Monday April 11, 2014. Nervous system and biological electricity III 1. No pre-lecture quiz 2. A review of Action potentials 3. Myelin 4. Synapses and neurotransmitters

The Action Potential Is a Rapid Change in Membrane Potential 1. Depolarization phase 2. Repolarization phase Threshold potential Resting potential 3. Hyperpolarization phase

Voltage-gated sodium channels allow the action potential to occur https://www.youtube.com/watch?v=ifD1YG07fB8

Voltage-gated channels Two important types: 1.) Na+ voltage gated channels 2.) K+ voltage gated channels How voltage-gated channels work At the resting potential, voltage- gated Na+ channels are closed. Conformational changes open voltage-gated channels when the membrane is depolarized.

Resting Potential - Both voltage gated Na+ and K+ channels are closed.

Initial Depolarization - Some Na+ channels open Initial Depolarization - Some Na+ channels open. If enough Na+ channels open, then the threshold is surpassed and an action potential is initiated.

Na+ channels open quickly. K+ channels are still closed. PNa+ > PK+

Na+ channels self-inactivate, K+ channels are open. PK+ >> PNa+

Emembrane ≈ E K+ PK+ > PK+ at resting state

Resting Potential - Both Na+ and K+ channels are closed.

Before the end of the semester you are going to learn how A. how the nerve cell interprets the information incoming from other neurons at the dendrites & axon hillock B. how the signal is propagated along the axon C. how the signal is transferred via neurotransmitters to the next neuron (or muscle as shown in this graph) In order to understand any of this, we need to understand the nature of biological electricity - which is the point of this lecture. Then, we’re going to discuss B (how the signal is propagated along the axon). Then A. and Then C.

Action Potentials Propagate because Charge Spreads down the Membrane PROPAGATION OF ACTION POTENTIAL Axon Neuron 1. Na+ enters axon. 2. Charge spreads; membrane “downstream” depolarizes. Depolarization at next ion channel 3. Voltage-gated channel opens in response to depolarization.

Why does the membrane potential increase during stage 3 of the action potential?   A. Both the voltage-gated Na+ channels and voltage gated K+ channels are open. B. All of the K+ channels (both leak and voltage gated) are open. C. The voltage gated Na+ channels are open, but the voltage gated K+ channels have not opened yet. D. The voltage gated Na+ channels are open, but the K+ channels (both voltage gated and leak) have not opened yet.

Why does the membrane potential decrease during stage 4 of the action potential? A. The voltage gated K+ channels open. B. The voltage gated Na+ channels open. C. The voltage gated K+ channels close. D. The voltage gated Na+ channels close. E. A and D

Action Potentials Propagate Quickly in Myelinated Axons Action potentials jump down axon. Action potential jumps from node to node Nodes of Ranvier Axon Schwann cells (glia) wrap around axon, forming myelin sheath Schwann cell membrane wrapped around axon

The process of coating axons with myelin is incomplete when humans are born. This is part of the reason why babies are uncoordinated and slow learners. Babies need lots of fat – not only for energy storage but also to myelinate their neurons.

Multiple Sclerosis (MS) Disease results in damage to myelin and impairs electrical signaling. Muscles weaken and coordination decreases.

Presynaptic Postynaptic

neurotransmitter Synaptic vesicle Neurotransmitter transporter Axon Terminal (pre-synapse) Voltage-gated Ca++ channel Neurotransmitter Receptor Synapse Don’t worry about this Dendrite (post-synapse)

ACTION POTENTIAL TRIGGERS RELEASE OF NEUROTRANSMITTER Na+ and K+ channels Presynaptic membrane (axon) Postsynaptic (dendrite or cell body) Action potentials 1. Action potential arrives; triggers entry of Ca2+. 2. In response to Ca2+, synaptic vesicles fuse with presynaptic membrane, then release neurotransmitter. 3. Ion channels open when neurotransmitter binds; ion flows cause change in postsynaptic cell potential. 4. Ion channels will close as neurotransmitter is broken down or taken back up by presynaptic cell (not shown).

Synapse animation https://www.youtube.com/watch?v=LT3VKAr4roo

Ion Channels on Post-synaptic Cell at Synapse Some only let Na+ pass through. Some let Na+/K+ pass through. Some only let K+ pass through. Some increase the permeability of Cl-.

Excitatory vs. Inhibitory Synapses Excitatory synapses cause the post-synaptic cell to become less negative triggering an excitatory post-synaptic potential (EPSP) Increases the likelihood of firing an action potential Inhibitory synapses cause the post-synaptic cell potential to become negative triggering an inhibitory post-synaptic potential Decreases the likelihood of firing an action potential

Postsynaptic Potentials Can Depolarize or Hyperpolarize the Postsynaptic Membrane Depolarization, Na+ inflow Hyperpolarization, K+ outflow or Cl– inflow Depolarization and hyperpolarization stimuli applied Excitatory postsynaptic potential (EPSP) Inhibitory postsynaptic potential (IPSP) EPSP  IPSP Resting potential

Neurons Integrate Information from Many Synapses Most neurons receive information from many other neurons. Axons of presynaptic neurons Dendrites of postsynaptic neuron Cell body of postsynaptic neuron Axon hillock Axon of postsynaptic cell Excitatory synapse Inhibitory synapse

Neurons Integrate Information from Many Synapses Postsynaptic potentials sum. Action potential Threshold Resting potential

Neurotransmitters More than 100 neurotransmitters are now recognized, and more will surely be discovered. Acetylcholine is important and one of the first ones discovered because its involvement in muscle movement. Dopamine and serotonin hugely important for many behaviors. The workhorses of the brain are glutamate, glycine, and γ-aminobutyric acid (GABA).

Acetylcholine Stimulates muscles Also found throughout nervous system Usually excitatory, but can be inhibitory depending on the receptor

Acetylcholine

Dopamine Excitatory (but sometimes inhibitory) depending on the location in the nervous system Associated with the reward system!! Requires a transport protein to inactivate

Dopamine

Serotonin Excitatory or inhibitory depending on area of CNS Ecstasy (MDMA) causes increased release Involved in sleep, appetite, mood Drugs like prozac (SSRIs – selective serotonin reuptake inhibitor) slows down transport protein Transporter also binds cocaine and amphetamines.

The Autonomic Nervous System Controls Internal Processes PARASYMPATHETIC NERVES “Rest and digest” SYMPATHETIC NERVES “Fight or flight” Constrict pupils Dilate pupils Stimulate saliva Inhibit salivation Slow heartbeat Cranial nerves Increase heartbeat Cervical nerves Constrict airways Relax airways Stimulate activity of stomach Inhibit activity of stomach Thoracic nerves Inhibit release of glucose; stimulate gallbladder Stimulate release of glucose; inhibit gallbladder Stimulate activity of intestines Inhibit activity of intestines Lumbar nerves Secrete epinephrine and norepinephrine (hormones that stimulate activity; see Chapter 47) Sacral nerves Contract bladder Sympathetic chain: bundles of nerves that synapse with nerves from spinal cord, then send projections to organs Relax bladder Promote erection of genitals Promote ejaculation and vaginal contraction

The Functions of the PNS Form a Hierarchy Central nervous system (CNS) Information processing Peripheral nervous system (PNS) Sensory information travels in afferent division Most information travels in efferent division, which includes… Somatic nervous system Autonomic nervous system Sympathetic division Parasympathetic division