5.3 Motor cortex and corticospinal tract
4 min read•august 14, 2024
The motor cortex and are key players in our ability to move. They work together to control our muscles, allowing us to perform everything from simple actions to complex tasks. Understanding their structure and function is crucial for grasping how our brains control movement.
Damage to these areas can lead to serious motor problems, like paralysis or difficulty with fine movements. But there's hope: our brains can adapt. Through plasticity, motor maps can change, helping us recover from injuries and learn new skills.
Motor Cortex Organization
Primary Motor Cortex Location and Somatotopic Organization
Top images from around the web for Primary Motor Cortex Location and Somatotopic Organization
Thalamus & Cerebral Cortex – KINES 200: Introductory Neuroscience View original
Is this image relevant?
Motor cortex - Wikipedia View original
Is this image relevant?
File:Human motor cortex.jpg - Wikimedia Commons View original
Is this image relevant?
Thalamus & Cerebral Cortex – KINES 200: Introductory Neuroscience View original
Is this image relevant?
Motor cortex - Wikipedia View original
Is this image relevant?
1 of 3
Top images from around the web for Primary Motor Cortex Location and Somatotopic Organization
Thalamus & Cerebral Cortex – KINES 200: Introductory Neuroscience View original
Is this image relevant?
Motor cortex - Wikipedia View original
Is this image relevant?
File:Human motor cortex.jpg - Wikimedia Commons View original
Is this image relevant?
Thalamus & Cerebral Cortex – KINES 200: Introductory Neuroscience View original
Is this image relevant?
Motor cortex - Wikipedia View original
Is this image relevant?
1 of 3
- Located in the precentral gyrus of the frontal lobe, immediately anterior to the central sulcus
- Organized somatotopically, different body parts are represented in specific areas of the cortex
- Somatotopic organization depicted as a distorted human figure called the
- Size of each body part in the motor homunculus corresponds to the amount of cortical area dedicated to controlling that body part, rather than its actual size
- Body parts requiring fine motor control (hands and face) have larger representations compared to body parts with less precise movements (trunk)
- Organized in a contralateral manner, left hemisphere controls the right side of the body and vice versa
Primary Motor Cortex Cellular Composition
- Contains large pyramidal neurons called , which give rise to the corticospinal tract
- Betz cells are the largest neurons in the cerebral cortex and are essential for the control of voluntary movements
- Betz cells receive inputs from other cortical areas and project their axons through the corticospinal tract to the spinal cord
- Damage to Betz cells or their axons can result in motor deficits and paralysis
Corticospinal Tract Function
Corticospinal Tract Anatomy and Organization
- Major descending motor pathway that originates from the and terminates in the spinal cord
- Essential for the control of fine, precise, and voluntary movements, particularly in the distal extremities (hands and feet)
- Axons descend through the , , , and before forming the pyramids
- At the medulla, majority of corticospinal fibers (90%) decussate to the contralateral side, forming the
- Remaining uncrossed fibers (10%) continue ipsilaterally as the and decussate at the level of the spinal cord
Corticospinal Tract Synaptic Connections and Clinical Significance
- Synapses directly or indirectly (via interneurons) with in the anterior horn of the spinal cord gray matter
- Lower motor neurons innervate skeletal muscles and are responsible for the execution of voluntary movements
- Damage to the corticospinal tract can result in contralateral weakness, , and (stroke or )
- Lesions above the decussation (cortex, internal capsule) cause contralateral deficits, while lesions below the decussation (spinal cord) cause ipsilateral deficits
Premotor and Supplementary Motor Areas
Premotor Cortex Functions
- Located anterior to the primary motor cortex in the frontal lobe
- Involved in the selection and preparation of movements based on sensory cues and learned associations
- Dorsal premotor cortex involved in the planning of reaching and grasping movements guided by visual information
- Ventral premotor cortex involved in the preparation of movements in response to visual and somatosensory cues
- Premotor cortex has reciprocal connections with the primary motor cortex and contributes to the corticospinal tract
Supplementary Motor Area (SMA) Functions
- Located anterior to the primary motor cortex in the frontal lobe
- Involved in the planning and coordination of complex, sequential, and bilateral movements
- Plays a role in the internal generation of movement sequences (speech production or playing a musical instrument)
- Involved in the timing and initiation of self-paced movements
- SMA has reciprocal connections with the primary motor cortex and contributes to the corticospinal tract
- Damage to the SMA can result in impairments in , coordination, and the execution of complex movements ()
Motor Maps and Plasticity
Motor Map Representation and Plasticity
- Motor maps refer to the topographic representation of movements in the primary motor cortex and other motor-related areas
