Human Biology: Nervous System

Lesson 1: Neurons to Reflex Arc

(Inquiry into Life pg. 319-325)

Today’s Objectives Analyse the transmission of nerve impulses,

including: Identify and give functions for: dendrite, cell body, axon,

axoplasm, axomembrane Differentiate among sensory, motor, and inter-neurons

with respect to structure and function Explain the transmission of a nerve impulse through a

neuron, using applicable terminology (see handout) Relate the structure of a myelinated nerve fibre to the

speed of impulse conduction, with reference to applicable structures (see handout)

Identify the major components of a synapse (see handout) Explain the process by which impulses travel across a

synapse Describe how neurotransmitters are broken down in the

synaptic cleft Describe the structure of a reflex arc (see handout), and

relate its structure to how it functions

Neurons

Learn Some Stuff With a partner, prepare a large diagram which

includes the following terms: Motor neuron, interneuron, sensory neuron, axon,

dendrite, cell body, node of Ranvier, myelin sheath, Schwann cell

Below each term write one or two sentences to describe the function of each.

Be sure to explain the difference between motor neuron, interneuron and sensory neuron.

Reading pages 320 – 321 will help!

Structures and Functions 1) Dendrites

Conduct a nerve impulse (message) towards a cell body

Many dendrites enter a cell body 2) Cell Body

Contains the nucleus and cell organelles needed to keep the cell alive

Only a single axon leaves a cell body Relays impulse from dendrite to axon

3) Axons Conduct a nerve impulse away from the cell body

Structures and Functions 4) Myelin Sheath

Protective coating of Schwann Cells around larger axons

5) Nodes of Ranvier Interrupted areas on the Myelin Sheath Speeds up transmission of impulse Animation

6) Motor End Plates In close proximity to muscles and organs From here the impulse is chemically transported to the

organs 7) Ganglia

A collection of cell bodies outside of the CNS

Types of Neurons (see animation)

1) Motor Neuron-Efferent Neuron: Moving toward a

central organ or point. -Relays messages from the brain or spinal cord to the muscles and organs.

Types of Neurons2. Sensory Neuron (see animation)

-Afferent Neuron: Moving away from a central organ or point. -Relays messages from receptors to the brain or spinal cord

Types of Neurons3. Interneuron (associated neuron or

Connector Neuron) (see animation)-Relays message from sensory neuron to motor neuron.-Make up the brain and spinal cord.

Bases of Bases of ComparisonComparison

Sensory Sensory NeuronNeuron

InterneuronInterneuron Motor NeuronMotor Neuron

Length of Length of FibersFibers

            

LocationLocation      

  FunctionFunction

Long Dendrites Long Dendrites and short and short AxonAxon

    Cell body and Cell body and

Dendrite Dendrite are outside are outside of the spinal of the spinal cord; the cord; the cell body is cell body is located in a located in a dorsal root dorsal root ganglion.ganglion.

    Conduct Conduct

impulse to impulse to the spinal the spinal cord.cord.

Short Short Dendrites Dendrites and short or and short or long Axonslong Axons

  

Entirely within Entirely within the spinal the spinal cord or CNS.cord or CNS.

            Interconnect Interconnect

the Sensory the Sensory neuron with neuron with an an appropriate appropriate Motor Motor Neuron.Neuron.

Short Short Dendrites Dendrites and long and long AxonsAxons

    Dendrites and Dendrites and

the cell the cell body are body are located in located in the spinal the spinal cord; the cord; the Axon is Axon is outside of outside of the spinal the spinal cord.cord.

    Conduct Conduct

impulse to impulse to an effector an effector (muscle or (muscle or gland)gland)

           

           

***A nerve is composed of long fibers of a number of Neurons***

Impulse Generation (Action Potential) Nerve impulses are electrical messages If we measure the voltage of a resting neuron

using a voltmeter, we will see a reading of -60 millivolts

Voltage is a comparison of electrical charge between two points

When the neuron is stimulated, the charge changes briefly to +40 millivolts (mv), then back to -60 mv

(-60 mv means that the inside is 60 mv more negative than outside)

If we hook up our voltmeter to a machine called an oscilloscope, we can see the change in voltage over a period of time

There is a difference in ion distribution on either side of the membrane of a neuron.

