C J Long
Overview of Topics
Activation & RF
Function of RF
Reticular Influence on Sensory Input
- Describe the difference in behavior following lesions in the reticular system and those in the posterior hypothalamus.
- How does the reticular system function to cause cortical activation (desynchronization)?
- Explain what Livingston means by the statement that the reticular system is necessary for perception.
- Explain what the statement "stimuli are weighted with regard to their activating influence" means.
- Explain how the DRAS influence receptor sensitivity and muscle tone.
- Discuss the significance of the research by Moruzzi & Magoun (1949) and Lindsley et.al.(1950).
General Description of Activation
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).
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
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
Recent Behavioral Research
- Adametz (1959): 15 cats with 1 & 2 stage lesions
- 1-stage -- all but one died; attention unimpaired
- 2-stage -- remained awake, no serious defects
- Doty, Beck, & Kooi (1959) similar; good recovery
- 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).
- Fuster (1958): discrimination facilitated and shorter reaction time when RF is stimulated (attention)
- Delgato, Roberts, & Miller, 1954 - punishment - inverted
U. (RF stimulation doesn't always improve discrimination.)
- Imamura and Kawamura (1962) demonstrated that posterior hypothalamus activated limbic system structures (emotion) but not cortex, whereas RF activated cortex, not limbic system.
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
|Reticular formation proper