To begin with, all voluntary movements are controlled by an area in the brain called the motor cortex. Interests in studying this area grew significantly in the 19th century. We may then wonder.... How can scientists know which brain region is for what function? The answer points to the knowledge called the electrophysiology, the field that studies the brain and behavior through electric current stimulation. The stimulation is introduced to a certain area of the brain with small metal probes. The field was started by using animals (primates mainly), such as Macaques, Cebus, and rhesus monkeys. Pioneering scientists who adopted this technique include Sherrington, Vogt & Vogt, Woolsey, and Evarts on primates. In 1940s at the Montreal Neurological Institute, Penfield produced the well-known somatotopic map known as the homunculus. He had been using metal probes to record neuronal activity directly from the brain during open brain surgeries.
Another non-trivial fact is that the motor cortex controls voluntary movements of the contralateral parts of the body. The decussation or crossing of motor tracts happens at the medulla or the brainstem. What this means is that your left motor cortex is responsible for the movement produced by the right arm, and vice versa. Such principle is found in both humans and primates; not sure whether it has any evolutionary reasons?!
The most important clue about the anatomical location of major brain areas is perhaps from the elegant work of K. Brodmann (c.1909) who defined the cerebral cortex in humans into 52 different areas which have the naming convention "BA-##". For example, the primary motor cortex is also known as the Brodmann Area 4, or BA-4. The motor cortex is located at the caudal region of the frontal lobe and divided into two major parts:
- Primary motor cortex, which is located along the precentral gyrus, at the anterior bank of the central sulcus and spread bilaterally down towards the lateral sulcus, or the Sylvian fissure. This area is analogous to BA-4. "M1" is the more popular name of the primary motor cortex.
- Secondary motor area, which is located anterior to the primary motor cortex. It is analogous to BA-6 and divided into two distinct regions: - Lateral region or premotor proper, further divided into dorsal (PMd) and ventral (PMv).
- The more medial area or the supplementary motor area (SMA).
Posterior to the central sulcus lies the postcentral gyrus. It is the location of another important brain region in motor behavior, i.e. the primary somatosensory cortex. This area is divided into four divisions: the BA-3a, 3b, 2, and 1, whose function is to receive somatosensory stimuli such as touch, vibration, pain, heat, and joint movement. It has a similar somatotopic organization as mentioned by Penfield. More interestingly, the central sulcus divides the brain into the actor (action) and receiver (sensing).
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Fig.1: The anatomical location of various areas related to sensorimotor behavior, taken from [1]. |
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| Fig.2: Somatotopic map of the post- and pre-central gyrus as discovered by Penfield; taken from [2]. |
The primary motor cortex is known as M1 or Area 4 according to Brodmann. In humans and primates, a low-intensity current is sufficient to stimulate contralateral limbs and produce simple movements as demonstrated by classic work of e.g., Penfield and Woolsey. This is the main reason why it is called 'primary'. Neurons associated with this area were sometimes called the pyramidal tract neurons (PTNs). The largest pyramidal neuron is known as Betz cells and located in layer V of the cerebral cortex.
A handful of these neurons synapse directly with the alpha motor neurons that control limb muscles and are thought to mediate skilled learning (Rathelot & Strick, 2005, 2009). This so-called monosynaptic connection is found in humans and primates. From an evolutionary perspective, lower-order mammals, e.g. platypus, hedgehogs, and rodents, do not have distinct M1. They have instead a vast layer of somatosensory areas, analogous to S1/SII/parietal areas in primates and humans (for more info see Krubitzer and Kaas, et al.) This may suggest that their voluntary activities are driven by low-level motor circuitry such as the brainstem and spinal cord.
What do these cells encode? Early studies in macaque revealed that certain cells are directional-dependent, fewer cells are directional-independent. For example: they fire during flexion, but not extension. What is more interesting is the existence of a preparatory firing stage ~100 msec before the onset of a movement (Evarts et.al., 1968, 1976). The primary motor cortex also encodes the amount of force exerted for making movement. Generally, more force means a higher discharge rate of neurons in M1. These early studies were done in awake behaving monkeys by treating cells in M1 as an individual.
Studies done in planar reaching movements demonstrate how PTNs have directional selectivity, certain groups of neurons are more active to produce movement in a certain direction (Georgopoulos, 1986). The author proposed the concept of a population vector whose direction more or less coincides with the direction of movement. In other words, multiple PTNs work in concert (ensemble of neurons) to produce movement in a particular direction. The direction population vector is also tuned towards the direction of the intended movement ~1000 msec, suggesting that the cells play a role during the preparation period. A follow-up study using a similar approach has shown that directional selectivity is also modulated by force (Kalaska, 1989).
