Somatic
Sensory and Pain Structures & Pathways
Laboratory
Wednesday, February
“Anatomy
without function is uninteresting; function without anatomy is meaningless.”
J.E. Rose,
ca 1950
Goals: • Solidify acquaintance
with the external features of the spinal cord, brainstem and cerebral hemispheres
• Comprehend
the organization of PNS and its relation to CNS with particular reference to
somatic sensation
•
Clarify the general internal organization of the spinal cord, brainstem,
thalmus and cerebral cortex with
emphasis on somatic sensation and pain
• Understand
the functional organization and topography of the following ascending systems:
•• fine
touch and position sense
••
touch, temperature and pain
••
pathways to the cerebellum and brain stem
###################################################
USE BRAINS, OVERHEAD PROJECTORS AND COMPOUND
MICROSCOPES.
###################################################
A. External Structure of Brain
B. Spinal Cord
1. External features, regional variation,
blood supply. Review the overall structure of the spinal cord including the anterior median fissure, the dorsolateral sulcus , the cervical and lumbar enlargements, and the filum
terminale. (See Heimer p. XX.) For blood supply review pp. 48-9 in Woolsey.
What is the explanation for the enlargements?
2. Segmental organization. Connections to
the periphery are “segmented” based on
the relation of the spinal nerves to the vertebral column. Review Heimer for
appearance; p.8 Woolsey for general scheme.
a.
Spinal nerves. Make sure you understand what a spinal root is.
b.
Peripheral nerves - axon Diameter Distribution Slides 1 & 2 are osmium tetroxide stained cross-sections of a
peripheral motor and sensory nerve respectively. Individual myelinated fibers appear as black
rings; unmyelinated fibers are
individually indistinguishable but are located in paler, orange stained
areas. How do the two sections differ? Examine
the myelinated axons and note the range and density of different size axons. Slide
2 shows examples of peripheral sensory myelinated fibers of all sizes (1 to 20
microns in diameter). Why do myelinated
fibers support faster conduction? Why do
larger diameter fibers support faster conduction?
c.
Spinal roots, dorsal root ganglia and
dorsal root entry zone. Slides 12 and 13 are sections from a
cat spinal cord stained for cytoskeleton and cell bodies respectively. These sections
show the anatomical relationship between the peripheral mixed spinal nerve, dorsal
root ganglion, dorsal and ventral
roots and the dorsal root entry zone into the spinal cord.
Figure 1. Microscope slide 13
Study these slides using the 4X lens on
your microscope. Use pp. 49, 136-40, 172-3
in Woolsey to guide you.
1) Identify the different levels of the
spinal cord from which the sections on these slides were taken. How do you know
this?
2) Identify
the gray matter of the spinal cord
(4X). Within the cell zone of the spinal cord find the dorsal horn, ventral horn and intermediate gray areas.
3) Identify the white matter of the spinal cord (4X). This is divided into three
bundles of axons called columns or fasciculi the dorsal, lateral and anterior columns. These are made up of myelinated and unmyelinated axons ascending
to and descending from other regions of the brain and spinal cord. At the end
of this lab review the principal direction of information flow in each of
these.
4) Lateral to the spinal cord, identify
the dorsal root ganglion, dorsal root and ventral root (4X). Using the 40X lens on your microscope,
examine the dorsal root ganglion cells. Neurons have large, round, clear nuclei
and a prominent nucleolus. Supporting cells have bean‑shaped, dark nuclei
that lack a distinct nucleolus. Note the
distribution of soma sizes, which roughly correlates with the distribution of
different diameter axons in the sensory nerve shown on slide 2. The small, more darkly staining ganglion
cells (not to be confused with the much smaller Schwann cell nuclei) support
the smallest peripheral fibers (A∂
and C fibers). The largest cells maintain the largest myelinated sensory
fibers that innervate muscle spindles and Golgi tendon organs.
5) Using the cytoskeleton stained slide,
trace the dorsal root fibers into the spinal cord. Note that many fibers course medially,
directly into the dorsal columns. These
are larger diameter fibers that send an ascending branch into the dorsal
columns. Collateral branches from these also
enter the dorsal horn from its medial aspect. Try to identify these fiber
bundles on your section.
