Beatrice Lang
Excitable cells are those that can be stimulated to create a tiny electric current. They play a crucial role in transmitting signals within the nervous system. The two main types of excitable cells are neurons and skeletal muscle cells. Neurons are the primary excitable cells of the nervous system and are electrically activated when the polarity shifts due to various ions coming in contact with the membrane. Muscle cells, on the other hand, contract in response to electrical signals. These include skeletal muscle cells, which are responsible for voluntary muscle movements, and cardiac muscle cells, which control involuntary contractions and are found only in the heart.
| Characteristics | Values |
|---|---|
| Definition | A brief disturbance of the normally alkaline cytoplasm of neurons, resulting from cation influx, which underlies the transient loss of cellular homeostasis and contributes to neuronal activity and sentience |
| Types | Neurons, nerve cells, skeletal muscle cells, cardiac muscle cells, astrocytes, myocytes, and some endocrine cells |
| Function | Motor control, sensory perception, cognitive processes, contraction, and transmission of signals within the nervous system |
| Action Potential | The activation of voltage-gated ion channels, resulting in a rapid and significant change in membrane potential for a short time (1 to 100 milliseconds) |
| Ion Channels | Na+, K+, Ca2+, Cl-, Mg2+ |
| Regulation | Extracellular electrolyte concentrations, associated proteins, voltage-gated ion channels, ion transporters, membrane receptors, and hyperpolarization-activated cyclic-nucleotide-gated channels |
| Excitatory Postsynaptic Potentials (EPSPs) | Generated by the opening of Na+ channels, leading to a reduction in resting potential and the generation of an action potential |
| Inhibitory Postsynaptic Potentials (IPSPs) | Induced by the opening of K+ or Cl- channels, producing inhibitory effects that counteract excitatory signals |
| Resting Potential | The electrical charge across the plasma membrane, typically around -70 millivolts in excitable cells |
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Neurons
The neuronal membrane has a negative charge on the inside compared to the outside, and this charge is produced by ions. The difference in charge creates a membrane potential, which functions as a battery, providing power to operate various "molecular devices" embedded in the membrane. In neurons, the membrane potential is influenced by diverse factors, including numerous types of ion channels, some of which are chemically gated, while others are voltage-gated.
The opening and closing of these ion channels lead to changes in the membrane potential, which can quickly be sensed by adjacent or distant ion channels, resulting in a regenerative feedback loop. This process is essential for transmitting signals between different parts of the neuron. For example, in an action potential, a stimulus causes the cell membrane to depolarize beyond a threshold level, leading to a rapid and significant change in the membrane potential that often reverses its polarity.
The most important regulators of neuronal excitability are the extracellular electrolyte concentrations of ions like Na+, K+, Ca2+, Cl-, and Mg2+, as well as associated proteins such as voltage-gated ion channels, ion transporters, membrane receptors, and hyperpolarization-activated cyclic-nucleotide-gated channels. Calcium ions (Ca2+), in particular, play a crucial role as a chemical messenger, activating various processes such as exocytosis, contraction, and gene expression.
Overall, the electrical excitability of neurons is a complex process involving the interaction of ion channels, membrane potentials, and various regulatory factors, enabling neurons to transmit signals and perform their essential functions in the nervous system.
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Muscle cells
The excitability of muscle cells allows them to respond to stimuli and maintain chemical potentials across their cell membranes. For example, in the reflex arc, action potentials in motor axons might carry the message from the spinal cord to the arm, telling the muscle fibers of the biceps to contract. In addition, muscle cells can be excited by pharmacological stimuli, which has been used for PMI estimation after subconjunctival injection of drugs.
The regulation of muscle cell excitability is influenced by extracellular electrolyte concentrations (e.g., Na+, K+, Ca2+, Cl-, Mg2+) and associated proteins. Voltage-gated ion channels, such as the voltage-gated chloride channel ClC-1, play a crucial role in muscle cell excitability. Mutations in this channel can lead to conditions like myotonia congenita (MC), causing muscle stiffness and impaired mobility.
Additionally, female steroid hormones have been found to regulate the expression of BK channels, impacting muscle relaxation during pregnancy. The K2P channel family, including TREK-1, TREK-2, and TRAAK, are expressed in the myometrium and may contribute to the regulation of membrane potential in both electrically excitable and non-excitable cells.
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Cardiac muscle cells
The primary function of cardiomyocytes is to contract, which generates the pressure needed to pump blood through the circulatory system. Each cardiomyocyte needs to contract in coordination with its neighboring cells, known as a functional syncytium. This coordination is vital for the efficient pumping of blood from the heart. If this coordination breaks down, the heart may not pump at all, as can occur during abnormal heart rhythms such as ventricular fibrillation.
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Skeletal muscle cells
The process of muscle contraction begins at the neuromuscular junction (NMJ), where a motor neuron meets a skeletal muscle fiber. Every skeletal muscle fiber is supplied by a motor neuron, and excitation signals from these neurons are necessary for the functional activation of skeletal muscle fibers to contract. The motor end-plate, located within the muscle fiber, contains acetylcholine (ACh) receptors. When the axon terminal releases ACh, it binds to these receptors, initiating the excitation-contraction process.
The high concentrations of calcium in skeletal muscle are stored in the sarcoplasmic reticulum (SR), a specialized organelle. The SR surrounds the myofibrils, facilitating the direct release of calcium at sites of actin and myosin overlap. The excitation of the muscle membrane is coupled with the release of calcium from the SR through T-tubules, which ensure the action potential reaches the interior of the cell. This arrangement of T-tubules and SR membranes is known as a triad.
The excitability of skeletal muscle tissues is important for the design of muscle tissue bioreactor systems and implantable muscle stimulators. For example, the chronic administration of taurine to aged rats has been shown to improve the electrical and contractile properties of skeletal muscle fibers. Additionally, the study of skeletal muscle excitability has forensic applications, as it can be used for post-mortem interval (PMI) estimation through electrical stimulation.
In summary, skeletal muscle cells exhibit electrical excitability by generating action potentials and releasing calcium ions, which ultimately leads to muscle contraction. This excitability is crucial for various applications, including muscle tissue engineering and forensic investigations.
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Astrocytes
The morphological and functional heterogeneity of astrocytes is influenced by neuronal heterogeneity through the release of specific neurotransmitters, ions, and neurotrophic factors. It is likely that this is also the case for the electrophysiological features of astrocytes. It is also possible that changes in astrocyte electrophysiology reflect shifts in the expression of ion transporters. For example, the K+-Cl− cotransporter (KCC2) modulates Cl− levels in neurons through the export of 1 K+ and 1 Cl− across the membrane. An increase in neuronal KCC2 expression early in development is believed to drive the shift of GABA from an excitatory to an inhibitory neurotransmitter. KCC2 is also expressed in astrocytes, and this expression increases across development.
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Frequently asked questions
Electrically excitable cells are those that can be stimulated to create a tiny electric current. They play a key role in transmitting signals within the nervous system.
There are two main types of excitable cells: neurons and skeletal muscle cells. Neurons are the primary excitable cells of the nervous system and are responsible for transmitting electrical signals, also known as nerve impulses or action potentials. Skeletal muscle cells are responsible for voluntary muscle movements.
Electrically excitable cells generate signals by opening or closing ion channels at one point in the membrane, producing a local change in the membrane potential. This change in the electric field can be quickly sensed by adjacent or distant ion channels in the membrane, which then open or close as a result, reproducing the signal.
In addition to neurons and skeletal muscle cells, cardiac muscle cells and certain endocrine cells also display electrical excitability. Cardiac muscle cells, found only in the heart, control involuntary contractions.
