The beauty of the Na + K + pump Na + K + pump Found along the plasma membrane of all cells. Establishes gradients, controls osmotic effects, allows for cotransport Nerve cells have a Na + K + pump and selective permeability to Na + and K + that set up a potential Na + K + pump transports 3 Na + out for every 2 K + in. Na + K + pump The setup Cotransport the result Cotransport p. 61 1
Cotransport The Na+K+ pump also establishes chemical gradients and ultimately influences electrical gradients Electrochemical gradient = electrical and chemical (concentration) gradient combined Electrochemical gradients of neurons Neurons and muscles are excitable cells With stimulation, potential across membrane changes from negative inside the cell to being positive inside 2
Membrane ECF ICF K + effects Concentration gradient for K + Electrical gradient for K + More K + diffuses out compared to the diffusion of Na + in K + would diffuse until it is balanced by its electrical gradient E K + = 90 mv Na + effects Gradient for Na + into the cell Na + would diffuse until balanced by its electrical gradient ECF ICF Concentration gradient for Na + Electrical gradient for Na + Na + and K + passive (leak) channels Na+ leak channel K+ leak channel E Na + = +60 mv 3
increase decrease Na + and K + movement together establish the resting potential (Passive) ECF More K+ leak channels Na + K + pump (Active) Outside Inside Large net diffusion of K + outward makes E K+ of 90 mv No diffusion of A Small net diffusion of Na + inward neutralizes some of the potential created by K + diffusion Na + channel K + channel (Passive) (Active) Resting membrane potential = 70 mv ICF Cell communication 4
Gated ion channels Gated channels allow specific ions to pass only when gates are open Note difference bw gated and leak channels Voltage-Gated Na + Channel Activation gate ECF Triggered by: potential change (voltage), chemical binding, temperature change, stretching Inactivation gate ICF Slow closing Closed but capable of opening Open (activated) Inactivated Voltage-Gated K + Channel ECF Na + and K + gated channels Depolarization causes: Na + gates to open, then slowly close Delayed opening of K + gates First Later ICF Delayed opening Closed Open Delayed opening 5
Graded potentials Graded potentials Graded potential Resting potential Below threshold Time Signal dies out over distance Magnitude of stimulus Stimuli applied Graded potentials Initial site of potential change Loss of charge Loss of charge Triggering an action potential Na + equilibrium potential At threshold, Na+ channels briefly open, which causes a large depolarization K + open during spike, and slowly close, resting potential returns Threshold potential Current flow Current flow Triggering event Resting potential K + equilibrium potential 6
1) Input zone receives incoming signals from other neurons. Dendrites Axon hillock 2) Trigger zone initiates AP s 3) Conducting zone conducts action potentials Axon terminals Dendrites Cell body 4) Output zone releases neurotransmitter that influences other cells. Axon Conduction of signal Conduction of signal interstitial fluid cytoplasm Na + Na + Na + 7
Conduction of signal Conduction of signal K + K + K + K + K+ K + Na + Na + Na + Na + K + Na + Na + Na + All or nothing Action potentials No degradation of signal over distance Conduction in one direction Refractory period Action potentials travel in one direction bc of the refractory period 8
Myelination unsheathed node Schwann cells of a myelin sheath axon Na + action potential resting potential resting potential K + Na + resting potential restored action potential resting potential Chemical Synapse Voltage-gated Ca 2+ channels Calcium influx causes vesicles to perform exocytosis Chemically gated Na +, K +, or Cl - channels 9
Neurotransmitters A synapse will use only one type of neurotransmitter Ex: dopamine, serotonin, epinephrine, GABA Neurotransmitters activate gated ion channels Excitatory synapse: Na+ channels Neurotransmitters are quickly removed once they bind to receptors Reuptake or inactivated Inhibitory synapse: K+ or Cl- channels Signal at the synapse excites or inhibits the postsynaptic neuron Excitatory synapse Inhibitory synapse Excitatory synapse: Causes an influx of Na + into postsynaptic neuron. This produces an EPSP and depolarizes the neuron. Inhibitory synapse: Causes an outflow of K + from the postsynaptic neuron. It can also cause an influx of CL - This produces an IPSP and hyperpolarizes the neuron. Activation of synapse Activation of synapse EPSP IPSP PSP= Postsynaptic potential 10
Temporal summation: PSPs occur close together in time from a single presynaptic neuron. Spatial summation: PSPs originate from several presynaptic inputs. 11
Some neuron shapes Drug effects If a drug affects the nervous system, it usually changes synapse function Hippocampus neuron Pyramidal neurons Drug molecules can: mimic neurotransmitters falsely stimulate neurotransmitter release block neurotransmitters, or their reuptake These drugs all mimic natural endorphin Purkinje neurons Bipolar neurons - retina How stimulants and sedatives work In a part of the brain stem (RAS), excitatory synapses (norepinephrine) cause wakefulness, inhibitory synapses (GABA) cause drowsiness How stimulants and sedatives work In a part of the brain stem (RAS), excitatory synapses (norepinephrine) cause wakefulness, inhibitory synapses (GABA) cause drowsiness Caffeine, amphetamines, ecstasy (MDMA) norepinephrine in RAS Alcohol, valium, barbiturates, & marijuana activate GABA receptors. 12