Neurons, Synapses, and Signaling  Overview  Neural Signals  Organization of Nervous Systems

Functions of Nervous Systems  Sensory input-conduction of signals from receptors to integration centers (CNS)  Integration-interpretation of signals and generation of proper response  Motor output-signals to effector cells (muscles, glands), carry out response

Composition of Nervous System Neurons  cells specialized for transmitting electrical and chemical signals  Composed of cell body, dendrites, and axons  Three types: sensory neurons, interneurons, and motor neurons  Arranged into circuits of two or more kinds of neurons: convergent, divergent, and reverberating circuits

Composition of Nervous System con’t. Supporting Cells (glial cells)  Structurally reinforce, protect, insulate and assist neurons  Do not conduct impulses  Outnumber neurons 10- to 50- fold  Types: astrocytes (blood-brain barrier), oligodendrocytes, Schwann cells

The Nature of Neural Signals  Signal transmission along a length of a neuron depends on voltages created by ionic fluxes across neuron plasma membranes  All cells have an electrical membrane potential: range from –50 to –100 mV; outside the cell is 0, inside is negatively charged  Neurons have about –70mV at rest

Ion Concentrations:  [Na+] 10x greater outside (main cation outside)  [K+] 30x greater inside (principal cation inside)  [Cl-] 12x greater outside  Other anions-100x inside Selective permeability of the membrane maintains these differences

Action Potential-Nerve Impulse Four phases  Resting phase-no channels open  Depolarizing phase-brief reversal of polarity, Na+ gates open  Na+ in, result:  -70mV to +30 mV  Repolarizing phase-Na+ gates close, K+ gates open, K+ out, +30 mV to –70 mV  Undershoot phase (refractory period)-K+ and Na+ are returned to proper concentrations, neuron insensitive to depolarizing stimuli

Action Potentials  “all-or-none” require a minimum stimulus or threshold  Self-propagating  Saltatory conduction-action potential “jumps” from one node of Ranvier to the next, skipping the myelinated regions of the membrane

Synapses  Tiny gap between terminal of an axon and beginning of a dendrite on the next neuron  Presynaptic cell-transmitting cell  Postsynaptic cell-receiving cell  Two types of synapses: electrical and chemical

Electrical Synapses  Electrical synapse  Less common  Travel through gap junctions  Example-giant neuron in crustaceans

Chemical Synapses  Separated by synaptic cleft  Ca 2+ enters cell stimulating cytoskeleton to move vesicles to the surface of the axonal knob  Neurotransmitter from the vesicle is released into the cleft  Neurotransmitter binds to receptor sites on the postsynaptic membrane, reestablishing the action potential

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