Image 1 of 10

Most Cephalic Level of the Spinal Cord

Note in this slide that the gray matter is much reduced and the lateral corticospinal tracts (pyramidal) are accumulating their fascicles in the lateral funiculi. These fibers have just crossed the midline in the lower medulla.

The fibers in the ventral corticospinal tracts (uncrossed) remain in a ventral position and form a major component of the ventral funiculi of the cord.

The substantia gelatinosa and dorsal lateral fasciculus are prominent at this level, and throughout the upper cervical cord as well, because these structures are overlapped by the spinal nucleus and the spinal tract of the trigeminal nerve which carry sensation from the ipsilateral face.

You should be able to identify the fasciculus gracilis, fasciculus cuneatus, and the spinothalamic tract.


A = Fasciculus Gracilis
B = Fasciculus Cuneatus
C = Substantia Gelatinosa
D = Lateral Corticospinal Tracts
E = Lateral Spinothalamic Tract
F = Ventral Corticospinal Tracts
Enlarged Annotated

Image 2 of 10

Most Caudal Level of the Medulla

In this slide one can see the beginning of the pyramidal decussation. The spinal nucleus and tract of the trigeminal nerve, the spinothalamic tract, fasciculus cuneatus and fasciculus gracilis, and the beginning of nucleus gracilis should also be identified on this slide.



A = Fasciculus Gracilis
B = Fasciculus Cuneatus
C = Nucleus and Tract of V
D = Pyramidal Decussation
E = Lateral Spinothalamic Tract
Enlarged Annotated

Image 3 of 10
Level showing the Pyramidal and Sensory Decussations

In this slide one can see the pyramidal decussation. Fasciculus gracilis has been replaced by nucleus gracilis and nucleus cuneatus is beginning to appear. Some of fasciculus cuneatus is still present at this level. The axons of the cells of these nuclei sweep ventrally as the sensory decussation, cross the midline and turn rostralward forming the medial lemniscus. Not many fibers have crossed at this level so the medial lemniscus is not easily identifiable. You should be able to identify the spinal nucleus and tract of the trigeminal nerve.



A = Nucleus Gracilis
B = Nucleus and Fasciculus Cuneatus
C = Nucleus and Tract of V
D = Pyramidal Decussation
E = Initial formation of Medial Lemniscus
Enlarged Annotated

Image 4 of 10
Level of the Rostral Sensory Decussation

This section shows the sensory decussation and the medial lemniscus. The cells of origin for these axons are found in nucleus gracilis and nucleus cuneatus.

Also here you should be able to identify the pyramids, the spinal nucleus and tract of the trigeminal, the inferior olivary nucleus and the central canal. Ventral to the midline you may observe a cluster of large multipolar neurons; they are the hypoglossal nucleus. Dorsal to this you can see the dorsal motor nucleus of the vagus.

Additional structures on this slide can be seen more easily on the next slide. These include the hypoglossal and dorsal motor nuclei. Note also that the spinothalamic and spinocerebellar tracts are continuous from the caudal levels of the spinal cord and do not deviate in position at this level.



A = Dorsal Spinocerebellar Tract
B = Central Canal
C = Nucleus and Tract of V
D = Ventral Spinocerebellar Tract
E = Spinothalamic Tracts
F = Medial Lemniscus
G = Inferior Olivary Nucleus
H = Pyramids
Enlarged Annotated

Image 5 of 10
Level of the Vagus Nerve

On this slide one can observe the beginning of the fourth ventricle dorsally. At this level the nucleus gracilis and nucleus cuneatus of lower levels have been replaced by the expansion of the hypoglossal nucleus and the dorsal motor nucleus of the Vagus.

Also on this slide you should be able to identify the inferior cerebellar peduncle. Many of the fibers that form this peduncle originate in the contralateral inferior olivary nucleus and cross the midline to enter the cerebellum.

Note that the Medial Lemniscus and Spinothalamic tract continue to travel rostrally.



A = 4th Ventricle
B = Hypoglossal Nucleus
C = Dorsal Nucleus of Vagus
D = Nucleus and Tractus Solitarius
E = Inferior Cerebellar PeduncleInferior Cerebellar Peduncle
F = Nucleus Ambiguus
G = Medial Lemniscus
H = Medial Lemniscus
Enlarged Annotated

Image 6 of 10
Level of CNs VIII and IX

In this slide you should note that the dorsal motor nucleus of the vagus and the hypoglossal nucleus are no longer present. Identify the dorsal and ventral cochlear nuclei and the roots of the vestibulocochlear nerve.

Also you should be able to identify the inferior cerebellar peduncle, inferior olivary nucleus, medial lemniscus, medial longitudinal fasciculus, the pyramids, and the nucleus and tractus solitarius.



