The Anatomy of the Central Control of Posture: Consistency and Plasticity
Introduction
BODY POSTURE usually means the maintenance of erect posture (antigravity movements), integration of body and limbs, synergy of the whole limb and orientation of body and head [21]. There exists a large number of different postures, each of which consists of a separate combination of muscle activity, and which may last for a considerable length of time.
The axial and proximal muscles, i.e. neck, shoulder, trunk, hip and back muscles, play a crucial role in postural control. They determine the position of the trunk, head and eyes, and thus of the visual field. Naturally, the visual field is also controlled by extrinsic eye muscles, which explains why the premotoneuronal systems of the axial and proximal muscles are intimately related to the extrinsic eye muscle premotor interneurons [13]. In Fig. 1 the posture and extrinsic eye muscle motor control systems represent the so-called medial component of the somatic or voluntary motor system [16].
Certain postures are steered by the somatic motor system, and others by the emotional motor system (EMS; [16]). An example of an EMS controlled posture is arching of the back by cats. This posture usually takes several seconds and represents a strong, fear related emotional response. Another example is the typical receptive posture of the female cat during mating, which also takes a relatively long time and can easily be elicited by rubbing the flanks or the skin of the lower back or perineum. However, this response only occurs when the female cat is in oestrus or `in heat'. Similar specific responses are found in female rats, hamsters, pigs, and many other female mammals, and they belong to the lateral component of the emotional motor system (Fig. 1). The present paper gives examples of the supraspinal postural control systems of the somatic or voluntary motor system as well as of the emotional motor system, but discusses first the location of the motoneurons innervating axial and proximal muscles and their propriospinal premotoneuronal control.
Section snippets
Axial muscle motoneurons and their propriospinal premotor interneurons
In fish almost all musculature is axial, and its motoneurons are located throughout the length of the spinal cord. In a phylogenetically later phase, certain species have developed limbs, which had to be innervated by motoneurons located lateral to the already existing axial muscle motoneurons. These extra groups of motoneurons form the enlargements of the caudal cervical (Fig. 2) and lumbosacral spinal cord.
Muscles never function independently, but always act in coordination with other
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