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The Reticular Formation, Limbic System and Basal Ganglia

February 19, 2012 2 comments

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 
  1. Sub callosal, cingulated and parahippocampal gyri
  2. Hippocampal formation
  3. Amygdaloid nucleus
  4. Mammillary bodies
  5. 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
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Ascending Tracts

February 10, 2012 Leave a comment

Introduction

They are located in the white matter and conduct afferent information (may or may not reach consciousness). There are two types of information:

  1. Exteroceptive : originates from outside the body (pain, temperature and touch
  2. Proprioceptive : originates from inside the body (from muscles and joints)

Normally there are three neurons in an ascending pathway:

  1. 1st order neuron: cell body is in the posterior root ganglion
  2. 2nd  order neuron: decussates (crosses to the opposite side) and ascends to a higher level of the    CNS
  3. 3rd neuron: located in the thalamus and passes to a sensory region of the cortex

Pain and temperature pathway: lateral spinothalmic tract

1st order neuron

Peripheral process extends to skin or other tissues and ends as free nerve endings (receptors). Cell body is situated in the posterior root ganglion. Central process extends into the posterior grey column and synapses with the 2nd order neuron.

2nd order neuron

The axon crosses obliquely to the opposite side in the anterior grey and white commissures within one spinal segment of the cord. It ascends in the contralateral white column as the lateral spinothalamic tract (LSTT).

As the LSTT ascends through the spinal cord new fibers are added to the anteromedial aspect of the tract (sacral fibers are lateral and cervical fibers are medial). The fibers carrying pain are situated anterior to those conducting temperature.

As the LSTT ascends through the medulla oblongata,  it’s  joined  by  the  anterior spinothalamic tract and the spinotectal tract and forms the spinal lemniscus. Spinal lemniscus ascends through the pons and the mid brain.

Fibers of the LSTT end by synapsing with the 3rd order  neurons  in  the  ventral  posterolateral nucleus  of  the  thalamus  (here  crude  pain  and temperature sensations are appreciated).

3rd order neuron

Axons pass through the posterior limb of the internal capsule and corona radiata  to reach the somatosensory  area  in  the  post  central  gyrus  of  the  cerebral  cortex.  From  here  information is transmitted to other regions of the cerebral cortex to be used by motor areas. The role of the cerebral  cortex  is  interpreting  the  quality  of  the  sensory  information  at  the  level  of  the consciousness.

Light (crude) touch and pressure pathway: anterior spinothalamic tract (ASTT)

1st order neuron

It is similar to the pain and temperature pathway.

2nd order neuron

The axon crosses obliquely to the opposite side in the anterior grey and white commissures within several spinal segments. It ascends in the contralateral white column as the anterior spinothalamic tract (ASTT). As the ASTT ascends through the spinal cord new fibers are added to the anteromedial aspect of the tract (sacral fibers are lateral and cervical fibers are medial).

As the ASTT ascends through the medulla oblongata, it’s joined by the lateral spinothalamic tract and the spinotectal tract and forms the spinal lemniscus. Spinal lemniscus ascends through the pons and the midbrain. Fibers of the ASTT end by synapsing with the 3rd order neurons in the ventral posterolateral nucleus of the thalamus (here crude awareness of touch and pressure sensations are appreciated).

3rd order neuron

Axons pass through the posterior limb of the internal capsule and corona radiata to reach the somatosensory area in the post central gyrus of the cerebral cortex. The sensations can be crudely localized. Very little discrimination is possible.

To read more click on this link to the full article: Ascending Tracts (pdf).