Brainstem Motor Function: An Overview

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Motor Function - Physiology | Mind Map

• Brainstem coordination and spinal cord coordination are key components.

Brainstem Coordination

• Includes the pons and medulla oblongata.
• Extension of the spinal cord into the cranial cavity.
• Similar to how the spinal cord controls motor and sensory function, the brainstem has these same responsibilities, but in the cranial region.
• Control of respiration by regulating breathing rate and rhythm.
• The brainstem controls cardiovascular circulation via blood pressure.
• Partial control of gastrointestinal functions, influencing digestion and peristalsis.
• It exerts control over many stereotyped motor patterns that are repetitive.
• Integrates sensory input from the vestibular system for equilibrium and balance.
• Eye movement control is managed by the vestibular system, such as gaze control.

Brainstem Motor Tracts

• The primary motor cortex signals are voluntarily sent to the brainstem and relayed to the spinal cord to impact motor function.
• Motor tract origins:
  • Red nucleus (midbrain) is the origin of the rubrospinal tract.
  • Pontine reticular formation of the pons is the origin of the pontine reticulospinal tract.
  • Medullary reticular formation of the medulla oblongata is the origin of the medullary reticulospinal tract.
  • Vestibular nuclei in the medulla oblongata are the origin of the vestibulospinal tract.

Major Motor Tracts

• Rubrospinal Tract:
  • Originates from the red nucleus in the midbrain.
  • Plays a role in coordinating movements.
  • These fibers synapse in the spinal cord.
  • Direct input from the cerebellum allows for faster reflexive control of movements.
  • Fibers decussate (cross over) in midbrain.
  • Located lateral and slightly anterior to the corticospinal tract in the spinal cord.
• Specific Functions (Cortiocspinal vs Rubrospinal)
  • While the rubrospinal assists the corticospinal, it is especially prominent or more important in development.
  • The spinocerebellum receives motor information.
  • The cerebellum receives input from the red nucleus, similar to those of the motor cortex, allowing for fine-tuning and coordination of movements.
• The pontine (medial) reticulospinal Tract:
  • Origin and Pathway: Originates in the pontine nuclei of the pons; descends through the ventral column (postural muscles of the limbs) of the spinal cord.
  • Innervates extensor muscles of the limbs.
  • The pontine works with the vestibular and cerebellar systems.
• The medullary (lateral) reticulospinal Tract:
  • Origin and Pathway: Originates in the medullary reticular formation of the medulla oblongata and descends in the ventral column of the spinal cord.
  • The tract contains both ipsilateral and contralateral fibers
  • Opposes the pontine reticulospinal tract by inhibiting extensor muscles and exciting flexor muscles.
  • Functional Importance: Balances the effects of the pontine reticulospinal tract for smooth postural control.
• Overview of Vestibulospinal Tract
  • Tracts originate from vestibular nuclei in the medulla oblongata that gets visual input and controls equilibrium.
  • Excites extensor muscles and inhibits flexor muscles.
  • Works in close association with signals to the antigravity muscles (muscles that maintain posture while standing).
  • The Lateral Vestibulospinal Tract controls limb extensors.
    • Originates from the lateral vestibular nucleus is in the medulla.
    • Descends ipsilaterally in "ventral white matter" of spinal cord.
    • Selectively controls excitatory signals to different antigravity muscles to maintain balance during standing in response to changes in head position
  • Both pontine and medullary reticulospinal tracts primarily control axial muscles of the trunks (postural and "gross" movements).
• Vestibular Nuclei
  • The vestibular nuclear complex consists of four subnuclei that integrate sensory signals for balance in the brainstem.
  • The medial vestibular nuclei receive input from the semi-circular canals.
  • The lateral vestibular nuclei receive input from the otolith organs.
• Role of the Vestibular System in Motor Control
  • Head position relative to gravity.
  • Head acceleration through space.
  • The vestibular system integrates information from the cerebellum, making it essential for motor coordination.
  • Unlike other motor control centers, the vestibular nuclei do not receive direct corticospinal projections (i.e., no descending voluntary signals are sent)

Vestibular Apparatus

• Located within a bony labyrinth within the temporal bone.
• Cochlea for hearing
• Otolith Organs - two chambers.
  • Detect linear acceleration and head tilt
• Semicircular Canals -Detect angular acceleration (rotation).
• Components:
  • Static tilt (e.g., holding the head at an angle)
  • Rotational acceleration (e.g., turning the head)
• Function:
  • Constantly sends signals to the oculomotor brainstem and cerebellum.
  • Stabilizes vision, ensuring that the eyes move in coordination with the head.
  • Pathways for vision: Oculomotor
  • Pathways for balance control: Cerebellum/Vestibulospinal tract.
• Membranous Labyrinth
  • Filled with endolymph, containing hair cells embedded in a gelatinous matrix.
  • Movement of endolymph causes fluid displacement, which bends the stereocilia (hair), either causing
• Depolarization
• Hyperpolarization
  • Sends signals to the CNS. The neural pathway begins at the vestibular ganglion.

