Questions: Properties of excitable tissues Transport across cell membrane Resting potential Action potential Excitability change at excitation

Questions: Properties of excitable tissues Transport across cell membrane Resting potential Action potential Excitability change at excitation

EXCITABLE TISSUES The tissues can change the properties under the stimulus influence. There are nervous, muscular tissues and a glandular epithelium tissue

PROPERTIES OF EXCITABLE TISSUES EXCITABILITY is the ability of tissue to react to the irritation with change of physiological properties and generation of process of excitation. Excitability of cells and tissues is a basic function of life. IRRITABILITY: 1 - the ability to receive and respond to a stimulus. 2 - the ability of the cell or tissue to generate and sometimes propagate action potentials. CONDUCTIVITY ability to conduct impulse. LABILITY ability to conduct certain quantity of impulses per time. It depends on rate of diffusion of an action potential. CONTRACTILITY ability to change the size (to contract) and tension.

Stimuli are any external and internal influences Classification of stimuli: By the nature : - the external: physical, chemical, biological - the internal: physiologically active substances By force : - subliminal - threshold - supraliminal The threshold strength is the minimal stimulus capable to cause tissue response. It is also called the threshold stimulation

Structure of membrane Membrane consists of: Lipid bilayer Protein Integral Proteins Transmembrane Channels Carries Pumps Receptors Enzymes Peripheral Proteins Cytoskeleton Glycocalyx

Transport Across Cell Membrane Passive transport (Doesn t Require Energy) Diffusion: Molecules move freely across a membrane from regions of higher concentration to regions of lower concentration to balance a concentration gradient. Diffusion of water is called osmosis. Facilitated diffusion is the spontaneous passage of molecules or ions across an impermeable or semipermeable cell membrane down their concentration gradient through specific transmembrane integral proteins.

Transport Across Cell Membrane Active transport (Requires Energy): Active transport is transport of molecules from regions of lower concentration to regions of higher concentration across a membrane using an energy-dependent transport proteins. Primary : Na-K-ATPase, Ca-ATPase, K-H-ATPase. Secondary : Glucose, Aminoacids, Ca.

Transport Across Cell Membrane Endocytosis is the process by which cells absorb molecules (such as proteins) by engulfing them. Pinocytosis: Endocytosis of dissolved liquid molecules. Phagocytosis: Endocytosis of undissolved solid matter. Exocytosis is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane.

Transport Across Cell Membrane

RESTING POTENTIAL There is an electrical potential of the cell membrane difference between the voltage inside and outside of the cells surfaces. More potassium is in a cytosol and more sodium is in extracellular fluid.

RESTING POTENTIAL The membrane has a property as the selective conductance. There are sodium channels is closed and potassium channels is opened, and it can leave cell. The inside of the membrane is usually around 60-70 mv more negative (- 60-70mV) compared to the outside. This polarization is accomplished by the specialized proteins in the cell membrane - ion channels and ion transporters. Using energy from ATP, they transport sodium out of the cell and potassium into the cell (also chlorine into the cell). As ions can leak through the membrane to some extent, the cell has to constantly use energy to maintain the resting membrane potential.

RESTING POTENTIAL It is caused: 1. Difference concentration of ions Na +, К +, Са 2+, Сl - between the voltage the inside and the outside of the cells surfaces of membrane. 2. Selective conductance of membrane. 3. Dissipation of ionic gradients is ultimately prevented by Na-K pumps, which extrudes Na+ from the cell while taking in K+. Because the pump moves Na+ and K+ against their net electrochemical gradients, energy of ATP is necessary.

Action Potential (AP) Action Potential is fast fluctuation of a membrane potential Components of an Action Potential

Physiological basis of AP When the threshold level is reached Voltage-gated Na+ channels open up Since Na conc outside is more than the inside Na influx will occur Positive ion coming inside increases the positivity of the membrane potential and causes depolarisation When it reaches +30, Na+ channels closes Then Voltage-gated K+ channels open up K+ efflux occurs Positive ion leaving the inside causes more negativity inside the membrane Repolarisation occurs

Physiological basis of AP Since Na+ has come in and K+ has gone out Membrane has become negative But ionic distribution has become unequal Na+/K+ pump restores Na+ and K+ conc slowly By pumping 3 Na+ ions outward and 2+ K ions inward

Action Potential

Action Potential The action potential is due to the opening of "gated" sodium (Na + ) channels. The inside of the cell becomes more positive This change of potential opens the "gates" of sodium channels. This occurs in an "explosive" manner leading to a sudden change from a negative potential across the membrane to a positive one (of about +50 mv). This potential difference results in closing the sodium channels and opening current "gated" potassium channels allowing outflow of potassium to areas outside the cell where potassium is at a lower concentration. As a consequence the potential across the membrane is lowered as the cells` inside of the becomes more negative and the outside more positively charged. Pumps also pump out sodium and potassium.

Excitability change at excitation Stages of excitability change: - Initial excitability - resting potential. - Excitability raises when sodium channels start to open. - Excitability is a very low (absolute refractory period), all sodiums` channels are opened. - Excitability is low (relative refractory period) some sodiums`channels are already closed. - Excitability is raised (Supernormality), almost all sodiums` channels are closed. - Excitability is low (Hyperpolarization), cell has got more potassium.

Literature Anatomy and physiology. - The McGraw Hill, Companies, 2003 www.mhhe.com/seeley6