6) Identify the dorsolateral sulcus. I Thinner fibers from the dorsal root enter Lissauer's tract (a.k.a. dorsolateral
tract) at this location. Lissauer's
tract appears as a pale, triangular‑shaped region that caps the dorsal
horn. Some dorsal root A∂ and C fibers ascend or descend
several segments within Lissauer's tract before sending collaterals directly
into the dorsal horn.
C. Spinal
Cord: Internal organization (pp. 136-140
in Woolsey)
Figure 2.
Schematic of divisions of spinal gray matter. (from Kandel, Schwartz & Jessell)
1.
Identify
the zones of the dorsal horn on slides
213, 250 with reference to your atlas. The lumbar segments of the cord are
often best for this purpose. Note the size and appearance of the cells in
different laminae (e.g., the cells of the substantia gelatinosa are small and
densely packed, while some of those in the nucleus proprius are large).
2.
Marginal zone.
The thin marginal or posteromarginal layer is composed of large cells
whose dendrites are arrayed tangentially within the lamina. The axons from these cells contribute to the
spinothalamic tract and other contralateral ascending projections.
3.
Substantia gelatinosa.
Most of the small cells of the substantia gelatinosa (SG) send their
axons to other segments of the spinal cord to end within SG of those segments.
In addition, axons from SG distribute over the dendrites of the marginal layer
neurons. SG is probably the most
critical region within the dorsal horn for sensory integration of, especially
(but not exclusively), inputs from pain fibers.
4.
Nucleus proprius.
Cells from nucleus proprius contribute to the spinothalamic tract,
spinoreticular tract, spinotectal tract, spinocervical tract ascending in the lateral
columns.
5. Identify nucleus dorsalis (a.k.a. Clarke's nucleus), which occurs only in
thoracic and upper lumbar segments of the cord (L1 - L3). It is an area of
large cells found lateral to the dorsal columns and in the ventral corner of
the dorsal horn. The nucleus dorsalis is the origin of the major fiber system
that sends information about the status of muscle spindle receptors to the
cerebellum: the dorsal spinocerebellar
tract (pp. 206-7 Woolsey). It is especially large in the upper lumbar
segments because this part of the nucleus receives afferent information from
all of the lower limb; the nucleus itself is not found below L2 or L3. (Since the nucleus is also not present above
T1, another system is required for equivalent input to the cerebellum from the
upper limb. The Ia and Ib axons from the arm project through the fasciculus
cuneatus of the dorsal columns to the lateral (or external) cuneate nucleus in
the lower medulla.) Projections to the
cerebellum are ipsilateral (see below).
6.
Identify motor neurons in the anterior horn of
the spinal cord. They are clustered into groups called motor nuclei. What is
the significance of this pattern? Are there differences between the groupings
and sizes of motor neurons in relation to spinal level? If so, why? The motor
neurons are divisible into 2 groups larger a motor neurons and smaller g
motor neurons that supply the work mucles and the muscles of muscle spindles
respectively. How does this correlate with axons seen in slide 1?
7.
Identify intermediolateral cells in the thoracic
spinal cord. What is there function and where do their axons travel?
D. Spinal Cord Ascending Somatic
Pathways.
1.
Identify
the segments containing both dorsal
column fasciculi: gracilis and cuneatus. At what level does the fasciculus
cuneatus first appear?
2.
Variations
by level: The cross‑sectional shape of the spinal cord varies at
different levels. The size of the dorsal
and ventral horns is larger in the cervical and lumbo‑sacral regions
(approximately C3 to T1 and L1 to S2, respectively) due to the greater number
of cells devoted to the innervation of the upper and lower limbs. Both the
sensory (dorsal) and motor (ventral) horns enlarge. Some cell groups are found only in restricted
parts of the cord. The most notable of these is the intermediolateral cell
column (lateral horn), which contains preganglionic sympathetic neurons
(restricted to T1 to L2) and preganglionic parasympathetic neurons (S2 to S4),
and the nucleus dorsalis (T1 to L3). The ratio of white matter (myelinated
axons) to gray matter (cells and cellular
processes) is much greater in the upper (e.g., cervical) segments than in the
lower (e.g., sacral) segments of the spinal cord.
