The Reticular Formation, Limbic System and Basal Ganglia
The Reticular Formation
It’s a ‘diffuse net’ which is formed by nerve cells and fibers. It extends from the neuroaxis spinal cord through medulla, pons, midbrain, subthalamus, hypothalamus and thalamus (spinal cord is relayed superiorly to the cerebral cortex).
Many afferent and efferent pathways project in and out of the RF from most parts of the CNS. The main pathways through the RF is poorly defined and difficult to trace using silver stains. Reticular formation can be divided into three columns : median, medial and lateral columns.
Functions of the Reticular formation
1. Control of skeletal muscles:
- RF modulates muscle tone and reflex activities (via reticulospinal and reticulo bulbar tracts). It is important in controlling muscles of facial expression when associated with emotions.
2. Control somatic and visceral sensation (influence can be excitatory or inhibitory)
3. Control of autonomic nervous system
4. Control of endocrine nervous system (hypothalamus and the pituitary)
5. Influence on the biological clock (rhythm)
6. The reticular activating system (arousal and level of consciousness are controlled by the RF)
When a person smiles for a joke, the motor control is provided by the RF on both side of the brain. The fibers from RF is separated from corticobulbar pathway (supply for facial muscles). If a patient suffers a stroke that involves corticobulbar fibers, he or she has facial paralysis on the lower part of the face, but is still able to smile symmetrically.
The Limbic System
|Limbic structures||Functions of the limbic system|
||1. Influence the emotional behavior:a. Reaction to fear and angerb. Emotions associated with sexual behavior
2. Hippocampus is involved in converting short term memory to long term memory (If the hippocampus is damaged, patient is unable to store long term memory – Anterograde amnesia)
The Basal Ganglia and their connections
Connections of the Basal Ganglia
Yellow arrow : Pallidofugal fibers
Caudate nucleus and the Putamen: main sites of receiving inputs
Globus pallidus: main site from which output leaves
Afferent and Efferent fibers
|Connections of the caudate nucleus and Putamen||Connections of the Globus pallidus|
|CS: CorticostriateTS: Thalamostriate
BS: Brainstem striatal fibers
SN: Striatonigral fibers
Functions of the Basal Nuclei
Basal Nuclei controls muscular movements by influencing the cerebral cortex (it doesn’t have direct control through descending pathways to the brainstem and spinal cord). It helps to prepare for the movements (enables the trunk and limbs to be placed in appropriate positions before discrete movements of the hands and feet).