1.9: Motor Control

Learning Objectives

1. Explain the divisions of the nervous system.
2. Explain how reflexes work.
3. Discuss how muscle strength is controlled.

Human motor control can be thought of simply as how your brain consciously and subconsciously controls your physical body. Motor control is the process by which the central nervous system plans, directs, and executes the movement of the body. This involves the coordination of using sensory information to move muscles and joints. In Chapter 6 the physiology of muscle contraction was detailed, in this chapter, we will review how those mechanisms are used for a human to perform coordinated physical tasks.

Figure 1. Central and Peripheral Nervous System The structures of the PNS are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the CNS are not obvious from this overall perspective and are best examined in prepared tissue under the microscope. Anatomy & Physiology. Authored by: OpenStax College. Provided by: Rice University. Located at: http://cnx.org/contents/14fb4ad7-39a...f2482e3e22@9.1

The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else (Figure 1). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. There are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery—meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.

Figure 2. Gray Matter and White Matter A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain. (credit: modification of work by “Suseno”/Wikimedia Commons)

Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). Figure 2 demonstrates the appearance of these regions in the brain and spinal cord. The colors ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue. Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin. Lipids can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. Gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray.

The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used, and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the CNS—for example, a frontal section of the brain or cross section of the spinal cord.

Usually, the first step in motor control is the input of a stimulus or conscious thought. Examples of these could be seeing a frisbee flying towards you, the smell of baking cookies, or the thought that what is currently on TV is boring. This initiates movement of the body to react to that stimulus, typically from the cerebral cortex in the brain that is responsible for higher-order cognitive processes. Higher-order cognition refers to mental processes like problem-solving, reasoning, creativity and decision-making. The brain processes the stimulus input and activates specific muscles in the body through motor neurons, which are specialized nerve cells that transmit stimulus from the brain to activate muscular contraction. The muscle contraction then moves the body and solves the problem. If a frisbee is flying toward you, you raise your arm to catch it. The nostril that could most strongly smell the cookies indicated the direction of the baking and your neck turns in that direction to allow a better vision of the food. The boring TV channel is replaced by pressing the button down on the remote resting in your hand.

Critical Thinking

How does this “automatic” movement help in day-to-day life? How would driving a car, writing with a pencil, or tying your shoes be if you had to think about each exact movement?

Once a movement has been initiated, it is carried out by the activation of specific muscle contractions to flex or extend joints and move the body. The activation of these muscles is controlled by motor neurons, which are specialized cells in the central nervous system that transmit signals to the muscles to contract. This is covered in more detail in Chapter 6. Movement and the activation of these motor neurons is influenced by a variety of factors covered below:

Figure 3. The pathway of sensory and motor neurons from the hand to the brain and back. Anatomy & Physiology. Authored by: OpenStax College. Provided by: Rice University. Located at: http://cnx.org/contents/14fb4ad7-39a...f2482e3e22@9.1

Reflexes are automatic involuntary responses to stimuli that do not require conscious thought or effort. If you touch something very hot your body will automatically pull your hand away through a series of reflex arcs. Reflex arcs are neural pathways that do not need to travel to the cerebral cortex for conscious thought. They directly link sensory receptors to motor neurons to allow for a quicker response. These automatic involuntary responses help maintain human upright balance and reduce physical tissue damage when we do something like accidentally touching a hot pan as the hand can be pulled back faster than if we had to think to do it.

Neurons that carry information from sensory receptors from all over the body towards the central nervous system are called afferent neurons. Efferent neurons transmit motor information from the central nervous system to the muscles of the body to trigger movement.

The brain gets sensory feedback from muscles to understand where and what the body is doing in space and in relation to objects around it.

Critical Thinking

Stand up and close your eyes. Move your body around and stop in unusual body positions. If you have a classmate, have them move your arms or hands around. Even with your eyes closed your brain knows where your limbs are and how the body is positioned. How do you think this is done?

