Brain Stem Control of Movement.
The brain stem plays a critical role in controlling movement, regulating muscle tone, and maintaining posture. It houses several nuclei and tracts that relay motor commands from higher brain centers (like the motor cortex) to the spinal cord, and also integrates sensory feedback to fine-tune movements.
- Descending Tracts: The brain stem gives rise to descending motor tracts that project to the spinal cord and influence lower motor neurons.
- Ventromedial Pathway: Controls axial muscles (trunk and head), important for posture and balance. Includes the pontine and medullary reticulospinal tracts.
- Dorsolateral Pathway: Controls distal muscles (limbs), involved in more skilled and voluntary movements. Includes the rubrospinal tract.
- Muscle Tone Regulation: Muscle tone is the baseline level of tension in a muscle at rest. The brain stem, particularly the reticular formation composed of 2 tracts that feed alpha & gamma motoneurons that are responsible for posture and muscle tone, plays a key role in modulating muscle tone.
- Medial Reticulospinal Tract: Excites axial and extensor motor neurons, increasing muscle tone and contributing to antigravity support.
- Lateral Reticulospinal Tract: Inhibits extensor motor neurons and excites flexor motor neurons, modulating muscle tone and facilitating movement.
- Posture Regulation: Maintaining an upright posture requires the integration of sensory information and coordinated muscle activity. The brain stem integrates inputs from various sources to achieve this:
- Vestibular System: Provides information about head position and movement in space.(3 semi-circular canals, utricle & saccule).
- Proprioceptors: Sensory receptors in muscles and joints that relay information about body position. (Rreceptors for knowing the body’s own movement).
- Visual System: Contributes to spatial awareness and orientation.
- Postural Reflexes: The brain stem coordinates a variety of postural reflexes, which are automatic, stereotyped responses to maintain balance and stability in response to changes in body position.
Pyramidal Tracts
The corticospinal and corticobulbar tracts are pyramidal pathways that descend through the pyramids of the medulla and directly connect with lower motor neurons, playing a vital role in regulating posture and voluntary movement:
- Ventromedial Tract:
- Involved in controlling axial muscles, such as those in the trunk and head.
- Supports tonic movements that help maintain posture.
- Dorsolateral Tract:
- Governs distal muscles, like those in the arms and legs.
- Responsible for phasic movements, enabling fine and precise actions.
Extrapyramidal Tracts
These tracts originate outside the pyramids and are involved in involuntary control of posture and reflexes:
- Rubrospinal Tract:
- Starts in the red nucleus (nucleus ruber) and projects to the lateral spinal cord.