Recent discoveries on macaques and humans help to elucidate more exciting findings. First, prolonged cortical microstimulation on monkeys is able to produce complex purposeful movements such as defensive actions, eating, finger manipulation, etc. (Graziano et al., 2005). This is as if the motor cortex encodes distinct behavior rather than direction or force per se. The primary motor cortex is not really a switch box, with a particular switch controls a particular muscle or muscle location. Instead, the same muscle can be activated by stimulation of different sites with varying stimulation intensity. Thus, the somatotopic organization is best seen in the context of producing purposeful behavior, rather than controlling the individual body part per se. Second, the somatotopic distinction has become blurry. Tracing studies in monkeys demonstrate how the representation of different body parts heavily overlaps or even crosses the central sulcus (Rathelot & Strick, 2005).
Lesion studies in macaques primarily point to the idea that M1 plays an important role in movement execution, controls, and dexterity. Refer to Kandel 5th ed, Chapter 37, [Box 37-1]. Recent neuroimaging studies also support the idea that M1 is involved in motor skill learning.
Roles of the Premotor Cortex
The existence of dorsal-ventral premotor cortex as a distinct entity was initially debatable. Due to the similarity in the movement produced upon stimulation, people such as Penfield and Woolsey thought that both areas are an integral part of the primary motor cortex. In actual fact, there are reciprocal anatomical connections between the two motor cortices. It was also found that stimulation of the premotor cortex requires a higher amount of current due to weaker synaptic connections with the lower motor tracts in the spinal cord.
In general, the role of the premotor cortex is to guide purposeful movements by combining sensory information from other brain areas. Both dorsal and ventral premotor receives heavy projections from the parietal cortex (somatosensory) and visual cortex. Neuronal discharge rate increases in a monkey that learns to associate a new visual cue to a movement. This happens between the appearance of the cue and the movement itself. Thus, the premotor cortex is related to the initiation of movements based on external, arbitrary cues. This has been repeatedly shown in electrophysiological studies in primates.
The ventral premotor cortex (PMv) is involved mainly in precision gripping. Another function of the PMv is in recognizing and understanding an action. Such work invokes patterns of activity in PMv and posterior parietal regions. Experiments with a macaque monkey watching another monkey grasping/manipulating an object appear to activate the same regions as if the monkey itself performs the task. The regions are known as area F5 or PMv in humans, and the inferior parietal lobule (IPL). The neurons involved are also called mirror motor neurons (Rizzolati et al., 1996, 2001). They are also involved in mental rehearsal. This is important for an athlete or a musician to rehearse their planned actions, for example.
Roles of the SMA and Pre-SMA
In the early days, SMA was considered to be the medial secondary motor area. Unlike the premotor cortex, the supplementary motor area is used to guide movement that is internally generated, e.g. making a memorized sequential digits movement or producing hand patterns when manipulating an object. The word "sequence" deserves attention, for if a monkey is given inhibitory drug muscimol to the SMA, the monkey is still able to make a movement but without the appropriate sequence (Shima & Tanji, 1998). SMA has connections to the spinal cord and is also important in planning complex movements and coordination of both hands known as bimanual coordination.
A region just anterior to SMA is called the pre-SMA. The inclusion of pre-SMA as a part of a motor system is without controversy. It is thought that the pre-SMA is important for conscious awareness or perception of the intention to move. Scientists regard pre-SMA as more cognitive than motoric, that is, it belongs to the prefrontal cortex. Anatomical studies in monkeys, mainly by P. Strick et al., found that, unlike SMA, pre-SMA does not send projections to M1 directly and has minimum access to the spinal cord.
Beneath the SMA and primary motor cortex, lies the cingulate motor area (CMA) that has a dense connection to motor areas. This is part of the cingulate cortex situated above the corpus callosum. The exact function is unknown, but cingulate areas are also part of the limbic system.
Connections to Motor Cortex
To produce a movement, the brain has to process the incoming sensory inputs. These inputs primarily come from the somatosensory and the visual system. We also require certain planning and attention in executing the action. The primary motor cortex receives inputs from the premotor areas, SMA, primary somatosensory cortex or S1, and parts of the superior parietal lobule, SPL. In turn, the premotor areas receive inputs from the prefrontal cortex and the parietal lobe, in particular, SPL. This has been confirmed by tracing studies in monkeys.
Of unique function is the parietal lobe of the brain, which contains two major areas apart from postcentral gyrus, i.e. the inferior parietal lobule (IPL, equivalent to BA-39 and 40) and superior parietal lobule (SPL or BA-5 and 7). These two regions are separated by the intraparietal sulcus. This lobe is especially well-known for multisensory integration as it receives information from the somatosensory, visual system (occipital lobe), and the auditory system (temporal lobe).
The parietal lobules and motor cortex are related to reaching and grasping, the two most common motor behavior of upper limbs. These two types of behavior utilize totally two different cortical pathways.
References
[1] Chapter 38 - Voluntary Movement. In Kandel E.R. et. al. (2000). Principles of Neural Science 4e, McGraw-Hill.
[2] Figure 18-6 (p.344). In Kandel E.R. et. al. (2000). Principles of Neural Science 4e, McGraw-Hill.
