Nervous System - Neurons...Neurons of the Peripheral Nervous System Neurons in the PNS are either carrying messages to or from the CNS Afferent = Sensory neurons = Neurons carrying

Nervous System - Neurons

Biol 105

Lecture 9

Chapter 7

Copyright © 2009 Pearson Education, Inc.

Outline

I. Nervous system function

II. Central and peripheral nervous system

III. Nervous system cells

IV. Myelinated neurons

V. Nerve signal transmission

VI. Nerve Synapse


Nervous Tissues

Nervous tissue functions to conduct

messages throughout the body.

When nerve cells are stimulated, an electrical

signal quickly travels through the nerve cell to

the nerve ending, triggering events


Nervous System

Includes nerve tissue and sense organs

Nervous system functions to:

Senses environment – receives

information from both outside and inside

the body

Processes the information it receives

Respond to information – sends out

orders


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

Sense organs


Central Nervous System

Peripheral


Nervous System Cells

Two types of nervous tissue cells

Neurons – the cells that are responsible for

transmitting messages

Neuroglial cells – cells that support the

neurons


Neuroglial cells

Microglia – immune system cells, engulf

bacteria and cellular debris

Astrocytes – provide nutrients to neurons

Oligodenrocytes and Schwann cells – form

myelin sheaths


Figure 4.6


Parts of a Neuron

Cell body – contains the nucleus, main body

of cell

Dendrites – projections from the cell body

that carry messages to the cell body

Axons – one large projection that carry

messages away from the cell body


Neuron


Neurons Have Dendrites, a Cell Body, and an

Axon

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


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

Efferent = Motor neurons = Neurons carrying

messages from the CNS


Interneurons in the Central Nervous System

Located between sensory and motor

neurons within the CNS

Interneurons integrate and interpret

sensory signals


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


The cell bodies of these neurons are located in the dorsal

root ganglion

1. Motor

2. Sensory


Neurons of the Nervous System

Figure 7.1

Interneuron

Sensory

receptor

for pain

Muscle

(effector)

Motor

neuron

Sensory

neuronCell

body

Impulse direction


These neuroglial cells provide nutrients to neurons

1. Microglia

2. Astrocytes

3. Oligodenrocytes

4. Schwann cells


These are projections of the neuron cell body that carry

messages to the cell body

1. Axons

2. Dendrites


Which of the following type of neuron would alert the brain

that you had touched a hot object?

1. efferent neuron

2. afferent neuron


What type of neuron is the arrow pointing to?

1. Sensory

2. Motor


Myelinated neurons

Neurons that have axons covered with

neuroglial cells that contain the protein myelin

are called myelinated neurons

Myelinated neurons are able to carry messages

faster than non-myelinated neurons


Functions of Myelin Sheaths

1. The main benefit of myelin sheaths is that

myelinated neurons are able to carry

messages faster than non-myelinated neurons

2. Myelin sheaths from Schwann cells also help

regenerate injured PNS neuron axons


Two Types of Cells that Myelinate neurons

Schwann cells and Oligodenrocytes are

wrapped around neuronal axons


Myelinated neurons

Schwann cells are found in the PNS

Oligodendrocytes are found in the CNS

Nodes of Ranvier are spaces on the axon

between the glial cells


Myelinated Neurons

http://www.youtube.com/watch?v=mOgHC5G8LuI


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.


Myelin Sheath

Figure 7.3b


Myelin Sheath

Figure 7.3c


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

Nerve contain Neuron axons are bundled

together

These bundles contain

Axons

Blood vessels

Connective tissue


Nerve

Figure 8.9d

(d) The anatomy of a nerve

Blood supply

Axons within a connective

tissue sheath

One axon

Connective tissue surrounding one nerve


An Ion is an atom that has gained or lost a

1. Neutron

2. Proton

3. Electron


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 Nerve Impulse Is an Electrochemical

Signal

A nerve impulse, or action potential, involves

sodium ions (Na+) and potassium ions (K+)

that cross the cell membrane through the 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


