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UNIT 5 - Afferent Pathways

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A. Spinothalamic System or Anterior Lateral System
B. Medial Leminiscal System
C. Spinocerebellar System

Each afferent pathway, from receptor to its final destination consists of a series of two or more neurons which are labeled in ascending order as primary (first order) neuron, secondary (second order) neuron, and tertiary (third order) neuron.

It should be remembered that not all of these pathways are as clear cut as they are represented here. Many collaterals are given off at various levels throughout the spinal cord and the brainstem. Also, not all of the primary neurons synapse at their level of entry into the spinal cord. They may ascend or descend 2-5 levels (spinal segments) before synapsing with the secondary neuron, or forming a specific pathway.

The major afferent fiber systems can be described on the basis of fiber size, fiber organization, speed of conduction, and destination.

The three classifications are the spinothalamic system, the medial leminiscal system, and the spinocerebellar system.

Links to related animations:

38. Afferent Pathways
44. Afferent Pathways

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Fig. 1 –

1. Dorsal (posterior) median sulcus 2. Dorsal (posterior) intermediate sulcus 3. Posterior median septum 4. Posterior funiculus 5. Posterior lateral sulcus 6. Substantia gelatinosa 7. Dorsal (posterior) gray horn, nucleus proprius 8. Lateral funiculus 9. Thoracic nucleus (dorsal nucleus of Clarke) 10. Intermediolateral gray column 11. Ventral (anterior) gray horn 12. Ventral funiculus 13. Anterior median fissure 14. Ventral white commissure 15. Intermediate gray substance 16. Medial longitudinal fasciculus 17. Anterior corticospinal tract 18. Tectospinal tract 19. Reticulospinal tract 20. Vestibulospinal tract 21. Spinotectal tract 22. Anterior spinocerebellar tract 23. Rubrospinal tract 24. Spinothalamic tract 25. Lateral corticospinal tract 26. Posterior spinocerebellar tract 27. Cuneate fasciculus 28. Gracile fasciculus

1-10 Transverse section of the spinal cord at midthoracic level, showing approximate positions of the main tracts, The Human Brain: Chapter 1: The Spinal Cord

A. Spinothalamic System or Anterior Lateral System

The spinothalamic system is composed mainly of small fiber types that are poorly, or non-myelinated and transmit information at low frequencies. These tracts for the most part carry less-specific information that is general and indistinct in nature, and is perceived on a conscious level.

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1. Lateral Spinothalamic Tract

Modality	Pain and Temperature
Receptors	Free Nerve Endings
Pathway	- Primary neuron runs from the receptor, enters the dorsal horn and synapses in the primary receiving area for exteroreceptor fibers (laminae I-IV, substantia gelatinosa). - Secondary neuron crosses to the contralateral side in the ventral white commissure, it then enters the central portion of the lateral funiculus where it forms a bundle which ascends as the lateral spinothalamic tract, and then synapses in the thalamus (VPL). - Tertiary neuron ascends through the internal capsule to the post central gyrus (area 1-2-3) of the cerebral cortex.

Fig. 2 –

 Figure 10.3. Major pathways for pain (and temperature) sensation. (A) The spinothalamic system. (B) The trigeminal pain and temperature system, which carries information about these sensations from the face, Neuroscience, Sensation and Sensory Processing, Pain, Central Pain Pathways: The Spinothalamic Tract

2. Ventral or Anterior Spinothalamic Tract

Modality	Less-specific Touch and Pressure
Receptors	Free Nerve Endings and Hair End Organs
Pathway	- Primary neuron runs from the receptor through the dorsal horn, and synapses in the primary exteroreceptive receiving area (laminae I-IV, Substantia Geratinosa). - Secondary neuron crosses to the contralateral side via the ventral white commissure to the lateral portion of the ventral funiculus forming a bundle, which ascends as the ventral spinothalamic tract, which synapses in the thalamus (VPL). - Tertiary neuron ascends through the internal capsule to the cerebral cortex (Post-central gyrus- area 1-2-3).

