actin: one of the proteins (with myosin) that provide the physical basis for muscular contraction.

agonist: a muscle whose contraction produces or facilitates a particular movement.

alpha motor neuron: a neuron whose axon forms synapses with extrafusal muscle fibers of a skeletal muscle; activation contracts the muscle fibers.

alpha motor unit: an alpha motor neuron and the extrafusal fibers it innervates.

antagonist: a muscle whose contraction resists or reverses a particular movement.

apraxia: difficulty in carrying out purposeful movements, in the absence of paralysis or muscular weakness.

basal ganglia: caudate nucleus, globus pallidus, putamen, and amygdala in the forebrain. The first three structures are important parts of the motor system and assemble word-forms into sentences. The caudate nucleus also controls impulses.

callosal apraxia: an apraxia of the left hand caused by damage to the anterior corpus callosum.

cardiac muscle: the muscle responsible for the contraction of the heart.

caudate nucleus: a telencephalic nucleus; one of the input nuclei of basal ganglia along with the putamen; involved with control of voluntary movement.

clasp-knife reflex: a reflex that occurs when force is applied to flex or extend the limb of an animal showing decerebrate rigidity; resistance is replaced by sudden relaxation.

constructional apraxia: difficulty in drawing pictures or diagrams or in making geometrical constructions of elements such as building blocks or sticks; caused by damage to the right parietal lobe.

corticobulbar pathway: a bundle of axons from the motor cortex to the fifth, seventh, ninth, tenth, eleventh, and twelfth cranial nerves; controls movements of the face, neck, tongue, and parts of the extraocular eye muscles.

corticorubral tract: the system of axons that travels from the motor cortex to the red nucleus.

corticospinal tract: the system of axons that originates in the motor cortex and terminates in the ventral gray matter of the spinal cord.

decerebrate: describes an animal whose brain stem has been transected.

decerebrate rigidity: simultaneous contraction of agonistic and antagonistic muscles; caused by decerebration or damage to the reticular formation.

dentate nucleus: a deep cerebellar nucleus; involved in the control of rapid, skilled movements by the corticospinal and rubrospinal systems.

direct pathway (in basal ganglia): the pathway that includes the caudate nucleus and putamen, the external division of the globus pallidus, the subthalamic nucleus, the internal division of the globus pallidus, and the ventral anterior/ventrolateral thalamic nuclei; has an inhibitory effect on movement.

endplate potential: the postsynaptic potential that occurs in the motor endplate in response to release of acetylcholine by the terminal button.

extension: a movement of a limb that tends to straighten its joints; the opposite of flexion.

extrafusal muscle fiber: one of the muscle fibers that are responsible for the force exerted by contraction of a skeletal muscle.

fastigial nucleus: a deep cerebellar nucleus; involved in the control of movement by the reticulospinal and vestibulospinal tracts.

flexion: a movement of a limb that tends to bend its joints; opposite of extension.

flocculonodular lobe: a region of the cerebellum; involved in control of postural reflexes.

gamma motor neuron: a neuron whose axons form synapses with intrafusal muscle fibers.

gamma motor system: gamma motor neuron and the intrafusal fiber it innervates.

globus pallidus: a telencephalic nucleus; the primary output nucleus of the basal ganglia; involved with control of voluntary movement.

Golgi tendon organ (GTO): the receptor organ at the junction of the tendon and muscle that is sensitive to stretch.

Huntington's disease: a fatal inherited disorder that causes degeneration of the caudate nucleus and putamen; characterized by uncontrollable jerking movements, writhing movements, and dementia. Also called Huntington's chorea.

indirect pathway (in basal ganglia): the pathway that includes the caudate nucleus and putamen, the internal division of the globus pallidus, and the ventral anterior/ventrolateral thalamic nuclei; has an excitatory effect on movement.

interposed nuclei: a set of deep cerebellar nuclei; involved in the control of the rubrospinal system.

intrafusal muscle fiber: a muscle fiber that functions as a stretch receptor, arranged parallel to the extrafusal muscle fibers, thus detecting changes in muscle length.

lateral corticospinal tract: the system of axons that originates in the motor cortex and terminates in the contralateral ventral gray matter of the spinal cord; controls movements of the distal limbs.

lateral group: the corticospinal tract, the corticobulbar tract, and the rubrospinal tract.

left parietal apraxia: an apraxia caused by damage to the left parietal lobe; characterized by difficulty in producing sequences of movements by verbal request or in imitation of movements made by someone else.

mesencephalic locomotor region: a region of the reticular formation of the midbrain whose stimulation causes alternating movements of the limbs normally seen during locomotion.

monosynaptic stretch reflex: quick stretching of a skeletal muscle fiber causes it to contract. Stretching lengthens the intrafusal fiber of a skeletal muscle fiber. The intrafusal fiber activates the alpha motor neuron which synapses on the same muscle, resulting in contraction. This reflex maintains upright posture and allows us to hold a heavy object. Neurologists test this reflex when they tap the patellar tendon to elicit a "knee jerk" response.

motor endplate: the postsynaptic membrane of a neuromuscular junction.

muscle spindle: specialized length sensor located within a skeletal muscle fiber that communicates with spinal neurons via group Ia afferents. Increases in skeletal muscle length cause a muscle spindle to fire.

motor unit: a motor neuron and its associated muscle fibers.

myofibril: an element of muscle fibers that consists of overlapping strands of actin and myosin; responsible for muscular contractions.

