Advanced Brainstem Pathways and Motor Control of Control of Locomotion and Eye Movements

The Reticular Formation and the Reticulospinal Pathways

The reticular formation is a complex meshwork of neurons and fibers present along the entire length of the brainstem, specifically located in the middle part. This system contains regions of functional significance, alternatively referred to as nuclear regions or nuclei.

Major motor centers within the reticular formation house upper motor neurons that project down to lower motor neurons in the spinal cord. These pathways are primarily involved in the control of:

• Locomotion
• Exploratory behavior
• Escape responses

These reticulospinal neurons receive various inputs from higher brain centers, including the cerebral cortex, the cerebellum, and the basal ganglia. Their outputs project to the central pattern generating (CPG) circuits situated in the spinal cord.

Major Nuclear Regions of the Reticular Formation

Mesencephalic Locomotor Region (MLR)

This is a heterogeneous region containing several nuclei located in the reticular formation of the midbrain. While not always easily illustrated, it is a critical region for movement coordination:

• Walking Initiation: Stimulation of this specific region causes an animal to begin walking.
• Gait Control: This region is capable of driving various locomotor gaits, such as the walk, trot, bound, and gallop.
• Speed Management: It is directly involved in controlling the speed of locomotion.

Pontine Reticular Nuclei

Located in the pontine region (pons) of the reticular formation, these neurons give rise to the pontine reticulospinal tract, also known as the medial reticulospinal tract.

• Antigravity Mechanism: This tract enhances the antigravity reflex mechanisms of the spinal cord.
• Postural Maintenance: It facilitates the ability of alpha motor neurons that innervate extensor muscles to resist gravity, allowing for the maintenance of a standing posture.

Medullary Reticular Nuclei

Situated lower in the brainstem within the medulla region of the reticular formation, these neurons give rise to the medullary reticulospinal tract, also called the lateral reticulospinal tract.

• Opposing Function: When activated, these neurons have the opposite effect of the pontine nuclei on alpha motor neurons.
• Dynamic Movement: They release extensors (which resist gravity) from reflex control, thereby enabling the performance of dynamic movements.
• Integrated Locomotion: The integration of the MLR, pontine, and medullary processes in the spinal circuits eventually produces appropriate locomotor movements.

Visual Sensory Processing and the Tectospinal Tract

Visual processing involves the transduction of light into membrane potential changes via photoreceptors in the neuronal retina. This process utilizes G protein-coupled receptor activation mechanisms.

Visual Pathway Overview

• Retinal Output: Membrane potential changes between neurons are transmitted to ganglion cells.
• Thalamic Relay: The majority of ganglion cell axons end in the lateral geniculate nucleus (LGN) of the thalamus.
• Cortical Perception: Neurons from the LGN project to the visual striate cortex in the occipital lobe, where visual perception occurs.

The Superior Colliculus

This brainstem region serves as a major visual-motor integration center. Located in the midbrain, it consists of four small structures referred to as the colliculi (or "little hills"), which form the tectum (roof) of the midbrain. The superior colliculus has two specific parts:

• Superficial Layers: Contain neurons that receive visual information from the eyes and maintain a map of the visual environment.
• Deeper Layers: Contain motor-related neurons involved in eye and neck movements.

The Tectospinal Tract

Neurons from the deeper layers of the superior colliculus project axons into the tectospinal tract with the following targets:

• Cervical Spinal Motor Neurons: These axons synapse with contralateral motor neurons in the ventral horn to control neck muscles, allowing for head movement.
• Extraocular Muscles: Other neurons project to the alpha motor neurons of extraocular muscles via gait centers to control eye movement.

Orienting Reflexes and Survival

The superior colliculus is responsible for orienting reflexes. These are primitive, automatic behavioral patterns that integrate eye and neck movements in response to sudden stimuli.

