Chapter 8: Action

Question 1

Alpha motor neuron

Answer 1

Book definition: “The neurons that terminate on muscle fibers, causing contractions that produce movements. Alpha motor neurons originate in the spinal cord and exit through the ventral root of the cord. (p. 330)”

Alpha motor neurons are located within the gray matter of the ventral horn of the spinal cord and project their axons to muscles in the body. Their location in the spine correlates with the location of the muscles they innervate – e.g., neurons in the upper spine projecting to arms, and neurons in lower spine to legs.

As the cell bodies of alpha motor neurons are located inside the spinal cord, and their axons outside, they are considered a part of both the central nervous system (CNS) and the peripheral nervous system (PNS).

An alpha motor neuron in combination with the muscle fibers it innervates is considered a motor unit, while an entire group of cell bodies that contract a single muscle is called a motor neuron pool.

Question 2

Apraxia

Answer 2

Book definition: “A neurological syndrome characterized by loss of skilled or purposeful movement that cannot be attributed to weakness or an inability to innervate the muscles. Apraxia results from lesions of the cerebral cortex, usually in the left hemisphere. (p. 336)”

Question 3

Ataxia

Answer 3

Book definition: “A movement disorder associated with lesions or atrophy of the cerebellum. Ataxic movements are clumsy and erratic, even though muscle strength is normal. (p. 333)”

Question 4

Basal ganglia

Answer 4

Book definition: “A collection of five subcortical nuclei: the caudate, putamen, globus pallidus, subthalamic nucleus, and substantia nigra. The basal ganglia are involved in motor control and learning. Reciprocal neuronal loops project from cortical areas to the basal ganglia and back to the cortex. Two prominent basal ganglia disorders are Parkinson’s disease and Huntington’s disease. (p. 47)”

The basal ganglia are located beneath the anterior portion of the lateral ventricles, near the thalamus. The nuclei in the basal ganglia include:

• The caudate nucleus: involved with working memory, learning, emotional response to visual beauty, and the integration of spatial information with motor movement. The latter includes body and limb posture, speed and accuracy of directed movements, and goal-directed action. Makes up the striatum in combination with the putamen.
• The putamen: a non-specialized structure involved with aiding other structures in several functions, including movement regulation and various types of learning. Makes up the striatum in combination with the caudate nucleus.
• The globus pallidus: involved with the unconscious regulation of voluntary movement, it allows for smooth, controlled movement and response to sensory feedback. Damage to this area can result in tremors or jerks, such as seen in Parkinson’s disease.
• The substantia nigra: Latin for “black substance,” it is divided into the pars compacta and pars reticulata. The prior serves mainly as an input to the basal ganglia circuit, supplying the striatum with dopamine, while the ladder serves as an output, conveying signals from the basal ganglia to other brain structures.

The several neurological conditions that are associated with basal ganglia dysfunction help outline the role of these structures. These include Parkinson’s disease (which involves a degeneration of dopamine-producing cells in the pars compacta of the substantia nigra) and Huntington’s disease (which primarily involves damage to the striatum), as well as Alzheimer’s, Tourette’s, OCD, and general addiction.

Question 5

Bradykinesia

Answer 5

Book definition: “Slowness in the initiation and execution of movements. Bradykinesia is a prominent symptom in Parkinson’s disease. (p. 358)”

Question 6

Brain-machine interface (BMI)

Answer 6

Book definition: “A device that uses the interpretation of neuronal signals to perform desired operations with a mechanical device outside the body. For instance, signals recorded from neurons or EEG can be used to move a prosthetic arm. (p. 352)”

Question 7

Central pattern generator

Answer 7

Book definition: “A neural network limited to the spinal cord that produces patterned motor outputs without descending commands from the cerebral cortex or sensory feedback. (p. 338)”

Question 8

Cerebellum

Answer 8

Book definition: “Also known as the ‘little cerebrum.’ A large, highly convoluted (infolded) structure located dorsal to the brainstem at the level of the pons. The cerebellum maintains (directly or indirectly) interconnectivity with widespread cortical, subcortical, brainstem, and spinal cord structures, and plays a role in various aspects of coordination ranging from locomotion to skilled, volitional movement. (p. 332)”

The cerebellum clings to the brainstem at the level of the pons and contains most of the brain’s neurons in a tightly folded layer of tissue. It resembles the forebrain, with the cerebellar cortex consisting of gray matter, and a center of white matter.

