Neuron organization and structure reflect function in information transfer The squid possesses extremely large nerve cells and is a good model for studying.

Name: Neuron organization and structure reflect function in information transfer The squid possesses extremely large nerve cells and is a good model for studying.
Uploaded: 2017-07-18T18:01:18+00:00
Duration: PTM32S31
Channel: Phillip Chandler
Description: Neuron organization and structure reflect function in information transfer The squid possesses extremely large nerve cells and is a good model for studying.

Neuron organization and structure reflect function in information transfer
The squid possesses extremely large nerve cells and is a good model for studying neuron function Nervous systems process information in three stages: sensory input, integration, and motor output

Nerves with giant axons Ganglia Brain Arm Eye Mantle Nerve Fig. 48-2
Figure 48.2 Overview of the squid nervous system

Sensors detect external stimuli and internal conditions and transmit information along sensory neurons Sensory information is sent to the brain or ganglia, where interneurons integrate the information Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity

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 An axon joins the cell body at the axon hillock Neurons Cell body contains nucleus Dendrites receive signals from sensory receptors Axon conducts nerve impulses Covered by myelin sheath Any long axon is also called a nerve fiber

Types of Neurons Motor Neurons Sensory Neurons Interneurons
Accept nerve impulses from the CNS Transmit them to muscles or glands Sensory Neurons Accept impulses from sensory receptors Transmit them to the CNS Interneurons Convey nerve impulses between various parts of the CNS

Dendrites Stimulus Presynaptic Nucleus cell Axon hillock Cell body
Fig. 48-4 Dendrites 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 Neurotransmitter

Synapse Synaptic terminals Postsynaptic cell Neurotransmitter
Fig. 48-4a Synapse Synaptic terminals Postsynaptic cell Neurotransmitter Figure 48.4 Neuron structure and organization For the Cell Biology Video Dendrites of a Neuron, go to Animation and Video Files.

A synapse is a junction between an axon and another cell
The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell) Most neurons are nourished or insulated by cells called glia

Cell bodies of overlapping neurons 80 µm Fig. 48-5c
Figure 48.5 Structural diversity of neurons 80 µm Cell bodies of overlapping neurons

Portion of axon Cell bodies of overlapping neurons Interneurons 80 µm
Fig. 48-5b Figure 48.5 Structural diversity of neurons Portion of axon 80 µm Cell bodies of overlapping neurons Interneurons

Dendrites Axon Cell body Sensory neuron Fig. 48-5a
Figure 48.5 Structural diversity of neurons Sensory neuron

Fig. 48-5d Figure 48.5 Structural diversity of neurons Motor neuron

Dendrites Axon Cell body Portion of axon Cell bodies of
Fig. 48-5 Dendrites Axon Cell body Portion of axon 80 µm Cell bodies of overlapping neurons Figure 48.5 Structural diversity of neurons Sensory neuron Interneurons Motor neuron

Ion pumps and ion channels maintain the resting potential of a neuron
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

Formation of the Resting Potential
In a mammalian neuron at resting potential, the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane These concentration gradients represent chemical potential energy Resting Potential The membrane potential (voltage) when the axon is not conducting an impulse The inside of a neuron is more negative than the outside -65 mV Due in part to the action of the sodium-potassium pump

The opening of ion channels in the plasma membrane converts chemical potential to electrical potential A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell Anions trapped inside the cell contribute to the negative charge within the neuron

150 mM 120 mM [K+] 140 mM [A–] 100 mM (a) OUTSIDE CELL INSIDE CELL
Fig. 48-6a OUTSIDE CELL [K+] 5 mM [Na+] 150 mM [Cl–] 120 mM INSIDE CELL [K+] 140 mM [Na+] 15 mM [Cl–] 10 mM [A–] 100 mM Figure 48.6a The basis of the membrane potential (a)

Key Na+ Sodium- potassium Potassium Sodium pump channel channel K+
Fig. 48-6b Key Na+ Sodium- potassium pump Potassium channel Sodium channel K+ OUTSIDE CELL Figure 48.6b The basis of the membrane potential INSIDE CELL (b)

[Na+] 150 mM [Cl–] 120 mM [K+] 140 mM [A–] 100 mM Fig. 48-6
Key Na+ Sodium- potassium pump Potassium channel Sodium channel K+ OUTSIDE CELL OUTSIDE CELL [K+] 5 mM [Na+] 150 mM [Cl–] 120 mM INSIDE CELL [K+] 140 mM [Na+] 15 mM [Cl–] 10 mM [A–] 100 mM Figure 48.6 The basis of the membrane potential INSIDE CELL (a) (b)

Action potentials are the signals conducted by axons
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 When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative This is hyperpolarization, an increase in magnitude of the membrane potential

