Voluntary movement of the skeletal muscles of the body is achieved through control by the central nervous system. The main areas of the cerebrum that are engaged with voluntary movement(s) are the primary motor cortex, the premotor and the supplementary motor cortices. Also in the cerebrum are the basal ganglia. Other structures equally involved in the voluntary movement of skeletal muscles are several subunits/components in the cerebellum and the brainstem. The roles of each of these assemblies are discussed below.
Cerebrum: The motor cortex
One of the most important afferent pathways engaged in voluntary movement, and originating from the motor cortex, is the corticospinal tract, which is involved in the movement of distal muscles. Corticospinal tracts also originate from the somatosensory regions of the cerebral cortex. The motor cortex is involved in the voluntary movement of skeletal muscles and has three major components the primary motor cortex (M1), which is located rostral to the primary somatosensory region and is believed to coordinate the appropriate combination of muscles required for the different movements (FIGURE 1); the premotor (PMR) and the supplementary motor regions (SMR) located laterally and medially, respectively, to the primary motor cortex and are two regions believed to be involved in the planning of the movement of the limbs. The parietal and prefrontal cortices are also engaged in the voluntary movements of muscles (Guyton & Hall 2000; Bear et al. 2001).
The corticorubral tract, which synapses with the rubrospinal tract in the red nucleus, present in the midbrain, is also involved in the motor control of distal limb muscles (Bear et al. 2001; Kandel et al. 2000).
Cerebrum: The basal ganglia
The basal ganglia, present in the cerebral cortex, are involved in the control of the voluntary movement of the skeletal muscles. The main structures of the basal ganglia involved in voluntary movement are the caudate and putamen (and nucleus accumbens- collectively known as the striatum), the globus pallidus interna (GPi) and externa (GPe), the substantia nigra pars reticulata (SNpr) and compacta (SNpc) and the subthalamic nucleus (STN). The caudate and putamen receive input synapses from the motor cortex and therefore function as mediators between the motor cortex and the other structures of the basal ganglia involved in voluntary movement. The GPi and the GPe receive input from the caudate and the putamen. The GPi, however, provides the main source of inhibitory stimuli to the motor cortex via the thalamus. This produces a stopping effect that positions a limb in a static orientation in space (Kandel et al. 2000; Molavi 1997). A disturbance in the neural transmission pathway in the basal ganglia will shortly be demonstrated, to depict its role in voluntary movement.
The cerebellum
The cerebellum is situated posterior to the brainstem and inferior to the cerebral cortex. It contributes to voluntary movement by coordinating the appropriate set of muscles that are required for a specific action. This is achieved through the integration of signals ‘to proceed in a certain activity’ from the motor cortex and proprioceptive signals coming from the peripheral ends (Molavi 1997).
The middle and inferior peduncles are routes that allow the descending pontine mossy fibres from the pons and the ascending fibres the inferior olive and proprioceptive axons, access to the cerebellum. The pontine fibres receive neural signals from the corticopontine fibres, which relay information to the cerebellum as to what the intended movement is (FIGURE 2). The relayed signals from the proprioceptive fibres ‘inform’ the cerebellum the location of the limb in space and therefore the appropriate proceeding/action may follow, while the ascending fibres from the inferior olive deal with the coordination of the limbs on the contralateral side to that of the olive (Molavi 1997; Harting 1997; Kandel et al. 2000).
The fastigial nucleus deals with maintaining balance while the interposed and dentate nuclei are engaged in voluntary movement(s). Neural signals to these two latter nuclei are relayed to the red nucleus and to the motor cortex via the thalamus (Molavi 1997). These motor and sensory relays are not perceived consciously but however facilitate voluntary movement(s).
The brainstem
The brainstem is part of the CNS that links the spinal cord to the cerebral cortex, and is located anterior to the cerebellum. It is made up of three major components; from the rostral to the caudal end these structures are the midbrain, pons and the medulla. As with the spinal cord, the sensory neurons are organised laterally in the brainstem, whereas the efferent neurons are located medially with respect to the afferent neurons. All the afferents have to ascend to the cerebellum and the cerebral cortex, via the brainstem, from the peripheral ends in order to be processed so that the appropriate voluntary response may be generated. This/these response(s) then descend(s) to the appropriate combination of skeletal muscles, and at the right time to result in coordinated movement (Zigmond 1999).
