BIOL 4380 MOTOR SYSTEM RECORDING

Motor Cortex Location: The primary motor cortex is located in the pre-central gyrus, directly anterior to the central sulcus. This region is critical for the execution of voluntary movements across various body parts, providing the brain's direct output to the spinal cord, which controls muscle contractions.

Motor Homunculus: The motor homunculus is a stylized representation of the human body within the motor cortex, where the size of each body part corresponds to the amount of cortical area dedicated to its motor control rather than its physical size. For instance, areas controlling the hands and face are disproportionately large, reflecting the finely controlled movements required for tasks involving these regions, such as writing or speaking.

Mapping Techniques: Motor mapping techniques include both invasive and non-invasive methods to pinpoint specific functionalities of the motor cortex. Invasive techniques, such as electrocorticography used during neurosurgery, allow for direct stimulation and mapping of cortical areas. Non-invasive methods, including Transcranial Magnetic Stimulation (TMS), utilize magnetic fields to induce electrical currents in the cortex, aiding in the evaluative mapping of motor functions without the need for surgical intervention.

Primary vs. Non-Primary Motor Cortex:

• Primary Motor Cortex: Directly controls muscle movements and is primarily responsible for executing voluntary movements. The neurons in this region send signals down the corticospinal tract to lower motor neurons in the spinal cord.

• Supplementary Motor Cortex: Plays an essential role in planning and coordinating sequences of movements, such as preparing to grasp and manipulate objects. This region is activated before movement initiation and is critical for learning and remembering motor sequences.

• Premotor Cortex: Engages in the planning and organization of movements, especially those that require sensory guidance and the integration of cues from the environment, contributing to the execution of complex motor patterns.

Reflexive vs. Voluntary Movement Control:

• Voluntary Control: Involves higher-order planning and is predominantly managed by the cortex. This control is crucial for executing complex tasks that require active engagement and cognitive input, such as playing a musical instrument.

• Reflexive Control: Involuntary movements or reflexes are organized by structures lower than the cortex, including the spinal cord, which allows for fast and automatic responses to stimuli without conscious thought, exemplified by the knee-jerk reaction.

Neuronal Dynamics in the Motor Cortex:

• Neuron Structures: Neurons in the motor cortex possess distinct input structures (dendrites), which gather signals from other neurons, and output structures (axons), which transmit signals to other destinations. Apical dendrites are particularly important as they facilitate long-range connectivity among neurons across different layers and regions of the cortex.

• Connection Pathways:

  • Corticobulbar Tract: This tract controls movements of the muscles in the head and neck and is considered an evolutionarily older pathway present in many vertebrates.

  • Corticospinal Tract: More advanced than the corticobulbar tract, it facilitates fine and dexterous movements, especially in humans and other primates. It extends from the motor cortex down to the spinal cord, playing a crucial role in voluntary movement.

Population Coding and Movement Representation:

• Population Code: This concept posits that motor commands are not encoded by individual neurons but rather through the collective activity of a population of neurons in the motor cortex. Each neuron's response contributes to the overall command for movement.

• Georgopoulos' Experiment: In experiments conducted on monkeys, researchers observed neuronal activity as the subjects moved their arms toward various targets. The study demonstrated that specific neurons in the motor cortex encoded both the direction and force of movements through their firing rates, necessitating vector averaging to accurately represent the combined activities of neuron populations in executing coordinated movements.

Upper and Lower Motor Neurons:

• Upper Motor Neurons: These originate in the motor cortex and descend through the brain and spinal cord. Damage to upper motor neurons can result in weakness, paralysis, and spasticity due to disrupted signals.

• Lower Motor Neurons: Located in the spinal cord, lower motor neurons receive signals from upper motor neurons and are responsible for directly innervating skeletal muscles, prompting contraction. Damage here leads to muscle atrophy and weakness.

Basal Ganglia and Motor Control:

• Structure: The basal ganglia consist of several key nuclei, including:

  • Striatum (comprised of the caudate and putamen) which integrates motor signals and cognitive functions

  • Globus pallidus (both internal and external segments) which regulates voluntary movement

  • Substantia nigra (including pars compacta and pars reticulata) which plays a significant role in movement regulation, particularly in reward-based motivation

  • Subthalamic nucleus which connects with other basal ganglia components to refine movement.

• Function: The basal ganglia function in a gating mechanism, selecting appropriate actions and integrating motivations for movement control.

  • Direct Pathway: Facilitates and promotes movement through excitation of the thalamus, enhancing the motor output to the cortex.

  • Indirect Pathway: An inhibitory mechanism that prevents unwanted movements, thus allowing for the smooth execution of motor tasks and prevention of disturbances during voluntary actions.

Clinical Observations in Motor Disorders:

• Parkinson's Disease: This neurodegenerative disorder results from the degeneration of dopaminergic neurons in the substantia nigra, leading to characteristic motor symptoms such as resting tremors, rigidity, and bradykinesia (slowness of movement). The loss of dopamine disrupts the balance of excitatory and inhibitory signals in the basal ganglia, exacerbating motor control issues.

• Deep Brain Stimulation (DBS): A surgical intervention for Parkinson's disease, DBS involves implanting electrodes in targeted brain areas, particularly within the subthalamic nucleus or globus pallidus. It offers a way to regulate abnormal electrical activity in the brain, alleviating symptoms and improving quality of life for patients.

Eye Movements and Control Mechanisms:

• Types of Eye Movements: Eye movements, particularly saccades (rapid movements), are crucial for visual scanning, focus, and the shift of attention between visual stimuli, allowing humans to engage dynamic environments.

• Neurons in Eye Control: Oculomotor neurons, which control the six extraocular muscles surrounding the eyes, exhibit variable firing patterns during eye movements, correlating with the amplitude and direction of those movements through rate coding mechanisms.

• Role of PPRF: The Parapontine Reticular Formation plays a central role in coordinating horizontal eye movements, working closely with the superior colliculus, which processes visual input and contributes to the planning and execution of saccadic movements.