The functions of this system made up of nerve cells are sensory input, integration, muscle and gland control, homeostasis, and mental activity.
Nervous tissue is the main tissue component of the two parts of the nervous system; the brain and spinal cord of the central nervous system (CNS) and the branching peripheral nerves of the peripheral nervous system (PNS), which regulate and control the functions and activity of the body.
It is made up of neurons, or nerve cells, that receive and transmit impulses, also known as glial cells or, more commonly, as glia (from the Greek, meaning glue), which help in the propagation of the nerve impulse, as well as providing nutrients to the neuron.
Nerve tissue is made up of different types of nerve cells , all with an axon, the long part of the cell that sends action potential signals to the next cell. Axon bundles make up the nerves.
Structure of nervous tissue
Nerve tissue is made up of neurons, also called nerve cells and glial cells. Generally, nerve tissue is classified into four types of tissue.
In the central nervous system (CNS), the types of tissues found are gray matter and white matter. In the peripheral nervous system (PNS), the types of tissues are nerves and ganglia. Tissue is classified by its neuronal and glial components.
Components of nervous tissue
Neurons are cells with specialized characteristics that allow them to receive and facilitate nerve impulses, or action potentials, through their membrane to the next neuron. They have a large cell body (soma), with cell projections called dendrites and an axon.
Dendrites are thin, branched projections that receive electrochemical (neurotransmitter) signaling to create a change in cell voltage. Axons are long projections that carry the action potential away from the cell body to the next neuron.
The bulbous end of the axon, called the axon terminal, is separated from the dendrite of the next neuron by a small space called the synaptic cleft.
When the action potential travels to the axon terminal, neurotransmitters are released across the synapse and bind to postsynaptic receptors, continuing the nerve impulse.
Neurons are classified functionally and structurally.
Sensory (afferent) neurons : relay sensory information in the form of an action potential (nerve impulse) from the peripheral nervous system to the central nervous system.
Motor (efferent) neurons : they relay an action potential from the central nervous system to the appropriate effector (muscles, glands).
Interneurons : cells that form connections between neurons and whose processes are limited to a single local area in the brain or spinal cord.
Multipolar neurons : they have 3 or more processes that come out of the soma (cell body). They are the largest type of neurons in the central nervous system and include motor neurons and interneurons.
Bipolar neurons: sensory neurons that have two processes that come out of the soma, a dendrite and an axon.
Pseudounipolar neurons: sensory neurons that have a process that divides into two branches, forming the axon and the dendrite.
Unipolar Brush Cells : These are excitatory glutamatergic interneurons that have a single short dendrite ending in a brush-shaped tuft of dendrils. These are found in the granular layer of the cerebellum .
Neuroglia encompasses the non-nerve cells in nerve tissue that provide several crucial support functions for neurons. They are smaller than neurons and vary in structure depending on their function.
Glial cells are classified as follows:
Microglial cells : microglia are macrophage cells that make up the primary immune system to the central nervous system. They are the smallest glial cell.
Microglia also work to protect the brain when neurons are injured or diseased by turning off malfunctioning neurons.
Astrocytes : Astrocytes are found in the brain and spinal cord and are 50 times more common than neurons. Astrocytes are not only the most abundant glia, they are also the most abundant cell type in the brain. Astrocytes are distinguished by their star shape.
They form a blood-brain barrier, this barrier prevents some substances from entering the brain and allows the entry of other people. The two main categories of astrocytes are protoplasmic astrocytes and fibrous astrocytes.
The main function of astrocytes is to provide structural and metabolic support for neurons. Additionally, astrocytes help signal between neurons and blood vessels in the brain. This allows blood flow to increase or decrease depending on the activity of the neuron.
Oligodendrocytes : cells of the central nervous system with very few processes. They form myelin sheaths on the axons of a neuron, which are lipid-based insulators that increase the rate at which the action potential can travel down the axon.
Oligodendrocytes are found in the white matter of the brain, while satellite oligodendrocytes are found in the gray matter. Satellite oligodendrocytes do not form myelin.
Glia NG2 : cells of the central nervous system that are distinct from astrocytes, oligodendrocytes, and microglia, and serve as precursors to oligodendrocyte development.
Schwann cells : the peripheral nervous system equivalent of oligodendrocytes, which help maintain axons and form myelin sheaths in the peripheral nervous system.
Schwann cells are neuroglia that surround some neuronal axons to form the myelin sheath in the structures of the peripheral nervous system.
Schwann cells help improve nerve signal conduction, aid nerve regeneration, and aid antigen recognition by T cells. Schwann cells play a vital role in nerve repair .
