Chapter 5 - Efferents Flashcards
Where are efferents located in the brainstem?
There is a close embryology between facial nerve motorneurons and the efferent system, such that in some animals like eels, the efferents are within the facial motor nucleus. In mammals and birds, these are separate, but close to the facial nucleus. There is also an incomplete separation in the efferent nucleus between cochlear and vestibular efferents.
Do efferents project ipsilaterally or contralaterally?
It depends on the animal. In chinchilla, roughly half project ipsilaterally, the other cross. In most other vertebrates, they project ipsilaterally. This indicates that caution should be exercised when comparing efferent function across species.
Where to efferents terminate?
On afferent processes and type 2 hair cells.
In mammals, what occurs when you electrically stimulate efferents centrally?
Electrical stimulation of the mammalian EVS centrally invariably results in excitation as reflected by an increase in afferent discharge (Goldberg and Fernández 1980 ; Marlinski et al. 2004 ; McCue and Guinan 1994 ). This excitation, which is best seen in response to high-frequency shock trains, is similar whether efferents on the ipsilateral or contralateral sides of the brain stem are stimulated separately or simultaneously (Goldberg and Fernández 1980 ; Marlinski et al. 2004 ).
Which units are more sensitive to efferent stimulation?
Irregular afferents are more sensitive. calyx units are by far the most sensitive to efferent electrical stimulation.
What are the dynamics of the afferent response when efferents are electrically stimulated?
For irregular afferents (afferents that are preferentially stimulated by efferents), similar to the dynamics of afferent stimulation direction, there is a peak that decays with kinetics of 10-100 ms, followed by a plateau. [picture from Goldberg/Fernandez 1980]
How are efferents stimulated by natural stimuli?
They appear to receive convergent stimuli to rotation, such that an efferent is excited by both inhibitory and excitatory stimuli, some call this a type III neuron.
Can efferents respond to other natural stimuli besides vestibular?
Yes! Efferent neurons also respond to non vestibular stimulation, including pressure applied to the skin (Precht et al. 1971 ; Schmidt 1963), passive movement of limbs (Precht et al. 1971; Schmidt 1963 ), and visual stimulation (Klinke and Schmidt 1970 ). Several workers have noted that efferents discharge in relation to active body movements (Gleisner and Henriksson 1964 ; Precht et al. 1971 ; Schmidt 1963), as do lateralline efferents (Roberts and Russell 1972 ; Russell 1971 ). In the toadfish, efferents are excited by a large variety of sensory stimuli (Highstein 1991 ;Highstein and Baker 1985). These have not been repeated in mammals yet, except one paper 1995 showing somatosensory responses in guinea pig (Marlinski).
What is a possible function of vestibular efferents in mammals?
Regardless of the mechanisms involved,
the efferent synapse may act as a filter, maximizing
the effects of high-frequency bursts of activity
in central efferent neurons and minimizing
the influence of lower, tonic discharge rates. So,
an input may be quite effective in exciting efferents,
yet have only a small or no influence on afferent discharge.