 At Rest: Na+ outside the Neuron

K+ and large negatively charged organic molecules inside the neuron.

The concentration of sodium ions Na+ is greater outside the axon than inside.

The concentration of potassium ions K+ greater inside the axon than outside.

This unequal distribution is due to the sodium-potassium pump which actively transports Na+ out and K+ into the axon

Action Potential The membrane is more permeable to K+ ions,

and some K+ diffuses back while Na+ does not. This unequal charge distribution, along with

of the large negative molecules, causes the inside to be more negative than the outside.

Na+ K+ Na+ Na+ __________________________ -- -- K+

K+ -- -- -- K+__________________________  Na+ K+ Na+ Na+

This situation is called Resting Potential. -60mv When the axon or dendrite is stimulated, sodium

gates open which allows some Na+ to enter the Axoplasm (interior). Now, the inside becomes more positive than the outside by 40 mv. (see video)

Na+ Na+ Na+

  -- K+ -- K+ K+ -- -- -- K+

  Na+ K+ Na+ Na+

This is called the Upswing Phase of the action potential. The charge changes from –60 mv to +40 mv. The change is called Depolarization.

  After the sodium gates have opened, then potassium gates open. K+ exit the axoplasm.

K+ K+

-- K+ Na+ -- Na+ Na+ -- K+ -- K+ -- Na+ K+ K+

K+ K+ This is called the Downswing Phase of the

action potential. The charge returns to –60mv. The change is called Repolarization.

  **Note: Charge is back to normal, but ions are

reversed

Finally, there is a Recovery Phase in which the sodium/potassium pump (ACTIVE TRANSPORT) returns Na+ to the outside and K+ to the inside.

This is called the refractory period. During the refractory period, another action

potential cannot be created.

Action Potential So far we have only been looking at one point

on the Axon or Dendrite The opening of the sodium gates in one area

causes the sodium gates in the next area to open

We get a wave motion (chain reaction) moving down the nerve fiber

Na+

+ + + + + + + + + +

- - - - - - - - - - -

K+

 

- - - - - + - - - - - - -

+ + + + + + + + + + + Na+

+ + + - - + + + + + + K+

+ + + + - - + + + + +

- - - - - + + - - - - - -

K+

Na+

1. RESTING POTENTIAL -Charge is –60mv

-Na+ outside -K+ inside 2. UPSWING OF ACTION POTENTIAL -Depolarization

-Charge from –60mv back to +40mv -Na+ moves inside -(Sodium gates opened)

3. DOWNSWING OF ACTION POTENTIAL -Repolarization

-Charge from +40mv back to –60mv -K+ moves outside -(Potassium gates opened)

 4. RECOVERY PHASE -Charge is –60mv

-Sodium/Potassium Pump -Moves Na+ out and K+ inside

  ****NOW NEURON CAN BE RE-STIMULATED ****

**REMEMBER THIS IS A WAVE MOTION DOWN THE NEURON**

Myelinated Vs Unmyelinated Fibers

Schwann cells

Nodes of Ranvier

Myelinated Neuron

Myelinated Neuron In vertebrates, where most long nerve fibres In vertebrates, where most long nerve fibres

have myelin sheath around them, the Schwann have myelin sheath around them, the Schwann cells restrict ion movement across the cells restrict ion movement across the axomembrane and the impulse “jumps” axomembrane and the impulse “jumps” between successive nodes of Ranvier, thus between successive nodes of Ranvier, thus speeding up the impulse. speeding up the impulse.

This type of “jumping” transmission is called This type of “jumping” transmission is called saltatory conduction. saltatory conduction.