Spinal Cord Coordination

• Spinal cord reflexes are not just for reflexes. They can also be affected by the central nervous system.

The Grey Matter as an Integrator

• Sensory Signal Entry and Processing
  • Sensory fibers enter the dorsal horn, forming ascending pathways to higher centers (e.g., thalamus, cerebral cortex), and affect primary motor neurons
  • When a muscle is stretched, the muscle spindle activates; also generating inhibitory signals toward antagonist muscles.

Muscle Spindle

• Now the spinal cord - Muscle Spindle
  • Also referred to as "intrafusial fibers".
  • Directly innervate to receptor located inside skeletal muscle.
  • It consists of 3 to 12 specialized muscle fibers.
• The larger muscle fibers outside the spindle are insulfusial fibers.
• Structure: -A proprioceptive sensory receptor that detects changes in muscle length.
• Function:
  • Transmits signals to the spinal cord via 2 types of afferent sensory fibers.
  • When the muscle is not stretched then there is baseline firing rate.
  • Muscle Stretch → Generating afferent impulses that travel to spinal cord.
  • Sensory fibers synapse directly with alpha motor neurons of the same muscle → Quadriceps Contract
  • The stretch reflex mechanism includes muscle stretch and contraction.
  • The importance of the stretch reflex is to increase muscle tone, counteract gravity, and maintain posture
• Role of Grey Matter
  • Example of the patellar tendon reflex (knee-jerk reaction): Tapping the patellar tendon stretches the quadriceps muscle, activating its muscle spindle. This activation causes the quadriceps, therefore raising the leg

Spinal Cord Coordination

• Key characteristics of the Golgi Tendon Organ (GTO)
  • Does not result in rapid reflexes like those in the brain.
• Structure:
  • Encapsulated sensory receptor located within tendons near the muscle-tendon junction.
  • Active contractions stimulate the Golgi Tendon Organ (GTO)
  • Passive stretching of the muscle whether due to internal or external forces will also stimulate contraction.
  • Protect Muscle and Bone:
  • When muscle tension is too high, afferent signals from the GTO excite inhibitory interneurons in the spinal cord to promote muscle relaxation.
• Functional Role of the Golgi Tendon Reflex
  • Detects Muscle contraction force when information from GTO enters dorsal horn and is integrated in spinal cord.
  • Integrated with info from muscle spindle.
  • Spinal cord also acts as a pattern generator and motor control center.
• Difference
  • Muscle spindle signals when stretched, whereas GTO signals strength of contraction
  • The spinocerebellar projects copy the information from the spinal cord.
    • Anterior carries internally (muscle spindle)
    • Posterior carries from external (GTO)
  • The dorsal column-medial lemniscus pathway signals touch (Messner's/pacinian corpuscles) and pain (free nerve endings)

Flexor & Crossed Extension Reflex

• A painful stimulus activates nociceptors, sending afferent signals to the spinal cord resulting in the Flexor (Withdrawal Reflex).
  • Excitation of ipsalateral flexor muscles leads to the withdrawal of the pained limb.
  • Inhibition of ipsilateral extensor muscles ensures the limb can flex and retract.
  • Stimulation of contralateral extensor muscles.
  • Inhibition of contralateral flexor muscles.
• Crossed Extensor Reflex:
  • Supports the body when the opposite limb is withdrawn because the limb feels a sudden increase in load
• Functional Importance from Step-on-a-Tack Reflex Example
  • Activate flexors to lift foot + other muscles provide balance.
• Major descending tracts
  • Lateral Tract:
    • Lateral Corticospinal (pyramidal)
    • Rubrospinal
  • Anterior/Ventral tract:
    • Anterior Corticospinal (pyramidal)
    • Vestibulospinal
    • Reticulospinal
    • Tectospinal

Motor Function Coordination-Descending Tracts

• Functional Differences in Tracts:
  • Lateral tracts
    • (e.g., corticospinal, rubrospinal) primarily control distal muscles for precise and voluntary movements.
  • Ventral tracts
    • (e.g., pontine reticulospinal, vestibulospinal) primarily control axial muscles (postural and gross motor control)
  • One exception is the medullary reticulospinal tract, which inhibits limb and axial muscles more than distal limb muscles.