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UNIT 11 - Efferent Pathways

View Unit 11 Learning Objectives

A. Introduction
B. Efferent Pathways of the Corticospinal Component
C. Efferent Pathways of the Corticoreticular Component
D. Postural Control System

A. Introduction

Neurologically, our motor control system is broken down into three components which interact simultaneously.   These components are as follows:  1.) the  corticospinal component which deals with functional activities,  2.) the corticoreticular component which deals with mobility skills and provision of background postural stability for all of the activities we perform,  3.) finally, our postural control mechanism which deals with our balance and equilibrium both proactively and reactively.   The following is a neurological and functional comparison between corticospinal and corticoreticular components.

CORTICOSPINAL (fiber system)	CORTICORETICULAR (fiber system)
1. Influenced by Area 4 & 8. 2. Direct route to Aplha and gamma motor neurons and cranial nerves. 3. Responsible for functional movements of skeletal musculature by allowing for fractionated control and voluntarycontrol of cranial nerve function. 4. Voluntary 5. Efferent Pathways; - Corticospinal Tracts - Corticobulbar Tracts	1. Influences by Area 4 & 6. 2. Indirect route to alpha and gamma meurons through the reticular formation and to cranial nerves. 3. Responsible for mobility skills and background postural stability and automatic cranial nerve function. 4. Automatic 5. Efferent Pathways; - Reticulospinal Tract - Reticulobulbar Tract

B. Efferent Pathways of the Corticospinal Component

The pathways for this fiber system descend directly to or near lower motor neurons.   Information in these pathways are relayed primarily through the internuncial pool with collaterals to the reticular formation and other sub-cortical and brain stem structures, before reaching the “final common pathway” (anterior horn cell).   These pathways allow for functional control over our skeletal musculature with emphasis on the digits of our upper extremity and voluntary control of movements dealing with gesturing and facial expression used to express our feelings and emotions. These functions are heavily dependent on information coming from our non-dominant parietal lobe and medial nucleus of the thalamus.

1.  Corticospinal Fibers

Corticospinal fibers originate in Area 4 primarily, as well as areas 6, 1, 2, 3, 5, 7, 8, and 19.   They course through the internal capsule and basilar portion of the brainstem and give off collaterals to various subcortical and brain stem structures dealing with motor control.   When reaching the lower border of the medulla 85% of the fibers from area 4 decussate and descend in the spinal cord within the contralateral lateral funiculus as the lateral corticospinal tract.   These fibers control the neck, trunk, and extremity musculature.   The remaining 15% of the fibers which do not decussate, descend in the ipsilateral ventral funiculus as the ventral corticospinal tract.   They control the musculature of the neck and the trunk.   The corticospinal tracts send fibers into the ventral horn of the gray matter, lamina 7 and 9 to contact the alpha and gamma motor neurons.

2.  Corticobulbar Tracts

The corticobulbar fibers/tracts originate in Area 4 and 8, and of the cerebral cortex.   They descend through the internal capsule and basilar portion of the brainstem side by side with the corticospinal fibers.   They exit within the brainstem at the level where the cranial nerve nuclei are found dealing with motor contol. This tract is both contralateral and bilateral in nature regarding motor functions controlled by cranial nerves.

NOTE: Major fibers from Areas 1, 2, and 3 of the cerebral cortex have been found to end in or near sensory nuclei such as the nuclei gracilis and cuneatus and the main sensory nucleus of the trigeminal nerve.   These fibers are believed to be a part of a feedback autoregulatory system, which allows the cerebral cortex to influence (inhibit or facilitate) its own sensory input. These fibers are found adjacent to the corticospinal fibers in the brainstem.

Fig. 1 –

Schematic pathway of corticospinal tract, Copyright (c) 2000 University of Bristol, Author: Department of Anatomy , http://137.222.110.150/calnet/UMN/page2.htm

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Fig. 2 –

Click for Printable PDF Figure 2.

