Neurons Synapses Signaling Neurons are nerve cells that

Neurons, Synapses, & Signaling § 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 (short-distance) § A neuron’s structure is closely tied to its function © 2014 Pearson Education, Inc.

Neuron Structure and Function § Most of a neuron’s organelles are in the cell body § Most neurons have dendrites, highly branched extensions that receive signals from other neurons § The axon is typically a much longer extension that transmits signals to other cells at synapses § The terminal branches of an axon pass information across the synapse in the form of chemical messengers called neurotransmitters § A synapse is a junction or space between two neurons © 2014 Pearson Education, Inc.

A Typical Neuron Cell Dendrites – receive signals Nucleus Cell body Presynaptic Cell = 1 st neuron Synapse Neurotransmitter © 2014 Pearson Education, Inc. Axon – sends the signal down the neuron to the Terminal branches at the end Terminal Branches Postsynaptic cell = 2 nd neuron

§ Information is transmitted along a pathway from a presynaptic neuron cell to a postsynaptic neuron cell (or effector cell) § Chemical signals called neurotransmitters cross the space = synapse between neurons. § Nervous systems process information in three stages involving the 3 types of neurons § sensory input (sensory neurons in PNS) § integration (associative neurons in CNS) § motor output (motor neurons in PNS) © 2014 Pearson Education, Inc.

Figure 49. 7 Cell body of sensory neuron in dorsal root ganglion CNS Quadriceps muscle Hamstring muscle Key Motor neurons Sensory neuron Motor neuron Interneuron © 2014 Pearson Education, Inc.

Nerve signals travel along a Specific Pathway: STIMULS: Receptor Sensory Neurons CNS motor neurons effectors: RESPONSE Central nervous system (CNS) Brain Cranial nerves Spinal cord Spinal nerves Peripheral nervous system (PNS) © 2014 Pearson Education, Inc.

Nerve signals = Action potential signals are sent to different regions of the Brain Eye Reticular formation Input from touch, pain, and temperature receptors © 2014 Pearson Education, Inc. Input from nerves of ears

§ Sensors detect external stimuli and internal conditions and transmit information along sensory neurons to the CNS § In the CNS (brain & spinal cord) interneurons = associative neurons integrate the information § Motor output leaves the CNS via motor neurons, which trigger muscle or gland activity = response © 2014 Pearson Education, Inc.

§ We have a complex nervous system that consists of § A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord § A peripheral nervous system (PNS), which carries information into and out of the CNS © 2014 Pearson Education, Inc.

Neurons send signals using ion pumps and ion channels, altering the resting potential § Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential § The resting potential is the membrane potential of a neuron that’s not sending signals § Changes in membrane potential act as signals © 2014 Pearson Education, Inc.

At Resting Potential, Neurons are NOT transmitting Signals § In a human neuron at resting potential, the concentration of K+ is highest inside the cell, while the concentration of Na+ is highest outside the cell § The inside of the membrane has a negative charge; the outside has a positive charge § Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane © 2014 Pearson Education, Inc.

Neuron Membrane at Resting Potential © 2014 Pearson Education, Inc.

A Nerve Signal = Action Potential. Sequence of Events: 1. A stimulus causes the opening of Na+ ion channels in the plasma membrane. 2. Na+ rushes in the cell = Depolarization occurs. This is the start of the action potential 3. Depolarization causes K+ ion channels to open and K+ rushes out = Repolarization occurs 4. Now, the Na+ K+ pump restores normal resting potential © 2014 Pearson Education, Inc.

Depolarization Action Potential Ions A stimulus causes Na+ ion channel gate to open Ion channel Gate closed: No ions flow across membrane. © 2014 Pearson Education, Inc. Gate open: Na+ ions flow in through channel.

§ Opening of Na+ ion channels triggers a depolarization because Na+ diffuses into the cell § An action potential has begun. This depolarization is a change in membrane’s polarity: + inside and outside § Action potentials have a constant magnitude or size because they are triggered by an all-or-none; either the ion gated channel opened or it didn’t § If the stimulus is strong enough to open the ion gated channels, threshold has been reached an action potential will begin © 2014 Pearson Education, Inc. -

Figure 48. 10 c Strong depolarizing stimulus The polarity of the Membrane switches: Becomes Positive on the inside and Negative on the outside +50 Membrane potential (m. V) Action potential is triggered by a depolarization that reaches the threshold. 0 − 50 − 100 © 2014 Pearson Education, Inc. Action potential Threshold Resting potential 0 1 2 3 4 5 6 Time (msec)

An action potential moves in a series of stages: 1. At resting potential most sodium (Na+) and potassium (K+) channels are closed 2. Gated Na+ channels open first and Na+ flows into the cell 3. The threshold is crossed, and depolarization has begun producing an action potential 4. During the falling phase, Na+ channels close and K+ channels open, and K+ flows out of the cell = repolarization occurs © 2014 Pearson Education, Inc.

Figure 48. 11 f Membrane potential (m. V) +50 3 0 − 50 − 100 © 2014 Pearson Education, Inc. Action potential 2 4 Threshold 1 Resting potential Time 5 1

§ During the refractory period after an action potential, a second action potential cannot be initiated until the Na+ K+ pump restores normal § Na+ is outside and K+ is inside = resting potential § Action potentials travel in only one direction: down the axon toward the synaptic terminals § At the terminal the action potential triggers the release of neurotransmitters into the synapse © 2014 Pearson Education, Inc.

Figure 48. 12 -1 Axon Action potential Na+ © 2014 Pearson Education, Inc. Plasma membrane Cytoplasm

Figure 48. 12 -2 Axon Plasma membrane Action potential Cytosol Na+ K+ Action potential Na+ K+ © 2014 Pearson Education, Inc.

Figure 48. 12 -3 Axon Plasma membrane Action potential Cytosol Na+ K+ Action potential Na+ K+ © 2014 Pearson Education, Inc.

§ Some axons are wrapped in myelin sheaths that have spaces called nodes of Ranvier § Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction § This allows action potential signals to be transmitted faster © 2014 Pearson Education, Inc.

Figure 48. 14 Schwann cell makes the myelin sheath Depolarized region (node of Ranvier) Myelin sheath Cell body Axon © 2014 Pearson Education, Inc.

Neurons communicate with other neuron cells at synapses § Action potentials are electrical signals along neuron membranes. § The action potential causes the release of the neurotransmitter by the axon terminal branches. § The chemical signal = neurotransmitter diffuses across the synapse and stimulates dendrites of the next neuron (postsynaptic = after the synapse) § A new action potential begins on the membrane of this postsynaptic neuron © 2014 Pearson Education, Inc.

Figure 48. 16 Presynaptic cell 1 Axon Postsynaptic cell Synaptic vesicle containing neurotransmitter Synaptic cleft Postsynaptic membrane Presynaptic membrane 3 K+ 4 Ca 2+ 2 Voltage-gated Ca 2+ channel © 2014 Pearson Education, Inc. Ligand-gated ion channels Na+

§ A number of toxins can disrupt a neurotransmission § These include the nerve gas, sarin, and the botulism toxin produced by certain bacteria § Death can result if signal transmission stops © 2014 Pearson Education, Inc.