Transcript Slide 1
The Action Potential & Impulse/Signal Propagation Learning Objective To know the sequence of events that occurs during an Action Potential. To link the events in an Action Potential to electric current traces on an oscilloscope. To understand how an impulse is passed along the neurone by a series of action potentials. The Action Potential • An action potential is a sudden change in the potential difference across a membrane. • It occurs at a specific point on the neurone. • The graph shows the electrical changes that occur during a single action potential. • The action potential begins when a stimulus causes a part of the neurone to become more permeable to Na+ • It is more permeable to Na+ because the stimulus has caused voltage sensitive Na+ gates at this part of the membrane to open. • As a result, Na+ ions diffuse rapidly into the neurone reducing the negativity at this part of the neurone (we can see this at point A). • In a positive feedback loop, this causes adjacent voltage-sensitive Na+ gates to open until the voltage difference across the membrane reverses. At point B the depolarisation of the membrane causes the voltage sensitive Na+ gates to close. After the Na+ gates close the voltage sensitive K+ gates open. The K+ gates open slower than the Na+ gates, allowing the K+ ions to flow out of the neurone (Point C). The K+ ions leaving causes the membrane to become repolarised, reaching the resting potential. The outward flow of K+ causes the K+ gates to close (Point D) Because K+ gates are slow to close a greater number of K+ ions than desired leaves the neurone, causing it to become hyperpolarised (Point E, a lower resting potential) In order to bring the resting potential back to normal the sodium potassium pumps begin moving Na+ ion out of the neurone and K+ ions back into it through active transport. • A new action potential will only be generated at the leading edge of the previous one; • Because the membrane behind it will be recovering/incapable of transmitting an impulse; • The membrane has to be repolarised and return to resting potential before another action potential can be generated; The Refractory Period • There is a time after depolarisation where no new AP can start – called the refractory period. – Time is needed to restore the proteins of voltage sensitive ion channels to their original resting conditions. – Na+ channels cannot be opened, as it can’t be depolarised again. WHY? – AP travel in one direction only. – Produces discrete impulses. – Limits the frequency of impulses. Learning Objectives: • How does an action potential pass along an unmyelinated axon? • How does an action potential pass along a myelinated axon? • What factors affect the speed of conductance of an action potential? • What is the refractory period? • What is meant by the “all or nothing” principle? Myelinated Neurones • The axons of many neurones are encased in a fatty myelin sheath (Schwann cells). • Where the sheath of one Schwann cell meets the next, the axon is unprotected. • The voltage-gated sodium channels of myelinated neurons are confined to these spots (called nodes of Ranvier). Na+ Sodium channel Na+ Nodes of Ranvier Na+ Myelinated Neurones • The inrush of sodium ions at one node creates just enough depolarisation to reach the threshold of the next. • In this way, the action potential jumps from one node to the next (1-3mm) – called saltatory propagation • Results in much faster propagation of the nerve impulse than is possible in unmyelinated neurons. Na+ Sodium channel Na+ Nodes of Ranvier Na+ AP – All or nothing • AP only happens if the stimulus reaches a threshold value. – Stimulus is strong enough to cause an AP – It is an ‘all or nothing event’ because once it starts, it travels to the synapse. • AP is always the same size • An AP is the same size all the way along the axon. • The transmission of the AP along the axon is the nerve impulse. How do we detect the size of a stimulus? • The number of impulses in a given time – the larger the stimulus, the more impulses generated. • By having neurones with different threshold values – the brain interprets the number and type of neurones and therby determines its size.