[3] Chapter 8 - Control of Movement. In Carlson, Neil R. (2013). Physiology of Behavior 11e, Pearson Education.
[4] The Brain: From Top to Bottom.
The existence of dorsal-ventral premotor cortex as a distinct entity was initially debatable. Due to the similarity in the movement produced upon stimulation, people such as Penfield and Woolsey thought that both areas are an integral part of the primary motor cortex. In actual fact, there are reciprocal anatomical connections between the two motor cortices. It was also found that stimulation of the premotor cortex requires a higher amount of current due to weaker synaptic connections with the lower motor tracts in the spinal cord.
In general, the role of the premotor cortex is to guide purposeful movements by combining sensory information from other brain areas. Both dorsal and ventral premotor receives heavy projections from the parietal cortex (somatosensory) and visual cortex. Neuronal discharge rate increases in a monkey that learns to associate a new visual cue to a movement. This happens between the appearance of the cue and the movement itself. Thus, the premotor cortex is related to the initiation of movements based on external, arbitrary cues. This has been repeatedly shown in electrophysiological studies in primates.
The ventral premotor cortex (PMv) is involved mainly in precision gripping. Another function of the PMv is in recognizing and understanding an action. Such work invokes patterns of activity in PMv and posterior parietal regions. Experiments with a macaque monkey watching another monkey grasping/manipulating an object appear to activate the same regions as if the monkey itself performs the task. The regions are known as area F5 or PMv in humans, and the inferior parietal lobule (IPL). The neurons involved are also called mirror motor neurons (Rizzolati et al., 1996, 2001). They are also involved in mental rehearsal. This is important for an athlete or a musician to rehearse their planned actions, for example.
Roles of the SMA and Pre-SMA
In the early days, SMA was considered to be the medial secondary motor area. Unlike the premotor cortex, the supplementary motor area is used to guide movement that is internally generated, e.g. making a memorized sequential digits movement or producing hand patterns when manipulating an object. The word "sequence" deserves attention, for if a monkey is given inhibitory drug muscimol to the SMA, the monkey is still able to make a movement but without the appropriate sequence (Shima & Tanji, 1998). SMA has connections to the spinal cord and is also important in planning complex movements and coordination of both hands known as bimanual coordination.
A region just anterior to SMA is called the pre-SMA. The inclusion of pre-SMA as a part of a motor system is without controversy. It is thought that the pre-SMA is important for conscious awareness or perception of the intention to move. Scientists regard pre-SMA as more cognitive than motoric, that is, it belongs to the prefrontal cortex. Anatomical studies in monkeys, mainly by P. Strick et al., found that, unlike SMA, pre-SMA does not send projections to M1 directly and has minimum access to the spinal cord.
Beneath the SMA and primary motor cortex, lies the cingulate motor area (CMA) that has a dense connection to motor areas. This is part of the cingulate cortex situated above the corpus callosum. The exact function is unknown, but cingulate areas are also part of the limbic system.
Connections to Motor Cortex
To produce a movement, the brain has to process the incoming sensory inputs. These inputs primarily come from the somatosensory and the visual system. We also require certain planning and attention in executing the action. The primary motor cortex receives inputs from the premotor areas, SMA, primary somatosensory cortex or S1, and parts of the superior parietal lobule, SPL. In turn, the premotor areas receive inputs from the prefrontal cortex and the parietal lobe, in particular, SPL. This has been confirmed by tracing studies in monkeys.
Of unique function is the parietal lobe of the brain, which contains two major areas apart from postcentral gyrus, i.e. the inferior parietal lobule (IPL, equivalent to BA-39 and 40) and superior parietal lobule (SPL or BA-5 and 7). These two regions are separated by the intraparietal sulcus. This lobe is especially well-known for multisensory integration as it receives information from the somatosensory, visual system (occipital lobe), and the auditory system (temporal lobe).
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| Fig-3: Input-output connections to motor cortex, taken from [1]. Part B shows pathways for reaching and grasping. |
- Reaching and pointing requires the knowledge of location through the higher-order visual system, then BA-5, to the dorsal premotor area, PMd. Reaching requires coordination of the upper and lower arm.
- Grasping or prehension requires the knowledge of the object, shape, surface, etc. Object manipulation activates BA-7, through the intraparietal sulcus (AIP) to the ventral premotor area, PMv. Fingers' coordination is prominent in grasping.
References
[1] Chapter 38 - Voluntary Movement. In Kandel E.R. et. al. (2000). Principles of Neural Science 4e, McGraw-Hill.
[2] Figure 18-6 (p.344). In Kandel E.R. et. al. (2000). Principles of Neural Science 4e, McGraw-Hill.
[3] Chapter 8 - Control of Movement. In Carlson, Neil R. (2013). Physiology of Behavior 11e, Pearson Education.
[4] The Brain: From Top to Bottom.
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