- Motor maps exhibit plasticity, they can reorganize and change in response to experience, learning, and injury
- Motor skill learning can induce changes in motor maps, such as an expansion of the cortical representation of the trained body part or movement
- Musicians who extensively train specific fingers may show an enlarged representation of those fingers in their motor cortex
- Motor map plasticity also occurs in response to injury or sensory deprivation
- Following a stroke or spinal cord injury, the motor cortex can reorganize, with adjacent areas taking over the function of the damaged region
- Phantom limb sensations in amputees may arise due to the invasion of the deafferented cortical area by neighboring representations
Mechanisms and Applications of Motor Map Plasticity
- Rehabilitation techniques () can exploit motor map plasticity to promote functional recovery after brain injury
- Mechanisms underlying motor map plasticity involve changes in synaptic strength ( and depression), unmasking of latent connections, and sprouting of new connections
- (TMS) can be used to study motor map plasticity by mapping the cortical representation of muscles before and after learning or injury
- Understanding motor map plasticity has implications for the development of targeted rehabilitation strategies and the promotion of recovery after brain injury or stroke
Key Terms to Review (33)
Acetylcholine: Acetylcholine is a neurotransmitter that plays a vital role in communication between neurons and is involved in various physiological functions such as muscle contraction, memory, and attention. It is found both in the central nervous system and the peripheral nervous system, influencing numerous neural pathways and processes.
Amyotrophic lateral sclerosis: Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that affects motor neurons in the brain and spinal cord, leading to muscle weakness, paralysis, and ultimately respiratory failure. The condition disrupts the principles of motor control by impairing voluntary muscle movements, severely impacting an individual’s ability to perform basic functions. As it progresses, ALS significantly affects the motor cortex and corticospinal tract, contributing to the overall understanding of neurodegenerative diseases.
Anterior corticospinal tract: The anterior corticospinal tract is a major pathway in the central nervous system that conveys motor commands from the cerebral cortex to the spinal cord, specifically targeting the axial and proximal muscles for voluntary movement. This tract is part of the larger corticospinal tract system, which is crucial for the fine control of voluntary motor activity. The anterior corticospinal tract primarily innervates muscles involved in posture and balance, facilitating coordinated movements of the trunk and proximal limbs.
Apraxia: Apraxia is a neurological condition characterized by the inability to perform purposeful movements, despite having the desire and physical ability to execute them. It results from damage to areas of the brain involved in planning and coordinating motor activities, particularly affecting the motor cortex and its connections. Individuals with apraxia may struggle with tasks like dressing, using tools, or even imitating movements, which highlights the importance of the motor cortex and the corticospinal tract in executing learned movements.
Betz cells: Betz cells are large pyramidal neurons located in the primary motor cortex, specifically in layer V, and are crucial for voluntary motor control. These cells play a significant role in the corticospinal tract, sending long axons down to the spinal cord to influence motor neurons that control skeletal muscles. Their size and unique structure allow them to transmit signals over long distances, making them integral for precise and coordinated movements.
Cerebral Peduncles: Cerebral peduncles are large bundles of nerve fibers located at the base of the brain that connect the forebrain to the hindbrain, specifically linking the cerebral cortex to the pons and the cerebellum. They play a crucial role in motor control and coordination, facilitating communication between different parts of the brain, especially in relation to voluntary movement.
Constraint-induced movement therapy: Constraint-induced movement therapy is a rehabilitation technique that aims to improve motor function in individuals with motor impairments by restricting the use of their unaffected limb, thereby forcing the use of the affected limb. This approach is based on the principle of neuroplasticity, where repeated practice and engagement of the affected limb can lead to functional improvements and brain reorganization. By promoting intensive practice of the impaired limb, this therapy leverages the brain's ability to adapt and recover following injury or stroke.
Contralateral Organization: Contralateral organization refers to the neurological principle whereby one side of the brain controls the opposite side of the body. This organization is crucial for motor functions, sensory perception, and integration of information, allowing the brain to coordinate actions and responses that involve the entire body efficiently.
