Trystan Casey
The human body is composed of four types of tissues: epithelial, connective, muscle, and nervous. Epithelial tissue forms a protective layer over the body's surfaces, lines its cavities, and creates glands. It also acts as a receptor for special senses like smell, taste, hearing, and vision. Nervous tissue, on the other hand, is responsible for transmitting electrical impulses, facilitating communication between different parts of the body, and controlling various body activities. This raises the question: are epithelial tissues capable of electrical excitation?
| Characteristics | Values |
|---|---|
| Epithelial tissue | Made of layers of cells that cover the surfaces of the body that come into contact with the exterior world, line internal cavities, and form glands |
| Acts as a covering, controlling the movement of materials across its surface | |
| Involved in the diffusion of ions and molecules | |
| Forms specialised junctions to create a barrier between connective tissues and free surfaces | |
| The exumbrellar epithelium of the hydromedusa, Euphysa japonica, is composed of a single layer of cells that are joined by gap junctions and simple appositions | |
| The average resting potential of epithelial cells is -46 mV | |
| The two-dimensional space constant of the epithelium is 1.3 mm | |
| The internal longitudinal resistivity of the cytoplasm and intercellular junctions is 196 omega cm | |
| The resistivity of both apical and basal cell membranes is greater than 23 k ohm |
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What You'll Learn
Electrical stimulation of excitable tissues
In the context of safety and efficacy, the physical basis for electrical stimulation of excitable tissue is well-studied. The process involves placing a metal electrode in a physiological medium like extracellular fluid (ECF), which forms an interface between the two phases. The charge is carried by electrons in the metal electrode phase and attached circuits. The electrode/tissue interface is a critical aspect of this process, with Faradaic and non-Faradaic charge transfer mechanisms being presented and contrasted.
Excitable tissues can be stimulated by electric fields through several mechanisms. For instance, a uniform, long, straight peripheral axon is activated by an electric field with a gradient parallel to the fiber axis. Cortical neurons in the brain are excited when the electric field, applied along the axon-dendrite axis, reaches a certain threshold. The bidomain model is a useful framework for understanding excitation events, with concepts like the activating function, virtual anodes and cathodes, and anode and cathode break and make stimulation.
Transcranial electrical stimulation (tES) is a non-invasive brain stimulation technique that has gained prominence in modulation central nervous system excitability in humans. It includes transcranial direct and alternating current stimulation (tDCS and tACS). This technique has been used to explore brain physiology, modelling approaches, cognitive neuroscience applications, and interventional approaches.
Furthermore, electrical stimulation has been studied in the context of cardiac tissue, with mathematical models describing the response of excitable tissues to electric fields. These models have been useful in predicting tissue behaviour, designing experiments, and interpreting findings.
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Electrical properties of an excitable epithelium
The exumbrellar epithelium of the hydromedusa, Euphysa japonica, is an example of an excitable epithelium. It is composed of a single layer of broad (70 micrometers), thin (1-2 micrometers) cells which are joined by gap junctions and simple appositions. Despite lacking nerves, the exumbrellar epithelium is excitable, and electrical stimulation can initiate a propagated action potential.
The average resting potential of the epithelial cells is -46 mV. The action potential, recorded with an intracellular electrode, is an all-or-nothing, positive, overshooting spike. The epithelial cells are electrically coupled. The passive electrical properties of the epithelium were determined from the decrement in membrane hyperpolarization with distance from an intracellular, positive current source.
The two-dimensional space constant of the epithelium is 1.3 mm, the internal longitudinal resistivity of the cytoplasm and intercellular junctions is 196 ohm cm, and the resistivity of both apical and basal cell membranes is greater than 23 k ohm.
The electrical properties of the exumbrellar epithelium of the hydromedusa have been studied to understand the passive electrical properties of this excitable tissue. The ionic channels in epithelial cell membranes play a role in epithelial conduction and its properties and functions.
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The four types of body tissues
While all living tissues respond to stimuli, excitable tissues like nerves and muscles have the unique ability to generate signals that can be swiftly transmitted to other cells. Excitable tissues can be electrically stimulated to supplement or replace functions lost in neurologically impaired individuals.
