Neuralation to Nerves

Neurulation is the process of formation of the neural tube and neural crest in vertebrates (Figure 9-15). The neural plate collapses in the middle and forms the neural tube. The neural crest originates in the mesoderm and forms between the neural tube and the epidermis (ectoderm). The neural crest then breaks up and forms individual stem cells. The stem cells produced in the neural crest migrate to different parts of the body and produce most of the unique vertebrate nervous systems and other organs. Although many of the genes in the neural crest are related or even the same as genes in cephalochordates, none of the genes that direct the movement of neural crest cells are present in cephalochordates or annelid worms. The system of neural crest cells migrating to locations in the body and forming nerves and organs is a novelty of the vertebrates. The cephalochordate neurulation process is similar but it is missing the neural crest, which specifies the uniquely vertebrate organs.

The vertebrate neural crest is divided into four regions: cranial, trunk, vagal, and cardiac. Cranial neural crest cells form the thymus (adaptive immune system) and the thyroid gland that controls blood calcium. As the name implies, cranial neural crest cells form the head and the skeleton, muscle, and cartilage in the face. Cranial neural crest cells also form cells that contribute to inner ear development. They form the cranial nerves that extend out from the brain, and they form Schwann cells that form myelin sheaths around the nerves in the peripheral nervous system.

Trunk neural crest cells form the trunk of sensory neurons along the spinal cord, the dorsal root ganglia. They form melanocytes that are in the skin, eye, bones, immune system, and elsewhere. They form neuroendocrine cells send message molecules from the nerve system into the blood. They form the sympathetic trunk of nerves along the vertebral column, which run from the head to the pelvis. The sympathetic nervous system changes the state of muscles to fright or flight status, such as increasing the heart rate. The parasympathetic nervous system changes the state of muscles to rest and digest, and feed and breed status (relaxed). The sympathetic and parasympathetic nervous systems can either operate through the central nervous system or they have much faster responses through the autonomous nervous system, which does not require processing in the central nervous system.

The vagal neural crest forms the enteric nervous system by forming hundreds of millions of nerves between internal organs and the sympathetic trunk and ultimately to the brain. These nerves govern gastrointestinal processes and some of the blood vessel sections. These cells also form the adrenal glands in the kidneys that produce hormones

The cardiac neural crest forms regions of the heart and cartilage and neurons in pharyngeal arches, which form the jaw, ears, and other structures in the head. The chambered heart is unique to vertebrates.

The cranial neural crest causes the formation of vertebrate sensory systems, such as the eye and ear. There are a series of cell fate decisions that lead to the development of the sensory organs. While the signaling process is unique to vertebrates, the signaling proteins are ancient or derived from more ancient proteins (BMP, Wnt, and FGF). Sensory organ formation begins with placodes (Figure 9‑11) that form on the surface of the ectoderm. The placodes are thought to have derived from ectodermal sensory cells in more ancient organisms.

Figure 9‑16 shows the human nervous system, but the basic components in the peripheral (PNS), enteric (ENS), and central (CNS) nervous systems were probably in place in Haikouichthys.[1] All vertebrates have olfactory, optic, oculomotor, trochlear, trigemal, abducens, facial, auditory, glossopharyngeal, vagus, accessory, and hypoglossal nerves, but invertebrates do not have these nerves. The pituitary gland (hypophysis) is behind the nose and is an important part of the vertebrate nervous system. It secretes hormones and controls appetite, sexual arousal, and many other glands through MSH. It also regulates reproduction, growth, and stress. It is likely that the pituitary gland and other endocrine glands were in Haikouichthys.[2]

Another difference between invertebrate and vertebrate nervous systems is that vertebrates have Schwann cells and mylenated nerves (Banner), which is a sheath over the nerve that allows nerve impulses to travel 100X faster than in invertebrates. Nerve signals jump from gap to gap in the myelin sheath, which is 100X faster than moving along the nerve. It is unknown whether the genes for myelination or the actual myelin sheaths were present in Haikouichthys.

References

[1] Gee, Henry, Across the bridge: understanding the origin of the vertebrates. University of Chicago Press, 2018.

[2] Gee, Across.

Banner

Vertebrate mylenated nerve (gaps). Credit. "Anatomy and Physiology" by the US National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program. Public domain.