C.Unilateral muscle atrophy and absent quadriceps (L3) and ankle jerk (S1) reflex activity.

D.Unremarkable incontinence and sexual function.

E.Gradual and unilateral onset.

VII Conus Medullaris Syndrome (Cord Segments S3 to C0)

usually results from an intramedullary tumor (e.g., ependymoma). It is characterized by:

A.Pain, usually bilateral and not severe.

B.Sensory distribution in a bilateral saddle-shaped area.

C.Unremarkable muscle changes; normal quadriceps and ankle jerk reflexes.

D.Severely impaired incontinence and sexual function.

E.Sudden and bilateral onset.

Posterior Spinothalamic spinocerebellar tract

tracts

Anterior spinocerebellar tract

Inferior olivary nucleus

Figure 7-3 Caudal medulla.

6.Nucleus ambiguus—lower motor neuron nucleus that sends fibers into CNs IX and X

7.Medial lemniscus

8.Inferior cerebellar peduncle

IV Cross Section Through the Rostral Medulla (Figure 7-5)

Dorsal and ventral cochlear nuclei

Pyramid

Figure 7-5 Rostral medulla.

1.Spinothalamic tracts (spinal lemniscus)

2.Spinal nucleus and tract of trigeminal nerve

3.Inferior cerebellar peduncle—contains olivocerebellar, cuneocerebellar, and posterior spinocerebellar tracts

VCross Section Through the Caudal Pons (Figure 7-6). The pons

has a posterior tegmentum and an anterior base.

Figure 7-6 Caudal pons.

Brainstem 63

1.Medial longitudinal fasciculus (MLF)

2.Abducent nucleus of CN VI (underlies facial colliculus)

3.Genu (internal) of CN VII (underlies facial nerve; facial colliculus)

4.Medial lemniscus

5.Corticospinal and corticonuclear tracts (in the base of the pons)

6.Facial motor nucleus (CN VII)

7.Spinal nucleus and tract of trigeminal nerve (CN V)

8.Spinothalamic tracts (spinal lemniscus)

9.Vestibular nuclei of CN VIII

10.Inferior and middle cerebellar peduncle

11.Central tegmental tract—fiber pathway traversing the reticular formation

VI Cross Section Through the Mid-Pons (Figure 7-7)

1.Raphe nuclei

2.Deep pontine nuclei and transverse pontine fibers

3.Superior cerebellar peduncle—main cerebellar outflow pathway, also contains decussating anterior spinocerebellar fibers

VII Cross Section Through the Rostral Pons (Figure 7-8)

1.Mesencephalic nucleus—unconscious proprioception for the head

2.Locus ceruleus—source of norepinephrine for the brain

3.Cerebral aqueduct—connects the third and fourth ventricles

4.Periaqueductal gray—involved in pain modulation, source of serotonin

Transverse pontine fibers

Central tegmental

tract

Deep pontine nuclei

Corticonuclear and

corticospinal tracts

Figure 7-7 Mid-pons.

Superior medullary velum

Middle cerebellar peduncle

Raphe nucleus

Deep pontine nuclei

Corticonuclear and

corticospinal tracts

Figure 7-8 Rostral pons.

VIII Cross Section Through the Caudal Midbrain (Figure 7-9).

The midbrain has a posterior tectum, an intermediate tegmentum, and a base. The cerebral aqueduct lies between the tectum and the tegmentum.

Medial lemniscus

1.Inferior colliculus—auditory relay nucleus

2.Trochlear nucleus—lower motor neuron nucleus that innervates the superior oblique

3.Cerebral aqueduct

4.Crus cerebri (basis pedunculi cerebri or cerebral peduncle)—composed of descending fibers.

IX Cross Section Through the Rostral Medulla (Figure 7-10)

1.Superior colliculus—visual relay nucleus

2.Oculomotor nucleus—lower motor neuron nucleus that innervates the majority of the extraocular musculature

3.Red nucleus—part of the primitive motor system, involved in coordination

4.Ventral tegmental area—source of dopamine, involved in the reward system

5.Spinothalamic and trigeminothalamic tracts

6.Medial lemniscus

XCorticonuclear Fibers project bilaterally to all motor cranial nerve nuclei except

the facial nucleus. The division of the facial nerve nucleus that innervates the upper face (the orbicularis oculi and above) receives bilateral corticonuclear input. The division of the facial nerve nucleus that innervates the lower face receives only contralateral corticonuclear input.