Proprioception refers to the ability to sense the position and movement of our own body parts without relying on visual or auditory cues. This is made possible by specialized receptors located in our muscles, tendons, and joints that provide information about the position, movement, and tension of our muscles and connective tissues. Muscle Spindles are small sensory receptors in muscle tissue that relay the change in length and speed of muscle fiber stretch to the central nervous system. This information is critical for controlling the precise movements of our body.

Golgi Tendon Organs are responsible for monitoring the tension or force that is applied to a muscle. They work by detecting the amount of stretch in the tendon caused by the contraction of the muscle fibers. When the tension in the tendon becomes too high, the Golgi tendon organs send a signal to the spinal cord, which in turn sends a signal to the muscle fibers causing them to relax. This is known as the Golgi tendon reflex.

The Golgi tendon reflex is an important protective mechanism that helps to prevent excessive tension or force from being applied to muscles, which could lead to injury. It is also involved in regulating muscle tone and maintaining posture. Proprioception allows us to perform coordinated movements and maintain balance and posture.

Kinesthetic sense, is the ability to sense the position and movement of our body parts in relation to each other and to the environment around us. It involves the integration of information from multiple sensory systems, including proprioception, vision, and the vestibular system (which provides information about balance and spatial orientation). Kinesthetic sense allows us to navigate through our environment, interact with objects, and perform complex tasks that require hand-eye coordination.

Together, proprioception and kinesthetic sense play crucial roles in motor control, movement planning, and body awareness. They are essential for activities such as sports, dance, and surgery, where precise control of movement and position is required. Disorders of proprioception and kinesthetic sense can lead to difficulties with movement control, balance, and posture, and can significantly impact a person's quality of life.

Muscle activation patterns are developed through a complex and dynamic process that involves a combination of genetic predispositions, experience, and neural plasticity.

At the genetic level, motor patterns are influenced by inherited traits that affect the development and function of the nervous system, including the formation and function of neural circuits that control movement. These genetic factors provide a foundation for motor development, but they do not determine the outcome.

Experience also plays a critical role in the development of motor patterns. Infants and children explore their environment and engage in various physical activities that challenge and refine their motor skills. Through repetition and practice, neural circuits involved in movement control are strengthened and optimized, leading to the development of more efficient and precise motor patterns.

In addition, neural plasticity plays an important role in the development of motor patterns. Neural plasticity refers to the brain's ability to reorganize and adapt in response to changes in the environment or experiences. Through this process, neural circuits involved in movement control can be modified and refined to optimize motor patterns.

Motor patterns are also shaped by feedback from sensory systems, such as vision and proprioception, which provide information about the body's position and movement in space. This feedback helps to refine motor patterns and improve their accuracy and precision.

The development of motor patterns is a complex and dynamic process that is influenced by a variety of factors. By understanding the interplay between genetics, experience, and neural plasticity, researchers can better understand how motor patterns are developed and how they can be optimized through early intervention and rehabilitation.

Critical Thinking

Balance test. Eyes open and eyes closed. Two feet vs one foot. Why is one harder?

Balance is the ability to maintain the body's center of mass over the base of support during static or dynamic movements. The visual system plays an important role in balance control by providing information about the body's position and movement in space.

Closing one eye can significantly impair balance because it reduces the amount of visual information available to the brain. With both eyes open, the brain uses the information from both eyes to create a three-dimensional image of the environment, which helps to accurately estimate the body's position and movement. When one eye is closed, the amount of visual information is reduced, which can result in inaccurate estimates of the body's position and movement and can make balance control more challenging.

In addition, closing one eye can also affect the vestibular system, which is responsible for detecting changes in head position and movement. The vestibular system relies on information from the inner ear, and it works together with the visual system to maintain balance. When one eye is closed, the vestibular system may receive conflicting information about the body's position and movement, which can further impair balance control.

Overall, closing one eye can make balance control more challenging because it reduces the amount of visual information available to the brain and can affect the vestibular system's ability to accurately detect changes in head position and movement. However, with practice and adaptation, the brain can learn to compensate for the loss of visual information and improve balance control when one eye is closed.