The cerebellum is critical for maintaining posture, walking, and performing coordinated movements. Fibers arriving from the brain carry information about motor outputs and sensory inputs about body position. Outputs either ascend to the thalamus and then the motor and premotor cortex, or descend to the spinal cord.

Apart from analyzing, coordinating and maintaining movement, the cerebellum has recently also been linked to aspects of language, attention, learning, mental imagery, and more.

Question 9

Corticospinal tract (CST)

Answer 9

Book definition: “Also pyramidal tract. A bundle of axons that originate in the cortex and terminate monosynaptically on alpha motor neurons and spinal interneurons in the spinal cord. Many of these fibers originate in the primary motor cortex, although some come from the secondary motor areas. The corticospinal tract is important for the control of voluntary movements. (p. 334)”

Question 10

Deep brain stimulation (DBS)

Answer 10

Book definition: “The electrical stimulation of brain structures via an implanted electrode. Stimulation of the subthalamic nucleus, one of the nuclei of the basal ganglia, is used as a treatment for Parkinson’s disease. (p. 362)”

Question 11

Effector

Answer 11

Book definition: “Any part of the body that can move, such as an arm, finger, or leg. (p. 330)”

Question 12

Endpoint control

Answer 12

Book definition: “A hypothesis concerning how movements are planned in terms of the desired final location. Endpoint control models emphasize that the motor representation is based on the final position required of the limbs to achieve the movement goal. (p. 340)”

Question 13

Extrapyramidal tracts

Answer 13

Book definition: “A collection of motor tracts that originate in various subcortical structures, including the vestibular nucleus and the red nucleus. These tracts are especially important for maintaining posture and balance. (p. 332)”

Question 14

Forward model

Answer 14

Book definition: “A theoretical construct referring to the idea that the brain generates predictions of expected events. In motor control, a forward model refers to the prediction of the expected sensory consequences of a movement. (p. 371)”

Question 15

Hemiplegia

Answer 15

Book definition: “A neurological condition characterized by the loss of voluntary movements on one side of the body. Hemiplegia typically results from damage to the corticospinal tract, either from lesions to the motor cortex or from white matter lesions that destroy the descending fibers. (p. 335)”

Question 16

Huntington’s disease

Answer 16

Book definition: “A genetic degenerative disorder in which the primary pathology, at least in the early stages of the diseases, is observed in the striatum (caudate and putamen) of the basal ganglia. Prominent symptoms include clumsiness and involuntary movements of the head and trunk. Cognitive impairments are also seen and become pronounced over time. Compare Parkinson’s disease. (p. 358)”

Question 17

Hypokinesia

Answer 17

Book definition: “A movement disorder characterized by the absence or reduction in the production of movement. Hypokinesia is a prominent symptom of Parkinson’s disease. (p. 358)”

Question 18

Ideational apraxia

Answer 18

Book definition: “A severe form of apraxia in which the patient’s knowledge about the intent of an action is impaired. For example, the patient may no longer comprehend the appropriate use for a tool, even though still capable of producing the required movement. Compare ideomotor apraxia. (p. 336)”

Question 19

Ideomotor apraxia

Answer 19

Book definition: “A form of apraxia in which the patient has difficulty executing the desired action properly. Patients with ideomotor apraxia appear to have a general idea about how the action should be performed and how tools are used, but they are unable to coordinate the movements to produce the action in a coherent manner. Compare ideational apraxia. (p. 336)”

Question 20

Mirror neuron

Answer 20

Book definition: “Neurons that show similar responses when an animal is performing an action or observing that action produced by another organism. Mirror neurons are hypothesized to provide a strong link between perception and action, perhaps providing an important basis for the development of conceptual knowledge. (p. 363)”

Question 21

Mirror system

Answer 21

Book definition: “A distributed network of neurons that respond not only to one’s own action but also to perceived actions. For instance, a mirror neuron responses when you pick up a pencil and when you watch someone else pick up a pencil. (p. 363)”

Question 22

Parkinson’s disease

Answer 22

Book definition: “A degenerative disorder of the basal ganglia in which the pathology result from the loss of dopaminergic cells in the substantia nigra. Primary symptoms include difficulty in initiating movement, slowness of movement, poorly articulated speech, and, in some cases, resting tremor. Compare Huntington’s disease. (p. 358)”

Question 23

Population vector

Answer 23

Book definition: “A statistical procedure to represent the activity across a group of neurons. Population vectors reflect the aggregate activity across the cells, providing a better correlation with behavior than that obtained from the analysis of individual neurons. For example, the population vector calculated from neurons in the motor cortex can predict the direction of a limb movement. (p. 342)”

Many cells in motor areas exhibit what is referred to as a preferred direction – i.e., that certain directions of movement result in a higher firing rate of a neuron. As this “certain direction” can be very broad, it is difficult to predict the direction of a movement (or “vector”) based on the activity of a single cell.

