What is the importance of action and the motor system? - Chapter 8

What holds the anatomy and control of motor structures?

To understand motor control, we have to consider the organization and function of much of the dorsal territory of the cerebral cortex, as well as much of the subcortex. The lowest level of the hierarchy centers on the spinal cord. Axons from the spinal cord provide the point of contact between the nervous system and muscles, with incoming sensory signals from the body going to ascending neurons in the spinal cord, and outgoing motor signals to the muscles coming from descending spinal motor neurons. At the top of the hierarchy are cortical regions that help translate abstract intentions and goals into movement patterns. Between the association areas and the spinal cord sit the primary motor cortex and brain structures, which with the basal ganglia and cerebellum, convert these patterns into commands to the muscles. 

What are the roles of muscles, motor neurons and the spinal cord?

Action, or motor movement, is generated by stimulating skeletal muscle fiber of an effector. An effector is a part of the body that can move. All forms of movement result from changes in the state of muscles that control an effector or group of effectors. Muscles consist of elastic fibers, tissue that can change in length and tension and are activated by motor neurons, which are the final neural elements of the motor system. Alpha motor neurons innervate muscle fibers and produce contractions of the fibers. Input to the alpha motor neurons comes from a variety of sources. Alpha motor neurons receive peripheral input from muscle spindles, sensory receptors embedded in the muscles that provide information about how much the muscle is stretched. 

The axons of the spindles form an afferent nerve that enters the dorsal root of the spinal cord and synapses on spinal interneurons that project to alpha motor neurons. Reflexes allow postural stability to be maintained without any help from the cortex. 

What are the subcortical motor structures?

There are 12 cranial nerves, essential for critical reflexes associated with breathing, eating, eye movements, and facial expressions that originate in the brainstem. The substantia nigra sends out direct projections down the spinal cord. This motor pathway is referred to as the extrapyramidal tract, meaning that this is not a part of the pyramidal part, the axons that travel directly from the cortex to the spinal segments. The cerebellum is a massive structure containing over 75% of all the neurons in the brain. Damage to the cerebellum from stroke, tumor or degenerative processes results in a syndrome called ataxia. Patients with ataxia have a lot of difficulty maintaining balance and producing well-coordinated movements. 

The basal ganglia is the other major subcortical motor structure next to the cerebellum and substantia nigra. It consists out of a collection of five nuclei: caudata nucleus, putamen, globus pallidus, subthalamic nucleus, and the substantia nigra. I will use the term motor areas to refer to cortical regions involved in voluntary motor functions, including planning, control and execution of movement. 

Which cortical regions are involved in motor control?

The motor cortex regulates the activity of spinal neurons in direct and indirect ways. The corticospinal tract (CST) consists of axons that exit the cortex and project directly to the spinal cord. The CST is also referred to as the pyramidal tract, because the mass of axons resembles a pyramid as it passes through the medulla oblongata. 

The primary motor cortex (M1) is located in the most posterior portion of the frontal lobe, spanning the anterior wall of the central sulcus and extending onto the precentral gyrus. M1 receives input from almost all cortical areas implicated in motor control. M1 includes two anatomical subdivisions: an evolutionairy older rostral region and a more recently evolved caudal region. Corticospinal neurons that originate in the caudal region may terminate on interneurons or directly stimulate alpha motor neurons. The latter known as corticomotorneurons or CM neurons, include prominent projections to muscles of the upper limb, and they support the dexterous control of our fingers and hands. 

The preeminent status of the primary motor cortex for movement control is underscored by the fact that lesions to this area, or to the corticospinal tract, produce a devastating loss of motor control. Lesions of the primary motor cortex usually result in hemiplegia, the loss of voluntary movements on the contralateral side of the body. Reflexes are absent immediately after a stroke that produces hemiplegia. But within a couple of weeks the reflexes return and are frequently hyperactive or even spastic. 

The lateral and medial aspects of Brodmann area 6 are referred to as premotor cortex and supplementary motor area (SMA). The secondary motor areas are involved in planning and control of movements, but they do not accomplish this feat alone. The premotor cortex has a strong reciprocal connection with the parietal lobe. 

The dorso-dorsal stream passes through the superior parietal lobe and projects to the dorsal premotor cortex. This pathway plays a dominant role in one of the most important motor activities: reaching. Patients with lesions in the dorso-dorsal stream have optic ataxia: they are unable to reach accurately for objects, especially those in their peripheral vision. 

