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Neurons and Nervous

Systems

34

Concept 34.1 Nervous Systems Consist of Neurons and Glia

Nervous systems have two categories of cells:

Neurons, or nerve cells, are excitable—they

generate and transmit electrical signals, called

action potentials.

Glia, or glial cells, provide support and maintain

extracellular environment.


Most neurons have four regions:

• Cell body—contains nucleus and

organelles

• Dendrites— carries signals, called nerve

impulses or action potentials, to the cell

body

• Axon—generates action potentials and

conducts them away from the cell body

• Axon terminal—synapse at tip of axon;

releases neurotransmitters


Neurons pass information at synapses:

• The presynaptic neuron sends the

message

• The postsynaptic neuron receives the

message

Figure 34.1 A Generalized Neuron


Glial cells, or glia, outnumber neurons in the

human brain.

• Glia do not transmit electrical signals but can

release neurotransmitters.

• Glia also give support during development,

supply nutrients, remove debris, and maintain

extracellular environment.

• Important in neuroplasticity—synapse

modification


Astrocytes are glia that contribute to the

blood–brain barrier, which protects the brain.

The blood-brain barrier is permeable to fat-

soluble compounds like alcohol and

anesthetics.

Microglia provide the brain with immune

defenses since antibodies cannot enter the

brain.


Oligodendrocytes are glia that insulate axons

in the brain and spinal cord.

Schwann cells insulate axons in nerves outside

of these areas.

The glial membranes form a nonconductive

sheath—myelin.

Myelin-coated axons are white matter and areas

of cell bodies are gray matter.

Multiple sclerosis is a demyelinating disease.

Figure 34.2 Wrapping Up an Axon (Part 1)

Figure 34.2 Wrapping Up an Axon (Part 2)


Neurons are organized into neural networks.

Afferent neurons carry sensory information

into the nervous system from sensory cells

that convert stimuli into action potentials.

Efferent neurons carry commands to effectors

such as muscles, glands—motor neurons are

effectors that carry commands to muscles.

Interneurons store information and

communicate between neurons.


Networks vary in complexity.

Nerve net—simple network of neurons

Ganglia—neurons organized into clusters,

sometimes in pairs, in simple animals

Brain—the largest pair of ganglia, found in

animals with complex behavior requiring more

information-processing

Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 1)

Concept 34.2 Neurons Generate and Transmit Electrical Signals

Neurons generate changes in membrane

potential—the difference in electrical charge

across the membrane.

These changes generate nerve impulses, or

action potentials.

An action potential is a rapid, large change in

membrane potential that travels along an axon

and causes release of chemical signals.


Voltage is a measure of the difference in

electrical charge between two points.

Electrical current in solution is carried by ions.

Major ions in neurons:

• Sodium (Na+) • Potassium (K+)

• Calcium (Ca2+) • Chloride (Cl–)

Different concentrations and charges inside and

out produce the membrane potential.


Membrane potentials can be measured in all

cells with electrodes.

Resting potential is the membrane

potential of a resting, or inactive, neuron.

The resting potential of a membrane is

between –60 and –70 millivolts (mV).

The inside of the cell is negative at rest. An

action potential allows positive ions to flow

in briefly, making the inside of the cell

more positive.

Figure 34.4 Measuring the Membrane Potential (Part 1)

Figure 34.4 Measuring the Membrane Potential (Part 2)


Ion channels and ion transporters in the

membrane create the resting and action

potentials.

Sodium–potassium pump—moves Na+

ions from inside, exchanges for K+ from

outside—establishes concentration

gradients

The Na+–K+ pump is an antiporter, or

sodium–potassium ATPase, as it requires

ATP.

Figure 34.5 Ion Transporters and Channels (Part 1)


Potassium channels are open in the resting membrane and are highly permeable to K+

ions—allow leak currents

K+ ions diffuse out of the cell along the concentration gradient and leave behind negative charges within the cell.

K+ ions diffuse back into the cell because of the negative electrical potential.

These two forces acting on K+ are its electrochemical gradient.

Figure 34.5 Ion Transporters and Channels (Part 2)


The equilibrium potential is the membrane

potential at which the net movement of an

ion ceases.

The Nernst equation calculates the value

of the equilibrium potential by measuring

the concentrations of an ion on both sides

of the membrane.


Some ion channels are “gated”—open and

close under certain conditions:

• Voltage-gated channels respond to

change in voltage across membrane

• Chemically-gated channels depend on

molecules that bind or alter channel

protein

• Mechanically-gated channels respond to

force applied to membrane


Gating provides a means for neurons to

change their membrane potentials in

response to a stimulus.

