BIOL 4380 MOTOR SYSTEM SLIDES

Motor Cortex Overview: The motor cortex is an integral part of the brain responsible for the planning, control, and execution of voluntary movements. This region is organized in a somatotopic manner, often depicted as a 'motor homunculus,' where specific areas correspond to various body parts. However, the size of these areas is not proportional to the physical dimensions of the body parts, but rather reflects the complexity and precision of the motor control they require.

• Distorted mapping on the homunculus highlights that regions such as the hands and face have a more significant representation due to the intricate motor skills needed for their movement.

Anatomy of the Motor Cortex:

• Primary Motor Cortex (PMC): Positioned in the precentral gyrus of the frontal lobe, the PMC plays a critical role in the execution of voluntary movements. It serves as the final output station for the motor signals descending from the cortex.

• Non-primary Motor Cortical Areas: In addition to the PMC, the motor system includes supplementary motor areas (SMA) and premotor cortices, which are crucial for the planning, coordination, and sequencing of complex movements, especially those that involve multiple body parts.

Betz Cells:

• Description: Betz cells are large pyramidal neurons found in the primary motor cortex. They are among the largest projection neurons in the human brain and play a vital role in transmitting motor signals from the motor cortex down to the spinal cord.

• Function: These cells are essential for the fine and skilled control of voluntary movements, especially in the limbs, facilitating dexterous tasks such as writing or playing musical instruments.

Corticobulbar and Corticospinal Tracts:

• Corticobulbar Tract: This tract is responsible for controlling motor functions of the head and neck, with its fibers terminating in the brainstem, influencing cranial nerve nuclei responsible for facial and neck movements.

• Corticospinal Tract: This major pathway projects fibers to the spinal cord, thereby controlling the movements of limbs and trunk. It is divided into lateral and anterior corticospinal tracts, with the lateral tract primarily responsible for fine motor control in the limbs.

Vector Averaging in Primary Motor Cortex:

• Directional Tuning: Neurons within the primary motor cortex exhibit specialized responses based on the direction of intended movement, firing more robustly for movements in their 'preferred direction.' This tuning is pivotal for preparing the motor system for action.

• Population Vector: This concept represents the combined activity of a population of neurons, allowing the motor cortex to leverage multiple signals to determine the overall direction of movement, enhancing the accuracy of motor execution.

Upper and Lower Motor Neurons:

• Upper Motor Neurons: These neurons originate in the motor cortex and descend through the corticospinal tract to synapse onto lower motor neurons in the spinal cord. Damage to these neurons can lead to a variety of motor deficits, including weakness or paralysis, hyperactive reflexes, and changes in muscle tone (e.g., spasticity).

• Lower Motor Neurons: Located in the anterior horn of the spinal cord, these neurons directly innervate skeletal muscles. Injuries to lower motor neurons result in flaccid paralysis and muscle atrophy, where the affected muscles lose strength and bulk.

Basal Ganglia Overview:

• Function: The basal ganglia are a group of nuclei located deep within the cerebral hemispheres that play a fundamental role in modulating motor control. They provide feedback to the motor cortex and contribute to refining movements, inhibiting unwanted or unintended movements, and facilitating desired motions.

• Components:

  • Input Nuclei: The striatum, comprising the caudate nucleus and putamen, serves as the main input region where most cortical inputs converge.

  • Intrinsic Nuclei: These include the globus pallidus (both external and internal segments), the subthalamic nucleus, and the substantia nigra, each contributing to the internal signaling pathways of the basal ganglia.

  • Output Nuclei: The internal globus pallidus and substantia nigra pars reticulata act as output regions, inhibiting the thalamus to suppress movement until appropriate signals arise.

Pathways in Basal Ganglia:

• Direct Pathway: This pathway facilitates movement by transmitting excitatory signals from the cortex to the striatum, which then sends inhibitory signals to the internal globus pallidus and substantia nigra reticulata, reducing inhibition on the thalamus and promoting motor output.

• Indirect Pathway: This pathway exerts an inhibitory effect on movement by activating the external segment of the globus pallidus, leading to inhibition of the subthalamic nucleus, which increases inhibition of thalamic neurons, effectively acting as a brake on unnecessary movements.

Parkinson's Disease:

• Overview: Parkinson's disease is characterized by the progressive degeneration of dopamine-producing neurons in the substantia nigra. This loss leads to a deficiency in dopamine levels that severely impairs motor control, presenting symptoms such as bradykinesia (slowed movement), rigidity, and resting tremors.

• Treatment: The primary treatment involves the administration of L-DOPA, a precursor to dopamine that the brain can convert. While it can temporarily alleviate symptoms, it does not prevent the progression of neurodegeneration, and its efficacy may diminish over time, requiring adjustments in therapy.

Eye Movements:

• Saccadic Movements: These are rapid, simultaneous movements of both eyes in the same direction, which are crucial for quickly shifting gaze to new visual targets. They ensure that important stimuli are brought into sharp focus on the retina.

• Control Mechanism: The amplitude (how far) and direction of these movements are carefully controlled by motor signals emanating from the oculomotor nuclei, facilitating accurate visual tracking.

• Superior Colliculus Role: The superior colliculus plays a critical role in integrating visual input and coordinating eye movements. It serves as a hub for converging sensory inputs with motor planning to ensure proper and timely direction of gaze towards visual stimuli.