Nervous system
Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. This emergence marked a turning point for animal life, allowing creatures to coordinate actions and sense environmental changes with precision. Sponges remain the only multicellular animals without any nervous system at all. They lack synaptic junctions connecting their cells, yet they possess homologs of genes that play key roles in synaptic function. Recent studies show sponge cells express proteins clustering together to form structures resembling postsynaptic density. These structures mediate simple actions like whole-body contraction through calcium waves rather than electrical synapses.
Radially symmetric organisms such as jellyfish and comb jellies developed diffuse nerve nets instead of centralized brains. In most jellyfish, this nerve net spreads evenly across the body, while in comb jellies it concentrates near the mouth. The network consists of sensory neurons picking up chemical signals, motor neurons activating contractions, and intermediate neurons detecting activity patterns. All other animal species, except for a few types of worms, evolved a nervous system containing a brain and central cord. The size of these systems ranges from a few hundred cells in the simplest worms to around 300 billion cells in African elephants.
The nervous system is defined by the presence of a special type of cell called the neuron. Neurons communicate with other cells via synapses, which are membrane-to-membrane junctions containing molecular machinery for rapid signal transmission. Many neurons possess an axon, a protoplasmic protrusion extending to distant parts of the body. Axons typically extend throughout the body in bundles called nerves. Even within a single human species, hundreds of different neuron types exist with varied morphologies and functions.
Alongside neurons, the nervous system contains specialized support cells known as glial cells or simply glia. Glial cells provide structural and metabolic support, maintain homeostasis, form myelin, and participate in signal transmission. In the human brain, the total number of glia roughly equals the number of neurons, though proportions vary across brain areas. A very important type of glial cell generates layers of fatty substance called myelin that wraps around axons. This insulation allows action potentials to transmit much more rapidly and efficiently. Recent findings indicate microglia and astrocytes serve as resident immune cells within the central nervous system.
In vertebrates, the nervous system divides into two main parts: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord. The spinal canal contains the spinal cord while the cranial cavity holds the brain. These structures are enclosed and protected by meninges, a three-layered membrane system including the tough dura mater. The brain is also shielded by the skull, and the spinal cord by vertebrae.
The peripheral nervous system comprises nerves connecting the central system to every other body part. Nerves transmitting signals from the brain are motor nerves, while those sending information to the central system are sensory nerves. The peripheral system splits into somatic and autonomic subsystems. The autonomic system further subdivides into sympathetic, parasympathetic, and enteric systems. Sympathetic activation mobilizes energy during emergencies, whereas parasympathetic activation occurs when organisms remain relaxed. The enteric system controls the gastrointestinal tract. Cranial nerves exit from the brain, while spinal nerves emerge from the spinal cord.
Nerves were large enough to be recognized by ancient Egyptians, Greeks, and Romans, yet their internal structure remained unknown until microscopes became available. It was not known that neurons are the basic units of the brain until approximately year 1900, according to Santiago Ramón y Cajal. The concept of chemical transmission in the brain remained undiscovered until around 1930, when Henry Hallett Dale and Otto Loewi conducted pioneering work. Scientists began understanding the action potential, the basic electrical phenomenon neurons use for communication, in the 1950s through research by Alan Lloyd Hodgkin, Andrew Huxley, and John Eccles.
The molecular revolution swept across US universities in the 1980s, leading to widespread knowledge of behavioral phenomena mechanisms by the 1990s thanks to Eric Richard Kandel. David H. Hubel and Torsten Wiesel revealed how neuronal networks code stimuli in the 1960s. These discoveries transformed neuroscience from a field studying gross anatomy into one examining molecular mechanisms. Modern neuroscience now integrates findings from genetics, biochemistry, and electrophysiology to explain complex behaviors and cognitive functions.
In vertebrates, the first sign of the nervous system appears as a thin strip of cells along the center of the back called the neural plate. This fold deepens and closes at the top to form a cylindrical structure known as the neural tube. The sequence from neural plate to neural tube is termed neurulation. As development proceeds, future peripheral systems appear as two strips of tissue called the neural crest running lengthwise above the neural tube. Hans Spemann and Hilde Mangold demonstrated in early 20th century experiments that nervous tissue formation is induced by signals from mesodermal cells called the organizer region.
Induction requires inhibition of bone morphogenetic protein genes, specifically BMP4. Two proteins named Noggin and Chordin inhibit BMP4, causing ectoderm to transform into neural tissue. A family of secreted signaling molecules called neurotrophins regulates neuron growth and survival across all bilateral organisms. Zhu et al identified DNT1, the first neurotrophin found in flies, sharing structural similarity with all known neurotrophins. These molecular mechanisms suggest common origins for nervous system formation in both arthropods and vertebrates dating back to their shared ancestor.
Damage to nerves can cause pain, loss of sensation, or loss of muscle control. Peripheral neuropathy may result from genetic conditions, metabolic issues like diabetes, inflammatory disorders such as Guillain-Barré syndrome, vitamin deficiency, infectious diseases including leprosy or shingles, or poisoning by heavy metals. If a nerve completely transects, it often regenerates, though long nerves may take months to heal. Carpal tunnel syndrome occurs when swelling or bruises compress nerves passing through tight bony channels.
Physical damage to the spinal cord typically produces permanent loss of function if nerve fibers are destroyed rather than merely swollen. Experimental studies show spinal nerve fibers attempt regrowth similar to peripheral nerves, but scar tissue usually prevents penetration. Neurological practice employs techniques from neuroscience and psychiatry to treat diseases using various forms of neurotherapy. The blood-brain barrier isolates the central system chemically, making it less susceptible to many threats while ensuring that any CNS damage carries more serious consequences than peripheral injuries.
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Common questions
When did the nervous system first arise in animal history?
Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. This emergence marked a turning point for animal life, allowing creatures to coordinate actions and sense environmental changes with precision.
Which multicellular animals lack a nervous system entirely?
Sponges remain the only multicellular animals without any nervous system at all. They lack synaptic junctions connecting their cells yet possess homologs of genes that play key roles in synaptic function.
What are the two main parts of the vertebrate nervous system?
In vertebrates, the nervous system divides into two main parts: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord while the peripheral nervous system comprises nerves connecting the central system to every other body part.
Who discovered that neurons are the basic units of the brain around 1900?
It was not known that neurons are the basic units of the brain until approximately year 1900 according to Santiago Ramón y Cajal. His work established the neuron as the fundamental building block of neural architecture.
How do glial cells support the function of neurons in the human brain?
Glial cells provide structural and metabolic support maintain homeostasis form myelin and participate in signal transmission. A very important type of glial cell generates layers of fatty substance called myelin that wraps around axons allowing action potentials to transmit much more rapidly and efficiently.