002_Nerve tissue and nerve system.pdf

1. nerve tissue and nerve system
lecturer
Paul A. Kobeza

2. Nerve tissue consists of 2 principal types of cells: neurons and supporting cells. The neuron is the structural and
functional/electrically excitable unit of the nervous system that receives, processes, and transmits electrical
signals to and from other parts of the nervous system via its cell processes. There are multiple types of neurons
that are classified based on their anatomic structure and function as sensory neurons, motor neurons, and
interneurons. The functional components of a neuron include dendrites (to receive signals), a cell body (to drive
cellular activities), an axon (to conduct impulses to target cells), and synaptic junctions (specialized junctions
between neurons that facilitate the transmission of impulses between neurons; they are also found between
axons and effector/target cells, such as muscle and gland cells). Supporting cells are called neuroglial cells and
are located close to the neurons; however, these cells do not conduct electrical signals. The CNS consists of 4
types of glial cells: oligodendrocytes, astrocytes, microglia, and ependymal cells, each having a different function.
In the PNS, the supporting cells are called peripheral neuroglia and include Schwann cells, satellite cells, and
various other cells having specific structures and functions. Schwann cells surround the processes of nerve cells
and isolate them from adjacent cells and the extracellular matrix by producing a lipid-rich myelin sheath, ensuring
the rapid conduction of nerve impulses. Satellite cells are similar to Schwann cells, but they surround the nerve
cell bodies. In the CNS, oligodendrocytes produce and maintain the myelin sheath. A nerve is composed of a
collection of bundles (or fascicles) of nerve fibers. Within the CNS, the brain and spinal cord tissue can be
classified as gray or white matter, depending on the tissue composition. White matter is most notably composed
of myelinated nerve fibers, whereas gray matter is made up of neuronal cell bodies.
General information

3. Neurons
Definition
Neurons are:
 Electrically excitable cells that receive, process, and transmit
signals throughout the body
 Important components of the CNS and PNS
Parts of a neuron
Neurons consist of 3 main parts:
Dendrites:
 Branched (tree-like) appendages (processes)
 Receive signals from:
o Axons of other neurons (via synapses)
o Sensory epithelial cells
o Environment
 Cytoplasm contains:
o Nissl bodies:
 Basophilic granular regions
 Made up of clusters of rough endoplasmic
reticulum and ribosomes
o Microtubules and neurofilaments
Cell body:
 Also called the soma or perikaryon
 Contains:
o Nucleus:
 Often large
 Pale staining
 Prominent nucleolus
 Some neurons are binuclear.
o Rough endoplasmic reticulum
o Ribosomes and polyribosomes that synthesize:
 Structural proteins
 Transport proteins
o Golgi bodies
o Lysosomes
o Mitochondria
o Microtubules and neurofilaments

4. Axon:
 Cylindrical process:
o Conducts nerve impulses to target cells
o Usually, only 1 axon is present.
o Collateral branches may be present to communicate with
many target cells.
 Structure:
o Connected to the cell body by the axon hillock (short, pyramid-
shaped area)
o Initial segment:
 Region between the axon hillock and point of myelination
 Site where an action potential is generated (or not)
 Contains a collection of different ion channels
o Myelinated axon contains:
 Myelin sheath: regional insulation produced by
specialized glial cells (Schwann cells and
oligodendrocytes)
 Nodes of Ranvier: gaps between myelin coverings that
facilitate rapid impulse conduction
o Axon terminal:
 End of the axon
 Several branches terminating in synaptic end bulbs
 Communication with target cells via synapses
 Components:
o Axolemma: plasma membrane covering
o Axoplasma contains cytoplasm and components such as:
 Abundant mitochondria → provide energy
 Microtubules → anterograde and retrograde transport
between the cell body and axon
 Neurofilaments → provide structural support to the cell
 Note: Nissl bodies are absent.