The ventro-dorsal stream passes through the inferior parietal lobe and projects to the ventral premotor cotex. This pathway is associated with producing both transitive gestures and intransitive gestures. Lesions along this processing stream can result in apraxia - a condition that affects motor planning, as well as the knowledge of which actions are possible with a given object. 

What are computational issues in motor control?

The spinal cord is capable of producing orderly movement, and the stretch reflex provides an elegant mechanism to maintain postural stability even in the absence of higher-level processing. Sherrington observed by looking at a cat whose spinal cord was disconnected from the spinal apparatus, the cat was still able, without the appropriate stimulus, to make movements like he/she was walking. Thus, neurons in the spinal cord could produce an entire sequence of actions without any descending commands or external feedback signals. These neurons have come to be called central pattern generators. They most likely evolved to trigger actions essential for survival, such as locomotion. 

But if the cortical neurons are not coding specific patterns of motor commands, what are they doing? Researchers did an experiment with monkeys that had been deprived of all somatosensory, or afferent, signals from the limbs. The monkeys were trained to simple point at a light. If, the animal generated a motor command specifying the desired position, it should have achieved this goal once the opposing force was removed. The results show that when the torque motor was on, the limb stayed at the starting location. As soon as it was turned off, the limb rapidly moved to the correct location. This experiment provides evidence that the location isn't the only thing being coded by humans. Although endpoint control reveals a fundamental capability of the motor control system, distance and trajectory planning demonstrates additional flexibility in the control processes. 

What are the hierarchical representations of action sequences?

Hierarchical representational structures organize movement elements into integrated chunks. Researchers originally developed the idea of 'chunking' when studying memory capacity, but it has also proved relevant to the representation of action. So, the motor system is also hierarchically organized. Subcortical and cortical areas represent movement goals at various levels of abstraction. 

What is the physiological analysis of motor pathways?

In this chapter we have stressed two critical points of movement: (1) motor control depends on several distributed anatomical structures, (2) these distributed structures operate in hierarchical fashion. 

Neuropsychologists have long puzzled over how best to describe cellular activity in the motor structures of the CNS. Stimulation of the primary motor cortex, either during neurosurgery or via TMS, can produce discrete movements about single joints, providing a picture of the somatotopic organization of the motor cortex. Research also shows that activity of the cells in the primary motor cortex correlates much better with movement direction than with target location. Many cells in the motor cortex show directional tuning, or exhibit what is referred to as a preferred direction. Directional tuning is not just observed in the primary motor cortex; similar tuning properties are found in cells in premotor and parietal cortical areas, as well as in the cerebellum and basal ganglia. We can assume that activity is distributed across many cells, each with its unique preferred direction. Researchers introduced the concept of population vector: each neuron can be considered to be contributing a 'vote' to the overall activity level. The strength of the vote will correspond to how closely the movement matches the cell's preferred direction: if the match is close, the cell will fire strongly. Thus, the activity of each neuron can be described as a vector, oriented to the cell's preferred direction with a strength equal to its firing rate. 

What are alternative perspectives on the neural representation of movement?

The population vector is dynamic and can be calculated continuously over time. After defining the preferred direction of a set of neurons, we can calculate the population vector from the activation of that set of neurons even before the animal starts to move. This shows that the population vector shifts in the direction of the upcoming movement well before the movement is produced, suggesting that at least some of the cells are involved in planning the movement and not simply recruited once execution of the movement has begun. Even though directional tuning and population vector have become cornerstone concepts in motor neurophysiology, it is also important tot consider that many cells do not show strong directional tuning. Even more puzzling, the tuning may be inconsistent: the tuning exhibited by a cell before movement begins may shift during the actual movement. Researchers are saying that we should take up on a radically different perspective on motor neurophysiology. Rather than viewing neurons as static representational devices, we should focus on the dynamic properties of neurons, recognizing that movement arises as the neurons move from one state to another. 

Although scientist refer to one part of the brain as the motor cortex and one part of the brain as the sensory cortex, we know that these areas are closely intertwined. People produce movements in anticipation of their sensory consequences: we increase the force used to grip and lift in anticipation of the weight we expect to experience. 

How does goal selection and action planning work?

Motor representation are hierarchically and need to encompass the goals of an action in addition to the activation patterns required to produce the movement necessary to achieve those goals. Including sensory information and feedback enables the motor cortex to have more than one option for achieving those goals. 

One hypothesis about how we set our goals and plan action is from Cisek. He says that in incorporates many of the ideas and findings that we are going to look at, providing a general framework for action selection. His affordance competition hypothesis is deeply rooted in an evolutionary perspective. Our ancestors evolved in a world where they engaged in interactions with a changing, and sometimes hostile environment that held a variety of opportunities and demands for action. To survive and reproduce, early humans had to be ever ready, anticipating the next predator etc. Many interaction don't allow time for carefully evaluating plans and goals, considering options: this is called serial processing. 

