Neurophysiology: Motor Cortex and Pyramidal Tracts

Practical and Tutorial Review: Week 2 Reflection

• Week 2 Practical/Tutorial Materials: The tutorial worksheet for Week 2 is officially available within the module materials. Students are provided with instructional videos detailing how to transform raw data into professional figures.

• Mock Data and Class Data: The materials include mock data sets as well as tables specifically for data collected during the live class session.

• Feedback Process: Completed worksheets should be submitted via email for feedback. This activity is designed to reinforce content assessed in the Week 4 post-lab quiz and offers a crucial opportunity for formative feedback.

Measuring Performance Variability

Specific metrics are used to evaluate the consistency and accuracy of motor performance:

• Absolute Error (AE): This represents the absolute difference between the intended target and the actual performance. It provides a measure of the magnitude of error without considering the direction of bias (positive or negative).

• Constant Error (CE): This represents the directional performance bias. It accounts for whether the performance was consistently above or below (positive or negative) the target.

• Variable Error (VE): This represents the consistency or inconsistency of the performance, statistically equivalent to the standard deviation of the scores.

Anatomical Terminology and Orientations

To describe the location of neural structures and the direction of movements, specific anatomical terms are employed:

• Anterior: Towards the front.

• Posterior: Towards the back.

• Superior: Above or situated higher.

• Inferior: Below or situated lower.

• Medial: Towards the midline of the body.

• Lateral: Towards the outer edge or side.

• Rostral: Towards the nose/beak (especially within the brain).

• Caudal: Towards the tail or posterior end.

• Ventral: Towards the front; in the context of the brain, this refers to the inferior surface.

• Dorsal: Towards the back; in the context of the brain, this refers to the superior surface.

Anatomical Planes of the Central Nervous System

Three primary planes are used to identify and distinguish between different neural structures:

• Frontal/Coronal Plane: Divides the structure into front and back portions.

• Sagittal Plane: Divides the structure into left and right portions.

• Horizontal/Axial Plane: Divides the structure into upper and lower portions.

• Clinical Utility: Utilizing different views and planes is essential for identifying damage or degeneration of specific structures at varying locations or depths.

The Nervous System: CNS and PNS

The nervous system is divided into two primary components:

• Central Nervous System (CNS): Composed of the brain and spinal cord. It acts as the "powerhouse," responsible for generating motor commands for voluntary movement and serving as the integration center for sensory information. It is physically protected by the skull and vertebrae.

• Peripheral Nervous System (PNS): Composed of connecting nerves (neurons) outside the CNS. It connects the CNS to the muscular systems. It contains both sensory axons (afferent signals from periphery to CNS) and motor axons (efferent signals from CNS to muscle). Unlike the CNS, the PNS is not protected by bone and is highly susceptible to damage.

Functional Classification of Neurons in the CNS

• Upper Motoneurons: These arise from premotor areas and connect the brain to the spinal cord via their axons, where they synapse with lower motoneurons. They are responsible for the planning of voluntary movement and are required for sophisticated movement patterns, coordination, and the interpretation of the environment.

• Lower Motoneurons (Alpha Motoneurons): These arise from the brainstem and spinal cord and directly innervate skeletal muscle, connecting the spinal cord to the muscles. They can unconsciously control complex movement patterns (reflexes) such as walking. Their cell bodies are located in the spinal cord ($SC$) and their axons extend to the periphery.

CNS Tissue Composition

• Grey Matter: Contains the dendrites and cell bodies of neurons.

• White Matter: Contains the axons of neurons.

• Regional Distribution:

  • Cerebrum: Grey matter is predominantly found in the outer layers (cortex).

  • Spinal Cord ($SC$): Grey matter is predominantly found in the inner layers (the "butterfly" shape).

Functional Lobes of the Cerebrum

• Frontal Lobe: Contains motor areas required for voluntary movement, motor planning, language, personality, and problem-solving.

• Parietal Lobe: Contains somatosensory areas; responsible for integrating sensory information such as touch, temperature, and pain.

• Occipital Lobe: Processes visual information; contains visual cortices to determine the location and depth of objects.

• Temporal Lobe: Involved in memory formation (containing the hippocampus) and auditory information processing.

Cerebral Topography and Landmarks

• Gyri: Raised ridges or areas on the surface of the lobes.

• Sulci: Troughs or fissures that separate the gyri.

• Key Fissures and Sulci:

  • Central Sulcus: Separates the frontal lobe from the parietal lobe. The pre-central gyrus (primary motor cortex) and post-central gyrus (somatosensory cortex) are located on either side of this sulcus.

  • Sylvian Fissure (Lateral Sulcus): Defines the border of the temporal lobe.

  • Parieto-occipital Sulcus: Separates the regions associated with visual processing from those associated with sensory processing.

Key Motor Areas of the Frontal Lobe

Stimulation of specific areas evokes distinct motor responses:

• Primary Motor Cortex (M1): Responsible for the execution of descending motor commands and distinct movements of contralateral limbs (the opposite side of the body). It has direct connections with the somatosensory cortex and premotor areas and undergoes extensive synaptic plasticity during motor learning. It contains Layer V Betz cells ("giant pyramids," discovered by Volodymyr Betz, 1834-1894), which are projection neurons that directly innervate alpha motoneurons.

• Premotor Cortex (PMC): Located rostral to M1, it is critical for motor planning and movement selection. Over $30\%$ of axons in the corticospinal tract arise here.

• Supplementary Motor Area (SMA): Located medially. It handles movements from both ipsilateral and contralateral muscles (both sides of the body) and can stop ongoing voluntary movement.

