Dorsal Columnmedial Lemniscus Pathway Figure 72 see also Figure

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A. Function. The dorsal column-medial lemniscus pathway mediates tactile discrimination, vibration sensation, form recognition, and joint and muscle sensation (conscious proprioception).

B. Receptors include Pacini's and Meissner's tactile corpuscles, joint receptors, muscle spindles, and Golgi tendon organs.

C. First-order neurons are located in the dorsal root ganglia at all levels. They project axons to the spinal cord through the medial root entry zone. First-order neurons give rise to:

1. The gracile fasciculus from the lower extremity

Gracile fasciculus Cuneate fasciculus j Descending tracts i

Dorsal spinocerebellar tract

Spinal Cord Entry Zone

Rubrospinal tract

Ventral

Figure 7-1. The major ascending and descending pathways of the spinal cord. The ascending sensory tracts are shown on the lejt, and the descending motor tracts arc shown on the right.

Rubrospinal tract

Ascending tracts

Gracile fasciculus Cuneate fasciculus

Dorsal spinocerebellar tract

Ventral spinocerebellar tract

Lateral spinothalamic tract

Ventral spinothalamic tract j Descending tracts i

Lateral corticospinal tract Hypothalamospinal tract

Ventral

Vestibulospinal tract corticospinal tract

Figure 7-1. The major ascending and descending pathways of the spinal cord. The ascending sensory tracts are shown on the lejt, and the descending motor tracts arc shown on the right.

Trigeminal nerve

Nucleus gracilis

Meissners corpuscle

Nucleus cuneatus

Internal arcuate fibers (neuron II)-

Cuneate fasciculus

Dorsal root ganglion cell (neuron I)-

Pacinian corpuscle

Hypothalamospinal Tract

Medulla

Pons

Medial lemniscus

Spinal trigeminal nucleus

-Decussation of medial lemniscus

-Gracile fasciculus

-Cuneate fasciculus

Cervical cord

Medial lemniscus

-Gracile fasciculus

Lumbosacral cord

Figure 7-2. The dorsal column-medial lemniscus pathway. Impulses conducted by this pathway mediate discriminatory tactile sense (e.g., touch, vibration, pressure) and kinesthetic sense (e.».. position, movement). The dorsal column system mediates conscious proprioception. (Adapted with permission from Carpenter MB, Sutin J: Human Neuroanatomy. Baltimore, Williams & Wilkins, 1983, p. 266.)

Medulla

Trigeminal nerve

Pons

Nucleus gracilis

Medial lemniscus

Thalamus Internal capsule Lentiform nucleus

Postcentral gyrus

Leg area Trunk area Arm area Head area Face area

Ventral posterolateral nucleus of thalamus (neuron III)

Spinal trigeminal nucleus

-Decussation of medial lemniscus

-Gracile fasciculus

-Cuneate fasciculus

Cervical cord

Medial lemniscus Midbrain

Medial lemniscus

Meissners corpuscle

-Gracile fasciculus

Lumbosacral cord

Nucleus cuneatus

Internal arcuate fibers (neuron II)-

Cuneate fasciculus

Dorsal root ganglion cell (neuron I)-

Pacinian corpuscle

Figure 7-2. The dorsal column-medial lemniscus pathway. Impulses conducted by this pathway mediate discriminatory tactile sense (e.g., touch, vibration, pressure) and kinesthetic sense (e.».. position, movement). The dorsal column system mediates conscious proprioception. (Adapted with permission from Carpenter MB, Sutin J: Human Neuroanatomy. Baltimore, Williams & Wilkins, 1983, p. 266.)

2. The cuneate fasciculus from the upper extremity

3. The collaterals for spinal reflexes (e.g., myotatic reflex)

4. The axons chat ascend in the dorsal columns and terminate in the gracile and cuneate nuclei of the caudal medulla

D. Second-order neurons are located in the gracile and cuneate nuclei of the caudal medulla. They give rise to axons and internal arcuate fibers that decussate and form a compact fiber bundle (i.e., medial lemniscus). The medial lemniscus ascends through the contralateral brain stem and terminates in the ventral posterolateral (VPL) nucleus of the thalamus.