5. Structure of neurons

6. Morphologycal characterization
Neurons may be classified by the number of processes (axon and
dendrites) attached to the cell body.
Multipolar neurons:
 Contain > 2 processes
 Include:
o A single axon on 1 end
o Many dendrites connecting to the cell body
 Most commonly found in:
o Brain
o Spinal cord
 Examples:
o Motor neurons
o Interneurons
o Pyramidal cells:
 Located in the cerebral cortex
 Axons project down to the spinal cord → communicate
with ventral horn cells → movement of skeletal muscles
o Purkinje cells:
 Located in the cerebellar cortex
 Responsible for controlling and coordinating motor
movements
 Mostly inhibitory neurons (release GABA)

7. Bipolar neurons:
 Have 2 processes:
o Axon projecting from 1 end of the cell body
o Dendritic tree extending from the other end
 Most commonly found in:
o Ganglia of cranial nerve (CN) VIII
o Retina
o Olfactory epithelium
Pseudounipolar neurons:
 Have a single process, which bifurcates near the cell body:
o One end contains dendrites.
o Signals travel directly to the axon at the other end.
 Function as sensory neurons
 Most commonly found in:
o Dorsal root ganglia
o CN ganglia
Unipolar neurons:
 Also known as monopolar neurons
 Contain only 1 extension from the cell body (axon)
 Dendrites can connect to the axon directly.
 Rarely found in vertebrates

8. Unipolar neurons have only one process emerging
from the cell body which causes them to appear T-
shaped. True unipolar cells are only found in
invertebrate animals, so the unipolar cells in humans
are more appropriately called “pseudo-unipolar” cells.
Human unipolar cells have an axon that emerges
from the cell body, but it splits so that the axon can
extend along a very long distance. At one end of the
axon are dendrites, and at the other end, the axon
forms synaptic connections with a target. Unipolar
cells are exclusively sensory neurons and have two
unique characteristics. First, their dendrites are
receiving sensory information, sometimes directly
from the stimulus itself. Secondly, the cell bodies of
unipolar neurons are always found in ganglia.
Sensory reception is a peripheral function (those
dendrites are in the periphery, perhaps in the skin) so
the cell body is in the periphery, though closer to the
CNS in a ganglion. The axon projects from the
dendrite endings, past the cell body in a ganglion,
and into the central nervous system.

9. Bipolar cells have two processes, which extend from
each end of the cell body, opposite to each other.
One is the axon and one the dendrite, forming a
straight line. Bipolar cells are not very common. They
are found mainly in the olfactory epithelium (where
smell stimuli are sensed), and as part of the retina.
Multipolar neurons are all of the neurons that are
not unipolar or bipolar. They have one axon and two
or more dendrites (usually many more). With the
exception of the unipolar sensory ganglion cells, and
the two specific bipolar cells mentioned above, all
other neurons are multipolar. Some cutting edge
research suggests that certain neurons in the CNS do
not conform to the standard model of “one, and only
one” axon.
Some sources describe a fourth type of neuron,
called an anaxonic neuron. The name suggests that it
has no axon (an- = “without”), but this is not accurate.
Anaxonic neurons are very small, and if you look
through a microscope at the standard resolution used
in histology (approximately 400X to 1000X total
magnification), you will not be able to distinguish any
process specifically as an axon or a dendrite. Any of
those processes can function as an axon depending
on the conditions at any given time. Nevertheless,
even if they cannot be easily seen, and one specific
process is definitively the axon, these neurons have
multiple processes and are therefore multipolar.
Neurons can also be classified on the basis of where
they are found, who found them, what they do, or
even what chemicals they use to communicate with
each other. Some neurons are named on the basis of
those sorts of classifications.

11. Functional classification
Neurons may also be classified based on their functional role and the
direction in which they transmit signals (toward or away from the
CNS).
Sensory neurons:
 Afferent signals: move toward the CNS
 Receive stimuli from:
o Within the body
o External environment
 Subdivided into:
o Somatic afferent neurons:
 Sense touch, temperature, pain, pressure, proprioception
 Stimuli obtained from receptors in skin, skeletal muscle,
tendons, and joints
o Visceral afferent neurons:
 Transmit visceral sensations (e.g., intestinal
distension, ischemia)
 Stimuli obtained from internal organs
Motor neurons:
 Efferent signals: move toward the periphery
 Conduct impulses to peripheral targets:
o Somatic (voluntary) efferent neurons innervate skeletal
muscles.
o Visceral efferent neurons innervate:
 Smooth muscles
 Glands
Interneurons:
 Connect and enable communication between neurons (sensory or
motor)
 Play a major role in some reflex arcs
 Comprise most of the neurons in the CNS

12. Synapses
Synapses are formed between two neurons, or between a neuron
and a target cell, such as a muscle cell.
Between two neurons, synapses can form between:
an axon and a dendrite (axodendritic)
an axon and an axon (axoaxonic)
an axon and a cell body (axosomatic)
Chemical synapses are common.
presynaptic terminal - part which
delivers the nerve impulse
postsynaptic terminal - part which
receives the impulse
synaptic cleft - gap between the
pre- and post synaptic membranes.
The presynaptic terminal can be
recognised in the EM, because it has
synaptic vesicles, that contain
neurotransmitter, and mitochondria.
The synapse formed between an
axon and a muscle fiber is called
a neuromuscular junction. This is a
chemical type of synapse.

