6.5 – Neurones and Synapses

Essential Idea: Neurons transmit the message, synapses modulate the message. 6.5 Neurons and Synapses Understandings: Neurons transmit electrical impulses   The myelination of nerve fibers allows for saltatory conduction Neurons pump sodium and potassium ions across their membranes to generate a resting potential An action potential consists of depolarization and repolarization of the neuron 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 Synapses are junctions between neurons and receptor or effector cells When presynaptic neurons are depolarized they release a neurotransmitter into the synapse A nerve impulse is only initiated if the threshold potential is reached Applications: Secretion and reabsorption of acetylcholine by neurons at synapses Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors Skill: Analyze oscilloscope traces showing resting potentials and action potentials

The human nervous system A. CNS – central nervous system – brain and spinal cord B. PNS – peripheral nervous system – composed of nerves 1. Nerves – bundles of neurons 2. Spinal nerves – 31 pairs from spinal cord 3. Cranial nerves – 12 pairs from brain stem

A. Basic functional unit of nervous system Neurons A. Basic functional unit of nervous system B. Can be sensory (take info to CNS), motor (take info away from CNS) or interneurons (within the CNS) There are about 85 billion neurons in the human body!!!!

III. Nerve Impulses A. The steps

1. Resting potential Annotate with them

Notice that 3 sodiums are moved out of the cell for every 2 potassiums that are moved in

Depolarization & Action Potential a. Must surpass threshold b. All or none response to stimulus

Repolarization & Hyperpolarization - Refractory period Flushing a toilet – have to wait before you can do it again

4. Return to resting potential

F. Propagation of nerve impulses 1. A nerve impulse is an action potential that starts at one end of a neuron and is then propagated along the axon to the other end of the neuron. 2. The propagation happens because the ion movements that depolarize one part of the neuron trigger depolarization in the neighboring part of the neuron. (Local currents) 3. Nerve impulses always move in one direction along neurons, because an impulse can only be initiated at one terminal of a neuron and can only be passed on to other neurons/different cell types at the other terminal.

G. Saltatory conduction 1 G. Saltatory conduction 1. In myelinated areas, ion movement is not necessary for signal transmission 2. Impulses have to “jump” from one node of Ranvier to the next (this does require ion movement 3. Increases speed of impulse and reduces energy expenditure

Synaptic Transmission Communication between neurons (pre-synaptic and post-synaptic) Neurotransmitters – chemical messengers

C. Neuron impulse reaches terminal buttons

Neonicotinoid insecticides Chemically similar to nicotine Binds to post-synaptic receptors, blocking acetylcholine Stops action potential Also, not broken down by acetylcholinesterase so receptor is permanently blocked Paralyzes insects -> death