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The World Health Organization (WHO) has developed a timeline called “motor milestones” to track human nervous system development in infancy.
Why are we using behaviors that, at first glance, are indicative of the musculoskeletal system instead of the nervous system, you ask?
Well, WHO has determined that the level of postnatal neurological development and “reorganization” of the central nervous system is reflected in skeletal flexibility.
Physical growth is also a good external measure of the developmental progress of the brain and spinal cord, which continues to grow postnatally (Hill, 2019a).
| Months Old | Milestone |
| 3.5-9.5 | Sitting without support |
| 4.5-12 | Standing with assistance |
| 5-14 | Hands-and-knees crawling |
| 5.5-14 | Walking with assistance |
| 6.5-17.5 | Standing alone |
| 7.5-18 | Walking alone |
A majority of what constitutes postnatal nervous system development in humans is the proliferation of white matter (glial cells) and gray matter (neurons) establishing new connections and reorganizing existing ones.
Three types of glial cells in the mammalian CNS:
- Oligodendrocytes: derived from neuroepithelium
- Astrocytes: derived from neuroepithelium
- Microglia: derived from the yolk sac mesoderm, more specifically the hematopoietic lineage that gives rise to monocytes and macrophages
Stages of Nervous System Development
Nervous system development is generally made up of 4 major stages (Staveley, n.d.):
- Specification of the neural cell identities, a fancy way of saying “differentiation” (neural cells organized into neural or glial cells).
- Neuron migration and axon outgrowth. There are a four different “templates” of neuron anatomy: unipolar, bipolar, pseudounipolar, and multipolar. These layouts determine the relative location of the cell body on the axon and number of dendrites, among other things.
These two stages are what ultimately form the organ structure of the peripheral and central nervous systems, creating the brain, spinal cord, retina and all nerves to be distributed throughout the body.
The following steps seal the deal by establishing connections to organs of other systems, powering up all functions of the body to bring Frankenstein’s monster to life.
- Synapse formation with target (neurons, muscles or gland cells). This determines what connections the neuron is going to have and ultimately what type of information it will be responsible for translating and distributing.
- Synaptic connection refinement. This process is essentially maintenance of the finer architecture of the nervous system. Think of it as the nervous system’s “self-cleaning” mechanism. Some of the tools used here are the removal of axon branches (which terminates a neuron’s ability to receive or transmit information) and apoptosis (programmed cell death). Remember that neurons are created over the duration of the human life – with synaptic refinement, the nervous system can keep a consistent supply of fresh, high-functioning neurons to support memory, learning, movement, sensory, and more.
Milestones of Postnatal Nervous System Development
Postnatal milestones of nervous system development are as follows (Chen et al., 2017):
| Milestone (in units of PND, Postnatal Day) | Structural and Sensory Developments |
| 1 | Corticospinal tracts reach cervical spinal cord segments. |
| 2 | Peak in development of rods in the retina |
| 3 | Synaptogenesis accelerates in the brain |
| 7 | Corticospinal tracts reach lumbar cord segments |
| 8 | Onset of myelination in the optic tract |
| 9 | Onset of myelination of neurons that specifically allow for sense of smell |
| 11 | External auditory meatus opens; Onset of myelination in the auditory tracts |
| 12 | Eyelids open |
| 13 | Onset of myelination in the hippocampus |
| 15 | Onset of myelination in the corpus callosum |
The corticospinal tract is a pathway composed of white matter. White matter is made up of glial cells, the most abundant cell type in the CNS. Glial cells can be further categorized into the following sorts: oligodendrocytes, astrocytes, ependymal, Schwan, microglia and satellite cells.
These all work to support and insulate neurons, aiding in the movement of electrical impulses through the entire system.
The corticospinal tract needs to extend down into the spinal cord in order to provide communicative capabilities between the central and peripheral nervous systems.
Those nerves which extend out of the spinal cord into distal parts of the body asssit in many musculoskeletal activities and sensory functions such as the ability to walk and information gathered by physical touch, respectively.
Myelination is the process by which neurons are surrounded by a fatty substance called myelin, which serves to insulate the neuron, increasing the rate at which the action potential (electrical impulse) moves along the axon to the axon terminal. It also helps to eliminate the possibility of the impulse of leaving the axon, ensuring that any information received or distributed by the nervous system is directed to the proper organ.
A large majority of myelination processes in the central nervous system are initiated either at or immediately after birth, starting with the brainstem, then spinal cord, and continuing up through the regions of the brain.
Brain Development
The whole of the nervous system is derived from the neural plate (this forms out of the embryonic ectoderm) which develops during the embryonic phase of development. This neural plate becomes the neural tube, which further develops into the major components of the nervous system.
There are three primary ventricles that give rise to the whole of the human brain. Those are the prosencephalon, diencephalon and mesencephalon.