13. Neuroglia
Neuroglia, also known as glial cells, are the most abundant cells in the CNS.
 Have multiple functions and provide a suitable environment for neuron activity
 Unlike neurons, neuroglia maintain the ability to undergo cell division.

14. Location
Neuroglia can be classified based on their location
within the nervous system.
 CNS:
o Astrocytes
o Ependymal cells
o Microglia
o Oligodendrocytes
 PNS:
o Schwann cells
o Satellite glial cells

16. Astrocytes
Location:
 CNS
 Subdivided into:
o Fibrous astrocytes: mainly in the white matter
o Protoplasmic astrocytes: mainly in the gray matter
Features:
 Largest of the neuroglia
 Star shaped due to multiple radiating processes
 Structures:
o Astrocytic endfeet:
 Form connections with other cells/structures
 Perivascular: surround capillaries (important component
of the blood–brain barrier)
 Perineuronal: surround neurons
o Glial filaments:
 Cytoplasmic components
 Bundles of intermediate filaments that reinforce cell
structure
 Contain glial fibrillary acid protein (GFAP) → important
marker
o Glycogen granules:
 Cytoplasmic component
 Can be broken down into glucose → energy

17. Functions:
 Connect neurons to:
o Capillaries
o Pia mater
 Control the environment:
o Regulating cerebral blood flow (via Ca2+ signaling)
o Buffering extracellular ion concentrations (e.g., K+)
o Clearing excess neurotransmitters
o Releasing neuroactive molecules
(e.g., enkephalins, endothelins, somatostatin)
 Transfer molecules to neurons:
o Ions from the blood (via endfeet)
o Lactate (after conversion from glucose)
 Proliferate and form glial scar tissue in damaged areas of the CNS

18. Protoplasmic Astrocytes
fibrous astrocytes

19. Ependymal cells
Location:
 CNS
 Form the epithelial lining of:
o Central canal of the spinal cord
o Ventricles
Features:
 Columnar epithelial cells
 Some are ciliated
 Generally have loose junctions
 Specialized types connect to capillaries:
o Choroid epithelial cells
o Tanycytes
 Choroid cells are connected together by tight junctions → create the
blood–CSF barrier
 Tanycytes have:
o Long processes
o Large endfeet
Functions:
 Cilia facilitate the movement of CSF.
 Choroid epithelial cells of the choroid plexus produce CSF.
 Tanycytes facilitate the transport of hormones.

20. Microglia
Unlike most neuroglia (which are derived from the neuroectoderm),
microglia are immune cells derived from the mesoderm.
Location:
 CNS
 Throughout the brain and spinal cord
Features:
 Small
 Elongated
 Short processes (when activated, processes retract → the cell appears
similar to a macrophage)
 Dense, elongated nuclei
Functions:
 Phagocytic cells that are important for:
o Inflammation:
 Release of inflammatory mediators
 Act as antigen-presenting cells
o Repair
o Removal of cellular debris
 Derived from monocytes

21. Oligodendrocytes and Schwann cells
The neuroglia listed below produce myelin but differ in their location
within the nervous system.
Oligodendrocytes:
 Location:
o CNS
o Cell processes wrap around axons.
o Subdivided into:
 Interfascicular oligodendrocytes: mainly found in white
matter
 Satellite oligodendrocytes: mainly found in gray matter

22.  Functions:
o 1 cell branches to myelinate many axons.
o Satellite oligodendrocytes:
 Not directly involved in myelination
 Possibly regulate extracellular fluid
Image of an oligodendrocyte in the process of myelinating axons

23. Schwann cells
Location:
PNS
Cell wraps around axons.
Functions:
1 cell forms myelin for 1 segment of an axon.
Plays a role in the regeneration of damaged axons
Myelin sheath:
Composed of:
Proteins
Lipids
Insulates axons → ↑ velocity of action potentials
Separates axons from the extracellular space
Myelination of an axon
Mammal. Spinal cord. Transverse section. 500X

24. Ultrastructure of the Cell, myelinated axon and Schwann cell

28. Satellite glial cells
 Location:
o PNS (ganglia)
o Cover neuronal cell bodies
 Functions:
o Not entirely known, but likely similar to astrocytes
o May include:
 Structural role
 Maintenance of chemical homeostasis
 Potential contribution to pain

29. Nerves

30. Nerve fibers
Nerve fibers
Nerve fibers are the axons of
neurons and can be classified
based on the presence/absence of
a myelin sheath.

