Nervous System: Neurons - Napa Valley College Pages 8... · Parts of a Neuron Cell body – the...

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Nervous System:

Neurons

Biology 105

Lecture 8

Chapter 7

Copyright © 2009 Pearson Education, Inc.

Outline

I. Nervous system overview and function

II. Central and peripheral nervous systems

III. Nervous system cells

IV. Myelinated neurons

V. Neuronal signal transmission

VI. Neuronal synapse


Nervous System

Includes nervous tissue and sensory organs

Nervous system functions to conduct

messages throughout the body:

Sense the environment – receives information

from both outside and inside the body.

Process the information it receives.

Respond to information – sends out orders.

When a neuron is stimulated, an electrical

signal quickly travels through the neuron,

triggering events.

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Two Parts of the Nervous System

1. Central Nervous System (CNS)

Brain and spinal cord

2. Peripheral Nervous System (PNS)

Nervous tissue outside brain and spine

Sensory organs


Central Nervous System

Peripheral


Nervous System Cells

Two types of cells found in nervous tissue:

Neurons – the cells that are responsible

for transmitting messages.

Glial cells – cells that support the

neurons.

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Glial Cells

Microglia – immune system cells, engulf

bacteria and cellular debris

Astrocytes – provide nutrients to neurons,

maintain balance of the extracellular

environment

Oligodendrocytes

and Schwann cells –

form myelin sheaths

Figure 4.6


Parts of a Neuron

Cell body – the main body of the cell.

Contains the nucleus.

Dendrites – many projections from the cell

body that carry messages to the cell body.

Axons – one large projection that carries

messages away from the cell body.

Copyright © 2009 Pearson Education, Inc. Figure 7.2

The cell body

integrates input

from other neurons.

Dendrites receive

information from

other neurons or

from the environment.

The cell body controls

the cell’s metabolic

activities.

An axon conducts the

nerve impulse away

from the cell body.

Axon endings release

chemicals called

neurotransmitters that

affect the activity of

nearby neurons or an

effector (muscle or gland).

Receiving portion of

neuron

Sending portion of neuron

Cell

body

Axon

endings

Nucleus

Neuron!

DIRECTION OF NEURONAL SIGNAL

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Neurons of the Peripheral Nervous System

Neurons in the PNS are either carrying

messages to or from the CNS.

Afferent = sensory neurons = neurons carrying

messages TO the CNS.

Arriving in the CNS.

Efferent = motor neurons = neurons carrying

messages FROM the CNS.

Exiting from the CNS.


Interneurons in the Central Nervous System

Located between sensory and motor neurons

within the CNS.

Interneurons integrate and interpret sensory

signals.

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Sensory Neurons

The afferent or sensory neuron cell bodies are

located in dorsal root ganglion.

Motor Neurons

The efferent or motor neuron cell bodies are

located in the gray matter of the spinal cord.

Their axons leave the CNS and go to the

skeletal muscles.


Neurons of the Nervous System


Q: The cell bodies of these neurons are located in the

dorsal root ganglion:

1. Motor

2. Sensory

Q: These glial cells provide nutrients to neurons:

1. Microglia

2. Astrocytes

3. Oligodendrocytes

4. Schwann cells

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Q: These are projections of the neuron cell body that

carry messages to the cell body:

1. Axons

2. Dendrites

Q: Which of the following type of neuron would alert

the brain that you had touched a hot object?

1. Efferent neuron

2. Afferent neuron


Q: What type of neuron is the arrow pointing to?

1. Sensory

2. Motor


Myelinated Neurons

Some glial cells form a substance called myelin

that is rich in lipid and contains proteins.

These glial cells wrap themselves around

neuronal axons.

The neurons that have axons covered with these

glial cells are called myelinated neurons.

Main function: myelinated neurons are able to

carry messages faster than non-myelinated

neurons.

Myelin is an excellent electrical insulator!

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Myelin Sheath

Figure 7.3b


Myelin Sheath

Figure 7.3c


Two Types of Cells Myelinate Neurons

Schwann cells are found in the PNS.

Oligodendrocytes are found in the CNS.

Schwann cells and oligodendrocytes both wrap

around neuronal axons.

Myelin sheaths from Schwann cells also help

regenerate injured PNS neuronal axons.

Nodes of Ranvier are spaces on the axon

between the glial cells.