The Prosencephalon
Besides sounding like a new map in World of Warcraft, the prosencephalon, also known as the forebrain, forms the cerebrum of the brain. The cerebrum, also called the telencephalon, is a secondary ventricle and encompasses the following organs:
- Olfactory bulbs
- Cerebral cortex
- Cerebral white matter
- Corpus callosum
- Basal nuclei
- Columns of fornix
- Body of fornix
- Hippocampus
- Amygdala
- Lateral ventricles
The cerebrum is divided into two halves, or “hemispheres,” which are connected by the corpus collosum, a collection of approximately 200 million axons that facilitate motor, sensory and cognitive functions of the brain. The cerebrum is home to many keystone nervous system functions such as learning, memory, and emotions.
The secondary ventricle that forms from the prosencephalon is known as the diencephalon, a portion of the brain that contains keystone regulatory organs. The thalamus, known as the “regulatory gateway,” receives and transmits nearly all sensory input throughout the body.
The hypothalamus governs processes essential to human survival including homeostasis, thirst, hunger, control of the autonomic nervous system (controls heartbeat and respiration, digestion, and more).
Sure, the prosencephalon gives you the cognitive ability which sets you apart from non-human animals, but without the diencephalon, you wouldn’t even be alive to use these cognitive skills!
The Mesencephalon
Also known as the “midbrain,” the mesencephalon is a primary ventricle that makes up a portion of the brainstem which connects the fore- and hindbrain. The mesencephalon is home to sensory functions such as sight and hearing and helps in movement as well (including movement of the eye). The components of the mesencephalon include:
- Tectum
- Tegmentum
- Cerebral peduncle
- Substantia nigra
- Crus cerebri
- Oculomotor and trochlear cranial nerves
The Rhombencephalon
The rhombencephalon, also called the hindbrain, gives rise to the secondary ventricles, metencephalon and myelencephalon. These two ventricles contain structures which are essential to life in some of the same ways that those of the of the prosencephalon are in that they control activities such as respiration, sleep, and movement.
The structures of the rhombencephalon include:
- Metencephalon (the collective name for the cerebellum and pons)
- Pontine nuclei
- Transverse pontine fibers
- Fourth ventricle
- Longitudinal fibers of the Pons
- Reticular formation
- Granule cells
- Purkinje cells
- Deep cerebellar nuclei
- Myelencephalon (everyone’s favorite, the medulla oblongata)
- Pyramids
- Trapezoid body
- Spinal tract of V
- Reticular formation
The cerebellum, pons, and medulla oblongata are central (no pun intended) to communication between the central and peripheral nervous systems. The pons connects the brainstem to the cerebral cortex and functions as the hub of communication between the cerebrum and the spinal cord.
Directly behind the pons sits the cerebellum, which coordinates muscle movements (primarily voluntary muscle movements) and balance.
The medulla oblongata is the physical connection between the brain stem and the spinal cord and is the control center of autonomic nervous system activities including heart rate and reflexes such as sneezing.
Development of the Spinal Cord
The spinal cord forms from two distinct structures, the alar plate and the basal plate. The alar plate develops afferent neurons, which aid in sensory functions, and the basal plate forms efferent neurons, which are the driving force behind motor functions (now that pun was intended).
All of the neurons that are distributed up and down the spinal cord can be divided into five distinct anatomy regions:
- Cervical (Head)
- Thoracic (Chest)
- Lumbar (Abdominal)
- Sacral (Pelvic)
- Coccygeal (Tailbone)
The 31 nerves that stretch down the spinal column are situated on either side of the spinal cord in pairs, and extend though the sides of the vertebrae out into the rest of the body. If you thought this was interesting – we’re just getting started.
The development of the nervous system is so incredibly complex and is a process that continues throughout an entire lifetime. The rate of nervous system development is at its highest rate in postnatal development into infancy, but continues to hold the same value in its importance to human survival, even as it slows with age.
References
- Chen, V. S., Morrison, J. P., Southwell, M. F., Foley, J. F., Bolon, B., & Elmore, S. A. (2017). Histology Atlas of the Developing Prenatal and Postnatal Mouse Central Nervous System, with Emphasis on Prenatal Days E7.5 to E18.5. Toxicologic pathology, 45(6), 705–744. doi:10.1177/0192623317728134
- Hill, M.A. (2019a, October 15) Embryology BGDA Lecture – Development of the Nervous System. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Lecture_-_Development_of_the_Nervous_System
- Hill, M.A. (2019b, October 16) Embryology Neural System – Postnatal. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Neural_System_-_Postnatal
- Staveley, B. E. (n.d.). Molecular & developmental biology (BIOL3530): Development of the nervous system [HTML]. Retrieved from http://www.mun.ca/biology/desmid/brian/BIOL3530/DB_10/DBNNerS.html