6.5 - Neurons and synapses www.gregadunn.com

The nervous system consists of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Neurons are the cells that make up the nervous system Neurons are the cells that make up the nervous system. These cells transmit electrical impulses (nerve impulses). Neuronal Anatomy: We will be talking about the movement of signals from one end of a neuron to the other. Key Terminology For Nervous Unit: Nerve Impulse: An electrical signal that travels from one end of a neuron to the other in order to convey a message. Potential: The difference in charge across the membrane of a neuron. Resting Potential: The negative charge registered when the nerve is not conducting a nerve impulse Depolarization: The change from the negative resting potential to the positive action potential. Repolarization: The change in electrical potential from positive during an action potential, back to the resting potential.

Neurons are the cells that make up the nervous system Neurons are the cells that make up the nervous system. These cells transmit electrical impulses.

Remember these? ECG of an Action Potential Neurons pump sodium (Na+) and potassium (K+) ions across their membranes to maintain a resting potential. Remember these? ECG of an Action Potential Resting potential is maintained by sodium-potassium pumps. These pumps use active transport to move ions across the neuronal membrane to maintain a constant resting potential of -70mV. Works through the activity of a Sodium/Potassium Pump: Fun Exercise: How long can you go without thinking of anything? Does this make sense with what you know about how nerves maintain RP?

An action potential consists of depolarization and repolarization of the neuron. An action potential is created through the opening of Voltage-Gated Ion Channels. These channel proteins open and close in finely tuned procedure to change the potential across the membrane of the neuron. The stimulus must increase the potential beyond the threshold potential to initiate an action potential. Click the pictures to the right to access the PHET simulation of a neuron. Use the simulation to observe how local currents change in a single part of an axon.

Nerve impulses are action potentials propagated along the axons of neurons. Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential. Steps to an Action Potential: Stimulus causes the membrane potential to exceed the threshold potential, leading to the opening of Na+ Voltage-Gated channels. Influx of Na+ ions leads to change in the membrane potential to +40mV (depolarization), causing the K+ Voltage-Gated channels to open. Depolarization of a local section of the neuron depolarizes neighboring sections of the neuron. Efflux of K+ ions eventually causes potential to return to -75mV (repolarization) in each section, closing all channels. Brief period before returning to -70mV is known as refractory period. Sodium/Potassium pump begins to pump Na+/K+ against concentration gradient to return neuron to resting potential (-70 mV). From McGraw Hill: http://goo.gl/tI2MD

The myelination of nerve fibers allows for saltatory conduction. Nerve Cells in humans have special coatings made out of fatty cells, known as Schwann Cells. These fatty cells “insulate” our neurons, allowing them to skip the action potential process for great lengths of the axon, in order to speed up the transmission process. Schwann Cell We estimate myelination makes our nerve cells 5000 times more efficient. Some organisms do not have this myelination….like squid! Poor guys….. Nodes of Ranvier

http://www.blackwellpublishing.com/matthews/channel.html

Synapses are junctions between neurons and between neurons and receptor or effector cells. Synapses work to convert signals conveyed by one type of neuron to another. Key Terms: Neurotransmitter: Any one of a family of chemical messengers that convey messages across a synapse Presynaptic Neuron: A neuron that releases a neurotransmitter into the synapse Postsynaptic Neuron: A neuron that receives a neurotransmitter. http://outreach.mcb.harvard.edu/animations/synaptic.swf

When presynaptic neurons are depolarized, they release a neurotransmitter into the synapse.

Secretion and reabsorption of acetylcholine by neurons at synapses. Acetylcholine is a neurotransmitter that is secreted in relation to muscular contraction in the peripheral nervous system. (Most muscle contractions are caused by this NT.) When secreted by the presynaptic neuron, it is metabolized into Acetic Acid and Choline, and the choline can be reuptaken. If the reuptake of choline is inhibited/blocked, then the cholinergic synapse is unable to continue to function. How does this play out in insecticides? (Click the bee to find out) Neurotransmitters are recycled through a reuptake procedure, by which the NTs that are secreted can be reabsorbed after use.