Fig. 3 –

Anterolateral Pathway, Copyright (c) 2000 University of Bristol, Author: Department of Anatomy http://137.222.110.150/calnet/Somat/page2.htm

Click for Printable PDF Figure 3.

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B. Medial Leminiscal System

The medial leminiscal system is comprised of heavily myelinated nerve fibers with relatively fast conduction rates. This tract carries information that is specific and distinct in nature and is perceived on a conscious level. This is phylogenetically a young fiber system developing with the neo-cortex.

Modality	A combination of exteroceptive and proprioceptive information, including vibratory sensation.
Receptors	Meissner's Corpuscles, Merkel's Disc, Pacinian Corpuscles, Ruffini End Organs, Golgi Massoni, and Golgi Type Endings
Pathway	- Primary neuron runs from the receptor through the dorsal horn, into the ipsilateral dorsal funiculus forming either the fasiculus gracilis, or cuneatus, which ascends to synapse in the nucleus gracilis, or cuneatus. - Secondary neuron crosses to the contralateral side of the myelencephalon and then ascends to the thalamus as the medial leminiscal tract within the medial leminiscus of the brain stem. - Tertiary neuron ascends through the internal capsule to the cerebral cortex. (post-central gyrus, area 1-2-3)

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Fig. 4 –

Medial Lemniscus Pathway, Copyright (c) 2000 University of Bristol, Author: Department of Anatomy, http://137.222.110.150/calnet/Somat/page2.htm - Figure 2.

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C. Spinocerebellar System

The spinocerebellar system consists of heavily myelinated large fibers with rapid conduction rates. This system carries information that is specific and distinct in nature, but unlike the first two systems, is not perceived on a conscious level. This system is divided into three major ascending tracts, the dorsal, ventral spinocerebellar and cuneocerebellar tract.

1a. Cuneocerebellar Tract - (Receives information above the level of T1)

Modality	Unconscious proprioception from individual muscles of the neck and upper extremity musculature.
Receptors	Muscle Spindles and Golgi Tendon Organs
Pathway	- Primary neuron runs from receptor through the dorsal horn, into the ipsilateral dorsal funiculus where it forms a bundle that ascends as the cuneocerebellar fibers within the fasiculus cuneatus, and synapsing in the accessory cuneate nucleus. - Secondary neuron ascends through the restiform body as the cuneo cerebellar tract to the ipsilateral cerebellar cortex.

1b. Dorsal Spinocerebellar Tract - Posterior Spinocerebellar - (Receives information from below the level of T1)

Modality	Unconscious proprioception from individual muscles of the trunk and lower extremity.
Receptors	Muscle Spindle and Golgi Tendon Organs
Pathway	- Primary neuron runs from receptor into the dorsal horn where it synapses in the primary proprioceptive receiving area (laminae V-VI, Clark's Column). - Secondary neuron enters the dorsal portion of the ipsilateral lateral funiculus and forms a bundle which ascends the spinal cord through the restiform body as the dorsal spinocerebellar tract, or posterior spinocerebellar tract, to the ipsilateral cerebellar cortex.

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Fig. 5 –

Click for Printable PDF Figure 5.

2. Ventral Spinocerebellar Tract - Anterior Spinocerebellar

Modality	Unconscious proprioception from groups of muscles within the lower extremity and trunk.
Receptors	Muscle Spindle and Golgi Tendon Organs
Pathway	- Primary neuron runs from the receptor into the dorsal horn where it synapses in the primary proprioceptive receiving area (laminae V-VI, Clark's Column). - Second order neuron via the ventral white commissure enters the ventral portion of the contralateral lateral funiculus, forming a bundle which ascends as the ventral spinocerebellar tract or anterior spinocerebellar tract.This tract enters the brachium conjunctivum and crosses over to the brachium conjuntivum of the contralateral side and then enters the cerebellum.
	NOTE: This pathway crosses the midline twice, so that impulses received on one side of the body will enter the ipsilateral cerebellar hemisphere

Fig. 6, –

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Fig. 7 –

Click for Printable PDF Figures 6 and 7.

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