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)

Clinical note

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
Sub callosal, cingulated and parahippocampal gyri Hippocampal formation Amygdaloid nucleus Mammillary bodies Anterior thalamic nucleus	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
Afferent	Efferent	Afferent	Efferent
CS: CorticostriateTS: Thalamostriate NS: Nigrostriate BS: Brainstem striatal fibers	SP: Striatopallidalfibers SN: Striatonigral fibers	SP: Striatopallidalfibers	Pallidofugalfibers

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).

Functional connections of the Basal Nuclei and how they influence muscle activities

 

REFERENCES: 

1. Ben Greenstein, Ph.D, Adam Greenstein, BSc (Hons) Mb, ChB Color Atlas of Neuroscience

2. Allan Siegel Ph.D, Hreday N. Sapru Ph.D Essential Neuroscience, 1st Edition

3. Stanley Jacobson, Elliot M. Marcus Neuroanatomy for the Neuroscientist

4. Patrick f. Chinnery Neuroscience for Neurologists

5. Dale Purves Neuroscience, 3rd Edition

6. Suzan Standring Gray’s Anatomy

7. Keith L. Moore, Arthur F. Dalley, Anne M. R. Agur Clinically Oriented Anatomy

8. Frank H. Netter Atlas of Human Anatomy

9. Walter J. Hendelman, M.D., C.M. Atlas of Functional Neuroanatomy

10. Mark F. Bear, Barry W. Connors, Michael A. Paradiso Neuroscience Exploring the Brain

11. Dale Purves et al. Principles of Cognitive Neuroscience

12. Eric R. Kandel et al. Principles of Neural Science