Excitability is a property of certain cells that allows them to respond to stimuli by generating electrical signals. This capability is found in various life forms, from single-celled organisms to complex animals and plants. At its core, excitability involves a cell’s membrane rapidly changing its electrical state. This regulated and transmissible signal allows for communication and coordination within an organism.
Nerve Cell Excitability
Nerve cells, or neurons, are designed to receive, process, and transmit information through electrical and chemical signals. A neuron at rest maintains a negative electrical charge inside compared to its surroundings, a state known as the resting potential. When a neuron receives a stimulus, such as a touch on the skin, it triggers the opening of specialized channels in its membrane.
This event allows positively charged ions to rush into the cell, rapidly reversing the electrical charge in a localized area. This sudden shift, called an action potential, creates an electrical impulse that travels down the length of the neuron. For instance, a sensory neuron in your fingertip generates an action potential that travels to your spinal cord when you touch a hot surface, initiating a reflex. The frequency of these impulses can encode the intensity of the stimulus.
Muscle Cell Excitability
Muscle cells also exhibit excitability, but their response leads to contraction. When a motor neuron signals a muscle fiber, it releases chemical messengers that trigger an electrical change in the muscle cell’s membrane, similar to an action potential. This electrical event is the initial step that leads to muscle contraction.
This excitation wave spreads across the muscle cell’s surface and into its interior. It triggers the release of stored calcium ions, which then interact with specialized proteins within the muscle fiber. This interaction causes the fibers to slide past one another, shortening the muscle cell and generating force. This process is responsible for voluntary movements, like lifting an object, and the involuntary beating of the heart.
Plant Excitability Examples
Several plants also exhibit excitability. In the Venus flytrap, for instance, an insect touching the sensitive trigger hairs on its leaves initiates an electrical signal. This signal, a type of action potential, propagates across the leaf and causes a rapid change in water pressure within the cells. This change in turgor pressure causes the trap to snap shut, capturing the prey.
A similar phenomenon occurs in the sensitive plant, Mimosa pudica, where a touch causes an electrical signal that makes its leaflets fold inward. While the mechanisms in plants involve ion fluxes that alter water movement, the principle is similar to that in animals. A stimulus triggers a propagated electrical response that leads to a rapid action.