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Questions and Answers
What is a primary function of the reticular activating system (RAS)?
What is a primary function of the reticular activating system (RAS)?
Which of the following factors is likely to increase RAS activity?
Which of the following factors is likely to increase RAS activity?
What distinguishes the pontine reticular system from the medullary reticular system?
What distinguishes the pontine reticular system from the medullary reticular system?
Which neurotransmitters are noted for producing an alerting response in the RAS?
Which neurotransmitters are noted for producing an alerting response in the RAS?
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What role does the pyramidal tract primarily serve?
What role does the pyramidal tract primarily serve?
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What is the impact of descending impulses from the cerebral cortex on the RAS?
What is the impact of descending impulses from the cerebral cortex on the RAS?
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What characterizes the medullary reticular system?
What characterizes the medullary reticular system?
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Which of the following statements about the brain stem is accurate?
Which of the following statements about the brain stem is accurate?
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Study Notes
Neurophysiology Lecture 6
- The brainstem is a complex extension of the spinal cord, controlling sensory, motor, and reflex functions. It also regulates cardiovascular, respiratory, and gastrointestinal functions.
- The brainstem plays a major role in controlling eye movements and maintaining posture against gravity.
The Arousal Mechanism (Reticular Formation)
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The reticular formation is a large structure within the brainstem.
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Functional divisions include the pontine reticular system ("facilitatory RF") and medullary reticular system ("inhibitory RF").
- The pontine system has ascending branches, part of the Reticular Activating System (RAS), controlling brain activity, consciousness, and alertness. It also has descending branches affecting gamma motor neurons.
- The medullary system's descending branches (lateral reticulospinal tract) inhibit gamma motor neurons.
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Factors influencing RAS activity:
- Sensory inputs (pain, proprioception) can arouse a person.
- Descending impulses from the cerebral cortex (emotions, voluntary actions) stimulate alertness.
- Epinephrine and norepinephrine from the adrenal medulla also stimulate alertness.
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Factors decreasing RAS activity:
- Impulses from sleep centers.
- Brain stem damage (e.g., vascular lesions, poisons, tumors, hypoxia).
- Drugs like barbiturates, which hyperpolarize neurons.
The Motor Cortex
- The motor cortex sends signals through descending tracts (pyramidal and extrapyramidal).
- The pyramidal tract (corticospinal tract):
- Controls complex, fine motor skills (e.g. fingers, toes, face).
- Influences muscle tone and deep reflexes.
- The extrapyramidal system:
- Maintains body position for fine movements.
- Influences muscle tone (some fibres are facilitatory, others inhibitory).
Upper and Lower Motor Neuron Lesions
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Upper Motor Neuron Lesion (UMNL):
- Cause: Often cerebrovascular accidents (strokes).
- Characterstics: Contralateral paralysis, hypertonia (spasticity), exaggerated reflexes (e.g., positive Babinski's sign).
- Additional Notes: Damage to both pyramidal and extrapyramidal fibers.
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Lower Motor Neuron Lesion (LMNL):
- Cause: Lesions in lower motor neurons, such as in poliomyelitis or nerve damage.
- Characterstics: Ipsilateral paralysis, hypotonia or atonia, absent or reduced reflexes, atrophy, and fasciculations.
Spasticity
- Spasticity results from an overactive facilitatory reticulospinal tract (VST).
- This creates an exaggerated stretch reflex that is difficult to override using voluntary efforts.
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Description
This quiz covers key concepts from Neurophysiology Lecture 6, focusing on the brainstem and the reticular formation. Explore how these structures regulate vital functions such as sensory processing, motor control, and consciousness. Understand the distinctions between the facilitatory and inhibitory roles of the reticular systems.