Figure 48.9 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 Hyperpolarizations Depolarizations –100 –100 –100 Figure 48.9 Graded potentials and an action potential in a neuron 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)
Fig. 48-9a Stimuli +50 Membrane potential (mV) –50 Threshold Resting potential Figure 48.9a Graded potentials and an action potential in a neuron Hyperpolarizations –100 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations

Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential
For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus

Fig. 48-9b Stimuli +50 Membrane potential (mV) –50 Threshold Resting potential Figure 48.9b Graded potentials and an action potential in a neuron Depolarizations –100 1 2 3 4 5 Time (msec) (b) Graded depolarizations

Production of Action Potentials
Voltage-gated Na+ and K+ channels respond to a change in membrane potential When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open A strong stimulus results in a massive change in membrane voltage called an action potential An action potential is generated only after a stimulus larger than the threshold Gated channel proteins Suddenly allows sodium to pass through the membrane Another allows potassium to pass through other direction

Strong depolarizing stimulus
Fig. 48-9c Strong depolarizing stimulus +50 Action potential Membrane potential (mV) –50 Threshold Resting potential Figure 48.9c Graded potentials and an action potential in a neuron –100 1 2 3 4 5 6 Time (msec) (c) Action potential

Fig. 48-UN1 Action potential +50 Falling phase Rising phase Membrane potential (mV) Threshold (–55) –50 Resting potential –70 Depolarization Undershoot –100 Time (msec)

An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold
An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane Action potentials are signals that carry information along axons

A neuron can produce hundreds of action potentials per second
Fig Key Na+ K+ +50 Action potential 3 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential Depolarization –100 Time Figure The role of volt A neuron can produce hundreds of action potentials per second The frequency of action potentials can reflect the strength of a stimulus An action potential can be broken down into a series of stages age-gated ion channels in the generation of an action potential Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop 1 Resting state

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

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

Fig 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 –100 Time Figure The role of voltage-gated ion channels in the generation of an action potential Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop 1 Resting state

Fig 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 –100 Time Figure The role of voltage-gated ion channels in the generation of an action potential Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop 5 Undershoot 1 Resting state

When an action potential is generated
At resting potential Most voltage-gated Na+ and K+ channels are closed, but some K+ channels (not voltage-gated) are open When an action potential is generated Voltage-gated Na+ channels open first and Na+ flows into the cell During the rising phase, the threshold is crossed, and the membrane potential increases During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close; resting potential is restored 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

Conduction of Action Potentials
An action potential can travel long distances by regenerating itself along the axon At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards Action potentials travel in only one direction: toward the synaptic terminals

Figure 48.11 Conduction of an action potential
Axon Plasma membrane Action potential Na+ Cytosol Figure Conduction of an action potential

Axon Plasma membrane Action potential Na+ Cytosol Action potential K+ Na+ K+ Figure Conduction of an action potential

Axon Plasma membrane Action potential Na+ Cytosol Action potential K+ Na+ K+ Figure Conduction of an action potential Action potential K+ Na+ K+

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Conduction Speed The speed of an action potential increases with the axon’s diameter In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS

Figure 48.12 Schwann cells and the myelin sheath
Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nodes of Ranvier Nucleus of Schwann cell Axon Myelin sheath 0.1 µm Figure Schwann cells and the myelin sheath

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

Myelinated axon (cross section)
Fig b Figure Schwann cells and the myelin sheath Myelinated axon (cross section) 0.1 µm

Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage- gated Na+ channels are found Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

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

Neurons communicate with other cells at synapses
At electrical synapses, the electrical current flows from one neuron to another At chemical synapses, a chemical neurotransmitter carries information across the gap junction Most synapses are chemical synapses A synapse is a region where neurons nearly touch Small gap between neurons is the synaptic cleft Transmission across a synapse is carried out by neurotransmitters Sudden rise in calcium at end of one neuron Stimulates synaptic vesicles to merge with the presynaptic membrane Neurotransmitter molecules are released into the synaptic cleft

Postsynaptic neuron Synaptic terminals of presynaptic neurons 5 µm
Fig Postsynaptic neuron Synaptic terminals of presynaptic neurons 5 µm Figure Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM)

The presynaptic neuron synthesizes and packages the 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

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

Synaptic Integration A single neuron is on the receiving end of
Many excitatory signals, and Many inhibitory signals Integration

Neurotransmitters The same neurotransmitter can produce different effects in different types of cells There are five major classes of neurotransmitters: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases

Table 48-1a Table 48.1

Table 48-1b Table 48.1

Neuron organization and structure reflect function in information transfer The squid possesses extremely large nerve cells and is a good model for studying.

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Neuron organization and structure reflect function in information transfer The squid possesses extremely large nerve cells and is a good model for studying.

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Presentation on theme: "Neuron organization and structure reflect function in information transfer The squid possesses extremely large nerve cells and is a good model for studying."— Presentation transcript:

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