The membrane is depolarized when Na+

enters the cell and the inside of the neuron

becomes less negative.

If gated K+ channels open and K+ leaves,

the cell becomes more negative inside and

the membrane is hyperpolarized.

Figure 34.6 Membranes Can Be Depolarized or Hyperpolarized


Graded membrane potentials are changes

from the resting potential.

Graded potentials are a means of

integrating input—the membrane can

respond proportionally to depolarization or

hyperpolarization.


Voltage-gated Na+ and K+ channels are

responsible for action potentials—sudden,

large changes in membrane potential.

At rest most of these channels are closed.

Local depolarization by gated channels in

dendrites produces a graded potential.

It spreads to the axon hillock, where Na+

voltage-gated channels are concentrated.


The membrane in the axon hillock may

reach its threshold—5 to 10 mV above

resting potential.

Many voltage-gated Na+ channels

(activation gates) open quickly and Na+

rushes into the axon.

The influx of positive ions causes more

depolarization, the membrane potential is

briefly positive, and an action potential

occurs.


The axon quickly returns to resting potential due to two things:

• Voltage-gated K+ channels open slowly and stay open longer—K+ moves out

• Voltage-gated Na+ channels (inactivation gates) close

Voltage-gated Na+ channels cannot open again during the refractory period—a few milliseconds.

Figure 34.7 The Course of an Action Potential (Part 1)

Figure 34.7 The Course of an Action Potential (Part 2)


An action potential is an all-or-none event—

positive feedback to voltage-gated Na+

channels ensures the maximum action

potential.

An action potential is self-regenerating

because it spreads to adjacent membrane

regions.


Axon diameter and myelination by glial cells

increase the speed of action potentials in

axons.

The nodes of Ranvier are regularly spaced

gaps where the axon is not covered by

myelin.

Action potentials are generated at the nodes

and the positive current flows down the

inside of the axon.


When positive current reaches the next

node, the membrane is depolarized—

another axon potential is generated.

Action potentials appear to jump from node

to node, a form of propagation called

saltatory conduction.

Figure 34.8 Saltatory Action Potentials (Part 1)

Figure 34.8 Saltatory Action Potentials (Part 2)

Concept 34.3 Neurons Communicate with Other Cells at

Synapses

Neurons communicate with other neurons or

target cells at synapses.

In a chemical synapse neurotransmitters

from a presynaptic cell bind to receptors in

a postsynaptic cell.

The synaptic cleft—about 25 nanometers

wide—separates the cells.


Synapses

In an electrical synapse, cells are joined

through gap junctions.

Gap junctions are made of proteins

(connexins) that create channels.

Ions flow through the channels—the action

potential spreads through the cytoplasm.

These action potentials are fast but do not

allow for complex integration of inputs.


Synapses

The neuromuscular junction is a chemical

synapse between motor neurons and

skeletal muscle cells.

An action potential causes voltage-gated

Ca+ channels to open in the presynaptic

membrane, allowing Ca+ to flow in.

The presynaptic neuron releases

acetylcholine (ACh) from its axon terminals

(boutons) when vesicles fuse with the

membrane.

Figure 34.9 Chemical Synaptic Transmission


Synapses

The postsynaptic membrane of the muscle

cell is the motor end plate.

ACh diffuses across the cleft and binds to

ACh receptors on the motor end plate.

These receptors allow Na+ and K+ to flow

through, and the increase in Na+

depolarizes the membrane.

If it reaches threshold, more Na+ voltage-

gated channels are activated and an

action potential is generated.

Figure 34.10 Chemically Gated Channels


Synapses

The postsynaptic cell must sum the excitatory and inhibitory input.

Summation occurs at the axon hillock, the part of the cell body at the base of the axon.

Spatial summation adds up messages at different synaptic sites.

Temporal summation adds up potentials generated at the same site, over time.


Synapses

Neurotransmitters are cleared from the cleft after release in order to stop their action in several ways:

• Diffusion

• Reuptake by adjacent cells

• Enzymes present in the cleft may destroy them

Example: Acetylcholinesterase acts on ACh.


Synapses

There are many types of neurotransmitters, and each may have multiple receptor subtypes.

For example, ACh has two:

• Nicotinic receptors are ionotropic and mainly excitatory

• Muscarinic receptors are metabotropic and mainly inhibitory

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

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