3.
Identify
the following ascending sensory pathways in the white matter: dorsal
columns (dorsal funiculi), containing the gracile and cuneate fasciculi lateral
funiculi; lateral funiculi, containing dorsal and ventral spinocerebellar
tracts; and the spinothalamic tract. (pp.184-189 Woolsey)
E. Illustrative
Neuropathology
Pathological material helps identify fiber systems because lesions or disease cause degeneration and/or demyelination that show the location of specific tracts.
Figure 3.
Cervical spinal cord from a case of tertiary syphilis (tabes dorsalis).
Slide 21 shows demyelination of the gracile
fasciculus in the cervical cord in a patient with tabes dorsalis
(syphilis). A manifestation of tertiary
syphilis is death of infected dorsal root ganglion cells. Accordingly, the central processes of spinal
ganglion cells degenerate. Axonal
degeneration distal to the cell soma leads to secondary loss of myelin known as
Wallerian degeneration. The infection kills dorsal root ganglia in spinal
segments from sacral to lumbar ganglia.
Thus, this slide illustrates the medial position of degenerating fibers
from lower spinal segments. These
contrast with the lateral location of normal fibers from higher segments. This slide provides an example of the
"topographic" organization within the dorsal columns.
Slides 22
& 23 contain spinal
cord tissue from a patient with subacute combined sclerosis. In this disease demyelination occurs in large
diameter axons of the long spinal cord pathways, i.e., the dorsal columns,
dorsal spinocerebellar tracts and
lateral corticospinal tracts. This condition occurs in pernicious anemia because of severe vitamin B12 deficiency. What happens to conduction in a myelinated axon when a patch of myelin is lost? Why does this happen?
E.
General Aspects of the Internal Organization of the Brain Stem
1.Look at the brainstem on you brain
specimens and review the external features with reference to the atlas (pp.
40-45 Woolsey). What are the functions of the superior colliculus, inferior
colliculus?
2.Referenced structures can be found in on
four series of slides with sections through the brainstem: three sets have
myelin staining (slides 51-58, 222-227)
and one set has cell and fiber staining (301-314). Use the images in the atlas for
identification (pp. 141-154 Woolsey). Lay these sets out in caudal to rostral
order to provide alternate views through some levels. In finding the nuclei and fiber tracts noted
below, we recommend following sequentially the course of each structure or set
of related structures on several adjacent sections.
3.For orientation identify the central
canal/ventricle/cerebral aqueduct. Review the general layout of sensory and
motor cranial nerves and nuclei as summarized in the atlas (pp. 174-183
Woolsey).
F. Dorsal Column/Meidal Lemniscal System -
Brainstem
1.
Dorsal Column Nuclei. The medial cuneate and gracile nuclei are paired structures in the
closed medulla and caudal part of the open medulla. The gracile nuclei are slender, singular
structures that lie close to the midline (pages 184-185, Woolsey). The medial cuneate nuclei are slightly
rostral, and are larger and more lobulated (pages 141-141, Woolsey).
2.
Medial Lemniscus. The secondary fibers from the dorsal
column nuclei cross to the contralateral side and ascend to the thalamusas the
medial lemniscus. As these fibers cross
to form the medial lemniscus, they arc ventrally and medially to pass
underneath the central canal as internal
arcuate fibers (pages 143-145, Woolsey).
Throughout much of the medulla, the medial lemniscus occupies a vertical
strip close to the midline, medial to the inferior olivary nucleus and, dorsal
to the pyramidal tract (page 142, Woolsey).
Sacral to cervical parts of the body, respectively, map onto ventral to
dorsal portions of the medial lemniscus.
In the caudal pons the bundle flattens across the dorsal surface of the
pontine grey (pages 146-149, Woolsey); this orientation persists up through the
midbrain (pages 150-154 Woolsey). Upon
assuming a horizontal alignment, the sacral representations are lateral, while
representations from the face, contributed by the trigeminal system (see
below), are medial.