Corticobulbar tract: The corticobulbar tract is a bundle of nerve fibers that connects the motor cortex of the brain to the cranial nerve nuclei in the brainstem. This tract plays a vital role in controlling the muscles of the face, head, and neck by facilitating voluntary movements and reflexes. It is an important pathway for the coordination of complex movements such as speech and facial expressions, linking higher brain functions with motor control.
Corticospinal tract: The corticospinal tract is a major neural pathway that connects the motor cortex of the brain to the spinal cord, facilitating voluntary motor control of the body's muscles. It plays a crucial role in executing precise and coordinated movements, allowing for the fine motor skills necessary for activities such as writing and playing musical instruments. The tract consists of upper motor neurons that originate in the motor cortex and descend through the brainstem and spinal cord, making critical synapses along the way.
Fine motor skills: Fine motor skills refer to the small, precise movements that involve the coordination of the small muscles in the hands, fingers, and wrists. These skills are essential for tasks that require accuracy, such as writing, sewing, or playing a musical instrument. Fine motor skills are heavily influenced by the motor cortex and the corticospinal tract, which are crucial in controlling these intricate movements and enabling smooth execution of detailed tasks.
Functional MRI: Functional MRI (fMRI) is a neuroimaging technique that measures and maps brain activity by detecting changes in blood flow and oxygen levels. This non-invasive method allows researchers and clinicians to observe brain functions in real-time, making it essential for understanding various neural processes related to cognition, emotion, and motor control.
Glutamate: Glutamate is the main excitatory neurotransmitter in the brain, playing a crucial role in sending signals between nerve cells. It's involved in various essential functions including synaptic transmission, plasticity, and learning processes, highlighting its significance across multiple neural pathways and mechanisms.
Hyperreflexia: Hyperreflexia is an exaggerated reflex response resulting from increased excitability of the spinal cord's reflex arcs. This condition often occurs due to damage or disruption in the pathways that modulate reflexes, particularly when the upper motor neurons are affected. When the motor cortex sends signals to initiate voluntary movement, a disruption can lead to an overactive response in the spinal cord, causing hyperreflexia.
Internal capsule: The internal capsule is a vital structure in the brain composed of white matter that contains a dense collection of axons. It plays a crucial role in connecting various regions of the cerebral cortex with the brainstem and spinal cord, facilitating communication between different parts of the nervous system. This structure is especially significant in motor control as it carries motor signals from the motor cortex to the lower motor neurons.
Lateral corticospinal tract: The lateral corticospinal tract is a major pathway in the central nervous system that carries motor signals from the cerebral cortex to the spinal cord, primarily involved in voluntary movement control. This tract is crucial for fine motor skills, particularly those requiring precision and coordination, such as writing or playing a musical instrument. It originates in the motor cortex, decussates (crosses over) at the junction of the medulla and spinal cord, and then descends to synapse with lower motor neurons in the spinal cord.
Long-term depression: Long-term depression (LTD) is a lasting decrease in the strength of synaptic transmission, occurring when synapses are repeatedly stimulated at a low frequency. This process is crucial for synaptic plasticity, allowing for the weakening of certain synaptic connections while strengthening others, which plays a vital role in learning, memory, and neural circuit refinement.
Long-term potentiation: Long-term potentiation (LTP) is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them synchronously. It plays a crucial role in synaptic transmission and is fundamental for various cognitive functions, including learning and memory, by increasing synaptic strength through biochemical changes.
Lower Motor Neurons: Lower motor neurons are the neurons that directly innervate skeletal muscles and are crucial for the execution of voluntary movements. They originate in the spinal cord and brainstem and send their axons to skeletal muscles, forming the final common pathway through which the central nervous system controls muscle contraction. The activity of lower motor neurons is influenced by inputs from upper motor neurons, local interneurons, and sensory feedback, making them integral to motor control.
Medulla Oblongata: The medulla oblongata is a crucial part of the brainstem located just above the spinal cord, responsible for regulating vital autonomic functions such as breathing, heart rate, and blood pressure. It acts as a relay station, connecting the higher regions of the brain to the spinal cord, ensuring communication and coordination between the central nervous system and peripheral body systems. Its involvement in reflexes like swallowing and vomiting highlights its role in basic survival mechanisms.