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 Nerve Impulse

Figure 7.5 (1 of 4)


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 take 3 Na+ ions out for

every 2 K+ ions into the cell = Na+/K+ pump

This is Active Transport – requiring ATP


Action Potential

An electrochemical signal conducted along

an axon. It is a wave of depolarization

followed by repolarization

Depolarization is caused by sodium ions

entering the axon

Repolarization is caused by potassium

ions leaving axon


Steps of an Action Potential

1. The axon is depolarized when voltage gated

sodium ion channels open and Na+ comes

rushing in, causing the inside of the neuron to

become positively charged


Action Potential

Figure 7.5 (2 of 4)


Steps of an Action Potential

2. The axon is repolarized when voltage gated

potassium ion channels open up and allow K+

to go out of the axon

This returns the membrane potential to be

negative on the inside of the neuron

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


The Nerve Impulse

Figure 7.5 (4 of 4)


The Nerve Impulse

Figure 7.6

Animation—The Nerve ImpulsePLAY

action_potential.htm


Action Potentials

It is an all or nothing response – if it is not

a great enough stimulation the channels

won’t open. The level of the action

potential is always the same.

The direction is always one way down the

axon. The sodium channels are

inactivated for awhile after the action

potential passes = refractory period.


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


When a neuron is depolarizing, which ions come into the

neuron?

1. Calcium (Ca++)

2. Sodium (Na+)

3. Potassium (K+)

4. Chlorine (Cl-)


When a neuron is depolarizing, the inside of the neuron cell

becomes

1. Positively charged

2. Negatively charged


Nerve Synapse

How are messages passed from one nerve to

the next or from the nerve 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

2. postsynaptic neuron is the receiving neuron

or the muscle

3. And the gap in between them = synaptic

cleft


Presynaptic neuron

Presynaptic neuron has synaptic

vesicles that contain neurotransmitters


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.



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


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


Synapse

Animation—The SynapsePLAY

the_synapse.htm


Neurotransmtters

Acetylcholine

Acts in both the PNS and the CNS as a

neurotransmitter

Causes voluntary muscles to contract

Acetylcholinesterase

Myasthenia gravis is an autoimmune

disease that attacks the acetylcholine

receptors, resulting in reduced muscle

strength


Important Concepts

Read Chapter 8

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

their functions

What are the two main divisions of nervous

system (CNS, PNS) and where each is found


Important Concepts

What are the parts and functions of a neuron

What are the three types of neurons (sensory,

interneuron and motor neurons) and their

functions, and where are they located

Where are the cell bodies are located for motor

and sensory nerve cells


What are schwann cells and oligodendrocytes and

what are their function

Where Schwann vs oligodendrocytes are found

What is the cause and 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 messages being conducted

through a neuron, starting with the resting stage

and ending with the next neuron or muscle being

stimulated.

Important Concepts


What ions enter and the leave the neuron during

the depolarization and repolarization steps of

action potential, what is the relative charge of the

inside vs the outside of the neuron during these

events, what is the order of events.

Components of the synapse

Function of neurotransmitters, how do they work,

where do they work, know the ions involved and

their functions.

Important Concepts


What is acetylcholine, where is it found, what effect

does it have, how is acetylcholine removed from

the synaptic cleft

What is the cause and effect of Myasthenia gravis

Important Concepts


Definitions

Afferent neurons, efferent neurons, dendrites, axons, sensory neurons, interneuron, motor neurons, myelin, myelin sheath, myelinated neurons, schwann cells, oligodendrocytes, nodes of ranvier, nerve, ions, ion channels, ligand gated ion channels, voltage gated ion channels, action potential, repolarization, depolarization, membrane potential, resting potential, sodium potassium pump, refractory period, synapse, synaptic cleft, synaptic vesicles, neurotransmitters, acetylcholinesterase, presynaptic neuron, postsynaptic neuron, stimulate, inhibit

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Nervous System - Neurons...Neurons of the Peripheral Nervous System Neurons in the PNS are either carrying messages to or from the CNS Afferent = Sensory neurons = Neurons carrying