It is Schwann cells that wrap around the nerve fiber. When it is myelinated, it is covered by Schwann cells. Impulse skips from node to node. (see video)

UnMyelinated Neuron

Each action potential starts the next Each action potential starts the next action potential, causing a wave along action potential, causing a wave along the entire neuron. the entire neuron.

In myelinated axons and dendrites, the impulse can travel up to 200m/s. In unmyelinated fibers, the impulse can be as slow as 0.5 m/s.

This difference in speed is because the action potential is able to jump over the myelin sheath. Depolarization only occurs at the nodes of Ranvier.

Synapse (see video) 

Each axon branches off and ends with a swelled tip or terminal knob that lies close to but not touching the dendrite of another neuron. (or an organ). The entire region is called a synapse.

 

Transmission of nerve impulses across a Transmission of nerve impulses across a Synaptic cleft is carried out by Synaptic cleft is carried out by chemicals called chemicals called NeurotransmittersNeurotransmitters. .

These substances are stored in vesicles These substances are stored in vesicles at the end of the axon. Noradrenalin at the end of the axon. Noradrenalin (speeds up activity) and acetylcholine (speeds up activity) and acetylcholine (slows down activity) are examples of (slows down activity) are examples of neurotransmitters.neurotransmitters.

When an impulse reaches the end of the axon like it usually would, not only does Na+ come into the axon, but Ca+2 as well since voltage gated calcium channels are opened.

This calcium binds with contractile proteins that pull the Neurotransmitter vesicles to the membrane surface. The vesicles join with the cell membrane, forcing the neurotransmitter into the cleft (exocytosis)

**Ca+2 causes the microfilaments to contract and pull the synaptic vesicles to the presynaptic membrane*

Neurotransmitter’s job is to increase the permeability of the sodium ions on the postsynaptic membrane.

The Neurotransmitter binds to The Neurotransmitter binds to specific receptor sites on the specific receptor sites on the dendrite of the next neuron. If dendrite of the next neuron. If enough transmitter substance is enough transmitter substance is received, the neuron will “fire” and received, the neuron will “fire” and continue the impulse.continue the impulse.

A neurotransmitter only has a short period to work once it has been released into the synaptic cleft.

Enzymes rapidly break down the transmitter substance to clear the synapse so the next impulse can be transmitted.

Monoamine oxidase breaks down noradrenaline and Acetylcholinesterase breaks down acetylcholine.(see video)

Pain killers such as Tylenol act as an enzyme to break down the neurotransmitter to decrease the pain impulse. A natural painkiller in the body is Prostaglandin.

An impulse can only travel across a synapse in one direction. Only the axon contains neurotransmitter vesicles, so the impulse can only travel one way

AXON DENDRITE across a synapse.  **** ALL OR NONE LAW (threshold): If enough

neurotransmitter is received by the postsynaptic fiber, it will fire 100%. (all). If not enough substance is received, it will not fire at all. (none)

There are excitatory and inhibitory neurotransmitters in the body. When two excitatory neurotransmitters work together to cause an action potential, it is called summation.

Reflex Arc Reflexes are automatic, involuntary responses to

changes occurring inside or outside the body. Some involve the brain (such as blinking the eye),

while others do not (such as moving your hand away from a hot object).

Why does the brain not have to be involved?

If it were, by the time the impulse traveled to the brain, the brain figured out what was happening, and sent a response to the body, serious damage

might occur.

So the body evolved a method of by passing the brain.

Stages of Reflex Arc  1. Receptor is stimulated and formulate

message. ie. nerve impulse  2. Sensory neuron takes the message to the

Central Nervous System. (spinal cord)

3. Interneuron passes the message to a motor neuron.

 

4. Motor Neuron takes the message away from the C.N.S. to the effector. (muscle/organ)

5. The muscle receives the message and contracts.

  ***The brain finds out later what had

happened***