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Nervous tissue is one of the four basic types of tissue that make up the animal body. Its basic function is to receive, transmit, and process the body’s internal and external stimuli, ultimately enabling an organism to relate to its environment. Through mechanisms not yet fully understood, nervous tissue is also responsible for the psychic and intellectual functions of humans, such as memory, cognition, and consciousness.
Nervous tissue allows the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body. In higher organisms, nervous tissue forms a structure of greater complexity called the nervous system.
Nerve cells associate with each other morphologically and functionally: neurons associate with each other to form intercommunicating nerve fibers. It can be said that the nervous tissue is a true functional syncytium: neurons can exchange messages thanks to two characteristics peculiar to them: excitability and conductivity.
Excitability means that the nerve cell can respond to external stimuli (physical and chemical) of various types, which are converted into nerve impulses. Conductivity means that the nerve impulse generated on a nerve cell can be transmitted to other cells in the form of a small electric current and at certain cell junctions called synapses.
Nervous tissue first appeared in worm-like organisms about 550-600 million years ago. This “classical doctrine” has recently been challenged by recent findings on the use of electrical signals in plants. Based on these findings, some scientists have proposed the existence of a plant nervous system and the creation of a scientific field called plant neurobiology. The inflexibility of the various positions in the scientific debate on both sides has led to a redefinition of the concept of the nervous system, using only physiological criteria and avoiding phylogenetic criteria.
Cells of the nervous system
The nervous system is essentially composed of two types of cells:
- neurons, which are actually responsible for receiving and transmitting nerve impulses;
- and glial cells, or neuroglia, which provide structural and functional support to the neurons.
Some connective cells of a fibrous nature, which provide structural support to the nervous tissue, are then sometimes incorrectly considered to be part of the nervous tissue.
Neurons have special structures that allow them to send signals to other cells quickly and accurately. These take the form of electrochemical impulses that travel along thin fibers called axons, which can connect directly to neighboring cells through electrical synapses or cause the release of chemicals called neurotransmitters at chemical synapses.
The cell that receives a synaptic signal from a neuron can be excited, inhibited, or otherwise modulated. Connections between neurons can form neural pathways, neural circuits, and large networks that generate an organism’s “perception of the world” and determine its behavior. In addition to neurons, the nervous system contains other specialized cells called glial cells or neuroglia (or simply glia), which provide structural and metabolic support.
Most multicellular animals have a nervous system, but it can vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges, placozoa, and mesozoa.
The nervous systems of ctenophores and cnidarians (which include anemones, hydras, corals, and jellyfish) consist of a single diffuse nervous network. All other animal species, except some types of worms, have a nervous system consisting of a brain, a central nervous cord (or two parallel cords), and nerves radiating from the brain and central nervous cord.
The size of the nervous system varies from a few hundred cells in the simplest worms to about 300 billion cells in African elephants.
The function of the central nervous system is to send signals from one cell to another, or from one part of the body to another, and to receive feedback.
Genetic defects, physical damage from trauma or toxicity, infection, or simply aging can cause the nervous system to malfunction.
The medical specialty that studies nervous system disorders, prevention, and treatment is called neurology.
In the peripheral nervous system, the most common problem is nerve conduction failure, which can have a variety of causes, including diabetic neuropathy and demyelinating diseases such as multiple sclerosis and amyotrophic lateral sclerosis.
Neuroscience, on the other hand, is the branch of science that focuses on the study of the nervous system.
Nervous system: Central and Peripheral
In vertebrates, the nervous system consists of two main parts:
- the central nervous system;
- the peripheral nervous system.
The central nervous system consists of the brain and spinal cord; the peripheral nervous system consists mainly of nerves.
Central nervous system: the brain and spinal cord
In an adult, the brain weighs an average of 1.3 to 1.4 kilograms and contains about 100 trillion nerve cells (neurons) and trillions of “support cells” called glia.
The spinal cord is about 43 cm long in an adult female and 45 cm long in an adult male, and weighs about 35-40 g. The vertebral column, the set of bones (of the back) that houses the spinal cord, is about 70 cm long, so the spinal cord is much shorter than the vertebral column.
The brain is composed of gray matter and white matter. Its interior consists mainly of white matter and is surrounded by a layer of gray matter, the cerebral cortex.