All sensory stimuli detected on one side of the body are relayed to the contralateral side of the cerebral cortex or the cerebellum while all the motor descending signals from the one side of the cerebellum and the motor cortex are conveyed onto the opposite side (Bhatnagar 2002).
The role of the basal ganglia in the control of voluntary movement
The substantia nigra pars compacta (SNpc) and the substantia nigra pars reticulata (SNpr), of the basal ganglia, are located caudal to the caudate and putamen bodies. These structures are interconnected by interneurons as shown in FIGURE 3. Loss of dopaminergic cells in the SNpc has been shown to result in the disturbance of the voluntary movement(s) of the skeletal muscles (Kandel et al. 2000; Molavi 1997). The account below describes the effects of changes to the basal ganglia and hence its role in the regulation of voluntary movements.
The loss of excitatory dopaminergic neural pathway leads to increased inhibitory neural activity in the indirect pathway (via D2 receptors), to result in less inhibition of the GPi and the STN nuclei. Simultaneously, a decreased inhibition from the SNpc leads to a decrease in the direct inhibitory pathway, through the action of D1 receptors. Subsequent results in the alteration of the activities of the direct and indirect pathways, is the increase in the inhibitory activity of the GPi to the motor cortex via the thalamus. Resultant weak excitatory signals arising from the motor cortex then descend to the skeletal muscles via the spinal cord (Kandel et al. 2000). Since the inhibitory pathway from the pedunculopontine nucleus is not altered by the increase in the inhibitory signals from the GPi, the normal levels of the inhibitory neurotransmitters relayed to the skeletal muscles, via the spinal cord, ultimate results in: -
Ø Rigidity due to simultaneous contractions of the extensors and flexors (skeletal muscles);
Ø Tremor caused mainly by an imbalance between inhibitory signals sent by the globus pallidus interna to the motor cortex via the thalamus and the excitatory action potentials originating from the cerebellum (FIGURE 4);
Ø And lastly, bradykinesia, which is the slow and laborious movement of the skeletal muscles. These are Parkinsonian effects and hence depict the role of the substantia nigra, and in particular the pars compacta, in regulating voluntary movement.
Therefore Parkinson’s disease has demonstrated the role of and the necessity of the substantia nigra pars compacta in the basal ganglia, in the control of voluntary movement in the skeletal muscles (Kandel et al. 2000; Zigmond et al. 1999; Molavi 1997).
The subthalamic nucleus plays a role in the regulation of the inhibitory process of the dopaminergic neurons to the thalamus, by increasing or decreasing the inhibitory process of the globus pallidus interna (to the thalamus). The subthalamic nucleus increases the inhibitory process of the GPi by increasing its glutaminergic (excitatory) stimulus to the GPi. The subthalamic nucleus itself is controlled by GABA-ergic inhibitory stimulus from the globus pallidus. In this way, the appropriate inhibition to the motor cortex is maintained (DeLong 2000; Kandel 2000).
Apart from lesioning dopaminergic neurons to the subthalamic neurons in order to determine the role of the this body, conformational experimental procedures have also been devised to selectively block dopaminergic D1 and D2 receptors only in this structure, in rats. The results have shown that increased rigidity of skeletal muscle occurred in the hind limb, which further confirms the necessity of the subthalamic nucleus in the control of voluntary movements of skeletal muscles (Hemsley et al. 2002). Similarly, I believe that using specific antagonist for either dopaminergic D1 or D2 receptors, the location and their function may be used to establish to what degree these receptors play in regulating cortical functions and hence the extent of loss or control of voluntary movement of the skeletal muscles.
If the inhibitory mechanism from the globus pallidus interna to the thalamus is impeded, the excitatory mechanism of the thalamus is increased.
This can lead to an increase in the excitatory stimulus by the motor cortex to the skeletal muscles of the body (Zigmond 1999; Kandel 2000). Similarly, lesions of the substantia nigra pars reticulata (or carefully introducing an antagonist for the dopamine receptors) can lead to the inability to control eye movements, leading to excess movement of the eyes, due to a disruption in the inhibitory pathway from the SNpr to the superior colliculus in the midbrain. The superior colliculus responsible for the control of the saccadic eye movements is thus rendered ineffective in its function (Zigmond 1999).
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