nervous system: neurons - napa valley college 9 - neuron… · neurons of the peripheral nervous...

Nervous System: Neurons

Biology 105 Lecture 9 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. Nerve signal transmission VI. Neuronal synapse


Nervous Tissues

Nervous tissue functions to conduct messages throughout the body.

When a nerve cell is stimulated, an electrical signal quickly travels through the nerve cell to the nerve ending, triggering events.


Nervous System

Includes nerve tissue and sensory organs

Nervous system functions to:

Sense the environment – receives information from both outside and inside the body.

Process 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 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. Neuroglial cells – cells that support the

neurons.


Neuroglial 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


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.


Neurons of the Nervous System


The cell bodies of these neurons are located in the dorsal root ganglion:

Motor

Senso

ry

50%50%1. Motor 2. Sensory


These neuroglial cells provide nutrients to neurons:

Micr

oglia

Astr

ocytes

Olig

oden

rocy

tes

Schwan

n cells

25% 25%25%25%1. Microglia 2. Astrocytes 3. Oligodendrocytes 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?

Senso

ry M

otor

50%50%

1. Sensory 2. Motor


Myelinated Neurons

Some neuroglial cells form a substance called myelin that is rich in lipid and contains proteins. These neuroglial cells wrap themselves around

neuronal axons. The neurons that have axons covered with these

neuroglial cells are called myelinated neurons.

Myelinated neurons are able to carry messages faster than non-myelinated neurons. Myelin is an excellent electrical insulator!


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 neuronal axons.


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.

Nodes of Ranvier are spaces on the axon between the glial cells.


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

A nerve contains many neuronal axons 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(n):

Neu

tron

Proton

Electro

n

33% 33%33%1. Neutron 2. Proton 3. Electron


How can an ion pass through a membrane?

Simple

diffusio

n

Facilit

ated d

iffusio

n

Active

transp

ort

Both 2 an

d 3

All of th

e 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 (= 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.


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 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.


The Nerve Impulse

Figure 7.5 (4 of 4)


The Nerve 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.


When a neuron is resting, sodium ions have a greater concentration:

insid

e the n

euro

n cell

outside

the n

euro

n cell

conce

ntrati

on 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?

Calc

ium (C

a++)

Sodium (Na+

)

Potassiu

m (K+)

Chlorin

e (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:

Positive

ly ch

arged

Neg

ative

ly ch

arge

d

50%50%1. Positively charged 2. Negatively charged


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.



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.


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.


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?


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


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

nervous system: neurons - napa valley college 9 - neuron… · neurons of the peripheral nervous...

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