The cone snail is a deadly predator. Why? Figure 48.1 What makes this snail such a deadly predator? For the Discovery Video Novelty Gene, go to Animation and Video Files.

Squid Nervous System Nerves with giant axons Ganglia Brain Arm Eye Mantle Nerve Figure 48.2 Overview of the squid nervous system

Information Processing Sensory input Sensor: Detects stimulus Integration Processing Motor output Figure 48.3 Summary of information processing Effector: Does response Peripheral nervous system (PNS) Central nervous system (CNS)

Neurons Dendrites Presynaptic cell Axon Synapse Postsynaptic cell Stimulus Nucleus Presynaptic cell Axon hillock Cell body Axon Synapse Synaptic terminals Figure 48.4 Neuron structure and organization For the Cell Biology Video Dendrites of a Neuron, go to Animation and Video Files. Postsynaptic cell Neurotransmitters

Structural diversity of neurons Dendrites Axon Cell body Portion of axon Figure 80 µm Cell bodies of overlapping neurons Sensory neuron Interneurons Motor neuron

Key Na+ Sodium- potassium Potassium Sodium pump channel channel K+ OUTSIDE CELL Figure 48.6b The basis of the membrane potential INSIDE CELL

Graded potentials and an action potential in a neuron Stimuli Stimuli Strong depolarizing stimulus +50 +50 +50 Action potential Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) –50 Threshold –50 Threshold –50 Threshold Resting potential Resting potential Resting potential Figure 48.9 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

Membrane potential (mV) Stimuli +50 Membrane potential (mV) –50 Threshold Figure 48.9b Graded potentials and an action potential in a neuron Resting potential Depolarizations –100 1 2 3 4 5 Time (msec) (b) Graded depolarizations – magnitude of the change varies with the strength of the stimulus.

Action potential Threshold Resting potential Strong depolarizing stimulus +50 Action potential Membrane potential (mV) –50 Threshold Figure 48.9c Graded potentials and an action potential in a neuron Resting potential –100 1 2 3 4 5 6 Time (msec) (c) Action potential = change in membrane voltage

The role of voltage-gated ion channels in the generation of an action potential Key Na+ K+ 3 Rising phase of the action potential 4 Falling phase of the action potential +50 Action potential 3 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential 2 Depolarization Figure 48.10 The role of voltage-gated ion channels in the generation of an action potential –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop 5 Undershoot 1 Resting state

Conduction of an Action Potential Signal Transmission Action potential Axon Action potential Plasma membrane Na+ Cytosol Action potential K+ Na+ Figure 48.11 K+ Action potential K+ Na+ K+

Schwann cells and the myelin sheath Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Figure 48.12 Schwann cells and the myelin sheath Nodes of Ranvier Nucleus of Schwann cell Axon Myelin sheath

Saltatory conduction Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon Figure

Chemical synapse neurotransmitter Ca2+ Ligand-gated ion channels 5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane Ca2+ 1 4 2 6 Figure Synaptic cleft 3 Ligand-gated ion channels

Summation of postsynaptic potentials Terminal branch of presynaptic neuron E1 E1 E1 E1 E2 E2 E2 E2 Postsynaptic neuron Axon hillock I I I I Threshold of axon of postsynaptic neuron Action potential Action potential Membrane potential (mV) Resting potential Figure 48.16 Summation of postsynaptic potentials –70 E1 E1 E1 E1 E1 + E2 E1 I E1 + I (a) Subthreshold, no summation (b) Temporal summation (c) Spatial summation (d) Spatial summation of EPSP and IPSP

Table 48.1

Action potential Resting potential Review Threshold (–55) +50 Falling phase Rising phase Membrane potential (mV) Threshold (–55) –50 Resting potential –70 Depolarization Undershoot –100 Time (msec)

You should now be able to: Distinguish among the following sets of terms: sensory neurons, interneurons, and motor neurons; membrane potential and resting potential; ungated and gated ion channels; electrical synapse and chemical synapse; EPSP and IPSP; summation. Explain the role of the sodium-potassium pump in maintaining the resting potential.

Describe the stages of an action potential; explain the role of voltage-gated ion channels in this process. Explain why the action potential cannot travel back toward the cell body. Describe saltatory conduction. Describe the events that lead to the release of neurotransmitters into the synaptic cleft.

Explain the statement: “Unlike action potentials, which are all-or-none events, postsynaptic potentials are graded.” Name and describe five categories of neurotransmitters.