These cells migrate to the site of injury and release growth factors to promote nerve regeneration. The Schwann cells then myelinate the newly generated nerve axons.
Schwann cells are being intensively investigated for their possible use in spinal cord injury repair.
Oligodendrocytes and Schwann cells indirectly assist in impulse conduction, since myelinated nerves can conduct impulses more rapidly than unmyelinated ones.
Interestingly, the white matter of the brain acquires its color from the large number of myelinated nerve cells it contains.
Satellite glial cell : lines the surface of the neural cell bodies in the ganglia (groups of cells in the nervous body grouped together or connected to each other in the peripheral nervous system).
Satellite sensory glial cells are involved in the development of chronic pain.
Enteric glia : found in the enteric nervous system, within the gastrointestinal tract.
Ependymal cells : Ependymal cells are specialized cells that line the brain ventricles and the central canal of the spinal cord. They are found within the choroid plexus of the meninges.
These hair cells surround the capillaries of the choroid plexus and form the cerebrospinal fluid (CSF). The functions of ependymal cells include the production of cerebrospinal fluid, the provision of nutrients for neurons, the filtration of harmful substances, and the distribution of neurotransmitters.
Classification of nervous tissue
In the central nervous system
Gray matter is composed of cell bodies, dendrites, unmyelinated axons, protoplasmic astrocytes (subtype of astrocytes), satellite oligodendrocytes (subtype of nonmyelinating oligodendrocytes), microglia, and very few myelinated axons.
The white matter is composed of myelinated axons, fibrous astrocytes, myelinating oligodendrocytes, and microglia.
In the peripheral nervous system
Ganglionic tissue is made up of cell bodies, dendrites, and satellite glial cells.
Nerves are made up of myelinated and unmyelinated axons, Schwann cells surrounded by connective tissue.
The three layers of connective tissue that surround each nerve are:
Endoneurium : Each axon or nerve fiber is surrounded by the endoneurium, which is also called the endoneurial tube, canal, or sheath. This is a thin, delicate, protective layer of connective tissue.
Perineurium : Each nerve bundle containing one or more axons is enclosed by the perineurium, a connective tissue that has a lamellar arrangement in seven or eight concentric layers.
This plays a very important role in the protection and support of nerve fibers and also serves to prevent the passage of large molecules from the epineurium into a bundle.
Epineurium : The epineurium is the outermost layer of dense connective tissue that surrounds the (peripheral) nerve.
A neuromuscular junction (or myoneural junction) is a chemical synapse formed by contact between a motor neuron and a muscle fiber.
It is at the neuromuscular junction that a motor neuron can transmit a signal to the muscle fiber, causing muscle contraction.
Muscles require innervation to function, and even just to maintain muscle tone, preventing atrophy.
Synaptic transmission at the neuromuscular junction begins when an action potential reaches the presynaptic terminal of a motor neuron, which activates voltage-gated calcium channels to allow calcium ions to enter the neuron.
Calcium ions bind to sensor proteins (synaptotagmin) in synaptic vesicles, causing fusion of the vesicle with the cell membrane and subsequent release of neurotransmitters from the motor neuron to the synaptic cleft.
In vertebrates, motor neurons release acetylcholine (ACh), a small molecule neurotransmitter that diffuses through the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs). ) in the cell membrane of the muscle fiber, also known as the sarcolemma.
Nicotinic acetylcholine receptors are ionotropic receptors, meaning that they serve as ligand-gated ion channels.
Binding of acetylcholine to the receptor can depolarize the muscle fiber, causing a cascade that ultimately produces muscle contraction. Neuromuscular junction diseases can be genetic and autoimmune in origin.
Genetic disorders, such as Duchenne muscular dystrophy, can arise from mutated structural proteins that comprise the neuromuscular junction, whereas autoimmune diseases, such as myasthenia gravis, occur when antibodies against nicotinic acetylcholine receptors are produced in the sarcolemma. .
Nerve tissue function
The nervous system is made up primarily of nervous tissue. Nervous tissue is made up of nerve cells that come in different forms and fulfill a variety of functions. The functions of the nervous system include the following:
- Sensory information.
- Control of muscles and glands.
- Mental activity
After looking at the components of nervous tissue and the basic anatomy of the nervous system, one comes to understand how nervous tissue is able to communicate within the nervous system.
The function of nervous tissue is to form the communication network of the nervous system by conducting electrical signals through the tissue. In the central nervous system, gray matter, which contains synapses, is important for information processing.
The white matter, which contains myelinated axons, connects and facilitates the nerve impulse between gray matter areas in the central nervous system.