Spatial Kinematics and Movement Selection

Premotor areas transform sensorimotor signals into detailed spatial instructions:

• Lateral PMC: Encodes the selection of movement based on external stimuli (e.g., visual cues).

• Medial PMC (SMA): Encodes the selection of movement based on internal stimuli (e.g., movements generated from memory).

• Neural Discharge: Neurons in these areas begin to discharge $1-2\,\text{seconds}$ prior to the onset of movement and are directionally activated.

Research Insights: Reaction Time Tasks

Studies by Crammond & Kalaska (2000) and Cisek et al. (2003) highlight the behavior of motor areas:

• PMC Activity: Neurons become active prior to movement onset, providing evidence of PMC involvement in motor planning. Different firing patterns are observed for movements in different directions, suggesting a directional bias.

• M1 Activity: Neurons are most active immediately prior to movement onset. The delay represents the processing time required by the CNS to generate movement. Some activity occurs prior to movement, possibly representing the "priming" of motor units. M1 also displays a directional bias.

Neuromuscular and Cortical Assessment Techniques

• Electroencephalography (EEG): Non-invasive (or invasive) electrodes measure electrical activity.

  • Pros: Can be inexpensive, easy to collect/analyze, and allows for recordings during exercise protocols.

  • Cons: Lacks detail for deep cortical areas (unless invasive) and is highly susceptible to Signal noise/artefacts.

• Positron Emission Tomography (PET): Uses radioactive tracers to assess metabolic activity and blood flow. It can map the expression of neurotransmitter receptors, including:

  • Serotonin: $5\text{-HTT}$, $5\text{-HT}_6$, $5\text{-HT}_4$, $5\text{-HT}_{2A}$, $5\text{-HT}_{1B}$, $5\text{-HT}_{1A}$.

  • Histamine: $H_3$.

  • Norepinephrine: $NET$.

  • Dopamine: $DAT$.

  • Opioid: $MOR$.

  • Cannabinoid: $CB_1$.

  • Acetylcholine: $VACHT$, $M_1$.

  • GABA: $GABA_A/BZ$.

  • Glutamate: $mGluR_5$, $NMDA$.

• Functional Magnetic Resonance Imaging (fMRI): Imaging technique with high resolution to assess blood flow changes (activation maps) during tasks like imagined force production.

• Other Methods: CT scans, Transcranial Electrical Stimulation, and Deep Brain Stimulation.

Transcranial Magnetic Stimulation (TMS)

TMS uses a magnetic coil to induce a magnetic field over the scalp, which generates an electrical current in the brain to non-invasively activate neurons.

• Motor Evoked Potential (MEP): The synchronous firing of corticospinal neurons detectable by EMG. MEPs reflect the excitability of the entire corticospinal pathway.

• Experimental/Clinical Use:

  • Clinical: Repetitive TMS ($rTMS$) can cause neuroplastic changes to address degeneration or damage; used for presurgical mapping of speech and motor functions.

  • Scientific Findings: Larger MEPs are observed during stronger voluntary contractions and higher stimulation intensities, indicating increased corticospinal activity and recruitment.

The Pyramidal Tracts

These tracts originate in cortical areas and travel through the internal capsule to the medulla oblongata (medullary pyramids):

• Corticobulbar Tract: Projects from premotor and primary motor areas to the brainstem. It decussates at the medulla and activates cranial nerves to control the neck (sternocleidomastoid/trapezius), tongue, mouth (chewing/speaking), and facial expressions.

• Corticospinal Tract (CST): The main output system of the CNS, comprising ~$1\,\text{million}$ axons (with ~$40\%$ from the motor cortex). It includes monosynaptic and indirect interneuron connections.

  • Lateral CST: ~$90\%$ of fibers cross (decussate) at the medulla. Controls extremity muscles (contralateral control).

  • Anterior CST: ~$10\%$ of neurons that do not decussate; controls axial and trunk muscles.

• Sensory Influence: The CST requires continuous tactile, visual, and proprioceptive inputs. Output is regulated by both the primary motor cortex and premotor/somatosensory areas.

Neurotransmitters and Post-Synaptic Potentials

• Glutamate: The primary excitatory neurotransmitter. It produces Excitatory Post-Synaptic Potentials (EPSP), which depolarize the neuron (making the membrane more positive) and increase the likelihood of an action potential.

• Gamma-amino butyric acid (GABA): The primary inhibitory neurotransmitter. It produces Inhibitory Post-Synaptic Potentials (IPSP), which hyperpolarize the neuron (making the membrane more negative) and decrease the likelihood of an action potential.

Electromyography (EMG) and Evoked Potentials

• EMG: Measures electrical activity of nerves and muscles to determine a person's ability to activate muscles and produce force. It can infer muscle weakness, motor unit activation, or tremors.

• M-wave (Motor wave): An evoked potential generated by electrical stimulation of peripheral nerve fibers. It reflects the excitability (responsiveness) of the nerve.

Case Study: Spinal Cord Injury (SCI)

Research by Bunday et al. (2014) investigated subcortical control after human spinal cord injury:

• Comparisons were made between controls and SCI patients (with/without Baclofen) during Index Finger Abduction and Precision Grip.

• In SCI patients, the slope of linear regression between MEP size and TMS intensity was utilized to assess the integrity and recruitment of the pathways.

Practical Application and Cognitive Check

• Diagnostic Inference: If a patient has a reduced ability to produce force in their right leg, the damage is most likely located in the Lateral Corticospinal Tract (given that it controls extremities and crosses to the contralateral side).