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Myelinated Neurons

Figure 7.3a

(a)

Cell

body

Dendrites

Myelin sheath

Node of

Ranvier

Nucleus

Schwann cell

In saltatory conduction, the

nerve impulses jump from one

node of Ranvier to the next.


Multiple Sclerosis (MS)

Caused by the destruction of the myelin sheath

that surrounds axons found in the CNS.

Can result in paralysis and loss of sensation,

including loss of vision.


Nerve

A nerve contains many neuronal axons

bundled together.

These bundles contain:

Axons

Blood vessels

Connective tissue

Figure 8.9d

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Q: An ion is an atom that has gained or lost a(n):

1. Neutron

2. Proton

3. Electron

Q: How can an ion pass through a membrane?

1. Simple diffusion

2. Facilitated diffusion

3. Active transport

4. Both 2 and 3

5. All of the above


The Neuronal Impulse is an Electrochemical

Signal

A neuronal impulse, or action potential,

involves sodium ions (Na+) and potassium ions

(K+) that cross the cell membrane through ion

channels.

Each ion channel is designed to allow only

certain ions to pass through.


Action Potential

Figure 7.4

Extracellular

fluid

Neuron plasma

membrane

Cytoplasm

Sodium-potassium pump

The sodium-potassium pump uses cellular energy (ATP) to

pump sodium ions out of the

cell and potassium ions into the cell

Continually open ion channels “Gated” ion channels Sodium-potassium pump

Ion channels

Ion channels can be open continuously or opened and closed by a molecular gate

Cross section

Axon membrane

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The difference in charge between the inside

and outside of the neuron is the membrane

potential.

Membrane Potential

A neuron that is not conducting a message is

said to be “resting”.

When a neuron is resting, there is more

sodium (Na+) outside the neuron cell and more

potassium (K+) inside the cell.

The inside of the cell has a negative charge

compared to the outside the cell.

Resting Membrane Potential


Resting Membrane Potential


The Neuronal Impulse

Figure 7.5 (1 of 4)

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Sodium-Potassium Pump

To maintain this resting membrane potential the

neuron pumps Na+ out of the cell and K+ into

the cell.

The transport proteins (= Na+/K+ pumps) move

3 Na+ ions out for every 2 K+ ions into the cell.

This is Active Transport and requires ATP!


Action Potential

Action Potential – an electrochemical signal

conducted along an axon.

An action potential is a wave of depolarization

followed by repolarization.

Depolarization is caused by sodium ions

entering the axon.

Repolarization is caused by potassium ions

leaving the axon.


Steps of an Action Potential

1. The axon is depolarized when voltage-gated

sodium ion channels open and Na+ comes

rushing in.

This causes the inside of the neuron to

become positively charged.

Figure 7.5 (2 of 4)

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Steps of an Action Potential

2. The axon is repolarized when voltage-gated

potassium ion channels open and allow K+ to

leave the axon.

This returns the membrane potential to

negative on the inside of the neuron.

3. The action potential travels down the axon.


Action Potential

Figure 7.5 (3 of 4)


Action Potential

After the action potential, the sodium-

potassium pump restores the original

conditions by pumping sodium (Na+) out of

the cell and potassium (K+) back into the cell.

Figure 7.5 (4 of 4)

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The Neuronal Impulse

Figure 7.6


Action Potentials

An action potential is an all-or-nothing

response:

If there is not enough stimulation, the ion

channels will not open and there will not be an

action potential.

The level of the action potential is always the

same.

An action potential always moves in the same

direction down the axon.

The sodium channels are inactivated for awhile

after the action potential passes = refractory

period.


Q: When a neuron is resting, sodium ions have a

greater concentration:

1. Inside the neuron cell

2. Outside the neuron cell

3. Concentration is the same

Q: When a neuron is depolarizing, which ions come

into the neuron?

1. Calcium (Ca2+)

2. Sodium (Na+)

3. Potassium (K+)

4. Chlorine (Cl-)

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Neuronal Synapse

How are messages passed from one neuron

to the next, or from a neuron to a muscle?

The junction between two neurons or

between a neuron and a muscle is called a

synapse.


Components of the Synapse

1. Presynaptic neuron is the transmitting

neuron.

It has synaptic vesicles that contain

neurotransmitters.

