Nervous System - Neurons Slides PDF/Lect09... · Sodium Potassium Pump To maintain this resting...

Nervous System - Neurons

Biol 105

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 nervous tissue and sensory organs.

Nervous system functions to:

Sense the environment – it receives

information from both outside and inside

the body.

Process the information it receives.

Respond to information – send 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


Figure 8.1 The nervous system


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.


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.

Axon – one projection that carries

messages away from the cell body (can

be very long).


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

Interneurons are located between

sensory and motor neurons within

the CNS.

Interneurons integrate and interpret

sensory signals.


Figure 8.1 The nervous system


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

Moto

r

Sen

sory

50%50%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

Mic

roglia

Ast

rocy

tes

Olig

oden

rocy

tes

Sch

wan

n cel

ls

25% 25%25%25%1. Microglia

2. Astrocytes

3. Oligodenrocytes

4. Schwann cells


These are projections of the neuron cell body that carry

messages to the cell body

Axo

ns

Den

drite

s

50%50%1. Axons

2. Dendrites


Which of the following type of neuron would alert the brain

that you had touched a hot object?

effer

ent n

euro

n

affer

ent n

euro

n

50%50%1. efferent neuron

2. afferent neuron


What type of neuron is the arrow pointing to?

Sen

sory

Moto

r

50%50%1. Sensory

2. Motor


Myelinated Neurons

Neurons that have axons covered with

neuroglial cells that contain the protein

myelin are called 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 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.


Myelin Sheath

Figure 7.3b


Myelin Sheath

Figure 7.3c


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.


Nerves

Nerves contain Neuron axons that

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

Neu

tron

Pro

ton

Ele

ctro

n

33% 33%33%1. Neutron

2. Proton

3. Electron


How can an ion pass through a membrane

Sim

ple d

iffusi

on

Fac

ilita

ted

diffus

ion

Act

ive

transp

ort

Both

2 a

nd 3

All

of the

above

20% 20% 20%20%20%

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 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.


NA/K ATPase


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 and allow K+ to

flow 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


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:

insi

de th

e neu

ron c

ell

outs

ide

the

neuro

n cel

l

conce

ntra

tion is

the

...

33% 33%33%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?

Cal

cium

(Ca+

+)

Sodiu

m (N

a+)

Pota

ssiu

m (K

+)

Chlo

rine

(Cl-)

25% 25%25%25%1. Calcium (Ca++)

2. Sodium (Na+)

3. Potassium (K+)

4. Chlorine (Cl-)


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

becomes

Posi

tivel

y ch

arged

Neg

ativ

ely

char

ged

50%50%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 the 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 may be 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.


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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