SYSTEM NEUROSCIENCE
,Nervous System Development
The nervous system develops through a sequence of highly coordinated steps. Neurons are
not simply produced randomly; they are born, positioned, differentiated, connected, and
refined into functional circuits.
A neuronal circuit is a group of interconnected neurons that performs a function, such as
visual processing, movement control, learning, or memory. These circuits are:
• precise: neurons connect to specific partners.
• complex: many neurons interact in organized networks.
• plastic: connections can change with activity and experience.
• clinically important: abnormal circuit development can contribute to neurological
and psychiatric disease.
At birth, many circuits are already functional, but they are not fully optimized.
• Postnatal experience, practice, sensory input, and activity refine them.
1. Embryogenesis
1.1. From fertilized egg to germ layers
After fertilization, the embryo undergoes rapid cell divisions.
• It forms a hollow ball-like structure called the blastula.
• During early embryogenesis, the embryo reorganizes into different germ layers.
The three main germ layers are:
Germ layer Main derivatives
Ectoderm Skin and nervous system
Mesoderm Muscle, bone, blood, notochord
Endoderm Gut and internal organs
Normally, the ectoderm tends to become skin. However, under the influence of signaling
molecules from nearby tissues, part of the ectoderm becomes neural tissue.
1.2. Neural induction
Neural induction is the process by which part of the ectoderm is instructed to become
nervous tissue.
• The key structure involved is the notochord, which forms from mesoderm.
• The notochord releases signaling molecules that influence the overlying ectoderm.
Important signals include:
• Sonic Hedgehog, Shh
• Noggin
• Other morphogens such as BMP inhibitors and Wnt-related signals
,These signals cause part of the ectoderm to become neuroectoderm, forming the neural
plate.
Neural induction converts ectoderm that would otherwise become skin into neural tissue.
1.3. Neurulation
Neurulation is the process by which the neural plate folds and forms the neural tube.
Steps:
1. The neural plate thickens.
2. The center sinks inward, forming the neural groove.
3. The edges rise up as neural folds.
4. The folds move toward each other.
5. They fuse in the midline to form the neural tube.
The neural tube is the embryonic structure that gives rise to the entire central nervous
system:
• brain
• spinal cord
• adult neural circuits
The neural tube does not close all at once. Temporary openings remain:
• Anterior neuropore
• Posterior neuropore
These close at different times, the closing from the neural tube closes in a posterior to
anterior gradient
1.4. Neural tube closure defects
If the neural tube fails to close correctly, severe malformations occur.
Failed closure Disorder
Posterior neuropore fails to close Spina bifida
Anterior neuropore fails to close Anencephaly
Spina bifida is a spinal dysraphism where the spinal cord and vertebral arches do not form
properly.
Anencephaly is a severe condition where major parts of the brain and skull fail to develop.
1.5. Early brain development
The neural tube changes shape and begins to specialize. The anterior part expands and
gives rise to major brain regions.
Important point:
Much of nervous system development already occurs during the first trimester.
, • Human brain development begins very early in utero, but unlike chimpanzees,
humans continue significant brain growth after birth.
This postnatal growth is not mainly because of producing many new neurons, but because
of:
• increased synapse density
• increased connectivity
• circuit refinement
• glial development
• myelination
1.6. Retina development
The retina is part of the nervous system
• It develops as an outgrowth or invagination of the diencephalic neural tube.
So the eye is not just a sensory organ separate from the brain. The retina is developmentally
a CNS structure.
2. Patterning
2.1. What is patterning?
Patterning means the molecular and cellular specification of different regions of
developing tissue.
It explains how one neural tube gives rise to different structures, for example:
• spinal cord
• hindbrain
• midbrain
• forebrain
• dorsal sensory areas
• ventral motor areas
Patterning occurs along major body axes:
Axis Meaning
Dorsal–ventral top–bottom patterning
Rostral–caudal / anterior–posterior head–tail patterning
A. Dorsal–ventral patterning
,Nervous System Development
The nervous system develops through a sequence of highly coordinated steps. Neurons are
not simply produced randomly; they are born, positioned, differentiated, connected, and
refined into functional circuits.
