Information Flow and the Neuron Chapter 37

Learning Objectives List the 4 functions of neural cells Describe the two types of neuronal cells Diagram the basic nerve cell and myelination Explain a basic neuronal circuit (SAINE)

Learning Objectives Describe the flow of an action potential across the axon of a neuron, including the action of Na+ and K+ gated ion channels, and understand how myelination adds to the transmission of the action potential Diagram the action of neurotransmission at synapses List the types of medications that can affect neurotransmission in humans

Organ systems: Nervous System

37.1 Neurons and the Nervous System Neurons are cells specialized for the reception and transmission of informational signals Neurons are supported structurally and functionally by glial cells Neurons communicate via synapses

4 Functions of the Animal Nervous System 1. Reception  Receives information about conditions in internal and external environment 2. Transmission  Transmits message along neurons

4 Functions of the Animal Nervous System 3. Integration  Integrates information to formulate appropriate response 4. Response  Sends out signals to muscles or glands

Neuron Structure Dendrites  Receive information  Conduct signals toward the cell body Axons  Conduct signals away from the cell body to another neuron or an effector

Fig. 37.3, p. 849 Dendrites Cell body Axon Nucleus Axon hillock Axon terminals

SAINE Sensory Afferent Inter Neuron Efferent

A Basic Neuron Circuit An afferent neuron, an interneuron, and an efferent neuron make up a basic circuit Circuits combine into networks that interconnect the peripheral and central nervous systems The Sensory, afferent, and efferent are the PNS The Interneurons are the CNS

Fig. 37.2, p. 848 External, for example, light, movement; or internal, for example, change in blood pressure, change in body temperature Stimulus Sensory receptors of afferent neurons in external or internal organs detect stimulus. Peripheral nervous system (PNS) Reception Afferent (sensory) neurons Transmission Central nervous system (CNS) Message travels along neuron. Interneurons Integration Neural messages sorted and interpreted. Interneurons Message travels along neuron. Efferent neurons Response Neural messages from efferent neurons transmitted to effectors. Effectors Action Transmission

Fig. 37.5, p. 850 Node of Ranvier Myelin sheath of Schwann cell Cytoplasm of axon Axon of neuron Plasma membrane of axon

How do neurons “work” Step 1- Resting potential Step 2- Threshold potential Step 3- Action potential Step 4- Refractory period (hyperpolarization)

Membrane Potential of a Cell Unequal distribution of positive and negative charges on either side of the membrane Establishes a potential difference across the membrane

37.2 Signal Conduction by Neurons Resting potential is the unchanging membrane potential of an unstimulated neuron – it is approx -70 mVolts Membrane potential changes from negative to positive during an action potential Action potential is produced by ion movements through the plasma membrane (Na+ and K+)

Resting Potential of Neurons (1) Na + /K + active transport pump  Sets up concentration gradients of Na + ions (higher outside) and K + ions (higher inside) (2) Open channel (passive) allows K + to flow out freely (3) Negatively charged molecules (proteins) inside cell can’t pass through membrane (semipermeable)

Fig. 37-8, p. 853 Na + /K + pump Voltage-gated K + channel (closed) Voltage-gated Na + channel (closed) Axon plasma membrane Axon Anions (negatively charged proteins, nucleic acids, and other large molecules) that cannot pass through membrane Charged Particle Concentrations (mM) Inside Outside Na K+K+ 5 A–A– Na + out Na + K+K+ K+K+ 2 K + in A–A– A–A– A–A– A–A– A–A– A–A– A–A– A–A– A–A– A–A–

Action Potential Generated when stimulus pushes resting potential to threshold value  Voltage-gated Na + and K + channels open in the plasma membrane Inward flow of Na + changes membrane potential from negative to positive peak Potential falls to resting value as gated K + channels allow ion to flow out

Fig. 37.9, p. 853 Refractory period Peak of action potential Threshold potential Resting potential Hyperpolarization Stimulus Depolarization Repolarization Time (msec) Membrane potential (mV)

Fig a (1), p. 854 Na + channel Outside cell K + channel Activation gate Inactivation gateCytoplasm 1. A stimulus raises the membrane potential to threshold. The activation gate of the Na + channel opens. Time (msec) Membrane potential (mV) Threshold potential Resting potential K+K+ Na +

Fig a (2), p Above the threshold, more Na + channels open and Na + flows inward along its concentration gradient, raising the membrane potential toward the peak of the action potential. Time (msec) Membrane potential (mV) K+K+ Na +

Fig a (3), p As the action potential reaches its peak, the inactivation gate of the Na + channel closes and the K + channel activation gate opens, allowing K + ions to flow outward. Time (msec) Membrane potential (mV) K+K+ Na +

Fig b (1), p The outward flow of K + along its concentration gradient causes the membrane potential to begin to fall. Time (msec) Membrane potential (mV)

Fig b (3), p. 855 Time (msec) Membrane potential (mV) 6 Closure of the K + activation gate stabilizes the membrane potential at the resting value.

