GENERAL PHYSIOLOGY OF EXCITABLE TISSUES Lecture in Normal

GENERAL PHYSIOLOGY OF EXCITABLE TISSUES Lecture in Normal Physiology

Normal Physiology l studies all the functions and processes, taking place in an organism (its systems, organs and tissues); the science also studies mechanisms of the functions’ regulation, those which make possible proper activities in different environment and at different inner state of the organism

Russian Physiologists I. M. Setchenov (1829 – 1905) I. P. Pavlov (1849 – 1936) 1904 Nobel Prize

EXCITABILITY l Irritability - All cells possess the ability to respond to environmental changes called stimuli. Response means changing shape, activity, etc. l Excitabily – Specific ability of certain cells to give special response to stimulation: changes in activity, metabolic processes and electrical charge.

Excitation is a complex biological reaction manifested as a combination of physical, physico-chemical, and chemical processes and functional changes; its imperative sign is a change in the electrical state of the cell-membrane. When stimulated, a cell changes from a state of rest to one of the physiological activity characteristic of it: a muscle fiber contracts, while a gland cell secretes. Only in nerve cells and nerve fibers excitation is encountered in its pure electric form.

CELL MEMBRANE The cell membrane is a protective sheath, enveloping the cell body. The thickness of the cell membrane varies from 75 A to 111 A. l This membrane separates the fluid outside the cell called extracellular fluid and the fluid inside the cell called intracellular fluid. l The cell membrane is a semipermeable membrane. So, there is free exchange of certain substances between the extracellular and intracellular fluids. l

STRUCTURE OF CELL MEMBRANE l The electron microscopic study reveals three layers of cell membrane: one central electron-lucent layer and two electron-dense layers. The two electron-dense layers are placed one on either side of the central layer. The central layer is called lipid layer as it is formed by lipid substances. The outer two layers are formed by proteins and are called protein layers. Cell membrane contains some carbohydrate molecules.

Lipid Layer of the Cell Membrane The central lipid layer is a bilayered structure. This is formed by a thin film of lipids. The major lipid is phospholipids. The phospholipids are the lipid substances containing phosphorus and fatty acids. l The phospholipid molecules are arranged in two layers. The outer part of the phospholipid molecule called head portion is soluble in water (hydrophilic). The inner part is the tail portion that is not soluble in water (hydrophobic). The two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of the membrane. The hydrophilic head portions of one layer face the extracellular fluid and those of the other layer face the cytoplasm. l

THE PROTEIN LAYER The protein layers of the cell membrane are the electrondense layers. These layers cover the two surfaces of the central lipid layer. The protein layers give protection to. the watery central lipid layer. The protein substances present in these layers are mostly the glycoproteins. Two types of protein molecules are present in the cell membrane: the integral proteins and peripheral proteins.

MEMBRANE PROTEINS The integral proteins are also known as transmembrane proteins. These proteins pass through the entire thickness of the cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane. l The peripheral proteins are otherwise called peripheral membrane proteins. These proteins are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane. The peripheral proteins are loosely bound with the cell membrane. So, these protein molecules can dissociate readily from the cell membrane. l

Functions of Membrane Proteins Integral proteins: Integral proteins provide the structural integrity of the cell membrane. Channel proteins: Some integral protein molecules form the channels for the diffusion of water soluble substances like glucose and electrolytes. So, these proteins are called the channel proteins. Carrier proteins: Some protein molecules help in the transport of substances across the cell membrane by means of active or passive (facilitated diffusion) transport. These are called carrier (pump) proteins. Receptor proteins: Some protein molecules serve as the receptor sites for hormones and neurotransmitters. Such proteins are known as receptor proteins. Enzymes: Some of the protein molecules form the enzymes which control chemical (metabolic) reactions within the cell membrane. Antigens: Some proteins act as antigens and induce the process of antibody formation.

Protein Channels l The characteristic feature of the protein channels is their selective permeability. That is, each channel can permit only one type of ion to pass through it. l Channel permeability depends on: ion charge, shape, size, water surrounding

Regulation of the Channels l Some of the protein channels are continuously opened, and most of the channels may be open or closed. l Continuously opened channels are called ungated channels (non-regulated). l The closed channels open only when it is required, and those are called gated channels.

Gated Channels The gated channels are divided into three categories: l voltage gated channels, l ligand gated channels, l mechanically gated channels. Voltage gated channels: These channels open whenever there is a change in the electrical potential. That is why these are called voltage gated channels. For example, in the neuromuscular junction, when action potential reaches axon terminal, the calcium channels are opened and calcium ions diffuse into the interior of the axon terminal from extracellular fluid in large number.