A = Dorsal Cochlear Nucleus
B = Nucleus and Tractus Solitarius
C = Medial Longitudinal Fasciculus
D = Inferior Cerebellar Peduncle
E = Ventral Cochlear Nucleus
F = Medial Lemniscus
G = Pyramids
Enlarged Annotated

Image 7 of 10
Level of the Lower Pons at the roots of CNs VI and VII

At this level one can see that the corticospinal tracts (pyramidal tracts) are broken up into many small fascicles. The transverse fibers that form the base of the pons here are related to the middle cerebellar peduncle.

The inferior olivary nucleus and the inferior cerebellar peduncle are not seen at this level. Also note that the medial lemniscus is no longer a strict midline structure, its fibers have now assumed a more ventral position. It may be difficult to identify here because the fibers forming the trapezoid body, related to the auditory system, interdigitate with them. Important fibers arise from all the vestibular nuclei and enter the medial longitudinal fasciculus and project to the nuclei of the extraocular muscles.

The cochlear nerve fibers terminate in the dorsal and ventral cochlear nuclei (Identified at the previous caudal level). The axons of these nerves course medially and superiorly across the midline and form the trapezoid body seen here. The fibers of the trapezoid body course rostralward and form the lateral lemniscus which ultimately ends in the nucleus of the inferior colliculus. These structures will be labeled at more rostral levels. Subsequently, the auditory signal is sent via the peduncle of the inferior colliculus to the medial geniculate nucleus of the dorsal thalamus. Auditory signals are transmitted via the sublenticular portion of the internal capsule to the transverse gyrus of Heschl within the temporal lobe.

Here you should also be able to identify the superior olivary nucleus which is reponsible for the efferent fibers in the cochlear nerve. These are largely inhibitory to the Organ of Corti.



A = Vestibular Nuclei
B = Medial Longitudinal Fasciculus
C = Abducens Nucleus
D = Facial Nucleus
E = Superior Olivary Nucleus
F = Trapezoid Body
Enlarged Annotated

Image 8 of 10
Level of the Nuclei and roots of the Trigeminal Nerve (CN V)

Three new structures can be seen at this level. The first is the motor nucleus of the trigeminal nerve which now occupies the same relative position as did the motor nucleus of the facial nerve at the lower level. The second is the chief sensory nucleus of the trigeminal nerve which now has replaced the nucleus and tract of the trigeminal nerve. The third new structure that can be seen is the lateral lemniscus. This bundle of fibers is found in the ventrolateral aspect of the tegmentum and represents the accumulated fibers of the auditory system.

You should also be able to identify all of the following:

  • Fourth ventricle
  • Middle cerebellar peduncles
  • Medial longitudinal fasciculus
  • Medial lemniscus
  • Pontine nuclei
  • Corticospinal tracts
  • Spinothalamic tracts


A = Motor Nucleus of V
B = Chief Sensory Nucleus of V
C = Lateral Lemniscus
Enlarged Annotated

Image 9 of 10
Level of the Rostral Pons

This section shows the prominent superior cerebellar peduncles as they leave the cerebellum. Also note the medial lemniscus, the spinothalamic tracts and the lateral lemniscus which are easily defined here because the fibers and nuclei of the trapezoid body are not present in the rostral pons.

Here you should be able to identify the following:

  • medial longitudinal fasciculus
  • trigeminal nerve roots
  • corticospinal tracts
  • pontine nuclei
  • pontocerebellar fibers
  • fourth ventricle
  • locus ceruleus

Note: The locus ceruleus is a nucleus located under the floor of the 4th ventricle. It belongs to the reticular formation.



A = Superior Cerebellar Peduncle
B = Locus Ceruleus
C = Spinothalamic Tract
D = Medial Lemniscus
E = Reticular Formation
Enlarged Annotated

Image 10 of 10
Level of the Inferior Colliculus

Beginning ventrally identify the cerebral peduncle. The orientation of the components of this structure are relatively similar throughout the midbrain. The pyramidal tracts which ultimately form the lateral and ventral corticospinal tracts of the spinal cord lie in a central position in the peduncle. The fibers that compose the most medial and most lateral areas of the cerebral peduncle originate in other motor areas of the cerebral cortex and ultimately synapse on the neurons of the pontine nuclei. Subsequently, the axons of the neurons of the pontine nuclei form the middle cerebellar peduncle.

The decussation of the superior cerebellar peduncle has already been indentified in the central tegmetum of the midbrain at the level of the inferior colliculus. This slide shows a large number of these fibers crossing the midline. The crescent-shaped bands of fibers that lie between the cerebral peduncles and the decussation of the superior cerebellar peduncles contain fibers of the medial lemnisci, the spinothalamic tracts, and the lateral lemnisci. The orientation of these fiber systems is the same throughout the pons and the midbrain. It is important that you be able to map them out.



A = Lateral Lemniscus
B = Spinothalamic Tract
C = Medial Lemniscus
D = Superior Cerebellar Decussation
E = Pontine Nuclei
F = Cerebral Peduncle
Enlarged Annotated