Summary Rest- active pump Na+ out, passive pump K+ in; more positive outside Stimulation causes active gated channel to open and Na+ goes in until… Threshold- all Na+ open quickly rises to peak Peak- Na+ gated close, K+ gated open to let K+ out Hyperpolarization- gates close and the pumps take over to reach resting potential again.

Propagation of Action Potential Action potentials are initiated by dendrite, and build in the axon hillock. Action potentials move along an axon as the ion flows generated in one segment depolarize the potential in the next segment

Fig b, p. 856 Time = 1 Adjacent area that was brought to threshold by local current flow; now active at peak of action potential New adjacent inactive area into which depolarization is spreading; will soon reach threshold Previous active area returning to resting potential; no longer active because of refractory period Remainder of axon still at resting potential – – – Membrane potential (mV)

Refractory Period Action potentials are prevented from reversing direction by a brief refractory period A segment of membrane that has just generated an action potential can’t produce another for a few milliseconds

Saltatory Conduction In myelinated axons, ions can flow across the plasma membrane only at nodes where the myelin sheath is interrupted Action potentials skip rapidly from node to node

Fig a, p. 857

Fig c, p. 857

Synapses Site where a neuron communicates with another neuron or effector Presynaptic cell  Neuron that sends a signal Postsynaptic cell  Neuron that receives a signal

2 Types of Synapses Electrical synapse  Impulses pass directly from sending cell to receiving cell Chemical synapse  Neurotransmitter released by presynaptic cell diffuses across synaptic cleft  Binds to receptors in the plasma membrane of postsynaptic cell

Neurotransmitters Released into synaptic cleft Bind to receptors in plasma membrane of postsynaptic cell Alter flow of ions across plasma membrane of postsynaptic cell  Push membrane potential toward or away from threshold potential

Types of Neurotransmitters Direct neurotransmitter  Binds to receptor associated with ligand-gated ion channel in postsynaptic membrane  Binding opens or closes the channel Indirect neurotransmitter  Binds to receptor in postsynaptic membrane  Triggers second messenger (leads to opening or closing of gated channel)

Neurotransmitter Release (1) Neurotransmitters are released from synaptic vesicles into the synaptic cleft by exocytosis Exocytosis  Triggered by entry of Ca 2+ ions into cytoplasm of axon terminal (through voltage-gated Ca 2+ channels opened by arrival of action potential)

Neurotransmitter Release (2) Neurotransmitter release stops when action potentials cease arriving at axon terminal Neurotransmitters removed from synaptic cleft  Broken down by enzymes  Taken up by axon terminal or glial cells

Types of Neurotransmitters Acetylcholine, amino acids, biogenic amines, neuropeptides, gases such as NO and CO Many biogenic amines and neuropeptides are also released into the general body circulation as hormones

Fig. 37.6a, p. 851 a. Electrical synapse Axon terminal of presynaptic cell Plasma membrane of axon terminal Plasma membrane of postsynaptic cell Gap junctions In an electrical synapse, the plasma membranes of the presynaptic and postsynaptic cells make direct contact. Ions flow through gap junctions that connect the two membranes, allowing impulses to pass directly to the postsynaptic cell.

Fig. 37.6b, p. 851 b. Chemical synapse Axon terminal of presynaptic cell Vesicle releasing neurotransmitter molecules Synaptic cleft Receptors that bind neurotransmitter molecules Plasma membrane of postsynaptic cell In a chemical synapse, the plasma membranes of the presynaptic and postsynaptic cells are separated by a narrow synaptic cleft. Neurotransmitter molecules diffuse across the cleft and bind to receptors in the plasma membrane of the postsynaptic cell. The binding opens channels to ion flow that may generate an impulse in the postsynaptic cell.

Fig , p. 861 Acetylcholine Amino acids Biogenic amines Aspartate Serotonin Glutamate GABA (gamma aminobutyric acid) Glycine Norepinephrine Neuropeptides Met-enkephalin Epinephrine Substance P Dopamine

Chemicals that affect neurotransmission Specific Serotonin Reuptake Inhibitors (SSRIs)— Fluoxetine Specific Norepinephrine Reuptake Inhibitors (SNRIs)— Reverse inward bound serotonin reuptake (MDMA) MAO inhibitors- block the breakdown of neurotransmitters (selegiline) Dopamine and serotonin antagonizers (risperidone, olanzapine) GABA enhancers- (topiramate)