Nerve & Action Potential - Physical Therapy PDF

Faculty of Physical Therapy October 29, 2016 1  The neuron  It is the structure unit of nervous system. Neuron consists of (1)Cell body: It contains cytoplasm, nucleus and cell organelles. (2) Cell Processes (axon and dendrites):...

Faculty of Physical Therapy October 29, 2016 1  The neuron  It is the structure unit of nervous system. Neuron consists of (1)Cell body: It contains cytoplasm, nucleus and cell organelles. (2) Cell Processes (axon and dendrites): Axon: Its function is to conduct impulses away from the cell body. A neuron has only one axon The axon ends up in several terminal branches. Dendrites: Increase the surface area of the cell. Transmit impulses from around of the cell to the cell body. 2 Neuron 3 Types of nerve fibers 1.According to the presence or absence of myelin sheath: 4 According to the presence or absence of myelin sheath 5 Types of nerve fibers 2. According to function: 6 According to function 7 Characters of nerve fibers 1- Excitability: It is the ability of the living organism to respond to a stimulus 2- Conductivity: It is the ability of the nerve to conduct an impulse away from the cell body through the axon to the nerve terminal in one direction. 3- Adaptation: Decrease impulse discharge with constant and continuous stimulus. 4- Not fatigued. 8  Stimulus  It is the changes which occur in the environment around the living organism. Types of the stimulus: ❖Electrical stimulus: by using galvanic current and it is preferred because ▪We can control its onset ▪We can control its intensity ▪We can control its duration ▪It resembles the natural stimulus. ▪It leaves the stimulated tissue without damage. ❖Chemical stimulus: O2, Co2, H+, acids and alkali ❖Mechanical stimulus: pain and movements. ❖Physical stimulus: cold and hot. ❖Electromagnetic stimulus: sun rays and light Membrane potential It is the electrical potential exists between the outer and inner surface of the cell membrane of all the cell of the body. Types of membrane potential Resting membrane potential. Action potential. 10 Resting membrane potential  It is the electrical potential difference due to unequal distribution of ions between the inner and outer surfaces of the cell membrane during rest. Values:  Nerve fibers: -70 m V.  Skeletal and cardiac muscle fibers: -90 m V. Causes: unequal distribution of ions across cell membrane as a result of: 1- Selective membrane permeability: during rest the membrane is more permeable to potassium ions (K+) more than sodium ions (Na+) through leakage channels. 2-Na+/K+-ATPase pump: it pumps 3 Na+ out of the cell and 2 K+ ions into the cell causing net deficit of positive ions inside the membrane. 3- Intracellular non-movable negatively charged proteins: the proteins in the inner surface of the cell membrane is negatively charges, and can’t move due to its large size creating negativity inside cell membrane. 11 Resting membrane potential 12 Resting membrane potential Since more positive Na ions (cations) are pumped outside than inside leaving the negative proteins and anions inside the cells. It leads to negativity inside and positivity outside the cell. 13 Resting membrane potential 14  MEMBRANE CHANNELS The cell membranes of nerves, like those of other cells, contain many different types of ion channels: (A) LEAK CHANNELS: during resting state, membrane permeability to K is more than Na, due to leak channels which is permeable to K 100 times than Na. (B) GATED CHANNELS 1- voltage-gated: stimulated by change in the voltage 2- ligand-gated: stimulated by binding with a substance.  Action potential Definition: It is the rapid change in the membrane potential after application of an effective stimulus Stages of action potential: 1: Latent period: It is the time between the application of the stimulus and the appearance of the response in which some positive ions flow inside membrane increase positivity of membrane potential. - When membrane potential rises toward zero or reach -65 mv reaching threshold level. 2: Depolarization: There is a rapid loss of polarity of the membrane. 3: Repolarization: This is the restoration of normal polarity of the cell membrane. 4: Hyperpolarization: This is occur when the membrane potential overshoots more to the negative side Its duration is about 40 ms. 16 Phases of the action potential 17 October 29, 2016 18  Depolarization  When stimulus applied to the nerve fiber decreases the negativity of the resting membrane potential.  It leads to open activation gates of voltage gated Na channels leads to flow of Na ions inside the membrane  It leads to increase positivity of membrane potential from - 90 mv to – 65 mv (threshold level)  Once membrane potential reaches threshold level, explosive flow of Na ions inside the membrane causes the overshoot phase (+35 mv)  More Na influx, more increase positivity of membrane potential, more increase in membrane permeability to Na ions.( positive feedback mechanism)  At the same time the activation gates open, the inactivation gates starts to close slowly to end depolarization phase. October 29, 2016 19  Repolarization  Once overshoots happens:  Inactivation gates of Na channels close  Voltage gated K channels open leads to efflux of K (flow of K outside the membrane).  