.
Somehow during development the time constant of the OMN must come to match the
time constant of the eyeball in the socket, for each individual.
Oculomotoneurons have the highest average spikes/second of any CNS cells. They
sing!
but: Ed Keller: cells in the deep layer of superior colliculus (one synapse away
from the OMN) can fire up to 1100 per second.
OMN motoneurons contacting muscle fibers. As with skeletal muscles contacted by spinal motoneurons, each muscle fiber in an EO muscle is contacted (after a period of development) by only ONE OMN motoneuron axon, and that axon may fan out to 10-100 other fibers, likely all of the same type, FG or SO.
see updated archive notes on muscle.
Dynamics of EO muscle contraction. What we've said so far assumes that extraocular muscle reacts instantly and smoothly to the change in firing rate of the OMN motoneurons driving them.