The Physiology of Neurons
The major function of the nervous system is the transmission, storage, and processing of information. This function is accomplished by the generation, conduction and integration of electrical activity and by the synthesis and release of chemical agents. Information is conducted from one region to another as electrical activity, commonly known as impulses, which are generated by neuronal cell bodies or axons and conducted by axons. Information is transmitted between cells by neurochemical agents that convey the signals from one cell to the next. The interaction of electrical activity in single cells and in groups of cells integrates this information. The physiology of neurons, axons and muscle fibers, are the basis for information transmission in the central and peripheral neural structures and for the transient symptoms and signs that accompany disease states.
The activity of the central and peripheral nervous systems never depends on the activity of a single neuron or axon but is always mediated by a group of cells or nerve fibers. Information is represented in the nervous system by a change in activity in a group of cells or fibers as they respond to some change in their input. Normal function in a single neuron as it participates in the processing of information is manifested as electrical potentials. Cell membranes separate ions into different concentrations on the exterior and interior of the cell. These concentration differences produce an electrical potential across the membrane called the membrane potential.
The membrane potential is the difference in electrical potential between the inside and the outside of a cell. All neurons, axons and muscle fibers have a membrane potential. Membrane potentials include resting potentials, action potentials and local potentials, such as synaptic potentials, generator or receptor potentials and electrotonic potentials. All membrane potentials result from ion flow through channels in the membrane.
The human body is approximately 60 percent water (by weight) in which a number of salts are dissolved. Salts in solution are dissociated into atoms with electrical charges (ions or electrolytes). The molecular arrangement and properties of cell membranes throughout the body create a barrier to the flow of certain ions through those membranes, resulting in a difference in proportion of ionic charges between the two sides of the membrane. A potential difference (or voltage; "potential" for short) is thus created across the membrane.
The concentration gradients are maintained by the cell membrane, a lipid bilayer that is relatively impermeable to sodium, potassium, chloride and calcium ions which are involved in the electrophysiological activity and signal transmission. The concentration of sodium, calcium, and chloride ions is higher extracellularly than that of potassium and impermeable anions is higher intracellularly. The equilibrium potential of each ion is the voltage difference across the membrane that exactly offsets the tendency of the ion to move down its concentration gradient. Ions can move across the cell membrane passively through ion channels or by adenosine triphosphate binding to carrier molecules. Some ion channels are open at rest, but most open or close in response to stimuli including changes in membrane potential (voltage gated channels) or chemical changes in the cytoplasm. The opening of a channel for a particular ion brings the membrane potential toward the equilibrium potential of that ion.
At a given time, the membrane potential is determined by the concentration gradient of the ions (which determines their respective equilibrium potentials) and by any changes in the permeability to individual ions across the membrane. The survival and excitability of the cell depend on the membrane potential. Maintaining this potential requires energy metabolism for the ATP dependent sodium/potassium pump. The resting potential is the baseline level of the membrane potential when the cell is at rest and not processing information. This potential depends primarily on the potassium channel. When a cell is active in the processing of information the membrane potential varies and are either local potentials or action potentials.
The membrane resting potential is created by the selective permeability of the cell membrane to certain ions in the extracellular fluid and intracellular fluid compartments. In the resting state, the cell membrane is virtually impermeable to sodium ions (Na+) and protein ions largely concentrated outside and inside the cell, respectively. It is much more permeable to chloride ions (Cl-) and potassium ions, which are largely concentrated in the extracellular and intracellular fluids respectively.
All cell membranes demonstrate a reasonably stable resting potential between the outside of the cell and the inside. Certain cells, specifically nerve and muscle cells, have excitable membranes. In response to a stimulus, the membrane potential of these cells undergoes a series of rapid changes, called an action potential, resulting in the formation of a nerve impulse. Nerve impulses are the currency of the nervous system.
A localized change in membrane potential results in current flow to surrounding areas of membrane. This current flow produces a small change in the membrane potential of adjacent areas. This change is called an electrotonic potential. Synaptic potentials are variation so the membrane potential that occur at synapses, the specialized areas where adjacent neurons are intimate contact.
