Neurons, Synapses, and Signaling Nervous System Neurons, Synapses, and Signaling

Sensory input Integration Sensor Motor output Effector Peripheral nervous system (PNS) Central nervous system (CNS)

NEURONS Presynaptic cell Postsynaptic cell Dendrites Stimulus Nucleus body Axon Synapse Synaptic terminals Postsynaptic cell Neurotransmitter

Transmit information from a sense receptor to the brain Dendrites Axon Cell body Portion of axon 80 µm Cell bodies of overlapping neurons Sensory neuron Interneurons Motor neuron Type of Neuron Function Sensory Neuron Transmit information from a sense receptor to the brain Interneuron Forms connections between neurons (i.e. relay neurons) Motor Neuron Transmits information from the brain to a muscle/gland

Glial Cells: neuron helper cells, nourish, insulate, regulate factors in neurons

Membrane Potentials - Inside of neuron is negative -Outside is positive -Involves sodium potassium pumps & ion channels OUTSIDE CELL [Na+] 150 mM [Cl–] 120 mM [K+] 5 mM INSIDE CELL [K+] 140 mM [A–] 100 mM [Na+] 15 mM [Cl–] 10 mM Each cell has a voltage (a difference in charge) across their membrane. Resting potential = -60 to -80 mV (millivolts)

Key Na+ Sodium- potassium pump Potassium channel Sodium channel K+ OUTSIDE CELL INSIDE CELL

Membrane potential (mV) Key Na+ Sodium channel Sodium channel K+ Potassium channel 3 Rising phase of the action potential 4 Falling phase of the action potential +50 Action potential Sodium channel 3 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential 2 Depolarization –100 Time Extracellular fluid Potassium channel Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop 5 Undershoot 1 Resting state

+50 +50 +50 Action potential Stimuli Stimuli Strong depolarizing stimulus Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) –50 Threshold –50 Threshold –50 Threshold Resting potential Resting potential Resting potential Hyperpolarizations Depolarizations –100 –100 –100 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6 Time (msec) Time (msec) Time (msec) (a) Graded hyperpolarizations (b) Graded depolarizations (c) Action potential Hyperpolarization (neuron is opening K+ channels which increases the negative charge) Depolarization ( neuron is opening Na+ channels thus decreasing the negative charge) Threshold (a set point of membrane potential measurement) Action Potential (wave of depolarization)

Action potentials are the nerve impulses (or signals) that carry information along an axon. Once initiated, the action potential has a magnitude that is independent of strength of the triggering stimulus. Because action potentials occur fully or not at all, they represent an all-or-none response to stimuli. This positive feedback loop of depolarization and channel opening triggers an action potential whenever the membrane potential reaches the threshold. Action Potentials

Conduction of A.P. At the site where an A.P. is initiated, Na+ inflow depolarizes the neighboring region of the axon membrane (past threshold) The neighboring area on the axon starts to depolarize as the original area is repolarizing by K+ outflow. Only moves in one direction due to this pattern of depolarizing and repolarizing BUT A.P. are not transmitted from neurons to other cells… Axon Plasma membrane Action potential Na+ Cytosol Action potential K+ Na+ K+ Action potential K+ Na+ K+

5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 6 Synaptic cleft 3 Ligand-gated ion channels Majority of synapses are chemical synapses (release of neurotransmitters) At end of the A.P. increased Ca2+ concentration causes N.T vesicles to fuse and enter the synaptic cleft. The neurotransmitter bind to the receptor proteins to depolarize the next cell (create a response.)

Neurotransmitters Common Neurotransmitters: Norepinephrine Epinephrine Dopamine Serotonin Nitric Oxide Neurotransmitters

Several factors affect the speed at which action potentials are conducted: Axon Diameter: Wider the axon, the fast the signal (wider=less resistance for ion flow) Myelin Sheath: insulation of the signal, signal jumps from node to node. Conduction Speed of A.P.

Helpful Links: 1. ) https://www. youtube. com/watch. v=ZscXOvDgCmQ 2

SUMMARY / REVIEW Large Concepts: Pathway of neural response Vocab: Sensory neuron Interneuron Motor neuron Synapse Action potential Sodium potassium pump Ion channels Resting potential Depolarization Hyperpolarization Threshold Refractory period Neurotransmitter Plasma membrane Large Concepts: Pathway of neural response Structure of a Neuron Types of Neurons A.P. Stages (Depolarize, Threshold, Repolarize…) Ions involved in A.P. Chemical & Electrical communication via neurons Speed,/strength of AP SUMMARY / REVIEW