A neuronal circuit is a group of interconnected neurons that performs a function, such as
visual processing, movement control, learning, or memory. These circuits are:
• precise: neurons connect to specific partners.
• complex: many neurons interact in organized networks.
• plastic: connections can change with activity and experience.
• clinically important: abnormal circuit development can contribute to neurological
and psychiatric disease.
At birth, many circuits are already functional, but they are not fully optimized.
• Postnatal experience, practice, sensory input, and activity refine them.
1. Embryogenesis
1.1. From fertilized egg to germ layers
After fertilization, the embryo undergoes rapid cell divisions.
• It forms a hollow ball-like structure called the blastula.
• During early embryogenesis, the embryo reorganizes into different germ layers.
The three main germ layers are:
Germ layer Main derivatives
Ectoderm Skin and nervous system
Mesoderm Muscle, bone, blood, notochord
Endoderm Gut and internal organs
Normally, the ectoderm tends to become skin. However, under the influence of signaling
molecules from nearby tissues, part of the ectoderm becomes neural tissue.
1.2. Neural induction
Neural induction is the process by which part of the ectoderm is instructed to become
nervous tissue.
• The key structure involved is the notochord, which forms from mesoderm.
• The notochord releases signaling molecules that influence the overlying ectoderm.
Important signals include:
• Sonic Hedgehog, Shh
• Noggin
• Other morphogens such as BMP inhibitors and Wnt-related signals
,These signals cause part of the ectoderm to become neuroectoderm, forming the neural
plate.
Neural induction converts ectoderm that would otherwise become skin into neural tissue.
1.3. Neurulation
Neurulation is the process by which the neural plate folds and forms the neural tube.
Steps:
1. The neural plate thickens.
2. The center sinks inward, forming the neural groove.
3. The edges rise up as neural folds.
4. The folds move toward each other.
5. They fuse in the midline to form the neural tube.
The neural tube is the embryonic structure that gives rise to the entire central nervous
system:
• brain
• spinal cord
• adult neural circuits
The neural tube does not close all at once. Temporary openings remain:
• Anterior neuropore
• Posterior neuropore
These close at different times, the closing from the neural tube closes in a posterior to
anterior gradient
1.4. Neural tube closure defects
If the neural tube fails to close correctly, severe malformations occur.
Failed closure Disorder
Posterior neuropore fails to close Spina bifida
Anterior neuropore fails to close Anencephaly
Spina bifida is a spinal dysraphism where the spinal cord and vertebral arches do not form
properly.
Anencephaly is a severe condition where major parts of the brain and skull fail to develop.
1.5. Early brain development
The neural tube changes shape and begins to specialize. The anterior part expands and
gives rise to major brain regions.
Important point:
Much of nervous system development already occurs during the first trimester.
, • Human brain development begins very early in utero, but unlike chimpanzees,
humans continue significant brain growth after birth.
This postnatal growth is not mainly because of producing many new neurons, but because
of:
• increased synapse density
• increased connectivity
• circuit refinement
• glial development
• myelination
1.6. Retina development
The retina is part of the nervous system
• It develops as an outgrowth or invagination of the diencephalic neural tube.
So the eye is not just a sensory organ separate from the brain. The retina is developmentally
a CNS structure.
2. Patterning
2.1. What is patterning?
Patterning means the molecular and cellular specification of different regions of
developing tissue.
It explains how one neural tube gives rise to different structures, for example:
• spinal cord
• hindbrain
• midbrain
• forebrain
• dorsal sensory areas
• ventral motor areas
Patterning occurs along major body axes:
Axis Meaning
Dorsal–ventral top–bottom patterning
Rostral–caudal / anterior–posterior head–tail patterning
A. Dorsal–ventral patterning
