Vestibular influences on cat lumbar paravertebral muscles

Task, muscle and frequency dependent vestibular control of posture

The vestibular system is crucial for postural control; however there are considerable differences in the task dependence and frequency response of vestibular reflexes in appendicular and axial muscles. For example, vestibular reflexes are only evoked in appendicular muscles when vestibular information is relevant to postural control, while in neck muscles they are maintained regardless of the requirement to maintain head on trunk balance. Recent investigations have also shown that the bandwidth of vestibular input on neck muscles is much broader than appendicular muscles (up to a factor of 3). This result challenges the notion that vestibular reflexes only contribute to postural control across the behavioral and physiological frequency range of the vestibular organ (i.e., 0-20 Hz). In this review, we explore and integrate these task-, muscle- and frequency-related differences in the vestibular system's contribution to posture, and propose that the human nervous system has adapted vestibular signals to match the mechanical properties of the system that each group of muscles controls.

Responses of thoracic spinal interneurons to vestibular stimulation

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 ms (minimum of approximately 3 ms), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.

Transneuronal tracing of neural pathways controlling an abdominal muscle, rectus abdominis, in the ferret

Abdominal muscles participate in generating a large number of behaviors and reflex responses, including expiration, coughing, sneezing, vomiting, postural control, production of speech, straining, facilitation of venous return to the heart, and reaction to vestibular stimulation. However, the only premotor neurons that have been conclusively shown to influence abdominal motoneurons are located in nucleus retroambiguus, the expiratory region of the caudal ventral respiratory group. In the present study, the neural circuitry controlling the activity of one abdominal muscle, rectus abdominis, was mapped using the transneuronal tracer pseudorabies virus (PRV) in the ferret. Injections of PRV into rectus abdominis labeled large presumed motoneurons in the ventral horn of T12-L4, and smaller presumed interneurons that were scattered in laminae VII, VIII, IX, and X of T4-L4. In addition, neurons in several areas of the medulla and caudal pons, including the retroambigual nucleus, medial and ventromedial reticular formation, nucleus prepositus hypoglossi, vestibular nuclei, and raphe nuclei, were infected by transynaptic passage of PRV from rectus abdominis motoneurons. Thus, the multifunctional roles of abdominal muscles appear to be coordinated by premotor neurons located in both the spinal cord and several regions of the brainstem.