Five questions (3 points each), due Thursday 31 March at 8:00 AM
1. What is gamma motoneuron co-activation and what is its effect on muscle spindle afferent activity?
Co-activation of gamma motoneurons refers to the simultaneous firing of alpha motoneurons and gamma motoneurons during a locomotor or other movement task. the alpha motoneurons innervate the extrafusal muscle fibers and the gamma motoneurons innervate the intrafusal muscle fibers in spindles. When co-activated, the muscle spindle and the muscle itself contract and shorten in parallel, thus the sensitivity of the spindle to muscle length and dynamic changes in length is maintained, even as the muscle shortens.
2. In an episodic biting task, like Figure 2 in Lund & Olsson 1983, under what conditions will group II primary sensory afferents from masseteric muscle spindles show enhanced discharge rate and under what conditions will they fall silent?
Discharge in group II (static) spindle afferents will be enhanced when the masseter muscle is lengthened, because the muscle spindle static fibers will sense the length. Group II afferents will also be enhanced when fusimotor drive is present (gamma motoneurons fire). Group II (static) spindle afferents will be silenced if jaw closing velocity is very rapid, in which case the intrafusal fiber shortening speed lags behind the extrafusal muscle shortening speed.
3. Why is the jaw opening reflex due to low-threshold afferents important under normal behavioral circumstances (i.e., not chewing), but should be inhibited during chewing?
Normally, tactile exploration or contact in the oral region would signal a defensive reaction in most animals that would prepare them for biting or defensive actions. During chewing, low-threshold oral afferents will be constantly stimulated by food in the mouth and chewing would be impossible if all these inputs elicited jaw opening.
4. What criteria define CPG neurons and what criteria define motor efferents?
Motoneurons are classically defined as central neurons that directly innervate muscle fibers/endplates. Motor efferents send axons to the periphery, conduct impulses from the CNS to the periphery during normal motor behavior, and cause discrete movments. CPG neurons: (1) display rhythmic activity in the form of oscillations or spike bursts in strict temporal relationship to the rhythmic behavior, (2) express synaptic or intrinsic properties that account for their rhythmic activity, (3) innervate or activate the motor apparatus, and (4) perturbing their normal timing of activity permanently resets motor rhythm at all levels.
5. What are three forms of delayed excitation from DSI/C2 neurons to VSI neurons?
Getting 1983, p.114, in VSI-A the integrative properties of the I-E synapse from C2 gives rise to delayed excitation. In VSI-B, there is excitation from C2 with simultaneous inhibition from DSI. Excitatory input from C2 must summate and override DSI inhibition. Also, depolarization of VSI-B activates A-current, which delays the onset of depolarization.