In membrane biology, the Hodgkin cycle is a key component of membrane physiology that describes bioelectrical impulses, especially prevalent in neural and

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

tissues. It was identified by British

physiologist Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a subdiscipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out chemical and ...

and

biophysicist Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations ...

Sir Alan Lloyd Hodgkin. The Hodgkin cycle represents a

positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate the effects ...

loop in which an initial membrane

depolarization In biology, depolarization or hypopolarization is a change within a cell (biology), cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolar ...

leads to uncontrolled deflection of the

membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. It equals the interior potential minus the exterior potential. This is th ...

to near V_Na. The initial depolarization must reach or surpass a certain threshold in order to activate voltage-gated Na⁺ channels. The opening of Na⁺ channels allows Na⁺ inflow, which, in turn, further depolarizes the membrane. Additional depolarization activates additional Na⁺ channels. This cycle leads to a very rapid rise in Na⁺ conductance (g_Na), which moves the membrane potential close to ''V''_Na. The cycle is broken when the membrane potential reaches to the sodium equilibrium potential and

potassium channel Potassium channels are the most widely distributed type of ion channel found in virtually all organisms. They form potassium-selective pores that span cell membranes. Potassium channels are found in most cell types and control a wide variety of ...

s open to re-polarize the membrane potential. This positive feedback loop means that the closer these voltage-gated Na⁺ channels are to each other, the lower the threshold of

activation In chemistry and biology, activation is the process whereby something is prepared or excited for a subsequent reaction. Chemistry In chemistry, "activation" refers to the reversible transition of a molecule into a nearly identical chemical or ...

Importance in cell physiology

An understanding of membrane physiology is needed in order to understand how cells communicate with one another. Signalling between cells, such as

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

s for example, revolves around changes in the electrical potentials across their membranes. In a healthy cell at rest, the

intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

area is usually negatively charged relative to the

extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

region. Depolarization refers to when the intracellular region is neutralized to be at the same

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

relative to the extracellular region. The

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

of sodium ions is intimately related to the electrical potential across a membrane. Depolarization often occurs via the influx of sodium ions to the intracellular region. Given that sodium ions have a positive charge, the intracellular region becomes less negatively charged relative to the extracellular region.

References

{{reflist Membrane biology