Cranial Nerves

Name	Type	Function
1. Olfactory	sensory	sense of smell
2. Optic	sensory	sense of vision
3. Occulomotor	motor	occular control
4. Trochlear	motor	occular control
5. Trigeminal	both	S= facial area, inside of mouth M= muscles of mastication
6. Abducens	motor	occular control
7. Facial	both	S= anterior 2/3 of tongue, hard and soft places M= all facial muscles except those of the eye andthe muscles of mastication
8. Vestibular Cochlear	sensory	vesitbular and auditory input
9. Glossopharyngeal	both	S= posterior 1/3 of tongue, larynx, pharynx, soft palate, epiglottis, esophagus and viscera M= muscles of swallowing and exerts control over ANS (viscera)
10. Vagus	both	S= same as #9 M= helps #9 with throat musculature and visceral control, as well as controlling cardiovascular and respiratory functioning
11. Spinal Accessory	motor	trapezius, sternocleidomastid, controls throat musculature for coughing, vomiting and gagging
12. Hypoglossal	motor	controls tongue musculature

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Fig. 2b –

Click for Printable PDF Figure 2b.

C. Efferent Pathways of the Corticoreticular Component

The corticoreticular component constitute pathways which have an indirect route.   These fibers extend from the neocortex, through the internal capsule, then to the reticular formation (central and lateral nuclei) before descending to the alpha and gamma motor neurons via the reticulospinal tracts.   Numerous collaterals are given off to various subcortical and brain stem structures dealing with motor control structures as they descend to the reticular formation.   The corticoreticular component carries information dealing with automatic movements which allows us to move from one posture to another as well as to provide background postural stability for any activity that we perform.   These automatic movements include gesturing and facial expressions that are used to express our feelings and emotions and are dependent on information from our non-dominant parietal lobe and medial nucleus of the thalamus.  Corticoreticular fibers also carry information from Area 4 dealing with automatic responses of cranial nerves.

1.  Corticoreticular Fibers

These fibers originate primarily in Area 4 & 6, as well as Areas 1, 2, 3, 5, 7, 8, 19 and 9-10-11-12 of the cerebral cortex.   They send information through the internal capsule to the central and lateral nuclei of the reticular formation and they give off collaterals to various subcortical structures and brain stem structures dealing with motor control.  This information is then relayed to the spinal cord via the reticulospinal tracts and to the cranial nerves via the reticulobulbar fibers.

2.  Medial and Lateral Reticulospinal Tracts

The lateral or medullary reticulospinal tract carries information to the alpha and gamma motor neurons of the contralateral extremity musculature.   The medial or pontine reticulospinal tract descends bilaterally to the musculature of the neck and trunk.

3.  Reticulobulbar Tract

These fibers connect with the cranial nerve motor nuclei in the brainstem for automatic control of a similar functions to those already described for the corticobulbar fibers in regard to our facial and throat musculature but not occular control. Information sent through these fibers originate from area 4 of the cerebral cortex.

D. Postural Control System

The Postural Control System is the third component of the motor control mechanism and monitors both our corticoreticular and corticospinal activities.   This mechanism is comprised of the vestibulocerebellum, the vestibular system, and the reticular formation.   This mechanism is responsible for both maintaining and regaining our balance and equilibrium.   Thus, it is both proactive and reactive.   Note, this system is active only if a person is voluntarily trying to move.   The following is a brief description of the tracts involved with our postural control mechanism.   These tracts have been covered in depth in previous units.

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1.  Lateral Vestibulospinal Tract

This tract deals with regaining our balance and equilibrium through ipsilateral control of trunk and extremity antigravity musculature as well as regulation of tone during high risk activities.

2.  Medial Vestibulospinal Tract - Descending MLF

This tract deals with regaining our head control through bilateral control of the neck antigravity musculature and ipsilateral regulation of tone during high risk activities.

3.  Ascending MLF

This tract controls occular movement bilaterally allowing visual fixation within our environment to help maintain a vertical position in space.

4.  Medial and Lateral Reticulospinal Tracts

These two tracts provide ipsilateral regulation of muscle tone throughout the body musculature when our posture is static in nature.   This tract also deals with regaining our balance through ipsilateral control of the non-antigravity musculature of the trunk and extremities and bilateral control of our neck musculature. Additionally, information dealing with ipsilateral proactive control of our body musculature and ipsilateral regulation of tone in our non-antigravity muscles during high risk activities is carried by these tracts.

Fig. 3 –

Click for Printable PDF Figure 3.

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