The Resting Membrane Potential All cells under resting conditions have a potential difference across their plasma membrane. This potential is the resting membrane potential. l By convention, extracellular fluid is assigned a voltage of zero, and the polarity of the membrane potential is stated in terms of the sign of the excess charge on the inside of the cell. l The magnitude of the resting membrane potential varies from about 50 to 90 m. V, depending upon the type of cell; in neurons, it is generally in the range of 60 to 75 m. V The membrane potential of some cells can change rapidly in response to stimulation, an ability of key importance in their functioning. l

The magnitude of the resting membrane potential is determined mainly by two factors : l (1) differences in specific ion concentrations in the intracellular and extracellular fluids, l (2) differences in membrane permeability to the different ions, which reflect the number of open channels for the different ions in the plasma membrane.

l Potential generated across the plasma membrane largely because of the movement of potassium out of the cell down its concentration gradient through open potassium channels, so that the inside of the cell becomes negative with respect to the outside.

Action potential l When a nerve or muscle cell is stimulated, Na-gated chanals in the plasma membrane open and Na instantly diffuses down its concentration gradient into the cell. These cations override the negative charges in the ICF, so the inside of the plasma membrane briefly becomes positive (up to +20 m. V).

This point Na gates close and K gates open. K rushes out of the cell, it diffuses down its electrical and concentration gradient. l The loss of positive potassium ions from the cell turns the inside of the membrane negative again. This quick up-and-down voltage shift, from the negative RMP to a positive value and then back to a negative value again, is called an action potential. l The RMP is a stable voltage seen in a “waiting” cell, whereas the action potential is a quickly fluctuating voltage observed in an active, stimulated cell. l

The first manifestation of the approaching action potential is a beginning depolarization of the membrane. After an initial 15 m. V of depolarization, the rate of depolarization increases. The point at which this change in rate occurs is called the threshold level. Thereafter, the tracing on the oscilloscope rapidly reaches and overshoots the isopotential (zero potential) line to approximately +35 m. V. It then reverses and falls rapidly toward the resting level. When repolarization is about 70% completed, the rate of repolarization decreases and the tracing approaches the resting level more slowly. The sharp rise and rapid fall are the spike potential of the axon, and the slower fall at the end of the process is the after-depolarization. After reaching the previous resting level, the tracing overshoots slightly in the hyperpolarizing direction to form the small but prolonged after-hyperpolarization.

Sodium-Potassium Pump The pump is a primary active transporter, because it uses the cellular energy of the terminal phosphate bond of ATP. The Na+-K+-pump transports 3 Na+ out of the cell and 2 K+ into the cell for each ATP hydrolysed. This is a net movement of positive ions out of the cell, and therefore called an electrogenic transport.

Absolute Refractory Period. During the action potential, a second stimulus, no matter how strong, will not produce a second action potential, and the membrane is said to be in its absolute refractory period. l This occurs because the voltage-gated sodium channels enter a closed, inactive state at the peak of the action potential. l The membrane must repolarize before the sodium channel proteins return to the state in which they can be opened again by depolarization. l

Relative Refractory Period Following the absolute refractory period, there is an interval during which a second action potential can be produced, but only if the stimulus strength is considerably greater than usual. This is the relative refractory period, which can last 10 to 15 ms or longer in neurons and coincides roughly with the period of after hyperpolarization. l During the relative refractory period, there is lingering inactivation of the voltage-gated sodium channels, and an increased number of potassium channels are open. If a depolarization exceeds the increased threshold or outlasts the relative refractory period, additional action potential will be fired. l

Excitability of different tissues l Excitability is the capacity of tissue to respond to stimulation consequently to generate an action potential

QUALITIES OF STIMULUS To excite a tissue, the stimulus must possess three characters : l 1. Intensity or strength l 2. Duration l 3. Steepness of increase in stimulus strength ( accommodation)

The intensity of stimulus is of five types: Subminimal stimulus Minimal stimulus Submaximal stimulus Maximal stimulus Supramaximal stimulus l The stimulus whose strength (or voltage) is sufficient to excite the tissue is called threshold or minimal stimulus.

The measure of excitability is the minimum strength of stimulus that produces excitation, and is known as the threshold of stimulation. l The higher the minimum strength of stimulus needed to cause a reaction, (the higher the threshold of stimulation) the lower the excitability; conversely, the lower the threshold of stimulation, the higher the excitability.

THE ALL-OR-NONE LAW l Through study of the relationship of stimulation effects to stimulus strength an all-or-none law was established, according to which subthreshold stimulation produces no excitation, while threshold stimuli immediately produce maximum excitation unaffected by a further increase in stimulus strength. l For a long time the all-or-none law was erroneously interpreted as the general principle of the reaction of excitable tissue. It was supposed that none meant the complete absence of excitation in response to a subthreshold stimulus, while all was considered a manifestation of the complete exhaustion of the potentials. Later research has shown that a stimulus of near-threshold strength causes a local excitation not capable of spreading, a local response, directly in the area stimulated.

