1-2 December 2014. What are the functions of the nervous system? Receive sensory input, interpret & make a decision, and effect a response Individual.

Nerve Impulses and Reflex Arcs

1-2 December 2014

What are the functions of the nervous system?Receive sensory input, interpret & make a decision, and effect a response

Individual neurons have two major functional properties: irritability and conductivity. Irritability = ability to respond to a stimulus and

convert it to a nerve impulse Conductivity = ability to transmit the impulse to

other neurons, muscles, or glands.

Neuron Functions

How do irritability and conductivity relate to the functions of the nervous system?

Irritability = ability to respond to a stimulus and convert it to a nerve impulse

What type of stimulus? ◦ Receptor neurons respond to specific stimuli (e.g. touch,

temperature, pressure, tension, sight, hearing, chemical) ◦ All other neurons (interneurons, motor neurons) respond to

neurotransmitters released by other neurons What is a nerve impulse?

◦ A quick switch in voltage potential (charge difference) across the membrane that travels all the way along the axon of the neuron

◦ Occurs due to flow of ions across the membrane◦ All – or - nothing

Irritability

Now to understand the process of a nerve impulse / action potential in much more detail …

Neurons at rest have an electrochemical gradient across the cell membrane, known as the resting potential.

The resting potential is approximately -70mV.

Resting Potential

Cell membrane

cytoplasm

Extracellular fluid

The resting potential is maintained by:

Sodium-potassium pump (3 Na+ pumped out for every 2 K+ pumped in)

Greater membrane permeability of K+

(K+ can diffuse back out to some degree)

Organic anions (- ions) within cell

Resting potential

Cell membrane

cytoplasm

Extracellular fluid

K+

Na+

Organic anion

This ‘rest state’ takes energyto create

An action potential involves the rapid depolarization and repolarization of the membrane.

Action Potential

When a stimulus is applied to a nerve, some Na+ gates open, allowing Na+ to diffuse in.

Action potential

Cell membrane

cytoplasm

Extracellular fluid

K+

Na+

Organic anion

Na / K pump

Na gate

K gate

Once a threshold is reached, all Na+ gates open, causing depolarization of the membrane.

When the membrane is depolarized, the inside of the membrane is more positively charged than the outside.

Action potential

Cell membrane

cytoplasm

Extracellular fluid

K+

Na+

Organic anion

Na / K pump

Na gate

K gate

Membrane repolarization occurs when Na+ gates close and K+ gates open, allowing net diffusion of K+ outside.

Repolarization returns the membrane to resting potential (more negatively charged inside)

Action potential

Cell membrane

cytoplasm

Extracellular fluid

K+

Na+

Organic anion

Na / K pump

Na gate

K gate

The K+ gates close and the resting potential is maintained by the Na+ / K+ pump

Action potential

Cell membrane

cytoplasm

Extracellular fluid

K+

Na+

Organic anion

Na / K pump

Na gate

K gate

Watch me!

Scholar with more siblings … At rest, what ions are most abundant outside the

cell, and which are most abundant inside the cell? Which side of the membrane is more negative at

rest?

Scholar with less siblings … Describe how the movement of ions causes

◦ Depolarization◦ Repolarization

Action Potential – Turn & Talk

Scholar with more siblings … At rest, what ions are most abundant outside the cell,

and which are most abundant inside the cell? Which side of the membrane is more negative at rest?

At rest, Na+ ions are mostly outside the membrane, while K+ and anions are mostly inside. The inside of the cell membrane is more negatively charged.

Scholar with less siblings … Describe how the movement of ions causes

◦ Depolarization – Na+ ions rush into the cell

◦ Repolarization – K+ ions rush out of the cell

Action Potential – Turn & Talk

The nerve impulsemoves along theaxon.

The change in voltage of one

area triggers thedepolarization of the next area.

Repolarization follows immediately.

Nerve Impulse Propagation

In myelinated neurons the impulse “jumps” from node to node, rather than traveling the whole length of the axon – makes the impulse transmission much more efficient.

Nerve Impulses

Watch me!

..and me!

Generation and propagation of nerve impulse along one neuron= irritability

Conductivity is the ability of one neuron to signal another. This occurs in an entirely different fashion at the synapse, or gap, between neurons.

Irritability vs. Conductivity

Synapses• The fluid-filled space betweenneurons is called a synapse

• Chemicals called neurotransmitters carry the nerve impulse across the synapse.

Synapses

1. The nerve impulse reaches the axon terminal.

2. Ca+ gates open, allowing Ca+ into the axon.

3. The Ca+ causesvesicles containingneurotransmitters to empty into the synapse

Synapses4. The neurotransmitters diffuse across the

synaptic cleft and binds with receptors of the next neuron.

5. Na+ channels openin the dendrites of the post-synapticneuron

6. Post-synaptic neuron depolarizes

7. Remainingneurotransmitter isbroken down.

Pre-synaptic neuronAction potential calcium ions release neurotransmitter release

Post-synaptic neuronNeutrotransmitter uptake sodium gates open action potential

Note: information travels as electrical signal within neurons and as chemical signal between them

Synapses

Watch me!

Reflex Arcs Reflexes

are rapid, predictable,involuntary responses to stimuli.

May be somatic or autonomic

Contain 5 elements

Watch me!

Closure What were our objectives today, and what did we

learn about them? What was our learner profile trait and how did we

use it? How does what we did today relate to our unit

question?

HOMEWORK: Quiz on Thursday / Friday!

Exit TicketCount off by three.1) Identify the steps that take place during an

action potential. Make a diagram of the cell membrane demonstrating the changes that occur during each step.

2) Draw a diagram of the voltage changes that occur during an action potential. Label the diagram with the changes that occur within the cell during each step.

3) Identify the steps that take place during synaptic transmission. Make a diagram of the events that within the pre- and post-synaptic neurons during each step.