Biology Main points/Questions 1.What does a neuron look like? 2.Why do membranes have charges? 3.How can these charges change?

Functions of the Nervous System Process and coordinate: –sensory input: from inside and outside body –motor commands: control activities of peripheral organs (e.g., skeletal muscles) –Integration – occurs in the central nervous system –higher functions of brain: intelligence, memory, learning, emotion

Coordinating all the different body systems and interacting with the external world are the job of the body’s control systems – the nervous system and the endocrine (hormone) system.

Aplysia (sea slug) neurons

Neurons are nerve cells that transfer information within the body Neurons use two types of signals to communicate: –electrical signals (long-distance) and –chemical signals (one cell to the next - short) Nervous systems process information in three stages: sensory input, integration, and motor output

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

White matter Spinal cord Sensory information Sensory neuron Motor neuron Interneuron Integration Response

Three types of neurons These stages use three basic types of neurons – – sensory – association and – motor

Neuron Structure and Function Most of a neuron’s organelles are in the cell body Most neurons have dendrites, that receive signals from other neurons The axon is typically a longer extension that transmits signals to other cells Many axons are wrapped by other cells (glial cells) to speed signaling

A typical neuron & formation of the myelin sheath Glial Cells

Big idea: Neuron membranes have a charge. Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential Messages are transmitted as changes in membrane potential The resting potential is the membrane potential of a neuron not sending signals

The Resting Potential Why do neurons have a resting potential? Lets look at one ion - potassium (K + ) – that is found in your neurons Cells have large amounts of potassium inside them and small amounts outside. Neurons have channels that let potassium cross the membrane – what does this do?

Electrochemical Gradients Figure 12–9c, d

Electrochemical Gradients Figure 12–9a, b

Inner chamber Outer chamber –90 mV 140 mM 5 mM KCI K+K+ Cl – Potassium channel (a) Membrane selectively permeable to K + Potassium stops moving when charge is -90mV – Why?

The Resting Potential Of course there are more charged ions and molecules inside a neuron Sodium (Na + ) is a key player in neuron signaling. There is lots of sodium outside the cell!

OUTSIDE CELL [K + ] 5 mM [Na + ] 150 mM INSIDE CELL [K + ] 140 mM [Na + ] 15 mM [A – ] 100 mM (a) Two key ions for neurons Other molecules and ions add negative charge to the inside of a neuron.

The Resting Potential In your neuron the concentration of K + is greater inside the cell, while the concentration of Na + is greater outside How do your neurons maintain this difference?

Neurons are constantly working to maintain “resting” conditions This is because the membrane leaks ions A neuron at rest contains many open K + channels and few open Na + channels; so lots of K + diffuses out of the cell Active resting in neurons

Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system

EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + [Na + ] low [K + ] high CYTOPLASM Cytoplasmic Na + binds to the sodium-potassium pump. 1

Na + binding stimulates phosphorylation by ATP. Na + ATP P ADP 2

Phosphorylation causes the protein to change its shape. Na + is expelled to the outside. Na + P 3

K + binds on the extracellular side and triggers release of the phosphate group. P P K+K+ K+K+ 4

Loss of the phosphate restores the protein’s original shape. K+K+ K+K+ 5

K + is released, and the cycle repeats. K+K+ K+K+ 6

K + constantly leaks out of the neuron The flow of K+ ions out of the cell helps to maintain the resting potential A neuron at rest has a potential about -70 mV

Big idea: Action potentials are the signals conducted by axons Signals are passed down an axon as spikes in membrane potential These spikes, that briefly reverse membrane polarity, are called action potentials These action potentials are the basic form of communication for neurons

(a) Gentle touch fires slowly 1 silent 2 2 1

Neurons contain gated ion channels that open or close in response to stimuli Membrane potential changes in response to opening or closing of these channels What would happen if K + permeability increased? Changing membrane potential

3 Conditions of Gated Channels 1.Closed, but capable of opening 2.Open (activated) 3.Closed, not capable of opening (inactivated)

Stimuli +50 Membrane potential (mV) –50 Threshold Resting potential Hyperpolarizations – Time (msec) (a) hyperpolarizations 0 15 When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative This is called hyperpolarization What if Na + gates open?

If gated Na + channels open and Na + diffuses into the cell This causes a depolarization, a reduction in the membrane potential Stimuli +50 Membrane potential (mV) –50 Threshold Resting potential Depolarizations – Time (msec) (b) depolarizations 15 0

If enough open the membrane in this region reaches threshold At this point a large number of Na + channels open and sodium pours in What would this do to membrane potential? Stimuli +50 Membrane potential (mV) –50 Threshold Resting potential Depolarizations – Time (msec) (b) depolarizations 15 0

Strong depolarizing stimulus +50 Membrane potential (mV) –50Threshold Resting potential – Time (msec) (c) Action potential Action potential 6 Membrane polarity flips! Then these channels shut & K + open Potential drops back as K + ions flow out This spike in charge is an action potential!

This flipping and returning of the membrane potential is passed along a neuron down it’s axon The action potential flows down the axon as depolarization is pushed ahead of the action potential (propagation)

Big idea: Action potentials starts with a slight of membrane (closer to 0mv) –often no action potential is fired if at ~ -50mv channels open – allowing to pour (in/out) depolarization Threshold isn’t hit Na + gated

Big idea: Action potentials at ~ -50mv gated channels open – allowing Na + to pour (out!) –this causes –They after a very short time (~1msec.) channels also respond to voltage – but they are – pours (in/out) – reversing the charge again –They shut after driving charge membrane potential to flip slam shut K+K+ much slower K+K+ below resting

Axon Plasma membrane Cytosol Action potential Na +

Axon Plasma membrane Cytosol Action potential Na + Action potential Na + K+K+ K+K+

Axon Plasma membrane Cytosol Action potential Na + Action potential Na + K+K+ K+K+ Action potential K+K+ K+K+ Na +

Because the sodium gates lock shut an action potential cannot move “backwards” During the refractory period after an action potential, a second action potential cannot be initiated The refractory period is a result of a temporary inactivation of the Na + channels

Figure 34.5 How an action potential is generated

Generation of Action Potentials

What happens at the end of the axon? Axons end at a synapse This is a small gap between one neuron and another (or sometimes another cell) Chemicals called neurotransmitters carry information across the gap

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

A synapse between two neurons

Voltage-gated Ca 2+ channel Ca Synaptic cleft Ligand-gated ion channels Postsynaptic membrane Presynaptic membrane Synaptic vesicles containing neurotransmitter 5 6 K+K+ Na +

The presynaptic neuron synthesizes and packages neurotransmitter in synaptic vesicles located in the synaptic terminal The action potential causes the release of the neurotransmitter The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell

Postsynaptic neuron Synaptic terminals of pre- synaptic neurons 5 µm