• Mechanism: A sudden bright visual input, such as a flash of light, causes an animal or human to orient the head and eyes toward the stimulus.
• Consciousness: These reflexes occur without conscious involvement and are faster than voluntary movements.
• Function: They are critical for animal survival, allowing for rapid response to environmental changes.

Classification of Eye Movements

There are $5$ major types of eye movements classified by their function in keeping images stable on the fovea, the central region of the eye responsible for the highest visual acuity.

Movements for Shifting Gaze

• Saccades: Very fast movements used to explore an environment or object. When saccades stop, the system enters a state of fixation where fast movement is actively inhibited.
• Smooth Pursuit: A voluntary movement where the eyes track a moving object entering the visual field.
• Vergence Movements: These are unconjugated movements (where eyes do not move in parallel). The eyes converge (move toward each other) when looking at close objects and diverge when looking at distant objects, ensuring the image remains projected on the same place in both retinas.

Movements for Stabilizing Gaze

• Vestibulo-Ocular Reflex (VOR): This reflex is driven by vestibular stimuli rather than visual ones. It responds to rapid head movements in any direction to keep eyes still in space. It is exceptionally fast because it travels from vestibular afferents via vestibular nuclei directly to extraocular motor neurons. This reflex functions even when the eyes are closed.
• Optokinetic Reflex (OKR): This is driven by visual stimuli and is sensitive to large, slow-moving stimuli. It involves a slow movement of the eyes following a stimulus, followed by a fast back-movement once the eyes hit the side of the orbits. It involves a long cortical neuronal loop (Retinal ganglion cells $\rightarrow$ visual cortex $\rightarrow$ parietal cortex $\rightarrow$ frontal cortex $\rightarrow$ brainstem $\rightarrow$ extraoculomotor neurons) and is therefore slower than the VOR.

Neural Control of Conjugated Eye Movements

Conjugated eye movements occur when the eyes move in parallel. The direction of movement is determined by two specific brainstem gait centers:

• Paramedian Pontine Reticular Formation (PPRF): Controls horizontal movements via the medial and lateral rectus muscles.
• Midbrain Center: Controls vertical movements.

Circuitry for Horizontal Movement (Example: Leftward Movement)

• Neurons in the left PPRF fire synchronously.
• They activate the left abducens nucleus, which stimulates the lateral rectus muscle of the left eye.
• Simultaneously, interneurons transmit information to the right oculomotor nucleus, which activates the medial rectus muscle of the right eye.
• To facilitate this, the left PPRF projects via inhibitory local circuit neurons to the contralateral abducens nucleus and the ipsilateral oculomotor nucleus to inhibit antagonistic muscles.

Hierarchical Control

• Reflexive Saccades: The superior colliculus receives visual information and passes it to the motor neuronal layers, which then signal the gait centers (e.g., PPRF).
• Voluntary Saccades: The cerebral cortex, specifically the frontal eye field in the frontal cortex, initiates the movement plan. This signal is sent to the superior colliculus, then to the gait centers, and finally to the extraocular motor neurons.

Summary of Descending Pathways and Vestibular Function

The Vestibular System

• Function: Provides balance and a sense of self-motion by detecting head position.
• Utricle and Saccule: Sense the direction of gravity and linear acceleration.
• Semicircular Canals: Sensors for angular acceleration to detect head rotation.

Descending Motor Pathways

The brain communicates with spinal cord motor neurons through two major groups:

1. Lateral Pathways: Includes the lateral corticospinal and rubrospinal tracts.
2. Ventromedial Pathways: Includes $4$ specific tracts:
   • Vestibulospinal Tracts: Control neck, back, and leg muscles to guide head movement while keeping the body upright against gravity.
   • Pontine Reticulospinal Tract: Maintains standing posture by enhancing antigravity reflexes in leg extensors.
   • Medullary Reticulospinal Tract: Liberates muscles from reflex control to allow for movement.
   • Tectospinal Tract: Originates in the midbrain; controls eye and neck muscles during orienting reflexes.