Adults complete a series of movements or activities almost mindlessly, such as showering, brushing their teeth, fixing their hair, tying their shoelaces, driving their cars, and cleaning their houses. The list could go on endlessly considering that almost all behavior involves movement, including chewing and swallowing – even squinting.
Now consider some of the most involved movements that take years of practice to perfect, the moves of an acrobat at a Cirque de Soleil show, the performance of a concert violinist, or the skill of Olympic figure skaters or professional basketball players.
From the seemingly simplistic methodical movements to the polished routines of skilled performers and athletes, moving the body in any way, for any purpose, in any setting, requires a complex motor network controlled by the body’s nervous system, and maintained through an intricate physiological balance that scientists today continue to investigate.
Neuroscientists and neuropsychologists research and study normal motor control and the diseases and injuries affecting this complicated physiological system.
What is the Nervous System?
In order to understand motor control within the body, neuropsychologists and neuroscientists must have in-depth knowledge of what is called the nervous system. To better understand the nervous system, anatomists have broken down the nervous system into two basic parts: the central nervous system (CNS) consisting of the brain and spinal cord; and the peripheral nervous system (PNS) consisting of every other part of the body connected to the CNS.
The PNS Sub-Systems or Branches:
Skeletal nervous system. This branch carries nerve impulses to muscles that individuals control, called skeletal muscles.
Autonomic nervous system. This branch carries nerve impulses to muscles that individuals can’t control, such as the heart and gut.
The Body’s Motor Network
Just as big cities like Los Angeles and New York have intricately connected, complicated systems for moving cars and trains into and around the cities, the body has a highly interconnected motor control network. This network establishes itself in the brain and connects to the spinal cord through distinct pathways.
Working as whole, the brain and spinal cord work to coordinate all voluntary movements, including the planning, preparation and execution of motor movements. And they work together to control motor learning or the acquisition of new motor skills – from learning to tie shoelaces, to becoming a concert violinist.
An essential part of this motor network is the neuronal structure located within the spinal cord that moves the nerve impulses from the brain to the spinal cord, and then to the skeletal muscles. This neuronal structure is built on highly specialized neurons called motor neurons.
Neurons of the Nervous System
Nerve impulses are carried by neurons within the body. Every neuron in the human body – billions in total – carries an electric signal.
In the case of the skeletal nervous system, neurons transmit data from the brain to the spinal cord’s motor neurons and toward the body’s muscles.
In the case of sensory neurons, nerve impulses are transmitted from the body’s sensory inputs (such as the skin) to the spinal cord, and then to the brain. For more information, see article on somatosensation.
Motor neurons located in the spinal cord synapse with neurons coming from pathways in the brain, and relay information to the relevant muscles. Scientists now know that the pathways that carry signals from the brain to the motor neurons in the spinal cord come from several different areas in the brain..
Three Main Brain Structures Related to Motor Control:
- The Cerebral Cortex. This is the brain’s outer surface known for its lumpy bumps (gyri) and folds (sulci). Scientists have localized areas of the cerebral cortex that control higher order functions, such as learning and memory, as well as areas that control muscles.
The cerebral cortex has a band or strip called the primary motor strip, or the M1. Similar to the somatosensory cortex strip or S1 (see article on somatosensation), the M1 is a topographical map with designated areas for controlling the trunk area, the legs, hands, and fingers. When stimulated, neurons descend from these designated areas down the brain to the spinal cord, synapsing with motor neurons and sending on the data to the specific muscle or muscles.
Two major pathways or tracts descend from the M1: The Corticospinal Tract and the Corticobular Pathway.
Another pathway that extends from the cortex to the spinal cord, the ventromedal pathway, is made up of several linked tracts. However, this pathway does not originate in the cortex’s M1, but in the brainstem or midbrain.
- The Cerebellum. The cerebellum lies outside of the cerebral cortex, and is connected to the brain stem. It is the body’s processing center for coordination, balance, equilibrium, and posture. Through numerous connections and nuclei, this part of the brain links to both the spinal cord and motor areas of the cortex.
- The Basal Ganglia. An area that lies just below the cerebral cortex called the subcortical region holds the basal ganglia. When scientists talk of the complexity of the brain’s processing of motor movements, it is often in reference to the basal ganglia.
Consisting of four main nuclei, the basal ganglia connect to the cerebral cortex – specifically the frontal lobes- and other areas of the brain. Recent research findings have found that the basal ganglia do not initiate direct movements, but in conjunction with the motor areas of the cortex, allow some activities and inhibit others. Researchers also think that this part of the brain enables “activity switching” or moving from one activity to another.
The four nuclei of the basal ganglia are the striatum, the pallidum, substantia nigra, and subthalamic nucleus.
What’s a Tract?
Neurons have long fibers or cables extending from them called axons. These cables often run side-by-side with each other in long strands, connecting one part of the nervous system to another. Bundles of axons are called tracts or nerves. These bundles are uniform in diameter, myelination, and conduction speed. Axons within one tract relay the same type of data and always in the same direction.
Ascending tracts take sensory information from the spinal cord to the brain. Descending tracts are made of motor fibers and carry information from the brain to the spinal cord.
What is Ganglia or Nuclei?
When a large number of the neurons’ cell bodies are clumped together and lie close to each other they form clusters or substances known as ganglia or nuclei. The cell bodies don’t actually touch, but sit minutely close to each other.
Spinal Cord Disorders
When a nerve gets damaged or injured, it takes days, months, and even years to heal. Some injuries never heal. And when a nerve can no longer relay signals that enable muscles to move, paralysis occurs.
