Model Answer
0 min readIntroduction
The central nervous system controls movement through a complex interplay of descending pathways originating in the brain. These pathways, often referred to as descending tracts, transmit motor commands to the spinal cord, ultimately influencing muscle activity. Understanding these tracts and the consequences of their disruption is crucial in clinical neurology. The lateral descending tracts are particularly important for controlling voluntary movements, especially of the distal limbs. Lesions to these tracts result in characteristic motor deficits, and understanding the specific deficits associated with each tract is vital for accurate neurological diagnosis.
Lateral Descending Tracts
The major lateral descending tracts are:
- Corticospinal Tract: This is the most important pathway for voluntary movement, particularly of the distal extremities. It originates from the cerebral cortex (primarily the motor cortex) and descends through the brainstem, crossing (decussating) in the medulla oblongata.
- Rubrospinal Tract: Originating in the red nucleus of the midbrain, this tract plays a role in motor coordination and control of limb movements, particularly flexion. It is more prominent in animals than in humans.
- Reticulospinal Tract: Originating in the reticular formation of the brainstem, this tract influences posture, muscle tone, and reflexes. It has both a lateral and medial component.
- Tectospinal Tract: Originating in the superior and inferior colliculi of the midbrain, this tract mediates reflexive movements of the head and eyes in response to visual and auditory stimuli.
- Vestibulospinal Tract: Originating in the vestibular nuclei of the brainstem, this tract maintains balance and posture by adjusting muscle tone and coordinating movements.
Effects of Lesions to Lateral Descending Tracts
The effects of lesions vary depending on the specific tract involved:
| Tract | Effects of Lesion |
|---|---|
| Corticospinal Tract | Ipsilateral hemiparesis/hemiplegia: Weakness or paralysis on the same side of the body as the lesion. More pronounced weakness in distal limb muscles. Spasticity: Increased muscle tone. Hyperreflexia: Exaggerated reflexes. Positive Babinski sign: Upward extension of the big toe upon plantar stimulation. |
| Rubrospinal Tract | Ipsilateral weakness: Primarily affecting flexor muscles. Clumsiness: Difficulty with coordinated movements. Less significant impact in humans compared to other animals. |
| Reticulospinal Tract | Ipsilateral weakness: Affecting posture and muscle tone. Difficulty with balance: Impaired ability to maintain equilibrium. Changes in muscle tone: Either increased or decreased depending on the specific component affected. |
| Tectospinal Tract | Impaired reflexive head and eye movements: Difficulty turning the head and eyes in response to visual or auditory stimuli. |
| Vestibulospinal Tract | Ipsilateral ataxia: Difficulty with coordination and balance. Postural instability: Difficulty maintaining an upright posture. Nystagmus: Involuntary eye movements. |
Sensory Loss in Lesion Around Central Canal of Spinal Cord
A lesion around the central canal of the spinal cord primarily affects the ascending sensory tracts. The central canal contains cerebrospinal fluid (CSF). Damage to this area typically impacts the spinothalamic tracts (pain and temperature) and the posterior columns (proprioception, vibration, and fine touch).
- Spinothalamic Tract Damage: Results in contralateral loss of pain and temperature sensation below the level of the lesion. This is because the fibers cross over in the spinal cord before ascending to the thalamus.
- Posterior Column Damage: Results in ipsilateral loss of proprioception, vibration, and fine touch below the level of the lesion. These fibers ascend ipsilaterally before crossing over in the medulla.
Therefore, a lesion around the central canal will cause a dissociated sensory loss – loss of pain and temperature on one side and loss of proprioception, vibration, and fine touch on the other side below the level of the lesion.
Conclusion
In conclusion, the lateral descending tracts are crucial for motor control, and their disruption leads to predictable patterns of weakness, spasticity, and reflex changes. Lesions around the central canal of the spinal cord primarily affect ascending sensory pathways, resulting in a characteristic dissociated sensory loss. A thorough understanding of these tracts and their clinical manifestations is essential for accurate neurological diagnosis and management. Further research into neuroplasticity and rehabilitation strategies continues to improve outcomes for patients with lesions affecting these pathways.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.