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Neuron Growth and Pathways
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Neural development in humans draws on both neuroscience and developmental biology
to describe the cellular and molecular mechanisms by which complex nervous systems
emerge during embryonic development and throughout life.
Key times of embryonic neural development include the birth and differentiation of
neurons from stem cell precursors, the migration of immature neurons from their
birthplaces in the embryo to their final positions, outgrowth of axons from neurons
and guidance of the motile growth cone through the embryo towards postsynaptic partners,
the generation of synapses between these axons and their postsynaptic partners, the
neuron pruning that occurs in adolescence, and finally the lifelong changes in
synapses which are thought to underlie learning and memory.
The neurodevelopmental process can generally be divided into two classes:
activity-independent mechanisms and activity-dependent mechanisms. Activity-independent
mechanisms are generally believed to occur as hardwired processes determined by genetic
programs played out within individual neurons. These include differentiation, migration
and axon guidance to their initial target areas. These processes are thought of as being
independent of neural activity and sensory experience. Once axons reach their target
areas, activity-dependent mechanisms come into play. Neural activity and sensory experience
will mediate formation of new synapses, as well as synaptic plasticity, which will be
responsible for refinement of the nascent neural circuits.
Neurogenesis (the birth of new cells) is the process by which new nerve cells are
generated. When this occurs, there is active production of new neurons, astrocytes,
glia, and other neural lineages from undifferentiated neural progenitor or stem
cells. The part that's unfortunate it's mostly an inactive process in most areas
of the adult brain.
For decades, scientists believed the brain was incapable of growing new cells.
It's now known that neurogenesis occurs throughout life, but only in certain parts
of the brain, including an area involved in learning and memory called the
hippocampus. The growth of new neurons is a lifelong process. In fact, the brain's
marvelous ability to "rewire" itself by sprouting neurons and reshaping their
connections is at the root of how you learn new information and gain fresh skills
throughout a lifetime.
Glial Cells
Glial cells are non-neuronal cells that provide support and nutrition, maintain
homeostasis, form myelin, and participate in signal
transmission in the nervous system. In the human brain, glia are estimated to
outnumber neurons by about 10 to 1.
Glial cells provide support and protection for neurons. They are historically
known as the "glue" of the nervous system. The four main functions of glial
cells are to surround neurons and hold them in place, to supply nutrients and
oxygen to neurons, to insulate one neuron from another, and to destroy pathogens
and remove dead neurons. They also modulate neurotransmission.
Some glia function primarily as the physical support for neurons. Others
regulate the internal environment of the brain, especially the fluid surrounding
neurons and their synapses, and provide nutrition to nerve cells. Glia also have
important developmental roles in guiding migration of neurons in early
development and producing molecules that modify the growth of axons and
dendrites.
Findings in the hippocampus and cerebellum have indicated that glia are also
active participants in synaptic transmission, regulating clearance of
neurotransmitter from the synaptic cleft, releasing factors such as ATP which
modulate presynaptic function, and even releasing neurotransmitters themselves.
Another unique type of glia, the oligodendrocyte precursor cells or OPCs, have
very well defined and functional synapses from at least two major groups of
neurons. The only notable differences between neurons and glia are the ability
to generate action potentials and the polarity of neurons, namely the axons and
dendrites which glia lack.
Many feel it to be inaccurate to consider glia as "glue" in the nervous system
as the name implies, but rather as a partner to neurons. They are also crucial
in the development of the nervous system and in processes such as synaptic
plasticity and synaptogenesis. Glia have a role in the regulation of repair of
neurons after injury. Astrocytes enlarge and proliferate to form a scar and
produce inhibitory molecules that inhibit regrowth of a damaged or severed axon.
In the peripheral nervous system (PNS), Schwann cells promote repair. After axon
injury, Schwann cells regress to an earlier developmental state to encourage
regrowth of the axon. This difference between PNS and central nervous system
(CNS) raises hopes for the regeneration of nervous tissue in the CNS, such as
spinal cord injury or severance.
Neural Pathways
A less anatomical but much more functional way of dividing the human nervous
system is classification according to the role that the different neural pathways
play, regardless of whether or not they cross through the CNS and/or PNS:
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The Somatic Nervous System (SNS) is responsible for coordinating
voluntary body movements like those are controlled consciously. |
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The Autonomic Nervous System (ANS) is responsible for coordinating
involuntary functions, such as breathing and digestion. |
In turn, these divisions of the nervous system can be further divided according
to the direction in which they conduct nerve impulses:
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Afferent system by sensory neurons, which carries
impulses from a somatic receptor to the CNS. |
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Efferent system by motor neurons, which carries
impulses from the CNS to an effector. |
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Relay system by interneurons (also called relay
neurons), which transmit impulses between the sensory and motor neurons
in both the CNS and PNS. |
The junction between two neurons is called a synapse. There is a very narrow gap
(about 20nm in width) between the neurons called the synaptic cleft. This is
where an action potential (nerve impulse) is transmitted from one neuron to the
next. This is achieved by relaying the message across the synaptic cleft using
neurotransmitters, which diffuse across the gap. The neurotransmitters then bind
to receptor sites on the neighboring (postsynaptic) neuron, which in turn
produces its own nerve impulse. This impulse is sent to the next synapse, and
the cycle repeats itself.
Nerve impulses are a change in ion balance between the inside and outside of a
neuron. Because the nervous system uses a combination of electrical and chemical
signals, it's incredibly fast. Although the chemical aspect of signaling is much
slower than the electrical aspect, a nerve impulse is still fast enough for the
reaction time to be negligible most of the time.
Speed is a necessary characteristic in order to quickly identify the presence of
danger, and avoid injury or death. An example of this could be a hand touching a
hot stove. Now if the nervous system was only comprised of chemical signals, the
nervous system wouldn't be able to signal the arm to move fast enough to escape
dangerous burns. So speed in the nervous system is essential to preventing harm
or death.
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