Terminologia textual

The central efferent pathways are responsible for the processing of afferent proprioceptive information ascending from the peripheral nervous system (Riemann and Lephart 2002b). There are two divisions; automatic (involuntary) and somatic (voluntary) nervous system and within these divisions two mechanisms; feedback and feedforward motor responses (Biedert, 2000). The motor components that comprise the efferent pathways can be divided into the central axis and two associated areas (Lundy Ekman, 2013). The central axis contains the spinal cord, brain stem and cerebral cortex (three levels of motor control). Motor output at these three levels are proceeded by afferent input, the response can be reflexive (spinal cord) or descending motor commands from the brain stem and/or cerebral cortex (Lephart et al., 1998). The associated areas are the cerebellum and the basal ganglia; these being responsible for modulation and regulation of motor commands (Biedert, 2000). Motor commands can both inhibit and facilitate sensory relay to peripherals. The following section will detail the three levels of motor control and the associated areas.

The Spinal Cord

The spinal cord contains three types of neuron: motor neurons, sensory neurons and interneurons (Bosco and Poppele, 2001). The motor neurons run through the ventral horn to

supply muscle fibres. Sensory nerve fibres enter the spinal cord via the dorsal horn (Stillman, 2000). The spinal cords main contribution to integration of afferent information is to provide the space in cord grey matter for synapses to occur between mechanoreceptor transmission and interneurons (Barrack et al., 1994). This sensory information may be transmitted to other interneurons, higher motor centres and other antagonistic motor neurones (Bosco and Poppele, 2001). The spinal cord is also responsible for the quickest response to peripheral afferent signals, the reflex response (Hewett et al., 2002). This response is necessary for protective reflexes in joint stability (Hewett et al., 2002).

Brain Stem

The brain stem had a significant role in postural equilibrium and autonomous movement through integration of visual, vestibular and all somatosensory sources (Lephart et al., 1998). It is under direct cerebral cortex command and provides the relay between the spinal cord and the cortex (Lundy-Ekman, 2013). Specifically, the medial neural pathway from the brain stem descending to the spinal cord influences axial and proximal muscles, whilst the lateral neural pathway controls distal muscles. Evidence also states the brain stem contributes to spinal reflexes and muscular tone (Lundy-Ekman, 2013).

Cerebral Cortex

The cerebral cortex is the highest level of motor control and is responsible for complex and discrete voluntary movements (Lephart et al., 1998). Efferent signals are transmitted from the cortex both directly to the spinal cord into interneurons and motor neurones or indirectly via the brain stem (Barrack et al., 1994). The major neural pathway from the cortex to the spinal cord is the corticospinal tract (Riemann and Lephart 2002a). Riemann and Lephart (2002a) describe three main areas of somatosensory management. The primary motor cortex is most directly responsible for muscle contraction, using information from several afferent pathways to determine the muscles that are activated, the muscular force of the movement and the direction of movement (Riemann and Lephart 2002a). The pre-motor area, as the name suggests, is indirectly responsible for muscular contraction, organising and preparing the motor commands and the supplemental area assists the primary motor area when programming complex, muscles group contractions (Riemann and Lephart, 2002a). The supplementary area works with the pre-motor area to control bi-lateral synergic movement (Lephart et al., 2000).

The cerebellum has a vital role in the correct sequencing of motor activity. This part of the brain “contains more nerve cells than the rest of the central nervous system combined” (Dye, 2000, p31) and as such may be expected to have this critical role. It is believed the cerebellum is responsible for feedback, feedforward and error correction processes. Bhanpuri et al., (2013) explains the cerebellum predicts body state (i.e. position, acceleration) from a copy of motor commands (known as an efference copy) plus previous knowledge on body movement. Hence perception of sensory information occurs in the cerebellum providing meaningful interpretation of sensory information (Lundy-Ekman, 2013, Proske and Gandevia, 2009). Afferent information is ascended to the cerebellum through the dorsal and ventral spinocerebellar tracts (Bosco and Poppele, 2001). The neural pathways along these tracts are the most rapidly conducting nerves in the entire neurological system reaching speeds of around 100m/s (Dye, 2000). The dorsal tracts provide information from the various mechanoreceptors (Bosco and Poppele, 2001). The ventral tract provides the efference copy of all neurological signals already sent to the spinal cord, and hence it is believed to monitor millions of motor unit contractions, for example agonist and antagonist muscle actions (Dye, 2000). This neurological copy is used to monitor and adjust motor activity; this is done by comparing the intended motor commands of the cerebral cortex to the actual musculoskeletal movement (Dye, 2000), for example during isometric contractions, in which receptors are stimulated, but no movement occurs (Rymer and D’Almeida, 1980).

The term “efference copy” was first proposed by Sperry (1950) and Von Holst and Mittelstaedt (1950). The CNS compares the efferent command with the expected afferent feedback; the reafference (Sperry, 1950). If the two signals match, i.e. if the efferent command minus the reafference equates to zero or a null point the motor act is perceived as successful. However any additional afferent feedback (known as exafference) from the external environment is reported to the sensory centres (the corollary discharge) and may be perceived as a sensation in its own right (Gandevia et al., 2006). This process allows the CNS to account for afferent activity arising from the motor act itself and hence provide a meaningful signal from muscle spindles (Proske, 2005). However, it is important to state there is no direct supporting evidence for a central subtraction process at this time (Proske, 2006).

The feedforward motor commands are not fully understood, but it is believed they play a vital role in preparing the body for impending movement. The lateral zone of the cerebellum is thought to facilitate this planning - “…neurones in the dentate nucleus manifest a copy of the next sequence of motor signals at a time when a current musculoskeletal movement is in

progress…” (Dye, 2000 p. 33). This predictive ability reduces the dependence on peripheral feedback that is time-delayed (Bhanpuri et al., 2013). This theory is supported by work from Bhanpuri et al., (2013); this research group compared the performance of simple, complex and complex disturbed tasks between patients with cerebellum damage and healthy controls. Results implicated an important role for the cerebellum during active predictive tasks, the control group performed better than the patient group in this condition. However, if the task was disturbed and hence unpredictable, both groups performed poorly, the cerebellum was unable to provide useful feedforward and preparatory information. The exact role of the cerebellum is not yet fully understood (Boisgontier and Swinnen, 2014) however, it is clear that this component of the central nervous system plays a vital role in motor control and activity.

Basal Ganglia

The basal ganglion has a direct connection with the cerebral cortex only and it is believed this area of the brain is responsible for higher order aspects of motor control, receiving input from all areas of the cerebral cortex, not just sensorimotor information (Riemann and Lephart, 2002a). Research has yet to fully explain the role of the basal ganglia in body homeostasis (Lundy-Ekman, 2013).