The Nerve Impulse and Reflex Arcs

Pages 234-239

http://highered.mheducation.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

Nerve impulse animation

Neurons have two functional properties

◦ Irritatability – respond to a stimulus and convert it into a nerve impulse

◦ Conductivity – ability to transmit the impulse to other neurons, muscles, or glands

Functional Properties of Neurons

© 2015 Pearson Education, Inc.

When neurons are in a “resting” state:◦ They are polarized

inside the plasma membrane is more negative than outside more potassium (K) inside the cell More sodium (Na) outside the cell

a gradient is created

the cell stays at rest

Waiting for a nerve impulse…

[Na+]

[K+]

© 2015 Pearson Education, Inc.

A stimulus depolarizes the neuron’s membrane

The membrane is now permeable to sodium sodium channels open sodium (Na) diffuses into the membrane the inside becomes more positive

This is how an action potential can be initiated

The potential for a nerve impulse…

© 2015 Pearson Education, Inc.

A nerve impulse (action potential) is generated: If the stimulus is strong enough If sodium influx great enough

If enough sodium enters the cell, the action potential (nerve impulse) starts and is propagated over the entire axon

Action potential generation

Na+

Na+

© 2015 Pearson Education, Inc.

the nerve impulse is propagated or it is not There are no partial impulses

Repolarization: the membrane is restored to resting potential

Negative inside, positive outside The sodium-potassium pump restores resting potential

using energy supplied by ATP Another impulse cannot take place until the entire

membrane has been repolarized

All-or-none response

K+

K+

© 2015 Pearson Education, Inc.

When the action potential reaches the axon terminal:1. calcium channels open2. vesicles containing neurotransmitters fuse with

the axonal membrane3. neurotransmitters diffuse across the synapse

and bind to receptors on the membrane of the next neuron

The amount of neurotransmitter released will determine if the action potential will continue ◦ This is known as graded potential

The Synaptic Events

Actionpotentialarrives.

Figure 7.10 How neurons communicate at chemical synapses.

Axon oftransmittingneuron

1

Receivingneuron

Dendrite

Axon terminal

Vesicles

Synapticcleft

Slide 2

Figure 7.10 How neurons communicate at chemical synapses.

Transmitting neuron Vesiclefuses withplasmamembrane.

Synapticcleft Ion

channelsNeurotransmittermolecules

Receiving neuron

Slide 3

2

Figure 7.10 How neurons communicate at chemical synapses.

Transmitting neuron Vesiclefuses withplasmamembrane.

Neurotrans-mitter isreleased intosynaptic cleft.

Synapticcleft Ion

channelsNeurotransmittermolecules

Receiving neuron

Slide 4

2

3

Figure 7.10 How neurons communicate at chemical synapses.

Transmitting neuron Vesiclefuses withplasmamembrane.

Neurotrans-mitter isreleased intosynaptic cleft.

Neurotrans-mitter bindsto receptoron receivingneuron’smembrane.

Synapticcleft Ion

channelsNeurotransmittermolecules

Receiving neuron

Slide 5

2

34

© 2015 Pearson Education, Inc.

The transmission of a nerve impulse down neuron is electrical

The transmission of a nerve impulse to next neuron is chemical

Signal transmission is electrochemical

© 2015 Pearson Education, Inc.

Somatic reflexes◦ Reflexes that stimulate the skeletal muscles◦ Example: pulling hand away from a hot object

Autonomic reflexes◦ Regulates smooth muscles, heart, and glands◦ Example: regulation of blood pressure, glands,

digestive system

The Reflex Arc

© 2015 Pearson Education, Inc.

Reflexes are rapid, predictable, involuntary responses to a stimulus

A reflex arc can include:1. Sensory receptor—reacts to a stimulus2. Sensory neuron—carries message to

integration center3. Integration center (CNS) –

processes information directs motor output

4. Motor neuron —carries message to an effector 5. Effector organ —muscle or gland to be

stimulated

The Reflex Arc and its components

Figure 7.11a Simple reflex arcs.

1

23

45

Stimulus at distalend of neuron

Skin

Receptor

Effector

Sensoryneuron

Motor neuron

Spinal cord(in cross section)

Integrationcenter

Interneuron