Epithelial tissues are one of the four types of body tissues, along with connective, muscle, and nervous tissues. Epithelial tissues act as coverings and control the movement of materials across their surface. They are sheets of cells that cover exterior body surfaces, line internal cavities, and form certain glands.
Connective tissues bind the various parts of the body together, providing support and protection. They form membranes that encapsulate organs and line movable joints, such as the synovial membrane in the shoulder, elbow, and knee joints.
Muscle tissues enable body movement by responding to stimulation and contracting. There are three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart.
Nervous tissues facilitate communication and coordination between different body regions and control many body activities. They are found in the brain, spinal cord, and nerves, and consist of neurons or nerve cells, which generate and conduct impulses.
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The role of nervous tissue
Nervous tissue is found in the brain, spinal cord, and nerves. It is responsible for coordinating and controlling many body activities, including muscle contraction, emotions, memory, and reasoning. Nervous tissue allows us to be aware of our environment and plays a role in everything we do. It stimulates muscle contraction, regulates our thoughts and memory, and controls our senses and movements.
The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. These cells have three principal parts: the dendrites, the cell body, and one axon. The cell body is the main part of the cell, carrying out general functions. Dendrites are extensions, or processes, of the cytoplasm that carry impulses to the cell body. The axon is another extension that carries impulses away from the cell body. Nervous tissue also includes cells that do not transmit impulses but support the activities of the neurons.
The nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises the brain and spinal cord, while the PNS is made up of a network of nerves branching out from the spinal cord. The PNS relays information from the CNS to the rest of the body, including the organs, limbs, fingers, and toes.
The nervous system works by sending electrical signals between the brain and other body parts. These signals tell us to breathe, move, speak, and see, among other functions. Electrical stimulation of excitable tissue, such as nerves, can be used to supplement or replace lost functions in neurologically impaired individuals. This stimulation aims to control the initiation and propagation of action potentials and the release of neurotransmitters, thereby influencing nervous system signaling.
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The structure and function of epithelial tissue
Epithelial tissue, or epithelium, is one of the four main types of body tissue, along with connective, neural, and muscular tissue. It forms the outer covering of the skin and lines the body's internal cavities, blood vessels, and organs. Epithelial tissue is made up of epithelial cells, which can vary in shape and arrangement depending on their location and function in the body.
There are three main types of epithelial cells: squamous, cuboidal, and columnar. Squamous epithelial cells are thin and flat, cuboidal epithelial cells are short and cylindrical, and columnar epithelial cells are long and cylindrical with their nucleus at the base. Epithelial cells can also be classified based on the number of layers they form. A simple epithelium is composed of a single layer of cells and typically has a secretory or absorptive function. A stratified or compound epithelium is made up of two or more layers of cells and primarily serves a protective function.
Epithelial tissues have a variety of functions, including protection, secretion, absorption, transportation, and receptor function. For example, the epithelial cells lining the respiratory tract have cilia that trap dust and other particles, preventing them from entering the lungs. Epithelial tissue in the inner ear, which contains stereocilia (specialized microvilli), is essential for hearing and balance. Additionally, the epithelial tissue in the nose, eyes, ears, and taste buds contains sensory receptors that help transmit signals from external stimuli to the brain.
Epithelial tissue is also capable of electrical excitation. The exumbrellar epithelium of the hydromedusa, Euphysa japonica, is an example of an excitable epithelium that lacks nerves. However, when electrically stimulated, it can initiate a propagated action potential.
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Frequently asked questions
Epithelial tissues are one of the four basic tissue types in the body, along with connective, muscle, and nervous tissues. Epithelial tissue creates protective boundaries and is involved in the diffusion of ions and molecules. It acts as a covering, controlling the movement of materials across its surface.
Yes, epithelial tissues are capable of electrical excitation. The exumbrellar epithelium of the hydromedusa, Euphysa japonica, is an example of an excitable epithelium that lacks nerves. However, when electrically stimulated, it initiates a propagated action potential.
The goal of electrical stimulation of excitable tissue is to control, by activation or inhibition, the initiation and propagation of action potentials and, subsequently, the release of neurotransmitters and nervous system signaling.