32. Myelinated:
 Generally thicker axons
 Axons are enveloped by the myelin sheath:
o PNS: formed when the Schwann cell wraps around axons
o CNS: formed by oligodendrite processes
 Nodes of Ranvier are present.
 Conduction of nerve impulses is faster.
 Appear white
 Includes group A and B fibers:
o Group A fibers are subdivided into:
 A-alpha: innervate primary receptors of the muscle
spindle and Golgi tendon organ
 A-beta: innervate secondary receptors of the muscle
spindle and cutaneous mechanoreceptors
 A-delta: free nerve endings that transmit pain stimuli
(pressure and temperature)
 A-gamma: motor neurons that control intrinsic activation
of the muscle spindle
o B fibers:
 Relay autonomic information
 Thinly myelinated

34. Unmyelinated:
 Generally thinner axons
 Not sheathed in myelin
o PNS:
 Axons lie within clefts of Schwann cells.
 Unlike in the case of myelinated fibers, 1 Schwann cell
may surround several axons.
o CNS:
 Not associated with oligodendrocytes
 Axons are separated by astrocyte processes.
 Nodes of Ranvier are absent.
 Conduction of nerve impulses is slower.
 Appear gray
 Includes group C fibers: relay information from thermal, mechanical,
and chemical stimuli

36. Nerve fibers

37. Peripheral nerves
 Nerves are formed by bundles (fascicles) of sensory and motor
nerve fibers.
 The fascicles are held together by layers of connective tissue.
 Epineurium:
o Outer layer of dense, fibrous connective tissue
o Comprises 30%‒75% of cross-sectional area
 Perineurium:
o Epithelium-like cells
o Wraps around fascicles
o Creates a barrier to protect nerve fibers
 Endoneurium:
o Innermost layer of loose connective tissue
o Surrounds groups of unmyelinated axons or
single, myelinated axons

38. Myelinated and Unmyelinated Axons
The Structure of a Peripheral Nerve

39. Ganglia
The neuronal cell bodies of nerve fibers can reside in
the CNS (brain, spinal cord, or cranial nerve ganglia)
or in the PNS (peripheral ganglia).
General:
 A ganglion is a collection of somas, which may also contain the
following:
o Satellite cells
o Connective tissue capsule
o Basement membrane
 Oval appearance

40. Some major types of peripheral ganglia:
 Dorsal root ganglia:
o Location: adjacent to the dorsal nerve root
o Contain:
 Sensory neuron cell bodies (usually pseudounipolar)
 Axons
 Satellite cells
 Autonomic ganglia:
o Location:
 Sympathetic: sympathetic trunk, close to the spinal cord
 Parasympathetic: near/within visceral organs
o Characteristics:
 Contains neuronal cell bodies with large dendritic trees
(multipolar)
 Satellite cells are less prominent.
 Enteric ganglia:
o Location: wall of the GI tract
o Characteristics:
 Very small compared with other ganglia types
 Lack connective tissue capsule

41. Dorsal Root Ganglion

42. Central Nervous System
White and gray matter
The tissue of the CNS (brain and spinal cord) has a characteristic
classification as white or gray matter.
 White matter:
o Contains myelinated nerve fibers
o Typically does not contain cell bodies
 Gray matter generally contains:
o Cell bodies and dendrites
o Neuroglia

43. Brain
Gray matter generally makes up the external layer of the brain.
Cerebellar cortex (outer layer) has 3 layers:
o Molecular cell layer (outer):
 Basket cells and stellate cells (multipolar, GABAergic
interneurons)
 Axons from the granule cell layer
o Purkinje cell layer (middle):
 Contains Purkinje cell bodies
 Dendrites extend into the molecular layer.
 Axons extend through the granule cell layer.
o Granule cell layer (inner):
 Granule cells (small neurons)
 Golgi cells (GABAergic interneurons)

45. Cerebral cortex
 Cerebral cortex (outer layer) has 6 layers:
o Molecular (outer) layer contains dendrites and axons from
other layers.
o External granular layer contains:
 Stellate cells
 Small pyramidal cells
o External pyramidal layer: contains pyramidal cell bodies
o Internal granular layer: similar to the external granular layer
o Internal pyramidal layer: contains more pyramidal cell bodies
o Multiform (inner) layer: contains fusiform cells
 Basal nuclei/ganglia: located deep within the cerebral white matter
White matter generally makes up the internal region of
the brain and contains:
 Nerve fibers
 Neuroglia (mostly oligodendrites)
 Blood vessels