Motor homunculus: The motor homunculus is a visual representation of the areas of the primary motor cortex that correspond to different body parts. This model illustrates how various regions of the body are controlled by specific neurons in the motor cortex, with more complex or skilled movements being represented by larger areas. It highlights the principle of somatotopic organization in the brain, emphasizing the relationship between motor control and the spatial arrangement of body parts.
Motor learning: Motor learning is the process of acquiring and refining motor skills through practice and experience, leading to a relatively permanent change in the ability to perform these skills. This process involves the brain and nervous system's adaptation to various factors such as feedback, task complexity, and environmental conditions, ultimately allowing for improved coordination and execution of movements.
Motor planning: Motor planning refers to the cognitive process that organizes and sequences movements to achieve a specific goal. It involves the formulation of motor commands that direct muscles and limbs to perform complex actions. This process is crucial for executing coordinated movements, as it integrates sensory information with prior experiences and requires involvement from several brain regions, particularly those related to motor control.
Motor unit: A motor unit is a functional entity consisting of a motor neuron and all the muscle fibers it innervates, working together to produce muscle contractions. This structure plays a crucial role in the coordination of muscle movements and force generation, influencing how muscles respond to various levels of neural input. Understanding motor units is essential for grasping how the nervous system communicates with muscles, especially in the context of voluntary movement control and fine motor skills.
Muscle contraction: Muscle contraction is the physiological process through which muscle fibers generate tension and shorten in response to neural stimulation, ultimately facilitating movement. This process involves a complex interaction of proteins within muscle cells, primarily actin and myosin, that slide past each other to create force. Proper functioning of muscle contraction is essential for voluntary movements, posture maintenance, and various bodily functions.
Neuroplasticity: Neuroplasticity is the brain's ability to reorganize itself by forming new neural connections throughout life, allowing it to adapt to changes, learn new information, and recover from injuries. This concept is fundamental to understanding how the brain develops and functions, emphasizing that it is not a static organ but rather a dynamic system capable of change in response to experience and environment.
Pons: The pons is a crucial structure located in the brainstem, situated between the medulla oblongata and the midbrain. It serves as a major pathway for communication between different parts of the brain and plays a key role in regulating functions such as breathing, sleep, and arousal. The pons is integral to motor control, linking the cerebellum with the cortex, making it vital for coordinating movements.
Primary motor cortex: The primary motor cortex is a crucial area of the brain located in the frontal lobe, specifically along the precentral gyrus, responsible for the planning and execution of voluntary movements. It plays a vital role in controlling motor functions and is closely linked with other brain areas that integrate sensory information and coordinate complex movements, making it essential for both basic and skilled tasks.
Spasticity: Spasticity is a condition characterized by increased muscle tone and stiffness, often resulting in exaggerated reflexes and involuntary muscle contractions. This condition typically arises from damage to the motor pathways in the central nervous system, particularly affecting the motor cortex and corticospinal tract, which play crucial roles in voluntary movement control.
Spinal cord injury: A spinal cord injury (SCI) is damage to the spinal cord that results in a loss of function, such as mobility or feeling. This condition can occur from trauma, diseases, or congenital disorders, leading to varying degrees of paralysis and loss of sensation depending on the injury's location and severity. Understanding SCI is essential as it affects not just physical abilities but also the neural pathways involved in reflex actions and voluntary movements.
Supplementary motor area: The supplementary motor area (SMA) is a region of the brain located on the medial surface of the frontal lobe, involved in planning and coordinating complex movements. It plays a critical role in the execution of voluntary movements and is also important for motor imagery and the sequencing of movements, which are essential for tasks requiring intricate motor skills.
Transcranial magnetic stimulation: Transcranial magnetic stimulation (TMS) is a non-invasive technique used to stimulate specific areas of the brain using magnetic fields. It is primarily employed to investigate the function of the motor cortex and has applications in treating various neurological and psychiatric disorders, showcasing its potential in neuroenhancement and the use of neurotechnology.
Upper motor neurons: Upper motor neurons are a type of neuron that originate in the motor cortex of the brain and project down to lower motor neurons in the spinal cord or brainstem. These neurons are crucial for the planning, initiation, and control of voluntary movements, forming a key part of the corticospinal tract that transmits signals to execute these movements.