Properties Of Excitable Tissues
Introduction
Excitability is the ability of a cell to generate electrical signals that enable it to integrate and transmit impulses such as action potential, receptor potential, and synaptic potential.
Excitable Tissues are tissues that can change their properties to generate electrical signals under the influence of a stimulus.
Types Of Excitable Tissues
There are 3 types of excitable tissues:
- Nervous tissues:
- The basic structural and functional unit of the nervous system is the neuron.
- The Nervous System is divided into the Central Nervous System and the Peripheral Nervous System.
- The nervous system consists of two major types of cells:
- Neurons: Respond to stimuli by communicating with other cells or neurons.
- Neuroglia: Provide support and protection for neurons.
- Muscular tissues:
- Muscle cells, also known as myocytes, form muscle tissue.
- There are 3 types of muscles:
- Skeletal muscle
- Cardiac muscle
- Smooth muscle
- Muscles respond to stimuli by contracting.
- Glandular epithelium tissues:
- Glandular epithelium cells form the glandular epithelium tissue that covers both the endocrine and exocrine glands of the body.
- This tissue responds to stimuli by secreting substances.
General Properties Of Excitable Tissues
All excitable tissues share the following properties:
1. Irritability
- All excitable tissues have the ability to react to irritation or stimuli.
- Stimuli change the physiological properties of excitable tissues and generate the process of excitation.
- Excitability of cells and tissues is a basic function of life.
2. Threshold
- The threshold is the minimal strength of stimulus required to cause a tissue response.
- It is also called the threshold stimulation and causes the membrane potential of excitable tissue to become less negative.
- Types of threshold include:
- Sub-threshold: Elicit graded potentials.
- Absolute threshold: Elicit action potentials.
- Supra-threshold: Elicit action potentials.
- Threshold is approximately −50 to −40 mV in most excitable cells.
3. Resting Membrane Potential (RMP)
- RMP is the potential difference that exists across all cell membranes.
- The inside of the cell is negative with respect to the outside.
- RMP is measured using microelectrodes and an oscilloscope.
- It is about -70 to -90 mV.
4. Action Potential
- Action potential is the fluctuation of a membrane potential that results in rapid depolarization and repolarization of excitable tissue.
- During the depolarization component of action potential:
- The inside of the cell becomes positive, and the outside becomes negative.
- An impulse is generated and spreads away from its origin.
5. Changes in Excitability During Excitation
- All excitable tissues exhibit changes in excitability at different stages:
- At resting stage: Excitability is at its initial level (resting membrane potential).
- At depolarization stage: Excitability rises as sodium channels start to open.
- At repolarization stage:
- Excitability is very low (absolute refractory period) – all sodium channels are open.
- Excitability is low (relative refractory period) – some sodium channels are already closed.
- After repolarization stage: Excitability is low (Hyperpolarization) – the cell has less potassium.
6. All-or-None Law
- The all-or-none law states that the strength of a response for an excitable tissue is not dependent upon the strength of the stimulus.
- An excitable tissue will always give the same maximum response and spikes of the same amplitude when stimulated.
- Exceptions occur if the membrane properties are affected by disease, toxins, or fatigue.
7. Propagation of Impulse
- All excitable tissues have the ability to propagate impulses (action potentials).
- When one area (part of the membrane) is depolarized, it creates a potential difference between the depolarized area and the adjacent area (membrane).
- This leads to the flow of local current produced by the depolarization process.
- This local current flow will cause the opening of voltage-gated Na+ channels in the adjacent membrane.
- Na+ influx will occur, and the adjacent membrane is depolarized.
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