Cell membranes separate ions into different concentrations on the exterior and interior of the cell. These concentration differences produce an electrical potential across the membrane, the membrane potential. The concentration gradients are maintained by the cell membrane, a lipid bilayer that is relatively impermeable to sodium, potassium, chloride, and calcium ions, the ions involved in electrophysiologic activity and signal transmission.
The activity of the central and peripheral nervous systems never depends on the activity of a single neuron or axon but is always mediated by a group of cells or nerve fibers. Information is represented in the nervous system by a change in activity in a group of cells or fibers as they respond to some change in their input. Normal function in a single neuron as it participates in the processing of information is manifested as electrical potentials. Cell membranes separate ions into different concentrations on the exterior and interior of the cell. These concentration differences produce an electrical potential across the membrane called the membrane potential.
The membrane potential is the difference in electrical potential between the inside and the outside of a cell. All neurons, axons and muscle fibers have a membrane potential. Membrane potentials include resting potentials, action potentials and local potentials, such as synaptic potentials, generator or receptor potentials and electrotonic potentials. All membrane potentials result from ion flow through channels in the membrane.
The human body is approximately 60 percent water (by weight) in which a number of salts are dissolved. Salts in solution are dissociated into atoms with electrical charges (ions or electrolytes). The molecular arrangement and properties of cell membranes throughout the body create a barrier to the flow of certain ions through those membranes, resulting in a difference in proportion of ionic charges between the two sides of the membrane. A potential difference (or voltage; "potential" for short) is thus created across the membrane.
The concentration gradients are maintained by the cell membrane, a lipid bilayer that is relatively impermeable to sodium, potassium, chloride and calcium ions which are involved in the electrophysiological activity and signal transmission. The concentration of sodium, calcium, and chloride ions is higher extracellularly than that of potassium and impermeable anions is higher intracellularly. The equilibrium potential of each ion is the voltage difference across the membrane that exactly offsets the tendency of the ion to move down its concentration gradient. Ions can move across the cell membrane passively through ion channels or by adenosine triphosphate binding to carrier molecules. Some ion channels are open at rest, but most open or close in response to stimuli including changes in membrane potential (voltage gated channels) or chemical changes in the cytoplasm. The opening of a channel for a particular ion brings the membrane potential toward the equilibrium potential of that ion.
At a given time, the membrane potential is determined by the concentration gradient of the ions (which determines their respective equilibrium potentials) and by any changes in the permeability to individual ions across the membrane. The survival and excitability of the cell depend on the membrane potential. Maintaining this potential requires energy metabolism for the ATP dependent sodium/potassium pump. The resting potential is the baseline level of the membrane potential when the cell is at rest and not processing information. This potential depends primarily on the potassium channel. When a cell is active in the processing of information the membrane potential varies and are either local potentials or action potentials.
The membrane resting potential is created by the selective permeability of the cell membrane to certain ions in the extracellular fluid and intracellular fluid compartments. In the resting state, the cell membrane is virtually impermeable to sodium ions (Na+) and protein ions largely concentrated outside and inside the cell, respectively. It is much more permeable to chloride ions (Cl-) and potassium ions, which are largely concentrated in the extracellular and intracellular fluids respectively.
All cell membranes demonstrate a reasonably stable resting potential between the outside of the cell and the inside. Certain cells, specifically nerve and muscle cells, have excitable membranes. In response to a stimulus, the membrane potential of these cells undergoes a series of rapid changes, called an action potential, resulting in the formation of a nerve impulse. Nerve impulses are the currency of the nervous system.
A localized change in membrane potential results in current flow to surrounding areas of membrane. This current flow produces a small change in the membrane potential of adjacent areas. This change is called an electrotonic potential. Synaptic potentials are variation so the membrane potential that occur at synapses, the specialized areas where adjacent neurons are intimate contact.
Cell membranes separate ions into different concentrations on the exterior and interior of the cell. These concentration differences produce an electrical potential across the membrane, the membrane potential. The concentration gradients are maintained by the cell membrane, a lipid bilayer that is relatively impermeable to sodium, potassium, chloride, and calcium ions, the ions involved in electrophysiologic activity and signal transmission.