In the peripheral nervous system, ganglionic tissue, which contains cell bodies and dendrites, contains relay points for impulses from nervous tissue. Nervous tissue, which contains bundles of myelinated axons, has nerve impulse / action potential.
The nervous system is subdivided into several overlapping ways. The central nervous system (CNS) is made up of the brain and spinal cord, which coordinates information from all areas of the body and sends nerve impulses that control all body movements.
The peripheral nervous system (PNS) consists of peripheral nerves that branch throughout the body. It connects the central nervous system with the rest of the body and is directly responsible for controlling the movements of specific parts of the body.
For example, just before arm movement, the central nervous system sends nerve impulses to the nerves in the peripheral nervous system of the arm, causing the arm to move.
Another subdivision of the nervous system is the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). The sympathetic nervous system is activated to stimulate a fight or flight response in an organism when that organism encounters a threat and must decide whether to fight or flee from it.
The nerves of the sympathetic nervous system have various effects on different parts of the body. Activation of the sympathetic nervous system causes the pupils of the eyes to dilate, inhibits digestion, increases the secretion of sweat and increases the heart rate.
In contrast, the parasympathetic nervous system is activated during times of “rest and digestion,” when an organism does not face an immediate threat.
The nerves of the parasympathetic nervous system work to stimulate activities that can occur at rest, such as digestion, waste excretion, and sexual arousal, and they also slow the heart rate.
The enteric nervous system (ENS) controls the gastrointestinal tract (digestive tract). This division of the nervous system, along with the sympathetic nervous system and the parasympathetic nervous system, is collectively known as the autonomic nervous system (ANS).
The autonomic nervous system regulates the activities that are carried out unconsciously; we don’t have to think about digesting food for it to happen, for example. In contrast, the somatic nervous system (SNS) controls voluntary body movements.
It is made up of afferent and efferent nerves that send signals to and from the central nervous system, causing voluntary muscle contraction.
Example of the function of nervous tissue
Imagine that you are about to take a shower in the morning before going to school. He has turned on the faucet to start the water as he prepares to step into the shower.
After a few minutes, wait for the water to have a comfortable temperature to go in. So you extend your hand into the stream of water.
What happens next depends on how your nervous system interacts with the water temperature stimulus and what you do in response to that stimulus.
It is found on the skin of the fingers or toes and is a type of temperature-sensitive sensory receptor, called a thermoreceptor. When you place your hand under the shower, the cell membrane of the thermoreceptors changes its electrical state (voltage).
The amount of change depends on the strength of the stimulus (how hot the water is). This is called the graduated potential. If the stimulus is strong, the cell membrane voltage will change enough to generate an electrical signal that will travel down the axon.
You have learned about this type of signaling before, regarding the interaction of nerves and muscles at the neuromuscular junction. The voltage at which this signal is generated is called the threshold, and the resulting electrical signal is called the action potential.
In this example, the action potential travels, a process known as propagation, along the axon from the axon mound to the axon terminals and into the bulbs at the synaptic end. When this signal reaches the end of the lamps, it causes the release of a signaling molecule called a neurotransmitter.
The neurotransmitter diffuses across the short distance from the synapse and binds to a receptor protein on the target neuron. When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new gradual potential begins.
If that gradual potential is strong enough to reach the threshold, the second neuron generates an action potential in its axon. The target of this neuron is another neuron in the thalamus of the brain, the part of the central nervous system that acts as a relay for sensory information.
At another synapse, the neurotransmitter is released and binds to its receptor. The thalamus then sends the sensory information to the cerebral cortex, the outermost layer of gray matter in the brain, where conscious perception of water temperature begins.
Within the cerebral cortex, information is processed between many neurons, integrating the water temperature stimulus with other sensory stimuli, with your emotional state (you are just not ready to wake up, the bed is calling you), memories (maybe lab notes you should study before an exam).
Finally, a plan is developed on what to do, whether it’s to turn up the temperature, turn off the entire shower and go back to bed, or get in the shower. To do any of these things, the cerebral cortex has to send a command to your body to move the muscles.
A region of the cerebral cortex is specialized to send signals to the spinal cord for movement. The upper motor neuron is located in this region, called the precentral gyrus of the frontal cortex, which has an axon that runs throughout the spinal cord.
At the level of the spinal cord where this axon forms a synapse, a gradual potential occurs in the cell membrane of a lower motor neuron. This second motor neuron is responsible for the contraction of muscle fibers.
Acetylcholine is released at this specialized synapse, which causes the muscle action potential to begin, following a large potential known as the end plate potential.
When the lower motor neuron excites the muscle fiber, it contracts. All of this happens in a split second, but this story is the basis for how the nervous system works.