49. Spinal cord
White matter (outermost layer):
 Contains bundles of parallel ascending and descending axons
(tracts)
 Organized into:
o Dorsal (posterior) column
o Lateral column
o Ventral (anterior) column
Gray matter (innermost layer):
 Dorsal horn (sensory):
o Sensory neuronal axons enter the spinal cord (cell bodies are
in ganglia).
o Interneurons
 Ventral horn (motor):
o Somatic motor neuron cell bodies
o Interneurons
 Lateral horn:
o Found only in the thoracic and lumbar regions
o Contains neurons of the sympathetic nervous system
 Also contains the central canal within the gray commissure, which is
lined with ependymal cells

50. Histology of the spinal cord stained with Luxol fast blue,
which stains myelinated fibers blue: Notice that, unlike
that in the brain, the white
matter (containing myelinated axons) is located in the
periphery and surrounding the gray matter (containing
mostly neurons with scant myelinated axons). The gray
matter has 3 regions containing neurons and
interneurons, namely, the dorsal horn (sensory), lateral
horn (sympathetic), and ventral horn (somatic motor), with
each having different functions.

52. Blood–brain barrier
The blood–brain barrier is an important structure that protects the
highly regulated CNS environment.
 Tight junctions:
o Anchor capillary endothelial cells together
o Create a relatively impermeable barrier to most substances
and pathogens
o Permeable to gases (e.g., O2, CO2)
o Other solutes may require specific transporters.
 Pericytes:
o Perivascular cells
o Regulate capillary function and immune cell entry into the CNS
 Podocytes from astrocytes encircle capillaries (perivascular
endfeet).
Cells and structures of the blood–brain barrier

54. Development of the nerve tissue
Neurulation
Neurulation begins in the fourth week of development
(around the 22-23 day). The neural folds fuse first in the
cervical region and continue to fuse in both cranial (head)
and caudal (tail) directions until only the very ends of the
tube remain open and connected with the amniotic cavity.
These openings are called neuropores, with the opening at
the cranial end of the embryo being the rostral neuropore,
and the opening at the caudal end being the caudal
neuropore. The rostral neuropore closes around day 25, and
the caudal neuropore closes approximately two days after.
The neural tube becomes vascularized around the time that
the neuropores close. Regions of the neural tube begin to
thicken, forming the brain and spinal cord, and the opening
within the tube begins to form the ventricles and central
spinal canal.
During this time in development, certain genes become vital
in ensuring accurate structural layout of the CNS: Sonic
hedgehog (Shh), the Pax genes, bone morphogenic
proteins, and a transforming growth factor (TGF-B) called
dorsalin. These components are all influential in the
appropriate dorsoventral patterning of the developing neural
tube.

55. Later Development
Spinal cord development
The caudal part of the neural tube (i.e. the neural
tube after the fourth pair of somites) becomes the
spinal cord. As the walls of the neural tube
thicken, the neural canal becomes smaller and
smaller, until only a very thin central canal
remains. The neuroepithelium surrounding this
canal transitions from pseudostratified columnar
ependymal epithelium (the cell layer surrounding
the ventricles, constituting the ventricular zone) to
instead form neurons and macroglia (including
astrocytes and oligodendrocytes) within the spinal
cord.

56. The formation of neurons from neuroepithelial cells occurs
when neuroepithelial cells in the ventricular zone
differentiate into primordial neurons called neuroblasts.
These neuroblasts form an intermediate zone called the
mantle layer in between the ventricular and marginal
zones. It is in this layer that neurons will eventually form
the gray matter of the spinal cord.

57. The primordial supporting cells of the CNS are called glioblasts or spongioblasts. As previously noted, these cells
also differentiate from neuroepithelial cells in the ventricular zone, but they do so after the neuroblasts have
already formed. After their formation, glioblasts migrate into the intermediate and marginal zones, where they
become astroblasts and oligodendroblasts. Eventually, astroblasts will form astrocytes and oligodendroblasts will
form oligodendrocytes. When neuroblasts and glioblasts are no longer being produced, the remaining cells
become ependymal cells. These cells will line the central canal of the spinal cord as the ependyma. The marginal
zone becomes the white matter of the spinal cord as axons develop and project into it from neuronal cell bodies
of the brain, ganglia, and spinal cord. In the late fetal period, once the CNS becomes fully vascularized, small
cells called microglia migrate into the CNS and can be found scattered throughout both the gray and white matter.
These derivatives of mesenchymal cells are mononuclear phagocytes that develop in the bone marrow.