G. Anterolateral System – Brain Stem
The anterolateral system consists of
the spinothalamic, spinoreticular and
spinomesencephalic tracts. From the
anterolateral funiculus of the spinal cord, fibers continue through the medulla
in the same position. In the closed
medulla this tract is lateral to part of the reticular formation known as the
lateral reticular nucleus (another cerebellar relay center) (pp. 186-187
Woolsey). In the open medulla, the tract
occupies a hilus found dorsal to the bump created by the inferior olivary
nucleus. Many fibers in the tract at this point are destined for brainstem
targets (e.g., raphe nuclei, periaqueductal grey, and portions of the reticular
formation). In the pons the tract is
difficult to identify because the middle cerebellar peduncle covers it
laterally and fibers associated with the auditory relay nuclei cross through it
. In the upper pons the tract adjoins
the lateral edge of the medial lemniscus with which it associates until
reaching termination targets in the thalamus.
H.
Trigeminal System
1.
Spinal Trigeminal Nucleus and Tract (see pp 188-189 Woolsey). This
structure is the caudal extension of a longitudinally arranged trigeminal
complex that starts in the pons. It
exists in three parts (see below). First
order, sensory projections to this nucleus course in the spinal tract of the trigeminal nerve. Identify this fiber tract
where it surrounds the lateral boundary
of the spinal trigeminal nucleus. In the closed medulla the nucleus lies
ventrolateral to the medial cuneate nucleus and displays a laminar array of
cells that resembles the dorsal horn (Figs. 40-41). Here the spinal trigeminal nucleus is called subnucleus caudalis Projections from this nucleus cross the
midline to join the spinothalamic tract, while information from peri‑oral
and oral structures projects bilaterally.
These connections convey pain and temperature information from the head
and face. Identify the substantia gelatinosa, marginal layer and nucleus
proprius regions of subnucleus caudalis.
Note the resemblance to the dorsal horn of the spinal cord. At the level
of the open medulla the spinal trigeminal nucleus loses its dorsal horn‑like
lamination and its name changes to subnucleus interpolaris. It contributes crossed and uncrossed
connections to the spinothalamic tract.
Most of this concerns information from low threshold cutaneous
receptors.
2.
Main or Principal Sensory Nucleus of
the Trigeminal Nerve This
nucleus exists in the rostral pons. It
is located at the level where the superior cerebellar peduncle lines the
lateral walls of the 4th ventricle. The
ventrolateral tip of this peduncle points toward the nucleus; the middle
cerebellar pedunclesurrounds it laterally and ventrally. The nucleus is rostral to the entry point of
the trigeminal nerve. Crossed
projections from this nucleus add to the medial aspect of the medial lemniscus
and convey discriminative touch information from the face and head. Cell groups in the nucleus representing peri‑
and oral structures send a separate, uncrossed projection to the thalamus. This ipsilateral bundle courses through the
dorso‑lateral corner of the central tegmentum and only joins the medial
lemniscus in the rostral midbrain.
Consequently, unilateral infarcts that interrupt the medial lemniscus
spare somatosensory sensations from the mouth.
3.
Mesencephalic
Trigeminal Nucleus and Tract This small nucleus first appears at the same
level as the principal sensory trigeminal nucleus. It lies between the ependymal lining of the
IVth ventricle and the fibers of the superior cerebellar peduncle. A thin,
underlying fiber tract, the mesencephalic trigeminal tract, marks its position.
This nucleus remains in approximately the same location through the midbrain where
its large sensory ganglion‑like cells pepper the lateral border of the
periaqueductal grey. The nucleus
contains displaced sensory ganglion cells that innervate muscle spindle
receptors in face musculature.
I.
Proprioception to the cerebellum
1.