Instead, a population vector can be measured from a group of neurons, averaging the sum of all the individual movement vectors into a single group vector. In other words, if ten neurons have the preferred direction of a 50 degree angle and another ten prefer to move at a 70 degree angle, the population vector will 60 degrees.

Studies show that activity in population vectors can be measured well before any movement is actually produced, indicating that at least some of the cells are also involved in planning movement. In addition, the pre-movement directional tuning of cells has also been shown to shift during actual movement.

Question 24

Preferred direction

Answer 24

Book definition: “A property of cells in the motor pathway, referring to the direction of movement that results in the highest firing rate of the neuron. Voxels have also been shown to have preferred directions in the fMRI studies, indicating that such preferences can even be measured at the cell population level of analysis. (p. 342)”

Many cells in motor areas exhibit what is referred to as a preferred direction – i.e., that certain directions of movement result in a higher firing rate of a neuron. As this “certain direction” can be very broad, it is difficult to predict the direction of a movement (or “vector”) based on the activity of a single cell.

Instead, a population vector can be measured from a group of neurons, averaging the sum of all the individual movement vectors into a single group vector. In other words, if ten neurons have the preferred direction of a 50 degree angle and another ten prefer to move at a 70 degree angle, the population vector will 60 degrees.

Studies show that activity in population vectors can be measured well before any movement is actually produced, indicating that at least some of the cells are also involved in planning movement. In addition, the pre-movement directional tuning of cells has also been shown to shift during actual movement.

Question 25

Premotor cortex

Answer 25

Book definition: “A secondary motor area that includes the lateral aspect of Brodmann area 6, just anterior to the primary motor cortex. Although some neurons in the premotor cortex project to the corticospinal tract, many terminate on neurons in the primary motor cortex and help shape the forthcoming movement. (p. 336)”

Question 26

Primary motor cortex (M1)

Answer 26

Book definition: “A region of the cerebral cortex that lies along the anterior bank of the central sulcus and precentral gyrus, forming Brodmann area 4. Some axons originating in the primary motor cortex form the majority of the corticospinal tract; others project to cortical and subcortical regions involved in motor control. The primary motor cortex contains a prominent somatotopic representation of the body. (p. 335)”

Question 27

Sensorimotor adaptation

Answer 27

Book definition: “A form of motor learning in which a learned skill is modified due to some change in the environment or agent. For example, a soccer player who adjusts her shot to compensate for a strong cross-wind is exhibiting a form of motor adaptation. (p. 367)”

Question 28

Sensorimotor learning

Answer 28

Book definition: “A term that refers to the acquisition of a new motor skill or capability. Motor learning can arise from maturation processes (e.g., infants crawling) or intense, dedicated practice (e.g., piano playing). (p. 368)”

Question 29

Spinal interneurons

Answer 29

Book definition: “A neuron found in the spinal cord. Many descending axons from the pyramidal and extrapyramidal tracts synapse on interneurons which, in turn, synapse on other interneurons or alpha motor neurons. (p. 331)”

Question 30

Substantia nigra

Answer 30

Book definition: “One of the nuclei that form the basal ganglia. The substantia nigra is composed of two parts: The axons of the substantia nigra pars compacta provide the primary source of the neurotransmitter dopamine and terminate in the striatum (caudate and putamen). The substantia nigra pars reticularis is one of the output nuclei from the basal ganglia. (p. 328)”

Latin for “black substance,” the substantia nigra is divided into the pars compacta and pars reticulata. The prior serves mainly as an input to the basal ganglia circuit, supplying the striatum (i.e., the caudate and putamen) with dopamine, while the ladder serves as an output, conveying signals from the basal ganglia to other brain structures.

Parkinson’s disease is the result of a degeneration of the dopamine-producing cells in the pars compacta.

Question 31

Supplementary motor area (SMA)

Answer 31

Book definition: “A secondary motor area that includes the medial aspect of Brodmann area 6, just anterior to the primary motor cortex. The SMA plays an important role in the production of sequential movements, especially those that have been well learned. (p. 336)”

Question 32

Visuomotor adaptation

Answer 32

Book definition: “A form of sensorimotor adaptation in which the visual feedback is altered, resulting in a mismatch between proprioception and vision. With practice, the motor system adjusts to compensate for the mismatch. (p. 369)”