Local potentials are 1. Graded, meaning that they vary in magnitude (voltage) according to the strength of the stimulus. A more intense or prolonged stimulus opens more ion gates than a weaker stimulus. Thus, more Na+ enters the cell and the voltage changes more than it does with a weaker stimulus.

Local potentials are 2. Decremental, meaning they get weaker as they spread from the point of stimulation. The decline in strength occurs because as Na+ spreads out under the plasma membrane and depolarizes it, K+ flows out and reverses the effect of the Na+ inflow. Therefore, the voltage shift caused by Na+ diminishes rapidly with distance. This prevents local potentials from having any long-distance effects.

Local potentials are 3. Reversible, meaning that if stimulation ceases, K+ diffusion out of the cell quickly returns the membrane voltage to its resting potential

STRENGTH-DURATION CURVE l l The excitability curve demonstrates the exact relationship between the strength and the duration of a stimulus. The shape of the curve is similar in almost all the excitable tissues. Following are some of the important points to be studied in the excitability curve: Rheobase =Threshold: This is the least possible, minimum strength (voltage) of stimulus which can excite the tissue. The voltage below this cannot excite the tissue, whatever may be the duration of stimulus. Utilization time: It is the minimum time required for a threshold strength stimulus to excite the tissue. Chronaxie: It is the minimum time, at which a stimulus with double threshold strength can excite the tissue.

The measure of excitability is the minimum time of doubled stimulus that produces excitation, and is known as the chronaxie. l The higher the chronaxie of stimulus needed to cause a reaction the lower the excitability; conversely, the lower the chronaxie, the higher the excitability.

ACCOMMODATION l Means the steepness of increase in stimulus strength. l The level of the threshold of nerve or muscle excitability depends not only on stimulus duration but also on the steepness of its increase.

The stimulus threshold is at its lowest value with rectangular current impulses characterized by maximum rapidity of increase in strength. If, however, stimuli increasing linearly or exponentially are used instead, thresholds prove to increase in inverse proportion to the rate of increase in current strength When the steepness of the rise is reduced below a certain minimum, no action potential appears, no matter how great the final strength of the current, because of the fact that during the period of increase in stimulus strength active changes occur in the tissue raising the threshold and interfering with stimulation. This phenomenon of adaptation of excitable tissue to a slowly increasing stimulus is known as accommodation.

Stimulus increase l Accomodation curves of a nerve fibre, measured by means of linearly increasing currents. Abscissa duration of increase in current strength; ordinate values of thresholds in milliampers. The inclination of the thin lines to the abscissa corresponds to the rate of increase of the stimuli.

Stimulation by direct electric current Electrical stimulus is commonly used for experimental and clinical purposes because of the following reasons: l It can be handled easily l The intensity of stimulus can be easily adjusted l The duration of stimulus can be adjusted l The stimulus can be applied to limited area on the issues l Damage caused to tissues is nil or least. The mechanism by which electricity stimulates is identical in principle for all types of stimuli, but is most distinctly seen when direct current of rectangular wave form is used.

LAW OF POLARITY OF STIMULATION l Direct current has a polar action on excitable tissue; when the direct current circuit is closed excitation always arises in a nerve or a muscle only at the anode (+), and when the circuit is opened only at the cathode (-).

Lability l Under the natural conditions of existence of an organism, however, rhythmic rather than isolated discharges of action potentials pass along nerve fibres. The discharge frequency of impulses in excitable tissues may vary within wide limits according to the strength of the stimulus applied, the properties and condition of the tissue, and the rate at which individual acts of excitation occur in the rhythmic series. l To characterize this rate Vvedensky formulated the concept of lability (functional mobility), which he understood as "the higher or lower rate of the elementary reactions that accompany the physiological activity of a given apparatus". l In his view the measure of lability is the largest number of action potentials an excitable substrate is capable of generating per second in response to frequently recurring stimuli.

It was initially supposed that the minimum interval between the impulses in a rhythmic series would correspond to the length of the absolute refractory period. To reproduce the rhythm of stimulation in a series of stimuli the interval between them must be somewhat longer than the absolute refractory period. In the motor nerve fibers of warm-blooded animals the absolute refractory period is about 0. 5 millisecond, whereas the maximum rhythm of stimulation is only about 1, 000 per second; in the nerve fibers of crustaceans the absolute refractory period lasts about one millisecond, while the maximum rhythm of stimulations is approximately 500 impulses per second. In the sensory fibres of the acoustic nerve and in Renshaw inhibitory cells more than 1, 000 impulses per second have been recorded.