- The white matter consists of myelin fibers, oligodendrocytes, fibrous astrocytes, and microglial cells. The white color is given by myelin;
- The gray substance contains the soma (cell body), myelin and myelinated fibers, protoplasmic astrocytes, oligodendrocytes, and microglial cells.
In transverse sections of the spinal cord, the white substance is on the outside and the gray substance is on the inside, where it takes an “H” shape.
In the central part of the H is a cavity, the central canal, which is a remnant of the neural tube covered with ependymal cells. The neural tube is a structure found in chordate embryos that gives rise to the central nervous system. Cylindrical in shape with a central cavity, the neural tube is derived from a thickened region of the ectoderm, the neural plate, by a process called neurulation.
Gray matter forms the anterior horns of the H, which contain motor neurons and give rise to the ventral roots of the spinal nerves. The dorsal horns of the H are also gray matter and receive sensory fibers from the neurons of the dorsal root ganglia.
- The anterior horn is made up of neurons responsible for motor functions (α motor neurons and γ motor neurons);
- The posterior horn is made up of neurons responsible for sensory functions, especially touch and pain.
The central nervous system is protected by the skull and spine, and also by connective tissue membranes called meninges. From the outermost, the meninges are:
- Dura mater;
- Pia mater.
Peripheral nervous system: nerves
Nerves are bundles of long fibers (or axons) that connect the central nervous system to every other part of the body.
Nerves that carry signals from the brain are called motor or efferent nerves, while those that carry information from the body to the central nervous system are called sensory or afferent nerves.
The spinal nerves are mixed because they perform both functions.
The peripheral nervous system is divided into two or three separate subsystems, depending on the classification:
- somatic (mediating voluntary movement)
- autonomic or vegetative (acting involuntarily on certain organs and glands as well as certain muscles);
- enteric (controlling only the gastrointestinal system) – which some consider to be autonomic.
The somatic nervous system consists of peripheral nerve fibers, which transmit sensory information to the central nervous system, and motor nerve fibers, which transmit to skeletal muscles.
The autonomic nervous system is further divided into sympathetic (thoracic-lumbar), which is activated in an “emergency” to mobilize energy, and parasympathetic (craniosacral), which is activated when the body is in a state of parasympathetic relaxation.
Nerves that come from the skull are called cranial nerves, while those that come from the spinal cord with the ganglia are called spinal nerves.
Sympathetic nervous system
The sympathetic nervous system originates in the spinal cord and stimulates the heart, dilates the bronchi, contracts the arteries, inhibits the digestive system, and prepares the body for physical activity.
The cell bodies of the first neuron (the preganglionic neuron) are located in the thoracic and lumbar tracts.
Axons from these neurons lead to a chain of ganglia located on either side of the spine (the latero-vertebral ganglia chain).
In the ganglionic chain, most neurons form synapses with another neuron (the postganglionic neuron).
The postganglionic neuron then projects to the “target”: a muscle (smooth or cardiac) or a gland.
In the sympathetic system, preganglionic fibers are short, while postganglionic fibers are long.
Parasympathetic nervous system
It is called the cranio-sacral autonomic system because it reports to the viscero-motor nuclei of the encephalic nerves and the sacral visceral effector columns.
The parasympathetic is a system that is predisposed to nutrition, digestion, sleep and rest.
The parasympathetic centers are located in the brain stem and the sacral part of the spinal cord.
In the brainstem there are nuclei for innervation of the salivary, nasal and lacrimal glands and all organs up to the left colic flexure, which is the boundary point between the middle and caudal intestines.
In this system, the preganglionic branches are long and reach the ganglia just outside or inside the organ to be innervated (hence the postganglionic fibers are very short).
In the heart, the parasympathetic nervous system is responsible for reducing the heart rate and pressure and causing vasoconstriction of the arteries of the heart (the coronary arteries).
A coronary constriction results in less blood supply to the heart.
In the digestive tract, the vagus represents the parasympathetic and acts by causing peristalsis and, at the gastric level, HCl secretion.
Enteric nervous system
The enteric nervous system is a tangle of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, gallbladder). In the various organs, it acts through plexuses (myenteric plexus and submucosal plexus).
Disorders of the nervous tissue
Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.
Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder.
Guillain-Barré syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.
- Atlas of Histology – University of Genoa, Italy. https://istologia.unige.it/en/nervous_tissue