Action Potentials and Conduction Lecture 7 PDF

LECTURE 7 ACTION POTENTIALS AND CONDUCTION  Terminology Associated with Changes in Membrane Potential ❑ Depolarization- a decrease in the potential difference between the inside and outside of the cell. ❑ Hyperpolarization- an increase in the...

LECTURE 7 ACTION POTENTIALS AND CONDUCTION  Terminology Associated with Changes in Membrane Potential ❑ Depolarization- a decrease in the potential difference between the inside and outside of the cell. ❑ Hyperpolarization- an increase in the potential difference between the inside and outside of the cell. ❑ Repolarization- returning to the resting membrane potential (RMP) from either direction. Gated ion channels Gated ion channels in the membrane open to a variety of stimuli A) Mechanical force, eg. sensory neurons. B) Chemical ligands, eg. neurotransmitters. C) Voltage, eg. changes in the resting Three Different membrane potential. Types of ion channels Graded and Action potentials When? ❑ Graded potentials are brought about by external stimuli (in sensory neurons) or by neurotransmitters released in synapses, where they cause graded potentials in the post-synaptic cell. Where? They occur at the postsynaptic dendrite in response to presynaptic neuron firing and release of neurotransmitter. The magnitude of a graded potential is determined by the strength of the stimulus. Action potentials are triggered by membrane depolarization to threshold  Gated Channels are involved in neuronal signaling In the nervous system: ❑ Different channel types are responsible for transmitting electrical signals over long and short distances: A. Graded potentials travel over short distances and are activated by the opening of 1) Mechanically gated channels (e.g. Sensory neurons) 2) Chemically gated channels (e.g. Neurotransmitters) B. Action potentials travel over long distances and they are generated by the opening of 1) Voltage-gated channels (i.e. changes in the resting membrane potential)  Graded Potentials ❑A graded potential depolarization is called: Excitatory postsynaptic potential (EPSP). ❑A graded potential hyperpolarization is called: Inhibitory postsynaptic potentials (IPSP). ❑They occur in the cell body and dendrites of the neuron. ❑The wave of depolarization or hyperpolarization which moves through the cell with a graded potential is known as local current flow.  Graded Potentials A neuron may receive greater than 10, 000 inputs from presynaptic neurons. Graded potentials are primarily generated by sensory input, causing a change in the conductance of the membrane of the sensory receptor cell. o It occurs due to depolarizations or hyperpolarizations whose strength is proportional to the strength of the triggering event. o It lose their strength as they move through the cell due to the leakage of charge across the membrane ▪ In the example below, an action potential is triggered when 3 depolarizing excitatory graded potentials summate to create a suprathreshold potential, ▪ whereas no action potential results when a hyperpolarizing inhibitory graded potential summates with the excitatory inputs to produce a subthreshold potential.  Action Potential (AP) Na+/K+-ATPase Has No Direct Role to Play in the Action Potential. They are initiated in an all-or-none manner when the summed graded potential exceed threshold voltage. They remain the same size as they travel along the axon over long distances. They are identical to one another. Occurs upon alteration of the permeability of Na+ and K+ ions through voltage-gated channels.  Graded Potential Action Potential Depending on the stimulus, graded potentials can be Action potentials always lead to depolarization of depolarizing or hyperpolarizing membrane and reversal of the membrane potentials. Amplitude is proportional to the strength of the stimulus Amplitude is all-or-none; strength of the stimulus is coded in the frequency of action potentials generated Amplitude is generally small (few mV to tens of mV) Large amplitude of about 100 mV Duration of graded potentials may be few millisecond to Action potentials duration is relatively short 3-5 ms second Ions involved are usually Na+, K+ and Cl- Na+/K+-ATPase Hasare No The ions involved Direct Na+ and Roleaction K+ (for neuronal to potential) Play in the Action Potential. No refractor period is associated with graded potentials Absolute and relative refractory periods are important aspects of action potentials. Refractory Periods Limit the Frequency of APs Absolutely refractory period- a second AP will not occur until the first is over. The gates on the Na+ channel have not reset. Relatively refractory period- a large suprathreshold graded potential can start a second AP by activating Na+ channels which have been reset.  Refractory Periods Limit the Frequency of APs Absolute refractory periods prevent back propagation of APs into the cell body. Refractory periods limit the rate at which signals can be transmitted down a neuron. Limit is around 100 impulses/s. The absolute refractory period also ensures one-way travel of an action potential from cell body to axon terminal by preventing the action potential from traveling backward.  Action Potential Conduction Movement of the AP along the axon at high speed is called conduction. A wave of action potentials travel down the axon. Each section of the axon is experiencing a different phase of the AP (see figure). Graded potential triggers AP. Opens voltage-gated Na+ channels. The Na+ spreads in all directions attracted by the -ve ions in adjacent regions (3,4). Opens Na+ channels and initiates AP in the adjacent region along the axon (4), but not in the cell body where there are no voltage-gated Na+ channels (3). K+ channels have opened in the initial segment (5) and the Na+ (6) ions cannot trigger an AP in that region since its absolutely refractory. Na+ ions initiate action potentials in segment (7).  Saltatory Conduction When depolarization reaches a node, Na+ enters the axon through open channels. At the nodes, Na+ entry reinforces the depolarization to keep the amplitude of the AP constant, but slows the current flow due to a loss of charge to the extracellular fluid. However, it speeds up again when the depolarization encounters the next node. The apparent leapfrogging of APs from node to node along the axon is called: Saltatory Conduction.  Factors Influencing Conduction Speed of APs The resistance of the membrane to current leak out of the cell and the diameter of the axon determine the speed of AP conduction. Large diameter axons provide a low resistance to current flow within the axon and this in turn, speeds up conduction. ❑ Myelin sheath which wraps around vertebrate axons prevents current leak out of the cells. Acts like an insulator, for example, plastic coating surrounding electric wires. ❑ However, portions of the axons lack the myelin sheath and these are called Nodes of Ranvier. High concentration of Na+ channels are found at these nodes.  Multiple Sclerosis In demylinating diseases, such as multiple sclerosis, the loss of myelin in the nervous system slows down the conduction of APs. Multiple sclerosis patients complain of muscle weakness, fatigue, difficulty with walking and a loss of vision. End