Chapter 44: Neurons and Nervous Systems CHAPTER 44 Neurons and Nervous Systems

Chapter 44: Neurons and Nervous Systems

CHAPTER 44Neurons and Nervous

Systems


Chapter 44: Neurons and Nervous SystemsNervous Systems: Cells and FunctionsNervous Systems: Cells and Functions

Neurons: Generating and Conducting NeNeurons: Generating and Conducting Nerve Impulsesrve Impulses

Neurons, Synapses, and CommunicationNeurons, Synapses, and Communication

Neurons in NetworksNeurons in Networks


Nervous Systems: Cells and Functions• Nervous systems consist of cells that Nervous systems consist of cells that

process and transmit information.process and transmit information.

33


Nervous Systems: Cells and Functions • Sensory cells transduce information Sensory cells transduce information

from the environment and body.from the environment and body.• This communicates commands to This communicates commands to

effectors such as muscles or glands.effectors such as muscles or glands.

44


Nervous Systems: Cells and Functions• The nervous systems of different The nervous systems of different

species vary, but all are composed of species vary, but all are composed of cells called neurons. cells called neurons.

Review Figures 44.1, 44.2Review Figures 44.1, 44.2

55


Figure 44.1

Figure 44.1Figure 44.1

figure 44-01.jpg


Nervous Systems: Cells and Functions• In vertebrates, brain and spinal cord In vertebrates, brain and spinal cord

form the central nervous system.form the central nervous system.

• They communicate with other body They communicate with other body tissues via the peripheral nervous tissues via the peripheral nervous system.system.

88


Figure 44.2


figure 44-02.jpg


Nervous Systems: Cells and Functions

• Neurons receive information mostly Neurons receive information mostly via their dendrites and transmit via their dendrites and transmit information over their axons. information over their axons.

• They function in networks. They function in networks.

Review Figure 44.3Review Figure 44.3

99


Figure 44.3 – Part 1

Figure 44.3 – Part 1Figure 44.3 – Part 1

figure 44-03a.jpg




figure 44-03b.jpg


Nervous Systems: Cells and Functions

• Information that neurons process is in Information that neurons process is in the form of electrical events in their the form of electrical events in their plasma membranes. plasma membranes.

• Where neurons and other cells meet, Where neurons and other cells meet, information is transmitted mostly by information is transmitted mostly by release of chemical signals called release of chemical signals called neurotransmitters.neurotransmitters.

1212


Nervous Systems: Cells and Functions• Glial cells physically support neurons and Glial cells physically support neurons and

perform many housekeeping functions. perform many housekeeping functions.

• Schwann cells and oligodendrocytes Schwann cells and oligodendrocytes produce myelin, which insulates neurons. produce myelin, which insulates neurons.

• Astrocytes create the blood–brain barrier. Astrocytes create the blood–brain barrier.


1313


Figure 44.4


figure 44-04.jpg


Neurons: Generating and Conducting Nerve Impulses• Neurons have an electric charge Neurons have an electric charge

difference across their plasma difference across their plasma membranes. membranes.

• This resting potential is created by ion This resting potential is created by ion pumps and channels. pumps and channels.


1515


Figure 44.5


figure 44-05.jpg


Neurons: Generating and Conducting Nerve Impulses • The sodium–potassium pump The sodium–potassium pump

concentrates Kconcentrates K++ ions on the insides and ions on the insides and NaNa++ ions on the outsides of neurons. ions on the outsides of neurons.

• Ion channels allow KIon channels allow K++ ions to leak out, ions to leak out, leaving behind unbalanced negative leaving behind unbalanced negative charges, leading to the resting potential. charges, leading to the resting potential.


1717


Figure 44.6


figure 44-06.jpg


Figure 44.7


figure 44-07.jpg


Neurons: Generating and Conducting Nerve Impulses• A potassium equilibrium potential A potassium equilibrium potential

exists when an electric charge that exists when an electric charge that develops across the membrane is develops across the membrane is sufficient to prevent net diffusion of sufficient to prevent net diffusion of potassium ions down their potassium ions down their concentration gradient. concentration gradient.

• This potential can be calculated with This potential can be calculated with the Nernst equation. the Nernst equation.



Figure 44.8


figure 44-08.jpg


Neurons: Generating and Conducting Nerve Impulses• The resting potential is perturbed The resting potential is perturbed

when ion channels open or close, thus when ion channels open or close, thus changing plasma membrane changing plasma membrane permeability to charged ions. permeability to charged ions.

• Thus, neurons become depolarized or Thus, neurons become depolarized or hyperpolarized in response to stimuli. hyperpolarized in response to stimuli.


2222




figure 44-09a.jpg




figure 44-09b.jpg


Neurons: Generating and Conducting Nerve Impulses• Rapid reversals in charge across Rapid reversals in charge across

portions of the plasma membrane, portions of the plasma membrane, resulting from opening and closing of resulting from opening and closing of voltage-gated sodium and potassium voltage-gated sodium and potassium channels, produce action potentials. channels, produce action potentials.

• These changes occur when the plasma These changes occur when the plasma membrane depolarizes to a threshold membrane depolarizes to a threshold level. level.



Figure 44.10


figure 44-10.jpg


Neurons: Generating and Conducting Nerve Impulses• Action potentials are conducted down Action potentials are conducted down

axons because of local current flowaxons because of local current flow

• This depolarizes adjacent regions of This depolarizes adjacent regions of membrane and brings them to membrane and brings them to threshold for the opening of voltage-threshold for the opening of voltage-gated sodium channels. gated sodium channels.