58. As the neuroepithelial cells multiply and differentiate, they
form thick walls, a thin roof, and floor plates within the
spinal cord. This results in the formation of the sulcus
limitans, a long, thin groove on each side of the spinal
cord that separates the alar plates/lamina (the dorsal
plates) from the basal plates/lamina (the ventral plates).
These plates span the entire length of the spinal cord. The
cell bodies in the alar plates develop into the dorsal gray
columns (the dorsal gray horns on cross-section), which
contain afferent nuclei that form the dorsal roots of the
spinal nerves. As the alar plates continue to grow, the
dorsal median septum is formed.
The ventral and lateral gray columns are formed from cell
bodies in the basal plates (the ventral and lateral gray
horns respectively on cross-section). The ventral roots of
the spinal nerves form from the axons of cell bodies in the
ventral horn as they project out of the spinal cord. Like the
dorsal median septum, the ventral median septum forms
with the enlargement of the basal plates, and eventually a
deep longitudinal groove called the ventral median fissure
will develop.

59. Spinal ganglia and meninges
The dorsal root ganglia (DRG) and unipolar neurons in the
spinal ganglia originate from cells of the neural crest. Parts of
these cells extend via the spinal nerves to somatic and visceral
structures. Here, they provide various types of receptors for
acquisition of sensory signals. The central processes of these
cells, the dorsal roots of the spinal nerves, project into the
spinal cord and assist in transmitting these signals to the brain
for interpretation.
The primordial meninges form from the mesenchyme that
surrounds the neural tube. The outer layer becoming the dura
mater and the inner layer (originating from neural crest cells)
becoming the leptomeninges, the arachnoid mater and pia
mater. By the fifth week of development, cerebrospinal fluid
(CSF) begins to form.
Spinal nerves and vertebral levels
At week eight of gestation, the embryonic spinal cord spans the
entire length of vertebral canal, and the spinal nerves pass
through the intervertebral foramina at the exact level that they
emerge from the cord. Due to different growth rates, however,
this relationship does not last: the embryo grows faster than
the cord, and with this continued growth the caudal end of the
cord becomes shorter and shorter compared to the length of
the embryo.
By 24 weeks, the spinal cord stops at the first sacral vertebra
(S1); which causes the end of the cord to rest around the
second or third lumbar vertebrae (L2, L3) in a newborn infant.
By adulthood, the cord stops at the lower border of the first
lumbar vertebra (L1). Because of this length disparity, the
spinal nerve roots in the lumbar and sacral cord project
obliquely from the spinal cord to their corresponding vertebral
levels below.

60. Myelination of the spinal cord begins in the late fetal period and
continues during the first postnatal year. The motor roots become
myelinated before the sensory roots. In the spinal cord, myelin sheaths
are formed by oligodendrocytes. This is unlike the peripheral nerves,
whose myelin sheaths are formed by the plasma membranes of neural
crest-derived Schwann (a.k.a. neurolemma) cells. These cells wrap
themselves around the axons of somatic motor neurons, presynaptic
and postsynaptic autonomic motor neurons, and somatic and visceral
sensory neurons.
Once myelination of the spinal cord takes place, the tissue looks white
on gross inspection. Because of this, these regions of myelinated
axons are referred to as the white matter of the spinal cord.

61. Development of the brain
The brain develops from the section of the
neural tube cranial to the fourth pair of somites.
Before the neural folds fuse, three vesicles can
be recognized at the rostral end of the neural
tube: the prosencephalon, mesencephalon,
and rhombencephalon.
Each will form the forebrain, midbrain, and
hindbrain respectively.In the fourth week of
gestation, the primitive brain bends ventrally
along with the head fold, forming the midbrain
and cervical flexures. Since parts of the brain
grow at different rates, the pontine flexure
forms in the opposite direction of the midbrain
and cervical flexures. In the fifth gestational
week, the prosencephalon divides into the
telencephalon and diencephalon, and the
rhombencephalon divides into the
metencephalon and myelencephalon, forming
five secondary brain vesicles. The sulcus
limitans of the spinal cord extends cranially
until the midbrain and forebrain meet, and the
alar and basal plates are recognizable up
through the midbrain only.