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Neurons & Neuroanatomy What are the characteristics of neurons important for Cognitive Neuroscience? What is the brain structure important for CogNeuro? Classes of neurons • Unipolar - One process extends from body, which can contain both dendrites and axon terminals • Bipolar - One axon and one dendrite process. • Multipolar - Many dendrite processes. • Pseudounipolar - Bipolar cells where the dendrite and axon processes have merged. • We’ll be primarily interested in multipolar cells. Neural membrane • There are specialized structures in the neural membrane that allow various elements to cross in and out of the cell – Ion channels: Proteins that cross the cell wall, creating pores that allow ions (Na+, K+, Cl-) to pass. – Specific to particular ions; more K+ channels – Nongated and gated – Ion pumps: Actively transport ions across the membrane, creating an electric gradient across the membrane (3 Na+ out; 2 K+ in) Cellular currents • There are two types of electrical currents that can pass through a neuron: • Active currents are ones that are caused by explicit chemical activity (opening and closing of ion channels); ex - at the synapse and across the surface of the axon • Passive currents are ones that simply pass through the cytoplasm, typically as a response to active currents; ex - within the cell body as a result of synaptic activity; within the axon as a result of action potentials. Action Potential • The signal a neuron generates down its axon is called an action potential. • All action potentials are the same magnitude (strength), so they are either on or off. • An action potential is only generated if the depolarization of the cell membrane crosses a threshold. • We determine how excited a neuron is by its firing rate - how many action potentials per second it generates. Depolarization of the AP • As opposed to the nongated ion channels discussed so far, action potentials are driven by gated channels that open in response to high voltage levels (the threshold). • In particular, gated Na+ channels are opened by membrane depolarization, which allows Na+ into the cell. This causes further depolarization, which opens more channels….. Repolarization of the AP • Depolarization also causes voltagegated K+ channels to open, but slightly after the Na+ channels. This drives K+ out of the cell, reestablishing the resting potential of the cell; this is called repolarization. • Because of the delay, K+ flows out after Na+ stops flowing in, so there is a brief period of hyperpolarization Neurotransmitter • Neurons communicate by sending chemical messages called neurotransmitters to other neurons. • These neurotransmitters travel from axon to either the dendrite or the cell body across the synapse. • Where a synapse is depends on what the connection type is – Excitatory: Axon to dendrite – Inhibitory: Axon to cell body