#08 Flashcards
During gastrulation the embryo converts from a bilaminar disk to a trilaminar embryo
comprised of three germ layers,
Endoderm: The innermost of the three primary germ layers of an embryo,
developing into the gastrointestinal tract, the lungs, and associated structures.
* Mesoderm: The middle embryonic germ layer, lying between the ectoderm and the
endoderm, from which connective tissue, muscle, bone, and the urogenital and
circulatory systems develop.
2
* Ectoderm: The outermost of the three primary germ layers of an embryo, from which
the epidermis, nervous tissue, and sense organs develop
The nervous system then develops in stages, some of which overlap in time:
- Neural induction and patterning: The neural plate forms from surface ectoderm,
then patterned into dorsoventral and anteroposterior domains - Neurulation: The neural plate folds up and the lateral edges fuse dorsally forming
the neural tube - Vesicle formation: The neural tube bends and expands to form characteristic
swellings called vesicles - Neuronal proliferation: Neuroblasts undergo mitosis in an area adjacent to
ventricles - Neuronal migration: Post-mitotic neurons travel from the subventricular zone to
their proper position within the maturing brain - Axon outgrowth and pathfinding: Neurons extend axons to innervate targets,
establishing roles in cell-cell signaling - Neuronal differentiation: Discrete populations of neurons acquire their unique
characteristics - Myelination
- Post-natal development
During the first stage, the neural plate forms from m, and is then
patterned into dorsoventral and anteroposterior domains.
m: surface ectoderm
Week 3: On the a side of the embryo, local mesodermal structures (e.g., the
notochord) secrete b which induce overlying c cells to
become d
a dorsal
b growth factors
c ectoderm
d neural plate
- Ectoderm cells secrete a which acts back on these same cells (i.e., in an autocrine fashion) to block neural development.
- The b, a dorsal mesoderm derivative, secretes noggin and chordin, two molecules which bind a and prevent its action
- Absent a signaling, ectoderm cells adopt a neural fate and become c.
a: Bone Morphogenic Protein 4 (BMP4)
b notochord
c neural plate
Tissues flanking the nervous system secrete growth factors which instill regional pattern
on the nervous system. This process begins at the neural plate stage but continues during
neurulation. The signals are usually a, defined as having dose-dependent
effect on the fate of responding cells; i.e., a higher dose induces one fate, and a lower
dose a different one.
a morphogens
Stage 2: neurulation
After neural induction, the neural plate folds up and the
lateral edges fuse dorsally forming the neural tube
In the adult, the spinal cord remains a tube while the
brain undergoes several characteristic bends and
swellings resulting from cellular proliferation
The most lateral domain of the neural plate becomes the
neural crest, a population of
pluripotent precursor cells.
Anencephaly
failure of the anterior neuropore to close, resulting in a major portion
of the brain, skull, and scalp being absent. This defect is generally incompatible with
life and infants delivered with this disorder typically do not live more than a few
hours or days after birth.
Rachischisis
failure of the posterior neuropore to close; the neural tube is not fully
“rolled up”. Rachischisis patients have motor and sensory deficits, chronic infections,
and disturbances in bladder function. The defect
often occurs with anencephaly.
Spina bifida:
Essentially a less severe form of
rachischisis, spina bifida defects involve
incomplete formation of the vertebrae and
meninges surrounding the spinal cord
Ventral root (motor) fibers of each spinal nerve innervate a group of skeletal muscles
called a
myotome.
Upper limb –
elbow flexors a
elbow extensors b
hand muscles c
a: C5-6
b: C7
c: C8/T1
Lower limb –
hip flexors a
knee extensors b
ankle dorsiflexors c
ankle plantarflexors d
a: L2
b: L3
c: L4
d: S2
Diaphragm – a
Pelvic diaphragm/perineum –
b
a C3/4/5
b S2/3/4
Many (not all!) dorsal root ganglion fibers
terminate in different portions of the
dorsal horn
Alar plate
Basal Plate
Dorsal- sensory
ventral-motor
Neural crast derivatives
-jaw, most of the skull, meninges
- melanocytes (skin, pigment)
-primary sensory neurons
-Shwann cells (myelination in PSN)
-post-ganglionic autonomic neurons
mesoderm cells differenciate into
somites
The neural tube closes at ~28th day of gestation, first at the a then at the b
anterior neuropore
posterior neuropore
Important dermatomes
fingertips
nipple
umbilicus (belly button)
toes
C6-8
T4
T10
L5/S1
dorsal pathway in spinal cord is involved in
pain and temperature signaling
lamina 1 posteromarginal nucleus
most pain and temperature signal
organization of dorsal horn
lamina1: posteromarginal nucleus
lamina 2: substantia gelatinosa
lamina 3-5: nucleus proprius
lamina 6: base of dorsal horn
organization of ventral horn
lamina 8: reticular core of ventral horn
lamina 9: LMN groups
a receives signals from certain muscle proprioceptors and so that we know where our leg, arm is
clarke’s nucleus
within the ventral horn, flexors are more a and extensors are more b
dorsal
ventral
Lower motor neurons in ventral horn are divided into two groups
Medial group: innervate trunk muscles
Lateral group: innervate limb muscles
Micturition pathway and bladder control
Visceral afferents follow parasympathetics back to S2-S4
Parasympathetic efferent neurons S2-S4 (via pelvic splanchnic nn.) contract detrusor/rectum wall
Postganglionic parasympathetic GVE neuron in wall of bladder stimulates detrusor muscle contraction…
Somatic control of the external urethral sphincter (via Onuf’s nucleus) lets you
“hold it in”
Injury above T12 vertebral level (L1/L2 spinal level) leads to
reflex
(spastic) bladder: incontinence
leakage
Injury below T12 leads to
flaccid (atonic) bladder: retention
hard to urinate
Explain how the adult spinal cord is supported and protected during movements of the vertebral column
- Central position of spinal cord prevents compression/stretching during vertebral column movements
- Epidural fat, CSF, and meninges (including denticulate ligaments and filum terminale) provide support and protection to the cord during movement
o Meninges includes: dura, arachnoid, and pia - Attachment of dural sheaths of spinal roots form the main support for keeping the spinal cord centrally placed and away from intervertebral joints Denticulate Ligaments*
A form the main support for keeping the
spinal cord centrally placed and away from
intervertebral joints
Attachment of dural sheaths of spinal
roots
Clarke’s nucleus (lamina 8) receives
signals from
certain muscle
proprioceptors
List three important “independent” motor nuclei found in the ventral horn and explain their basic functions
- Spinal accessory nucleus C1-C5 – origin of the spinal accessory nerve (CN XI) for somatomotor control of the Trapezius and Sternocleidomastoid mm.
- Phrenic nucleus C3-C5 – origin of the phrenic nerve for somatomotor control of the Diaphragm
- Onuf’s nucleus S1-S4 – origin of the pudendal nerve for somatomotor control of the external (voluntary) urethral and anal sphincters
Important neural cell coloumns are:
Clarke’s nucleus
IML
Lateral group LMN
C8-L3
T1-L2
C4-T1, L2-S2