A better idea is to develop multiple plans in parallel. The affordance competition hypothesis proposes that the processes of action selection and specification occur simultaneously within an interactive neural network, and they evolve continuously. Even when performing one action, we are already preparing the next one. Then there is the competition part. At some point, one option wins out over the other competitors. An action is selected an executed. This selection process involves many parts of the motor pathway, where interactions within frontoparietal circuits have a prominent role. 

There are cells in the premotor cortex that have been shown to represent action goals more abstractly. Some cells are preferentially activated when the animal reaches for an object, other cells become active when the animal makes a gesture to hold an object. 

What are the representational variations across motor areas of the cortex?

The lateral premotor cortex is more heavily connected with the parietal cortex - a finding consistent with the hypothesis that this region plays a role in sensory-guided action. The SMA has strong connections with the medial frontal cortex, and is likely to bias or influence action selection and planning that are based on internal goals and personal experiences. The SMA has also been hypothesized to play an important role in more complex actions, such as those involving sequential movements or those requiring coordinated movements of the two limbs. Damage in the SMA can lead to impaired performance on tasks that require integrated use of the two hands, even though the individual gestures performed by either hand alone are unaffected. Lesions in SMA can also result in alien hand syndrome, a condition in which one limb produces a seemingly meaningful action but the person denies responsibility for the action. 

The lateral premotor cortex is part of a network for stimulus-guided movement, whereas the more medial supplementary motor area is important for movements based on internal goals and personal experience, including skilled movements that require coordination between the two hands. Parietal motor areas also show topography; different regions of the intraparietal cortex are associated with hand, arm, and eye movements. Therefor the parietal motor representations are more goal oriented, whereas premotor-motor representations are more closely linked to the movement itself. The conscious awareness of the movement appears to be related to the neural processing of action intention rather than the movement itself. 

What links are there between action and perception?

The most common known link between perception and action is neurons that are called mirror neurons. These are neurons that are active during action but also active during action perception. You might suppose that the activity in MN's reflects the similar visual properties of the action and perception conditions. Additional experiments ruled out this hypothesis: (1) the same MN that is activated when the monkey cracks a peanut itself is activated when the monkey merely hears a peanut being cracked, (2) MNs are also active when a monkey watches someone reach behind a screen for a peanut but cannot see grasping of the peanut. In fact, there doesn't even need to be a peanut behind the screen, as long as the monkey thinks there's a hidden peanut. The work on a mirror neuron network has revealed the intimate links between perception and action, suggesting that our ability to understand these actions of others depends on the neural structures that would be engaged if we were to produce the action ourselves. 

Coaches and sport psychologists recognize the intimate relationship between action observation an action production. The skier can mentally visualize his movements of the slope. This process is thought to strengthen perception-action links. 

How do you recoupe motor loss?

Lesions to the primary motor cortex or the spinal motor neurons can result in hemiplegia, or in hemiparesis, a unilateral weakness. Such lesions severely impact the patient's ability to use the affected limbs on the contralesional side: unfortunately, these patients rarely regain significant control over the limbs. 

How do you regain movement after loss of motor cortex?

Two critical questions for the clinician and patient are, what is the patient's potential for recovery? what is the best strategy for rehabilitation? A panel looked at biomarkers for predicting motor recovery from stroke. Some biomarkers look promising when the assessment is performed after the patient has recovered from the critical initial post-stroke period, outperforming the location of the lesion and even the behavioral asymmetry between the affected and unaffected limbs. 

The main treatment, traditionally, to regain motor function is physical therapy, a behavioral intervention that seeks to retrain the affected limbs. But physical therapy only produces modest recovery. Also the idea that more therapy is better, has been contradicted with research. A different behavioral method is based on the idea that the brain may favor short-term solutions over long-term gains. Scientists are currently seeking novel interventions that more directly target specific neural mechanisms. 

What is the brain-machine interface?

Can neural signals be used to control a movement directly with the brain, bypassing the intermediate stage of muscles? Could you plan an action in your motor cortex, somehow connect those motor cortex neurons to a computer, and send the planned action to a robot, which would fold the laundry? These systems are called brain-machine interfaces using decoding algorithms to control prosthetic devices with neural signals. 

