The function of the cranial nerves and their motoneurons is to control individual ocular muscles. However, all eye positions and eye movements require the coordinated activity of all eye muscles and higher centers are needed to orchestrate specific movements of the eyes to specific positions of gaze. This orchestrated activity is performed by specific groups of cells in the reticular formation (located in the brainstem underneath the cranial nerves). These supra nuclear regions receive input from almost everywhere in the brain. Inputs come from the cortex, spinal cord, muscle stretch receptors, visual stimuli, neck proprioception, and vestibular inputs. This information is integrated by the cerebellum and automated subroutines are selected by the cerebellum to respond in a reflex manner to many stimuli. They serve to perform dedicated subroutines just like you have sections in a computer program that can be called at any time to perform a specific function. You may want your computer to make a beeping sound when something else has been accomplished like the clock reaches the top of the hour. Similarly, you may want your eyes to move leftward when something suddenly appears in the left field of view. These subroutines are in a category of activities called reflexes. They occur automatically when they are called upon, either involuntarily by a vital stimulus, or voluntarily as a result of some conscious decision. In either case, the response occurs in a reflexive fashion without any need for conscious organization. For example, we don't need to think how to voluntarily move our eyes together in a conjugate motion or to increase our accommodation. We only need to will these changes which is equivalent to calling up the subroutine. These automated subroutines are accomplished by nuclei that send both excitatory and inhibitory inputs to several motor nuclei such as the three cranial nerves. The multiple axons are called interneurons. In some cases the resulting activity can be quite complex. Locomotor Reflexes Good examples of complex activities which call upon automated subroutines are found in postural reflexes during walking or locomotor behavior. For example, there are stretch reflexes by the limbs when the muscles experience an increase in load. Lean your weight onto one foot and the muscles contract automatically to support the load. If you produce a painful stimulus like heat there is a withdrawal or flexor reflex that pulls the limb away. At the same time the opposite limb extends to help get away from the stimulus (cross extensor reflex). When we walk or run there are rhythmic stepping and diagonal stepping or mark time reflexes in which stepping occurs diagonally between the fore and hind limbs. For example the right hind limb and left forelimb move backward together while the left hind limb and right forelimb move forward together. This is what we do when we walk and run or throw a bowling ball. This results in a twisting of the torso with the hip twisting clockwise while the shoulders twist counterclockwise. The effect of this is to keep the center of body gravity stable while we move forward instead of swaying from side to side. Body sway would cause imbalance and falls. Keep in mind that we make conscious decisions to walk and run but that these activities are conducted in a reflex or automated manner once they are initiated. We don't have to think about which arm to swing forward when we step forward on our right foot. Four Examples of Locomotor Reflexes
- Stretch reflex, load reflex, knee-jerk reflex: Lean on one foot and muscles contract.
- Flexor reflex: Withdraw rapidly from painful noxious stimulus.
- Cross-extensor reflex: Works with flexor to push away from painful stimulus with uninjured limb. Agonist-antagonist interaction follows Sherington's law.
- Diagonal stepping (mark time) reflex: Twisting of torso when walking causes diagonal yoking of hind and for limbs and keeps center of gravity stable. This is analogous to Hering's law of conjugate eye movements.
Oculomotor Reflexes The oculomotor system also has preprogrammed activities or behaviors such as conjugate movements of the eyes. Our eyes move together even when one is covered (Hering's Law). This is similar to the mark time and crossed extensor reflex. Similarly there is coordinated actions between agonists and antagonists during saccades and other types of eye movements. This is like the agonist-antagonist interactions of skelital muscle. Accommodation and pupillary constriction are largely consensual. Finally, the horizontal and vertical vergence systems maintain precise eye alignment in all directions of gaze, even when one eye is covered. OKN and VOR are analogous to load or stretch reflexes where body rotation is the load and the eyes respond to resist the motion and remain stationary in space. All of these are automated coordinated activities that are willed into action but controlled subconsciously by brainstem subroutines. Six Examples of Oculomotor Reflexes
- Yoked conjugate movements of the two eyes (Hering's Law)
- Reciprocal innervation of agonist-antagonist pairs (Sherington's Law)
- VOR (vestibulo-ocular reflex)
- OKN (optokinetic nystagmus)
- Consensual accommodation and pupil constriction
- Coupling between accommodation and convergence (AC/A and CA/C)
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