Neurophysiology Lecture 6

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)

• The reticular formation is a large structure within the brainstem.

• 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.

• 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.

• 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

• 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.
• 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.