2727




figure 44-11a.jpg




figure 44-11b.jpg




figure 44-11c.jpg


Neurons: Generating and Conducting Nerve Impulses• Patch clamping allows us to study Patch clamping allows us to study

single ion channels. single ion channels.


3131


Figure 44.12


figure 44-12.jpg


Neurons: Generating and Conducting Nerve Impulses• In myelinated axons, the action In myelinated axons, the action

potentials appear to jump between potentials appear to jump between nodes of Ranvier, patches of plasma nodes of Ranvier, patches of plasma membrane not covered by myelin. membrane not covered by myelin.


3333




figure 44-13a.jpg




figure 44-13b.jpg


Neurons, Synapses, and Communication

• Neurons communicate with each other Neurons communicate with each other and other cells at specialized junctions and other cells at specialized junctions called synapses, where plasma called synapses, where plasma membranes of two cells come close membranes of two cells come close together.together.

3636


Neurons, Synapses, and Communication • The classic chemical synapse is the The classic chemical synapse is the

neuromuscular junction, a synapse neuromuscular junction, a synapse between a motor neuron and muscle cell. between a motor neuron and muscle cell.

• Its neurotransmitter is acetylcholine, Its neurotransmitter is acetylcholine, which causes a depolarization of the which causes a depolarization of the postsynaptic membrane when it binds to postsynaptic membrane when it binds to its receptor. its receptor.


3737


Figure 44.14


figure 44-14.jpg


Neurons, Synapses, and Communication• When an action potential reaches an When an action potential reaches an

axon terminal of the presynaptic cell, it axon terminal of the presynaptic cell, it causes the release of neurotransmitters. causes the release of neurotransmitters.

• These chemical signals diffuse across These chemical signals diffuse across the synaptic cleft and bind to receptors the synaptic cleft and bind to receptors on the postsynaptic membrane. on the postsynaptic membrane.


3939


Figure 44.15


figure 44-15.jpg


Figure 44.16


figure 44-16.jpg


Neurons, Synapses, and Communication• Synapses between neurons are either Synapses between neurons are either

excitatory or inhibitory. excitatory or inhibitory.

• Excitatory responses are caused by Excitatory responses are caused by membrane depolarization. membrane depolarization.

• Inhibitory responses are caused by Inhibitory responses are caused by hyperpolarization of membranes.hyperpolarization of membranes.

4242



• A postsynaptic neuron integrates A postsynaptic neuron integrates information by summing its synaptic information by summing its synaptic inputs in space and time. inputs in space and time.


4343




figure 44-17a.jpg




figure 44-17b.jpg


Neurons, Synapses, and Communication• Ionotropic neurotransmitter receptors are ion Ionotropic neurotransmitter receptors are ion

channels. channels. • Metabotropic receptors influence the Metabotropic receptors influence the

postsynaptic cell through various signal postsynaptic cell through various signal transduction pathways that involve G transduction pathways that involve G proteins. proteins.

• These pathways can result in changes in ion These pathways can result in changes in ion channels, alterations of enzyme activity, and channels, alterations of enzyme activity, and gene expression. gene expression.

• Actions of ionotropic synapses are generally Actions of ionotropic synapses are generally faster than those of metabotropic synapses. faster than those of metabotropic synapses.


4646


Figure 44.18


figure 44-18.jpg



• Electrical synapses pass electric Electrical synapses pass electric signals between cells without the use signals between cells without the use of neurotransmitters. of neurotransmitters.

• Connexons make physical contact Connexons make physical contact between the cells.between the cells.

4848



• There are many different There are many different neurotransmitters and even more neurotransmitters and even more receptors. receptors.

• The action of a neurotransmitter The action of a neurotransmitter depends on the receptor to which it depends on the receptor to which it binds. binds.

Review Table 44.1Review Table 44.1

4949


Table 44.1 – Part 1

Table 44.1 – Part 1Table 44.1 – Part 1

table 44-01a.jpg


Table 44.1 – Part 2

Table 44.1 – Part 2Table 44.1 – Part 2

table 44-01b.jpg



• Glutamate binds to ionotropic and Glutamate binds to ionotropic and metabotropic receptors, and may be metabotropic receptors, and may be involved in learning and memory. involved in learning and memory.


5252


Figure 44.19


figure 44-19.jpg


Neurons, Synapses, and Communication• With repeated stimulation, a neuron With repeated stimulation, a neuron

can become more sensitive to its can become more sensitive to its inputs. inputs.

• Since this increased sensitivity can last Since this increased sensitivity can last a long time, it is called long-term a long time, it is called long-term potentiation, or LTP.potentiation, or LTP.

• Properties of the NMDA glutamate Properties of the NMDA glutamate receptor appear to explain LTP. receptor appear to explain LTP.


5454


Figure 44.20


figure 44-20.jpg



• In chemical synapses, the transmitter In chemical synapses, the transmitter must be cleared rapidly from the must be cleared rapidly from the synapse. synapse.

• Some poisons and drugs block or slow Some poisons and drugs block or slow the clearance of transmitter from the the clearance of transmitter from the synapse.synapse.

5656


Neurons in Networks

• Neurons work together in networks to Neurons work together in networks to accomplish specific tasks. accomplish specific tasks.

• The networks use all of the The networks use all of the mechanisms we have discussed in this mechanisms we have discussed in this chapter.chapter.

5757

Documents

Chapter 44: Neurons and Nervous Systems CHAPTER 44 Neurons and Nervous Systems