The spine is covered by a thick backbone, yet injuries to the cord still occur. Spinal cord injuries result in the displacement of vertebrae, and axons are actually torn apart. Injuries to the spinal cord at or above the fifth cervical vertebra – or in the neck - prevents sensation and motor control of all four limbs and trunk, resulting in quadriplegia.
- Myasthenia Gravis. The neurotransmitter acetylcholine (ACh) is released from motor neurons, as it should be, but the number of receptor cells on muscles that pick up and process the ACh are at a lower level than normal. This results in muscle weakness or fatigue, especially in the head and neck region. Drooping eyelids are an early symptom of the disorder.
- Multiple Sclerosis (MS). Demyelination occurs in this disorder, meaning that the white matter – a fatty insulation substance - surrounding the axons in some neurons becomes reduced. (Myelinated nerve fibers have faster nerve conduction than non-myelinated nerves.) Recent research in MS suggests that the axons themselves could be injured in addition to the reduced myelination.
- Motor Neuron Disease. Motor neurons in the spinal cord and cranial nerves die, and individuals begin to lose all muscle control. Usually motor neurons responsible for respiration and swallowing become affected, resulting in death in only a few years of disease onset.
Disorders that fall within the range of cortical damage refer to areas of the brain’s cerebral cortex that have been injured. Disorders related to cortical abnormalities include the following:
- Hemiplegia. This disorder results from a loss of blood supply to the primary motor strip or M1 of the brain caused by an aneurysm, hemorrhage or clot. The damage causes an individual to lose control over parts of the body on the opposite side of the brain damage. Other causes of hemiplegia are accidental head injury, epilepsy, and tumor.
- Cerebral Palsy. This disorder usually results from trauma to the fetus or during birth process. It is exhibited by motor disturbances such as voluntary and involuntary movements, and unusually tensed muscles.
- Apraxia. Apraxia results when damage to a specific area of the brain causes an individual to be unable to perform a certain action. For example, damage to a part of the left frontal lobe can result in an individual’s inability to stick out his or her tongue, or to blow a kiss. Inother cases, an individual cannot produce a correct movement when asked, but they are able to make the movement voluntarily. In still other cases, individuals lose the ability to know what to do with certain tools, such as hammers or toothbrushes. There are many forms of apraxia and many different ways to classify this disorder. Many of the classifications depend on the area of the cerebral cortex that is damaged or injured.
Diseases and disorders that affect the brain’s basal ganglia, a grouping of nuclei that reside just below the brain’s cerebral cortex are called subcortical disorders. These disorders progress slowly, and usually result in a gradual loss of motor control and function. Individuals with these disorders are still able to perform certain movements, but the movements may be poorly coordinated and unregulated.
These diseases are associated with damage, disease, or disorder to parts, segments, or components of the basal ganglia or within the connections and pathways from the basal ganglia to the cerebral cortex and thalamus.
These diseases or disorders are well known and researched, yet definitive causes or cures still elude scientists. They include the following:
- Parkinson’s disease
- Huntington’s disease
- Tourette syndrome
- Obsessive-compulsive disorder
Career in Studying Motor Control and Movement Disorders
Neuropsychologists and neuroscientists usually focus their research within a specific topic area, such as understanding multiple sclerosis or Parkinson’s disease. Clinical neuropsychologists administer assessments to individuals with brain injuries and disorders affecting motor control and movement. Some neuroscientists work exclusively in laboratories with animals investigating the physiological functioning of the complex motor control system.
Usually a Ph.D. is required for fields that specialize in motor control and movement disorders. Some positions are available for those with master’s degrees, involving the assistance or laboratory management for other scientists.
Researching A Genetic Link to Lou Gehrig’s Disease
Scientists and researchers have worked for years to find the cause of amyotrophic lateral sclerosis (ALS), better known as Lou Gehrig’s disease, a usually fatal neuromuscular disease that attacks nerve cells responsible for voluntary muscle movements.
The muscles of those with ALS weaken and deteriorate. When the muscles in the diaphragm and chest wall waste away, individuals can’t breathe, requiring the full time use of a ventilator. ALS researchers say that the disease affects 1 or 2 out of 100,000 people annually.
But researchers at the University of Pennsylvania believe they have discovered a possible genetic link to this devastating disease. Their finding links back to another Penn discovery made in 2006 that showed people with ALS have abnormally large deposits on their brains of a protein called TDP-43.
Because yeast cells share a number of genes with human genes, the Penn researchers started with these unlikely cells. They exposed the yeast cells to TDP-43. At first, the cells died just as human cells die when exposed to the protein. But after they genetically modified the yeast in 5,500 ways, they discovered that some of the genetic modifications made the cells die faster – but some made the cells survive.
Penn geneticist Aaron Gitler discovered that disabling one gene called PBP1 made some cells survive the assault of TDP-43, and that gene is the equivalent to the human gene ataxin-2.
Moving the research to fruit flies, the researchers discovered that disabling ataxin-2 in the flies’ nervous system also helped the flies survive an assault of TDP-43.
In humans, ataxin-2 is also involved in a disease called spinocerebellar ataxia type-2 (SCA2), which causes coordination and walking problems. Geneticists call this disease a “genetic stutter,” meaning that a three-letter genetic code of CAG is repeated more than the normal 22 or 23 times. In SCA2, the code is repeated more than 34 times.
The Penn researchers hypothesized that perhaps in ALS patients, the code is longer than normal but not as long as in SCA2.
In their research, they compared 915 ALS patients to a group of healthy individuals, and they found that those with ALS are much more likely to have the genetic stutter in their ataxin-2 genes – longer than 22 or 23 but shorter than 34.
In Huntington’s disease, the same stutter appears in a different gene.
The researchers caution that the gene ataxin-2 is probably not the cause of ALS but a possible risk factor.