A homunculus is a representation of the human body within the brain, specifically in the primary motor and sensory cortex. The term "homunculus" comes from the Latin word for "little man." In the primary motor cortex, the homunculus is a distorted representation of the body's motor functions, with different areas of the cortex controlling different body parts. The amount of space devoted to each body part is proportional to the complexity and precision of its movement. For example, the areas responsible for the hands, fingers, and face are relatively larger than the areas for the torso or legs, reflecting the greater precision and complexity of their movements.

Similarly, in the primary sensory cortex, there is a homunculus that represents the body's sensory functions, with different areas of the cortex processing information from different parts of the body. The amount of space devoted to each body part is again proportional to the complexity and density of its sensory receptors.

The homunculus is important because it provides a detailed map of how the brain processes sensory and motor information from the body. By understanding the organization of the homunculus, researchers can better understand how the brain controls movement and processes sensory information, and how these processes may be disrupted in conditions such as stroke or traumatic brain injury. The homunculus is also important in clinical settings, as it provides a way to map the location of sensory or motor deficits in patients, allowing for more targeted rehabilitation interventions.

Critical Thinking

How do we engage and control muscle contraction? How do you not snap a pencil in half when writing. Even though your hand is strong enough to do so.

The ability to control muscle strength is due to the fine-tuning of motor commands sent from the brain to the muscles. These commands are generated in the primary motor cortex and are transmitted through descending motor pathways in the spinal cord to the muscles. The strength of the motor command determines the force produced by the muscle.

In order to perform fine motor skills like using a pen, the motor commands sent to the muscles need to be precisely controlled to produce the appropriate level of force. The brain achieves this control by activating specific groups of motor neurons in the spinal cord that innervate the muscle fibers. The number and frequency of motor neurons activated determine the force produced by the muscle.

When performing fine motor tasks, the brain can precisely adjust the number and frequency of motor neurons activated to produce the appropriate level of force for the task. This is achieved through a process called motor unit recruitment, where the brain activates only a small number of motor units (a motor neuron and the muscle fibers it innervates) to produce a low level of force. As the required force increases, the brain activates additional motor units to produce a greater force.

This precise control of motor unit recruitment allows for the execution of fine motor skills without overexerting the muscles and risking damage to the object being manipulated, such as a pen. Additionally, the brain can monitor the amount of force being produced and adjust the motor commands accordingly to maintain the appropriate force level for the task. This helps prevent accidental over-exertion and breakage of objects.

The challenge of upright walking:

Upright walking, also known as bipedalism, is challenging because it requires several adaptations to the anatomy and physiology of the human body. These adaptations include changes to the structure and function of the spine, pelvis, legs, and feet.

One reason why upright walking is evolutionarily rare is that it requires a significant reconfiguration of the body's anatomy and physiology. In order to walk upright, the spine must be reconfigured to form an S-shaped curve, which places the center of gravity over the pelvis and allows for better balance while walking. The pelvis must also be modified to be shorter and broader, which helps support the weight of the upper body while walking. The legs and feet must also be adapted to bear the weight of the body and provide stability during movement.

These adaptations take time to evolve and are not easily achieved through natural selection. In addition, bipedalism requires a higher level of coordination and balance than quadrupedal movement, which is the typical mode of locomotion for most mammals. This increased demand for coordination and balance may have also made bipedalism a difficult trait to evolve.

Despite these challenges, the benefits of bipedalism may have made it an advantageous trait for early hominids. Upright walking allowed for greater efficiency in travel and foraging, as well as better surveillance of predators and prey. Overtime, natural selection may have favored individuals with adaptations for upright walking, leading to the evolution of modern humans and our unique ability to walk upright.

Motor control is a complex process involving the coordination of neural structures and pathways to produce coordinated movements of the body. Understanding the neural mechanisms of motor control is essential for enhancing athletic performance and rehabilitation from injury. Through motor learning, individuals can acquire and refine motor skills to perform daily activities, participate in sports, and recover from injury.

If the motor control topics discussed in this chapter captured your attention then a future as a biomechanistic may be for you.

In Chapter 10 we explore personal and group training.