It leads to increase the negativity of the membrane potential toward the resting potential (repolarization phase)  conductance of K ions outside remains longer causing the increase in membrane potential negativity than normal (hyperpolarization or undershoots)  The importance of hyperpolarization: inactivation gates of Na channels open voltage gated K channels close October 29, 2016 20 October 29, 2016 21   After single action potential, a minute change in concentration gradient happens.  But, after 100,000 to 50 million action potentials (impulses), there is great concentration ion difference enough to cease the generation of other action potentials.  So, the nerve fiber needs to restore its concentration gradient.  Na-K atpase pump is responsible to restore the resting membrane potential.  It extrudes 3 Na ions and enters 2 K ions. October 29, 2016 22 Phases of the action potential 23 Mechanism of Action Potential 24 Excitability changes during action potential (1) Absolute Refractory Period : In this period the nerve is never undergoing to develop action potential. No stimulus can excite it, whatever its strength. It corresponds to ascending limb of A.P. and the upper l/3 of descending limb It happens in the repolarization phase where all inactivation gates of Na channels closed. (2) Relative refractory period: In this period the excitability is partially recovered. It corresponds to the remaining part of descending limb of A.P. (mainly in hprepolarization) - But it needs a strong stimulus than usual (supra stimulus) to develop an action potential. 25  Relative Permeability of Ions 26 Refractory periods 27 Mechanism of impulse propagation - At any point action potential develops can propagate in any direction along the nerve fiber. - The Na ions flow 1 to 3 millimeter to adjacent parts to develop action potential. - Propagation of action potential along nerve fiber is a nerve impulse. - The ratio of action potential to threshold of excitation should be greater than 1 (safety factor for propagation) 28   There are two ways of conduction along the nerve fibers:- (1) In unmyelinated N.F: sweeping conduction (2) In myelinated N.F: saltatory conduction October 29, 2016 29 Mechanism of impulse propagation 30 Mechanism of impulse propagation (1) Sweeping conduction = self propagation On application of a stimulus to unmyelinated N.F. reverse polarity occurs. an action potential will developed at that point of stimulation The neighboring area becomes depolarized up to the firing level, an action potential will developed at that point The process is repeated and so on. It is a slow type It consume relatively large amount of energy. 31 Mechanism of impulse propagation (2) Saltatory conduction It is a node to node conduction in which the impulse is conducted from a node of Ranvier to another. The impulse is said to Jump from one node to another. It is fast type of conduction. It consumes less energy during recovery. Voltage gated Na+ channels are concentrated at nodes of Ranvier. 32 Myelinated and unmyelinated nerves 33  The neuromuscular junction It is the junction between a motor neuron and a skeletal muscle fibres. Structure: 1.The motor neuron (presynaptic axon) 2.Synaptic cleft 3.Motor end plate (postsynaptic membrane) 34  The neuromuscular junction Structure: 1.The motor neuron(presynaptic axon) As the motor neuron approaches the skeletal muscle, it loses its myelin sheath and, it divides into axon terminals which contains: i.Mitochondria to supply energy for synthesis of acetylcholine. ii.Acetylcholine vesicles. iii.Voltage gated Ca++ channels. 35  The neuromuscular junction 2.Synaptic cleft It is a space between the neuron terminal and the muscle in which acetylcholine is released. The cleft contains acetyl cholinesterase enzyme. 3.Motor end plate It is the part of the muscle cell membrane under the axon terminal. It contains: i.Cholinergic receptors for binding with acetylcholine. ii.Acetylcholine gated Na+ channel. 36 The neuromuscular junction 37 The neuromuscular junction 38  The neuromuscular junction Mechanism of neuromuscular junction 1. On arrival of an impulse to motor neuron, it leads to increase the permeability to the Ca++. 2. The diffuse of the Ca++ to the interior of the axon, leads to rupture of vesicles and release of acetyl choline (A.Ch) in synaptic cleft. 3. The released A.Ch attached to A.Ch receptor on the postsynaptic membrane. 4. This process leads to increase in Na+ influx, which leads to the depolarization of the postsynaptic membrane (END PLATE POTENTIAL) 5. The action potential produced is conducted along the muscle fibers. 6. The muscle action potential initiates muscle contraction. 7. ACh is degraded to choline and acetate by acetylcholinesterase (AChE); choline is taken back into the presynaptic terminal 39  Clinical Case An 18-year-old college woman comes to the student health service complaining of progressive weakness. She reports that occasionally her eyelids “droop” and that she tires easily, even when completing ordinary daily tasks such as brushing her hair. She has fallen several times while climbing a flight of stairs. These symptoms improve with rest. 40