Brain-machine interfaces are also called BMI's. BMI's offer a promising avenue for rehabilitation of people with mostly severe movement disorders, such as those resulting from, for instance, a spinal cord injury. In the early BMI systems the decoders were built from recordings of neural activity made while the animal produces movements. The output of these decoders was then used to drive the prosthetic device. More recent work has revealed that the brain's plasticity enables it to spontaneously learn how to adapt neural activity to control an arbitrary decoder, eliminating the need for a training phase to build the decoder. This insight is essential if BMI systems will be useful for individuals who have lost the ability to move their limbs by themselves. 

How does movement initiation of basal ganglia work?

When multiple plans are present in the cortex, how do we decide which plan to execute? The basal ganglia plays a critical role in movement initiation. The afferent fibers to the basal ganglia terminate in the striatum, composed in primates of two nuclei: the caudate and the putamen. Processing within the basal ganglia takes place along two pathways that originate with GABAergic projection neurons from the striatum. The direct pathway involves fast, direct, inhibitory connections from the striatum to the basal ganglia. The indirect pathway takes on a slower, roundabout route to the basal ganglia. Stiatal axons inhibit the external segment of the globus pallidus, which in turn inhibits the subthalamic nucleus and GP. The final internal pathway of note is the projection from the pars compacta of the substantia nigra to the striatum, known as the dopamine pathway. The substantia nigra excites the direct pathway by acting on one type of dopamine receptor (D1), and inhibits the indirect pathway by acting on a different type of dopamine receptor (D2). 

When the direct pathway is activated, it sends inhibitory signals to the target neurons in the output nuclei of the basal ganglia, which results in the inhibition of inhibiting signals to the thalamus. This sum effect is disinhibition of the thalamus, resulting in increased excitation of the cortex. So, activation of the direct pathway will promote movement if the disinhibition is along a circuit that terminates in the primary motor cortex. 

Activation along the indirect pathway will result in increased inhibition from the basal ganglia and as such, reduces excitation of the cortex. This puzzling arrangement seems to be an important mechanism for helping the motor system both to maintain stability and to rapidly change when the situation changes. So, the basal ganglia can be seen to play a critical role in the initiation of actions. 

What are disorders of the basal ganglia?

Huntington's disease is a hereditary neurodegenerative disorder. Patients with this gene develop symptoms in the fourth or fifth decade of life, experiencing rapid progression and die within 12 years of onset. Within a year from onset movement abnormalities are noticed: clumsiness, balance problems, and a general restlessness. The excessive movements, or hyperkinesia, can be understand by considering how the pathology affects information flow through the basal ganglia. The striatal changes occur primarily in inhibitory neurons forming the indirect pathway.  

Parkinson's disease is the most common and well-known disorder affecting the basal ganglia, results from a loss of dopamine-producing neurons in the substantia nigra (SN). As with most brain tissue, these neurons atrophy with age. Motor symptoms of Parkinson are related to locomotion called hypokinesia and bradykinesia. Hypokinesia is a reduced ability to initiate voluntary movements, bradykinesia refers to a slowing in the rate of movement. At the extreme end of these symptoms lies akinesia, the total absence of voluntary movement. One of the biggest breakthroughs in neurology occurred with the development of L-dopa, a synthetic precursor of dopamine. But, over time, the dopamine-producing cells continue to die off and striatal neurons become sensitized to L-dopa, so the amount of required medication tends to increase. More recently, the success of invasive techniques such as pallidotomy and deep brain stimulation, where an electrode is placed in the skull to initiate the signal for movement by an Parkinson's disease patient, has inspired neurosurgeons to consider similar interventions for other disorders. 

How do you learn and perform a new skill?

People frequently attribute motor learning to the lower levels of the hierarchical representation of action sequences. We speak of 'muscle memory' as if our muscles have learned how to respond in a way that seems automatic. The fact that we have great difficulty verbalizing how we perform these skills reinforces the notion that learning is noncognitives. 

When people are acquiring new action, the first effects of learning likely will be at a more abstract level. Learning a skill takes practice, and becoming very skilled at anything requires a lot of practice. Our motor system has some basic movement patterns that can be controlled by subcortical circuits. Learning a new skill can involve building on these basic patterns, linking together a series of gestures in a novel way. 

When you come off a boat, you feel that you first few steps are wobbly, it takes a moment or two to become acclimated to the stability of the dock, and to abandon you rolling gait. You sea legs are a form of sensorimotor adaptation. We cannot simply switch back to the normal state, but rather must relearn how to control our limbs in the absence od a visual or force distortion. Sensimotor learning is improvement, through practice, in the performance of motor behavior. The acquisition of a motor skill involves the formation of new movement patterns that can result in changes in both structure and connectivity.