myosin: one of the proteins (with actin) that provide the physical basis for muscular contraction.

neuromuscular junction: synapse between the terminal buttons of an alpha motor neuron and a muscle fiber.

pontine nucleus: a large nucleus in the pons that serves as an important source of input to the cerebellum.

prefrontal cortex: the neocortex of the frontal lobes rostral to the supplementary motor area and premotor cortex that develops goals and is involved in attention and planning.

premotor cortex: a region of motor association cortex of the lateral frontal lobe, rostral to the primary motor cortex. The lateral part of area 6 that adjusts existing motor programs using sensory input. Also called premotor area.

primary motor cortex (M1): the precentral gyrus (area 4) executes programmed motor plans by activating central pattern generators (CPGs) in the spinal cord.

putamen: a telencephalic nucleus; one of the input nuclei of the basal ganglia along with the caudate nucleus; involved with control of voluntary movement.

pyramidal tract: an alternate term for the corticospinal tract.

reticulospinal tract: a bundle of axons that travels from the reticular formation to the gray matter of the spinal cord; controls the muscles responsible for postural movements.

rubrospinal tract: the system of axons that travels from the red nucleus to the spinal cord; controls independent limb movements.

skeletal muscle: one of the striated muscles attached to bones.

smooth muscle: nonstriated muscle innervated by the autonomic nervous system, found in the walls of blood vessels, in the reproductive tracts, in sphincters, within the eye, in the digestive system, and around hair follicles.

somatotopic organization: a topographically organized mapping of parts of the body that are represented in a particular region of the brain.

striated muscle: skeletal muscle; muscle that contains striations.

supplementary motor area (SMA): medial part of area 6 that develops and executes programs that control sequences of individual movements.

sympathetic apraxia: a movement disorder of the left hand caused by damage to the left frontal lobe; similar to callosal apraxia.

tectospinal tract: a bundle of axons that travels from the tectum to the spinal cord; coordinates head and trunk movements with eye movements.

ventral anterior nucleus: one of the two thalamic nuclei that receive projections from the basal ganglia and send projections to the motor cortex.

ventral corticospinal tract: the system of axons that originates in the motor cortex and terminates in the ipsilateral ventral gray matter of the spinal cord; controls movements of the upper legs and trunk.

ventrolateral nucleus: one of the two thalamic nuclei that receive projections from the basal ganglia and send projections to the motor cortex.

ventromedial group: the vestibulospinal tract, the tectospinal tract, the reticulospinal tract, and the ventral corticospinal tract.

vermis: the portion of the cerebellum located at the midline; receives somatosensory information and helps control the vestibulospinal and reticulospinal tracts through its connections with the fastigial nucleus.

vestibulospinal tract: a bundle of axons that travels from the vestibular nuclei to the gray matter of the spinal cord; controls postural movements in response to information from the vestibular system.

akinesia (a ki NEE zee ah)
dimer (DY mur)
Golgi tendon organ (GOHL ji)

Group 1: (a) Differentiate extrafusal from intrafusal muscle fibers. (b) Explain the function of an alpha motor neuron. (c) How is an alpha motor neuron related to a motor unit? (d) What do gamma motor neurons do?

Group 2: (a) What causes skeletal muscles to contract? (b) Why does a muscle twitch last longer than the action potential in the alpha motor neuron that triggered it? (c) Why isn't muscular contraction all or none?

Group 3: (a) Explain the distinction between the functions of the stretch receptors of the intrafusal muscle fibers and those of the Golgi tendon organ. (b) Describe the patellar reflex. (c) Describe the polysynaptic inhibitory reflex shown in Figure 8.7.

Group 4: (a) When the brain initiates limb movement, which two sets of motor neurons in the spinal cord are activated? (b) Why are they activated?

Group 5: Describe the motor functions of the: (a) prefrontal cortex. (b) supplementary motor area. (c) premotor cortex. (d) primary motor cortex.

Group 1: (a) Define limb apraxia in your own words. (b) Explain the difference between apraxia and paralysis. (c) Explain constructional apraxia.

Group 2: Summarize the lesion site and physical disabilities for each of the following types of limb apraxia: (a) callosal apraxia. (b) sympathetic apraxia. (c) left parietal apraxia.

Group 3: (a) Briefly describe the primary symptoms of Parkinson's disease. (b) Explain why the destruction of the GPi during a pallidotomy might relieve the symptoms of Parkinson's disease.

Group 4: (a) Briefly describe the main symptoms of Huntington's chorea. (b) Summarize possible causes of this disorder.

Group 5: (a) What role does the cerebellum play in movement? (b) How does the reticular formation affect movement?

riding a bicycle

The putamen contains procedural information for skills like riding a bicycle. The putamen sends information to the premotor cortex which instructs the primary motor cortex to "move it." The primary motor cortex then orders the needed muscles to contract.

the "20% of your brain myth

The myth that you only use 20% of your brain comes from the observation that a tumor has to slowly kill 80% of cells in the hand region of the motor strip to produce weakness or paralysis. Sudden death of 30% of these same neurons during a stroke would also produce paralysis (Calvin & Ojemann, Conversations with Neil's brain, 1994).

the left hemisphere controls left and right hands

MRI studies have shown that regardless of handedness, the left hemisphere is critical in the control of left and right hands, while the right hemisphere is only critical for control of the left hand. This explains why left hemisphere damage is more disruptive than right hemisphere damage. Left-handed people require more cortical activity to move their dominant hand (Georgopolous, Science, July 1993).