Peripheral Nervous System
Time period: day 27 to birth
Introduction
The peripheral nervous system
develops in tandem with the brain and spinal cord. It connects the central
nervous system to structures of the body as they form and includes the spinal
nerves, cranial nerves and autonomic nervous system.
This process begins with
neurulation (see Chapter 17), when ectoderm is induced by the notochord to form
neuroectoderm. This neuroectoderm in turn produces neuroblasts (primitive
neurons) and neural crest cells.
Neural crest cells migrate out
from the neural tube, passing towards multiple targets throughout the embryo
(see Chapter 18). Some neural crest cells only migrate a little way from the
developing spinal cord, collect together and differentiate to form neurons of
the dorsal root ganglia (Figure 45.1). Located bilaterally to the spinal
cord, the dorsal root ganglia send afferent processes back towards the alar
plate of the spinal cord (see Figure 44.1), eventually passing to the dorsal
horn. The dorsal root ganglia also send processes out to run alongside
processes of neurons of the ventral root. Their combined bundle of neuronal
axons become the spinal nerve.
Dorsal root ganglia contain the
cells bodies of sensory neurons from afferent spinal nerve fibres. It could be
considered that these ganglia are grey matter of the spinal cord that have
moved out to the peripheral nervous system.
Neurons in the (ventral) basal
plates of the developing spinal cord send fibres outwards from the cord to meet
the fibres from the dorsal root ganglion. Fibres grouping to leave the spinal
cord form the ven- tral horn, the motor fibres of the spinal nerves (Figures
44.1 and 45.1). The mixed collection of motor and sensory fibres split again
almost as soon as they meet, forming two bundles of fibres: the dorsal ramus
and the ventral ramus. The dorsal ramus passes dorsally to the deep
axial musculature of the back, the synovial vertebral joints and the skin of
this region. The ventral ramus sends fibres to the ventral and lateral parts of
the trunk and to the
upper and lower limbs, depending
upon the spinal level.
Neural crest cells migrate to the
new axons that extend away from the central nervous system and wrap themselves
around them, differentiating to become neurolemmal (Schwann) cells.
Myelin within these cells causes the fibres to appear white. This is apparent
from around week 20.
Spinal nerves develop at the
level of each somite, and the segmented organisation of embryonic development
can be seen in the adult segmented pattern of cutaneous dermatomes (see Chapter
22).
A dermatome is a region of skin
that is predominantly supplied by the sensory component of one spinal nerve.
The dermatomes are named according to the spinal nerve that supplies them (see
Figure 22.5).
Some neural crest cells migrating
out from the neural tube collect dorsolaterally to the dorsal aorta in the
thorax. They differentiate and become chains of sympathetic ganglia connected
by longitudinal nerve fibres running cranially and caudally (Figure 45.2).
Neuroblasts migrate from the ganglia cranially into the neck and caudally into
the abdomen and pelvis to complete the sympathetic trunks.
Other neural crest cells migrate
ventrally to the aorta to form the preaortic ganglia such as the coeliac
and mesenteric ganglia, while others migrate towards organs such as the lungs,
heart and gastrointestinal tract to form sympathetic organ plexuses.
Sympathetic neurons of the developing
spinal cord in the inter- mediolateral cell column (intermediate or lateral
horn) of thoracolumbar segments T1–L2 (Figure 45.3) send axons out from the
spinal cord through the ventral root to each trunk ganglion. These axons form
the white ramus communicans passing between the spinal nerve and the
sympathetic ganglion (Figure 45.2). These preganglionic sympathetic fibres
either synapse with neurons in the ganglion, ascend or descend to synapse in a
ganglion of a different level, or pass through the ganglion/ganglia without
synapsing to run towards preaortic (or prevertebral) ganglia or organs as
splanchnic nerves.
Postganglionic fibres pass
onwards to viscera or group together to pass back to a spinal nerve as a grey
ramus communicans (unmyelinated). Grey ramus communicans are found at all
spinal levels and allow postganglionic fibres to pass with other nerves to
reach structures, for example in the limbs.
Neurons of the parasympathetic
nervous system form in the brainstem and sacral part of the spinal cord. The
parasympathetic nuclei in the brainstem contribute preganglionic fibres to the
oculomotor (CN III), facial (CN VII), glossopharyngeal (CN IX) and vagus (CN X)
nerves. Preganglionic parasympathetic fibres of the sacral spinal cord form
pelvic splanchnic nerves.
Postganglionic parasympathetic
neurons differentiate from neural crest cells and can be found either in
ganglia or near the viscera.
Cranial nerves develop in a
similar way to the spinal nerves, with motor nuclei differentiating from the
neuroepithelium and sensory nuclei forming outside the brain.
The olfactory nerve (CN I) and
the optic nerve (CN II) are linked with the telencephalon and diencephalon,
respectively. The olfactory nerve connects to the olfactory bulb, a growth from
the prosencephalon. The oculomotor nerve (CN III) originates from the dorsal
midbrain (mesencephalon) and the trochlear nerve (CN IV) from the ventral
midbrain (metencephalon). Similarly, CN V–VIII arise from the metencephalon and
CN IX–XII arise from the myelencephalon.
Cranial nerves IV–VII and IX–XII
develop from the hindbrain which has been divided into eight sections called rhombomeres
by day 25 (Figure 45.4). Rhombomeres appear ventral to the cephalic flexure
by day 29 and will form the motor neurons for these cranial nerves (note that
these are the cranial nerves with motor function, hence the skipping of CN
VIII).
Neural crest cells form the
parasympathetic neurons for CN III, VII, IX and X.
The development and structure of
the cranial nerves is very al nerves, but remember that not all cranial erves
carry both sensory and motor neurons.
