1-2 december 2014. what are the functions of the nervous system? receive sensory input, interpret...
TRANSCRIPT
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.