2. Postsynaptic neuron is the receiving

neuron or the muscle.

3. And the gap in between them = synaptic

cleft.


Synaptic Transmission

Figure 7.8 (1 of 3)

Nucleus

Impulse

Synaptic

knob

Axon

Dendrites

Cell

body

Synaptic

cleft

Synaptic

vesicle

Impulse

Membrane of

postsynaptic neuron

Step 1: The impulse reaches

the axon ending of the

presynaptic membrane.

Step 2: Synaptic

vesicles release

neurotransmitter

into the synaptic

cleft.

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Figure 7.8 (2 of 3)

Neurotransmitter

Receptor (of sodium ion

channel) on postsynaptic membrane

Step 3: Neurotransmitter

diffuses across synaptic cleft.

Synaptic

vesicle



Figure 7.8 (3 of 3)

Step 5: Sodium ion channels open.

Step 4: Neurotransmitter molecules bind to receptors on the postsynaptic neuron.

Step 6: Sodium ions enter the postsynaptic neuron, causing depolarization and possible action potential.


1. The action potential gets to the end of the presynaptic axon.

2. The action potential triggers Ca2+ to enter the presynaptic axon terminal.

3. The Ca2+ triggers synaptic vesicles located at the axon terminal to merge with the neural membrane.

Transmission Across Synaptic Cleft

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4. The synaptic vesicles release the neurotransmitters into the synaptic cleft.

5. These neurotransmitters travel across the synaptic cleft to the postsynaptic neuron (or the muscle).

6. Neurotransmitter binds to receptors on the postsynaptic neuron (or muscle).




7. These receptors are ligand-gated sodium

ion channels which allow Na+ to enter the

postsynaptic neuron (or muscle) and

triggers an action potential in the

postsynaptic neuron (or muscle

contraction).

8. Once the neurotransmitters are released,

they need to be destroyed or contained

quickly or they will continue to stimulate

the nerve.


Neurotransmitters

Acetylcholine

Acts in both the PNS and the CNS as a

neurotransmitter.

Causes voluntary muscles to contract.

Acetylcholinesterase: an enzyme that breaks

down excess acetylcholine in the synaptic cleft.

Myasthenia gravis is an autoimmune disease

that attacks the acetylcholine receptors,

resulting in reduced muscle strength.

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Important Concepts

Read Chapter 7

What are the functions of nervous system?

What are the two types of cells in nervous

tissue? (Neuroglial cells and neurons!)

What are the three types of neuroglial cells and

what are their functions?

What are the two main divisions of the nervous

system (CNS, PNS) and where are they each

found?


Important Concepts

What are the parts and functions of a neuron?

What are the three types of neurons (sensory,

interneuron, and motor neuron) and their

functions, and where are they located?

Where are the cell bodies located for motor and

sensory neuronal cells?

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What are Schwann cells and oligodendrocytes,

and what are their functions?

Where are Schwann cells and oligodendrocytes

found?

What is the cause and what are the effects of

multiple sclerosis?

What are the parts of a nerve?

Important Concepts


How do ions pass through membranes?

What is the function of the sodium-potassium

pump?

What are the steps of signal transmission

through the nervous system, starting with the

resting stage of one neuron and ending with

the next neuron or muscle being stimulated?

Important Concepts


Which ions enter and exit the neuron during the

depolarization and repolarization steps of an

action potential?

What is the relative charge of the inside versus

the outside of the neuron during these events,

and what is the correct order of these events?

What are the components of a synapse?

What is the function of neurotransmitters? How

do they work and where do they work?

Important Concepts

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What is acetylcholine, where is it found, what

effect does it have, and how is acetylcholine

removed from the synaptic cleft?

What is the cause and effect of Myasthenia

gravis?

Important Concepts


Definitions

Afferent neuron, efferent neuron, dendrite, axon, sensory neuron, interneuron, motor neuron, myelin, myelin sheath, myelinated neuron, Schwann cell, oligodendrocytes, nodes of Ranvier, nerve, ion, ion channel, ligand-gated ion channel, voltage-gated ion channel, action potential, repolarization, depolarization, membrane potential, resting potential, sodium-potassium pump, refractory period, synapse, synaptic cleft, synaptic vesicle, neurotransmitter, acetylcholinesterase, presynaptic neuron, postsynaptic neuron, stimulate, inhibit

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