E. Third-order neurons are located in the VPL nucleus of the thalamus. They project through the posterior limb of the internal capsule to the postcentral gyrus, which is the primary somatosensory cortex (Brodmanns areas 1, and 2).

F. Transection of the dorsal column-medial lemniscus tract

1. Above the sensory decussation, transection results in contralateral loss of the dorsal column modalities.

2. In the spinal cord, transection results in ipsilateral loss of the dorsal column modalities.

III. LATERAL SPINOTHALAMIC TRACT (Figure 7-5; see also Figure 8-1)

A. Function. The lateral spinothalamic tract mediates pain and temperature sensation.

B. Receptors are free nerve endings. The lateral spinothalamic tract receives input from fast- and slow'conducting pain fibers (i.e., A 8 and C, respectively).

C. First-order neurons are found in the dorsal root ganglia at all levels. They project axons to the spinal cord through the dorsolateral tract of Lissauer (lateral root entry zone) to second-order neurons.

D. Second-order neurons are found in the dorsal horn. They give rise to axons that decussate in the ventral white commissure and ascend in the contralateral lateral funiculus. Their axons terminate in the VPS nucleus of the thalamus.

E. Third-order neurons are found in the VPL nucleus of the thalamus. They project through the posteiior limb of the internal capsule to the primary somatosensory cortex (Brodmanns areas I, and 2).

F. Transection of the lateral spinothalamic tract results in contralateral loss of pain and temperature below the lesion.

IV. LATERAL CORTICOSPINAL TRACT (Figure 7-4; see also Figure 8-1)

A. Function. The lateral corticospinal tract mediates voluntary skilled motor activity, primarily of the upper limbs. Ir is not fully myelinated until the end of the second year (Babinski s sign).

B. Fiber caliber. Approximately 90% of the fibers He between 1 and 4|xm, and 4% lie above 20|xm (frot i the giant cells of Betz).

C. Origin and termination

1. Origin. The lateral corticospinal tract arises from layer V of the cerebral cortex from three cortical areas in equal aliquots:

a. The pre motor cortex (Brodmanns area 6)

b. The primary cortex (Brodmanns area 4)

Cerebral cortex (postcentral gyrus)

Medial lemniscus

Medial lemniscus

Crus cerebri

Free nerve endings

Crura Cerebri Transection

Pons

Medulla

Lateral spinothalamic tract

Midbrain

Red nucleus

Ventral white commissure

Neuron I (dorsal root ganglion cell)

Neuron II

Medial lemniscus

Ventral white commissure

Corpus callosum Thalamus

Internal capsule

Neuron I (dorsal root ganglion cell)

Pons

Medulla

Lateral spinothalamic tract

Medial lemniscus

Crus cerebri

Free nerve endings

Neuron II

Midbrain

Axons of neurons in posterior limb of internal capsule

Ventral posterolateral nucleus (VPL) Neuron III

Red nucleus

Cerebral cortex (postcentral gyrus)

Figure 7-3. The lateral spinothalamic tract. Impulses conducted by this tract mediate pain and thermal sense. Numerous collaterals are distributed to the brain stem reticular formation. (Reprinted with permission from Carpenter MB, Sutin J: Human Neuroanatomy. Baltimore, Williams & Wilkins, 1983, p. 274.)