Spinocerebellar Tract and Inferior
Cerebellar Peduncle The
dorsal spinocerebellar tract from nucleus dorsalis (Clarkes' nucleus) in the
spinal cord relays proprioceptive information from the lower limb to the
cerebellum (pp. 206-207). In the open
medulla this tract disappears as a separate structure. The fibers join with the outflow from the
lateral cuneate nucleus (see below) to form part of the inferior cerebellar
peduncle. The inferior cerebellar
peduncle contains projections to the cerebellum from a vast array of brainstem
nuclei (e.g., inferior olivary nucleus, vestibular nuclei, various nuclei of the
reticular formation, etc.). The ventral spinocerebellar tract parallels that of
the spinothalamic tract. Its presence as
a separate bundle of axons cannot be detected on your slides.
2.
Lateral
(External) Cuneate Nucleus The lateral nuneate nucleus relays
proprioceptive information from the upper limb to the cerebellum . The major projection joins the ipsilateral inferior cerebellar peduncle and a
minor projection crosses with the internal arcuate fibers to join the medial
lemniscus.
J. Brain Stem Nuclei Implicated in Pain
Control Mechanisms
1.
Raphe nuclei (raphe magnus) These nuclei occupy the midline
throughout the center of the brainstem. Serotonergic connections to the
brainstem nuclei and spinal cord arise from these nuclei.
2.
Nuclei of the Periaqueductal Grey This collection of nuclei surrounds
the cerebral aqueduct of sylvius in the midbrain. Many of these cells interconnect with the
raphe nuclei in a circuit concerned with modulation of sensory signals from
pain receptors.
K. Thalamus
1.
General
aspects of the Thalamus. Study slides 307-312,
221-224. Use a general diagram such as XX
in Heimer to review the general organization of the thalamus. It is divided in
to distinct nuclei named by position. Recall the location and function of the
lateral and medial geniculate nuclei
(see pp. 40-45, 155-162, 192-193, 196-197 Woolsey).
2.
Ventral Posterior Medial and Lateral
Nuclei (VPM and VPL of
the thalamus). These nuclei appear in the ventral and posterior third of the
thalamus. They lie close to the junction with the midbrain where the medial
lemniscus and spinothalamic (see below) tracts coalesce before entering the
thalamus. The ventroposterior nuclei
appear more darkly stained than surrounding nuclei in fiber stained sections
because of the heavier myelination of incoming axons from the medial lemniscus
and spinothalamic tract (pages 160-161 Woolsey). Identify ventroposterior
medial nucleus for the face (see
trigeminal system, below) and ventroposterior
lateral nucleusfor the rest of the body.
Fiber bundles separate these nuclei.
L. Cortex
1.
Review the
principal lobes, gyri and sulci of the cerebral hemisphere (pp. 20-21, 24-25,
36, 37 Woolsey). Locate the central sulcus on your brain’s slices (plastic
embedded slices). Review the location of the histologically defined areas as
described by Brodmann (p.12 Woolsey).
2.
Slide 230
has three sections across the central sulcus. What distinguishes precentral
(motor) cortex from post central (somatosensory) cortex? Is the cell body
pattern in the latter the same all across the postcentral gyrus?
BEFORE LEAVING THIS LABORATORY...
Know the location of the major
ascending somatosensory tracts in the brainstem. For each somatosensory tract, know its cells
of origin, submodality, topographic organization, and the side of the body
represented.
cervical and lumbosacral enlargements
cuneate and gracile nuclei
dorsal and ventral spinocerebellar
tracts
dorsal columns (gracile and cuneate
fasciculi)
dorsal horn
dorsal root
dorsal root ganglion
dorsal spinocerebellar tract
dorsolateral sulcus
intermediate grey
internal arcuate fibers
lateral (external) cuneate nucleus
lateral funiculi
Lissauer's tract (dorsolateral tract)
main or principal sensory nucleus of
the trigeminal nerve
medial lemniscus
mesencephalic trigeminal nucleus and
tract
nucleus dorsalis
nucleus proprius
posteromarginal zone
spinal nerve roots
spinal tract of the trigeminal nerve
spinal trigeminal nucleus, subnucleus
caudalis
spinothalamic tract
Spinothalamic tract
substantia gelatinosa
ventral horn
ventroposterior lateral nucleus (VPL)
ventroposterior medial nucleus (VPM)