General Description of Activation

Activation refers to cortical desynchronization.
Arousal has referred to behavioral processes (Feldman & Waller, 1962).

Anatomy of Reticular Formation

• Neurons forming meshwork - spinal cord to thalamus.
  • Reticulated lattice-work of neurons in ventral core of brain stem.
  • Reticulated areas extend from the lower spinal cord to the diencephalon, but the area from the brain stem to the thalamus is called the reticular formation proper
• Neurons are neither sensory nor motor
• Phylogenetically old

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Two Types of Reticular Formation: Ascending and Descending Reticular Activating Systems

Ascending Reticular Activating System (ARAS)

• Fibers in brain stem which receive collaterals from sensory fibers projecting to thalamus
• Fibers relay this information to the archi-thalamic nuclei, and project this information to the cortex in a diffuse manner
• Reticular formation responds to stimuli from all sensory systems
• Effect of different senses is weighted as to activating properties.

Descending Reticular Activating System (DRAS)

• Receives input from sensory collaterals, cerebellum and vestibular system and cortex
• Influences the motor system by sending information to the motor neuron pool via the reticulospinal tract
• Tonic influence on motorneurons Gamma Efferents (tone)
• Provides background upon which pyramidal systems play
• Influences receptors (Granit, 1955).

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Function of Reticular Formation (electrophysiological data)

• Allen (1932) RF plays a role in general inhibition and excitation
• Single unit recording -- multibranched fibers channel impulses from number of different sensory systems to a single neuron (convergence).
• Permits integrative activity ascribed to the system as a whole
• Overlap of sensory collaterals is so great that specificity of input cannot be maintained
• Cells respond to several different sensory systems but not all

• Research on Physiology of Activation
  • Bremer (1935) midbrain transection (cerveau isole); sleep-like EEG
  • Moruzzi and Magoun (1949) stimulation immediate or long-lasting arousal
  • Lindsley et. al. (1949-50) RF lesions -- sleep
  • French et. al. (1952)
    • Neurons in RF respond to all sensory stimulation
    • Reciprocal connections with cortex

Other Functions of the Reticular Formation

• Reflexive control of visceral responses (GI secretions, vasomotor tone, respiration)
• Habituation and arousal
• ARAS exerts tonic inhibitory influence on sensory input

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Reticular Influence on Sensory Input

Sensory input is distorted in awake animals (Evoked Potential -- reduced amplitude). Effects appear to be integrated by RF

• Evoked potential (EP) is smaller when awake.
• RF is not as active when asleep
• Therefore RF must reduce amplitude of recorded EP but add what is necessary for perception

Livingston (1959) suggests that sensory input alone is not sufficient for perception

• Needs to be integrated with other cortical activity
• This is facilitated by activation of cortex by RF
• RF function necessary for perception

RF can influence sensory input at several points:

• Receptor -- influences receptor sensitivity
• In sensory nucleus of thalamus
• At cortex

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Recent Behavioral Research

1. Adametz (1959): 15 cats with 1 & 2 stage lesions
   • 1-stage -- all but one died; attention unimpaired
   • 2-stage -- remained awake, no serious defects

2. Doty, Beck, & Kooi (1959) similar; good recovery

3. Feldman and Waller (1962)
   • Bilateral RF lesions -- sluggish behavior; all survived
   • Bilateral PH lesions -- Ss in coma
   • Cortical desynchronization, not behavioral arousal with posterior hypothalamic lesions. (The reverse was true for RF lesions).

4. Fuster (1958): discrimination facilitated and shorter reaction time when RF is stimulated (attention)

5. Delgato, Roberts, & Miller, 1954 - punishment - inverted U. (RF stimulation doesn't always improve discrimination.)

6. Imamura and Kawamura (1962) demonstrated that posterior hypothalamus activated limbic system structures (emotion) but not cortex, whereas RF activated cortex, not limbic system.

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Pharmacological Properties of ARAS

• Sensitive to adrenalin
• Respiratory center in RF (CO2 sensitive)
• Bradley, 1958
  • Barbiturates depress
  • Amphetamines excite
  • Cholenergic drugs - physiostigmine
  • Chldoropromazine - collaterals

Terms to Know