Thalamus

Posterior limb internal capsule

Lenticular nucleus

Caudate nucleus (head)

Lateral corticospinal tract (crossed axons of neuron I)

Motor end plates

Pons

Thalamus

Posterior limb internal capsule

Lenticular nucleus

Caudate nucleus (head)

Lateral corticospinal tract (crossed axons of neuron I)

Motor end plates

Nucleus Tract

Pons

Large pyramidal cells of Betz

Genu of internal capsule

Anterior limb of internal capsule

Corticospinal tract

Midbrain

Longitudinal fibers in basilar portion of pons

Medulla

Ventral corticospinal tract (uncrossed axons of neuron I)

Spinal cord

Ventral white commissure

Figure 7-4. The lateral and ventral corticospinal (pyramidal) tracts. These major descending motor pathways mediate volitional motor activity. The cells of origin are located in the premotor, the motor, and the sensory cortices. CN = cranial nerve. (Reprinted with permission from Carpenter MB, Sutin J: Human Neuroanatomy. Baltimore, Williams & Wilkins, p. 285.)

Motor cortex (precentral gyrus)

Large pyramidal cells of Betz

Genu of internal capsule

Anterior limb of internal capsule

Corticospinal tract

Midbrain

Longitudinal fibers in basilar portion of pons

Medulla

Medulla

Pyramidal decussation

Ventral corticospinal tract (uncrossed axons of neuron I)

Spinal cord

Ventral white commissure

Figure 7-4. The lateral and ventral corticospinal (pyramidal) tracts. These major descending motor pathways mediate volitional motor activity. The cells of origin are located in the premotor, the motor, and the sensory cortices. CN = cranial nerve. (Reprinted with permission from Carpenter MB, Sutin J: Human Neuroanatomy. Baltimore, Williams & Wilkins, p. 285.)

Ophthalmic artery

Ophthalmic artery

Oculosympathetic Tract

Figure 7-5. The oculosympathetic pathway. Hypothalamic fibers project to the ipsilateral ciliospinal center of the intermediolateral ceil column at T-l. The ciliospinal center projects preganglionic sympathetic fibers to the superior cervical ganglion. The superior cervical ganglion projects perivascular postganglionic sympathetic fibers through the tympanic cavity, cavernous sinus, and superior orbital fissure to the dilator muscle of the iris. Interruption of this pathway at any level results in Horner's syndrome. CN = cranial nerve.

Figure 7-5. The oculosympathetic pathway. Hypothalamic fibers project to the ipsilateral ciliospinal center of the intermediolateral ceil column at T-l. The ciliospinal center projects preganglionic sympathetic fibers to the superior cervical ganglion. The superior cervical ganglion projects perivascular postganglionic sympathetic fibers through the tympanic cavity, cavernous sinus, and superior orbital fissure to the dilator muscle of the iris. Interruption of this pathway at any level results in Horner's syndrome. CN = cranial nerve.

C. The primary sensory cortex (Brodmanns areas 3, 1, and 2) d. Arm, face, and foot areas. The arm and face areas of the motor homunculus are found on the lateral convexity; the foot region of the motor homunculus is found in the paracentral lobule (see Figure 23-2).

2. Termination. The lateral corticospinal tract terminates contralateral^, through interneurons, on ventral horn motor neurons.

D. Course of the lateral corticospinal tract

1. Telencephalon. The lateral corticospinal tract runs in the posterior limb of the internal capsule in the telencephalon.

2. Midbrain. The lateral corticospinal tract runs in the middle three-fifths of the crus cerebri in the midbrain.

3. Pons. The lateral corticospinal tract runs in the base of the pons.

4. Medulla. The lateral corticospinal tract runs in the medullary pyramids. Between 85% and 90% of the corticospinal fibers decussate in the pyramidal decussation as the lateral corticospinal tract. The remaining 10% to 15% of the fibers continue as the anterior corticospinal tract.

5. Spinal cord. The lateral corticospinal tract runs in the dorsal quadrant of the lateral funiculus.

E. Transection of the lateral corticospinal tract

1. Above the motor decussation, transection results in contralateral spastic paresis and Babinski's sign (upgoing toe).

2. In the spinal cord, transection results in ipsilateral spastic paresis and Babinski's sign.

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