Action potential and synapes.
Describe how membrane potential can be altered in response to a stimulus.
Changes in the membrane potential occur because neurons contained gated ion channels. Gated Ion Channel: ion channels that open or close in response to stimuli. When gated ion channels are stimulated to open, ions flow across the membrane changing the membrane potential. The increase in magnitude of the membrane potential is hyperpolarization. This makes the inside of the membrane more negative. Hyperpolarization results from any stimulus that increases the outflow of positive ions of the inflow of negative ions. A reduction in the magnitude of membrane potential is depolarization. Depolarization often occurs with sodium channels. If a stimulus causes the gated sodium channels to open, the membrane's permeability to NA+ increases, NA+ diffuses into a cell.
Distinguish between electrical and chemical synapses, and describe how signals cross synapses.
In most cases, action potentials are not transmitted from neurons to other cells. However, information is transmitted, and this transmission occurs at synaptic terminals. Electrical synapses, contain jap junctions, which do allow electrical current to flow directly from one neuron to another. In both vertebrates and invertebrates, electrical synapses synchronize the activity of neurons responsible for certain rapid, unvarying behaviors. Ex: Electrical synapse associated with giant axons of squids are lobsters facilitate swift escapes from danger. Majority of synapse are chemical synapse These involve the release of chemical neurotransmitter by the presynaptic neuron. At each terminal, the presynaptic neuron synthesize the neurotransmitter and packages it into multiple membrane enclosed comparted called synaptic vesicles. The arrival of an action potential at a synaptic terminal depolarizes the plasma membrane, opening voltage-gated channels that allow CA+ to diffuse into the terminal. The resulting rise in CA concentration in the terminal membrane, releasing the neurotransmitter. Once released, the neurotransmitter diffuses across the synaptic cleft, the gap that separate the presynaptic neuron from the postsynaptic cell. When reached the postsynaptic membrane, the neurotransmitter binds to and activates a specific receptor in the membrane. Chemical synapes are critical for learning.
What is the refractory period?
The sodium channels remain inactivated during the falling phase and the early part of the undershoot. As a result, if a secound depolarizing stimulus occurs during the period, it will unable a second action potential. This downtime where the 2nd action potential can't be trigger is the refactory period. The refacotory period ensures that all signals in a axon travel in one directions from cell body to the axon terminals.
saltatory conduction?
the process in which impulses jump from node to ndoe.
Distinguish between excitatory and inhibitory postsynaptic potentials, and between spatial and temporal summation.
At some synapes, the ligand gated ion channel is permeable to both K+ and Na+ When the channel opens, the membrane potential depolarizes toward a value roughly midway between EK & ENA. Since this depolarization brings the membrane potential toward the threshold, its called a excitatory postsynptic potential. Other synapes, the ligand gated ion channel is selectivtly permeable for only K+ & Cl. When this channel opens, the postsynpatic membrane hyperpolarizes. This is the inhinitory response. Because it causes the membrane potential to be more negative moving it away form the threshold. This is the essence of the intergatrion of the nervous system.
What is an action potential?
When depolarization shifts the membrane potential suffiently, the result is a massive change in membrane voltage. The membrane voltage is called an action potential. Action potentials have a constant magnitude and doesn't vary. They can regenerate in adjacent regions of the membrane. Therefore, they can spread along axons making them well suited for transmitting a signal over a long distance.
What is a graded potential?
When the response to hyperpolarization or depolarization is just a shift in membrane potential. Grade potential has a magnitude that varies with the strength of the stimulus. A larger stimulus causes a greater change in the membrane potential. A larger stimulus produce a larger hyperpolarization. Which causes K+ to move out the neuron. Making the cell more negative. A larger stimulus produces a large depolarization. Which causes NA+ to move in making the cell less negative since positive ions are moving in. Graded potential induces a small electrical current that leaks out the neuron as it flows along the membrane. As they leak the move further from neuron and time passes, the grade potential decays.
what is the function of the ligand gated ion channel during a synapse.
At chemical synapes, the receptor protien that binds and responds to neurotransimitter is a ligand-gated iion channel. These receptors are clustered in the membrane of the postsnypatic cell, directly opposite of the synpatic terminal. Binding the neurotransmitter to a particular part of the receptor opens the channel and allows specific ions to diffuse the postsynpatic membrane. This result is a postsynaptic potential, a graded potential in the postynaptic cell.
Using acetylcholine as an example, describe how neurotransmitters transmit the action potential signal.
Acetylocholine: a common neurotransmitter in both invertabrates and vertabrates. It is vital for nervous system functions that incude muscle stimulation, memory formation, and learning. In vertabtrates, there are two major classes of acetylcholine receptor. One is a ligand-gated ion channel, which function at the vertabrate neuromuscular junction, the site where a motor neuron forms a synapes with a skeletal muscle cell. When Ach is released by a motor neuron bind to this receptor, the ion channel opens, producing an EPSP. This excitatory activity is soon terminated by acetylcholinesterase, an enzyme in the synaptic cleft that hydrolyzes the neurotransmitter. A metabotropic acetylcholine receptor is found at locations that include the vertabrate CNS and heart. In heart muscle, acetylcholine released by neurons activates a signal tranduction pathway. The G protiens in the pathwat inhibit adenyly cyclase and open potassium channels in the muscle cell membrane. Both effects reduce the rate at which the heart pumps. The effect of acetylcholine in heart muscle is inhibitory rather than excitatory.
Describe how action potentials are propagated, and sometimes accelerated, along axons. Explain why action potentials are unidirectional and always travel without loss of signal.
At the site where action potential is intiatiated, usually the axon hillock, Na+ inflow during the rising phase creates an electrical current that depolarizes the neighboring region of the axon membrane. The depolarization is large enough to reach the threshold, causing an action potential in the neighboring region. This process is repeated many time along the length of the axon. Since the action potential is an all of none event, the magnitude and duration of the action potential are the same at each potential. The net result is the movement of a new impulse from cell body to synaptic terminals. Much like domino cascading. The action potential move along the axon towards the synaptic terminals because immediately behind the traveling zone of depolarization cause by NA+ inflow is a zone of repolarization caused by K+ outflow. In the repolarized zone the sodium channel remain inactivated. So the inward current that depolarizes the axon membrane ahead of the axon potential cannot produce another action potential behind it. This prevent actions potentials from returning back toward the cell body of the neuron. For most neurons, the interval between the onset of an action potential and the beginning of the undershoot is only 1-2 milliseconds. Thus, a neuron can produce action potentials as at a rate of hundreds per second. So in hearing, louder sounds result in more frequent action potentials in neurons connecting the ear to the brain.
Spatial Summation Vs. Temporal summation.
Temporal summation: when two excitatory posynaptic potentials occur at single synapse in rapid succession. Here the membrane potential hasn't had the change to come back to the resting potential before the 2 excitatory potential. Thus the two exitatory potentials add together. Excitatory potentials that are produced nearly simultaneously by different synapse on the same postsynaptic neuron are spatial summation. Thru these summations, several ESPSs can combine to depolarize the membrane at the axon hillock to threshold, causing the postsynpatic neuron to produce an action potential. This can also happen with inhibitory post synpatic potentials.. two ISPS can occur simitanuouty. Thru summation, an IPSP can counter the effect of an ESPS. The interplay between multiple IPSP and ESPS is the essence of intergration in the nervous system. The axon hillock is the neuron's intergrating center, the region where the membrane potential at any instant represents the summed effect of all EPSP and IPSP. Whenever the Axon Hillock reaches threshold, an action potential is generated and travels along the axon to its synpatic terminals.
